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CHAPTER

6

Synaptic & Junctional

Transmission

OBJECTIVES

After studying this chapter, you should be able to:

■

Describe the main morphologic features of synapses.

■

Distinguish between chemical and electrical transmission at synapses.

■

Define convergence and divergence in neural networks, and discuss their implications.

■

Describe fast and slow excitatory and inhibitory postsynaptic potentials, outline the

ionic fluxes that underlie them, and explain how the potentials interact to generate

action potentials.

■

Define and give examples of direct inhibition, indirect inhibition, presynaptic inhi-

bition, and postsynaptic inhibition.

■

Describe the neuromuscular junction, and explain how action potentials in the

motor neuron at the junction lead to contraction of the skeletal muscle.

■

Define and explain denervation hypersensitivity.

INTRODUCTION

The all-or-none type of conduction seen in axons and skeletal

muscle has been discussed in Chapters 4 and 5. Impulses are

transmitted from one nerve cell to another cell at
synapses
(Fig-

ure 6–1). These are the junctions where the axon or some other

portion of one cell (the
presynaptic cell
) terminates on the den-

drites, soma, or axon of another neuron (Figure 6–2) or, in some

cases, a muscle or gland cell (the
postsynaptic cell
). Cell-to-cell

communication occurs across either a
chemical
or
electrical syn-

apse.
At chemical synapses, a
synaptic cleft
separates the terminal

of the presynaptic cell from the postsynaptic cell. An impulse in

the presynaptic axon causes secretion of a chemical that diffuses

across the synaptic cleft and binds to receptors on the surface of

the postsynaptic cell. This triggers events that open or close chan-

nels in the membrane of the postsynaptic cell. In electrical syn-

apses, the membranes of the presynaptic and postsynaptic

neurons come close together, and gap junctions form between the

cells (see Chapter 2). Like the intercellular junctions in other tis-

sues, these junctions form low-resistance bridges through which

ions can pass with relative ease. There are also a few conjoint syn-

apses in which transmission is both electrical and chemical.

Regardless of the type of synapse, transmission is not a sim-

ple jumping of an action potential from the presynaptic to the

postsynaptic cell. The effects of discharge at individual synaptic

endings can be excitatory or inhibitory, and when the postsyn-

aptic cell is a neuron, the summation of all the excitatory and

inhibitory effects determines whether an action potential is

generated. Thus, synaptic transmission is a complex process

that permits the grading and adjustment of neural activity nec-

essary for normal function. Because most synaptic transmis-

sion is chemical, consideration in this chapter is limited to

chemical transmission unless otherwise specified.

Transmission from nerve to muscle resembles chemical syn-

aptic transmission from one neuron to another. The

neuromus-

cular junction,

the specialized area where a motor nerve

terminates on a skeletal muscle fiber, is the site of a stereotyped

transmission process. The contacts between autonomic neurons

and smooth and cardiac muscle are less specialized, and trans-

mission in these locations is a more diffuse process. These forms

of transmission are also considered in this chapter.