THE CENTRAL NERVOUS SYSTEM 57
thin, dense stripe on the superolateral margin of the lateral ventricle on higher cuts, where it should not be confused with heterotopic cortical tissue. The claustrum can be seen on CT and MRI (Fig. 2. 3c) as a high-attenuation stripe, separated from the putamen by the external capsule and from the insula by the extreme capsule. Magnetic resonance imaging MRI is sensitive to paramagnetic substances such as iron, which may be deposited in the globus pallidus, giving this a different signal' intensity from the lentiform nucleus. The connecting fibres between the lentiform and caudate nuclei that cross the anterior limb of the internal capsule and are responsible for the name corpus striatum can be seen on MR imaging. Ultrasound examination of the neonatal brain (Fig. 2. 8) The thalami and caudate heads can be seen on coronal images. Parasagittal scans are angled to show most of the lateral ventricle on one image. These show the head of the caudate and the thalamus forming the floor of the lateral ventricle. The caudothalamic groove between these is the commonest site of haemorrhage in preterm infants. Strands of grey matter connect the head of the caudate nucleus with the putamen of the lentiform nucleus across the anterior limb of the internal capsule. The resulting striated appearance gives rise to the term corpus striatum. The function of the corpus striatum is not well understood. It is part of the extrapyramidal system, and influences voluntary motor activity. Cortical afferents enter the putamen and caudate nucleus, which send efferents to the globus pallidus. This in turn sends efferents to the hypothalamus, brainstem and spinal cord. Claustrum This thin sheet of grey matter lies between the putamen and the insula. It is separated medially from the putamen by the external capsule and bounded laterally by a thin sheet of white matter (the extreme capsule) just deep to the insula. The claustrum is cortical in origin but its function is unknown. Radiological features of the basal ganglia CT and MRI On axial CT or MRI the head of the caudate nucleus can be seen projecting into the anterior horn of the lateral ventricle on slices taken at ventricular level (see Fig. 2. 3c). The head of the caudate nucleus is usually more radiodense than the lentiform nucleus or the thalamus, especially in older subjects. The body of the caudate nucleus is seen as a
THALAMUS, HYPOTHALAMUS AND PINEAL GLAND The structures around the third ventricle include the thalamus, hypothalamus and pineal gland. Together with the habenula these form the diencephalon.
Thalamus These paired, ovoid bodies of grey matter lie in the lateral walls of the third ventricle, from the interventricular foramen anteriorly to the brainstem posteriorly. Each has its apex anteriorly and a more rounded posterior end called the pulvinar. The thalamus is related laterally to the internal capsule and, beyond that, to the lentiform nucleus. The body and tail of the caudate nucleus are in contact with the lateral margin of the thalamus. The superior part of the thalamus forms part of the floor of the lateral ventricle. The thalamus is attached in approximately 60% of cases to the thalamus of the other side by the interthalamic adhesion or massa intermedia. This is not a neural connection. Most thalamic nuclei are relay nuclei of the main sensory pathways. Medial and lateral swellings on the posteroinferior aspect of the thalamus are called the geniculate bodies. The medial geniculate body is attached to the inferior colliculus and is involved in the relay of auditory impulses. The lateral geniculate body is attached to the superior colliculus and is involved with visual impulses. The thalamus receives its blood supply from thalamostriate branches of the posterior cerebral artery.
