The Action Potential, Synaptic Transmission, and Maintenance of Nerve Function

1. Nongated ion channels establish the resting membrane potential of neurons; voltage-gated ion channels are responsible for the action potential and the release of neurotransmitter. 2. Ligand-gated ion channels cause membrane depolarization or hyperpolarization in response to neurotransmitter. 3. Nongated ion channels are distributed throughout the neuronal membrane; voltage-gated channels are largely restricted to the axon and its terminals, while ligand-gated channels predominate on the cell body (soma) and dendritic membrane. 4. Membrane conductance and capacitance affect ion flow in neurons. 5. An action potential is a transient change in membrane potential characterized by a rapid depolarization followed by a repolarization; the depolarization phase is due to a rapid activation of voltage-gated sodium channels and the repolarization phase to an inactivation of the sodium channels and the delayed activation of voltage-gated potassium channels. 6. Initiation of an action potential occurs when an axon hillock is depolarized to a threshold for rapid activation of a large number of voltage-gated sodium channels. 7. Propagation of an action potential depends on local current flow derived from the inward sodium current depolarizing adjacent regions of an axon to threshold. 8. Conduction velocity depends on the size of an axon and the thickness of its myelin sheath, if present. 9. Following an action potential in one region of an axon, that region is temporarily refractory to the generation of another action potential because of the inactivation of the voltage-gated sodium channels. 10. When an action potential invades the nerve terminal, voltage-gated calcium channels open, allowing calcium to enter the terminal and start a cascade of events leading to the release of neurotransmitter. 11. Synaptic transmission involves a relatively small number of neurotransmitters that activate specific receptors on their postsynaptic target cells. 12. Most neurotransmitters are stored in synaptic vesicles and released upon nerve stimulation by a process of calciummediated exocytosis; once released, the neurotransmitter binds to and stimulates its receptors briefly before being rapidly removed from the synapse. 13. Metabolic maintenance of neurons requires specialized functions to match their specialized morphology and complex interconnections. K E Y C O N C E P T S