No. 24 1. Cerebellum 2. Diencephalon

Ⅱ. The Cerebellum The cerebellum is the second of large portion of the brain lying behind the pons and medulla oblongata, and its median portion is separated from them by the cavity of fourth ventricle. It is located in the posterior cranial fossa and is covered by the tentorium of cerebellum.

Ⅰ) External Features and divisions 1. External features of the cerebellum and its morphological lobes The cerebellum is composed of two lateral cerebellar hemispheres connected in the midline by a structure called the vermis. The surface of the cerebellum consists of a thin cortex of gray matter. The cortex dips deeply below the apparent surface of the cerebellum in a manner similar to the fissures and sulci of the cerebrum.

The surface of cerebellum is composed of numerous narrow transverse folds, laminae or cerebellar folia. Some deeper fissures divide the organ into lobes or lobules. The horizontal fissure extends around the lateral and posterior borders of each hemisphere and marks the junction of the superior and inferior surfaces of the cerebellum. The tonsils of cerebellum are two elevated masses on the inferior surface of the hemispheral portion just behind the flocculonodular and nearby the foramen magnum.

The cerebellum presents three regions according to the external features. ① Flocculonodular It consists of both flocculi, their peduncles and the nodule. ② Anterior lobe It is that part of the superior surface rostral to the primary fissure. ③ Posterior lobe The remainder of the cerebellum on both surfaces constitutes the posterior lobe. Anterior and posterior lobes constitute the corpus of cerebellum.

2. Functional region of cerebellum The cerebellum can be divided by its connections and phylogeny into three portions. ① Vestibulocerebellum or archicerebellum The flocculonodular lobe is predominantly vestibular in its connections and constitutes the oldest part of the cerebellum. ② Spinocerebellum or paleocerebellum The anterior lobe and the rostral part of the inferior vermis are predominantly spinocerebellar in its connections. ③ Cerebrocerebellum or neocerebellum The posterior lobe is predominantly corticopontocerebellar in its connections.

Ⅱ) Internal Structure 1. The cerebellar cortex Structurally, the cerebellum consists of a cerebellar cortex or surface layer of gray matter, a medullary center of white matter, and four pairs of central nuclei embedded in the medullary center. From the surface to the white matter of the folium, these are the molecular layer, the Purkinje cell layer, and the granular layer.

2. The central nuclei of the cerebellum Four pairs of nuclei are embedded deep in the medullary center, they are the fastigial, globose, emboliform, and dentate nuclei. The dentate nucleus is the largest one and lies most laterally. It receives the fibers from the cerebellar cortex. The efferent fibers arise from the dentate nucleus forming the major part of the superior cerebellar peduncle to reach the red nucleus and the thalamus of contralateral side.

Ⅲ) The fibrous connections and functions of cerebellum 1. The fibrous connections and functions of vestibulocerebellum or archicerebellum (1) Fibrous connections ① Afferent fibers The vestibulocerebellar fibers from the vestibular nuclei and vestibular nerve to the vestibulocerebellum from the inferior cerebellar peduncles.

② Efferent fibers The fibers proceed from the vestibulocerebellum (flocculonodular lobe), relay in the fastigial nucleus and get to the vestibular nuclei and the reticular formation in the medulla oblongata and pons, then through the vestibulospinal tract and medial longitudinal fasciculus, control the muscles of trunk and extraocular muscles. (2) Function The vestibulocerebellum is concerned with the maintenance of equilibrium and body posture.

2. The fibrous connections and functions of spinocerebellum or paleocerebellum ① The afferent fibers The fibers of anterior and posterior spinocerebellar tract go through the superior and inferior cerebellar peduncles respectively and get to the spinocerebellum. ② The efferent fibers The efferent fibers from the cortex of spinocerebellum, relay in the globose and emboliform, get to the red nucleus of opposite side.

(2) Function The spinocerebellum influences muscle tone and synergy of muscles during stereotyped movements such as those of postural changes and locomotion.

3. The fibrous connections and function of cerebrocerebellum or neocerebellum ① The afferent fibers The afferent fibers from the contralateral pontine nucleus and inferior olivary complex form the pontocerebellar tract and olivocerebellar tract respectively. They get to the cerebrocerebellum through the middle and inferior cerebellar peduncles.

