White Matter of Cerebrum

 WHITE  MATTER  OF  CEREBRUM

• Nerve fibres forming the white matter of cerebrum are derived from the axons of pyramidal cells in the cerebral cortex.
• They are classified into 3 groups –
• Association fibres
• Projection fibres
• Commissural fibres 

ASSOCIATION  FIBRES

• These fibres connect the cortical areas of the same hemisphere.
• They consist of two sets – Short & Long fibres.

Short  Association  Fibres :-

• They connect the adjacent gyri.
• They are intra-cortical or sub-cortical in arrangement.

Long  Association  Fibres :-

• They connect the gyri which are separated from one another by considerable interval.
• They are arranged in 5 bundles.

Cingulum –

• It is an arched bundle lies within the cingulate gyrus.
• The fibres belong to limbic lobe and form a part of Papez circuit.

Uncinate Fasciculus –

• It is an arched bundle and hooks around the stem of lateral sulcus.
• It connects the Broca’s area (44 & 45) with the psychical cortex (38) of temporal pole and higher auditory association area (22) of superior temporal gyrus.

Superior Longitudinal Fasciculus –

• It connects the frontal eye field (8) and occipital eye field (18 & 19).

Inferior Longitudinal Fasciculus –

• It connects the temporal lobe with occipital lobe (18 & 19).

Fronto-occipital Fasciculus –

• It connects the frontal and occipital lobes.
• It lies medial to the corona radiata and forms a sub-callosal bundle.

PROJECTION  FIBRES

• These fibres connect the cerebral cortex with sub-cortical grey matter of basal ganglia, thalamus, nuclei of brain stem and spinal cord.
• They include cortico-fugal and cortico-petal fibres.
• The projection fibres of allocortex are represented by Fimbria & Fornix of hippocampal formation.
• The neo-cortical projection fibres form Corona radiata & Internal capsule.

FIMBRIA :-

• Axons of pyramidal cells of hippocampus form output fibres known as alveus.
• Alveus fibres converge along the medial margin of hippocampus and form fimbria hippocampi.
• Fimbria proceeds backwards overlapping the dentate gyrus and reach the splenium of corpus callosum where it continuous with fornix.

FORNIX :-

• It forms the sole efferent projection fires of hippocampus.
• It begins as a continuation of alveus and fimbria.
• On reaching the splenium of corpus callosum fimbria divides into dorsal & ventral fornix.

Dorsal  Fornix –

• It surrounds the splenium and continuous with gyrus fasciolaris & indusium griseum.

Ventral  Fornix –

Crura of fornix –

• Ventral fornix sweeps forwards below the splenium around the pulvinar end of thalamus and divides into a pair of crura of fornix.
• Medial margins of both the crura are connected by transverse fibres called commissure of fornix or hippocampal commissure.
• Crura along with commissure of fornix known as body of fornix.

Body of fornix –

• It is triangular in outline with apex directed in front.
• Upper surface is lined by ependyma of lateral ventricle and connected with corpus callosum by bilaminar septum pellucidum.
• Lower surface is separated from the ependyma of third ventricle by tela choroidea containing choroid plexus and internal cerebral veins.
• Lower surface is separated from the thalamus by choroidal fissure through which choroid plexus projects into the lateral ventricle.
• At interventricular foramen the apex of the body diverges into a pair of columns of fornix.

Columns of fornix –

• Each column is separated by anterior commissure into pre & post commissural parts.
• Pre-commissural fornix is continuous with para-terminal gyrus.
• Post-commissural fornix reaches the mamillary body.

Connections  of  Fornix –

• Fibres of dorsal fornix synapse with gyrus fasciolaris, indusium griseum, cingulate gyrus and reach the septal nuclei of septum pellucidum through corpus callosum.
• Pre-commissural fornix connects with paraterminal gyrus, pre-optic and anterior hypothalamic nuclei.
• Post-commissural fornix terminates in anterior nucleus of thalamus, hypothalamic nuclei, mamillary body and habenular nucleus.

