Download File

Unit 6



Muscle makes up nearly half of the body’s
mass
Essential function of muscle is contraction or
shortening
Responsible for all body movement

Three types of muscle



Skeletal
Cardiac
Smooth

Skeletal and smooth are elongated- muscle
fibers



Not cardiac
The ability of muscle to shorten or contract
depends on myofilaments
Terminology

Myo- and mys- (muscle)
Sacro- (flesh)

Skeletal muscle fibers are packaged into the
organs skeletal muscles that attach to the
body’s skeleton
Huge, cigar-shaped, multinucleate cells; some 30 cm
 Some fibers of large, hardworking muscles are so big
and coarse that they can be seen with the naked eye


Also known as striated muscle because of
obvious stripes

Also voluntary muscle- it is the only muscle
type subject to conscious control

Also activated by reflexes (without our “willed
command”)
*When you think of skeletal muscle, remember
skeletal, striated, and voluntary



Can contract rapidly and with great force, but
tires easily and must rest after short periods of
activity
Muscle fibers are soft and fragile
The reason they are not ripped apart is that
fibers are bundled together by connective
tissue

Provides strength and support





Each muscle is enclosed in a delicate connective
tissue sheath called endomysium
Several sheathed muscle fibers are then
wrapped by a coarser fibrous membrane called
perimysium
Forms a bundle of fibers called a fasicle
Many fasicles bound together by a tougher
“overcoat” called epimysium which covers the
entire muscle
The epimysia blend into strong tendons or
sheetlike aponeuroses, which attach muscles
indirectly to bones, cartilages, or connective
tissue coverings



Tendons provide durability and conserving
space
Mostly tough collagenic fibers, they cross
rough bony projections, which would tear the
more delicate muscle tissues
More tendons than fleshy muscles can pass
over a joint




Has no striations
Involuntary, cannot consciously control it
Found in visceral organs- stomach, urinary
bladder, and respiratory passages
Propels substances along a definite tract,
pathway, within the body
* Best described using- visceral, nonstriated,
involuntary


Cells are spindle-shaped, single nucleus,
surrounded by a scant endomysium
Arranged in layers




One running circularly and the other longitudinally
The two layers alternately contract and relax,
changing the size and shape of the organ
Slow and sustained
“Lumbers along tirelessly”




Only found in the heart
Pump, propelling blood into the blood vessels
and to all tissues of the body
Striated
Involuntary
*cardiac, striated, and involuntary




Cushioned by small amounts of endomysium
and arranged in spiral
Cardiac muscle fibers are branching cells joined
by special junctions called intercalated discs
Contracts at a fairly steady rate set by the
heart’s pacemaker
But it can be stimulated by the nervous system
to speed up
* we will concentrate on skeletal muscle
 Producing Movement


Maintaining Posture



All movement is due to muscle contractions
Making tiny adjustments so that we can maintain an
erect or seated posture
Stabilizing Joints
Generating Heat


By-product of muscle activity
As ATP is used to power muscle contraction, ¾ of
energy escapes as heat



Sarcolema- plasma membrane, many oval
nuclei seen just beneath this
Myofibrils- long ribbon like organelles that are
pushed aside by the nuclei
Myofibrils are aligned with alternating dark
(A) and light (I) bands, giving the striped
appearance

Think of the second letter to help remember each
band



The I band has a midline interruption, a darker
area called the Z disk
The A band has a lighter central area called the
H zone
The M line in the center of the H zone contains
tiny protein rods that hold thick filaments
together


Sarcomeres- chains of tiny contractile units,
aligned end-to-end, like boxcars, all along the
myofibrils
Myofilaments within sarcomeres actually
produce banding pattern
Two types of myoflimalents
 Thick filaments- myosin filaments, a protein
Contain ATPase enzymes, split ATP to generate the
power of muscle contraction
 Thick filaments extend the entire length of the dark
band
 Mid parts are smooth
 Ends contain myosin heads called cross bridges
when they link the thick and thin filaments together
during contraction


Thin filaments- contain actin, a contractile
protein
Some regulatory proteins that play a role in allowing
or preventing myosin head-binding to actin
 Anchored to the Z disk
 H zone lacks actin filaments


Muscle cells’ functional properties
Excitability, responsiveness or irritability- ability to
receive and respond to stimulus
 Contractility- ability to shorten
 Extensibility- ability of muscle cells to be stretched
 Elasticity- ability to recoil and resume their resting
length after being stretched







