Skeletal Muscle Physiology

Question 1

What are the overall physiological functions of skeletal muscle?

Answer 1

1. Maintenance of posture
2. Purposeful movement in relation to external environment
3. Respiratory movements
4. Heat production
5. Contribution to whole body metabolism

Question 2

What are the three types of muscles in human body?

Answer 2

1. Skeletal Muscle
2. Smooth Muscle
3. Cardiac Muscle

Question 3

What are the common things between all muscles?

Answer 3

they are capable of developing tension and producing movement through contraction

Question 4

Which muscles are striated muscle? How are they different?

Answer 4

Question 5

What are the dark bands and light bands in striated muscle under microscope?

Answer 5

Dark bands (thick filaments) are myocin, light bands (thin filaments) are actin

Question 6

How is skeletal muscle contraction initiated?

Answer 6

Neurogenic - neuromuscular junction (ACh)

Question 7

How are skeletal muscle fibres organised?

Answer 7

Into motor units

Question 8

What does ‘motor unit’ mean?

Answer 8

a single alpha motor neurone and ALL the skeletal muscle fibres it innervates

Question 9

What does the number of muscle fibres per motor unit depend on?

Answer 9

It depends on the function served by the muscle:
- muscles for finer movements –> fewer fibres per unit (precision) eg.intrinsic hand muscles, extra-ocular muscles
- muscles for power –> more fibres per unit eg.thigh muscles

Question 10

Describe the overall structure and the levels of organisation of skeletal muscle

Answer 10

Whole muscle (whole organ) - attached to skeleton by tendons –> form lever system allowing movement
Muscle fibre (one cell) - extend the entire length of muscle
Myofibril (specialised intracellular structure)
Sarcomere (functional unit of organ)

Question 11

What is the functional unit of skeletal muscle? What does it mean by ‘functional unit’?

Answer 11

Sarcomere is the functional unit
*Functional unit = smallest unit of that organ capable of producing function of the organ

Question 12

What is one sarcomere consist of?

Answer 12

A-band:
Made up of 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 don’t 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 13

What is the molecular-basis of skeletal muscle contraction?

Answer 13

Excitation-contraction coupling = the process whereby the surface action potential results in activation of the contractile structures of the muscle fibre

Calcium is released from lateral sacs of SR (and entirely from SR in skeletal muscle) when action potential spreads down T tubule

After contraction, Ca++ actively taken up by SR when there is no longer local action potential

Question 14

What are the roles of Ca ++ and ATP?

Answer 14

Calcium is required for switching on cross bridge formation between actin and myosin (only required for contraction) - Ca++ binds to troponin-tropomyosin complex, so binding site for myosin on actin is exposed

Ca++ is link between excitation and contraction

ATP allows interaction between actin and myosin, it is required for BOTH contraction and relaxation - breaking of cross-bridge and active uptake of Ca++ into SR are both active processes
- The lack of ATP after death is cause for rigor mortis (corpse being tense)

Question 15

What is the tension developed by a skeletal muscle being influenced by?

Answer 15

1. The number of muscle fibres contracting
   - Motor unit recruitment: stimulation of more motor units –> stronger contraction
   - Asynchronous motor units recruitment during submaximal contractions helps prevent muscle fatigue
2. The tension developed by each contracting muscle fibre
   - ‘frequency of stimulation’ & ‘summation of contractions’
   - length of muscle fibre at onset of contraction
   - thickness of fibre

Question 16

How does ‘frequency of stimulation’ & ‘summation of contractions’ bring about increased tension in skeletal muscle?

Answer 16

In skeletal muscle, the duration of action potential is much shorter than the duration of resulting twitch. It is therefore possible to summate twitches
to bring about a stronger contraction through
repetitive fast stimulation of skeletal muscle

Question 17

What is a tetanus?

Answer 17

A maximal sustained contraction resulting from very rapid stimulations giving muscle no chance to relax

Tetanus is not achievable in cardiac muscle, since the action potential lasts almost as long as twitch (long refractory period)

Question 18

When does maximum tetanic muscle tension develop?

Answer 18

When muscle is at its optimum muscle length (= around resting muscle length) before onset of contraction

Question 19

What are the two primary types of skeletal muscle contraction?

