Anatomy & Physiology

Anatomy & Physiology Study Guide

13.12 Postsynaptic Potentials Postsynaptic potentials are graded potentials that originate in the postsynaptic membrane in response to a neurotransmitter. Two main types of postsynaptic potentials originate at neuron-to- neuron synapses: excitatory postsynaptic potentials and inhibitory postsynaptic potentials. An excitatory postsynaptic potential , or EPSP , is a graded depolarization produced by the arrival of a neurotransmitter at the postsynaptic membrane. An EPSP results from the opening of chemically gated membrane channels that lead to depolarization of the plasma membrane. Because it is a graded potential, an EPSP affects only the area immediately surrounding the synapse. An inhibitory postsynaptic potential , or IPSP , is a graded hyperpolarization of the postsynaptic membrane. While the hyperpolarization continues, the neuron is said to be inhibited because a larger-than-usual depolarizing stimulus must be present to bring the membrane potential to the maximum limit. Summation A single EPSP will not result in an action potential. Individual EPSPs combine through the process of summation, which connects the effects of all the graded potentials that incfluence one portion of the plasma membrane. The graded potentials may be EPSPs, IPSPs, or both. Two forms of summation exist: temporal summation and spatial summation. Temporal summation is the addition of stimuli occurring in rapid succession at a single synapse that is active repeatedly. A typical EPSP lasts about 20 msec, but under maximum stimulation an action potential can reach the synaptic knob each millisecond. A second EPSP arrives before the effects of the first EPSP have disappeared. Every time an action potential arrives, a group of vesicles discharges ACh into the synaptic cleft; every time more ACh molecules arrive at the postsynaptic membrane, further chemically gated channels open, and the degree of depolarization increases. In this way, a series of small movements can eventually bring the initial segment to the maximum limit. Spatial summation occurs when simultaneous stimuli created at different locations have a cumulative effect on the transmembrane potential. In other words, spatial summation involves multiple synapses that are active simultaneously. 13.13 Chapter Thirteen Review An introduction to neural tissue: • The nervous system involves all the neural tissue in the body. The basic functional unit is the neuron. The nervous system has anatomical and functional divisions: • The anatomical divisions of the nervous system are the central nervous system (the brain and spinal cord) and the peripheral nervous system (all the neural tissue outside the CNS). Bundles of axons (nerve fibers) in the PNS are called nerves.

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