Snapshot A 36-year-old male presents with ascending muscle weakness. The patient is generally healthy, with no pertinent past medical history. He states he recently recovered from diarrhea that has lasted over a week. Neurological examination is notable for weakness in the lower extremities and an abscence of patellar reflexes. (Botulinum toxin) Introduction Action Potential Overview initiation neuronal membrane potential becomes more positive → opening of voltage gated Na+ channels login to view 2 more bullets propagation action potential generated at one portion of neuron → excitation of adjacent neuronal membrane login to view 1 more bullet neuromuscular junction when the propagated action potential reaches the presynaptic neuron, it becomes depolarized login to view 2 more bullets released ACh act on nicotinic receptors (a ligand-gated ion channel) on the motor end plate login to view 1 more bullet Action Potential Initiation Initiation initial depolarization → opening of voltage gated Na+ channels increased Na+ inflow → opening of more voltage gated Na+ channels (positive feedback loop) login to view 1 more bullet clinical correlate login to view 3 more bullets Action Potential Propagation Propagation depolarized region of the neuronal membrane → excitation of adjacent neuronal membrane myelinated neurons Schwann cells myelinates axons of the peripheral nervous system (PNS) login to view 10 more bullets myelinated neuron examples login to view 2 more bullets non-myelinated neurons transmit potentials via Na+ channels that exist along the entire length of the axon (no Nodes of Ranvier) login to view 1 more bullet Neuromuscular Junction Neuromuscular junction propagated action potential reaches presynaptic neuron → opens voltage gated Ca2+ channels clinical correlate login to view 3 more bullets Ca2+ influx → ACh release with the help of SNARE proteins clinical correlate login to view 4 more bullets released ACh act on nicotinic receptors (ligand gated ion channels) of the motor end plate clinical correlate login to view 2 more bullets depolarization travels along the muscle cell via T-tubules, which are invaginations that allow for the skeletal muscle to be depolarized dihydropyridine receptors are then depolarized mechanically coupled to ryanodine receptors (Ca2+ release channels) on the sarcoplasmic reticulum login to view 6 more bullets