258
pu(E FLOCK
determining the rate of transmitter release. The sensitivity of these
synapses to change in electric potential is such that if the afferent synapse
on hair cells works by a similar mechanism then the receptor potential
recorded in hair cells is large enough to be causative in the synaptic
process.
V. INITIATION OF NERVE IMPULSES
In the neuromuscular junction, as well as in other neurochemical
synapses, each synaptic vesicle contains a fixed amount of transmitter substance capable of producing one unit of potential change in the postsynaptic membrane-a miniature postsynaptic potential. Each miniature
potential represents a depolarization of the postsynaptic terminal which
travels along its nonmyelinated portion down to the point where the
myelin sheath starts. This site is electrically excitable; when a threshold
level of depolarization is reached the nerve action potential is triggered.
Each miniature potential is generally of subthreshold size; however, the
integrative capacity of the nonmyelinated terminal makes possible the
summation of several miniature potentials to reach threshold. The spontaneous release of transmitter is random in time and so the firing of a
nerve potential is probabilistic.
What is said here may apply also to excitation of sensory nerve fibers
in lateral line organs as well as in the inner ear; in fact, postsynaptic
potentials have recently been recorded from the saccular nerve in fish
by Furukawa and Ishii (1967). Spontaneous firing of the nerve in
Xenopus is probabilistic; that is, the histogram of successive spike intervals shows a Poisson distribution (Harris and Milne, 1966). When the
organ is stimulated, nerve action potentials are triggered by each cycle
of a periodic stimulus (Fig. 15); in other words, the probability of the
nerve firing becomes time-locked to the stimulus. The underlying mechanism may be a synchronization of transmitter release to the stimulus
causing an increased probability of nerve firing. There may as well be
an actual increase in release of transmitter during part of the stimulus
cycle. An increase in intensity is signaled by an increasing number of
spikes occurring during each cycle as well as by shortening of the latency
between the stimulus and the first spike ( Fig. 16).
According to their response two types of nerve fibers can be distinguished. Some fibers are excited when the cupula is displaced in one
direction along the axis of the canal, other fibers are excited by displacement in the opposite direction; this is true in canal organs (Sand, 1937) as
pu(E FLOCK
determining the rate of transmitter release. The sensitivity of these
synapses to change in electric potential is such that if the afferent synapse
on hair cells works by a similar mechanism then the receptor potential
recorded in hair cells is large enough to be causative in the synaptic
process.
V. INITIATION OF NERVE IMPULSES
In the neuromuscular junction, as well as in other neurochemical
synapses, each synaptic vesicle contains a fixed amount of transmitter substance capable of producing one unit of potential change in the postsynaptic membrane-a miniature postsynaptic potential. Each miniature
potential represents a depolarization of the postsynaptic terminal which
travels along its nonmyelinated portion down to the point where the
myelin sheath starts. This site is electrically excitable; when a threshold
level of depolarization is reached the nerve action potential is triggered.
Each miniature potential is generally of subthreshold size; however, the
integrative capacity of the nonmyelinated terminal makes possible the
summation of several miniature potentials to reach threshold. The spontaneous release of transmitter is random in time and so the firing of a
nerve potential is probabilistic.
What is said here may apply also to excitation of sensory nerve fibers
in lateral line organs as well as in the inner ear; in fact, postsynaptic
potentials have recently been recorded from the saccular nerve in fish
by Furukawa and Ishii (1967). Spontaneous firing of the nerve in
Xenopus is probabilistic; that is, the histogram of successive spike intervals shows a Poisson distribution (Harris and Milne, 1966). When the
organ is stimulated, nerve action potentials are triggered by each cycle
of a periodic stimulus (Fig. 15); in other words, the probability of the
nerve firing becomes time-locked to the stimulus. The underlying mechanism may be a synchronization of transmitter release to the stimulus
causing an increased probability of nerve firing. There may as well be
an actual increase in release of transmitter during part of the stimulus
cycle. An increase in intensity is signaled by an increasing number of
spikes occurring during each cycle as well as by shortening of the latency
between the stimulus and the first spike ( Fig. 16).
According to their response two types of nerve fibers can be distinguished. Some fibers are excited when the cupula is displaced in one
direction along the axis of the canal, other fibers are excited by displacement in the opposite direction; this is true in canal organs (Sand, 1937) as