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SPPA 2050 Speech Anatomy and Physiology
What is Anatomy and Physiology? A Brief Introduction
This course focuses on the integrated study of
descriptive anatomy and physiology associated with
normal speech production. The term descriptive
means that you will use words and pictures to
acquire and represent your knowledge of the body.
Anatomy refers to the study of the structure of
organisms and the relation of their parts. Our
general notion about anatomy, derived largely from
television programs such as CSI and ER, is that it
involves the dissection and description of body
parts. However, anatomy is a vast discipline that
includes large things like bones and very small
things like cells and cell organelles. Within the
domain of anatomy, there are a number of
specializations. A few examples include
Systemic anatomy: Considers the body as a
composition of a number of distinct systems, each
with relatively homogeneous tissue and/or function.
Regional/topographical anatomy: Considers the
relations between structures that are within a region
of the body. For example, the head may be studied
as a whole even though it contains portions of a
number of body systems.
Clinical/applied anatomy: Concerned with the study
of anatomy that has relevance for practical sciences
such as medicine, surgery, or speech pathology.
Radiologic anatomy: The study of anatomy revealed
through imaging techniques. Imaging techniques
allow the investigation of non-visible parts without
harming or killing the organism.
However,
different techniques will reveal different body parts
with varying levels of resolution. Examples of
imaging technology include magnetic resonance
imaging (MRI), computerized tomography (CT),
and positron emission tomography (PET).
Microscopic anatomy: Concerned with the structure
that cannot be seen with the naked eye (in contrast
to gross anatomy). This will include the study of
cells and their parts (cytology) and the study of
body tissue (histology). As the name implies, this
involves the use of microscopes of various types
(e.g. light, polarized light, electron etc).
Developmental anatomy/embryology: Deals with
the growth and development of an organism.
Pathologic anatomy/pathology: Concerned with the
effect of disease on anatomical structure. These
may include gross or microscopic changes.
Comparative anatomy: The study of structure across
more than one species. For example, we might
compare the tongue of the human with the tongue of
the frog.
Anthropological anatomy: The anatomical study of
persons in history or across race and ethnicity.
Our anatomical investigations over the semester
will delve into many of these areas.
Physiology usually refers to the functioning of
living organisms or their parts. Physiology, like
anatomy, is also a broad discipline, which includes
specializations. For example, one could study the
physiology of small structures such as the cell or its
components (cellular physiology), larger bodily
systems
(e.g.
neurophysiology,
respiratory
physiology) or how disease influences function
(pathophysiology).
Why study Anatomy and Physiology Together? A Brief Rationale
Often, we study anatomy and physiology together
because we want to understand why people (and
their bodies) operate, behave, and react the way
they do. In a discipline like Communicative
Disorders, where clinicians deal daily with speech,
language, and hearing behaviors that are disordered
in some way. A creed underlying clinical practice
is that disordered behaviors can be remediated,
moved closer to some state of order. A related,
natural assumption is that all behaviors ordered or
not, take some part of their root in the structure of
the object that generates them. After all, we say that
SPPA 2050 Speech Anatomy and Physiology
machines that do a good job at what they are
supposed to do – cars that run well or lawnmowers
that really cut the grass – are well-designed and
well-built. Thus, we can argue that we study
anatomy because we want to understand how,
when, and to what degree we can “move” behaviors
from some disordered, undesirable state, to some
ordered state, more easily tolerated by clients and
their communities.
In general, when we integrate the study of the
structure of things, and their operation, we usually
try to work out some “map” between their form and
function. We borrow the idea of a map from
mathematics, and use the term to refer to some
relational statement that links form and function, in
the best of all worlds, in some cause-and-effect
way. The fundamental value of such mapping
statements can be illustrated by the hypothetical
example that follows.
We can use lengths of tubing to produce sounds.
You can create a sound by slapping the palm of
your hand against the end of a piece of steel tubing
whose length is a good deal greater than its
diameter. The tube acts as a resonator for the sound
your hand produces. The tube amplifies a tone
whose pitch (i.e., perceived frequency) depends
upon the geometry of the cavity inside the
resonator. The geometry formed by a uniform piece
of tubing is easy to describe, using only two
features. The cavity has a length L, and a crosssectional area A. L and A completely describe the
“anatomy” of the resonating cavity. For
convenience, we can use the symbol F to represent
the pitch of the tone created by the tube. Thus, F is a
variable that describes the resonator’s function.
After a little experimentation, it is easy enough to
establish that F depends upon, and is inversely
proportional to L, but is independent of A (as long
as L is significantly bigger than A). When the tube
is long (and L is a high number), the pitch of its
tone is low (and F is a low number). Conversely,
when the tube is short, the pitch of its tone is high.
This is a handy fact to know about the relationship
between the structure (L and A) and the function (F)
of the tube. Now, all of this information may be
fine if you plan to assemble a Steel Tube Marching
Band, but what does this have to do with Anatomy,
Physiology, and Communicative Disorders? This is
a concrete example of how, by generating a simple
map between form and function, we can take a
rational approach to altering the function of these
simple tubes. Similarly, we study speech anatomy
and physiology, together, because we hope to
understand maps that might exist between how
people are built, and how they behave. These maps
may be used to provide rational bases for
therapeutic intervention when (speech) behaviors
are not what we, or our clients, would like. If that’s
not motivation, I don’t know what is.
Terminology and Reference Frames: Knowing Up from Down, Front from Back, In from Out, etc.
Your gross anatomical obligations in SPPA 2050
are basically of two types. On the one hand, you
have an obligation to learn the classical names of
body parts (e.g., umbilicus for belly button; clavicle
for collar bone; gluteus maximus for what’s often
beneath the seat of your pants). You might consider
this the “French” part of anatomy. You have to
learn -- by memorizing! -- new, correct names (e.g.,
in French, chat and chapeau) for many things for
which you may already have familiar names (e.g.,
English cat and hat). Of course, we learn French so
that we can communicate with the French when
necessary. We learn anatomical terminology so that
we can communicate with other health and alliedhealth professionals. There is no way to avoid the
French part of anatomy. This is simply a burden
you are obligated to bear.
On the second (anatomical) hand, you also have an
obligation, to describe the “geography” of the body.
In effect, this obligation boils down to describing
where body parts, and parts of parts, are with
respect to one another. Thus, your anatomical
education will involve a second important
SPPA 2050 Speech Anatomy and Physiology
vocabulary, in addition to the classical lexicon for
body parts. This second vocabulary includes a
simple set of terms that convey direction, and some
rules of thumb that allow you to interpret these
direction terms.
Establishing a frame of reference suitable for
anatomical geography is a more interesting problem
than you might imagine, largely because certain
choices have to be made, and defended. At least
one feature of the necessary frame of reference is
determined by the object we want to describe. The
body is three-dimensional, and thus, the frame of
reference we use to describe its geography must
also be three-dimensional. Two familiar threedimensional reference-frame options are possible.
