Synaptic transmission, the process by which electrical signals are transferred between cells in the nervous system, occurs at specialized junctions called synapses. There are two main types of synapses: electrical and chemical. Originally, synaptic transmission was thought to only involve neurons, but it is now understood that glia also play a role in the process. In addition, neurotransmitters released at synapses can have effects beyond the immediate area. This has led to a broader definition of synaptic transmission and the recognition of other mechanisms by which cells in the nervous system communicate with each other. In this chapter, we will first discuss the traditional view of synaptic transmission and then explore the impact of nontraditional neurotransmitters on our understanding of communication in the nervous system.

The generation and conduction of action potentials involve rapid transmission from one node of Ranvier to the next, known as saltatory conduction. Receptors respond selectively to specific forms of stimulus energy within their receptive fields. Receptor potentials result from the transduction of sensory stimuli and can alter the firing patterns of afferent neurons if they surpass threshold levels. Electrical synapses have specific characteristics, while chemical synapses involve various specializations in the presynaptic and postsynaptic elements. The sequence of events connecting the arrival of an action potential at the presynaptic terminal to the entry of calcium, as well as the release of neurotransmitter, is important in synaptic transmission. The quantal hypothesis of synaptic transmission is supported by the presence of miniature end plate potentials. EPSPs and IPSPs differ in terms of their underlying ionic conductances, effect on membrane potential, and neuronal firing probability. Despite having a reversal potential equal to or more positive than a neuron's resting potential, IPSPs can still inhibit neuronal activity. Synaptic effects can change over time due to various mechanisms. Determining whether a substance is a neurotransmitter involves specific criteria, and there are major excitatory and inhibitory neurotransmitters. Different classes of neurotransmitter receptors exist.

References:
1. Hille B. Ion Channels of Excitable Membranes. 3rd ed. Sunderland, MA: Sinauer Associates; 2001.
2. Hodgkin AL, Huxley AF. A quantitative description of membrane current and its application to conduction and excitation in nerve. J Physiol. 1952;117:500-544.
3. Johnston D, Wu SM-S. Foundations of Cellular Neurophysiology. Cambridge, MA: MIT Press; 1994.
4. Sakmann B, Neher E. Single-Channel Recording. 2nd ed. Philadelphia: Springer; 1995.
Hille B. Ion Channels of Excitable Membranes. 3rd ed. Sunderland, MA: Sinauer Associates; 2001.
Hodgkin AL, Huxley AF. A quantitative description of membrane current and its application to conduction and excitation in nerve. J Physiol. 1952;117:500-544.
CHAPTER 5 Generation and Conduction of Action Potentials be conducted very rapidly from one node of Ranvier to the next. This makes the action potential appear to jump from node to node in a form of conduction called saltatory conduction.
9.
A receptor responds preferentially to a particular form of stimulus energy. Its receptive field is that part of a sensory domain in which energy can affect the receptor.
10.
Receptor potentials are the result of transduction of sensory stimuli. These potentials reflect the specific parameters of the stimulus and, if they exceed threshold, alter the action potential firing patterns of the afferent neurons.
Johnston D, Wu SM-S. Foundations of Cellular Neurophysiology. Cambridge, MA: MIT Press; 1994.
Sakmann B, Neher E. Single-Channel Recording. 2nd ed. Philadelphia: Springer; 1995.
Upon completion of this chapter the student should be able to answer the following questions: 1.
What are the characteristics of electrical synapses?
2.
What are the specializations found in the presynaptic and postsynaptic elements of a chemical synapse?
3.
What sequence of events connect the arrival of the action potential at the presynaptic terminal to the entry of calcium?
4.
What sequence of events connect the entry of calcium at the presynaptic terminal to release of neurotransmitter?
5.
What is the quantal hypothesis of synaptic transmission, and how does the presence of miniature end plate potentials support this hypothesis?
6.
Why is the reversal potential of a typical EPSP near 0 mV?
7.
What distinguishes EPSPs and IPSPs in terms of underlying ionic conductances, effect on membrane potential, and neuronal firing probability?
8.
How does an IPSP still inhibit a neuron when its reversal potential is equal to or more positive than the neuron’s resting potential?
9.
What are the mechanisms by which synaptic effects can change over time?
10.
What are the criteria for determining a substance is a neurotransmitter, and what are the major excitatory and inhibitory neurotransmitters?
11.
What are the major classes of neurotransmitter receptors?
ynaptic transmission is the major process by which electrical signals are transferred between cells within the nervous system (or between neurons and muscle cells or sensory receptors). Within the nervous system, synaptic transmission is usually conceived of as an interaction between two neurons that occurs in a point-to-point manner at specialized junctions called synapses. Two main classes of synapses are distinguished: electrical and chemical. However, as the list of chemical neurotransmitters has grown and as understanding of their mechanisms of action has increased, the definition and conception of what constitutes synaptic transmission has had to be refined and expanded. We no longer think of synaptic transmission as a process that involves only neurons, but now realize that glia form an important element of the synapse and that signaling occurs between neurons and glia. Moreover, in some cases, neurotransmitter released at a synapse will act over a widespread territory rather than just at the synapse from which it is released. Thus, we must either generalize the definition of synaptic transmission or consider classically defined synaptic transmission as but one of several mechanisms by which cells in the nervous system communicate with each other. In this chapter we first describe the classic conception of synaptic transmission (electrical and chemical) and then introduce some of the nontraditional neurotransmitters and discuss how they have forced modifications in our conception of chemical communication between cells in the nervous system.