BS2014: Skeletal Muscle Physiology

Connective tissue in skeletal muscle

Fascia — connective tissue surrounding skeletal muscle

Epimysium — surrounds entire muscle organ

Perimysium — surrounds fascicles (bundles of fibres)

Endomysium — surrounds individual fibres

Structural organisation of skeletal muscle

skeletal muscle → fascicle → fibre → myofibril → sarcomere

Fascicle

a bundle of muscle fibres

Muscle fibre

an individual muscle cell

Myofibril

rod-like structures that run the length of the muscle fibre

Sarcomere

functional contractile unit of muscle containing myosin (thick) and actin (thin) filaments

Motor unit

alpha motor neuron + muscle fibres it innervates

Neuromuscular junction

point at which the alpha motor neuron and muscle fibres meet

Excitation-contraction coupling

how an action potential leads to muscle contraction

T-tubules

pores in the sarcolemma which allow APs to travel down into the muscle interior to cause contraction

Role of DHPRs in T-tubules

voltage gated channels that sense changes in membrane potential in the t-tubules and change shape to activate RyR1 when an action potential is detected

Role of ryanodine receptors in the SR

release calcium from SR into the sarcoplasm

Triad

3-part junction composed of a t-tubule and 2 terminal cisternae of the SR

Ca2+-dependent regulation of contraction

• at low calcium, tropomyosin covers actin binding sites (at rest)

• once calcium enters the sarcoplasm, it binds to troponin C, changing its shape

• the change in shape pulls on tropomyosin, exposing the actin binding sites

• allowing binding of myosin to actin

• allowing for muscle contraction by cross-bridge cycling

Cross-bridge cycling

• ATP is bound to myosin head

• ATP is hydrolysed to forming ADP + Pi

• Pi is released from myosin, moving its head from the relaxed state to the active state

• allowing the myosin head to bind to the actin binding site

• the ADP molecule is then released causing the myosin head to perform the power stroke, pulling the actin molecule towards the centre of the sarcomere

• shortening the sarcomere and producing a contraction

Cross-bridge cycling regulation

another ATP molecule binds to the myosin head to detach it from actin

Why does rigor mortis occur

in death, new ATP cannot be produced therefore a cross-bridge cannot be released and therefore the muscles (of the body) are chronically contracted

Main 2 regulators of strength of muscle contraction

• AP firing rate

• motor unit recruitment

Incomplete tetanus

a sustained but inconsistent skeletal muscle contraction caused by high frequency of action potentials, where the muscle partially relaxes between APs

Tetanus

a sustained smooth contraction of skeletal muscle caused by high frequency motor unit stimulation preventing relaxation between APs

Types of motor unit

• type IIx — fast twitch, high force, fast fatigue

• type IIa — fast twitch, moderate force, fatigue resistant

• type I — slow twitch, low tension, fatigue resistant

All-or-none principle

if a stimulus exceeds the threshold required to initiate muscle contraction, all muscle fibres within that motor unit will contract, if not, none will contract

Henneman’s size principle theory

motor units are always recruited from slow to fast twitch based on the required force (type I → type IIa → type IIx)

Collateral sprouts

new axon branches generated by healthy, neighbouring motor units to reinnervate and save muscle fibres left behind by dead motor units

Downside of collateral sprouts

• a dead motor unit can be innervated by any motor unit type regardless of size

• leading to changes in contractile and metabolic properties

• leading to inefficient contraction

Muscle glycogen storage location(s)

• sub-sarcolemma (under cell surface membrane)

• intramyofibrillar (between contractile filaments)

• intermyofibrillar (between myofibrils)

Most important muscle glycogen storage location

the intramyofibrillar store

Role of skeletal muscle stem (satellite) cells

upon injury, satellite cells are activated to infiltrate the muscle fibre and initiate muscle repair

Nuclear domain theory

a single nucleus can only manage and support protein synthesis for a limited volume of cytoplasm in a muscle fibre

Muscle memory

the rapid regain of muscle size and strength after inactivity due to new nuclei acquired in previous training permanently retained in muscle fibre