• MUSCULAR SYSTEM: MUSCLE ORGANIZATION AND STRUCTURE
Muscle fiber = long multinucleate skeletal muscle cell
Fascicle = a bundle of muscle fibers held together by a thin sheet of fibrous connective tissue called fascia
(these are visible to the naked eye). This fascia is called endomysium. The fascia also contains collagen that
helps make the muscle continuous with the tendons.
Bundle of Fascicles = held together by fascia. This fascia is called perimysium.
Whole muscle = held together by fascia. This fascia is called epimysium.
Motor neurons = neurons that innervate skeletal muscle fibers
Motor unit = a motor neuron can divide and form neuromuscular junctions with several (sometimes
thousands) of skeletal muscle fibers. The motor neuron plus the skeletal muscle fibers it innervates make up
a motor unit. When a motor neuron fires, all of the muscle fibers that it innervates will contract.
Graded response = the number of motor neurons that fire (and thus the number of motor units that are
recruited) can vary according to need. This is how the body provides “gradations of response”. To lift heavier
objects, more motor units are used.
Muscles that are used for more refined movements have more motor units and thus finer control of the
movement. The gluteus maximus (butt) has few motor units per square centimeter as compared to the
muscles of the hand.
The strength of a muscle is directly proportional to the cross-sectional diameter of the muscle.
Generally, a muscle can contract up to one half of its total length. The longer the muscle, the further it can
contract. Longer muscles provide for a greater range of motion.
Parallel = longitudinal with the bundles arranged parallel and end to end (long skinny muscle)
Parallel muscles have a small cross section (weak), but a relatively large range of motion.
Unipennate muscles = muscle fibers are at an angle which results in a greater cross-section with greater
power, but less range of motion;
Bipennate muscles = muscle fibers are arranged in double rows of fibers (like a feather) for even greater
power but with even less range of motion than unipennate muscles.
Convergent (or Radiate) muscles = strongest arrangement of fibers with the greatest cross section and
smallest range of motion.
Muscles are named by their:
1. location (e.g. temporalis, subscapularis)
2. shape (e.g. deltoideus, rhomboideus)
3. size (e.g. gluteus maximus, gluteus minimus)
4. direction of fibers (e.g. rectus = straight through abdominis, external obliques = across)
5. number of bellies or heads (e.g. biceps, triceps, quadriceps (ceps = heads))
6. action (e.g. flexion, extension, adduction, abduction)
• TYPES OF MUSCLE
- Muscles are made of specialized fibrous cells that contain the proteins actin and myosin. These fibers are
packed together into myofibrils which in turn are packed together to form a muscle. Muscle cells are
characterized by having the ability to be excited (input from the nervous system) and to contract.
- In vertebrates, there are three basic tvpes of muscle tissue:
1. Skeletal muscle = moves bones, respiratory organs, eyes, mouth, etc.
- skeletal muscle fibers are long, cylindrical multinucleate muscle cells or muscle fiber
- skeletal muscle is striated and is under voluntary control
2. Smooth muscle = circle the digestive tract, blood vessels, respiratory and urinary ducts
- smooth muscle cells are long, tapering strands that are not multinucleate (=uninucleate)
- smooth muscle is not striated and is under involuntary control
3. Cardiac muscle = heart muscle only
- cardiac muscle is composed of the fast acting, uninucleate muscle cells that make up the heart
- cardiac muscle is not really striated and is under involuntary control
- due to specialized connections between cardiac muscle cells, the cells which make up a
chamber of the heart can beat in unison for the efficient pumping of blood
- All muscle cells contain a modified endoplasmic reticulum called a sarcoplasmic reticulum which stores
calcium ions and plays an important role in muscle function.
The basic functional unit of myofibrils (muscle cells) is the sarcomere. A sarcomere extends between the Z line
striations in skeletal muscle and is composed of thick filaments (myosin) and thin filaments (actin). As a
muscle contracts, the distance between the Z lines decreases (the sarcomere becomes shorter) while the Z
lines become farther apart as the muscle relaxes. Muscles contract according to the SLIDING FILAMENT
THEORY as described below.
• Excluding the heart muscle will only contract upon stimulation from the nervous system. If the nerve
supply to a muscle is damaged, the muscle will atrophy. Skeletal muscle is innervated by motor neurons
that originate in the spinal cord of vertebrates and branch so that each nerve terminal off of the axon of a
single nerve cell innervates a single muscle fiber. Each motor neuron may innervate one, a few or
thousands of muscle cells depending on their location within the body. The motor neuron and all of the
muscle cells that it innervates is called a motor unit. The total force a muscle exerts depends on the
number of motor units recruited (used) by that muscle.
• In addition to myosin and actin, there are accessory proteins attached to the actin filaments that play a
critical role in muscle cell contraction. Calcium binds to troponin and alters the conformation of the
troponin-tropomyosin complex to allow for the binding of actin and myosin which allows the thick and
thin filaments to slide past one another.
