Page 233 - Anatomy and Physiology of Farm Animals, 8th Edition

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are not in direct contact. A small but to sodium which allows this positive ion to
follow its concentration and electrical
distinct separation exists, and the space
VetBooks.ir between is the synaptic cleft. Neurotrans­ gradients by flowing to the cell’s interior.
neurotransmitters
cause
mitters released by the presynaptic neuron
Inhibitory
must diffuse across the synaptic cleft membrane hyperpolarization by increasing
to have their effect on postsynaptic cells. the membrane’s permeability to either
However, this diffusion occurs almost potassium (a positive ion that will flow out
instantaneously because of the very small of the cell) or chloride (a negative ion that
size of the cleft (average 20 nm, or about a will flow into the cell). This makes the cell’s
millionth of an inch). interior more negative, a hyperpolarization
Neurotransmitters bind to cell mem­ of the cell membrane that therefore reduces
brane receptors on postsynaptic cells. At the excitability of the postsynaptic cell.
synapses between neurons, the binding The amount of excitatory neurotransmit­
most commonly changes the postsynaptic ter released by a single action potential in a
membrane’s permeability to ions (either single presynaptic neuron is a constant.
directly or indirectly via second messen­ However, it is also typically insufficient to
gers), and this in turn produces a change in depolarize the postsynaptic neuron to the
the membrane potential of the postsynaptic threshold voltage at which another action
neuron. Synapses and the neurotransmit­ potential can be elicited. Therefore, to reach
ters that depolarize the postsynaptic neuron threshold voltage in a postsynaptic neuron,
are excitatory synapses and excitatory the change in voltage produced by multiple
neurotransmitters, while synapses and single action potential events must be added
neurotransmitters that hyperpolarize the together, or summated. Summation is classi­
postsynaptic neuron are inhibitory. fied as either spatial or temporal (Fig. 11‐6).
Typically, excitatory neurotransmitters Spatial summation occurs when simulta­
cause membrane depolarization by neous or nearly simultaneous neuro­
increasing the membrane’s permeability transmitter release occurs at more than

(A) (B)

a
b

d
c

Axon Axon

Figure 11-6. (A) Spatial summation. Each of multiple synapses (a through c) fire simultaneously and
the additive effects of each synapse brings the postsynaptic neuron to threshold. (B) Temporal summa­
tion. Repeated, rapid firing of a single synapse (d) allows sufficient depolarization to bring the postsynaptic
neuron to threshold.