430
M. V. L. BENNETT
residual potential across the stalk face following its brief spike is
ascribable to voltage drop across this face produced by activity of the
nonstalk face.
What is the significance of this sequence of activity? The resistance
of the two faces at rest is about equal. The brief spike in the stalk face
is apparently a rapid way to turn on delayed rectification in this membrane, and by increasing its conductance to increase external current
flow. Electron microscopic observations show that both faces have a
moderate number of branching tubules that increase their surface areas
(Mathewson et al., 1961). The proliferation is slightly greater in the
nonstalk face. The similarity in areas is consistent with the suggestion
that both surfaces increase their conductances during a response.
Actually, the stalk face may not be capable of generating an all-or-none
response. The response of this face to depolarization (applied by an
external electrode) apparently is graded, and a propagated spike cannot
be set up by external stimuli that depolarize this face and hyperpolarize
the nonstalk face. It is even possible that this membrane only exhibits
delayed rectification and totally lacks an inward current mechanism.
The initial maximum seen in some external records would then be
ascribable to capacitative current as in cup-type electrocytes of rajids
(Fig. 15). The steep rise and fall of the potential across the nonstalk
face makes this explanation unlikely, but it does not contradict available
data.
From the location of the synapses and the observation that spikes
can propagate along the body of the cell, it appears that a PSP sets up
a spike in the stalk that propagates to invade the rest of the cell. As
would be expected, neurally evoked responses in the body of the cells
are similar in shape to directly evoked ones. Transmission at the synapses
is cholinergic in that curare blocks neural excitation of the organ and
a high concentration of cholinesterase is found histochemically at the
synapses ( Couteaux and Szabo, 1959).
Activity of the stalk can be recorded by electrodes placed (under
visual control) close to the rosette, i.e., the site where the stalk joins
the disc-shaped part of the cell. External stimulation in this region can
excite the cell even if the polarity is such as to depolarize the stalk face
(Fig. 55). External to the rosette opposite the stalk (upper trace) an
initially positive potential is observed apparently resulting from activity
of the stalk, because it is much smaller a short distance away (lower
trace). If the stimulus strength is increased, repetitive firing is produced
(Fig. 53B), which is never observed with stimulation of either polarity
distant from the rosette. If the stimulus strength is increased further,
the impulses fail to invade the body of the cell, but small biphasic
M. V. L. BENNETT
residual potential across the stalk face following its brief spike is
ascribable to voltage drop across this face produced by activity of the
nonstalk face.
What is the significance of this sequence of activity? The resistance
of the two faces at rest is about equal. The brief spike in the stalk face
is apparently a rapid way to turn on delayed rectification in this membrane, and by increasing its conductance to increase external current
flow. Electron microscopic observations show that both faces have a
moderate number of branching tubules that increase their surface areas
(Mathewson et al., 1961). The proliferation is slightly greater in the
nonstalk face. The similarity in areas is consistent with the suggestion
that both surfaces increase their conductances during a response.
Actually, the stalk face may not be capable of generating an all-or-none
response. The response of this face to depolarization (applied by an
external electrode) apparently is graded, and a propagated spike cannot
be set up by external stimuli that depolarize this face and hyperpolarize
the nonstalk face. It is even possible that this membrane only exhibits
delayed rectification and totally lacks an inward current mechanism.
The initial maximum seen in some external records would then be
ascribable to capacitative current as in cup-type electrocytes of rajids
(Fig. 15). The steep rise and fall of the potential across the nonstalk
face makes this explanation unlikely, but it does not contradict available
data.
From the location of the synapses and the observation that spikes
can propagate along the body of the cell, it appears that a PSP sets up
a spike in the stalk that propagates to invade the rest of the cell. As
would be expected, neurally evoked responses in the body of the cells
are similar in shape to directly evoked ones. Transmission at the synapses
is cholinergic in that curare blocks neural excitation of the organ and
a high concentration of cholinesterase is found histochemically at the
synapses ( Couteaux and Szabo, 1959).
Activity of the stalk can be recorded by electrodes placed (under
visual control) close to the rosette, i.e., the site where the stalk joins
the disc-shaped part of the cell. External stimulation in this region can
excite the cell even if the polarity is such as to depolarize the stalk face
(Fig. 55). External to the rosette opposite the stalk (upper trace) an
initially positive potential is observed apparently resulting from activity
of the stalk, because it is much smaller a short distance away (lower
trace). If the stimulus strength is increased, repetitive firing is produced
(Fig. 53B), which is never observed with stimulation of either polarity
distant from the rosette. If the stimulus strength is increased further,
the impulses fail to invade the body of the cell, but small biphasic