Download Chapter 6: The Muscular System

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Chapter 10: The Muscular System
(Part 1: Microscopic Anatomy and Physiology of Muscles)
Thought provoking questions to consider:
 How do muscles grow during adulthood?
 Why do we “feel” & “look” bigger when lifting (pumped up)?
 Why do muscles get sore? Consider Lactic acid and muscles tears.
Prefixes of importance:
 Myo, mys = muscle
 Sarco = flesh
Muscles have the unique ability to take chemical energy (ATP) and convert it into mechanical energy.
40% of the mass of the average human body is muscle.
Review of Muscle Histology
Tissue – groups of cells that are similar in structure and function (two or more tissue types combine
to form an organ. Typically, all four tissue types are found in each organ).
Introduction to Tissues: Basic Characteristics (each tissue type has many subclasses or varieties
and therefore each has many functions in addition to those listed)
o Nervous - control
o Muscle - movement
o Connective - support
o Epithelial - covering
Muscle Tissue (p. 86; Fig. 3.18)
Basic Characteristics
 Muscle tissue is composed of cells that are highly specialized to contract and shorten.
They are well vascularized, and responsible for most types of body movement
 Muscle cells are composed of myofilaments (actin and myosin) that bring about movement
or contraction in all cell types
3 Types of Muscle Tissue
*Note: The “muscular system” generally refers to skeletal muscle, but the other two types of
muscle tissue are listed below for comparison:
Smooth Muscle Tissue
 Found in the walls of the hollow organs (visceral organs) of the digestive tract, urinary tract,
circulatory system (except heart) and respiratory tract; Also in the arrector pili muscles of the skin
 Non-striated muscle that is involuntary
 Cells are spindle shaped and uninucleate, found in longitudinal and circular layers
 Few mitochondria exist in each cell yet do not tire due to slow contractions
 Smooth muscle lacks the connective tissue sheaths found in skeletal muscle
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Smooth Muscle Contraction
The fibers of a smooth muscle are arranged in sheets and layers. Typically one
layer runs circularly and the other longitudinally so they may alternately
contract and relax moving substances through hollow organ. The contraction
of smooth muscle in this fashion is referred to as peristalsis. Peristaltic
contractions are responsible for the movement of objects and fluids in the
uterus, urinary bladder, and rectum.
Smooth muscle contains thick and thin filaments just as skeletal muscle but they are found in smaller
amounts. Actin and myosin are also present and ATP is used for energy. Synchronization of smooth
muscle is accomplished through gap junctions.
Special Features of Smooth Muscle Contraction
Smooth muscle responds to stretching. When stretched, smooth muscle
will begin to contract more rigorously. However, after a short period
of time the smooth muscle will adapt to the stretching and again slow
contractions. This process is known as the stress-relaxation response.
If relaxation did not occur, you would have no control over holding
your urine until you have time to visit the restroom. Also, your food
would be rushed through your digestive tract so quickly you would not
have time to digest what you ate for dinner.
Cardiac Muscle Tissue
 Found only in the walls of the heart
 Striated muscle that is involuntary
 Cells are uninucleate
 Does not tire
 Adaptable contraction rates
 Cells are arranged in figure 8 shape bundles
 Branching cells connected by intercalated discs
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Skeletal Muscle –muscle that is attached to bones of the skeleton. As they contract they
cause the body to move (voluntary). The cells of the skeletal muscles are long, cylindrical,
striated and multinucleate.
 May tire easily and need periods of rest following strenuous activity.
 Very adaptable at lifting objects (the same muscles can be used to lift a paper clip
or to lift a 100# weight)
Cardiac Muscle
Smooth Muscle
Skeletal Muscle
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Skeletal Muscle Functions
 Movement (mobility of individual)
 Maintains Posture (even though we may think we are relaxed when sitting are muscles are
constantly contracting and relaxing)
 Stabilizes Joints (muscle tone strengthens joints)
 Generates Heat (heat is needed to maintain homeostasis, most of it is generated by skeletal
muscle)
Anatomy of Muscle Tissue
Each skeletal muscle is a discrete organ that contains all four tissue types.
There are several connective tissue wrappings that connect and hold skeletal muscle tissue together. As a
whole, these connective tissue coverings work together with the tendons and ligaments of the skeletal
system. When a muscle contracts, the pressure it exerts is transferred to the coverings and to the tendons
to create the movement of bone.
