Functional Anatomy of Skeletal Muscles and Contraction

Epimysium

Outside layer of the muscle belly

Fascicle

Bundles of muscle fibres (fascicles) inside epimysium

Perimysium

Outside layer of the fascicle

Muscle fibre

Smaller units of muscle fascicle

Myofibril

Smaller units of muscle fibre, contains sarcomeres

Myosin

Thick protein filaments, contains cross bridges that attaches to actin filaments when stimulated by calcium

Actin

Thin protein filaments that provide a binding site for cross bridges

Sarcomere

Smallest unit of muscles where concentric/eccentric contractions occur

H zone

Space between actin filaments, changes in length during contractions

I band

Contains only the actin filaments adjacent to each sarcomere

A band

Centre of the sarcomere, length of the myosin

Z line

Both ends of a sarcomere. Distance between two ends vary depending on contractions

Calcium

When relaxed, tropomyosin blocks these binding sites; When stimulated, calcium binds to troponin, revealing the binding site

Force-velocity

Higher force at a lower velocity; gives more time for motor units to be recruited

Force-length

Relaxed length provides greatest force. Contracted = greatest overlap of actin/myosin, reducing potential to contract. Lengthened - insufficient overlap of actin/myosin, reducing potential to contract

Sensory neuron

Detects stimuli and sends message to the brain

Brain

Analyses and sends messages (action potential) to the spinal cord

Spinal cord

Transmits messages between the brain and the peripheral nervous system (body)

Motor neuron

Receives message (action potential) from spinal cord, delivers it to target muscles

Motor unit

A motor neuron and the fibre it innervates. Coordinates strength of contraction

Dendrites

Detects impulse from sensory receptors, delivers it to cell body

Cell body

Contains nucleus, directs neuron activities, sends msg to axon

Axon

Transmits msg from cell body to muscles

All or none principle

When a motor unit receives stimulation/impulse that exceeds the threshold, all muscle fibres associated will contract at maximum potential. If threshold is not reached, nothing will occur

Size of motor unit

Small motor units are used for precision, low threshold. Large motor units have high threshold, used for force/power

Fibre recruitment

Motor units are recruited depending on intensity. Type I motor units → type IIA → type IIB. Lowest threshold → highest. Low intensity stimulus → high intensity stimulus

Frequency of impulse

Amount of motor units recruited can be increased by increasing stimulus size or the frequency at which they are sent

Type I muscle fibre

Red, Slow contraction speed, Low force production, High resistance to fatigue, Aerobic activity, Small size of motor neuron, Triglyceride/glycogen fuel source

Type IIA muscle fibre

White, Fast contraction speed, High force production, Medium resistance to fatigue, Long term anaerobic activity, Large size of motor neuron, CP/glycogen fuel source

Type IIB muscle fibre

White, Very fast contraction speed, Very high force production, Low resistance to fatigue, Short term anaerobic activity, Very large size of motor neuron, CP/glycogen fuel source