Physiology
Muscle Physiology
Unlock FRCOphth Part 1 Study Notes to access this content.
Get access- Presynaptic neurotransmitter release
- Occurs via exocytosis
- Controlled by calcium influx
- Quantal
Post-synaptic potentials spread through muscle via the transverse tubular system, an extension of the sarcolemma
- Acetylcholine binds to postsynaptic receptors on the motor end-plate
- This triggers sodium ion influx producing a depolarisation.
An excitatory action potential is produced when a threshold is reached and sodium channels are activated bringing about a massive influx of sodium ions
The density of sodium channels is highest at the initial segment of the axon (the “axon hillock” of motor neurones or the first node of Ranvier for myelinated sensory neurones).
The membrane potential reaches positive values, reaching a threshold triggering an action potential
Repolarisation later occurs due to a decrease in sodium conductance and slow increase in potassium conductance allowing potassium to leave the cell
The tubular system is a continuation of the sarcoplasmic reticulum and is a calcium ion reservoir
Action potential stimulates calcium release from this tubular system by opening DHP receptors
- Calcium binds to troponin C associated with tropomyosin on the actin filament.
- A conformational change occurs to displace tropomyosin
- The displacement of tropomyosin frees the myosin binding sites in the actin grooves
- The myosin can combine with actin to form “cross-bridges”
Myosin magnesium-dependent ATPase generates energy which effects the movement of the myosin heads and muscle contraction
- Calcium accumulates back in the SR via the action of ATPase
- Striated muscle (and cardiac muscle): organised into units called sarcomeres
- The region where two Z lines overlap
- A band: overlapping myosin and actin
- H band: myosin only
- Skeletal muscle does not require an intrinsic pacemaker like cardiac muscle
- Resting membrane potential is about -90mV: potassium contributes most to this
- Muscle spindles
- Intrafusal fibres
- Innervated by type Ia, type II and gamma fibres
- Respond to rate of change in muscle length
- Action potential length
- Cardiac: 200ms
- Smooth: 50ms
- Striated/skeletal: 10ms