Muscle

Question 1

What is a motor unit?

Answer 1

A motor unit is a motor neuron and all the muscle fibers it innervates.

Question 2

What is a neuromuscular junction?

Answer 2

A neuromuscular junction is the site on a muscle cell where an α motor neuron forms a synapse.

Question 3

What is the endoplasmic reticulum of muscle called?

Answer 3

Sarcoplasmic reticulum

Question 4

What are the mitochondria of muscle called?

Answer 4

Sarcosomes

Question 5

State the contractile proteins.

Answer 5

Actin and myosin

Question 6

State the structural proteins.

Answer 6

Titin and dystrophin

Question 7

State the regulatory proteins.

Answer 7

Troponin complex and tropomyosin

Question 8

State the function of the troponin complex.

Answer 8

The troponin complex blocks myosin-binding sites on actin filaments to prevent muscle contraction.

Question 9

What are the 3 subunits of the troponin complex? State their function.

Answer 9

TnC binds to Ca2+ to produce a conformational change in TnI (allowing for muscle contraction as the complex is moved off the myosin-binding sites on actin, leading to cross bridge formation).
TnT binds to tropomyosin, to form a troponin-tropomyosin complex.
TnI binds to actin to hold the troponin-tropomyosin complex in place.

Question 10

What part of a sarcomere is shortened when a muscle is contracted?

Answer 10

A sarcomere is shortened by the pulling in of the Z-lines at each extreme of the sarcomere.

Question 11

Describe the cross-bridge cycle.

Answer 11

At rest, myosin is energised. ATP is cleaved into ADP, Pi and energy by myosin ATPase. Energy is stored within the myosin protein. Ca2+ binds to TnC. The resulting conformational change makes it so that the myosin binding site on actin is exposed. Myosin cross bridges are bound to actin. Pi is released from myosin upon binding with actin. Myosin cross bridges bend, pulling the thin filament inward and releasing energy through the power stroke. Sarcomere length decreases, translating to a muscle contraction. After the power stroke, myosin binds a new ATP molecule. Actin and myosin detach. ATP hydrolysis occurs to re-energise myosin. The cross bridge detaches and returns to original conformation. This process will repeat if Ca2+ and ATP are present. Fatigue and lack of ATP will cause contractions to stop.

Question 12

What are the factors that affect graded tension in the muscle?

Answer 12

Motor unit recruitment and development of tension by each fibre.

Question 13

What is fatigue in muscles?

Answer 13

Fatigue is the inability to maintain muscle tension.

Question 14

What marks the end of a muscular contraction?

Answer 14

The removal of all Ca2+ from the cytosol.

Question 15

What are the factors that affect the tension developed by muscle fibers?

Answer 15

Frequency of stimulation, length of fibres at onset of contraction, thickness of fiber, extent of fatigue

Question 16

What is a muscle twitch?

Answer 16

A muscle twitch is a contraction.

Question 17

Differentiate the 3 varying results of adjusting the frequency of muscular stimulation.

Answer 17

If a muscle fiber is restimulated after it has completely relaxed, the 2nd twitch is the same magnitude as the 1st. If a muscle fiber is restimulated before it has completely relaxed, summation occurs and a larger than normal tension is produced. If a muscle fiber is stimulated so rapidly that there is no time to relax at all, a maximal sustained contraction known as tetanus occurs.

Question 18

What type of muscle fibers are trained in bodybuilding and what is this hypertrophy due to?

Answer 18

Muscular hypertrophy mainly effects on fast-glycolytic fibers. Hypertrophy is due to increased actin and myosin synthesis.

Question 19

What is the purpose of muscle fatigue?

Answer 19

To prevent the muscle from reaching a point where it can no longer produce ATP. Lack of ATP is muscular rigor.

Question 20

What are the 3 types of muscle fibers? Differentiate.

Answer 20

Slow-oxidative (Type I)
Fast-oxidative (Type IIa)
Fast-glycolytic (Type IIx)
Fast vs. slow is based on the rate of myosin ATPase activity. Oxidative vs. glycolytic is predicated on the fact that net ATP is greater in oxidative fibers, hence less fatigue.

Question 21

What is isotonic contraction?

Answer 21

Isotonic contraction is when the load remains constant but the muscle length changes.

Question 22

What is isometric contraction?

Answer 22

Isometric contraction is when the muscle is prevented from shortening, so tension develops at a constant muscle length.

Question 23

What is isokinetic contraction?

Answer 23

Isokinetic contraction is when the velocity of shortening remains constant as the muscle length changes.

Question 24

What are muscle spindles?

Answer 24

Muscle spindles are stretch receptors found in skeletal muscle.

Question 25

How do muscle spindles detect changes in skeletal muscle?

Answer 25

When a muscle stretches, so do the spindles. This depolarises the primary sensory axons and causes them to fire action potentials. Primary sensory axons synapse with alpha motor neurons which cause muscle to contract to resist this stretching. When the muscle contracts, the spindle slackens, but it needs to be taut to remain sensitive to stretch. Hence, activation of gamma-motor neurons causes the distal ends of spindle to contract, keeping the central part taut.

Question 26

From where do alpha motor neurons receive input such that movement is coordinated?

Answer 26

Primary sensory axons, descending axons from the brain and collaterals of lower motor neuron axons.

Question 27

Describe the withdrawal reflex.

Answer 27

Nociceptor fibers stimulate flexors and cause relaxation of extensors to withdraw limb on ipsilateral side. Nociceptor fibres stimulate extensors and cause relaxation of flexors to stabilise limb on contralateral side and ensure weight-bearing.

Question 28

Differentiate between the location of muscle spindles and GTOs.

Answer 28

Muscles spindles are parallel to muscle fibres whereas GTO are in series.

Question 29

Where do GTOs synapse?

Answer 29

GTOs are innervated by Ib sensory axons. They synapse on the interneurons of the ventral horn.

Question 30

How do GTOs work?

Answer 30

GTOs respond to changes in muscle tension. The increased tension in the tendon stretches the GTOs. The stretch intensity increases the action potential frequency.

Question 31

Where is the motor cortex located?

Answer 31

The motor cortex is located on the frontal lobe, anterior to the central sulcus.

Question 32

Corticospinal pathway (pyramidal tract) -Note the origins -Note the structures passed through -Note crossover (decussation) -Note ventral origin of lower motor neurons

Answer 32

-Primary motor cortex (Brodmann area 4) in the precentral gyrus -corona radiata, internal capsule, cerebral peduncles, pons, medullary pyramids -90% of the fibers cross oppositely, forming the lateral corticospinal tract. 10% remain uncrossed continuing as the anterior corticospinal tract. -terminates at the ventral horn and synapses with lower motor neurons which exit via ventral roots

Question 33

Brainstem pathway -Note the origins -Note the structures passed through -Note crossover (decussation) -Note ventral origin of lower motor neurons

Question 34

What is the effect of damage at the motor neurons?

Answer 34

Flaccid (LMNs) paralysis (UMNs), no reflexes

Question 35

What is the effect of damage at the cerebellum/basal nuclei?

Answer 35

Uncoordinated voluntary movement

Question 36

What is the effect of damage at the higher cortical command centres?

Answer 36

Inertia, inability to translate voluntary planning into motor activity

Question 37

Describe the opposing action of alpha and gamma motor neurons.

Answer 37

Alpha motor neuron activation alone would decrease 1a sensory activity as muscle spindle would be slack; contraction would cease as muscle spindle would be flaccid. Gamma motor neuron activation alone would increase 1a sensory activity as muscle spindle (centre) is stretched; induces contraction of muscle. Balance between these is required to maintain muscle tone