endplaadid

Endplaadid are specialized regions of the postsynaptic membrane at the neuromuscular junction of skeletal muscle. They lie opposite the presynaptic motor nerve terminal and form the primary site where neural signals are converted into muscle activity.

Structure and composition: Endplaadid feature a high density of acetylcholine receptors clustered at the tops of deep postsynaptic folds, increasing the surface area available for neurotransmitter interaction. The synaptic cleft contains extracellular matrix and enzymes, notably acetylcholinesterase, which rapidly degrades acetylcholine to terminate the signal. The basal lamina beneath the endplate helps organize the junction and anchors receptors.

Function: On nerve stimulation, acetylcholine is released into the synaptic cleft and binds to acetylcholine receptors on the endplate, opening ion channels. This generates a depolarizing end-plate potential that, if sufficient, triggers an action potential in the muscle fiber and leads to contraction.

Development and plasticity: Agrin released by the motor neuron binds to the MuSK–Lrp4 receptor complex on the muscle, promoting clustering of acetylcholine receptors and maturation of the postsynaptic apparatus. Endplaadid undergo remodeling during development and can adapt with activity or disease, reflecting plasticity at the neuromuscular junction.

Clinical relevance: Disruption of endplaadid underlies several neuromuscular disorders, most notably myasthenia gravis (antibodies against acetylcholine receptors or MuSK) and Lambert-Eaton myasthenic syndrome, as well as various congenital myasthenic syndromes. Toxins that affect acetylcholine release or receptor function also impact endplates.

See also: Neuromuscular junction, acetylcholine receptor, myasthenia gravis.