Visual Space Biasing by the
Nucleus Isthmus of the Frog
by David D. Olmsted (Copyright - 2000, 2006. Free to use for personal and
Last Revised September 30, 2006
Connections of the Nucleus
The principle characteristic of the nucleus isthmus is its representation
of the space around the frog. This is in contrast to the tectum which only represents
the visual field of one eye. The nucleus isthmus is located in the tegmentum
between the tectum and the cerebellum. It is reciprocally connected with the tectum
and its only inputs come from the tectum. The projections of the nucleus isthmus
to the tectum use acetylcholine as their neurotransmitter and these projections
seem to be the only source of cholinergic inputs to the tectum (Desan, et al - 1987)
Figure 1 shows the projections of the nucleus isthmus (dark area) to the tectum.
The small dots represent the existence of isthmo-tectal fibers 24 hours after 3H-proline
injection into the nucleus isthmus in the frog Rana pipiens. The projections
on the same (ipsilateral) side
terminate in a continuous band from the middle to superficial layers. In contrast the projections are less numerous to the opposite
(contralateral) side to such a degree that they separate into two bands corresponding
to layers A (superficial) and 8 (middle) of Potter (who defined a layering scheme).
The projections to the opposite
side first travel towards the head (rostally) so they can cross over near the preoptic
area of the hypothalamus in a region called the supraoptic decussation. From there
the projections travel back towards the tectum.
Projections of the Nucleus Isthmus.
Lower right figure shows the locations of the slices. cb - cerebellum, ct - transverse commissure, di - diencephalon, ni - nucleus isthmus, oc - optic chiasm, po - preoptic area, sd - supraoptic decussation, tect. - tectum, tel - telencephalon, teg - tegmentum. (Gruberg and Udin - 1978)
The nucleus isthmus is organized
like a shell with a cell dense rim layer surrounding a less cell dense inner region
as shown in figure 2. A gap (hilus) exists in the rim which is pointed out
by the arrow in the figure through which most of the axon fibers pass. The dendrites
of the rim neurons generally lie in the horizontal plane and extend towards the
hilus. The nucleus isthmus consists of approximately 8,000 neurons (Gruberg and
Udin - 1978).
Interestingly cells which project to the hypothalamus surround the
nucleus isthmus and one can only wonder if nucleus isthmus projection fibers make
contact with them on the way to the tectum? This might be the case for stimulation
of the optic tract stimulates some of these cells (see figure 10 below).
Top View Through the Middle of the Nucleus Isthmus Showing its Rim Region in the Frog Rana pipiens. Noseward (rostal) direction is up, medial direction is to the left. (Gruberg and Udin - 1978)
The nucleus isthmus represents the total 3D space surrounding the frog.
This is shown in figure 3 where it is compared to the more limited one-sided (but multisensory)
spatial representation in the tectum. The figure shows the retinal topographic representations
from the frog Rana pipeins on the surface of the tectum and nucleus isthmi.
The visual field is represented by arrows in view A. Notice how the order of the
left / right visual field representation is reversed in the nucleus isthmi on each
side of the brain. The nucleus isthmus seems to be homologous with the parabigeminal
nucleus in mammals (Gruberg and Udin - 1978)
The Two Nucleus Isthmi Represent the Space Surrounding the Frog. (Gruberg and Udin - 1978)
In contrast the projections of the retina to the insides of the nucleus isthmus
are not so topographically organized when compared to the tectum. This is shown
in figure 4 which shows slices of the nucleus back (caudal) to front (rostal).
Inner Neurons of Isthmi Nuclei are Only Semi-Topographically Organized.
D - dorsal, L - lateral, M - medial, V - ventral. (Gruberg and Udin - 1978)
Effects of Lesions to the Nucleus Isthmus
As shown in figure 5 the result of full or partial
lesions in one nucleus isthmus is to form a general behavioral deficit in the region of space represented by the
destroyed part of the nucleus isthmus which is always contralateral (opposite) to the lesioned nucleus isthmus. It is
almost (but not quite) like a blind spot (scotoma) in the visual field. In figure 5 the outer numbers represent the order of placing
a cricket in each radial sector for two minutes. The inner numbers give the number
of responses (orienting, snapping) by the frog Rana pipiens. Notice the
generally greater number of responses when the stimulus is novel and the lack of
responses in the lower right quadrant.
Location of the General Behavioral Deficit after Lesioning the Nucleus Isthmus on the Left Side of the Brain. (Gruberg, et al - 1991)
Even a partial destruction of the nucleus isthmus that leaves up to 70% of it intact
will produce a scotoma in nearly the whole visual field of one eye (120 degrees)
as shown in figure 6. Lessor damage produces a wide variation in the size of the
Not only did the scotoma affect prey acquisition behavior
but it also prevented the frog from responding to threatening behavior (a large
looming object) in the affected area.