Answer 19

1. Isotonic contraction = tension remains constant while length changes
   a. body movements
   b. for moving objects
2. Isometric contraction = tension develops at constant muscle length
   a. supporting objects in fixed positions
   b. maintaining body posture

Question 20

Are skeletal muscle fibres all the same? What are the differences?

Answer 20

No.

Main difference = enzymatic pathways for ATP synthesis & resistance to fatigue

Greater capacity to synthesis ATP = More resistant to fatigue

Activity of myosin ATPase –> determines speed of energy availability –> speed of contraction

Each motor unit usually contains one type of muscle fibres

Question 21

What are the metabolic pathways for ATP supply?

Answer 21

Transfer of high energy phosphate from creatine Phosphate to ADP - immediate source for ATP

Oxidative phosphorylation: main source when O2 is present

Glycolysis: main source when O2 is not present

Question 22

What is the functional relevance of muscle fibre types and metabolism? What are the types?

Answer 22

Slow oxidative type I fibres (also known as slow-twitch fibres) are used mainly for prolonged relatively low work aerobic activities e.g. maintenance of posture, walking

Fast oxidative (Type IIa) fibres (also known as intermediate-twitch fibres) use both aerobic and anaerobic metabolism and are useful in prolonged relatively moderate work activities e.g. jogging

Fast glycolytic (Type IIx) fibres (also known as fast-twitch fibers) use anaerobic metabolism and are mainly used for short-term high intensity activities e.g. jumping

Question 23

What is the activity of motor unit being influenced by?

Answer 23

Motor nerves receive inputs from brain (UMNs) and various receptors (eg. withdrawal reflex, stretch reflex); inputs can be excitatory/inhibitory

Question 24

What does ‘reflex action’ mean? What is the role of reflexes in coordinated movement?

Answer 24

reflex action = a stereotyped response to a specific stimulus

it is the simplest form of coordinated movement; important for localising lesions in the motor system

Question 25

What is the stretch reflex arc and what is its value?

Answer 25

Stretch reflex = a negative feedback that resists passive change in muscle length to maintain optimal resting length of muscle (simplest monosynaptic reflex)

It helps to maintain posture

Question 26

What are some examples of tendon reflexes?

Answer 26

Question 27

What is the role of gamma-motor neurones?

Answer 27

Muscle spindles have their own efferent (motor) nerve supply

The efferent neurons that supply muscle spindles are called gamma (γ) motor neurons

The γ−motor neurons adjust the level of tension in the muscle spindles to maintain their sensitivity when the muscle shorten during muscle contraction

The contraction of intrafusal fibres does not contribute to the overall strength of muscle contraction

Question 28

What might impairment of skeletal muscle function be caused by?

Answer 28

(1) intrinsic muscle disease
(2) neuromuscular junction disease
(3) pathology of lower motor neurons
(4) disruption of input to motor nerves (eg. UMNs)
(going from bottom to top)

Question 29

What are some causes and symptoms of intrinsic muscle disease?

Answer 29

1. Genetically determined myopathies
   Congenital myopathies: characteristic microscopic changes leading to reduced contractile ability of muscles

Chronic Degeneration of contractile elements - muscular dystrophy

Abnormalities in muscle membrane ion channels e.g. myotonia

2. Acquired myopathies
   Inflammatory myopathies e.g. polymyositis, inclusion body myositis
   Non-Inflammatory myopathies e.g. fibromyalgia

Endocrine mypoathies e.g. Cushing syndrome, thyroid disease

Toxic myopathies e.g. alcohol, statins

Question 30

Symptoms of muscle disease?

Answer 30

Muscle weakness / tiredness

Delayed relaxation after voluntary contraction (myotonia)

Muscle pain (myalgia)

Muscle stiffness

Question 31

What are some investigations used in the diagnosis of neuromuscular disease?

Answer 31

1. Electromyography (EMG)
   Electrodes detect the presence of muscular activity
   Records frequency and amplitude of muscle fibres action potentials
   EMG findings not pathognomonic of specific disease - will not provide the definitive diagnosis
   EMG helps differentiate primary muscle disease from muscle weakness caused by neurological disease
   Nerve conduction studies usually done at the same time as an EMG
2. Nerve conduction studies
   Determine the functional integrity of peripheral nerves
3. Muscle enzymes
   e.g. Creatine kinase (CK)
4. Inflammatory markers
   e.g. C-reactive protein (CRP), plasma viscosity (PV)
5. Muscle biopsy