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PROJECTION FIBRES
THE INTERNAL CAPSULE
By
Prof. Dr. Muhammad Imran Qureshi
PROJECTION FIBRES
These fibres connect the cerebral cortex to the subcortical centres (such as the corpus
striatum, thalamus, brainstem) and the spinal cord.
They are of two types:
1. Corticofugal fibres: Go away from the cortex to centres in the other parts of the
CNS. (Cortical Efferents)
2. Corticopetal fibres: Come to the cerebral cortex from the other centres in the
CNS. (Cortical Afferents)
The Projection fibres of neocortex constitute the corona radiata and internal capsule
while those of the archicortex and paleocortex constitute the fimbria and fornix.
The most important bundles of projection fibres are the internal capsule and fornix.
The Internal Capsule
The Internal Capsule is a compact
bundle of projection fibres that lies
between the Thalamus and
Caudate nucleus medially and the
Lentiform nucleus laterally.
Rostrally, these fibres fan out to
form the Corona Radiata.
Caudally, they condense and
continue as the Crus Cerebri of the
midbrain.
These ascending (corticopetal
/sensory) and descending
(corticofugal/motor) fibres of the internal
capsule chiefly interconnect the cerebral
cortex with the brainstem and spinal
cord. These fibres are mainly responsible
for the sensory and motor innervation of
the opposite half of the body.
As there is a high concentration of motor
and sensory nerve fibres within the
internal capsule, even a small lesion
there may produce a widespread
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paralytic effects and sensory loss in the opposite half of the body.
The arrangement of fibres can be easily remembered if it is realized that any group of
fibres within the capsule takes the most direct path to its destination. Thus:
Fibres to and from the anterior part of the frontal lobe pass through the anterior limb of
the internal capsule.
Those to and from the posterior part of the frontal lobe, and from the greater part of the
parietal lobe, occupy the genu and posterior limb of the capsule.
Those to and from the temporal lobe occupy the sublentiform part of the capsule.
Those to and from the occipital lobe pass through the retrolentiform part of the capsule.
Some fibres from the lowest parts of the parietal lobe accompany the temporal fibres
through the sublentiform part of the capsule.
Ascending Fibres
These are predominantly Thalamocortical fibres which go from the thalamus to all
parts of the cerebral cortex.
Fibres to the frontal lobe constitute the anterior thalamic radiation (or frontal thalamic
peduncle).
They pass through the anterior limb of the internal capsule. The fibres arise mainly from
the medial and anterior nuclei of the thalamus. The anterior thalamic radiation also
carries fibres from the hypothalamus and limbic structures to the frontal cortex.
Fibres travelling from the ventral posterior nuclei of the thalamus to the somatosensory
area (in the postcentral gyrus) constitute the superior thalamic radiation (or the
superior / dorsal thalamic peduncle). These fibres occupy the genu and posterior limb of
the capsule. These fibres are the third order sensory neurons responsible for conveying
somesthetic sensations to the cerebral cortex. This radiation also contains some fibres
that go from the thalamus to parts of the frontal and parietal lobes adjoining the
postcentral gyrus.
Fibres from the thalamus to the occipital
lobe constitute the posterior thalamic
radiation (or the posterior, or caudal,
thalamic peduncle). This includes the
optic radiation from the lateral
geniculate body to the visual cortex.
These radiations lie in the retrolentiform
part of the internal capsule. The
retrolentiform part also contains some
fibres passing from the thalamus to the
posterior part of the parietal lobe.
Fibres from the thalamus to the temporal lobe constitute the inferior thalamic radiation
(or ventral thalamic peduncle). It includes the acoustic radiation from the medial
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geniculate body to the acoustic area of the cerebral cortex. These fibres pass through
the sublentiform part of the internal capsule.
Descending Fibres
Corticonuclear and Corticospinal fibres:
Corticonuclear fibres (for motor cranial nerve nuclei) pass through the genu of the
internal capsule.
Corticospinal fibres form several discrete bundles in the posterior limb. The fibres for the
upper limb are most anterior, followed (in that order) by fibres for the trunk and lower
limb.
Corticopontine fibres:
Frontopontine fibres are the most numerous. They
pass through the anterior limb, genu, and posterior
limb of the internal capsule.
Parietopontine fibres pass mainly through the
retrolentiform part. Some fibres also pass through
the sublentiform part.
Temporopontine fibres pass through the
sublentiform part.
Occipitopontine fibres pass through the
retrolentiform part.
Corticothalamic fibres:
These pass from various parts of the cerebral
cortex to the thalamus. They form part of the
thalamic radiations.
Fibres from Cerebral Cortex to the Brainstem nuclei:
Corticonuclear fibres to cranial nerve. They pass through the genu and posterior limb.
Corticorubral fibres pass through the posterior limb. Occipitotectal and Occipitocollicular
fibres pass through the retrolentiform part.
Fibres of the subthalamic fasciculus: pass transversely through the posterior limb
(intersecting the vertically running fibres). These fibres connect the subthalamic nucleus
to the globus pallidus.