Size of Scotoma vs. the Completness of Nucleus Isthmus Lesion. (Gruberg, et al - 1991)
Significantly, persistent trials of approximately
80 tests per sector turned up a few responses to prey stimuli in the deficit sector
(scotoma region). This is significant because similar lesions in the tectum never show
any responses from the visual field so affected (Gruberg, et al - 1991) indicating that the signals
from the nucleus isthmus might be more of a biasl.
The neural visual fields in the tectum expand with isthmal
nucleus lesions as shown
in figure 7. The left column in the figure shows the neural visual fields
in the tectum of the frog Rana pipiens unaffected by nucleus isthmus lesions
while the column on the right shows the mirror image visual fields in the scotoma
area from the opposite side of the tectum which were affected by the lesions. Notice
that visual field “C” has been broken into two pieces. This evidence tends to support
the idea that the nucleus ishmus produces some sort of bias.
The Neural Visual Fields in the Tectum that Correspond to the Scotomas of Damaged Nucleus Ishmi Increase in Size. (Gruberg, et al - 1991)
As mentioned above the nucleus isthmus is the only source of acetylcholine
to the tectum (Desan, et al - 1987) yet surprisingly its effect is inhibitory. Acetylcholine is the major neurotransmitter to muscles where
its effect is excitatory. Curare is a poison because it paralyzes the muscles by
blocking acetylcholine. Figure 8
shows an experiment in which curare was applied to the tectal area at time = 0.
Section A - (from left to right then down):
- (time at -1 minute) Normal response of a moving straight edge through this tectal
neuron's visual field
- (time at 1 minute) Enhanced neural response due to a 45 second pulse of the acetylcholine
blocker curare beginning at time 0
- (time at 4 minutes) An example of spontaneous neural bursting in the absence of
a stimulus which started after 2 minutes
- (time at 30 minutes) Back to the normal neural response to stimuli.
- (time at -1 minute) Normal neural response of a moving straight edge stimulus
- (time at 1 minute) Reduced neural response due to a 30 second pulse of acetylcholine
applied at 0 time
- (time at 15 minutes) Back to the normal neural response to stimuli.
Take away some of the inhibition and the visual fields expand.
Consequently the nucleus isthmus seems to keep the tectum neurons (or at least the
neurons) in a mid-level responsive state which can be adjusted either up
or down depending on the situation. The purpose of this bias may be to adjust the tectal
visual field to various eye ball positions so that the frog's tongue always targets
the same position.
Application of curare or strychnine to the tectum on one side of the brain produced only turning behavior while its application to both tecta produced
the full orientating, jumping, and snapping sequence. The receptors to acetylcholine
are also activated by nicotine. In contrast atropine (muscarinic - cholinergic blocking
drug) has no effect.
The Acetylcholine Bias to Tectal Neurons Provided by the Nucleus Isthmus. (Stevens - 1973)
Another significant feature found by
Gruberg, et al (1991) is that approximately half their frogs recovered and regained
full visual responsiveness over their complete visual field. The recovery seems
to be random and not correlated to the size of the isthmal nucleus lesion. This
recovery would seem to indicate some neuron regrowth in the nucleus isthmus on the
opposite side of the brain which was consequently able to take over the function
of the damaged side (remember that both isthmi nuclei have complete spatial field
maps). The time to recovery took between 5 and 203 days depending on the frog.
Neuron Responses in the Nucleus Isthmi
The various neuron shapes found in the nucleus isthmus are shown in figure 9. This
figure gives the scale of the neuron's dendritic field relative to the size of the
Representative Neurons in the Nucleus Isthmus. From the toad Bufo bufo. D -dorsal, M - medial, V - ventral (Wu - 1995)
Optic tract stimulation causes a variety of responses
in the neurons of the nucleus
isthmus as shown in figure 10. For the nucleus on the same side (ipsilateral) it
can inhibit cell types a and c from figure 9 and excite and then inhibit (b) or excite (d). In contrast its effect on the
contralateral nucleus isthmus is mostly inhibitory.
Intra-cellular Responses of Some Figure 9 Neurons to Optic Tract Stimulation.
IR - ipsilateral optic tract simulation, CR - contralateral optic tract stimulaion of the same cell. (Wu - 1995)
Optic tract stimulation is mostly
inhibitory on neurons adjacent to the nucleus isthmi as well as shown in figure
11. These neurons have a high probability of sending projections
to the hypothalamus. Figure 11 shows the intracellularly recorded responses
of “a” and “b” type cells shown in section B to contralateral (CR) optic tract
stimulation and ipsilateral (IR) optic tract stimulation. Cells of the “a” type
were spontaneously active and this activity was interrupted by optic tract stimulation.
Cells of the “b” type were first inhibited and then excited.
Responses of Neurons (Possibly Hypothalamic Projecting) Adjacent to the Nucleus Ishmus to Optic Tract Stimulation.
NI - nucleus isthmus, OT - optic tectum, PTG - posterodorsal tegmentum, V - ventricle (Wu - 1995)
Desan, P.H., Gruberg, E.R., Grewell, K.M., and Eckenstein,
F. (1987). Cholinergic Innervation of the Optic Tectum in the Frog Rana pipiens.
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