One of these is a spherical coordinate system, in
which the position of any object is defined in terms
of a distance from some central point of origin, and
two angles of inclination or deflection. (A spherical
coordinate system is merely an “extended” version
of the more familiar two-dimensional polar
coordinate system we sometimes use to define the
location of one place on the locally flat Earth with
respect to some other place). In simple polar terms,
we might easily say that South Haven, MI is about
36 miles due west of Kalamazoo. This type of
statement is easy and familiar to make and
understand. An alternative and possibly more
familiar three-dimensional frame of reference is the
ever popular Cartesian coordinate system in
which the position of any object is defined in terms
of three displacements from some point of origin,
along mutually orthogonal axes.
(A threedimensional Cartesian system is merely an
“extended” version of the familiar two-dimensional
Cartesian system we often use to describe where a
library might be with respect to the neighborhood
gas station (e.g., 2 blocks south, and 3 blocks west.)
Two facts (?) suggest that a Cartesian system might
be preferable in anatomy. In general, people seem to
be mentally more “at home” with Cartesian than
spherical systems. Moreover, a spherical system
demands a specific well-defined center point, from
which all distances are measured. It is hard to
imagine what point or object in the body might
serve that role. The origin that is also necessary for
a Cartesian system is less troublesome to define.
We now take on the chore of building up a threedimensional Cartesian frame of reference to be used
in our studies. It is important that we place the
frame of reference locally in the body, using body
parts to define its axes. This is because we want the
axes of the frame of reference to stay with the body
as it moves about in space. In local body terms, it is
important that up is always the same up. The
location of the diaphragm should always be the
same with respect to the lungs, whether the body is
standing, lying, or on its head. We insure that this
will be so, as long as we establish a local, bodybased frame of reference. Described below are three
orthogonal planes that you will become very, very
familiar with over the next few months. Note the
body position of the poor soul who is cut up into
various pieces by these planes. He is in what we
call the anatomical position, standing upright, face
forward, arms to the sides, with the palms of the
hands forward and thus parallel to the face.
The first plane, known as the mid-sagittal plane,
cuts the body into (ideally symmetric) right
(dextral) and left (sinistral) halves. Since any plane
can be defined by any three non-collinear points, we
can locate the mid-sagittal plane in the body by two
points defining the “line” that corresponds to the
mid-sagittal suture, joining the right and left parietal
bones of the calvaria. We can then designate our
third “point” to be in the vicinity of any of those
landmarks in the middle of the body of which there
is only one: e.g., the diastema (a fancy word for
gap) between the frontmost of the maxillary teeth;
the “tip” of the xyphoid process at the lower margin
of the sternum; the symphysis between the pubic
bones belonging to the right and left halves of the
pelvis; the lower-most “tip” of coccyx. The plane
containing the line of the sagittal suture, and any or
all of these mid-line “points” is the mid-sagittal
plane. All planes parallel to the mid-sagittal plane
are also sagittal planes, though there can be only
one mid-sagittal plane.
SPPA 2050 Speech Anatomy and Physiology
Our second plane must be perpendicular to the midsagittal plane, and might be placed to cut the body
roughly into front (ventral or anterior) and rear
(dorsal or posterior) “halves”, passing through the
coronal suture “line”, running right to left across the
external surface of the calvaria. The coronal suture
represents the immobile joint between the frontal
bone and parietal bones, and provides a basis for the
name of our second reference plane, typically the
coronal (or sometimes, frontal) plane. A third
transverse plane, perpendicular to both the midsagittal and coronal planes, can be used to cut the
body into top (rostral, superior or cephalic) and
bottom (caudal or inferior) “halves”. Often, the
transverse plane is centered in the vicinity of the
umbilicus, though this is not a necessary placement.
In fact, if we want to choose a frame of reference
well-suited to the description of positions of the
articulators (e.g., tongue, jaw and lips, relative say
to the head), we might place the transverse plane in
the vicinity of the caudal edges of the maxillary
teeth, a plane dentists sometimes refer to as the
maxillary occlusal plane.
Note that in the foregoing paragraphs, certain new
terms -- dextral, sinistral, ventral, dorsal, rostral,
and caudal -- were introduced to refer to directions
associated with our three-dimensional Cartesian
frame of reference. From this point on, these terms
will provide unambiguous ways to indicate
direction senses. Note that all lines perpendicular to
the mid-sagittal plane “point” dextrally or
sinistrally; all lines perpendicular to the coronal
plane point ventrally (forward) or dorsally
(backward); and, all lines perpendicular to the
transverse plane point rostrally (up) or caudally
(down).
The mid-sagittal and coronal planes
intersect to form a rostral-caudal axis; the midsagittal and transverse planes intersect to form a
ventral-dorsal axis; and, the coronal and transverse
planes intersect to form a dextral-sinistral axis.
The following are several terms that are common in
descriptive anatomy that you will immediately
begin to use. Get familiar with them and talk to
your friends using them.
Planes of reference
Sagittal plane: Divides the body into right (dextral)
and left (sinistral) halves. The mid-sagittal refers to
a specific plane which is in the midline.
Frontal (Coronal) plane: Divides the body into front
(anterior) and back (posterior) parts
Transverse (Horizontal) plane: Divides the body
into upper (superior) and lower (inferior) parts
General Anatomical Terms
Ventral (Anterior): toward the front of the body
Dorsal (Posterior): away from the front of the body
Superficial (External): toward the surface
Deep (Internal): away from the surface
Superior: upper
Inferior: lower
Rostral (Cranial): toward the head
Caudal: toward the tail
Medial: toward the axis or midline
Lateral: away from the axis for midline
Proximal: toward the body or toward the root of a
free extremity
Distal: away from the body or the root of a free
extremity
Central: pertaining to or situated at the center
Peripheral: toward the outward surface or part
Dextral: right
Sinistral: left
SPPA 2050 Speech Anatomy and Physiology
Levels of Organization of the Human Body
An organism such as a human being is a highly
complex structure that may be organized at a
number of levels. Below are some traditionally
defined levels of biological organization starting
small and moving to the level of the organism.
Atoms, Molecules and Ions
Atoms are considered to be the building blocks of
matter. Atoms have a common structure made of
protons, neutrons and electrons. The number of
protons and neutrons in a given atom will define
“what” that atom is. For example, hydrogen has a
single proton and neutron. Atoms bind together in
various ways to produce structures called
molecules. For example, a water molecule is made
of two hydrogen atoms and one oxygen atom. If an
atom or molecule has an unequal number of
electrons and protons, they will carry a positive or
negative charge and are termed ions. Atoms,
molecules and ions are the building block of cells,
the basic unit of life.
Cells
The cell is the smallest biologic unit that is
considered to be living. The study of cells is called
cytology. Cells are quite small (measured on the
order of microns or thousandths of a mm) and
plentiful (about 100 trillion in the average human).
