Medical brain notes 24

  THE SUBTHALAMIC NUCLEUS         As explained above the subthalamic nucleus which has traditionally been described as a part of the subthalamic region is now grouped functionally with the basal nuclei which are considered.         The subthalamic nucleus is shaped like a biconvex lens. It lies between the thalamus above, and the substantia nigra below (Figs. 13.2, 13.28). The upper end of the red nucleus is close to it. The nucleus is closely related to the zona incerta, the fascicularis lenticularis intervening between the two.         The fibres connecting the subthalamic nucleus to the globus pallidus form a bundle called the  subthalamic fasciculus  which passes through the internal capsule.

  DEVELOPMENT OF THE DIENCEPHALON        The establishment of the diencephalon has been described. We have seen that with the great enlargement of the telencephalon, the diencephalon comes to lie on the medial side of the cerebral hemisphere. The lateral wall of the diencephalon is subdivided by the appearance of the epithalamic and hypothalamic sulci. The central part lying between the two sulci enlarges to form the thalamus. The part above the epithalamic sulcus remains small and forms the epithalamus which is represented by the habenular nuclei and the pineal body. The part below the hypothalamic sulcus forms the hypothalamus.

  The Basal Nuclei          The basal nuclei (or basal ganglia) are large masses of grey matter situated in the cerebral hemispheres. Classically, the following have been included under the definition of basal nuclei (Fig. 13.2). All these are telencephalic in origin. 1.                Caudate nucleus. 2.                Lentiform nucleus , which consists of two functionally distinct parts, the  putamen  and the  globus pallidus . 3.                Amygdaloid nuclear complex . 4.                The  claustrum  is often included among the basal ganglia.          Various other terms commonly used for some of the above nuclei are as follows. The caudate nucleus and the lentiform nucleus together constitute the  corpus striatum.  This consists of two functionally distinct parts. The caudate nucleus and the putamen form one unit called the  striatum,  while the globus pallidus forms the other unit, the  pallidum.          Recent researches have shown that a number of masses of grey matter, o

  The Caudate Nucleus          The caudate nucleus is a C-shaped mass of grey matter (Fig.14.1). It consists of a large head, a body and a thin tail. The nucleus is intimately related to the lateral ventricle. The head of the nucleus bulges into the anterior horn of the ventricle and forms the greater part of its floor (Fig.20.3). The body of the nucleus lies in the floor of the central part (Fig. 20.2); and the tail in the roof of the inferior horn of the ventricle (Fig. 20.5). The anterior part of the head of the caudate nucleus is fused, inferiorly, with the lentiform nucleus. This region of fusion is referred to as the  fundusstriati . The fundus striati is continuous, inferiorly,with the anterior perforated substance. The anterior end of the tail of the caudate nucleus ends by becoming continuous with the lentiform nucleus. It lies in close relation to the amygdaloid complex.          The body of the caudate nucleus is related medially to the thalamus, and laterally to the interna

  The Lentiform Nucleus          The lentiform nucleus lies lateral to the internal capsule. Laterally, it is separated from the claustrum by fibres of the external capsule. (Note that these capsules are so called because they appear, by naked eye, to form a covering for the lentiform nucleus). Superiorly, the lentiform nucleus is related to the corona radiata, and inferiorly to the sublentiform part of the internal capsule. Some other relationships are evident in Fig. 13.2. The lentiform nucleus appears triangular (or wedge shaped) in coronal section. It is divided, by a thin lamina of white matter, into a lateral part, the  putamen;  and a medial part, the  globus pallidus.  The globus pallidus is further subdivided into medial and lateral (or internal and external) segments. The Claustrum         This is a thin lamina of grey matter that lies lateral to the lentiform nucleus. It is separated from the latter by fibres of the external capsule. Laterally, it is separated by a thin laye

  Connections of the Basal Nuclei          The various basal nuclei have numerous connections, but no useful purpose is served by enumerating the afferents and efferents of each nucleus. An integrated view of the corpus striatum, along with the substantia nigra and the subthalamic nucleus, is necessary. A scheme showing the main connections is given in Fig. 14.3. The striatum (caudate nucleus and putamen) receive afferents from the following. 1.                The entire cerebral cortex. These fibres are glutamatergic. 2.                The intralaminar nuclei of the thalamus. 3.                The pars compacta of the substantia nigra. These  fibres  are  dopaminergic.  (Some dopaminergic fibres arise from the retrorubral nuclei lying behind the red nucleus). 4.                Noradrenergic fibres are received from the raphe nuclei (in the reticular formation of the midbrain). 5.                Serotoninergic fibres are received from the locus coeruleus.         The afferents from the

  Ventral striatum and pallidum          On the basis of recent investigations some masses of grey matter lying in the region of the anterior perforated substance are now described as the ventral striatum. In Fig. 14.4 we see the anterior part of the caudate and lentiform nuclei. Inferiorly, the two nuclei fuse to the form the fundus striati. Immediately below the fundus striati we see the olfactory tubercle (in the anterior perforated substance). More medially, we see a mass of grey matter called the  nucleus accumbens . Note that this nucleus is closely related to the caudate nucleus (superolaterally) and to the septal nuclei medially. The ventral striatum consists of the nucleus accumbens and the olfactory tubercle.             A coronal section through the brain a little posterior to the plane of Fig. 14.4 is shown in Fig. 14.5. Note the anterior commissure running laterally just below the head of the caudate nucleus. It cuts through the globus pallidus. The part of the globus pall

  Abnormal Movements          Various kinds of abnormal movements are seen in neurological disorders involving the basal nuclei (basal ganglia), the subthalamic nucleus and the cerebellum. These may take the form of involuntary shaking ( tremor ) of the hands, head or other parts of the body, because of rapid alternating contraction of opposing groups of muscles. In some instances the tremor comes on when the patient tries to perform voluntary movement ( intention tremor ). Another type of abnormal movement consists of a slow twisting of a limb, or of the face or neck ( athetosis ). Sudden, jerky, shock-like movements involving any part of the body ( myoclonus ) may also occur. These can cause objects held in the hand to be thrown away ( hemiballism ). Sometimes different, complex, involuntary movements occur in succession, particularly in the distal parts of the limbs ( chorea ).         It has not been possible to precisely correlate specific abnormal movements with disease in specif

  DEVELOPMENT OF THE CORPUS STRIATUM         The corpus striatum is a derivative of the thickened basal part of the telecephalon. Nerve fibres of the developing internal capsule cut through the developing corpus striatum and divide it into medial and lateral parts. The medial part becomes the caudate nucleus. The lateral part forms the lentiform nucleus.