• EXCITATION-CONTRACTION COUPLING AND RELAXATION OF MUSCLE
(THE SLIDING FILAMENT MODEL)
1. An electrical signal (action potential) is carried down a motor neuron to a group of muscle fibers (skeletal
2. The electrical signal causes the release of a neurotransmitter that is stored at the end (terminal) of the
motor neuron. The neurotransmitter crosses the neuromuscular junction (synapse) to bind to the muscle
3. Receptors in the muscle cell membrane bind to the neurotransmitter and the muscle cell becomes
4. The excited muscle cell then releases calcium from the sarcoplasmic reticulum where the calcium is
5. The calcium binds to the thin actin filaments and allows them to bind to the thick myosin filaments.
6. Using ATP energy (from mitochondria in the muscle cell), the myosin heads bend back and forth to slide
the actin filaments past the myosin filaments.
7. This shortens the sarcomere that in turn shortens (contracts) the entire muscle.
• THREE MAJOR TYPES OF MUSCLE FIBERS:
(differ in their speed of contraction and the enzyme pathway used to make ATP)
1. Slow-oxidative (Type I) fibers = slower myosin-ATPases = slow twitch = red fiber
2. Fast-oxidative (Type IIa) fibers = fast myosin-ATPases = fast twitch = red fiber
3. Fast-glycolytic (Type IIb) fibers = fast myosin-ATPases = fast twitch = white fiber
#1 and #2 have a greater capacity to form ATP by oxidative phosphorylation and thus are more resistant to
#3 relies on the anaerobic process of glycolysis which only produces a little ATP (2 ATPs) and thus fatigues
In humans, many muscles are combinations of the various fiber types. The postural muscles of the back and
legs have a high proportion of slow-oxidative fibers that are specialized for maintaining low-intensity
contractions for long periods of time. Arm muscles have a high proportion of fast-glycolytic fibers specialized
for performing rapid, forceful movements. The proportions of different fiber types differs in different
individuals. Those with more fast-glycolytic fibers make good sprinters while those with more slow-oxidative
fibers perform better in endurance events.
• MORE ON MUSCLES AND SOME MUSCULAR PROBLEMS
linea alba = the midline of collagenous connective tissue that longitudinally divides the rectus abdominis
muscles. It is formed from the fusion of the three aponeuoroses (external oblique, rectus abdominis, and
transverse abdominis); see figure 10-15 in the text
external oblique aponeurosis = the large, most superficial flat tendon onto which the external abdominal
obliques insert; see figure 10-15 in the text
lumbodorsal fascia = the large sheet of fascia on the dorsal side in the lumbar region from which the internal
obliques originate; see figure 10-15 in the text
origins of the diaphragm = xiphoid process, ribs 7-12 & costal cartilages, and anterior sides of lumbar
vertebrae; see figure 10-16 in the text
insertion of the diaphragm = central tendinous sheet of the diaphragm
action of the diaphragm = contraction expands the thoracic cavity (during inhalation) and compresses the
tendonitis (or tendinitis) = an inflammed tendon, usually resulting from strain on the tendon
tennis elbow = lateral humeral epicondyiltis = an inflammation of the tissue at the lower end of the
humerus at the elbow joint, caused by the repetitive flexing of the wrist against resistance. It may result from
athletic activity or manual manipulation of tools (e.g. grocery store scanners). Treatment includes anti-
inflammatory drugs, rest, correction of body mechanics.
tendon sheath = elongated bursae that reduce friction between the tendons that run on the anterior and
posterior sides of the wrist
extensor retinaculum = a wide band of thickened connective tissue (fascia) on the posterior side of the wrist
that holds all of the extensor muscle tendons of the wrist in place
flexor retinaculum = a wide band of thickened connective tissue (fascia) on the posterior side of the wrist
that holds all of the flexor muscle tendons of the wrist in place
carpal tunnel = a conduit for the median nerve and the flexor tendons formed by the carpal bones and the
carpal tunnel syndrome = a common painful disorder of the wrist and hand, induced by compression on the
median nerve between the inelastic carpal ligament and other structures within the carpal tunnel. Most often
seen in women, especially in pregnant and in menopausal women. Symptoms may result from inflammation
of the tendons, trauma, synovial inflammation, tumor, or rheumatoid arthritis. The median nerve innervates
the palm and the radial side of the hand; compression of the nerve causes: weakness, pain with opposition of
the thumb or palmar flexion, burning, tingling or aching which sometimes radiates up the arm. Lack of use
can lead to weakness and muscle atrophy. Treatment includes the use of a splint and anti-inflammatory
drugs or steroids. Some patients require surgical division of the carpal ligament to relieve nerve pressure.
MUSCLES OF THE ROTATOR CUFF = The head of the humerus SITS in the rotator cuff =
S = supraspinatus
I = infraspinatus
T = teres minor
S = subscapularis