Because cells are living, they have a lifespan. A
cell may live as long as the organism itself, as is the
case of many neural cells. Alternatively, a given
cell may lead a relatively short life and be subject to
frequent replacement (e.g. skin cells). Cells contain
a number of organelles that are necessary to sustain
life activities.
Tissue
When cells and intercellular material (i.e. various
molecules) are combined to produce some
functional arrangement, we refer to it as tissue. The
study of tissue is called histology. It is generally
agreed that there are four basic types of tissue.
Epithelial Tissue: The main function of epithelial
tissue is to cover the other bodily tissues. Epithelial
tissue is characterized by a paucity of intercellular
materials. The outer surface of the body is covered
by epithelial tissue proper (i.e. skin). The inner
surface of the body is covered with endothelial
tissue. This includes the gastrointestinal tract and
the lining of the lungs. Epithelial tissue proper and
endothelial tissue is in direct contact with the
environment. Body cavities such as the thoracic
and abdominal cavities are lined with mesothelial
tissue.
Connective Tissue: Connective tissue is a large
category of tissues whose principle function is to
provide structural support for the organism. In
contrast to epithelial tissue, connective tissue is
characterized by a lot of intercellular material. This
is commonly referred to as the extracellular matrix.
There are many types of connective tissue.
Loose Connective Tissue
Areolar tissue
Adipose tissue (FAT)
Dense Connective Tissue
Tendons–attaches muscle to bone/cartilage
Ligaments–attaches bone to bone
Fascia – other supportive tissue
Special Connective Tissue
Cartilage
Bone
Vascular tissue
Blood
Lymphatic tissue
Muscle Tissue: Muscle is a tissue that has the
capacity to generate force through contraction of
tissue elements. There are three broad categories of
muscle tissue that are differentiated based on their
anatomical structure and function.
SPPA 2050 Speech Anatomy and Physiology
Striated muscle:
movement
Associated
Smooth muscle: Associated
movement (e.g. GI system)
with
with
voluntary
Muscular System*
Muscles and tendons
Myology
involuntary
Cardiac muscle: Heart muscle
Nervous System*
Brain, spinal cord, nerves, ganglia and sense organs
Neurology
Nervous Tissue: Nervous tissue is a specialized
tissue designed to generate, propagate and transmit
electrochemical signals.
Respiratory System*
Air passages and lungs
Pulmonology
Organs or Tissue Aggregates
When two or more tissues are combined to produce
a functional unit, it is termed an organ. Examples
of organs include the heart, the lungs, the kidney
and the liver.
Digestive System*
Digestive tract and associated glands/organs
Gastroenterology
Systems
A system refers to a complex, organism-wide
functional arrangement of organs/tissue aggregates.
Outlined below are some primary bodily systems
with a brief description of the tissues involved and
the name given to the discipline that studies these
systems. Those marked with an asterisk are
particularly relevant for speech and swallowing.
Skeletal System*
Bones and related cartilage
Osteology
Articular System*
Joints and ligaments
Arthrology
Cardiovascular (Circulatory) System*
Heart, blood vessels, blood and lymphatic system
Cardiology, Angiology
Reproductive System
Genital tracts
Gynecology, Urology
Endocrine System
Ductless glands of the body
Endocrinology
Urinary System
Kidneys and urinary passages
Urology
Integumentary System
Skin, nails and hair
Dermatology
Selected body systems relevant for Communication Disorders
The broad goal of this section is to provide a basic
overview of selected bodily systems that are
particularly relevant to the study of speech language
pathology. We start with the circulatory system.
For most of you this will be a review of information
you probably learned in biology class. However, it
is worth reviewing since in class and in the
laboratory you will frequently come across arteries
and veins that provide the blood supply to key
anatomical structures. In many cases, you will not
be responsible for their names and what they
supply. This is not true for the blood vessels within
the central nervous system. I will want you to know
this information in more depth because many
communication disorders can result from problems
with the circulatory system.
However that
SPPA 2050 Speech Anatomy & Physiology
information will be handled in the last unit of the
course. Second, there will be a brief introduction to
the nervous system. We will return to this topic in
much more detail at the end of the semester. Our
goal at this point is to simply provide enough
information to get you through the next few units.
Finally, we will move onto the muscular system.
Since speech is the product of bodily movement, it
is important for you to understand the basic function
of muscle and how it is organized.
The Circulatory System
All cells in the body require nutrients and oxygen to
function properly. Cellular function also produces
products such as carbon dioxide that needs to be
removed from the cell. Without this important
function, the cells will die. As a result, there need
to be a mechanism to deliver key chemicals to and
from the cells of the body. Blood serves as a
chemical transport system and the circulatory
system is responsible for controlling blood flow
through the body. The circulatory system consists
of the heart, arteries, veins and capillaries.
The heart is principally a muscular structure that
acts as a pump to move blood through the vascular
(arteries, veins and capillaries) system. Arteries
serve to take blood away from the heart. Veins
return blood to the heart. Capillaries or capillary
beds are small microscopic vessels that
communicate between very small arteries
(arterioles) and very small veins (venules). Arteries
and veins have a different histological structure.
Arteries have thick, firm walls that contain
connective and (smooth) muscle tissue. Veins are
thinner with less connective and muscle tissue.
When in the laboratory, you will quickly note the
differences
in these structures, which helps in identification.
The arteries retain their shape in the absence of
blood. Veins tend to collapse. The Arterial and
venous systems are each tree-like structures in that
large vessels progressively branch into smaller and
smaller vessels. The arterial tree and venous tree
are connected by capillary beds. The main trunk of
the arterial tree is a huge artery called the aorta.
The main trunk of the venous tree is structure called
the vena cava. Names are given to the branches of
each. Over the course of the semester, you may be
required to learn some of these branches. In
general, blood pumps from the heart to a network of
arteries that get progressively smaller and smaller
until they feed into capillaries, which then feed into
the venous system which returns the blood to the
heart. Capillaries are the site at which the important
chemical exchange with surrounding tissue occurs.
Therefore, every anatomical structure in the body
will have a capillary bed that receives its blood
supply by a particular artery(s) and drained by a
particular vein(s).
In humans, the heart has four chambers. The left
atrium and ventricle are interconnected as are the
right atrium and ventricle. Blood enters the heat via
the atria (plural for atrium) and leaves the heart via
the ventricles. Following birth, the left and right
side of the heart work as separate pumps that
control two distinct circulatory circuits.
