L6 Skeletal Muscle Structure and Function

Question 1

Identify and describe the structure and/or function of The Skeletal muscle (gross)

Answer 1

Connective tissue
Epimysium - surrounds entire muscle
Perimysium - surrounds bundles of fibres
Endomysium - surrounds individual muscle fibres
External lamina - below endomysium
Sarcolemma - muscle cell membrane

Question 2

Identify and describe the structure and/or function of the muscle cell fibre

Answer 2

Basement membrane
Plasmalemma (cell membrane)
Cytoplasm (sarcoplasm)

Question 3

Identify and describe the structure and/or function of the sarcomere

Answer 3

myofibrils
striations
I band - just actin
A band - all of the myosin (plus actin that overlaps with it)
Z line 
M line - down the middle of the myosin
H zone - just myosin

Question 4

Identify and describe the structure and/or function of Protein filaments

Answer 4

Actin
Myosin
Nebulin (actin binding protein)
Titin - protein than is responsible for the passive elasticity of muscle

Question 5

What do satellite cells do?

Answer 5

role is growth and repair of muscle by increasing the number of nuclei. More nuclei allows for greater protein synthesis.
Important for strength training adaptations

Question 6

Describe the basis or force generation (sliding filament theory)

Answer 6

Muscle shortening occurs due to the movement of the actin filament over the myosin filament.
Formation of cross-bridges between actin and myosin filaments produce a power stroke.
Results in a reduction in the distance between z lines of the sarcomere

Question 7

Identify and describe the structure and/or function of the neuromuscular junction

Answer 7

Junction between the motor neuron and muscle fibre

• Motor end plate
• Neuromuscular cleft
• Acetylcholine is released form the motor neuron

Question 8

What does the Excitation-Contraction Coupling graph look like?

Answer 8

Answer - look up in book

but graph consists of membrane potential, calcium, tension and stimulus

Question 9

What are the different modes of action? (4)

Answer 9

Static - muscle force but with no change in length
Dynamic - concentric or eccentric
Isotonic - muscle tension = same. length decreases
Isokinetic - length decreases with constant velocity

Question 10

Describe the sequence of events in muscle action excitation contraction coupling

Answer 10

1. generation of action potential causes axon to release acetylcholine. This diffuses across the synaptic cleft where it attaches to ACh receptors on the sarcolemma
2. Muscle AP depolarizes the transverse tubules at the sarcomeres A-I junction
3. Depolarisation of T-tubule system causes Ca2+ release from lateral sacs of sarcoplasmic reticulum.
4. Ca2+ binds to troponin-tropomyosin in actin filaments, releasing the inhibition that prevented actin + myosin from binding.
5. Actin binds to myosin-ATP which activates myosin ATPase to split ATP - creating tension
6. ATP binds to myosin cross bridge which breaks the actin-myosin bond which actin comes away from the cross bridge. The filaments slide past each other + the muscle shortens
7. When Ca2+ concentration remains high the cross bridge activation continues
8. Muscle stimulation stops and Ca2+ decreases –> it moves back into the sarcoplasmic reticulum through active transport
9. Ca2+ removal restores inhibitory action of troponin-tropomyosin. In presence of ATP, acting and myosin remain in dissociated, relaxed state

Question 11

What does force generation depend on?

Answer 11

1. Nature of the neural stimulation of motor units - frequency of stimulation (simple twitch, summation, tetanus)
2. Number of motor units recruited - more motor units = greater force
3. Initial muscle length –> there is an ideal length for force generation, as it increases cross-bridge formation

Question 12

A muscle twitch consists of 3 sections. What are they and how long to they last?

Answer 12

Latent period ~5ms
Contraction ~ 40ms
Relaxation ~50ms

Question 13

What is the relationship between Torque (force) and stimulus frequency?

Answer 13

As stimulus frequency increases, so does torque, which eventually begins to plateau

Question 14

what is the relationship between length and tension?

Answer 14

linear increase in relative tension as you increase length. Then small increase to optimal length. Then linear decrease /¬\ ish

Question 15

What is the relationship between velocity and tension.

Answer 15

As velocity increases tension decreases in a curve.

Lighter loads move quicker

Question 16

What is the relationship between velocity and power

Answer 16

Inverted U

At any given velocity, the power generated is greater in a muscle that has a higher percentage of fast twitch fibres