Introduction to the Frog Forebrain (Telencephalon)
by David D. Olmsted (Copyright - 2000, 2006. Free to use for personal and
educational purposes)
Last Revised September 2, 2006
Introduction
Figure 1
Cross-sections of the Frog Rana pipiens Forebrain (chapter three of Parent - 1986)
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Unlike the forebrain (telencephalon
or cerebrum) of mammals, birds, and fish, the forebrain of amphibians is similar across all three orders of amphibians: the anurons (frogs and toads), the urodiles
(salamanders and newts), and the caecilians. The telencephalon consists of
two tubular hemispheres which are shown in cross-section in figure 1. Notice
that the neural cells as indicated by the dots tend to cluster close to the ventricles
(empty spaces) leaving the outside regions free for fibers of passage. Also notice
that the medial (inner) wall is much thicker and more developed than the outer wall.
Section “A” is the most nose-ward (rostal) while “D” is the most tailward (caudal).
The left side of each drawing shows the neural cell density while the right side
shows the regional names. AL - lateral amygdala, AM - medial amygdala, AS - nucleus
accumbens, DP - dorsal pallium (hippocampus), EN - entopeduncular nucleus, LFB -
lateral forebrain bundle, LP - lateral pallium (hippocampus), LS - lateral septum,
MFB - medial forebrain bundle, MP - medial pallium (hippocampus), MS - medial septum,
OTU - olfactory tubercle, PO - preoptic area of hypothalamus, STRd - dorsal striatum,
STRv - ventral striatum.
Medial Pallium
The medial pallium (MP) is the evolutionary antecedent to the mammalian
hippocampus based upon its location and interconnections. The Hippocampus defines the relative
spatial context and later in evolution the temporal context for this hierarchy.
The medial pallium receives projections
from all parts of the forebrain except the striatum (STR), lateral amygdala (AL)
and the preoptic area (POA). In return it projects to all telencephalic area except
the striatum. It also receives projections from the anterior thalamic nucleus and
the raphe in the medulla (brain stem).
Four major fiber systems connect the medial
pallium with the other brain regions. They are the dorsal association tract interconnecting
it with the dorsal pallium, the fornix, the pallial or hippocampal commissure interconnecting
it with the other hemisphere, and the ascending anterior thalamic tract bringing
fibers in from the anterior thalamus.
Lateral Pallium
The lateral pallium seems
to be the main olfactory pattern characterizing region since it receives most of
the secondary olfactory projections and projects back to the ipsilateral (same side)
olfactory bulb and tubercle. It also receives projections from the medial and dorsal
palliums and possibly from the dorsal thalamus. It also projects to the striatum
and the septum.
Dorsal Pallium
The dorsal pallium (DP) has no precise lateral boundaries
and seems to be an interconnecting zone between the medial and lateral palliums
since it sends and receives inputs from both. It also receives secondary olfactory
fibers and fibers from the anterior thalamus. It consists mostly of bipolar neurons
with some priform cells and intermediate neurons (Capanna and Clairambault - 1974).
Amygdala
The lateral amygdala is the main target of the accessory olfactory bulb
and it also receives inputs from the medial pallium. It projects to the hypothalamus,
thalamus, and possibly the brain stem. The medial amygdala is reciprocally connected
with the ipsilateral (same side) septum and the medial palliums of both sides. In
contrast to the lateral section the medial section receives projections from the
hypothalamus.
Septum
The septum is the other well developed area of the medial telencephalic
wall and like the amygdala it is divided into medial and lateral sections based
upon neural cell densities. The dendrites of these cells project and arborize among
the medial forebrain bundle fibers. The septum receives bilateral projections from
the brainstem, the hypothalamus, and the medial pallium just above it (dorsal) it.
Its projections give rise to the medial forebrain bundle which reach the ventral
thalamus, the preoptic area, and other hypothalamic areas.
Olfactory Tubercle
Also
called the diagonal band of Broca since it probably evolved and divided into both
the olfactory tubercle and the nucleus of the diagonal band in mammals. It receives
bilateral secondary olfactory inputs from the medial palliums and in turn projects
to the ipsilateral (same side) medial pallium.
Striatum
The striatum (STR) is composed
of the lateral striatum (which some researchers divide into a dorsal and ventral
part based upon neural cell density) and the nucleus accumbens (AS). It receives
projections from the ipsilateral (same side) thalamus and from the entopeduncular
nucleus of both sides. Both inputs arrives via the lateral forebrain bundle. No
projections come from the palliums. The striatum sends projections to the ventral
thalamus, preoptic area, the posterior tuberculum, and the brainstem.
Entopeduncular Nucleus
The entopeduncular nucleus is located in and dorsal to the lateral forebrain
bundle from which it receives most of its striatal inputs. It sends projections
to the optic tectum, lower medulla, and cervical spinal cord but mostly back to
the striatum.
Types of Neurons
Figure 2
The Types of Neurons in the Frog Forebrain (Capanna and Clairambault - 1974)
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No cortical pyramidal neurons are found in the amphibian forebrain indicating it
developed later in evolution. Instead the multipolar neuron (a) is the most common
and it is found throughout the forebrain (figure 2). The neurons are as follows:
a - medial pallium multipolar neuron from Bufo viridis, b - medial septum multipolar
neuron from Bufo bufo, c - Striatal intermediate neuron from Bufo virdis, d - lateral
pallium piriform neuron from Bufo virdis, e - dorsal pallium bipolar neuron from
Bufo bufo. Axons are labeled by “n”.
Some of these multipolar
neurons have spines and some don’t. Piriform neurons are more common in the lateral
walls (lateral pallium and striatum) than in the medial walls. Bipolar neurons are
less common and are found in the superficial layers of the walls where they seem
to act as association neurons.
References
Capanna, E. and Clairambault, P (1974)
Neuronal Typology in the Anuran Telencephalon: a Golgi Study. Acta Anat. 89:321-332
Parent (1986). Comparative Neurobiology of the Basal Ganglia. John Wiley & Sons:
New York