Thalamus is a large ovoid mass of gray matter, situated bilaterally in diencephalon. Both thalami form 80% of diencephalon. Thalami on both sides are connected in their rostral portions by means of an intermediate mass. Caudal portions are more widely separated by corpora quadrigemina.
THALAMIC NUCLEI
Thalamic
nuclei are classified by two methods:
A.
Anatomical clasasification
B.
Physiological classification.
ANATOMICAL
CLASSIFICATION
Thalamus
on each side is divided into five main nuclear groups by ‘Y’-shaped
internal medullary lamina.
1. Midline nuclei
2. Intralaminar nuclei
3. Medial mass of nuclei
4. Lateral mass of nuclei
5. Posterior group of
nuclei.
1. Midline Nuclei
Midline nuclei are a group
of small nuclei, situated on the medial surface of thalamus near the midline.
2. Intralaminar
Nuclei
Intralaminar nuclei are
smaller nuclei present in the medullary septum of thalamus.
3. Medial Mass of
Nuclei
Medial mass of nuclei are
situated medial to septum and it comprises two nuclei:
i. Anterior nucleus
ii. Dorsomedial nucleus.
4. Lateral Mass of
Nuclei
This group of nuclei are
situated lateral to septum. Lateral mass of nuclei are again divided into two
subgroups:
i. Dorsal
group of lateral mass with two nuclei:
a. Dorsolateral nucleus
b. Posterolateral nucleus
ii.
Ventral group of lateral mass with three nuclei:
a. Ventral anterior
nucleus
b. Ventral lateral nucleus
c. Ventral posterior
nucleus. It consists of two parts:
• Ventral
posterolateral nucleus
• Ventral
posteromedial nucleus.
5. Posterior Group
of Nuclei
Posterior group of nuclei are
the continuation of lateral mass of nuclei. It has two
subgroups:
i. Pulvinar
ii.
Metathalamus which consists of two structures:
a. Medial geniculate body
b. Lateral geniculate
body.
Thalamic
reticular nucleus
Thalamus also includes
thalamic reticular nucleus, which is a thin layer of neurons covering the
lateral
aspect of thalamus. It is
separated from thalamus by external medullary lamina. It receives information
from reticular formation, cerebral cortex and other thalamic and sends
inhibitory signals to other thalamic nuclei.
PHYSIOLOGICAL
CLASSIFICATION
On the basis of functions and
their projections, thalamic nuclei are classified into
five groups. This type of
classification
is also called Bondok classification. Five groups of thalamic nuclei are:
1.
Specific sensory relay nuclei
2.
Specific motor nuclei
3.
Association or less specific nuclei
4.
Non-specific nuclei
5. Limbic system nuclei.
CONNECTIONS OF THALAMIC NUCLEI
THALAMIC RADIATIONS
Thalamic
radiation is the collection of nerve fibers connecting thalamus and cerebral
cortex. It contains
both
thalamocortical and corticothalamic fibers. All these fibers between thalamus
and cerebral cortex pass through internal capsule. Fibers of
thalamic radiation are divided into four groups,
which are called thalamic peduncles or thalamic stalks. Thalamic peduncles are:
1. Anterior (frontal)
thalamic peduncle or radiation
2. Superior (centroparietal)
thalamic peduncle or radiation
3. Posterior (occipital)
thalamic peduncle or radiation
4. Inferior (temporal)
thalamic peduncle or radiation
ANTERIOR (FRONTAL)
THALAMIC PEDUNCLE OR RADIATION
Anterior thalamic peduncle
connects the frontal lobe of cerebral cortex with medial and lateral thalamic
nuclei. It contains mostly motor nerve fibers.
SUPERIOR
(CENTROPARIETAL) THALAMIC PEDUNCLE OR RADIATION
Fibers of this peduncle
connect postcentral gyrus (somesthetic area) of parietal lobe and adjacent area
in frontal cortex with
lateral mass of thalamic nuclei. It contains mainly
the sensory fibers.
POSTERIOR (OCCIPITAL)
THALAMIC PEDUNCLE OR RADIATION
Posterior thalamic peduncle connects
occipital lobe of cerebral cortex with pulvinar and
lateral geniculate body. It contains the
nerve fibers concerned with vision.
INFERIOR (TEMPORAL) THALAMIC
PEDUNCLE OR RADIATION
Fibers of this peduncle connect
temporal lobe and insula with pulvinar and medial geniculate body. This
peduncle contains the nerve fibers concerned with hearing
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