9. THE MAUTHNER CELL
271
tion of the spinal cord; toward its caudal end they eventually taper and
disappear. En route they give off numerous collaterals. [In Protopterus
the Mauthner axons may be over 200 p in diameter (see Smith, 1955).]
Of all the various nerve inputs converging on to the Mauthner neuron,
one (the only one in this category) is outstandingly clear, constant, and
easily traceable; it is that from the ipsilateral VIIIth cranial nerve
(Figs. l b and e ) , This long-established fact, the large size of many of
the myelinated fibers composing this input, and the directness of their
route to the lateral dendrite (see Fig. 4 ) has had an important influence
on both the experimental and the speculative approach to the problem
of Mauthner cell function.
111. THE SELECTIVE ACTIVATION OF
MAUTHNER NEURONS
A. Choice of Stimulus
Although the Mauthner cell can presumably be excited orthodromically by a variety of stimuli and experimentally by selective stimulation of the ipsilateral VIIIth nerve, there is no component of that or
any other nerve which is known to supply only the Mauthner cell, and
orthodromic excitation therefore is not an adequate means of selective
activation of a Mauthner cell. The only certain method, stimulation
via a microelectrode which has penetrated the Mauthner cell itself, is
not a practicable one for many types of experiment. The method which
has found most use is the relatively easy one of directly exciting the
Mauthner axon in the spinal cord, i.e., antidromic excitation. Because
the axon is so large its threshold to electrical shocks delivered to the
whole spinal cord is low (generally much lower than that of other
fibers), and experimentally no important difficulties have been caused
by coincidental excitation of other axons.
The great advantage of antidromic excitation over orthodromic, however, is the precision it confers when exact timing of the Mauthner impulses is important. The presence of “synaptic noise” ( sporadic fluctuasubjacent medulla). Note decussation of Mauthner axons ( M axons). From Furshpan
and Furukawa (1962). ( b ) The principal circuitry involved in Mauthner Reflex.
Note: Only the left Mauthner cell, and thus the right Mauthner axon, is excited
(excited cells and fibers are indicated by thick continuous lines and inhibitory
fibers by dashed lines). Only the VIIIth nerve connections and collaterals deriving
from the excited Mauthner cell are shown.
271
tion of the spinal cord; toward its caudal end they eventually taper and
disappear. En route they give off numerous collaterals. [In Protopterus
the Mauthner axons may be over 200 p in diameter (see Smith, 1955).]
Of all the various nerve inputs converging on to the Mauthner neuron,
one (the only one in this category) is outstandingly clear, constant, and
easily traceable; it is that from the ipsilateral VIIIth cranial nerve
(Figs. l b and e ) , This long-established fact, the large size of many of
the myelinated fibers composing this input, and the directness of their
route to the lateral dendrite (see Fig. 4 ) has had an important influence
on both the experimental and the speculative approach to the problem
of Mauthner cell function.
111. THE SELECTIVE ACTIVATION OF
MAUTHNER NEURONS
A. Choice of Stimulus
Although the Mauthner cell can presumably be excited orthodromically by a variety of stimuli and experimentally by selective stimulation of the ipsilateral VIIIth nerve, there is no component of that or
any other nerve which is known to supply only the Mauthner cell, and
orthodromic excitation therefore is not an adequate means of selective
activation of a Mauthner cell. The only certain method, stimulation
via a microelectrode which has penetrated the Mauthner cell itself, is
not a practicable one for many types of experiment. The method which
has found most use is the relatively easy one of directly exciting the
Mauthner axon in the spinal cord, i.e., antidromic excitation. Because
the axon is so large its threshold to electrical shocks delivered to the
whole spinal cord is low (generally much lower than that of other
fibers), and experimentally no important difficulties have been caused
by coincidental excitation of other axons.
The great advantage of antidromic excitation over orthodromic, however, is the precision it confers when exact timing of the Mauthner impulses is important. The presence of “synaptic noise” ( sporadic fluctuasubjacent medulla). Note decussation of Mauthner axons ( M axons). From Furshpan
and Furukawa (1962). ( b ) The principal circuitry involved in Mauthner Reflex.
Note: Only the left Mauthner cell, and thus the right Mauthner axon, is excited
(excited cells and fibers are indicated by thick continuous lines and inhibitory
fibers by dashed lines). Only the VIIIth nerve connections and collaterals deriving
from the excited Mauthner cell are shown.