Chapter 9 Muscle Tissue
I. Muscle types- characteristics & appearance (review this from chpt 4)
A. Skeletal :40% of body weight
B. Smooth: “ visceral muscle”
C. Cardiac
II. Functions & Characteristics Muscle tissue
A. Functions
1.Maintenance of posture
2.Joint Stabilization
3.Movement
4. Generate body heat
5. Support soft tissues
6. Regulate enter/exit of materials (eg) smooth muscles of digestive system, urinary, respiratory etc.
B. Characteristics
1. Contractility: contracts forcefully (gets shorter & makes a pulling force as it contracts)
2. Excitability: Nerve signals or other factors can excite muscle cells – cause electrical impulses to travel along the fibers
3. Extensibility: able to stretch
4. Elasticity: after stretch, muscle can recoil passively
5. Skeletal & smooth muscle cells are called fibers. (but not cardiac muscle cells)
6. All three types of muscle depend on contraction of myofilaments- actin & myosin.
Contraction = muscle fibers shorten
7. Muscles contain nervous tissue, blood vessels & connective tissue
C. Terminology: “sarco- flesh” “lemma”= sheath
Sarcoplasm: cytoplasm of muscle cells
Sarcolemma: the plasma membrane
sarcoplasmic reticulum : the endoplasmic reticulum of muscle cells
III. Skeletal Muscle
A. Organization of Skeletal Muscle
1. endomysium
- around a muscle fiber
2. perimysium
- create fascicles
3. .epimysium
- deep fascia- fascia fibers blend into tendons & aponeuroses for stability & attachment.
4. All 3 sheaths are continuous with each other & with the tendon which attaches muscle to bone.
5. The sheaths provide muscle with elasticity & carry blood vessels & nerves.
B. Cells
1. Myoblasts fuse in development to form a multinucleated fiber.
2. Myosatellite cells: do not fuse & remain on periphery. Undifferentiated & proliferate in damaged skeletal muscle to repair tissue. Some fuse to form fibers, others stay on periphery. Badly damaged skeletal muscle cannot completely regenerate & forms scar tissue.
3. Muscle fiber: contracts
C. Microanatomy of a muscle fiber
1. myofibrils–2 types of protein fibers
a) actin(thin filament)
b) myosin (thick filament) with myosin heads
Myosin heads bind to actin and “pull” actin across its surface in a power stroke.
The myosin head acts as an ATPase enzyme, cleaving ATP to get its energy.
2. Sarcomere
a) Z-line: proteins which anchor actin filaments. A sarcomere is between 2 Z-lines.
b) I band: actin only
c) A band: overlap of actin & myosin
d) H zone: myosin only
e) M line: midline of sarcomere
3. Sliding filament theory
a) Contraction:In the presence of Ca+2,myosin heads bind to actinpulls actin across itself -results in contraction, or shortening of the sarcomere.
- theZ lines move inwards, the I band and the H zone become smaller.
b) Relaxation:Myosin heads would be attached to actin unless ATP is hydrolyzed. In other words, ATP is required to release myosin heads from actin so that another power stroke can be made.
4. Types of contraction:
a) Concentric contraction: muscles shorten as it does work.
b) Eccentric contraction: muscle generates force as it lengthens. Mechanism less understood. Essential for controlled movements & resistance to gravity.
5. Neuromuscular junction
- motor neuron’s axon terminals come very close to, but do not physically touch the muscle fiber.
- Acetylcholine (Ach), a neurotransmitter, is stored in vesicles in the axon terminal of the motor neuron.
- themotor neuron releases acetylcholine in response to an action potential (impulse).
- Ach is released into the synaptic cleft where it binds to Ach receptors on the muscle fiber.
- An impulse travels down transverse tubules (T-tubules) of the muscle fiber
- the impulse stimulates sarcoplasmic reticulum to release its Ca+2.
- Myofibrils contract.
5. Motor unit
a). All the muscle fibers controlled by a single motor neuron.
Fine control (eye mov’t)= few fibers per unit.
Less precise mov’t (thigh) = hundreds or 1,000’s of fibers /unit
b) Muscle fibers of different motor units are intermingled for even force.
c) Amount of tension on muscle:
1) Frequency of stimulation
2) # of motor units involved
D. Size of muscle
1. Hypertrophy: increase in size
2. Atrophy: decrease in size
3. Muscle tone=at rest, some motor units are active. Not enough to cause movm’t tense muscle
E.Muscle fiber types
1. Characteristics:
a. Fast, slow & intermediate muscle fibers found in almost all muscles in different amounts.
b. All fiber types in a motor unit are the same.
c. % of fast vs slow fibers in muscle is genetically determined! (born marathoner or sprinter?)
d. Characteristics of muscle fibers change with conditioning
e. Repeated, intense workouts promote fast fibers
f. Endurance training promotes slow fibers.
g. Cross-training = using both types of exercise for endurance & muscle hypertrophy.
2. Categorizing fiber types
a) oxidative fibers: use oxygen to produce ATP
b) glycolytic fibers: make ATP anaerobically via glycolysis.
c) Speed of contraction: fast or slow
d) classifications are
Slow Oxidative fibers (SO), Fast Glycolytic Fibers (FG), Fast Oxidative fibers (FO)
* possible to transform muscle types through training, but revert back to original type after training.
F. Skeletal Muscle Attachments
Origin:Insertion pulled toward the origin. Usually the more proximal attachment
Insertion:bone (or CT or skin) which moves. Usually the more distal attachment
* the terms origin and insertion are being phased out. They are now using “proximal attachment & distal attachment”
Action:the main movement that occurs when the muscle contracts.
Prime mover (agonist): a muscle that does most of the work in an action
Synergists :
a) add extra force eg) biceps brachii & brachialis muscles have same action
b)fixator: A fixator holds the bone still that the agonist is attached to.
For example, during elbow flexion and extension the shoulder muscles hold the scapula (shoulder blade) still (both the biceps and triceps attach to the scapula).The fixators hold the scapula still by increasing tension, but keeping the same length.
Antagonist: works opposite the synergists.