motor end plate

STRUCTURE: The NMJ has three parts: (1) presynaptic terminal of the alpha motor neuron (contains ACh vesicles, voltage-gated Ca2+ channels, mitochondria); (2) synaptic cleft (contains acetylcholinesterase, AChE, anchored to basal lamina); (3) postsynaptic motor end plate (sarcolemma with junctional folds, which increase surface area, and nAChRs concentrated at fold crests; voltage-gated Na+ channels are concentrated in the depths of the folds).

PHYSIOLOGY (sequence): Action potential reaches the nerve terminal -> opening of voltage-gated Ca2+ channels -> Ca2+ influx -> SNARE-mediated (synaptobrevin, syntaxin, SNAP-25; synaptotagmin as Ca2+ sensor) vesicle fusion and exocytosis of ACh -> ACh diffuses across the cleft and binds the nicotinic (Nm, muscle-type) receptor, a ligand-gated cation channel (pentamer: 2 alpha, beta, delta, epsilon in adults; gamma replaces epsilon in fetal receptor). Both ACh-binding sites (on the alpha subunits) must be occupied to open the channel. -> Na+ influx (and K+ efflux; net depolarization, reversal potential about 0 mV) -> end plate potential (EPP; graded, local, non-propagating). Under normal conditions the EPP always exceeds threshold (large safety factor) -> opens voltage-gated Na+ channels in the adjacent sarcolemma -> muscle action potential -> T-tubule -> DHPR/RyR1 coupling -> Ca2+ release from sarcoplasmic reticulum -> contraction. ACh is degraded by AChE into acetate and choline; choline is taken back up into the presynaptic terminal by a Na+-choline cotransporter to resynthesize ACh via choline acetyltransferase (ChAT). Miniature end plate potentials (MEPPs) are small depolarizations (~0.4 mV) from spontaneous release of single ACh vesicles (quanta); the EPP is the summation of many quanta.

CLINICAL CORRELATIONS: - Myasthenia gravis: autoantibodies against postsynaptic nAChR (type II hypersensitivity) -> receptor internalization/destruction, complement-mediated damage to the end plate, simplification of junctional folds, and a reduced EPP. Fatigable weakness (ptosis, diplopia, dysphagia, proximal weakness) that worsens with use; associated with thymoma/thymic hyperplasia. Dx: anti-AChR Ab, anti-MuSK Ab, edrophonium test, decremental response on repetitive nerve stimulation. Tx: pyridostigmine (AChE inhibitor), immunosuppression, thymectomy; plasmapheresis/IVIG for crisis. - Lambert-Eaton myasthenic syndrome: antibodies against presynaptic P/Q-type voltage-gated Ca2+ channels -> decreased ACh release (presynaptic); associated with small cell lung cancer; weakness improves with use; incremental response on high-frequency stimulation. - Botulism: Clostridium botulinum toxin cleaves SNARE proteins -> blocks ACh release -> flaccid paralysis. Tetanus toxin acts on inhibitory interneurons (spastic paralysis), not at the NMJ. - Organophosphates (e.g., parathion) irreversibly inhibit AChE -> excess ACh -> cholinergic crisis (DUMBBELSS), with depolarization block at the end plate and muscle fasciculations, then paralysis. Tx: atropine plus pralidoxime (reactivates AChE if given before "aging"). - Neuromuscular blocking drugs: Nondepolarizing (rocuronium, vecuronium, cisatracurium, atracurium, pancuronium) are competitive nAChR antagonists, reversed by AChE inhibitors (neostigmine) or sugammadex (encapsulates rocuronium/vecuronium). Depolarizing (succinylcholine) is an nAChR agonist that causes persistent depolarization (Phase I block: fasciculations, then paralysis; not reversed, potentiated by AChE inhibitors); metabolized by plasma pseudocholinesterase (prolonged paralysis with atypical/deficient enzyme). Risks: hyperkalemia (burns, denervation injury, crush, upregulated extrajunctional receptors), malignant hyperthermia (with volatile anesthetics; treat with dantrolene, which blocks RyR1). - Denervation: nAChRs proliferate extrajunctionally over the whole sarcolemma (upregulation), explaining denervation hypersensitivity and succinylcholine-induced hyperkalemia. - Congenital myasthenic syndromes: mutations in nAChR subunits, rapsyn, or AChE-anchoring collagen Q (ColQ). - Curare/alpha-bungarotoxin: bind nAChR (competitive/irreversible blockade). Black widow spider venom (alpha-latrotoxin) causes massive presynaptic ACh release. Hemicholinium blocks choline uptake; vesamicol blocks vesicular ACh uptake.

HIGH-YIELD POINTS: Muscle-type nicotinic receptor (Nm) is a ligand-gated ion channel (ionotropic), different from neuronal ganglionic (Nn) nicotinic receptors (blocked by hexamethonium, trimethaphan). The NMJ is a 1:1 synapse for each muscle fiber (each fiber has one end plate, each motor neuron innervates many fibers = motor unit). Smooth/cardiac muscle lack an end plate. Neurotransmitter: ACh (cholinergic, somatic), with no inhibitory modulation at the NMJ.