Connective Tissue Layers of the skeletal muscles:
 Epimysium – layers of connective tissue that line the entire outside of a muscle;
o this tough connective tissue layer blends into tendons or into sheet-like aponeuroses
which attach muscles indirectly to bone
 Fascicles – bundles of muscle fibers that are arranged within the muscle itself
 Perimysium - layers of connective tissue that line the outside of a fascicle
 Endomysium - layers of connective tissue that line the outside of an individual muscle fiber
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Microscopic Anatomy of a Skeletal Muscle Fiber
Each muscle fiber is an elongated cell with multiple nuclei. They are about tens times larger in diameter
than most other body cells (10 to 100μm) and they can reach up to more than 30cm in length. Even
though they are considered to be only one cell, each muscle fiber is the result of hundreds of embryonic
cells fusing together.
The cell membrane of a muscle fiber is referred to as a sarcolemma, and the cytoplasm is referred to as a
sarcoplasm.
Myofibrils – long ribbon-like organelles that act as the contractile elements of a muscles fiber.
 There are literally hundreds to thousands of these structures found in each muscle fiber (cell).
They are so tightly packed organelles such as the mitochondria and nuclei are forced in between
them.
Striations – the striations of a muscle can be seen and described in great detail microscopically.
 Overall, the striations are light (I) bands and dark (A) bands that are evident on the myofibrils.
These light and dark bands are responsible for muscle contraction.
The I band has a midline interruption, a darker area called a Z line
(which is a membrane). The I band contains only the thin filaments
and is an area that includes parts of two adjacent sarcomeres. The A
band has a lighter central area called the H zone (an area where thin
filaments are not present, but the tick filaments are).
Sarcomeres – chains of tiny contractile units aligned end to end along the length of the myofibrils
(contain thick and thin filaments called myofilaments)
Myofilaments – structures found within the striations of a myofibril. These structures are known more
commonly as either thick or thin filaments.
Thick filaments (myosin filaments) – contain the protein Myosin and ATPase enzymes that
release phosphate groups from ATP to produce energy.
o Thick filaments extend the entire length of the dark A band and contain myosin heads
(projections) that link thick and thick filaments together.
Thin filaments (actin filaments) – contain the protein Actin as well as regulatory proteins
(tropomyosin and troponin) that allow or prevent myosin heads from binding to actin.
Tropomyosin and troponin occupy the myosin binding site during relaxation of muscles.
Troponin-Tropomyosin Complex - is a complex of three proteins that is integral to muscle contraction.
Troponin is attached to the protein tropomyosin and lies within the groove between actin filaments in
muscle tissue. In a relaxed muscle, tropomyosin blocks the attachment site for the myosin crossbridge,
thus preventing contraction. When the muscle cell is stimulated to contract, mechanisms cause the
concentration of calcium in the sarcoplasm to rise. Some of this calcium attaches to troponin, causing a
conformational change that moves tropomyosin out of the way so that the cross bridges can attach to actin
and produce muscle contraction.
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Skeletal Muscle Contraction at the Cellular Level
Initially, muscle contraction is stimulated by an electrical impulse (action potential) from the somatic
(voluntary) nervous system. This triggers the sliding of filaments to cause contraction. When the actin
and myosin filaments slide past one another the H zone and I band disappear.
The sliding filament theory of contraction – proposed by Hugh Huxley in 1954, suggests that the thin
filaments slide past the thick filaments and the actin and myosin sites are able to bind and cause
contraction. (The thick and thin filaments only slightly overlap in a relaxed muscle).
What causes the filaments to slide?
When muscle fibers are activated by the nervous system the myosin heads attach to myosin
binding sites on the thin filaments. As the thick and thin filaments move past each other the
myosin heads release and reattach creating a ratchet like method of contraction that pulls the thin
filaments closer to the center of the sarcomere.
What role does calcium play?
Calcium is stored within the sacs of the sarcoplasmic reticulum and is released when an action
potential sweeps down the sarcolemma. The calcium removes tropomyosin and troponin exposing
the myosin binding sites on the thin (actin) filament allowing myosin heads to attach. The
continuous attachment and release of the myosin heads slide the filaments past one another
shortening the muscle fiber. Once the action potential stops, the calcium returns to the storage
area and the cell relaxes.
What role does ATP play? ATP provides the energy needed to release and reattach the each
myosin head.
