Synaptic potentials are graded and can depolarize or hy-perpolarize the receiving cell membrane. For example, if a neurotransmitter binds to a postsynaptic receptor and opens sodium ion channels, the ions diffuse inward, depolarizing the membrane, possibly triggering an action potential. This type of membrane change is called an excitatory postsynaptic potential (EPSP), and it lasts for about 15 milliseconds.
If a different neurotransmitter binds other receptors and increases membrane permeability to potassium ions, these ions diffuse outward, hyperpolarizing the membrane. Since an action potential is now less likely to occur, this change is called an inhibitory postsynaptic potential (IPSP).
Within the brain and spinal cord, each neuron may receive the synaptic knobs of a thousand or more axons on its dendrites and cell body. Furthermore, at any moment, some of the postsynaptic potentials are excitatory on a particular neuron, while others are inhibitory (fig. 10.19).
The integrated sum of the EPSPs and IPSPs determines whether an action potential results. If the net effect is more excitatory than inhibitory, threshold may be reached, and an action potential triggered. Conversely, if the net effect is inhibitory, no impulse is transmitted.
Summation of the excitatory and inhibitory effects of the postsynaptic potentials commonly takes place at the trigger zone, usually in a proximal region of the axon, but found in some dendrites as well (see fig. 10.6). This region has an especially low threshold for triggering an action potential; thus, it serves as a decision-making part of the neuron.
H Describe a synapse.
Explain the function of a neurotransmitter. H Distinguish between EPSP and IPSP. D Describe the net effects of EPSPs and IPSPs.
Shier-Butler-Lewis: III. Integration and 10. Nervous System I: © The McGraw-Hill
Human Anatomy and Coordination Basic Structure and Companies, 2001
Physiology, Ninth Edition Function
Axons of presynaptic neurons
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