CORONA  RADIATA :-

• Projection fibres from the entire neocortex converge towards the corpus striatum in the form of fan-shaped fibres known as corona radiata.
• It is intersected by corpus callosum and anterior commissural fibres.
• It intervenes between the fronto-occipital fasciculus medially and superior longitudinal fasciculus laterally.
• It is continuous below with the fibres of internal capsule.

INTERNAL  CAPSULE :-

• It is a compact band of neocortical projection fibres.
• It is V-shaped on horizontal section with the concavity directed laterally.
• It is continuous above with the corona radiata and below with the crus cerebri of mid brain.

Relations :-

• Medially – Head of caudate nucleus, Thalamus.
• Laterally – Lentiform nucleus.

Parts :-

From before backwards –

• Anterior limb- It intervenes between the head of caudate nucleus and the lentiform nucleus.
• Genu- It forms an angulation between the anterior and posterior limbs.
• Posterior limb- It occupies the interval between thalamus medially and the lentiform nucleus laterally.
• Sub-lentiform part- It extends below the lentiform nucleus.
• Retro-lentiform part- It extends backwards as a continuation of posterior limb along the lateral wall of posterior horn of lateral ventricle.

Fibres  Passing  Through :-

Anterior Limb –

• Fronto-pontine fibres
• Anterior thalamic radiation fibres
• Medial fore brain bundle fibres
• Cortico-striate fibres

Genu –

• Cortico-nuclear / cortico-bulbar fibres
• Cortico-reticular fibres
• Superior thalamic radiation fibres

Posterior Limb –

• Cortico-spinal tract fibres
• Cortico-rubral tract fibres
• Cortico-striate fibres
• Fronto-pontine fibres
• Parieto-pontine fibres
• Superior thalamic radiation fibres
• Nigro-striate fibres
• Thalamo-striate fibres

Sub-lentiform Part –

• Auditory radiation fibres
• Optic radiation fibres (from Meyer’s loop)
• Temporo-pontine fires
• Parieto-pontine fibres

Retro-lentiform Part –

• Optic radiation fibres
• Posterior thalamic radiation fibres
• Parieto-pontine fibres
• Occipito-pontine fibres

Arterial  Supply :-

• Central branches of cerebral arteries supply the internal capsule.
• The central arteries are end-arteries.

Anterior limb is supplied by –

• Striate branch of Anterior cerebral artery
• Recurrent branch of Anterior cerebral artery (Artery of Heubner)
• Striate branch of Middle cerebral artery (Charcot’s artery of cerebral haemorrhage)

Genu is supplied by –

• Recurrent branch of Anterior cerebral artery
• Striate branch of Middle cerebral artery
• Direct branches from Internal carotid artery

Posterior limb is supplied by –

• Striate branch of Middle cerebral artery
• Anterior choroid artery branch
• Postero-lateral branches of posterior cerebral artery

Sub-lentiform part is supplied by –

• Striate branch of Anterior cerebral artery
• Anterior choroid artery branch
• Posterior cerebral artery branch

Retro-lentiform part is supplied by –

• Postero-lateral branches of posterior cerebral artery

Applied  Aspects :-

Stroke –

• Haemorrhage within the internal capsule due to rupture of arterial branches, in the presence of high blood pressure and arterio-sclerosis, causing sudden collapse of the individual.
• It produces paralysis (Hemiplegia) as the pyramidal tract fibres are located in the genu and posterior limb.
• Signs – spasticity, increased tendon reflexes and positive Babinski’s response.
• Sensory loss may be produced when superior thalamic radiation fibres are involved.

COMMISSURAL  FIBRES

• These fibres connect the wide areas of cerebral cortex of two hemispheres across the middle line.
• Most of the fibres connect identical areas of two cerebral hemispheres – Homotopical fibres.
• Some fibres connect the non-identical areas – Heterotopical fibres.
• 5 sets of commissural bands are present in the brain –
• Corpus callosum
• Anterior commissure
• Posterior commissure
• Habenular commissure
• Hippocampal commissure

CORPUS  CALLOSUM :-

• It is the largest band of commissural fibres of the neocortex.
• It connects the areas of cerebral cortex of two hemispheres except the primary visual area (17) and hand foot somesthetic areas (3,2,1).
• It forms an arched band with upward convexity.

