General Characteristics and Histological Appearance
In skeletal muscle, neuroeffector junctions are known as neuromuscular junctions. The entire complex consisting of nerve terminals and the underlying muscle region is called the motor end plate. It is crucial to clearly distinguish these effector structures from neuromuscular spindles, which perform an exclusively sensory (receptor) function.
On histological slides, the motor end plate is best visualized using silver impregnation. Against the background of muscle tissue, myelinated nerve fibers stain dark brown. At the contact site, the axon branches extensively, forming terminal arborizations that appear as characteristic convoluted dark structures on the muscle surface.
Synapse Formation and Ultrastructure
The formation of a neuromuscular junction begins when a motor neuron axon approaches a muscle fiber. In the immediate vicinity of its target, the axon loses its myelin sheath and divides into several terminal branches. These branches deeply invaginate the muscle fiber. The muscle cell membrane (sarcolemma) is not breached, but rather caves inward following the contours of the nerve endings.
The plasma membrane of the nerve terminal becomes the presynaptic membrane. The cytoplasm of the terminals contains a vast number of mitochondria, which meet the high energy demands of the synapse. Synaptic vesicles containing the neurotransmitter acetylcholine are also concentrated here. The enzyme choline acetyltransferase operates within the synapse to continuously replenish acetylcholine stores. The mechanism of action for these cholinergic synapses is ionotropic.
Molecular Mechanism of Exocytosis
Neurotransmitter release into the synaptic cleft is a strictly regulated process driven primarily by calcium ions and specialized proteins:
- Activation: Voltage-gated $Ca^{2+}$ channels are embedded in the presynaptic membrane. Upon depolarization, they open; calcium enters the cell and activates protein kinase along with the protein synaptotagmin.
- Mobilization of the Reserve Pool: At rest, vesicles are anchored to the cytoskeleton by the protein synapsin. Upon phosphorylation by protein kinase, synapsin detaches from the vesicles, freeing them for transport.
- Fusion: Synaptotagmin firmly binds the vesicle to the presynaptic membrane. Subsequently, the protein synaptoporin, influenced by synaptotagmin, forms a fusion pore between the vesicle and the membrane, initiating neurotransmitter exocytosis.
Postsynaptic Membrane and Contact Zone
The postsynaptic membrane consists of specialized folds of the sarcolemma that invaginate and surround the axon terminal. Its defining feature is the presence of numerous junctional folds, which dramatically increase the surface area for contact.
Embedded within the postsynaptic membrane are specific nicotinic acetylcholine receptors (nAChRs) coupled to cation channels. When acetylcholine binds to these receptors, the channels open, triggering muscle depolarization. To terminate the signal, the enzyme acetylcholinesterase rapidly degrades the neurotransmitter molecules within the synaptic cleft.
In the underlying sarcoplasm (directly beneath the nerve terminal), a prominent accumulation of mitochondria and muscle cell nuclei is observed. On histological sections of the junction area, the nuclei of glial cells—Schwann cells (neurolemmocytes) accompanying the axon—can also be identified.