9. THE MAUTHNER CELL
325
3. ELECTRICAL INHIBITION
By a chance which surely cannot be fortuitous, an extraordinary and
at present unique inhibitory mechanism exists at the Mauthner cell,
and its particular relevance to the present discussion lies in the fact that
it involves no “synaptic delay.” It is an electrical inhibition, and it was
discovered and analyzed by Furukawa and Furshpan ( 1963).
The electrical inhibition is manifested as a hyperpolarization of the
Mauthner cell in the axon hillock region not far from that where the
impulse is generated during orthodroniic excitation. [This was shown
by Furshpan and Furukawa (1962) to be probably in the axon, somewhat distal to the axon hillock.] An extracellular electrode located in
this region records a transient and often irregular positive potential
when the opposite VIIIth nerve or when either Mauthner cell is excited.
The spatial distribution of this positive field, which is the sign of the
electrical inhibition, was shown to conform closely to that of the axon
cap; this is a histologically identified region around the axon hillock
of the Mauthner cell, which stains somewhat differently from surrounding tissue with many staining procedures and is bounded by a roughly
spherical shell of glial cells ( Bartelmez, 1915). Running through the
axon cap is the narrow neck of the Mauthner axon, and it is at the outer
limit of the axon cap that this axon acquires its myelin sheath. Enclosed
in the axon cap are many nerve fibers and a few fine Mauthner cell
dendrites. Many of the axons coming to the Mauthner cell in this region
make irregular spirals round the axon neck before apparently ending on
the axon hillock and cap dendrites, while other axons run through the
axon cap without spiraling and also end apparently directly on the cell
(see Fig. 4 ) . The diameter of the cap may be 50-100 p.
The positive electric field recorded in this region when either Mauthner cell or the opposite VIIIth nerve is excited acts effectively as an external anode outside the Mauthner cell and is the direct cause of its hyperpolarization. Figure 37A shows records made within the axon cap during
contralateral VIIIth nerve excitation; when the cell was fired antidromically so that the axon hillock spike coincided with the peak of the external positivity, the impulse was blocked. Clearly a very powerful
inhibition was operating at that time. The mechanism of this inhibition
is best appreciated from viewing Fig. 37B. In this figure it is shown how
such an external positivity ( Furukawa and Furshpan’s “extrinsic hyperpolarizing potential” or EHP) caused by the activation of the collateral inhibitory pathway, actually succeeds in hyperpolarizing the
local Mauthner cell membrane. When the electrode was inserted into
the cell in the same region (the direction of the spike is then reversed),
325
3. ELECTRICAL INHIBITION
By a chance which surely cannot be fortuitous, an extraordinary and
at present unique inhibitory mechanism exists at the Mauthner cell,
and its particular relevance to the present discussion lies in the fact that
it involves no “synaptic delay.” It is an electrical inhibition, and it was
discovered and analyzed by Furukawa and Furshpan ( 1963).
The electrical inhibition is manifested as a hyperpolarization of the
Mauthner cell in the axon hillock region not far from that where the
impulse is generated during orthodroniic excitation. [This was shown
by Furshpan and Furukawa (1962) to be probably in the axon, somewhat distal to the axon hillock.] An extracellular electrode located in
this region records a transient and often irregular positive potential
when the opposite VIIIth nerve or when either Mauthner cell is excited.
The spatial distribution of this positive field, which is the sign of the
electrical inhibition, was shown to conform closely to that of the axon
cap; this is a histologically identified region around the axon hillock
of the Mauthner cell, which stains somewhat differently from surrounding tissue with many staining procedures and is bounded by a roughly
spherical shell of glial cells ( Bartelmez, 1915). Running through the
axon cap is the narrow neck of the Mauthner axon, and it is at the outer
limit of the axon cap that this axon acquires its myelin sheath. Enclosed
in the axon cap are many nerve fibers and a few fine Mauthner cell
dendrites. Many of the axons coming to the Mauthner cell in this region
make irregular spirals round the axon neck before apparently ending on
the axon hillock and cap dendrites, while other axons run through the
axon cap without spiraling and also end apparently directly on the cell
(see Fig. 4 ) . The diameter of the cap may be 50-100 p.
The positive electric field recorded in this region when either Mauthner cell or the opposite VIIIth nerve is excited acts effectively as an external anode outside the Mauthner cell and is the direct cause of its hyperpolarization. Figure 37A shows records made within the axon cap during
contralateral VIIIth nerve excitation; when the cell was fired antidromically so that the axon hillock spike coincided with the peak of the external positivity, the impulse was blocked. Clearly a very powerful
inhibition was operating at that time. The mechanism of this inhibition
is best appreciated from viewing Fig. 37B. In this figure it is shown how
such an external positivity ( Furukawa and Furshpan’s “extrinsic hyperpolarizing potential” or EHP) caused by the activation of the collateral inhibitory pathway, actually succeeds in hyperpolarizing the
local Mauthner cell membrane. When the electrode was inserted into
the cell in the same region (the direction of the spike is then reversed),