Muscles are stimulated by nerve impulses
Motor unit- stimulates one neuron and all the
skeletal muscle cells
Axon- long, threadlike extension of the neuron
Axon terminals- end of the axon that reaches
the muscles and branches
Forms junctions with the sarceloma called
neuromuscular junctions
Synaptic cleft- gap between, filled with
interstitial fluid





When a nerve impulse reaches the axon
terminals, a chemical called a neurotransmitter
is released
The specific neurotransmitter that stimulates
skeletal muscle is acetylcholine or Ach
Diffuses across the synaptic cleft and attaches
to receptors
Na+ rushes into the muscle cell and K+ leaves
More Na+ enters than K+ leaves




Gives the cell interior excess of positive ions
and reverses the electrical conditions of the
sarcolemma
The upset generates an electrical current called
an action potential
Once begun, the action potential is unstopalbe
Result is contraction




While the action potential is occurring, Ach is
broken down to acetic acid and choline by
enzymes (acetylcholinesterase AChE)
Single nerve impulse produces only one
contraction
Prevents continued contractions
Muscle cell relaxes until stimulated by the next
round of acetylcholine





https://www.youtube.com/watch?v=7wM5_a
Un2qs
https://www.youtube.com/watch?v=HJj3jUV
DFFo
https://www.youtube.com/watch?v=D3JkAe
838Zo
Simplified
https://www.youtube.com/watch?v=RcweKl
4_OVw





After activated by the nervous system
Myosin heads attach to binding sites on the
thin filaments, and sliding begins
Each cross bridge attaches and detaches several
times during a contraction, pull thin filament
toward the center of the sarcomere
Thin filaments cannot slide backwards as the
cycle repeats again
https://www.youtube.com/watch?v=StFddjOmR4
Actin myofilaments prevent myosin binding
called tropomyosin
Action potential sweeps along the
sarcolemma, muscle cell excited, Ca2+
released
Calcium acts as the final trigger for contraction
1.
2.
3.
1.
4.
Change their shape and their position on the thin
filaments
Exposes myosin binding sites on the actin to
which the myosin heads can attach- troponin
5.
6.
7.
8.
9.
10.
11.
Myosin heads are “cocked”
Physical attachment of myosin to actin
“springs the trap” causing the myosin heads to
pivot
Thin filaments pulled toward the center of the
sarcomere
ATP provides energy to release and recock
each myosin head so its ready for the next step
Calcium reabsorbed into the SR storage areas
Resume original shape and position and block
myosin
The muscle cell relaxes and settles back to its
original length

Graded Responses
“all-or-none” law applies to the muscle cell, not the
whole muscle
 A muscle cell will contract to its fullest exten when
stimulated adequately
 However, the whole muscle reacts to stimuli with
graded responses, or different degrees of shortening
1. By changing the frequency of muscle stimulation
2. By changing the number of muscle cells being
stimulated at one time


Muscle Response to Increasingly Rapid
Stimulation




Muscle twitches (single, brief, jerky contractions)
Result from certain nervous system problems, not
the way our muscles normally operate
When muscle is stimulated so rapidly that no
evidence of relaxation is seen and contractions are
completely smooth and sustained, the muscle is
fused, or complete, tetanus
Until this point is reached, the muscles is unfused, or
incomplete, tetanus

Muscle Response to Stronger Stimuli


The goal of tetanus is to produce smooth and
prolonged muscle contractions
How forcefully a muscle contracts depends on how
many of its cells are stimulated


Muscles only store 4 to 6 seconds worth of ATP
3 pathways for ATP regeneration
1.
Direct phosphorylation of ADP by creatine
phosphate
1. Creatine phosphate (CP) high-energy molecule,
regenerates ATP, more CP ( 15 seconds)
2.
3.
Aerobic respiration- metabolic pathways uses oxygen
Anaerobic glycolysis and lactic acid formation
1. When oxygen is not present, pyruvic acid converted to
lactic acid, 2.5 times faster than aerobic, provide ATP
needed for 30-60 second stenuous muscle activity
2. Uses large amounts of glucose

Muscle Fatigue and Oxygen Deficit




Fatigue occurs when it is unable to contract even
though it is still being stimulated
Begins to tire until if finally ceases reacting and stops
contracting
Result of oxygen deficit- not able to take in oxygen
fast enough
Breathes rapidly and deeply after activity until
enough oxygen is needed to get rid of the
accumulated lactic acid and make ATP reserves

