White matter of the hemispheres

THE CENTRAL NERVOUS SYSTEM 53

WHITE MATTER OF THE HEMISPHERES

There are three types of fibre within the cerebral hemispheres: • Commissural fibres, which connect corresponding areas of the two hemispheres; • Association (arcuate) fibres, which connect different parts of the cortex of the same hemisphere; and • Projection fibres, which join the cortex to lower centres.

Commissural fibres

The corpus callosum (Figs 2. 4 and 2. 5) The corpus callosum is a large midline mass of commissural fibres, each of which connects corresponding areas of both hemispheres. It is approximately 10 cm long and becomes progressively thicker towards its posterior end. Named parts include the: • Rostrum - this is the first part, which extends anteriorly from the anterior commissure (vide infra); • Genu - this is the most anterior part where it bends sharply backwards; • Trunk (body) - this is the main mass of fibres extending from the genu anteriorly to the splenium posteriorly. It lies below the lower free edge of the falx cerebri. The anterior cerebral vessels run on its superior surface; and • Splenium - this is the thickened posterior end.

In cross-section, fibres from the genu that arch forward to the frontal cortex on each side are called forceps minor, and fibres from the splenium passing posteriorly to each occipital cortex are called forceps major. Fibres extending laterally from the body of the corpus callosum are called the tapetum. These form part of the roof and lateral wall of the lateral ventricle.

Fig. 2. 5 MRI scan of brain: midline sagittal image.

1. Frontal lobe 2. Parietal lobe 3. Occipital lobe 4. Rostrum of corpus callosum 5. Genu of corpus callosum 6. Body of corpus callosum 7. Splenium of corpus callosum 8. Septum pellucidum 9. Foramen of Monro 10. Fornix 11. Massa intermedia of thalami 12. Third ventricle 13. Supraoptic recess of third ventricle 14. Suprapineal recess of third ventricle 15. Pineal gland 16. Optic chiasm 17. Midbrain 18. Interpeduncular cistern 19. Aqueduct of Sylvius 20. Quadrigeminal plate (superior and inferior colliculi) 21. Quadrigeminal plate cistern 22. Fourth ventricle 23. Vermis of cerebellum 24. Pons 25. Tonsil of cerebellum 26. Prepontine cistern 27. Medulla oblongata 28. Odontoid process 29. Cisterna magna 30. Clivus 31. Pituitary 32. Tentorium cerebelIi 33. Spinal cord

Anterior commissure (Fig. 2. 6) This is a bundle of fibres in the lamina terminalis in the anterior wall of the third ventricle. The fibres pass laterally in an arc indenting the inferior surface of the globus pallidus. The anterior commissure is part of the olfactory system and connects the olfactory bulbs, the cortex of the anterior perforated substance and the piriform areas.

Habenular commissure

This small commissure is situated above and anterior to the pineal body. It unites the habenular striae, which are fibres from the olfactory centre that pass posteriorly along upper surface of each thalamus and unite in a ' U' configuration in this commissure.

Posterior commissure This is situated anterior and inferior to the pineal body It connects the superior colliculi, which are concerned w i th light reflexes (see brainstem). Hippocampal commissure This is the commissure of the fornix (see below).

Projection fibres

These fibres join the cerebral cortex to lower centres. Some are afferent and some efferent. They are called the internal capsule, where they lie lateral to the thalamus and the corona radiata as they fan out between the internal capsule and the cerebral cortex.

Internal capsule (see Fig. 2. 3c) This contains sensory fibres from the thalamus to the sensory cortex and motor fibres from the motor cortex to motor nuclei in the brainstem, corticobulbar tracts and, in the spinal cord, the corticospinal (pyramidal) tracts.

In cross-section, the internal capsule has an anterior limb between the caudate and lentiform nuclei and a posterior limb between the lentiform nucleus and the thalamus. Both limbs meet at a right-angle called the genu.

Fig. 2. 6 Axial T2 image to show the anterior commissure.

The anterior limb is composed mainly of frontopontine fibres. The genu and the anterior two-thirds of the posterior limb contain motor fibres. The most anterior fibres at the genu are those of the head. Fibres to the arm, hand, trunk, leg and perineum lie progressively more posteriorly. Haemorrhage or thrombosis of thalamostriate arteries supplying this area leads to paralysis of these muscles.