Shape and boundaries of the Internal Capsule
In a horizontal section of the cerebral hemisphere, the internal capsule appears as a “V”
shaped compact bundle of white fibres with its concavity directed laterally. It is bounded
medially by the caudate nucleus and thalamus, and laterally by the lentiform nucleus.
Parts of the internal capsule
The internal capsule is divided into following five parts:
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1.
Anterior limb: It lies between the head of caudate nucleus medially and the
anterior part of the lentiform nucleus laterally.
2.
Posterior limb: It lies between
the thalamus medially and the
posterior part of the lentiform nucleus
laterally.
3.
Genu: It is the bend between
the anterior and posterior limbs with
concavity of the bend facing laterally.
4.
Retrolentiform part: It lies
behind the lentiform nucleus.
5. Sublentiform part: It
lies below the lentiform
nucleus.
Constituent fibres of the Internal
Capsule
Motor fibres:
Corticopontine fibres: They originate
from the cerebral cortex of all the lobes
of the cerebral hemisphere and form
the largest single group of projection
fibres in the internal capsule (about
2/3rd of the total fibre component).
They are named according to the lobe
from which they arise, e.g.
frontopontine, parietopontine,
occipitopontine and temporopontine,
arising from frontal, parietal, occipital
and temporal lobes respectively.
The frontopontine fibres are most numerous and pass through the anterior limb, genu,
and posterior limb.
The parietopontine and occipitopontine fibres pass through the retrolentiform part.
The temporopontine fibres pass through the sublentiform part.
The corticopontine fibres relay (synapse) in the ipsilateral pontine nuclei.
The fibres arising from pontine nuclei cross the midline to relay in the cortex of the
opposite cerebellar hemisphere, thus forming the cortico-ponto-cerebellar pathway.
The cortico-ponto-cerebellar pathway is most recent in development and best
developed in man.
Pyramidal fibres: These arise in the cerebral cortex and relay in the lower motor
neurons of the brainstem and spinal cord.
They are of two types:
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1.
Corticonulear fibres synapse with the contralateral motor nuclei of the cranial
nerves innervating the head and neck muscles.
They occupy the genu of the internal capsule.
2.
Corticospinal fibres synapse
with the anterior horn cells of the
opposite half of the spinal cord, which
innervate the muscles of the upper
limb, trunk and lower limb.
The corticospinal fibres form several
discrete bundles in the anterior two
third of the posterior limb.
The fibres for the upper limb
are most anterior, followed in that
order, by the fibres for the trunk and
the lower limb.
Extrapyramidal fibres arise in the
cerebral cortex and relay into the
subcortical grey matter belonging to
the extrapyramidal system, (The red
nucleus, the corpus striatum, the
substantia nigra, etc.) They are
named according to their destinations,
e.g. corticorubral, corticostriate,
corticonigral, etc. respectively.
Most of the extrapyramidal fibres occupy the position near the corticospinal fibres in the
internal capsule, and are therefore affected in the lesions of the posterior limb.
Sensory fibres:
These are mostly thalamocortical fibres, which radiate from thalamus in differnt
directions to reach the widespread areas of the cerebral cortex and constitute most of
thalamic radiation.
Most of the thalamocortical fibres, are the 3rd order sensory neurons of various
sensory pathways conveying somesthetic sensations to the cerebral cortex.
The other fibres contributing the formation of thalamic radiation are corticothalamic
fibres.
According to the direction of these fibres the thalamic radiation is divided into following
subgroups:
Anterior thalamic radiation: These fibres are directed anteriorly and connect the
anterior and dorsomedial nuclei of thalamus to frontal cortex.
Superior thalamic radiation is directed superiorly. Its fibres pass through anterior limb
of internal capsule and connect the ventral tier of thalamic nuclei with the sensory cortex
of the frontal and parietal lobes.
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Posterior thalamic radiation is directed posteriorly.
The fibres of posterior thalamic radiation pass through the retrolentiform part of internal
capsule and connect the lateral geniculate body to the primary visual cortex, of the
occipital lobe forming optic radiation (geniculo-calcarine tract).
Inferior thalamic radiation is directed inferiorly.
Its fibres pass through sublentiform part of internal capsule and most of them connect
the medial geniculate body with the primary auditory area of the temporal lobe forming
auditory radiation.
Part
Motor Fibres
Sensory Fibres
Anterior Limb
Frontopontine
Anterior Thalamic
Radiation
Genu
Frontopontine,
Corticonuclear &
Corticospinal fibres for
Head & Neck
Superior Thalamic
Radiation (Anterior part
only)
Posterior Limb
Frontopontine,
Corticospinal
(Pyramidal) fibres for
Upper limb, trunk &
Lower Limb,
Corticorubral
(Extrapyramidal) fibres
Superior Thalamic
Radiation
Retrolentiform Part
Parietopontine &
Occipitopontine fibres
Posterior Thalamic
(Optic) Radiation
Sublentiform Part
Parietopontine &
Temporopontine fibres
Inferior Thalamic
(Auditary) Radiation
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