Muscle Physiology

What is the organisation of skeletal muscle?

Muscle → Fascicle → Muscle fibre → Myofibril → Sarcomere

What connective tissues surround skeletal muscle?

Epimysium (muscle), Perimysium (fascicle), Endomysium (fibre)

What is the sarcolemma?

The plasma membrane of a muscle fibre

What is a T-tubule?

An invagination of the sarcolemma that carries action potentials into the fibre

What is the sarcoplasmic reticulum (SR)?

Specialised ER that stores Ca2+

What is a triad?

One T-tubule flanked by two terminal cisternae of the SR

What is a sarcomere?

The basic unit of contraction between two Z lines

What are the bands of a sarcomere?

A band, I band, H zone, Z line, M line

What happens to the I band during contraction?

It shortens

What happens to the H zone during contraction?

It shortens

What happens to the A band during contraction?

It remains the same

What proteins make up thin filaments?

Actin, tropomyosin, troponin

What proteins make up thick filaments?

Myosin

What blocks myosin binding sites on actin?

Tropomyosin

What does Ca2+ bind to?

Troponin

What happens when Ca2+ binds troponin?

Tropomyosin moves, exposing binding sites

What is a motor unit?

One motor neuron and all muscle fibres it innervates

What neurotransmitter is released at the NMJ?

Acetylcholine (ACh)

What enzyme breaks down ACh?

Acetylcholinesterase

What happens when an action potential reaches the NMJ?

Ca2+ channels open and ACh is released

How many ACh molecules are needed to open the receptor?

Two

What ions pass through the ACh receptor channel?

Na+, K+, Ca2+

What is the end plate potential (EPP)?

Local depolarisation caused by Na+ influx

What is the safety factor at the NMJ?

EPP is ~3× larger than needed to trigger an action potential

What causes NMJ fatigue?

Depletion of ACh vesicles during high-frequency stimulation

How does excitation–contraction coupling begin?

Action potential travels along sarcolemma and T-tubules

What receptor is on the T-tubule?

Dihydropyridine (DHP) receptor

What receptor is on the SR?

Ryanodine receptor

What happens when DHP is activated?

It activates the ryanodine receptor

What is released from the SR?

Ca2+

Why is Ca2+ essential for contraction?

It enables actin–myosin binding

What are the steps of the cross-bridge cycle?

Bind → Power stroke → Detach (ATP) → Reset

What happens during the power stroke?

Myosin pulls actin towards the centre

What causes myosin to detach from actin?

ATP binding

What happens when ATP is hydrolysed?

Myosin head is re-cocked

What are the three roles of ATP in muscle?

Detach myosin, re-cock myosin, pump Ca2+ into SR

What happens when ATP is depleted?

Muscle remains contracted (rigor)

What are the three energy sources for muscle?

Creatine phosphate, glycolysis, aerobic respiration

What is creatine phosphate used for?

Rapid ATP regeneration

What type of metabolism is glycolysis?

Anaerobic

What does glycolysis produce?

2 ATP and pyruvate (or lactic acid without oxygen)

What causes muscle fatigue during glycolysis?

Lactic acid and pH decrease

How much ATP does aerobic respiration produce?

~36 ATP per glucose

What is myoglobin?

Oxygen-binding protein in muscle

Why is myoglobin important?

Facilitates oxygen storage and diffusion

What is the Cori cycle?

Lactate → liver → glucose → returned to muscle

What is oxygen debt?

Oxygen required after exercise to restore normal state

What are red muscle fibres (Type I)?

Slow, fatigue-resistant, oxidative, high myoglobin

What are white muscle fibres (Type II)?

Fast, powerful, glycolytic, low myoglobin

What determines muscle force?

Frequency, recruitment, and muscle length

What is a muscle twitch?

Response to a single stimulus

What are the phases of a twitch?

Latent, contraction, relaxation

What is temporal summation?

Increased force when stimuli are close together

What is incomplete tetanus?

Sustained contraction with partial relaxation

What is complete tetanus?

Sustained contraction with no relaxation

What is recruitment?

Activation of more motor units to increase force

Which motor units are recruited first?

Small motor units

Why are small motor units recruited first?

They have lower activation thresholds

What is the length–tension relationship?

Maximum force occurs at optimal sarcomere length

What is treppe (staircase effect)?

Gradual increase in tension with repeated stimulation

What is isotonic contraction?

Muscle changes length while contracting

What is isometric contraction?

Muscle produces tension without changing length

What is a muscle spindle?

Sensory receptor that detects stretch

What does a muscle spindle cause?

Reflex contraction

What is a Golgi tendon organ (GTO)?

Sensory receptor that detects tension

What does the GTO cause?

Relaxation to prevent damage

What is muscle tone?

Continuous low-level contraction due to spindle activity

What is hypertrophy?

Increase in muscle fibre size

Does hyperplasia occur in humans?

No significant increase in fibre number

Which fibres hypertrophy most?

Type II fibres

What happens to muscle with ageing?

Loss of motor units and muscle mass

Why does muscle weaken with age?

Neuron loss and fibre atrophy