9. THE MAUTHNER CELL
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tion like that in the spinal cord relating to the trunk and tail movement (Figs. 9 and 10). If this cranial response occurred, then the sudden
intake of water into the mouth and the sudden expulsion of water from
the gills could still cause a movement by a “jet propulsion” effect,
which might well serve to remove the animal from immediate danger
even in the absence of the tail flip.
IX. THE FUNCTIONS OF THE MAUTHNER CELLS
A. Swimming and Equilibration: Two
Improbable Functions
1. FATIGUE IN THE MAUTHNER CELL SYSTEM
a. Excitatory and InhibitoTy Mechanisms. Certain parts of the neuronal system involved in the Mauthner reflex show marked “fatigue”
with repetitive activation [ i.e., their response ( s ) becomes progessively
reduced]; they are: (i) the collateral inhibition of the Mauthner cells
in the brain (both the electrical and the chemical components); (ii)
the excitation of the spinal and cranial motoneurons; and, to a lesser
extent, (iii) the crossed inhibition in the spinal cord. Figure 40 shows
an experiment in which the progressive reduction of the collateral electrical inhibition (the EHP), with increasing frequency cf stimulation of
the Mauthner cell causing it, paralleled that of the tail muscle response
on the side of the excited Mauthner axon. Figure 41 shows fatigue of
the collateral chemical inhibition ( L C I ) of the Mauthner cell, and
Fig. 42 shows fatigue affecting the ventral root response in the trunk
region. Sometimes many seconds ( u p to 10) may be required for complete recovery of any of these component systems after even a single
activation. Some of the fatigue, certainly of the excitation of the primary
motoneurons, seems to be of the presynaptic mechanism (Fig. 43), although a form of postsynaptic “desensitization” cannot be excluded.
Since the recovery timc is very variable and sometimes can be as small
as 1-2 sec, this fatigue might be partly dependent on a reduced local
blood circulation following operation. However, even in the best preparations, e.g., those in which only the brain and not the spinal cord or
ventral roots was exposed (as in the preparation which gave the results
shown in Figs. 40 and 44) fatigue in all the systems mentioned above
was always readily demonstrable. Interestingly, the crossed inhibition in
the spinal cord was less susceptible to fatigue than was the excitation
(Fig. 44). The Mauthner cell and its axon can follow rates of stimulation of 100/sec or higher however.
331
tion like that in the spinal cord relating to the trunk and tail movement (Figs. 9 and 10). If this cranial response occurred, then the sudden
intake of water into the mouth and the sudden expulsion of water from
the gills could still cause a movement by a “jet propulsion” effect,
which might well serve to remove the animal from immediate danger
even in the absence of the tail flip.
IX. THE FUNCTIONS OF THE MAUTHNER CELLS
A. Swimming and Equilibration: Two
Improbable Functions
1. FATIGUE IN THE MAUTHNER CELL SYSTEM
a. Excitatory and InhibitoTy Mechanisms. Certain parts of the neuronal system involved in the Mauthner reflex show marked “fatigue”
with repetitive activation [ i.e., their response ( s ) becomes progessively
reduced]; they are: (i) the collateral inhibition of the Mauthner cells
in the brain (both the electrical and the chemical components); (ii)
the excitation of the spinal and cranial motoneurons; and, to a lesser
extent, (iii) the crossed inhibition in the spinal cord. Figure 40 shows
an experiment in which the progressive reduction of the collateral electrical inhibition (the EHP), with increasing frequency cf stimulation of
the Mauthner cell causing it, paralleled that of the tail muscle response
on the side of the excited Mauthner axon. Figure 41 shows fatigue of
the collateral chemical inhibition ( L C I ) of the Mauthner cell, and
Fig. 42 shows fatigue affecting the ventral root response in the trunk
region. Sometimes many seconds ( u p to 10) may be required for complete recovery of any of these component systems after even a single
activation. Some of the fatigue, certainly of the excitation of the primary
motoneurons, seems to be of the presynaptic mechanism (Fig. 43), although a form of postsynaptic “desensitization” cannot be excluded.
Since the recovery timc is very variable and sometimes can be as small
as 1-2 sec, this fatigue might be partly dependent on a reduced local
blood circulation following operation. However, even in the best preparations, e.g., those in which only the brain and not the spinal cord or
ventral roots was exposed (as in the preparation which gave the results
shown in Figs. 40 and 44) fatigue in all the systems mentioned above
was always readily demonstrable. Interestingly, the crossed inhibition in
the spinal cord was less susceptible to fatigue than was the excitation
(Fig. 44). The Mauthner cell and its axon can follow rates of stimulation of 100/sec or higher however.