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Sarcoplamic Reticulum – endoplasmic reticulum of a muscle cell that is responsible for storage of
calcium when a muscle is not contracting
Transverse (T) Tubule – when an action potential sweeps down the entire length of the sarcolemma of a
muscle cell, the T-Tubule is responsible for carrying the action potential deep within the muscle cell so
that it may act on all myofibrils within
Rigor Mortis – after death, muscles begin to stiffen within 3 to 4 hours and reach peak rigidity at about
12 hours. After 12 hours, the muscles begin to relax once again and are completely relaxed within the
next 48 to 60 hours. This is caused by the lack of fluctuations of oxygen, ATP, and calcium after death.
In rigor mortis, the actin and myosin fibers begin to bind, causing slight contraction of muscle. Actin and
myosin are proteins; proteins break down over time and are replaced in living tissue. The muscles relax
after rigor mortis because the actin and myosin proteins that are causing contraction begin to break down
and cannot be replaced.
Why are muscles Red?
Muscle fibers also have a unique pigment (myoglobin) that is not found
in other tissues and cells. Myoglobin is a red pigment that is able to hold
large amounts of stored oxygen. Because muscles are so active, they
need larger amounts of oxygen and nutrients to be transported in, and
large amounts of metabolic wastes to be transported out. (Myoglobin is
very similar to the oxygen transporting pigment in blood called
hemoglobin)
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Stimulation and contraction of single skeletal muscles
 Irritability – the ability to receive and respond to a
stimulus
 Contractility – ability to forcibly shorten when an
adequate stimulus is received
 Motor unit – one neuron and all the muscle cells that it
stimulates
 Neuromuscular junction – junction of a neuron and the
sarcolemma (the neuron does not actually touch the
sarcolemma, but it comes very close)
 Synaptic cleft – the area or gap between the neuron and
sarcolemma that is filled with interstitial fluid
 Neurotransmitter – chemical that transmits signals
from neuron to neuron or neuron to sarcolemma
 Acetylcholine (ACh) – a specific neurotransmitter that stimulates skeletal muscles by diffusing
across the synaptic cleft and attaching to receptor membrane proteins.
o Once ACh has created an action potential it is degraded by enzymes, resulting in only one
contraction per nerve impulse. If this did not occur the muscle would continue to contract!
With the introduction of ACh, the sarcolemma becomes temporarily permeable to sodium ions. The ions
rush into the muscle cells creating a difference in charge across the sarcolemma and in turn creating an
electrical current called an action potential. The action potential sweeps down the sarcolemma and results
in contraction of the muscle cell.
The events that return the cell to its resting state include (1) diffusion of potassium ions out of the cell and
(2) activation of the sodium-potassium pump, the active transport mechanism the pumps sodium ions out
of the cells and potassium ions into the cells.
The sodium-potassium pump is an active transport mechanism that transports three sodium ions (Na+) out
of the cell for every two potassium ions (K+) that it transports into the cell.
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Skeletal Muscle Contraction at the Organ Level
 Muscle tension – the force exerted on an object by a contracting or
lengthening muscle
 Load – the force exerted on a contracting or lengthening muscle by an
object
 Isotonic Contraction – tension overcomes the load and the muscle
shortens
 Isometric (same measure) – tension develops and the load is not moved
(the muscle does not shorten or lengthen)
 Graded Responses – different degrees of shortening of a muscle that are
produced by changing the speed of the muscle contraction or changing the
number of muscle cells being stimulated
 Tetanus – muscle is stimulated extremely rapidly with no evidence of
relaxation, resulting in sustained contraction
Muscle Stimulation and contraction – when a muscle receives a single response, it will contract
quickly and then return to its resting state. When a muscle receives continuous stimuli it will have
shorter relaxation periods. If contraction is still needed and muscles are still receiving stimuli,
Calcium will begin to build up within the muscles, the muscles will then reach a sustained quivering
contraction. This contraction cannot continue indefinitely and the muscle then begins to fatigue, in
which the muscle is not able to produce enough ATP to sustain contraction.
Muscle Tone – the slightly contracted state of relaxed muscles.
 This phenomenon does not produce muscle movement, but rather keeps muscles firm, healthy, and
ready to respond. Muscle tone also assists in posture and
stabilizing joints
Isotonic Muscle Contractions: (muscle moves the load) thin filaments
are sliding past thick filaments
a) Concentric Contraction – as the muscles contracts it
shortens and does work
b) Eccentric Contraction - as the muscles contracts it
lengthens (known as muscle braking) Eccentric contractions
place the body in position to contract concentrically.
