White Matter (Medulla) Dr. Zeenat Zaidi Cerebrum

White Matter Underlies the cortex Contains: Nerve fibersNerve fibers ( predominantly myelinated) NeurogliaNeuroglia Blood vesselsBlood vessels The nerve fibers originate, terminate or sometimes both, within the cortex

origintermination three typesDepending on their origin & termination, these nerve fibers are classified into three types: A.Association Projection B. Projection C. Commissural

Association Fibers Unite different parts of the same hemisphere Are of two kinds: Short association fibersShort association fibers: those connecting adjacent gyri, Long association fibers:Long association fibers: those connecting more distant gyri

Short Association Fibers Lie immediately beneath the gray substance of the cortex Connect together the adjacent gyri.

Long Association Fibers connect distant areasLong fibers travel through white matter to connect distant areas of cerebral cortex primary sensory areas association areas of the cerebral cortex to each otherLink the primary sensory areas in parietal, temporal and occipital lobes to the association areas of the cerebral cortex, and to each other

Superior longitudinal fasciculus: Superior longitudinal fasciculus: connects the frontal, parietal, temporal and occipital lobes Uncinate fasciculus: Uncinate fasciculus: connects frontal to temporal lobe, contributing to the regulation of behavior Arcuate fasciculus: Arcuate fasciculus: connect gyri in frontal to temporal lobes, important for language function Wernicke’s Area Broca’s Area Arcuate Fasciculus

Cingulum: Cingulum: connects frontal & parietal lobes to the para- hippocampal gyrus and adjacent temporal gyri Inferior longitudinal fasciculus: Inferior longitudinal fasciculus: connects occipital to temporal pole & contributes to visual recognition

Commissural Fibers Connect the corresponding regions of the two hemispheres Include: Corpus callosumCorpus callosum Anterior commissureAnterior commissure Hippocampal commissure (commissure of fornix)Hippocampal commissure (commissure of fornix) *(Posterior commissure, not a cerebral commissure) Corpus Callosum F P

fibrous bridge median longitudinal fissureIs a fibrous bridge located in the depth of the median longitudinal fissure Connects the two cerebral hemispheres together ShorterShorter craniocaudally than is the hemisphere Cranial end is nearer to the frontal pole of hemisphere as compared to caudal end to the occipital pole

corpus callosumThe fibers in the corpus callosum connect the corresponding regions of the two hemispheres with each other (except the inferior part of the temporal lobes) CCCC

Fibers linking the two frontal poles with each other, curve forward & form u-shaped anterior forceps (forceps minor) Fibers linking the two occipital poles with each other, curve backward & form u-shaped posterior forceps (forceps major) F P O CCCC Anterior forceps Posterior forceps

Parts of Corpus Callosum Splenium Body Genu Rostrum

Anterior Commissure Bundle of fibers runs transversely in front of the anterior columns of fornix Connects the inferior and middle temporal gyri & the olfactory regions of the two hemispheres Anterior column of fornix fornix IVF

Hippocampal Commissure Bundle of fibers runs transversely between the crura of the fornix Connect the two hippocampi with each other ( note that hippocampo- mamillary fibers do not cross)

Posterior Commissure Rounded band of white fibers Crossing the midline on the dorsal aspect of the upper end of the cerebral aqueduct (located between superior colliculus & pineal body) Connects the left and right midbrain. Plays important role in the bilateral pupillary reflex SC P IC Cerebral aqueduct

Projection Fibers Fibers running vertically through the hemispheres Consist of: Cortical afferent fibers conveying impulses to the cerebral cortex: (mainly thalamo-cortical fibers) Cortical efferent fibers carrying impulses away from the cortex to the lower centers: (corticostriate, corticobulbar, corticopontine, corticospinal, & descending autonomic fibers)

Deeper to the cortex, these fibers are arranged radially as the corona radiata Then the fibers converge to form a sheath, called the internal capsule, that passes between the thalamus and the basal ganglia Continue in the: Crus of the midbrain Basilar part of pons Pyramid of medulla oblongata Continue in the spinal cord as the corticospinal tracts corona radiata

Internal Capsule Bundle of projection fibers, passes through the interval between the thalamus and the basal ganglia BG Th

5 parts Has 5 parts: L 1.Anterior limb: between caudate (C) & lentiform (L) nuclei 2.Genu TH L 3.Posterior limb: between thalamus (TH) & lentiform nucleus ( L ) 4.Retrolenticular part: caudal to lentiform nucleus 5.Sublenticular part: below lentiform nucleus (can not be seen in this section ) C Th L

Anterior limbAnterior limb contains: Thalamocortical projections that connect mediodorsal nucleus of thalamus with the prefrontal cortex Frontopontine fibers GenuGenu contains: Corticobulbar fibers which connect the cortex with cranial nerve motor nuclei in the brainstem

Posterior limbPosterior limb contains: Corticospinal Corticobulbar Thalamocortical projections from: VPN to the primary somatosensory cortex VAN & VLN to motor regions of cortex

Retrolenticular part contains thalamocortical projections: Geniculocalcarine fibers (visual radiation), from the lateral geniculate nucleus of thalamus to the visual cortex in the occipital lobe & few Geniculotemporal fibers (auditory radiation) from the medial geniculate nucleus of thalamus to the auditory cortex in the temporal lobe

Sublenticular partSublenticular part contains thalamocortical projections: geniculo- temporal fibers (auditory radiation) from the medial geniculate nucleus of thalamus to the auditory cortex in the temporal lobe

Clinical Notes Bilateral lesion of the inferior longitudinal fasciculus (as in carbon monoxide poisoning), leads to: Loss of identification of the:Loss of identification of the: Nature of objects (object agnosia)Nature of objects (object agnosia) Individual faces (prosopagnosia)Individual faces (prosopagnosia) The elementary vision remains intactThe elementary vision remains intact Damage to corpus callosum leads to split-brain syndrome. The two half of the brain behave relatively autonomously Damage to splenium of corpus callosum leads to posterior disconnection syndrome of alexia (cannot understand written material) without agraphia (can speak and write without difficulty)

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