② The efferent fibers The efferent fibers arising from the cerebrocerebellum, relay in the dentate nucleus, and get to the contralateral red nucleus, ventral intermediate nucleus and ventral anterior nucleus of dorsal thalamus. (2) Function The cerebrocerebellum ensures the coordination of muscles for accuracy of nonstereotyped movements.

Ⅳ) Clinical expression after cerebellar damnification 1. Archicerebellar syndrome. (1) Imbalance of equilibrium The archicerebellar syndrome is characterized by the disturbance of equilibrium. The patient is unsteady when standing, walks on a wide base, the gait resembles that of a drunken individual (cerebellar gait) and sways from side to side even falls backward or to either side. (2) Quiver of eyeball

2. Paleocerebellar syndrome It is characterized by the disturbance of postural reflex, in which the subject staggers or cannot coordinate normal walking movement.

3. Neocerebellar syndrome It has the following characteristic signs: (1) Ataxia: It is due to incoordination of the related muscles. For instance, the patient cannot touch the tip of his nose with a finger without looking at this finger, because he cannot coordinate movement with his sense of where a body part is located. Adiadochokinesia, rapidly successive movements, such as alternately pronating and supinating the forearms are poorly performed. (2) Reducing of muscle tone. (3) Intention tremor.

Ⅲ. The Diencephalon The diencephalon, being almost entirely surrounded by the hemispheres of the cerebrum, exposes only the ventral surface of the diencephalon to view in a diamond-shaped area containing hypothalamic structures. The diencephalon is divided into symmetrical halves by the slitlike third ventricle. The medial surface of the diencephalon forms the wall of the third ventricle. Thalamic medullary stria (stria terminalis). The diencephalon consist of five components or regions on each side, named the: dorsal thalamus, epithalamus, subthalamus, metathalamus, and hypothalamus.

Ⅰ) The Thalamus (dorsal thalamus) Each of the two thalami is a large ovoid mass of gray matter, making up four-fifths of the diencephalon.

1.The nuclei of dorsal thalamus The internal medullary lamina, consisting mainly of a band of myelinated fibers, divides the thalamus into three nuclear groups. the anterior nuclear group, the medial nuclear group, the lateral nuclear group.

(1) The anterior nuclear group is enclosed by a bifurcation of the lamina and forms a rostromedial swelling known as the anterior tubercle. (2) The medial nuclear group contains the large dorsomedial nucleus. (3) The lateral nuclear group consists of ventral and dorsal tiers of nuclei which have been identified because of differing fiber connections. Three nuclei are recognized in the ventral tier: Ventral anterior nucleus, Ventral lateral nucleus, Ventral posterior nucleus. The ventral posterior nucleus consists of ventral posteromedial nucleus and ventral posterolateral nucleus.

2. The nucleus classified on the basis of phylogeny On the basis of phylogeny, connections with other parts of the brain, and function, the thalamic nuclei may be classified according to the following scheme. (1) Nonspecific projection nuclei (archithalamus) They are composed of midline nuclei, intralaminar nuclei, and reticular nucleus. The midline nuclei lie in the periventricular gray matter of the thalamus and in the interthalamic adhesion.

In the central part of the thalamus, the internal medullary lamina partially encloses the intralaminar nuclei, including the well-developed centromedian nucleus. Nonspecific projection nuclei receive the afferent fibers from the olfactory brain and reticular formation of brain stem. The ascending fibers of reticular formation pass through these nuclei and project extensive area of cerebral cortex, forming ascending reticular activating system,

(2) Specific projection (relay) nuclei (paleothalamus) They include the ventral anterior, lateral, and posterior nuclei, and send the fibers to sensory and motor areas of cortex. The ventral posteromedial nucleus receives the fibers from the trigeminothalamic tract.

The ventral posterolateral nucleus receives the fibers from the medial lemniscus and spinothalamic tract. The nerve fibers leave the ventral posteromedial and posterolateral nuclei form the thalamocortical tract, traverse the internal capsule and medullary center of the cerebral hemisphere, and end in the general sensory area of cortex in the parietal lobe

(3) Association nuclei (neothalamus) They include anterior nucleus, medial nucleus, and dorsal tier of lateral nucleus. The dorsal tier consists of the pulvinar, lateral posterior nucleus and lateral dorsal nucleus.