Behind these fibres on the posterior limb and on the retrolentiform part of the internal capsule are parietopontine and occipitopontine fibres and the sensory fibres. More posteriorly are the visual fibres that extend towards the occipital pole as the optic radiation. Most posterior of all are the auditory fibres.

Radiological features of the commissural and projection fibres

Plain films of the skull

Calcification of the habenular commissure is a common finding on skull radiographs. It is found anterior and superior to the pineal gland if this is also calcified. Typically the calcification is C-shaped, w i th the open part of the letter facing backwards. Some authors suggest that this calcification is in the choroid plexus of the third ventricles the taenia habenulare - rather than in the commissure.

CT and MRI

The corpus callosum cannot be well seen on axial CT slices. The internal capsule is seen as a V-shaped low-attenuation or high T1 signal structure (see Fig. 2. 3c) between the caudate and lentiform nuclei anteriorly and the lentiform and thalamus posteriorly. The rostrum, genu, body and splenium can be seen on sagittal MRI (Fig. 2. 5). The anterior and posterior commissures can also be seen on this view. A line joining the anterior and posterior commissures, the AC-PC line, is used as a reference in image-guided procedures. On coronal MRI scans (Fig. 2. 7) the body of the corpus callosum and the tapetum can be seen superior to the lateral ventricles and the internal capsule can be seen lateral to the thalami. On this view the anterior commissure may also be visible inferior to the third ventricle, but this commissure is best seen as an arc of fibres on axial MRI

(Fig. 2. 6).

Ultrasound examination of the neonatal brain (Fig. 2. 8) The corpus callosum can be seen on midline sagittal scans as a thin band of tissue between the pericallosal artery in the pericallosal sulcus superiorly, and the fluid of the cavum septum pellucidum inferiorly. It is seen below the interhemispheric fissure on coronal scans, where both its upper and lower surfaces are perpendicular to the beam. On coronal scans the internal capsule can be lateral to the thalamus.

1. Superior sagittal sinus 2. Interhemispheric fissure 3. Tapetum 4. Body of corpus callosum 5. Septum pellucidum 6. Fornix 7. Third ventricle

8. Sphenoid sinus 9. Sylvian fissure 10. Insula 11. External capsule 12. Lentiform nucleus 13. Head of caudate nucleus 14. Internal capsule

Basal ganglia (Fig. 2. 9) This subcortical grey matter includes: • The corpus striatum - the caudate and lentiform nuclei; • The amygdaloid body; and • The claustrum.

Caudate nucleus

This nucleus is described as having a head, body and tail. Its long, thin tail ends in the amygdaloid nucleus. The caudate nucleus is highly curved and lies within the concavity of the lateral ventricle. Thus its head projects into the floor of the anterior horn and its body lies along the body of the lateral ventricle. Its tail lies in the roof of the inferior horn of this ventricle.

Lentiform nucleus

This is shaped like a biconcave lens. It is made up of a larger lateral putamen and a smaller medial globus pallidus. Medially, it is separated from the head of the caudate nucleus anteriorly, and from the thalamus posteriorly by the internal capsule. A thin layer of white matter on its lateral surface is called the external capsule.

Fig. 2. 8 Ultrasound of infant brain: (a) coronal image through the frontal horns of the lateral ventricles; (b) coronal image through body of the lateral ventricle; (c) midline sagittal image; (d) parasagittal image through lateral ventricle.

1. Interhemispheric fissure 2. Sulci 3. Frontal horn of lateral ventricle 4. Corpus callosum 5. Caudate above, lentiform nucleus below, separated by internal capsule. (These three are not distinguished separately) 6. Sylvian fissure 7. Brainstem 8. Parahippocampal gyrus of temporal lobe 9. Choroids plexus in atrium of lateral ventricle 10. Calcarine sulcus 11. Genu of corpus callosum 12. Cavum septum pellucidum 13. Cingulate gyrus 14. Midbrain 15. Pons 16. Medulla 17. Fourth ventricle 18. Vermis of cerebellum 19. Third ventricle 20. Body of lateral ventricle 21. Hippocampus 22. Fornix