Skeletal Muscle Physiology 1

Question 1

3 types of muscle?

Answer 1

Skeletal

Cardiac

Smooth

Question 2

Different ways of categorising smooth muscle by appearance and innervation?

Answer 2

Striated muscles:

Skeletal

Cardiac

Voluntarily controlled (somatic NS):

Skeletal

Involuntarily controlled (ANS):

Cardiac

Smooth

Question 3

What is striation?

Answer 3

Seen under a light microscope as alternating dark bands (myocin thick filaments) and light bands (actin thin filaments) in striated muscles

Question 4

Physiological functions of skeletal muscles?

Answer 4

Maintenance of posture

Purposeful movement in relation to external environment

Respiratory movements

Heat production

Contribution to whole body metabolism

Question 5

What is a MOTOR UNIT?

Answer 5

A single α-motor neuron and all the skeletal muscle fibres it innervates

Question 6

Variation in the no. of fibres per motor unit?

Answer 6

Muscles which serve fine, precise movements, e.g: external eye muscles, intrinsic hand muscles and those for facial expression, have fewer fibres per motor unit

For muscles where power is more important than precision, e.g: thigh muscles, there are 100s-1000s muscle fibres per motor unit

Question 7

Levels of organisation in a skeletal muscle?

Answer 7

The muscle is the whole organ that consists of muscle fibres/cells

Each cell contains specialised intracellular organelles called myofibrils,, which are, in turn, made up of alternating thick and thin filaments

Actin and myocin filaments, within each myofibril, are arranged into sarcomeres (functional units of the muscle)

Question 8

Compare the appearance of skeletal and cardiac muscle?

Answer 8

Both are striated

Question 9

Compare the initiation & propagation of contraction in skeletal and cardiac muscle?

Answer 9

Skeletal muscle:

• Neurogenic initiation of contraction
• Motor units
• Neuromuscular junctions
• NO gap junctions

Cardiac muscle:

• Myogenic (pacemaker potential) initiation of contraction
• No neuromuscular junction
• Gap junctions present

Question 10

Compare excitation contraction coupling in skeletal and cardiac muscle?

Answer 10

In skeletal muscle, Ca2+ is released entirely from the sarcoplasmic reticulum

In cardiac muscle, Ca2+ from ECF and sarcoplasmic reticulum (Ca2+ induced Ca2+ release)

Question 11

Compare graduation of contraction in skeletal and cardiac muscle?

Answer 11

Skeletal muscle:

1. Motor unit recruitment
2. Summation of contractions

Cardiac muscle: Frank-Starling mechanism

Question 12

How can an action potential transmitted in an α-motor neuron cause muscle contraction?

Answer 12

Excitation contraction coupling is the process whereby the surface action potential results in activation of the contractile mechanism of the muscle fibre

Question 13

Describe Ca2+ release in skeletal muscles

Answer 13

Ca2+ is released from the lateral sacs of the sarcoplasmic reticulum when the surface a.p spreads down the transverse T-tubules (invaginations of the membrane)

Question 14

In skeletal muscles, what is the neurotransmitter at the neuromuscular junction?

Answer 14

Acetycholine

Question 15

Label the image

Answer 15

Question 16

What is the functional unit?

Answer 16

Smallest component of an organ that is capable of performing all the function of that organ, e.g: nephrons in the kidneys and sarcomeres in skeletal muscles

Question 17

Where are sarcomeres found?

Answer 17

Between two Z-lines (connect the thin filaments of 2 adjoining sarcomeres)

Question 18

Four zones of sarcomeres?

Answer 18

A-band: thick filaments along with portions of thin filaments that overlap in both ends of thick filaments

H-zone: lighter area within middle of A-band where thin filaments do not reach

M-line: extends vertically down middle of A-band within the centre of H-zone

I-band: consists of remaining portion of thin filaments that do not project in A-band

Question 19

Describe the sliding filament theory

Answer 19

Muscle tension is produced by sliding of actin filaments over myocin filaments, muscle shortens and produces force

Question 20

What does force generation depend upon?

Answer 20

ATP-dependent interactions between thick and thin filaments power cross-bridge formation; ATP is required for both contraction and relaxation Ca2+ is required to switch on cross-bridge formation

Question 21

Describe how cross-bridge formation occurs

Answer 21

ATP binds to the myocin head; it is broken down into ADP + Pi, by an enzyme (ATP Mg2+)

When Ca2+ is present, it moves the troponin-tropomyosin complex from the actin binding site

Question 22

What is rigor complex?

Answer 22

No ATP present in death, so relaxation of muscles cannot occur

Question 23

Why is ATP required during relaxation of smooth muscles?

Answer 23

To release cross bridges

To pump Ca2+ back into the sarcoplasmic reticulum

Question 24

Steps in nervous stimulation of skeletal muscle contraction?

Answer 24

1. ACh released into NMJ and binds to motor end plate
2. a.p is stimulated and is propagated across the surface membrane and down T-tubules
3. a.p in T-tubule triggers Ca2+ release from the sarcoplasmic reticulum
4. Ca2+ ions released from the lateral sacs bind to troponin on actin binding sites; troponin-tropomyosin complex is physically moved
5. Myocin cross-bridges attach to actin and bend, pulling actin to the centre of the sarcomere (energy is provided by ATP)
6. Ca2+ is actively taken up by the SR when there is no longer an a.p (ATP required)
7. Ca2+ no longer bound to troponin, so tropomyosin covers actin binding site again; this ends contraction and actin slides passively back to its original resting position