21 - Chapter 21 THE SYNAPSE are called afferent neurons and...

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Chapter 21. THE SYNAPSE Neurons communicate with each other via cell - cell structures known as synapses . There may be thousands of such structures on the dendritic tree of a given neuron. In such a situation, the potential exists for thousands of input neurons to have the opportunity to influence the activity of a single output neuron. The former (inputs) are called afferent neurons and the latter (output) is the efferent neuron . At the terminal part of an axon, neurons are nonmyelinated and the axon is enlarged into a synaptic bouton (Figure 21-1). The axolemma is therefore naked at this location and it is intimately associated with an exposed part of the dendrite of the efferent cell. Figure 21-1. The synapse. Synapses provide for loci of communication and integration between neurons. The incoming (afferent neuron) terminates with the bouton (b) arising from a segment of naked (non-myelinated) axon (a) at the end of the myelinated portion (m). Immediately adjacent to the bouton is the receptive post-synaptic membrane (psm) in the dendritic portion of the efferent neuron (e). The two cells are separated by the synaptic cleft (c), an extracellular space only about 0.2 microns in width. An incoming action potential, generated by Na + influx in the axon (triangles represent Na + channels, leads to Ca 2+ taking over as the inward current carrier in the bouton (dots). Stores of neurotransmitter, held in vesicles (n) exist close to the pre-synaptic membrane (p) of the bouton. When local intra-bouton concentrations of Ca 2+ increase (arrival of the AP), the vesicles fuse with the membrane and release their content of neurotransmitter into the cleft. These molecules diffuse across the cleft, then bind to and open Na + channels (ch) in the post-synaptic membrane. The transient influx of Na + that results at the post- synaptic membrane gives rise to excitatory post-synaptic potentials (EPSP) as described in the text. 2007 version – page 159
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A. Electrophysiology and integration at synapses The membrane in the bouton region is slightly different from the remainder of the axolemma in that calcium ions (Ca 2+ ) rather than Na + serve as the inward current carrier for depolarization. Ca 2+ ions are very important in signal transduction (described in Chapter 13), in part because [Ca 2+ ] i is kept at very low levels (~ 10 -7 M ) compared to ~ 10 -3 M concentrations in extracellular fluids; any influx of Ca 2+ can rapidly and profoundly increase intracellular concentrations. At the bouton, Ca 2+ ions enter the cell through voltage-gated Ca 2+ channels and flow down about a 10,000-fold concentration gradient from outside to inside. Within the bouton, vast stores of neurotransmitter compounds are stored, packaged in membrane-bounded secretory vesicles. When an action potential passes along the axon and reaches the bouton region, there is influx of Ca 2+ , and the intracellular concentration of this ion is rapidly increased. When intracellular concentrations of Ca
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This note was uploaded on 03/23/2009 for the course ANSCI 1110 taught by Professor Brucecurrie during the Fall '08 term at Cornell.

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21 - Chapter 21 THE SYNAPSE are called afferent neurons and...

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