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3-28-06
Muscular System
 Muscular cells (fibers) rapidly change their shape
 They are contractible, excitable, elastic
 For every joint there are at least two muscles. Reason- Muscles only actively
shorten (they pull but don’t push)
 Tendons attach muscle to bone
Biceps Brachii & Triceps
Antagonistic muscles = two muscles that oppose each others action
Muscle Cell
*mitochondria- synthesize ATP
*myofibril- active part of muscle
*T-tubule- bring action potentials from surface to down deep into the core of the
muscle cell
*Sarcoplasmic reticulum- filled with Ca2+
Myofibrils get activated to shorten (contract) when [Ca2+] in the cytoplasm goes up.
Motor Neuron
*axon terminal- filled with neurotransmitter
*acetycholine receptor
-65mV>>>>-55mV>>>>-50mV
Depolarization
Motor Neuron
*motor end plate- has ACH receptors (nicotinic), no voltage gated channels
*The sarcoplasmic reticulum stores calcium until a nerve action potential will cause the
release of Calcium.
*DHP protein (shorten when depolarized by AP)
Steps to increase Ca2+ in a muscle cell
1.
2.
3.
4.
5.
Motor neuron release acetycholine ACH motor end plate
ACH binds to ACH receptors (nicotinic)
ACH receptor gate opens allows Na+ to enter cell
Na+ influx activates voltage gated Na+ channels letting Na+ into cell
Na+ depolarizes cell, spreading action potential to towards ends, away from
motor end plate, down T-tubules
6. A.P. reaches DHP protein, shortens, pulls open ryanodine channel allowing Ca2+
flow from sarcoplasmic reticulum to cytoplasm
Myofibril
Actin- made of protein subunits (thin filament)
Myosin- made from protein subunits (thick filaments)