The
systemic circuit sends blood to and from all of the
tissues of the body. A main function of the
systemic circuit is to provide a supply of oxygen
(O2) and other nutrients to the body and remove
products of cellular metabolism (namely carbon
dioxide or CO2) from the same tissues. Blood that
has a relatively high concentration of O2 and a low
concentration of CO2 is pumped from the left
ventricle of the heart through the arterial system to
the body. As the blood passes through the capillary
beds, O2 diffuses into the tissues to allow cellular
metabolism to occur. Simultaneously, CO2 and
other metabolites diffuse into the blood. The blood
then returns to the heart via the venous system with
a relatively low concentration of O2 and a relatively
high concentration of CO2. It enters the right atrium
of the heart. The pulmonary circuit is responsible
for moving this O2-depleted, CO2 rich blood (which
gives the blood a dull, bluish appearance) to the
lungs where CO2 can be removed from the blood
SPPA 2050 Speech Anatomy & Physiology
and O2 can diffuse into the blood. Blood leaves the
right ventricle of the heart into the pulmonary artery
which transports the blood to the lungs. Once gas
exchange has occurred, the, now O2 rich blood
(which has a bright red appearance) returns to the
heart via the pulmonary vein and enters the left
atrium of the heart. The blood pumps out of the
heart via the left ventricle into the systemic circuit
and the process repeats itself. In summary, a blood
cell is on a continuous journey that takes the
following general route: left ventricle-systemic
arterial system-capillary bed-venous system-right
atrium-right ventricle-pulmonary artery-pulmonary
capillary bed-pulmonary vein-left atrium-left
ventricle. It should be noted that in the systemic
circuit, which is much larger than the pulmonary
circuit, arteries always contain O2 rich blood and
veins always contain O2 depleted blood. The
opposite is true for the pulmonary circuit.
The Nervous System
The nervous system (NS) is the source of
stimulation (at least internal to the body) that causes
muscles to contract, and eventually, causes all
movements to occur. The NS is a huge topic, and in
this course we can really only cover it in a very
rudimentary way. There are several ways to slice
and dice the NS, to make sense of it. From a
structural (i.e., anatomical) point of view, we can
subdivide the NS into central and peripheral parts.
In most reference sources, the central nervous
system (or CNS for short) includes the brain and
spinal cord. The peripheral nervous system (or the
PNS for short) includes the bush-like
conglomeration of cranial and spinal nerves, all
their many sub-branches, and all peripheral sensors.
i. Cytology of the Nervous System
There are two broad cell types found in the nervous
system. They are the neuron, and a group of cells
collectively termed glia. The neuron typically gets
all the press. To be sure, the neuron is the key to
NS function, allowing us to do all the thinking,
acting, (and speaking) that we do. Without it, I
wouldn’t be able to stand in front of the class and
entertain you the way I do (yeah right!). But as is
often true in life, the star gets all the attention, and
the supporting cast is barely acknowledged. To
illustrate, the second edition of Kandel &
Schwartz’s Principles of Neural Science (the
“bible” of the nervous system) devotes a meager 15
pages of a 900-page text to the topic of glial cells.
This seems even more meager when we learn that
glial cells outnumber neurons by 10-50 times. Our
remedy for this injustice is to begin our discussion
of the NS cells with glia.
There are 3 main types of glial cells: The Schwann
cell, the oligodendrocyte, and the astrocyte. Glial
cells (glia= “glue” in Greek) are considered to serve
a number of functions. First, glial cells provide
firmness and structure to the nervous system, much
like connective tissue does in other parts of the
body, and separates and insulates groups of neurons
from each other. Second, some glial cells (i.e.
astrocytes) have projections or “end feet” which
serves to provide a physical barrier between the
brain and the blood supply to the brain (blood-brain
barrier). Third, glial cells provide a variety of
“janitorial” services including (a) scavenging for
cell debris that comes about from cell death and
injury and (b) cleaning up potassium ions and
chemical transmitters that float around in the
extracellular space following neural activity.
Fourth, glial cells provide myelin (i.e.
oligodendrocyte in the CNS & Schwann cell in the
PNS), the sheath that insulates nerve axons. Fifth,
glial cells play an important role in guiding
neuronal migration and axon growth during
development. Finally, there is some more recent
SPPA 2050 Speech Anatomy & Physiology
evidence that glial cells have the potential to
communicate in ways similar to neurons, suggesting
that their NS role may be more than simply
supportive.
It may be that a comprehensive
understanding of the nervous system will not be
possible without a much-improved description of
glial cell function.
The speech and hearing professional often hears
about glial cells when things in the nervous system
go wrong. Brain tumors often originate in glial
tissue. The astrocytoma and glioblastoma are two
types of tumors that can affect normal speech and
hearing function.
ii. Somatic vs. Autonomic Nervous System
From at least one particular functional (i.e.,
physiological) point of view, we might also
subdivide the NS into autonomic and somatic
portions or systems. The autonomic subdivision of
the NS is usually associated with involuntary motor
behavior, involving “vegetative” functions that
maintain life (e.g., digestion, cardiovascular action,
respiration, glandular regulation). The autonomic
system is usually subdivided into sympathetic and
parasympathetic sub-systems, either on the basis of
anatomical considerations, or functional ones. For
example, most neurons and related nerve trunks that
drive the sympathetic system are concentrated in the
thoracic and lumbar regions of the spine -- hence
the corresponding descriptive phrase thoracolumbar
division. Neurons that drive the parasympathetic
system are concentrated in cranial and sacral
regions -- hence, the descriptive phrase craniosacral
division. Some texts will suggest that the
sympathetic and parasympathetic divisions of the
autonomic system have antagonistic effects, in the
sense that the sympathetic system is said to be
involved mostly in “alerting” reactions (e.g.,
accelerating the heart rate, constricting peripheral
blood vessels), while the parasympathetic system is
said to be involved in “calming” reactions (e.g.,
slowing the heart rate, dilating the blood vessels).
Since we have declared speech to be a voluntary
act, we will largely ignore any other information
about, or reference to the autonomic portion of the
NS (though there is potentially a bit of an interest
overlap, given our concern with respiration. Most
of the time, we hope and assume that respiration
happens “automatically,” and that we stay alive by
breathing but never thinking about it or having to
try to do it. In that sense, respiration for life seems
to be vegetative and involuntary. You will come to
learn, however, that speech respiration is
significantly different from life respiration. The
somatic subdivision of the NS is far more central to
our interests, since it is usually associated with
conscious, voluntary motor behavior.
iii. Afferent and Efferent Fibers of the PNS
In general, we can say that there are two main
functions served by the conducting channels (e.g.,
nerves) in any major subdivision of the nervous
system, including the peripheral somatic part.
Efferent nerve fibers conduct information away
from the central nervous system. Afferent fibers
conduct information toward the central nervous
system. When we speak of efferent nerves, we
usually do so in a context of the so-called motor
fibers that conduct excitatory impulses out to the
striated muscles, supplying and “driving” them to
contract. When we speak of afferent nerves, we
usually refer to conducting fibers that relay
information back to the CNS, from sensory
mechanisms distributed about the periphery. The
four main types of peripheral sensory mechanisms
(also, called receptors) are the mechanoreceptors,
photoreceptors,
chemoreceptors,
and
thermoreceptors. The names given to these receptor
families suggest the types of stimuli that excite
SPPA 2050 Speech Anatomy & Physiology
them. Mechanoreceptors relay information about
movement (kinesthesia), touch-pressure, sound (hair
cells), and pain (nociception) back to the CNS.
