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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 1|Page 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 2|Page 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 āVā 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: 3|Page 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: 4|Page 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. 5|Page 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 6|Page