Parts :-

From behind forwards –

• Splenium- It is the enlarged posterior end lies 6 cm in front of occipital pole.
• Trunk / Body- It is the lengthiest part.
• Genu- It is an abrupt bent situated about 4 cm behind the frontal pole.
• Rostrum- A prolonged thin part from the genu directed downward and backward, continuous with the lamina terminalis.

Relations :-

Upper Convex Surface –

• It forms the floor of median longitudinal fissure.
• Related with the lower free margin of falx cerebri containing inferior sagittal sinus.
• It is covered by a sheet of grey matter known as indusium griseum which continue as gyrus fasciolaris over the splenium.
• On each side it is overlapped by cingulate gyrus separated by callosal sulcus and related to anterior cerebral vessels.

Lower Concave Surface –

• It is lined by ependyma of lateral ventricle.
• It is attached to septum pellucidum through which it is connected to body of fornix.
• Splenium comes in direct contact with the crura of fornix.
• Gap between the crura of fornix and ependymal roof of third ventricle is known as transverse fissure which transmits the tela choroidea of third ventricle and great cerebral vein.
• Trunk forms the roof of central part and anterior horn of lateral ventricle.
• Genu forms the anterior wall of anterior horn of lateral ventricle.
• Rostrum forms the floor of anterior horn of lateral ventricle partly.

Connections :-

• Rostrum fibres connect the orbital surfaces of both the hemispheres.
• Genu fibres connect the medial and lateral surfaces of frontal lobes as a curved bundle known as Forceps minor.
• Trunk fibres connect the wide areas of cerebral cortex.
• Some of trunk fibres are known as Tapetum which forms the roof and lateral wall of posterior horn & roof of inferior horn of lateral ventricle.
• Splenial fibres extend backwards as a curved elongated bundle known as Forceps major which connects the occipital lobes.

Functions :-

Transfer of learning process –

• Through the fibres of corpus callosum the memory is utilised by both the hemispheres for motor expression.
• When the right hand is doing some precision work the left hand will have some knowledge about the activities of right hand through corpus callosum.
• Corpus callosum transfers memory traces from the trained hemisphere to the uneducated hemisphere.

Transfer of speech function –

• Wernicke’s area (22), angular gyrus (39), supramarginal area (40) and Broca’s areas (44, 45) of both the hemispheres are connected through corpus callosum.

Applied  Aspects :-

• When the left (dominant) hemisphere is damaged before the age of 6-8 years, speech centres are transferred to the right (non-dominant) hemisphere through corpus callosum.
• Congenital absence or surgical transection of corpus callosum exhibits little disturbance of functions.
• Commissurectomy is sometimes made, in order to confine the epileptic discharge to one hemisphere, in severe epilepsy.

ANTERIOR  COMMISSURE :-

• It is a small compact bundle of commissural fibres of the neocortex and allocortex.
• It stretches across the middle line in the substance of lamina terminalis.
• It lies in front of columns of fornix in the anterior wall of third ventricle.
• Laterally it splits into anterior and posterior bundles.

Anterior bundle –

• It connects the anterior perforated substance, olfactory tracts, entorhinal cortex of limbic system.

Posterior bundle –

• It connects the anterior part of middle and inferior temporal gyri.

POSTERIOR  COMMISSURE :-

• It forms a bundle of fibres traversing across the ventral lamina of pineal stalk.
• These fibres fan out bilaterally and connect pretectal nuclei, nuclei of posterior commissure.
• Lesions in this commissure may reduce the consensual pupillary light reflex.

HABENULAR  COMMISSURE :-

• It is formed by the crossed fibres of stria medullaris through the dorsal lamina of pineal stalk.
• They connect the habenular nuclei of both sides.

HIPPOCAMPAL  COMMISSURE :-

• Crura of the fornix are connected by transverse fibres known as commissure of fornix or hippocampal commissure.
• Along with the crura it forms the body of fornix.
• These fibres connect the efferent projection fibres from hippocampus of both sides.
• The commissure is attached to corpus callosum through septum pellucidum.
• It forms the roof of third ventricle and separates it from lateral ventricle.