Types of Muscle Contractions

Isotonic contractions- myofilaments are successful in
their sliding movements, muscle shortens, and
movement occurs
 Ex. Bending the knee, rotating the arms, smiling

Isometric Contractions- myofilaments try to slide,
but the muscle is pitted against some more or less
immoveable object
 Ex. pushing against a wall, muscles can’t shorten to
straight arms

Muscle Tone- continuous partial contractions


Continue to contract at rest
Effect of Exercise on Muscles



Muscle inactivity leads to muscle weakness
“Use it or lose it”
Regular exercise increases muscle size, strength and
endurance
 Aerobic or endurance exercise- jogging, biking=
stronger, more flexible, greater resistance to fatigue
 Resistance or isometric- weight lifting, increase muscle
size

Every muscle is attached to bone at no fewer
than two points



Origin- attached to immovable or less moveable bone
Insertion- attached to the moveable bone, when
muscle contracts, insertion moves toward the origin
Body movement occurs when muscles contract
across joints

Flexion- movement in sagittal plane, decreases
the angle of the joint and brings two bones
closer together


Extension- opposite of flexion


Hinge joints, also ball-and-socket joints
Greater than 180° would cause hyperextension
Rotation- movement of a bone around its
longitudinal axis

Ball-and-socket joints

Abduction- moving a limb away from the
midline, or median plane



Also applies to the fanning movement of fingers or
toes when they are spread apart
Adduction- opposite of abduction, toward
Circumduction- combination of flexion,
extension, abduction, and adduction
commonly seen in ball-and-socket joints such
as the shoulder


Dorsiflexion (up) and plantar flexion (down)up and down movement of the foot at the
ankle
Inversion (medially) and eversion (latterally)movement of the sole of the foot

Supination (turning backward) and pronation
(turning forward)- movement of the radius
around the ulna



Supination- palm faces forward
Pronation- palm faces posteriorlly
Opposition- between the metacarpal 1 and the
carpals, movement of the thumb




Muscles can only pull as they contract
Muscles are arranged so that whatever one
muscle can do, other muscles can reverse
The muscle that has the major responsibility for
causing a particular movement is called the
prime mover
Muscles that oppose or reverse a movement are
antagonists



Synergists help prime movers by producing
the same movement or by reducing
undesirable movements
A muscle crossing two or more joints,
contraction will cause movement in all the
joints crossed unless stabilized
Fixators-specialized synergists, hold a bone still
or stabilize the origin of a prime mover so all
the tension can be used to move the insertion
bone


Muscle naming bases on several criteria which
focuses on structure and function
Direction of muscle fibersRectus (straight)- running parallel to the midline
 Oblique (slant) to the imaginary line


Relative size of the muscle


Maximus (largest), minimus (smallest), longus (long)
Location of the muscle-some named for the
bone with which they are associated

Number of origins


Location of the muscle’s origin and insertion



Bi-, tri-, and quad- it has that many points of origin
Named for their attachments sites
Shape of the muscle- different shapes
Action of the muscle- named for their actions

Flexor, extensor, adductor


Fasicle arrangements differ structurally
Circular- arranged in rings, found surrounding
external body openings which close by
contracting





Sphincters (squeezers)
Convergent- converge toward a single insertion
tendon
Parallel- run parallel to the long axis of the
muscle
Fusiform-spindle shaped muscle, expanded
belly
Pennate- short fasicles obliquely to central
tendon








Frontalis- covers frontal bone, forhead
Occipitalis- posterior of the skull
Orbicularis Oculi- circles around the eyes
Orbicularis oris- around lips
Buccinator- runs across cheek, flattens the
cheek like when whistling
Zygomaticus- extends from corner of the
mouth to the cheek bone, “smiling muscle”
Masseter- cover the lower jaw
Temporalis- fan-shaped muscle overlying
temporal bone, synergist of the masseter


Platysma- covers the anterolateral neck, the
front, originates from the connective tissue on
the chest muscles, produce downward sag of
the mouth
Sternocleidomastoid- two headed muscle



one found on each side of the neck
Of the two heads one arises from the sternum and
the other arises from the clavicle
If one muscle contracts, the head is rotated toward
the shoulder on the opposite side and tilts the head
to its own side



