Muscle physiology, mechanism of muscle contraction.

1. Muscle Tissue 2
Nerve impulse (action potential)
Motor units
Neuromuscular junctions
Physiology of Muscle contraction (sliding filament theory)

2. Nerve impulse
Necessary for muscle contraction
Also known as action potential
Momentary change in electrical potential due to rapid
changes in ion concentration

3. Motor neuron
Neuron with cell body located in the brain or spinal cord
Ends at neuromuscular junction / synaptic end bulb
Motor unit
The functional unit of a skeletal muscle
Consists of a motor neuron and the muscle fibers it stimulates
muscles with large ratio motor units (1 neuron: many muscle
fibers) provide powerful contractions but cannot provide delicate
control
muscles with small ratio motor units (1 neuron: few muscle fibers)
provide delicate control for very precise movements

7. Resting potential
Ion pumps actively maintain concentration
gradients
[Na + ] much higher outside cell
[K + ] much higher inside cell
3 Na + transported out for every 2 K + transported in
Results in net – 70 mV charge inside nerve cell; cell
is polarized

8. Action potential
Nerve impulse
Na + channels open, Na + rush in to nerve axon
Internal cell environment no longer polarized
K + channels open; K + rush out of nerve
Repolarizes cell
Process travels down length of cell axon

13. Neuromuscular junction
Meeting point at which motor neuron meets muscle fiber
Synapse
Region of communication between two neurons or a neuron
and a target cell
Neurotransmitter
Chemical that crosses synaptic cleft (gap) allowing for
communication between two cells
Acetylcholine (ACh) is the neurotransmitter released from
synaptic end bulb of motor neurons

16. Nerve impulse initiates contraction
Nerve impulse arrives at NMJ
Acetylcholine (Ach) released via exocytosis across synaptic cleft
ACh stimulates Na + / K+ ion channels to open, muscle cell depolarizes
Depolarization of sarcolemma causes Ca2+ release
Ca2+ allows myosin / actin crossbridge and ultimately, sarcomere
shortening

23. Key players of muscle contraction
Myosin
~300 molecules comprise each thick filament
Imagine two golf club twisted together
Actin
Thin filament molecules extending from anchoring points in the Z
disc
Titin
Extends from Z disc to M line
One of largest molecules known to exist; molar mass = 3million
grams

24. Key players of muscle contraction
Tropomyosin
Part of thin filament; blocks sites where actin / myosin bind
Troponin
Holds tropomyosin in position.
When Ca2+ is present, troponin changes shape (conformational
change)
Linked to tropomyosin, actin/mysoin binding site is uncovered
when Ca2+ is present in sarcoplasm.
Myomesin
Forms M line of sarcomere

30. Sarcomere regions
Z line (disc)
Lateral terminus of sarcomere; separate adjacent
sarcomeres
A band
Region where thick filaments present; darker
appearance
some overlap of thin filament in relaxed sarcomere
more overlap of thin filament in contracted sarcomere

31. Sarcomere regions
I band
Region where thin filaments (no thick filaments) are present
Lighter in appearance
Narrowed in contracted sarcomere
H zone
Center of A band, thick filaments only
Disappears during sarcomere contraction
M line
Central to sarcomere, myomesin proteins anchor myosin and
titin proteins