Photoreceptors-- obviously critical to the visual
system -- respond to light. Chemoreceptors convey
information about chemical processes (e.g., the
changing “balance” between oxygen and carbon
dioxide in the bloodstream that occurs during
respiration). Thermoreceptors signal temperature
and its changes. Mechanoreceptors, and to a lesser
extent chemoreceptors, play significant roles in
speech production. Photo- and thermoreceptors do
not.
Two of the better-known mechanoreceptors that are
important for kinesthesia* (sensation of movement,
via the lengths of muscles, angles formed at joints,
and forces applied to bones at tendinous interfaces)
are the muscle spindle and the Golgi tendon organ.
The spindle is a remarkable sensory mechanism,
actually with its own muscle fibers and efferent
motor supply that lies in parallel with other fibers in
a (“parent”) muscle. When a muscle containing
spindles is lengthened and stretched, the spindles
(acting in their “passive” mode: i.e., assuming that
their own muscle fibers do not also contract) will
also be stretched, and will then discharge impulses,
proportional to the stretch, that project back to that
portion of the CNS where the motor neurons of the
parent muscle reside. These impulses are excitatory
to the parent muscle (and sometimes, some or all of
its synergists). All else being equal, the spindle
discharge will cause the parent muscle to contract,
relieving the stretch on the spindles and
surrounding, parallel muscle fibers.
You are
probably already familiar with the actions of muscle
spindles, though you may not know it. For
example, spindles in the superficial quadriceps
femoris muscle of the ventral portion of the thigh
underlie the familiar patellar tendon reflex (the ever
popular knee-jerk response that we all can elicit
when we “play doctor,” applying a gentle hammer
to the tendon just below the knee cap or patella). In
general, we can say that spindles provide
information about muscle length, and the rate of
change of muscle length. These are important
sensations, for at least two reasons. First, it is
possible that we might sense the positions of parts
of our body by “working out” the joint geometry
implied by concurrent lengths of all our muscles.
Spindles signal muscle length, and thus, provide a
basis for this type of geometrical calculus. Second,
it is also important to remember that the forcegenerating capability of striated muscle depends
upon muscle length. An awareness of the ability of
some muscle to generate force, for some future act,
might easily be conveyed to the CNS -- and to the
“Smart Little Person” who lives “in there”
somewhere, who makes you do whatever you do -by spindles that signal the current length of the
muscle.
*The terms kinesthesia and proprioception are
sometimes used as synonyms, sometimes not.
According to The Bantam Medical Dictionary,
kinesthesia is defined as the “sense that enables the
brain to be constantly aware of the position and
movement of muscles in different parts of the
body,” while proprioception is mediated by “a
specialized sensory nerve ending... that monitors
internal changes in the body brought about by
movement and muscular activity.” Given only these
two statements, it is hard to tell the terms apart. For
the near term, in SPPA 2050 we will use the two
terms interchangeably.
The Golgi tendon organ is not as structurally
elaborate, or as functionally complicated as the
muscle spindle. The organ lies in the tendon of a
muscle, near its bony origin and/or insertion, and
senses the force generated within a contracting
muscle. Its discharge is proportional to force, and is
ultimately inhibitory to the parent muscle. In short,
the Golgi organ “encourages” (in a passive,
unintelligent way) the parent muscle to stop
contracting. A decrease in the level of contraction
of the parent would relieve strain transmitted to the
sensor resulting from active or passive force
generated within the muscle and sensed at the
tendon. Golgi tendon organs are germane to SPPA
2050 interests in the general sense that we are
interested in body movements (that cause variations
SPPA 2050 Speech Anatomy & Physiology
in air pressures and flows) that are themselves
caused by forces generated within contracting
muscle.
iv. Nerves of the peripheral nervous system
We usually subdivide the major nerves of the
peripheral nervous system first into two groups,
cranial and spinal.
In most books, there are
thirty-one (31) spinal nerves, and all of these are
mixed with afferent and efferent fibers. Each spinal
nerve takes its identifying initial-and-number from
the regions of the spinal vertebra where it exits the
spinal cord. We say that there are five types of
vertebra – from top to bottom, Cervical (neck),
Thoracic (thorax), Lumbar (curve of the lower
back), Sacral (actually a midline vertebral complex
integrated into the pelvis), and Coccygeal (your tail)
– and correspondingly, five types of spinal nerves,
identified by capital letters and Arabic numerals,
and the same five main adjectives Cervical (eight in
all, designated C1-8), Thoracic (twelve in all,
designated T1-12), Lumbar (five in all, designated
L1-5), Sacral (also five in all, designated S1-5), and
Coccygeal (only one, referred to merely as the
coccygeal nerve). Those spinal nerves that are of
greatest concern in speech production are in the
cervical, thoracic, and lumbar regions. These
nerves provide motor supply to various thoracic and
abdominal muscles relevant for respiration.
In broad terms, motor supply information -defining which major nerve goes to which muscle -is not all that hard to remember. Generally, the
relative locations of muscles along a rostral-tocaudal axis through the body will give you some
idea of their relative "level" of innervation (e.g.,
Cranial, supplies the head and neck, Cervical
supplies the neck and upper limbs, etc). If you have
difficulty remembering the motor supply for some
specific muscle in the body, it is usually reasonable
to guess at the answer, with the following top-down
rule of thumb in mind: the higher the muscle along
a rostral-to-caudal axis, the more likely the higher
the supply. Of course, there are always some
surprises (e.g., the ever-popular Diaphragm) that
seem to be obvious counter-examples to the top-
down rule-of-thumb, but even considering those,
you might still bat about .500 by guessing the level
of innervation from the approximate location of a
muscle along the rostral-caudal axis of the body.
There are twelve (12) cranial nerves, usually
identified by a Roman numeral that is sometimes
preceded by a capital letter “C.” Seven have
relevance for speech and hearing. We will get into
some of these later. I’d like you to be able to
identify all of them, although not in the same level
of detail. On the website there is a table that
outlines all 12 cranial nerves and identifies those
relevant for speech in bold. These nerves will be
revisited at least two more times throughout this
course. First, as we learn about the peripheral
structures (in particular, the muscles) involved in
respiration, phonation, and articulation, we will
learn the innervation of these structures. Second, as
we learn more about central nervous system
structure and function, we will learn in more detail
where these nerves originate in the brain.
And now, for a few words about plexuses...
