Muscle tissue 2

Muscle Tissue 2

Nerve impulse (action potential)Motor units

Neuromuscular junctionsPhysiology of Muscle contraction (sliding filament theory)

Nerve impulseNecessary for muscle contractionAlso known as action potential

Momentary change in electrical potential due to rapid changes in ion concentration

Motor neuronNeuron with cell body located in the brain or spinal cordEnds at neuromuscular junction / synaptic end bulb

Motor unitThe functional unit of a skeletal muscle Consists of a motor neuron and the muscle fibers it stimulatesmuscles 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

Resting potentialIon pumps actively maintain concentration gradients[Na + ] much higher outside cell[K + ] much higher inside cell3 Na + transported out for every 2 K + transported inResults in net – 70 mV charge inside nerve cell; cell is polarized

Action potentialNerve impulseNa + channels open, Na + rush in to nerve axonInternal cell environment no longer polarizedK + channels open; K + rush out of nerveRepolarizes cellProcess travels down length of cell axon

Neuromuscular junctionMeeting point at which motor neuron meets muscle fiber

SynapseRegion of communication between two neurons or a neuron and a target cell

NeurotransmitterChemical that crosses synaptic cleft (gap) allowing for communication between two cells

Acetylcholine (ACh) is the neurotransmitter released from synaptic end bulb of motor neurons

Nerve impulse initiates contractionNerve 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

Key players of muscle contractionMyosin

~300 molecules comprise each thick filamentImagine two golf club twisted together

ActinThin filament molecules extending from anchoring points in the Z disc

TitinExtends from Z disc to M lineOne of largest molecules known to exist; molar mass = 3million grams

Key players of muscle contractionTropomyosin

Part of thin filament; blocks sites where actin / myosin bind

TroponinHolds 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.

MyomesinForms M line of sarcomere

Sarcomere regionsZ line (disc)

Lateral terminus of sarcomere; separate adjacent sarcomeres

A bandRegion where thick filaments present; darker appearancesome overlap of thin filament in relaxed sarcomeremore overlap of thin filament in contracted sarcomere

Sarcomere regionsI band

Region where thin filaments (no thick filaments) are presentLighter in appearanceNarrowed in contracted sarcomere

H zoneCenter of A band, thick filaments onlyDisappears during sarcomere contraction

M lineCentral to sarcomere, myomesin proteins anchor myosin and titin proteins