The Neuromuscular Junction

What triggers a contraction of the muscle cell?

calcium ions

synaptic vesicles

membranous organelles containing neurotransmitter substances; found within the axon terminals of neurons

Acetylcholine Binds to Receptor Sites

Acetylcholine binds to receptor sites of chemically regulated ion channels on the motor end plate. This causes the channels to open, permitting an influx of Na ions and a small efflux of K ions. This ion exchange causes a local depolarization of the motor end plate.

Breakdown of Acetylchlone

Acetylcholine diffuses away from its receptor site and the ion channel closes. Acetylcholine is then broken down by the enzyme acetylcholinesterase

Neuromusular Junction Activity

1. Action potential arrives at the axon terminal; acetylcholine is released 2. Depolarization of motor end plate 3. Action potential propagates along sarcolemma and down T tubules 4. Contraction of sarcomeres

Troponin

A protein of muscle that together with tropomyosin forms a regulatory protein complex controlling the interaction of actin and myosin and that when combined with calcium ions permits muscular contraction

Tropomyosin

Blocks myosin binding sites on actin molecules, preventing cross-bridge formation, which prevents contraction in a muscle.

Fusion of Synaptic Vesicles

The calcium ions cause several synaptic vesicles to fuse with the membrane of the axon terminal

Action Potential Propagation

The depolarization of the motor end plate initiates an action potential which propagates along the sarcolemma in all directions and down T tubules

Release of Acetylcholine

The neurotransmitter acetylcholine contained with the vesicles is liberated by exocytosis into the synaptic cleft. Additionally, calcium ions are pumped out of the axon terminal

Arrival of Action Potential at Axon Terminal

When the action potential arrives at the axon terminal, the voltage change of the membrane opens voltage-regulated calcium channels, allowing calcium ions to enter the axon terminal

polarized

a condition in which a difference in electrical charge exists across a cell membrane

depolarization

a decrease in the negative resting membrane potential; the voltage that exists across a plasma membrane

motor neuron

a single nerve cell that extends from the brain or spinal cord to a muscle or gland; it stimulates multiple muscle cells

receptor site (binding site)

a specific region on a protein to which another molecule becomes attracted and attaches

action potential

an electrical signal consisting of the depolarization and subsequent repolarization of a nerve or muscle cell membranel travels along the membrane and functions as a signal to initiate an activity

acetylcholinesterase

an enzyme located in the synaptic cleft that rapidly degrades the neurotransmitter acetylcholine

The motor neuron and the sarcollema of the muscle cell...

do not actually touch

Skeletal muscle cells contact as a result of...

impulses from motor neurons

voltage-regulated calcium channels

integral proteins in cell membranes that respond to changes in membrane voltage by changing shape, which opens or closes calcium-specific channels in the proteins; when the channels are open, calcium ions move from one side of the membrane to the other

exocytosis

the bulk transport of material out of the cell by first fusing a membrane-bound vesicle with the plasma membrane; when the membrane ruptures, the contents of the vesicle are released into the extracellular space

axon

the elongated process of a nerve cell that carries impulses away from the nerve cell body

motor end plate

the folded portion of the sarcolemma in close contact with the synaptic ending of the axon terminal

acetylcholine

the neurotransmitter released by a motor neuron at the neoromuscular junction one of the principal neurotransmitters of the peripheral nervous system

neuromuscular junction

the place where a motor neuron stimulates a muscle cell

propagate

the process of reproducing and spreading the action potential along the sarcolemma and down the T tubules

What does the action potential cause?

the release of calcium ions from the terminal cisternae into the cytosol

synaptic cleft

the space between the axon terminal and the membrane of the target cell; the space between the axon terminal and the folded region of the muscle cell membrane called the motor end plate

axon terminal

the swollen, distal end of an axon; contains neurotransmitter substance within synaptic vesicles

resting membrane potential

the voltage that exists across a cell's plasma membrane when the cell is at rest; cell interior is relatively more negative that the cell exterior; measured in millivolts