Certain nerves emerge separately from the central
nervous system, but then “nest up,” cross, join,
communicate, and intermingle to form indistinct
“bundles.” These “bundled nerve nests” (my term,
not anatomically standard) are called plexuses. A
plexus represents a mixture of nerve roots that are
truly distinct somewhere, but not where the bundles
have "formed." The cervical spinal nerves, as a
group, seem to be especially guilty of plexomania
(again, my own editorial term, and definitely not a
standard in anatomy). It is the custom in several
texts to identify two major plexuses in the cervical
spinal region -- the cervical (usually said to include
roots from C1-C4) plexus, and the brachial (usually
said to include roots from C5-C8). Perhaps the
most famous nerve derived from the cervical plexus
is the so-called phrenic nerve (formed from parts of
SPPA 2050 Speech Anatomy & Physiology
three cervical nerves, C-3, C-4, and C-5) that
supplies (efferent) motor excitation to the
diaphragm. (The careful reader will immediately
notice a “contradiction” even in the last two
sentences -- to wit, we say that the cervical plexus
includes tracts from spinal nerves C1-C4, and then
in the next breath, we say that the phrenic nerve,
with contributions from C3-C5, is a component of
the cervical plexus. Take this set of comments as a
hint that the situation with plexuses is hardly ever
clear, or straightforward. For example, the brachial
plexus is itself often subdivided into quite a few
major branches -- the long thoracic, the
thoracodorsal, the pectoral [medial and lateral]
branch, the inferior and superior branches [often
given other names], and so on. Oh, woe. Different
texts say different things about plexuses. Don't get
upset, or huffy. Just deal with the situation as best
you can.)
The Muscular System
Muscle is the motor that “causes” all movement (as
long as we (a) ignore the troublesome concept of
free will; (b) take for granted some prior activity of
the nervous system; and (c) disavow all other
stimuli and forces “external” to the moving
organism [e.g., wolves, elves, the wind, and
whatnot]!). In humans, there are three kinds of
muscle: (1) heart or cardiac muscle; (2) smooth (or
“involuntary” muscle, e.g., that moves the gut, and
the walls of blood vessels and certain of the “tubes”
that make up the bronchial tree); and, (3) striated (or
“voluntary”) muscle (e.g., the biceps brachii,
diaphragm, and the ever-popular gluteus maximus).
Sometimes, striated muscle is also called skeletal
muscle. In the version of Speech Science you
encounter in this class, striated muscle is more
germane to our interests than other kinds of muscle,
because we have declared speech to be a voluntary
activity. A main purpose of this handout is to be
sure you are aware of the basis for the name
“striated” that is given to many muscles in the body
. This information provides important insight about
the structure and function of this type of muscle.
[After this point, in this muscular introduction, the
term muscle will be used simply to mean striated
muscle.]
One of the pedagogical attractions of muscle is that
it provides a good context for demonstrating that
form and function are inter-related. If we simply
examine the appearance of muscle, and couple our
observations with a few simple “circumstantial”
facts about its function, we can begin to make good
guesses about how and why muscle works as it
does. At a gross level, a muscle is something like a
package. The basic, standard package consists of a
fleshy or meaty belly, wrapped in a membrane
(fascia) called epimysium, attached to bone or
cartilage at each of two ends by tendon. The bony
attachment that is less mobile and often nearer the
center of the body is referred to as the origin of the
muscle. The attachment that is more mobile is
usually referred to as the insertion. For example,
the masseter muscle (look for this muscle in Netter)
runs across the lateral surface of the lower jaw (i.e.,
mandible) -- a muscle’s line of action is usually the
geometric line drawn through its “points” of origin
and insertion. For the masseter, the origin is
distributed across the zygomatic arch and temporal
process of the zygomatic bone, and the insertion is
distributed across the mandibular ramus and angle.
Thus, its line of action runs from rostral and ventral,
to caudal and dorsal. The masseter can generate
force that “acts” along this line, and in this general
direction. Muscles move bones (or whatever they
happen to be attached to) not by pushing, but by
pulling on them. If we have two bones that form a
joint, like leaves of a hinge -- roughly, not unlike
the joint formed between the mandible and skull -and a muscle running between them, then one bone
(We usually say the mandible.) will move toward
the other (the rest of the skull) when the muscle is
stimulated and allowed to “contract.” In this sense,
contraction means that the muscle shortens in
length. In simple geometric terms, the fact that the
muscle shortens “explains” why one bone moves
SPPA 2050 Speech Anatomy and Physiology
closer to the other. The mandible moves toward
(also, approximates, flexes or elevates toward) the
skull when the masseter muscle contracts and
shortens. The mandible moves away (also, extends
or depresses away) from the skull, only if other
muscles (e.g., anterior belly of the digastric,
mylohoid, and/or geniohyoid muscles – look for
them on different plates) contract and shorten to
pull it in that direction.
In a more general sense, the term contraction is
often used to refer to stimulation of muscle, and a
subsequent generation of force by the muscle,
independent of whether there is any change in
muscle length. Thus, we speak of isometric
contractions that produce force but do not involve
any change in length of the muscle, and usually no
movement of the structures against which the
muscle pulls. We contrast these contractions with
those that are anisometric, which create force and
do involve some change in muscle length. It may
interest you to know that a muscle can contract (in
the sense of produce force) while shortening or
lengthening. The extent to which any muscle
contraction will cause movement will always
depend upon the balance between the “pulling
forces” generated within the muscle itself, and other
forces that act on the structure that is pulled.
Across many joints (e.g., formed by bone 1-- say,
the skull -- and bone 2 -- say, the mandible, from
the preceding example) there will be one muscle
that moves bone 2 one way with respect to bone 1,
and an “opposing” muscle that moves bone 2 back
the other way with respect to bone 1. The two
muscles are mutually antagonistic. Their actions
oppose one another. Each muscle is also said to be
an agonist for its respective action. Often, there are
multiple muscles that act about a joint, all
potentially capable of producing the same motion.
Each of these muscles is potentially an agonist for
the motion, though one muscle among the group
may “dominate” when the action is performed. We
usually refer to “helper muscles” as synergists.
Synergists may “operate” about the same joint as
the “main mover,” or they may be distal to the joint,
but still assist in performing some action even they
do not (and cannot?) “cause” that action directly.
A key, basic question to ask about contracting
muscle is this: “Where does the force come from?”
The answer is not “The Dark Side”. Instead, the
answer comes to us from an examination of muscle
structure. Suppose, out of idle curiosity, we start to
(gently) pull a muscle apart. We find that a muscle
belly separates relatively nicely into bundles of
fibers that run more or less the length of the muscle.
These bundles are called fasciculi, and each is
wrapped in a membrane called perimysium. In turn,
each fasciculus can also be subdivided
longitudinally into a collection of muscle fibers,
each wrapped separately in membranes referred to
as endomysium. If we pull away the membranous
cover that surrounds each muscle fiber, we then
expose a collection of many small filaments, called
myofibrils, running the length of the fiber, all lying
side by side. One interesting fact about myofibrils
is that they are not individually wrapped, in any
way analogous to muscle fibers, fasciculi, or even
(whole) muscles. A second interesting fact about
the myofibrils is that if we view them in
longitudinal section under a fairly high-powered
microscope, we can see a pattern of “repeating
stripes,”where the stripes run at right angles to the
long direction of the myofibrils.
