WHITE FIBRES IN THE BRAIN

SABIHA M HAQIIMC

Learning objectives Classify white fibres in the CNS Describe each type of fibres, based on

function and location Describe the clinical manifestations of a lesion

of internal capsule

Types of white fibres in the brainCommisural fibresAssociation fibresProjection fibres

Association fibres, Important Facts Most named association bundles

(superior longitudinal, arcuate, inferior longitudinal, inferior occipitofrontal, uncinate, and superior occipitofrontal fasciculi) interconnect lobes on the same side

The cingulum, fornix, and stria terminalis are association bundles of the limbic system

Association fibres

Commisural fibres Connect the same areas of brain on two

sides1. Largest commisure is corpus callosum

which connests neocortex of two sides2. Anterior commisure3 & 4. Posterior and habenular commisures

connect the old cortex on two sides

Commisural fibres The corpus callosum and anterior

commissure, which interconnect symmetrical cortical regions, exchange information between the left and right sides.

After transection of the commissures, a task that is newly learned with one hand cannot be performed by the other.

Sensory data that enter only the right hemisphere cannot be put into words because of disconnection from the language areas in the left hemisphere.

Commisural fibres (red)

Commisural fibres

Commisural fibres

Corpus callosum, parts

Anterior commisure

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)

PROJECTION FIBRES Most projection fibers pass through the

internal capsule. All parts of the internal capsule contain

thalamocortical and corticothalamic fibers.

Motor fibers, including those of the pyramidal system, descend in the posterior limb of the internal capsule. A small infarct in this area can cause contralateral hemiplegia.

Internal capsule

PROJECTION FIBRES

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 the Crus Cerebri of the midbrain.

The Internal Capsule

The Internal Capsule 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.

Even a small lesion there may produce a widespread paralytic effects and sensory loss in the opposite half

The Internal Capsule 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.

The Internal 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.

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:

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.

Parts of the internal capsule

3. Genu: It is the bend between the anterior and posterior limbs with concavity of the bend facing laterally.

4. Sublentiform part: It lies below the lentiform nucleus.

5. Retrolentiform part: It lies behind the lentiform nucleus.

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 radiationThey pass through the anterior limb of

the internal capsule.

Corona Radiata

Ascending Fibres

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

Ascending Fibres 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.

Ascending Fibres 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 geniculate body to the acoustic area of the cerebral cortex.

These fibres pass through the sublentiform part of the internal capsule.

Descending Fibres1. 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.

Descending Fibres2. Corticopontine fibres:

a) Frontopontine fibres are the most numerous. They pass through the anterior limb, genu, and posterior limb of the internal capsule.

b) Parietopontine fibres pass mainly through the retrolentiform part. Some fibres also pass through the sublentiform part.

c) Temporopontine fibres pass through the sublentiform part.

d) Occipitopontine fibres pass through the retrolentiform part.

Descending Fibres3. Corticothalamic fibres:

These pass from various parts of the cerebral cortex to the thalamus.

They form part of the thalamic radiations

Constituent fibres of the Internal Capsule

Constituent fibres of the Internal Capsule

Sensory Fibres Motor Fibres PartAnterior Thalamic Radiation

Frontopontine Anterior Limb

Superior Thalamic Radiation (Anterior part only)

Frontopontine,Corticonuclear & Corticospinal fibres for Head & Neck

Genu

Superior Thalamic Radiation

Frontopontine, Corticospinal (Pyramidal) fibres for Upper limb, trunk & Lower Limb, Corticorubral (Extrapyramidal) fibres

Posterior Limb

Posterior Thalamic (Optic) Radiation

Parietopontine & Occipitopontine fibres

Retrolentiform Part

Inferior Thalamic (Auditary) Radiation

Parietopontine & Temporopontine fibres

Sublentiform Part