Nerve impulses also known as action potentials, travel from the brain or spinal cord to trigger the contraction of skeletal muscles. An action potential propagates down a motor neuron to a skeletal muscle fiber. The site where a motor neuron excites a skeletal muscle fiber is called a neuromuscular junction. This junction is a chemical synapse consisting of the points of contact, between the axon terminals of a motor neuron and the motor end plate of a on sale wedding options With Lace Appliques ... skeletal muscle fiber. The events at the neuromuscular junction occur in 7 coordinated steps.
1. An action potential travels the length of the axon of a motor neuron to an axon terminal.
2. Voltage gated calcium channels open and calcium ions diffuse into the terminal.
3. Calcium entry causes synaptic vesicles to release Acetylcholine via exocytosis
4. Acetylcholine diffuses across a synaptic cleft and binds to acetylcholine receptors which contain ligand-gated cation channels.
5. These ligand-gated cation channels open.
6. Sodium ions enter the muscle fiber, and potassium ions exit muscle fibers. The greater inward flux of sodium ions relative to the outward flux of potassium ions causes the membrane potential to become less negative.
7. Once the membrane potential reaches a threshold value, an action potential propagates along the sarcolemma. Neurotransmission to a muscle fiber ceases when Acetylcholine is removed from the synaptic cleft, this removal occurs in two ways:
1. Acetylcholine is diffuses away from the synapse.
2. Acetylcholine is broken down by the enzyme -acetylcholinesterase- to acetic acid and choline. Choline is then transported to the axon terminal for the resynthesis of acetylcholine.