BIO 201: Chapter 3.5 Neuromuscular Junction

neuromuscular junction

specialized structure (synapse) that allows for communication to occur between a cell of the peripheral nervous system (a neuron) and a cell/fiber of skeletal muscle

center, one

the neuromuscular junction is located at the ____ of each muscle fiber and there is only ____ per fiber

motor end plate

the neuromuscular junction is also referred to as this because it involves motor/efferent signals

synaptic cleft

small and narrow space that separates the two tissues of the neuromuscular junction

excitation

process in which the action potential in the nerve fiber leads to the action potential in the muscle fiber

pre-synaptic (neural) side

ends with synaptic knob

synaptic knob

bulbous swelling that contains synaptic vesicles filled with the neurotransmitter acetylcholine

voltage-gated

gate that opens due to depolarization in the membrane

exocytosis

process in which the vesicle in the cytoplasm of a cell fuses with the plasma membrane and releases its contents from the cell

excitation step 1

action potential travels down the motor neuron to the synaptic knob that contains synaptic vesicles with acetylcholine

excitation step 2

presence of an action potential opens the voltage-gate calcium channels at the axon terminal

excitation step 3

calcium ions enter the synaptic knob and causes about 60 synaptic vesicles to undergo exocytosis to the pre-synaptic cell membrane and releasing about 10,000 molecules of acetylcholine into the synaptic cleft

acetylcholine receptors

receptors that acetylcholine molecules bind to that are found in the junctional folds

junctional folds

increase the surface area of the membrane where acetylcholine receptors are concentrated

excitation step 4

released acetylcholine diffuses across the synaptic cleft and binds to acetylcholine receptors found in the junctional folds positioned on the post-synaptic membrane of the skeletal muscle

depolarization

membrane becoming more positive

ligand-gated

gate that opens when a neurotransmitter binds to the receptors

excitation step 5

acetylcholine receptors open as ligand-gated channels to allow sodium to enter the muscle cell while potassium flows out → depolarization

excitation-contraction coupling

events that link action potentials on the plasma membrane of the skeletal muscle to activation of the myofilaments preparing them for contraction

true

true or false: the transfer of an action potential from the neuron to the skeletal muscle fiber, followed by the production of muscular force occurs almost instantaneously

excitation-contraction coupling step 1

begins with the muscle action potential within the muscle fiber

excitation-contraction coupling step 2

muscle action potential travels down the muscle plasma membrane into the transverse tubules

transverse tubules

continuation of the muscle’s plasma membrane that penetrates deep into the fiber

excitation-contraction coupling step 3

muscle action potential signals the terminal cisternae of the sarcoplasmic reticulum to release its calcium stores into the cytoplasm of the skeletal muscle fiber

excitation-contraction coupling step 4

within the sarcomere of the fiber, free calcium binds to troponin C to remove tropomyosin and expose the myosin binding sites on actin

troponin c

protein that helps removes another protein called tropomyosin

contraction

step in which the muscle fiber develops tension and may shorten

sliding filament theory

the mechanism of contraction is called …

contraction step 1

myosin head must have an ATP molecule bound to it to initiate contraction and then myosin ATPase hydrolyzes ATP into ADP and phosphate so that energy released “cocks” the head in an extended high energy position

contraction step 2

cocked myosin head binds to the exposed active site on the actin filament to form a cross-bridge

contraction step 3

myosin releases ADP and phosphate and flexes into a bent, low energy position tugging the actin filament along with it → power stroke

• head remains bound to actin until it binds a new ATP

contraction step 4

binding of the new ATP to myosin destabilizes the myosin-actin bond, breaking the cross bridge and myosin is prepared to repeat the whole process again