Skeletal Muscle Structure and Function

Question 1

What are the functions of muscle?

Answer 1

Force production for locomotion, postural support, breathing, and heat production during cold stress.

Question 2

Organization of muscle

Answer 2

muscle, fascicles, muscle fibers, myofibrils, myofilaments (actin and myosin).

Question 3

What is the role of the connective tissue in muscle?

Answer 3

Connective tissue layers connect and transfer force to tendon which transfers force to bone

Question 4

What are the 3 layers of connective tissue?

Answer 4

Epimysium: covers entire muscle

Perimysium: covers fascicles (bundles of fibers) and contains nerve and blood vessels.

Endomysium: covers individual muscle fibers.

Question 5

What is the primary content in a muscle fiber?

Answer 5

Sarcolemma: membrane

Sarcoplasm: cytoplasm

Nuclei: contain many

Mitochondria: densely packed

Question 6

What is the sarcoplasmic reticulum?

Answer 6

Serves as storage for calcium and surrounds each myofibril and runs parallel.

Question 7

What is the smallest functional unit of muscle?

Answer 7

Sarcomere

Question 8

Name the two principal contractile proteins (myofilaments) in skeletal muscle

Answer 8

Thin filament (actin)

Thick filament (myosin)

Question 9

What is the neurotransmitter released at the neuromuscular junction?

Answer 9

Acetylcholine

Question 10

What is the Excitation-contraction coupling sequence?

Answer 10

1. Motor neuron stimulation of muscle fiber:
   - Nerve impulse transmitted from cns to motor neuron toward the muscle fiber
   - Release of Ach from motor neuron to nmj
   - Ach binds to its receptor on the sarcolemma
2. Muscle fiber action potential:
   - Action potential transmitted down T-tubules which leads to the release of calcium from sarcoplasmic reticulum.
3. Crossbridge cycling:
   - Calcium binds to troponin pulling tropomyosin off binding site and myosin heads attach to the actin filament.

Question 11

What is the process of muscle contraction (crossbridge cycle)?

Answer 11

1. Reset: ATP on myosin head is hydrolyzed by myosin ATPase to ADP + Pi (energized)
2. Attach: when motor nerve is stimulated, calcium is released and binds to troponin, tropomyosin moves, exposes myosin binding site on actin, energized myosin forms a strong bond with actin.
3. Pull: ADP release causes head to relax while still bound to myosin which allows it to pull on actin (power stroke, shortens sarcomere)
4. Release: a new ATP binds to myosin, allowing myosin to release from actin.

Question 12

What is the role of myosin ATPase in the process of muscle contraction?

Answer 12

Each myosin head can move and bind actin when ATP is hydrolyzed by myosin ATPase.

Question 13

What is the requirement of ATP for muscle contraction and relaxation?

Answer 13

Contraction: ATP is necessary to move the myosin head toward myosin.

Relaxation: binding of new ATP is necessary for the release of myosin from actin + calcium is actively pumped back into SR and ATP is required.

Question 14

What are the types of muscle actions?

Answer 14

Isotonic (dynamic) contractions: muscle changes length and moves a load (force is greater than resistance)

Isometric (static) contractions: tension in the muscle increases but the muscle neither shortens nor lengthens (cross bridges form but actin is not moving and resistance is greater than force).

Question 15

What is the motor unit organization/structure?

Answer 15

Consists of motor neurons together and all the muscle fibers it stimulates + multiple motor units support an individual muscle, not just one.

• Each muscle fiber is innervated by only 1 motor neuron.
• Not all motor units will be activated at the same time.

Question 16

What is the size principle?

Answer 16

Motor units are recruited according to their size

Question 17

What are the main factors that determine the ability for muscles to produce force?

Answer 17

• The amount of force generated by a single fiber is related to the number of actin/myosin cross-bridges formed.
• The amount of force generated by an entire muscle is dependent on several factors including the number
• (increasing the strength of the stimulus recruits more motor units = more muscle fibers = more force) and types of muscle fibers recruited (increasing the strength of the stimulus recruits fast twitch motor units which innervate fast twitch/type II fibers),
• Length of the muscle during contraction aka sarcomere length. (ideal length is resting or slightly stretched which enhances the interaction between actin and myosin),
• Frequency of stimulation of the motor units, and the velocity of contraction.