• Each cerebral hemisphere includes a large volume of white matter, sometimes called the medullary center, that accommodates vast numbers of axons running to and from all parts of the cortex

• Axons that establish connections between the cortex and subcortical gray matter continue into the internal capsule.

Cerebral White Matter

• Three types of axons are present in the cerebral white matter

• Association fibers are confined to a hemisphere and connect one cortical area with another.

• Many of these fibers accumulate in named longitudinally running bundles that can be displayed by dissection.

• Most named association bundles (superior longitudinal, arcuate, inferior longitudinal, inferior occipitofrontal, uncinate, and superior occipitofrontal fasciculi) interconnect lobes.

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

• 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.

• Commissural fibers connect the cortices of the two hemispheres; most are located in the corpus callosum, and the remainder are located in the anterior commissure.

• Projection fibers establish connections between the cortex and such subcortical structures as the corpus striatum, thalamus, brain stem, and spinal cord.

• They are afferent (corticopetal) or efferent (corticofugal) with respect to the cortex.

• Most corticopetal projection fibers originate in the thalamus; some ascend from nuclei in the hypothalamus and brain stem

ASSOCIATION FIBRES

• Association fibers are the most numerous of the three types of fiber noted.

• Operative procedures, vascular accidents, and lesions that transect the fasciculi may lead to dysfunction by disconnecting functionally related regions of the cerebral cortex.

• The cingulum, which is most easily displayed by dissection in the cingulate gyrus, is an association fasciculus of the limbic lobe.

• The axons in this longitudinal bundle run in both directions and interconnect the cingulate gyrus, parahippocampal gyrus of the temporal lobe, and septal area below the genu of the corpus callosum.

The cingulum

• easily displayed by dissection in the cingulate gyrus

• an association fasciculus of the limbic lobe.• axons in this longitudinal bundle run in both

directions and interconnect the cingulate gyrus, parahippocampal gyrus of the temporal lobe, and septal area below the genu of the corpus callosum.

Superior longitudinal fasciculus

• also known as the arcuate fasciculus• runs in an anteroposterior direction above the

insula, and many of the fibers turn downward into the temporal lobe.

• It is similar to the other large association bundles in that it consists of axons of various lengths that enter or leave the fasciculus at any point along its course.

• provides important communications between cortices of the parietal, temporal, and occipital lobes and the cortex of the frontal lobe.

• These provide a pathway whereby interpreted sensory signals (especially visual and proprioceptive) from the parietal cortex influence the formulation in the frontal lobe of neuronal programs for appropriate movements.

• provide a pathway whereby interpreted sensory signals (especially visual and proprioceptive) from the parietal cortex influence the formulation in the frontal lobe of neuronal programs for appropriate movements

• The arcuate fasciculus also includes fibers that connect the receptive (sensory) and expressive (motor) language areas

• An inferior longitudinal fasciculus, beneath the lateral and ventral surfaces of the occipital and temporal lobes, is difficult to demonstrate by dissection.

• The inferior occipitofrontal fasciculus and uncinate fasciculus are components of a single association system

• The fibers are compressed into a well-defined bundle below the insula and lentiform nucleus.

• The longer part of the fiber system, extending the length of the hemisphere, is the inferior occipitofrontal fasciculus.

• The uncinate fasciculus is the part that hooks around the stem of the lateral sulcus to connect the frontal lobe, especially cortex on its orbital surface, with cortex in the region of the temporal pole.

The superior occipitofrontal fasciculus/subcallosal bundle

• located deep in the hemisphere • Its fibers spread out to cortex of the frontal

lobe and to cortex in the posterior part of the hemisphere

Arcuate fibers

• Connect adjacent gyri. • Oriented at right angles to the gyri and bend

sharply under the intervening sulci. • Spread of activity along a gyrus or sulcus is

provided by other subcortical association fibers and by axons within the cortex

COMMISSURES

Corpus Callosum

• In laboratory animals, commissural fibers from an area of cortex in one hemisphere have been shown to terminate in the corresponding area and in cortex closely related functionally with that area, in the other hemisphere.

• In laboratory animals, commissural fibers from an area of cortex in one hemisphere have been shown to terminate in the corresponding area and in cortex closely related functionally with that area, in the other hemisphere.

• The hand areas of the primary somatosensory cortices and large parts of the primary visual areas are notable in that they are not directly connected by commissural fibers.

• They communicate functionally, however, through callosal fibers that connect the adjacent association areas..

• Much of the cortex of the temporal lobe makes its commissural connections by way of the anterior commissure rather than the corpus callosum

• The body/trunk of the corpus callosum is the compact part of the commissure in and near the midline.

• As they pass laterally, the callosal fibers intersect association bundles and projection fibers

• The trunk of the corpus callosum is considerably shorter than the hemispheres; this accounts for the enlargements of the ends, which are the splenium posteriorly and the genu anteriorly

• The splenium and the radiations that connect the occipital lobes constitute forceps major. a

• the genu and the radiations that connect the frontal lobes form the forceps minor.

• The genu tapers into the rostrum of the corpus callosum, which is continuous with the lamina terminalis forming the anterior wall of the third ventricle.

• Callosal fibers that form a thin sheet over the temporal horn of the lateral ventricle constitute the tapetum (see Fig. 16-6), which provides some of the communication between the cortices of the temporal lobes.

Projection fibres/internal capsule

• Interconnections between cerebral cortex and Subcortical structures travel through the Internal capsule

• compact bundle of fibres in the cleft between lenticular nucleus [laterally] and thalamus and head of the caudate nucleus [medially]

• Site for fibres leaving and entering thalamus• Other fibres passing through are

corticopontine, corticobulbar and corticospinal

• Also fibres going to parts of basal nuclei• All fibres fan out as corona radiata and reach

cortical destinations

Parts

• Based on relationship of each part to lenticular nucleus

• Anterior limb- between lenticular nucleus and head of caudate nucleus

• Posterior limb- between lenticular nucleus and thalamus

• Genu – junction between the two

• Retrolenticular- posterior to lentiform nucleus• Sublentiform – inferior to the lenticular

nucleus• Curved sheet – not all parts seen in one

section

Components

Anterior limb• Fibres interconnecting anterior nucleus and

the cingulate gyrus and most of those interconnecting the dorsomedial nucleus and prefrontal cortex

• Also frontopontine fibres- to ipsilateral pontine nuclei

Posterior limb

• Fibres connecting VA and VL with motor and premotor cortex

• Corticospinal and corticobulbar fibres • Somatosensory fibres from VPL/VPM to

postcentral gyrus• Corticospinal fibres

Genu

• Transition zone between anterior and posterior limbs

• Frontopontine fibres• Fibres interconnecting VA and VL with motor

and premotor cortex• Corticobulbar fibres to motor nuvlei of cranial

nerves

Retrolenticular part

• Most fibres connecting thalamus with posterior portions of cerebral hemispheres [ bidirectional between parietal-occipital-temporal association cortex and the pulvinar/LP complex

• Also include optic radiation• Corticopontine fibres [parietal lobe]

Sublenticular part

• Remainder of optic radiation• Auditory radiation

• 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.