Ganong's Review of Medical Physiology, 23rd Edition

116
SECTION II
Physiology of Nerve & Muscle Cells

SYNAPTIC TRANSMISSION:

FUNCTIONAL ANATOMY

TYPES OF SYNAPSES

The anatomic structure of synapses varies considerably in the
different parts of the mammalian nervous system. The ends of
the presynaptic fibers are generally enlarged to form
terminal
boutons (synaptic knobs)
(Figure 6–2). In the cerebral and
cerebellar cortex, endings are commonly located on dendrites
and frequently on
dendritic spines,
which are small knobs
projecting from dendrites (Figure 6–3). In some instances, the
terminal branches of the axon of the presynaptic neuron form
a basket or net around the soma of the postsynaptic cell (basket
cells of the cerebellum and autonomic ganglia). In other loca-
tions, they intertwine with the dendrites of the postsynaptic cell
(climbing fibers of the cerebellum) or end on the dendrites di-
rectly (apical dendrites of cortical pyramidal cells). Some end
on axons of postsynaptic neurons (axoaxonal endings). On av-
erage, each neuron divides to form over 2000 synaptic endings,
and because the human central nervous system (CNS) has 10
11

neurons, it follows that there are about 2
×
10
14
synapses. Ob-

viously, therefore, communication between neurons is ex-

tremely complex. It should be noted as well that synapses are

dynamic structures, increasing and decreasing in complexity

and number with use and experience.

It has been calculated that in the cerebral cortex, 98% of the

synapses are on dendrites and only 2% are on cell bodies. In

the spinal cord, the proportion of endings on dendrites is less;

there are about 8000 endings on the dendrites of a typical spi-

nal neuron and about 2000 on the cell body, making the soma

appear encrusted with endings.

PRESYNAPTIC & POSTSYNAPTIC

STRUCTURE & FUNCTION

Each presynaptic terminal of a chemical synapse is separated

from the postsynaptic structure by a synaptic cleft that is 20 to

40 nm wide. Across the synaptic cleft are many neurotrans-

mitter receptors in the postsynaptic membrane, and usually a

postsynaptic thickening called the

postsynaptic density

(Fig-

ures 6–2 and 6–3). The postsynaptic density is an ordered

complex of specific receptors, binding proteins, and enzymes

induced by postsynaptic effects.

Inside the presynaptic terminal are many mitochondria, as

well as many membrane-enclosed vesicles, which contain neu-

rotransmitters. There are three kinds of

synaptic vesicles:

small,

clear synaptic vesicles that contain acetylcholine, glycine, GABA,

or glutamate; small vesicles with a dense core that contain cate-

cholamines; and large vesicles with a dense core that contain

FIGURE 6–1
Synapses on a typical motor neuron.
The neuron
has dendrites
(1),
an axon
(2),
and a prominent nucleus
(3).
Note that
rough endoplasmic reticulum extends into the dendrites but not into
the axon. Many different axons converge on the neuron, and their ter-
minal boutons form axodendritic
(4)
and axosomatic
(5)
synapses.
(6)
Myelin sheath.
(Reproduced with permission from Krstic RV:
Ultrastructure of the
Mammalian Cell.
Springer, 1979.)

2

6

6

1

5

4

3

1

FIGURE 6–2

Electron photomicrograph of synaptic knob (S)

ending on the shaft of a dendrite (D) in the central nervous system.

P, postsynaptic density; M, mitochondrion. (

×

56,000).

(Courtesy of DM

McDonald.)

MMM

SSS

PPP

DDD