Neuromuscular Junction & Excitation

Innervation

Connection of a skeletal muscle fiber to a neuron; all skeletal muscle fibers are innervated.

Motor neuron

Neuron that communicates with and stimulates skeletal muscle fibers.

Neuromuscular junction (NMJ)

Synapse between a motor neuron and a skeletal muscle fiber; consists of the axon terminal, synaptic cleft, and motor end plate.

Axon terminal (synaptic bulb)

Swollen end of a motor neuron's axon that contains synaptic vesicles filled with neurotransmitter.

Synaptic vesicles

Membrane-bound sacs in the axon terminal that contain neurotransmitter.

Acetylcholine (ACh)

Neurotransmitter released by motor neurons to stimulate skeletal muscle fibers.

Synaptic cleft

Narrow space between the axon terminal and muscle fiber into which ACh is released; contains enzymes that break down ACh.

Motor end plate

Specialized folded region of the muscle fiber's sarcolemma containing many receptors for ACh.

Three parts of the neuromuscular junction

Axon terminal → synaptic cleft → motor end plate.

Excitation phase

Phase of skeletal muscle contraction in which ACh released by a motor neuron stimulates the sarcolemma of a muscle fiber, leading to an action potential.

Excitation — Step 1

A neuronal action potential arrives at the axon terminal and opens voltage-gated Ca2+ channels.

Excitation — Step 2

Ca2+ enters the axon terminal by following its electrochemical gradient.

Excitation — Step 3

Ca2+ entry triggers exocytosis of synaptic vesicles, releasing ACh into the synaptic cleft.

Excitation — Step 4

ACh diffuses across the synaptic cleft and binds to ligand-gated cation channels in the motor end plate.

Excitation — Step 5

ACh binding opens the ligand-gated cation channels; more Na+ enters the muscle fiber than K+ exits, producing a net gain of positive charge.

Excitation — Step 6

The entry of cations locally depolarizes the sarcolemma, producing an end-plate potential.

End-plate potential

Local depolarization of the sarcolemma in the area of the motor end plate caused by cation movement through ACh-activated channels.

Excitation phase sequence

Action potential reaches axon terminal → voltage-gated Ca2+ channels open → Ca2+ enters → synaptic vesicles release ACh → ACh binds ligand-gated cation channels → channels open → net positive charge enters → end-plate potential occurs.

Acetylcholinesterase (AChE)

Enzyme in the synaptic cleft that rapidly degrades and inactivates ACh.

Why must a motor neuron keep releasing ACh for continued stimulation?

ACh is broken down almost immediately by acetylcholinesterase, so the neuron must continue firing action potentials and releasing new ACh to generate repeated end-plate potentials.

What is the difference between ACh and AChE?

ACh is the neurotransmitter that stimulates the muscle fiber; AChE is the enzyme that breaks down ACh and ends its effect.

What ion triggers ACh release from the axon terminal?

Ca2+.

What ion movement primarily causes the end-plate potential?

More Na+ enters the muscle fiber than K+ exits, creating a net gain of positive charge and depolarizing the motor end plate.

Three major phases of skeletal muscle contraction

Excitation → excitation-contraction coupling → contraction.

Excitation-contraction coupling phase

Transmits excitation from the sarcolemma to the myofilaments through the release of Ca2+ from the sarcoplasmic reticulum.

Contraction phase

The sliding-filament mechanism occurs and the sarcomere contracts.