Muscular System – Notes
Chapter 6
Myology: the study of muscles
If all your muscle pulled in one direction, you could lift almost 25 tons.
Muscle Tissue
Smooth muscle: nonstriated, involuntary, walls of hollow organs and GI
tract, attached to hair follicles
Cardiac muscle: striated, involuntary, autorhythmic because of built in
pacemaker, heart wall
Skeletal muscle: voluntary, striated, attached to bone skin or fascia
Endomysium: separates individual myofibrils
Perimysium: separates the muscle tissue into fasicles
Epimysium: closely surround the skeletal muscle, under the fascia
Aponeurosis: fibrous or membranous sheet connecting a muscle
and the part it moves.
Fascia – separates individual skeletal muscles from adjacent
muscles; composed of fibrous connective tissue
Tendon – attaches muscle to bone
Muscle Functions
1. movement – internal and external
2. stability / posture
3. control of body openings
4. heat production – involuntary contractions of skeletal muscle
(shivering)
Characteristics of muscles
1. excitability (irritability) - respond to chemicals released from nerve cells
2. conductivity – ability to propagate electrical signals over membrane
3. contractility – ability to propagate electrical signals over membranes
4. extensibility – ability to be stretched without damaging the tissue
5. elasticity – ability to return to original shape after being stretched
Gross Anatomy of Skeletal muscle
Origin - - point of attachment on the stationary bone
Insertion - point of attachment on the moving bone
1
Belly – main part of the muscle body
Microscopic Anatomy of Skeletal muscle
Multinucleate with the nuclei located under the sarcolemma
Sarcolemma – plasma membrane
Sarcomere – muscle functional unit, extends from Z line to Z line
Sarcoplasm – filled with myofibrils and myoglobin(red-colored, oxygen
binding protein)
Myofibrils – long ribbon-like organelles
Myofilaments
Myosin (thick)
Actin (thin)
Sarcoplasmic reticulum – system of tubular sacs similar to smooth ER,
stores calcium, release of calcium triggers muscle contraction
Striations
A-bands – thick filaments
I-bands – thin filaments
H-zone – middle of a relaxed sarcomere
Z-bands – anchor the thin filaments
Muscle Contraction
Motor unit – one neuron and all the skeletal muscle cells it stimulates
Neuromuscular junction – axonal terminals + sarcolemma; don’t touch ,
synaptic cleft
Neurotransmitter – (ACh) – released from the axonal terminals and
diffuse across the synaptic cleft to bind with receptors on the sarcolemma;
causes the membrane to become permeable to sodium ions which rush
into the cell and causes the cell to become depolarized.
action potential – depolarization causes an electrical current; once begun
it is unstoppable and travels over the entire surface of the sarcolemma
Sliding filament theory
 Action potential causes the release of calcium from the SR
which causes the binding sites on the actin to be revealed
 Actin – thin filaments, contain myosin binding sites which
are revealed by the release of calcium by the SR
2

Myosin – myosin heads form cross bridges with the binding
sites on the actin filaments; heads pull the thin filaments
toward the center of the sarcomere (power stroke), Z lines
come closer together and the sarcomere shortens; power
stroke occurs several times during a contraction cycle;
contraction will occur as long as there is ATP available and
high Ca level.
1.
2.
3.
4.
5.
Nerve impulse
ACh released
Ca released
cross bridges form
power stroke
Relaxation occurs
 Diffusion of potassium out of the cell
 Activation of the sodium-potassium pump which moves
the sodium and potassium ions back to their original
positions
Rigor mortis – after death CA ions leak out of the SR causing the cross bridges
to form, eventually enzymes will begin to digest the decomposing cells
Creatine Phosphate – excess ATP within resting muscle used to form creatine
phosphate, quick break down provides energy for the creation of ATP, sustains
maximal contraction for 15 sec; use of supplements could cause gain in muscle
mass, but could shut down bodies own synthesis
Action of muscle groups
Prime mover (agonist) – responsible for contractions
Synergist – contract to stabilize nearby joints
Antagonist – relax to smooth the action of the agonists
Fixator – stabilize the origin of the prime mover
Intrinsic muscle – fully contained within the organ
Extrinsic muscle – not fully contained within the organ, but acting on it
Contraction
All or none response – applies to the muscle cell, not to the whole muscle; it
will contract all the way or not at all.
Graded responses – different muscle cells react to stimuli with different degrees
of shortening either by different speeds or by changing the number of cells being
stimulated.
3
Twitch – single brief jerky contractions
Tetanus – when the muscle is stimulated so rapidly that the contractions are
completely smooth and sustained
Energy for contraction
1. Direct phosphorylation of ADP by creatine phosphate
2. Aerobic respiration
3. Anaerobic respiration
muscle fatigue – inability to contract after prolonged activity; depletion of
creatine phosphate, decline of calcium, insufficient oxygen, build-up of lactic acid
and ADP, insufficient ACh
isometric contrations – muscle do not shorten; lifting heavy objects
isotonic contrations – muscle shortens and movement occurs, most common;
bending the knee
muscle tone – involuntary contraction of a small number of motor units,
essential for maintaining posture; important in maintaining blood pressure
Directions of Movement















Flexion - brings a body part forward so that the angle between the
parts is decreased
Extension – straightening a joint so that the angle between the
parts is increased
Dorsiflexion – flexing the foot at the ankle (bending the foot
upwards)
Plantar flexion – extending the foot at the ankle (bending the foot
downwards)
Hyperextension – extensive extension of the parts of a joint
Abduction - moves an appendage laterally from the midline.
Adduction - moves an appendage toward the midline.
Rotation – moving a part around an axis (twisting head from side
to side)
Circumduction - movement of an appendage in a circle around a
joint.
Pronation - rotating the palm of the hand downward.
Supination - rotating the palm of the hand upward.
Inversion - turning the toes of the foot inward.
Eversion - turning the toes of the foot outward
Retraction – moving a part backwards (pulling the chin backward)
Protraction – moving a part forward (thrusting the chin forward)
4
Disorders
Muscular hypertrophy – muscles that are forcefully exercised and tend to
enlarge; ex. weight lifting when a muscle lifts more than 75% of its maximum
tension
Atrophy – muscle that are not exercised and therefore shrinks; there is a
reduction in the capillary networks and the number of mitochondria
Muscular dystrophy – progressive crippling disease of unknown cause in which
the muscles gradually weaken and atrophy; hereditary; most common is
Duchenne muscular dystrophy (DMD) caused by a sex-linked recessive allele,
mainly a disease of males
Sprain – partial or complete tearing of tendon connecting muscle to bone or
fascia surrounding the muscle
Rigor mortis – partial contraction that causes the joints to become fixed; results
from an increase in membrane permeability to calcium ions and a decrease in
ATP in muscle fibers which prevents relaxation; actin and myosin filaments
remain linked together until the muscles begin to decompose
Hernia – condition in which the abdominopelvic viscera protrude through a weak
point in the muscular body wall; hiatal hernia the superior curveture of the
stomach protrudes through the diapragm into the thoracic cavity
Myasthenia gravis – progressive autoimmune disorder that blocks Ach
receptors at the neuromuscular junction; this weakens the muscle; more common
in women between 20-40 with possible line to thymus gland tumors; could lead to
paralysis of respiratory muscles; treated with steroids that reduce antibodies that
prevent binding of Ach.
Naming Skeletal muscles
Direction the muscle fibers run
Size, shape, action, number of origins or locations
5