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OBJECTIVES

Objectives

After studying this chapter, the student should be able to:

  • Define neurotransmission types: synaptic and volume transmission.

  • Distinguish synaptic transmission at electrical and chemical synapses.

  • Identify the morphologic features of chemical synapses.

  • Categorize the different types of neurotransmitters (NTs).

  • Diagram the mechanisms of NT synthesis, storage. and release.

  • Describe the types of NT receptors and their functions.

  • Compare and contrast fast/direct and slow/neuromodulatory synaptic transmission.

  • Diagram the mechanisms for NT removal from the synapse.

  • Illustrate how synaptic transmission responses are integrated.

  • Explain how volume transmission can influence neuronal function.

  • Identify psychoactive drug targets in neurotransmission.

OVERVIEW OF NEUROTRANSMISSION & SYNAPTIC TRANSMISSION

Neurotransmission includes the processes by which neural cells communicate with other neural or target cells. The term neurotransmission is often used synonymously with synaptic transmission, where a neuron communicates with its target cell at a specialized junction called the synapse. Classical synaptic transmission is also called wiring or point-to-point or wired transmission, and involves neurotransmitter (NT) release by a presynaptic neuron and generation of a rapid response in the target cell. However, neurotransmission and synaptic transmission are much more complex than originally envisioned. Neurotransmission includes both synaptic/wiring and volume transmission. In volume transmission, signaling molecules, called neuromodulators, released by neuronal, glial, or endothelial cells into the extracellular fluid (ECF) or cerebrospinal fluid (CSF), undergo short- or long-distance diffusion and activate receptors found on many regions of neural cells. Synaptic/wiring transmission includes both fast/direct electrical responses and also slow/indirect responses called neuromodulation that produce short-term changes in membrane potential and long-term changes in metabolism, excitability, and gene expression, as well as feedback to the presynaptic axon.

Synaptic transmission mediates many different effects, from the generation of electrical responses in target neurons, to activation of muscle contraction, to secretion of hormones from glands. Investigated for over a century, synaptic transmission is a major focus of basic and clinical neuroscience research today. Errors in synaptic transmission have been implicated in many nervous system disorders. In addition, dynamic and long-term changes in synaptic transmission, called synaptic plasticity, have been proposed to underlie learning and memory. This chapter will concentrate primarily on synaptic transmission but also highlights several current concepts in volume transmission and how synaptic activities during neurotransmission can be integrated.

As described in the previous chapters, the output signal from most neurons is the action potential (AP), an electrical signal that is conducted along the axon to the presynaptic terminus or bouton. In some special sensory neurons that lack axons, the output signal is a graded potential. All synapses possess a gap, between 20 and 40 nm in diameter, called the synaptic cleft. Two different mechanisms are used by neurons to transmit the presynaptic signal to the postsynaptic target. At electrical synapses, the presynaptic and postsynaptic membranes are connected by gap junction channels, where the presynaptic current flows directly to the postsynaptic neuron. At chemical synapses, depolarization of the ...

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