Move the vertebral column
Anterior thorax muscles
Abdominal wall


Pectoralis Major- covers upper part of the chest
Intercostal Muscles- deep muscles found
between the ribs, help raise the rib cage when
breathing

Internal and external

Muscles of the Abdominal Girdle
Rectus abdominis-most superficial, run from pubic
to rib cage, main function to flex the vertebral
column
 External oblique- lateral walls of the abdomen, rotate
and bend laterally
 Internal oblique- fibers run at right angles to those of
the external obliques, function the same
 Transversus abdominis- deepest muscle, fibers run
horizontal across the abdomen, compresses contents


Trapezius- most superfical of the posterior
neck
Each runs from the occipital to the end of the
thoracic vertebrae
 Elevate, depress, adduct, and stabilize the scapula


Latissimus Dorsi- cover lower back

Lower spine and ilium and sweep superiorly to the
proximal end of the humerus

Erector Spinae- prime mover of back extension




Deep muscles of the back, span entire length of
vertebrae
After injury to back, muscle go into spasms, common
source of lower back pain
Quadratus Lumborum- form part of the
posterior abdominal wall, flexes the spine
laterally
Deltoid- fleshy, triangle-shaped muscles that
form the rounded shape of your shoulders


Favorite spot for small (<5 mL) injections
Prime mover for arm abduction



Muscles of the Humerus That Act on the Forearm
All anterior arm muscles cause elbow flexion
In order of decreasing strength:

Biceps Brachii- originates by two heads,
shoulder girdle and radial tuberosity




Prime mover
Brachialis- lifts the ulna as the biceps lifts the
radius
Brachioradialis- weak muscle, arises on the
humerus and inserts into the distal forearm
Triceps Brachii- only muscle on the posterior
humerus




3 Heads- should girdle, proximal humerus, and
olecranon process of the ulna
Prime mover of ulna extension
Antagonist of biceps brachii
“Boxer muscle”



Largest and strongest muscles
Origin and insertion are often interchangeable
Many thigh attachments to the pelvic girdle

Gluteus Maximus- most of the flex of the buttock



Powerful hip extensor acts to bring the thigh in a straight
line with the pelvis
Not very important in walking, more important in
jumping and climbing stairs
Gluteus Medius- runs from the ilium to the femur
beneath the gluteus maximus
Steadies the pelvis during waling
 Important site for giving intramuscular injections (>5mL)
 Part of each buttock overlies the sciatic nerve
 Divide the buttock into four equal quadrants, the gluteus
medius is a safe site


Iliopsoas- fused muscle composed of two
muscles, iliacus and psoas major


prime mover of hip flexion
Adductor Muscles- medial side of each thigh


They adduct thighs together
Become flabby very easily

Hamstring Group
Biceps femoris, semimembranosus, and
semitendinosus
 Originate on the ischial tuberosity, run down the
thigh to insery on both sides of the proximal tibia
 Prime movers of thigh extension and knee flexion
 Felt at the back of the knee



Sartorius- not to important, most superficial
Quadriceps Group- rectus femoris and 3 vastus
muscles, originate from the femur, extends the
knee powerfully, kicking


Rectus femoris crosses two joints, flex the hip
Intramuscular injection site, infants

Tibialis Anterior- superficial anterior leg, runs
to the tarsals,



Extensor Digitorum Longus- prime mover of
tow extension and a dorsiflexor of the foot
Fibularis Muscles- longus, brevis, tertius



Dorsiflex and invert the foot
Plantar flexes and everts the foot
Gastrocnemius- two-bellied curve of the
posterior leg, prime mover of plantar flexion,
lifts heal, walking hard if torn
Soleus- doesn’t affect knee movement, plantor
flexor







In developing embryo, muscles is laid down in
segments and then invaded by nerves
Movement of the fetus occurs by the 16th week
After birth, movements are gross reflex types
because the nervous system must mature
before the baby can have control
Control proceeds in proximal/distal direction
Because of blood supply, resistant to infection
As we age, connective tissue increases and
muscle tissue decreases
More muscle mass = increased strength

Muscular Dystrophy a group of inherited
muscle-destroying diseases that affect specific
muscle groups


Muscles enlarge due to fat and connective tissue
deposit, muscle fibers degenerate
Myasthenia gravis- drooping of the upper
eyelids, difficulty in swallowing and talking,
muscle weakness and fatigue

Shortage of acetylcholine receptors