Muscle physiology 1

Question 1

What is a motor unit?

Answer 1

A single motor neurone innervating muscle fibres of one type

Question 2

What is a motor neurone pool?

Answer 2

All motor neurones that supply one whole muscle

Question 3

What makes up the neuromusculcar junction?

Answer 3

Terminal bouton of the axon

Motor end plate of the muscle fibre

Question 4

Which neurotransmitter acts on the NMJ and at which receptors?

Answer 4

Acetylcholine

On the nicotinic receptors

Question 5

Describe the structure of a muscle fibre

Answer 5

Multinucleated = formed from many mononucleated cells

Includes the:

• sarcoplasm = cytoplasm of muscle fibre
• sarcolemma = muscle fibre membrane
• sarcoplasmic reticulum = endoplasmic reticulum
• motor end plate = part of sarcolemma contacting axon

Question 6

What is the function of the sarcoplasmic reticulum?

Answer 6

Release Ca2+ on stimulation

Question 7

Describe the structure of the sarcolemma

Answer 7

Muscle fibre memrane consists of two parts:

• outer coat = thin polysaccharide/collagen layer
• inner plasma membrane

T-tubules - deep invaginations into sarcolemma to allow fast depolarisation

Question 8

Describe the motor end plate of the muscle fibre?

Answer 8

Contains nicotinic receptors and in contract with axon

Consists of in-foldings to increase surface area

Question 9

Describe the mechanism of action potential across the NMJ

Answer 9

The action potential reaches the axon terminal causing Ca2+ to flood in

Ca2+ binds to vesicles => vesicles exocytose releasing ACh into synaptic cleft which then bind to nicotinic receptors of the motor end plate of the muscle fibre

This causes Ca2+ influx into musce fibre sarcoplasm which then generates action potential as a result of threshold

Question 10

Describe the mechanism of action potential through muscle fibre

Answer 10

The end plate potential spreads over sarcolemma

This potential then penetrates transverse tubules causing more Na+ influx into sarcoplasm

Ca2+ released from sarcoplasmic reticulum which then enters myofibrils as a result of Na+ flooding into sarcoplasm

Ca2+ binds to troponin and pull tropomyosin out of the way clearing the myosin binding site

Myosin head then binds to actin forming a cross bridge leading to contraction

Question 11

Describe the make up of a muscle in term of its smaller structures

Answer 11

Made up of many fascicles/bundles surrounded by epimysium

• fascicle = many fibrils surrounded by perimysium
• fibril = many myofibrils
• myofibril = many myofilaments
• myofilament = single protein chain

Question 12

What are the proteins which make up myofibrils?

Answer 12

Myosin

Actin

Troponin

Tropomyosin

Question 13

What type of protein is myosin?

Answer 13

A thick protein with multiple heads

Question 14

What type of protein is actin?

Answer 14

A thin filament

Question 15

What type of protein is tropomyosin?

Answer 15

Lies over actin filaments and blocks myosin binding sites

Question 16

What type of protein is troponin?

Answer 16

When bound by Ca2+ it changes its shape and then pulls tropomyosin out of the way

Question 17

What is a sarcomere?

Answer 17

Contractile unit of a muscle

• extends from one Z line to the next

Question 18

Describe the bands of a sarcomere

Answer 18

A bands - dark - constant length of myosin on contraction

I bands - light - reduced length of myosin on contraction

Question 19

Describe the cross bridge cycle

Answer 19

The process of muscle contraction

1. ATP binds to myosin head => myosin-actin dissociation
2. Myosin head splits ATP to ADP and P => myosin head recocks
3. Myosin head binds to actin and P dissociates
4. Power stroke occurs as ADP dissociates

Question 20

Why does rigor mortis occur?

Answer 20

No ATP => myosin head can’t dissociate from actin

=> stiffness (rigor mortis)

Question 21

What are the four sources of ATP in skeletal muscle?

Answer 21

Phosphocreatine

Anaerobic respiration

Aerobic respiration

Free fatty acids oxidation