3.Functions of thalamus The thalamus is a relay station under the cortex. The thalamus is a complex center for regulation of somatic motion, emotion, memory, and so on.

Thalamic syndrome: The thalamic syndrome is essentially a disturbance of these aspects of thalamic functions, subsequent to a lesion involving the thalamus or its connections. The symptoms vary according to the location and extent of the lesion. The threshold for touch, pain and temperature is usually raised on the opposite side of the body, but when the threshold is reached the sensations are exaggerated, perverted and exceptionally disagreeable. For example, the prick of a pin may be felt as a severe burning sensation, and even music that is ordinarily pleasing may be disagreeable. There is spontaneous pain in some instances, which may become intractable to analgesics.

Ⅱ) The metathalamus It includes medial geniculate body and lateral geniculate body. The medial geniculate body is the relay station of acoustic pathway, and gives rise to the acoustic radiation. The lateral geniculate body is the relay station of visual pathway, and gives rise to the optic radiation.

Ⅲ) The epithalamus It occupies the caudal roof of the third ventricle together with adjacent areas. It includes the right and left habenular nuclei, each situated deep to the floor of a habenular trigone. The epithalamus also includes the habenular commissures. The pineal body, an endocrine gland in mammals, is attached to the posterior commissure on the midline.

Ⅳ) The subthalamus The subthalamus is situated immediately ventral to the dorsal thalamus, in part lateral to the hypothalamus, and emerges caudally with the tegmentum of midbrain. The region includes the rostral extension of the red nucleus and substantia nigra, the prominent subthalamic nucleus. Caudally the medial part of the nucleus overlies rostral portions of the substantia nigra. It is a component of the extrapyramidal system.

Hypothalamic structures include: mammillary bodies, tuber cinereum, Ⅴ) The Hypothalamus As the name indicates, the hypothalamus lies below the thalamus, where it forms part of the walls and floor of the third ventricle. Like the thalamus, the hypothalamus is composed of several nuclei, each of which is involved with specific functions. 1. Morphology Hypothalamic structures include: mammillary bodies, tuber cinereum, infundibulum, optic chiasma, hypophysis..

The mammillary bodies are distinct swellings on the ventral surface. Located just anterior to the mammillary bodies is the tuber cinereum. Extending downward from the tuber cinereum is the stalklike infundibulum. Anterior to the infundibulum is the optic chiasma, which is formed by the decussation (crossing) of some of the neurons in the optic nerves. Inferior to the infundibulum is the hypophysis.

2. Division and main nuclei The hypothalamus consists of four areas. Anteroptic area: Supraoptic area: Includes paraventricular nucleus and supraoptic nucleus. Tubar area: Includes infundibular nucleus. Mammillary area. Includes mammillary nucleus.

3. Fibrous connections of hypothalamus (1) The connection with the limic system (2) The connection with the brain stem and spinal cord (3) The connection with the dorsal thalamus (4) The connection with the hypophysis Hormones produced by neurons in the hypothalamus have been sent to the posterior lobe (neurohypophysis) through paraventriculohypophyseal tract and supraopticohypophyseal tract.

4. Functions of hypothalamus (1) Hypothalamus is a center of endocrine system. The hypothalamus also regulates the release of the hormones of the pituitary gland and thus, to a large extent, it controls the endocrine system. (2) The hypothalamus controls many vital processes, most of them associated with the autonomic nervous system. Some of the hyopothalamic nuclei have been shown experimentally to regulate sympathetic activity; others control parasysmpathetic functions. (3) The hypothalamus is involved in regulating body temperature, water balance, appetite, gastrointestinal activity, sexual activity, and even emotions such as fear and rage.

The third ventricle The third ventricle is a narrow cavity in the diencephalon. It is between the two dorsal thalami and hypothalamus. Cerebrospinal fluid enters the third ventricle from each lateral ventricle through the interventricular foramen. The fluid leaves the third ventricle by way of the mesencephalic aqueduct, through which it reaches the fourth ventricle.