These stripes are the basis for the name striated.
What do they represent? Each myofibril is itself
composed of a collection of “stringy” protein
molecules or filaments. There are basically two
types, called actin and myosin, that also lie side by
side. Neither type runs the length of the myofibril.
Instead, both filament types are discontinuous,
“running” for a little bit, and then stopping, and
then running for a bit more, and stopping again, and
so on. In a way, the regularly repeating “spans” of
actin and myosin molecules are responsible for the
striped appearance of muscle in a longitudinal
section. We can divide each myofibril along its
length into units called sarcomeres, each containing
some myosin and some actin filaments, whose
lengths partly overlap one another within the
SPPA 2050 Speech Anatomy and Physiology
sarcomere, but not completely. It is helpful to think
of a myofibril as a string, made up of a series of
cylindrical barrels or beads (i.e., the sarcomeres)
joined end to end.
Each sarcomere is about the same length as any
other, and each sarcomere looks like any other,
from the point of view of its stripes (i.e., its bands
of light and dark “color”). By convention, the
“boundary line” shared by any two adjacent
sarcomeres is referred to as a Z line. Thus, each
sarcomere is the collection of material (actin,
myosin, and whatever else -- but certainly not air! -that occupies intracellular space) lying between any
two adjacent Z lines. Along the length of each
sarcomere viewed in longitudinal section, there are
these bands of color: (1) a relatively dark region
(the A band) spanning maybe the middle two-thirds
of the distance between Z lines, that may be slightly
lighter in a narrow region (known as the H band)
near the very middle of the sarcomere; and, (2) two
relatively “lightish” regions (each half of an I band
“shared” by two adjacent sarcomeres), one at each
end of the sarcomere, falling between the end of the
A band and the nearest Z line.
If we cut through or across a sarcomere,
transversely or at right angles to the long axis of the
myofibril, we discover that the bands correspond to
regions filled by different proportions and
arrangements of actin and myosin filaments. In the
area of the I band, we find actin but no myosin
filaments. In the area of the H band we find myosin
but no actin filaments. In the darker portions of the
A bands, we find both types of filaments apparently
overlapping one another. We also come to realize
that the A band corresponds to the length of the
myosin filaments lying in the central portion of the
sarcomere. There are many fine illustrations of this
verbal description of the anatomy of a sarcomere,
especially in physiology texts.
In the early 1950s, with the advent of electron
microscopy, muscle biologists began to see directly
for the first time that interesting things would
happen to the lengths of some (but not all) “color
bands” in sarcomeres when the length of a muscle
would change (e.g., as during anisometric
contraction).
We now know that if a muscle
shortens, the lengths of the sarcomeres (i.e., the
distances between adjacent Z lines), and their H
bands and I bands also shorten, while the lengths of
the A band remains the same. Similarly, if a muscle
lengthens, the lengths of the sarcomeres, and their H
bands and I bands, lengthen, but again the lengths
of the A band remains the same. One way to
understand these changes is to assume that actin and
myosin filaments slide past one another as muscle
length changes, increasing or decreasing their
overlap (with muscle shortening or lengthening,
respectively). This inference, completely correct, is
typically referred to as the Sliding Filament Theory
of Muscle Contraction.
We also know that the amount of force generated by
contracting muscle after stimulation (e.g., from an
electric shock, or more naturally, from nervous
system activity), depends upon muscle length. This
interdependence between muscle length and
contractile force is known as the length-tension
property of muscle. (Tension is another term that is
often used to refer to the force generated by
contracting muscle.) For many years, it has been
known that at relatively short or relatively long
lengths, a muscle produces less (active) force for
the same level of stimulus applied to it than at
intermediate lengths within the muscle’s
“operating” range (i.e., its range of possible
lengths). It is relatively easy, and correct, to leap to
the idea that the force-generating ability of muscle
depends in some way upon the degree of overlap of
actin and myosin filaments within its sarcomeres.
There are small “projections” that “stand off” the
myosin filaments. When a muscle is stimulated,
electrochemical events follow that allow these
projections to “bind” to receptor sites on the actin
filaments. These “bindings,” usually referred to as
cross bridges, are the “source” of (active) force we
receive from contracting muscle. Momentarily,
after stimulation, it is as though the myosin
filaments “lock” onto the actin filaments. The
arrangement of the myosin projections that “lock”
into actin binding sites will actually pull and slide
SPPA 2050 Speech Anatomy and Physiology
the actin filaments toward the center of the
sarcomere, causing the muscle to shorten, as long as
no force greater than that produced by the
contraction is applied to both ends of the muscle.
We thus gain some insight into the length-tension
property of muscle -- a key feature of striated
muscle function -- by examining the form of its
sarcomeres (i.e., the degree of overlap of actin and
myosin) at the moment the muscle is stimulated to
contract. At some optimum, intermediate length of
the sarcomere, the number of cross bridges that can
form between actin and myosin filaments is at a
maximum. This length corresponds to the “peak”
of the “active portion” of the so-called lengthtension curve (roughly speaking, a curve that looks
something like the bell-shaped curve representing
the normal distribution in statistics). At sarcomere
lengths greater than this optimum length, the
filaments do not overlap as much, and fewer cross
bridges (and thus, less contractile force) are
possible. At sarcomere lengths less than the
optimum length, filaments over-lap too much, in the
sense that actin filaments from opposite ends of the
sarcomere “crowd” (i.e., overlap) one another, and
myosin filaments collide against the Z-line
boundary at each end of the sarcomere.
The stimulus that excites muscle, and causes it to
contract, comes from nerve cells called motor
neurons. Each motor neuron, in the brain or spinal
chord, has a nerve fiber called an axon -- if you like,
a kind of “insulated electrical chord” -- that leads
away from it and toward the part of the muscle the
neuron stimulates. Individual neurons don’t “drive”
whole muscles. Instead, each neuron is responsible
for stimulating some specific subset of the muscle
fibers in a muscle, usually distributed across some
portion of the muscle rather than lying together,
side by side, in a clump. We often say that a pool
of motor neurons within the central nervous system
is responsible for driving all the muscle fibers in
any particular muscle. Some neurons are “tied” by
their axons to single muscle fibers, while other
neurons are “tied” to, and responsible for
stimulating, many (e.g., even hundreds of) muscle
fibers. We refer to the combination of a motor
neuron, its associated axon, and the collection of
muscle fibers it stimulates (or innervates), as a
motor unit (often abbreviated MU). The “size” of
the motor unit depends upon the number and cross
diameters of all fibers that belong to it, and in an
obvious way, is related to the amount of force the
motor unit will deliver when it contracts. This is
merely because the total cross sectional area of the
muscle fibers belonging to the unit is a
straightforward index of the number of cross
bridges that can form, as long as all muscle fibers
belonging to all motor units within the same muscle
are about the same length.