When doing exercises such as parallel squats, the quadriceps
muscles contract eccentrically on the downward motion and
eccentrically with the upward motion.
Isometric Muscle Contractions: (the muscle neither lengthens or
shortens with tension) the thin filaments are not sliding past the thick
filaments.
 However, actin is binding to the myosin sites and contraction is occurring. The actin is not
moving on to the next myosin site to create sliding.
 Posture is maintained by isometric muscle contractions. Also, during squats (or similar exercises)
any slight pause is maintained by isometric contraction)
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Energy for Muscular Contractions
Muscles store enough ATP to contract for only 4 - 6 seconds. When that ATP is used, more needs to be
created by:

Aerobic respiration (36 ATP per glucose are produced within the mitochondria)
Glucose + Oxygen = Carbon Dioxide + Water + ATP
95% of ATP used during rest and light exercise is produced by Aerobic Respiration

Interactions of ADP with creatine phosphate (CP + ADP = Creatine + ATP)

Using stored glycogen through anaerobic glycolysis (2 ATP per glucose and produces lactic acid).
This is an anaerobic process because the muscles bulge when they have been used for a long time,
constricting blood vessels and not allowing oxygen to get to the cells.
Most of the lactic acid that is produced within the muscles is washed out of the muscles within
thirty minutes after use and is transported in the blood. The liver can transfer lactic acid into
pyruvic acid and then can be used again for energy.
Aerobic pathways are more efficient, producing more ATP per glucose molecule. However,
anaerobic processes are 2.5 times faster at producing ATP. Therefore, during strenuous exercise
ATP can be produced very quickly for energy.
Muscle Fatigue – ATP stores have been diminished. Even though the muscle
may still be receiving messages to contract it is unable to do so because there is
no ENERGY due to an oxygen debt and lactic acid is increasing!
Athletics and ATP
 40 % of the ATP that is used in your body is converted to actual work
60% of the ATP that is used is converted to heat
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Divers and weightlifters get most of their ATP by aerobic means because the motion is usually
finished before other mechanisms can begin
Sprinters (running and swimming) get most of their ATP by anaerobic means because so much
ATP is needed very quickly
Marathon runners get their ATP from aerobic means because the body needs to be efficient and
use energy slowly to sustain ability
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Types of Muscle Fibers
 Slow oxidative fibers (red fibers) – contracts slowly, relies on oxygen, has a high endurance,
is fatigue resistant, has little power, has many mitochondria, and a rich blood supply (marathon
runner, posture muscles)

Fast oxidative fibers (pink fibers) – contracts quickly, relies on oxygen, has a moderate
endurance, expresses moderate fatigue (Sprinting)

Fast glycolytic fibers (white fibers, very little myoglobin)– contracts quickly, has few
mitochondria, uses anaerobic glycolysis, low endurance, and fatigues quickly (hitting a
baseball)
Within muscles of an average person there is a mixture of all three types of fibers. The amount of
each is genetically controlled. Marathon runners have 80% red fibers. Weight lifters have equal
amounts of each fiber. Sprinters have 60% fast oxidative (pink) fibers.
Adaptations of Skeletal Muscle to Exercise
 More myoglobin is produced
 More mitochondria are created
 Capillaries within the muscles increase in number
Additional Benefits of Aerobic Activity
 Increases overall body metabolism
 Increases neurotransmitter coordination
 Increases gastrointestinal mobility and elimination
 Enhances the strength of the skeleton
 Enhances cardiovascular & respiratory system functioning, delivering oxygen & nutrients
 Increases the size of the heart which pumps more blood per contraction
 Clears the vessel walls of fatty deposits
Aerobic Exercise is not meant to increase muscle mass. Muscle mass is better increased by resistance
exercises such as weight lifting, which is primarily anaerobic. Increased muscle bulk is typically the
result of fast glycolytic fibers increasing in size. It is also believed that muscles increase in size due to
the slight longitudinal splitting or tearing of muscle fibers and the subsequent growth of these fibers.
(another cause of pain after rigorous a workout)
Hypertrophy – increase in muscle mass.
 Increased muscle size and strength results from the enlargement of individual muscle cells
(they make more contractile filaments), rather than increase in number. The amount of
connective tissue also increases
Atrophy – decrease in muscle mass (can occur at the rate of 5% per day when muscles are
immobilized)
Training for Sports and Exercise
 Stretch immediately after warming up the muscle
 Allow a day of rest in between workouts to allow muscles to repair
themselves
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