The motor unit is the smallest functional unit within
striated muscle. We say this for the following
reason: When a stimulus from a motor neuron is
delivered to its muscle fibers, at some time we
might call t0, essentially all sarcomeres within all
myofibrils within all affected muscle fibers respond
in the same way by contracting simultaneously,
forming the maximum number of cross bridges
subject only to the degree of overlap between actin
and myosin filaments. If the stimulus is “isolated”
(i.e., a “one-shot instruction” to contract), the motor
unit responds with a twitch, a brief increase in force
that dies out over time. It takes a little time after the
stimulus at t0 for the contractile force to build up,
and then a little bit of time for the force derived
from that stimulus to dissipate. How much time
depends upon the specific chain of electrochemical
events that allows formation and dissociation of the
actin-myosin cross bridges. Some motor units, even
within the same muscle, twitch faster than others
because of their intrinsic biochemistry. The “twitch
speed” -- in effect, an index of how rapidly force
builds up in a MU after it is stimulated -- is often
described in terms of the time to peak tension after
the moment of stimulation. The fastest “twitching”
motor units in the human body (e.g., those MUs in
muscles that control positioning of the eye, and
other tiny structures) reach peak tension in about 15
ms. Very slow twitching motor units (e.g., in leg
muscles that are responsible for maintaining
posture) reach peak tension in about 150-200 ms.
SPPA 2050 Speech Anatomy and Physiology
Any specific motor unit will twitch (almost) exactly
the same way each time it is stimulated to contract.
The force that its contraction can contribute to some
behavior depends solely upon the number of
possible cross bridges that can form amongst all of
its fibers’ myofibrils’ sarcomeres. Over time, we
can obtain more force from an MU than the amount
given by its isolated twitch, if we stimulate the MU
to contract at a rate faster than the time it takes for
its isolated twitch response to die out. Twitch
responses from closely repeating stimuli, delivered
to the same motor unit, will “ride up on top of one
another,” summing to create a bigger force (at some
time after the initial stimulation) than is possible
from an isolated twitch. Conceptually, then, one
way to vary or grade the force output from
contracting muscle is to vary or grade the rate at
which motor units within the muscle are stimulated.
When this happens -- and it happens all the time
when we voluntarily increase the strength of
contraction of a muscle -- we say that we have
increased the firing frequencies of the active MUs.
Another way to grade the force output of muscle,
conceptually speaking, is to vary the number of
MUs that are being stimulated at any one time. Not
all motor neurons within the pool associated with
some whole muscle will send their excitatory
instructions to their respective motor units at the
same time, in either a narrow or broad sense. At
any one time, some proportion of the motor units in
the “parent” muscle may be contracting. The
greater the number of units that are contracting, the
greater the total force output from the muscle.
Conversely, the lower the number of units
contracting, the lower the force output. During
voluntary muscle contractions, where total force
increases over time, the number of motor units that
are “recruited” to the task also increases over time.
This is a phenomenon that is easy to demonstrate
(with appropriate machinery, to “listen in” on the
concurrent activity of several contracting MUs), and
enlightening to perceive.
Top-10 List of Things to Remember about
(Striated) Muscle:
10. A few simple “size facts” about muscle. It
comprises ~40 % of body weight. Fibers are 10100 microns in diameter and range from 1 to 120
mm long. MUs can contain up to 100s of fibers.
Sarcomeres are ~3 microns long.
9. A muscle belly, wrapped in epimysium and
connected by tendon to bone, is made of parallel
fibrous bundles called fascicles, wrapped in
perimysium, made of parallel bundles called fibers,
wrapped in endomysium,
made of parallel
filaments called myofibrils, made of (serial) chains
of repeating sections called sarcomeres.
8. The repeating pattern of "stripes" that cannot be
seen by the naked eye, but that give striated
(voluntary) muscle its name, represent repeating
arrangements of thick (myosin) and thin (actin)
filaments within every sarcomere along the length
of a myofibril.
7. When muscle contracts (and shortens), actin
filaments attached to both ends of each sarcomere,
and myosin filaments arrayed in the central part of
each sarcomere, slide past one another.
6.
Only (anisometric) muscle contractions
involving changes in muscle length can cause
movement (of one body part with respect to
another). There are special "contractions" involving
changes in force but no change in length that are
identified by the special name isometric.
5. The active force arising from muscle when it is
stimulated depends specifically upon the degree of
overlap (and thus, the extent of cross-bridge
formation) between actin and myosin filaments
within sarcomeres. This fact is at the root of the
Length-Tension Property of every muscle.
4. All sarcomeres within the myofibril, and all
myofibrils within the muscle fiber, respond (and
contract) in the same way when the fiber is
SPPA 2050 Speech Anatomy and Physiology
stimulated. But only a subset of fibers within a
fascicle contract when the fascicle is stimulated, if
that stimulus is "delivered" to the fascicle by means
of one or some of the pool of motor neurons
supplying the muscle as a whole.
3. The set of muscle fibers all innervated by the
same motor neuron, the neuron itself, and its
associated axon (nerve fiber) make up what is
known as the Motor Unit (MU). This is the
minimal functional unit within muscle, which
responds in an all-or-none fashion to stimuli applied
to it. If too weak a stimulus is applied (actually and
initially, directly the the motor neuron), the MU will
not respond (and contract) because the motor
neuron will not “send a signal” to its muscle fibers
to contract.
If a sufficiently strong, isolated
stimulus is applied, the MU will respond with an
invariant twitch whose amplitude (peak force) and
time history are exactly the same for every
sufficient stimulus (assuming that the MU has not
been “fatigued” by too many preceding stimuli).
MUs vary in several interesting ways, within and
across muscles of the body:
(a) Some MUs are “large,” with large numbers of
fibers and thus, a capability for large twitch
tensions. Other MU's are small.
(b) Some MUs (fight-or-flight?) fatigue rapidly
(lose the ability to produce any tension, even when
stimulated) when driven rapidly and repeatedly;
other MUs (postural units?) are highly resistant to
fatigue.
(c) Some MU's, usually in muscles that move small
loads, have relatively fast times to peak tension in
their twitch response (15-30 ms); other MU's
contract a good deal more slowly (100-150 ms, to
peak tension).
2. The active force generated by contracting muscle
can be "graded" (increased or decreased) by either
or both of two mechanisms:
(a) changes in firing frequency of active MU's,
and/or (b) increases (or decreases) in the number of
MU's recruited to task.
And the #1 fact to remember about muscle is...
1. Muscle is the motor by which all voluntary
movement takes place.