13. Regulation of sexual function

In animals, hypothalamus plays an important role in maintaining the sexual functions, especially in females. A decorticate female animal will have regular estrus cycle provided the hypothalamus is intact. In human being also, hypothalamus regulates the sexual functions by secreting gonadotropin releasing hormones. Tuberal and posterior hypothalamic nuclei are involved in the regulation of sexual functions.

14. Regulation of response to smell

Posterior hypothalamus along with other structures like hippocampus and brainstem nuclei, is responsible for the autonomic responses of body to olfactory stimuli. The responses include feeding activities and emotional responses like fear, excitement and pleasure.

15. Role in circadian rhythm

The body rhythm to 24 hours dark - light cycle is called circadian rhythm. The supraoptic and anterior nuclei of hypothalamus play an important role in regulation of circadian rhythm.

Cerebellum physiology

Cerebellum consists of a narrow, worm like central body called vermis and two lateral lobes, the right and left cerebellar hemispheres.

Cerebellar hemispheres are the extended portions on either side of the vermis.

DIVISIONS OF CEREBELLUM

I Anatomical:

On the basis of structure, the whole cerebellum is divided into three portions.

1. Anterior Lobe

This lobe includes lingula, central lobe and culmen. It is separated from posterior lobe by the primary fissure.

2. Posterior Lobe

This consists of lobulus simplex, declive, tuber, pyramid, uvula, paraflocculi and the two portions of hemispheres-ansiform lobe and paramedian lobe.

3. Flocculonodular Lobe

This includes nodulus and the lateral extension on either side called flocculus. It is separated from rest of the cerebellum by posterolateral fissure.

II. Phylogenetic:

Depending upon phylogeny, the cerebellum is divided into two divisions.

1. Paleocerebellum

This is the phylogenetically oldest part of cerebellum. It includes two divisions, archicerebellum and paleocerebellum proper.

a). Archicerebellum—Flocculonodular lobe

b) Paleocerebellum proper—It includes lingula, central lobe, culmen, lobulus simplex, pyramid, uvula and paraflocculi

2. Neocerebellum

This is the phylogenetically newer portion of cerebellum. It includes declive, tuber and lateral portions of hemispheres, lobulus ansiformis and lobulus paramedianus.

III. Functional:

Based on the functions, the cerebellum is divided into three divisions.

1. Vestibulocerebellum

This includes flocculonodular lobe that forms the archi-cerebellum.

2. Spinocerebellum

This includes lingula, central lobe, culmen, lobulus simplex, declive, tuber, pyramid, uvula and paraflocculi and medial portions of cerebellar hemispheres.

3. Corticocerebellum

This includes the lateral portions of hemispheres.

STRUCTURE

Cerebellum is made up of outergray matter or cerebellar cortex and an inner white matter. The white matter is formed by afferent and efferent nerve fibers of cerebellum. The gray masses called cerebellar nuclei are located within white mater.

GRAY MATTER

Gray matter or cerebellar cortex is made up of structures arranged in three layers. Each layer of gray matter is uniform in thickness and appearance throughout cerebellum. The three layers of gray matter are:

1. Outer molecular or plexiform layer

2. Intermediate Purkinje layer and

3. Inner granular layer

1. Molecular or Plexiform Layer

It is the outer most layer of cortex having the cells arranged in two strata. The superficial stratum contains few star shaped cells known as stellate cells. The deep stratum contains basket cells. In addition to stellate and basket cells, the molecular layer has the following structures:

a) Parallel fibers, which are the axons of granule cells, present in granular layer

b) The terminal portions of climbing fibers.

c) Dendrites of Purkinje cells and Golgi cells.

Molecular layer also contains the following cellular junctions:

a) The dendrites of stellate cells and basket cells synapse with parallel fibers, which are the axons of granule cells

b) The axons of stellate cells end on the dendrites of Purkinje cells. However, the axon of basket cell descends down into the Purkinje layer and forms the transverse fiber that ends on the soma of Purkinje cells

c) The dendrites of Purkinje cells synapse with climbing fibers and parallel fibers

d) The dendrites of Golgi cells situated in granular layer enter the molecular layer and end on parallel fibers.

2. Purkinje Layer

It is situated in between outer molecular layer and inner granular layer. It is the thinnest layer having a single layer of flask shaped Purkinje cells. The Purkinje cells are the largest neurons in the body. The dendrites of these cells ascend through the entire thickness of molecular layer. These dendrites terminate either on climbing fibers or the parallel fibers. The axons of the basket cells form the transverse fibers and end on the soma of Purkinje cells. The axons of Purkinje cells descend into the white matter and terminate on the cerebellar nuclei and vestibular nuclei via cerebellovestibular tract. The Purkinje cells are termed as "Final common path" of cerebellar cortex because the impulses from different parts of cerebellar cortex are transmitted to other parts of brain only through Purkinje cells.

Fig.24. Structure of cerebellar cortex. (+) = Excitation, (-) = Inhibition

3. Granular Layer

This layer is placed in between Purkinje layer and the white matter. It is formed by interneurons namely granule cells and Golgi cells. The total number of interneurons in this layer is about half the number of all neurons in the whole nervous system.

The axon of the granule cell ascends into molecular layer and forms the parallel fiber, which synapses with dendrites of Purkinje cells, stellate cells, basket cells and Golgi cells. The dendrites of granule cells and the axon and few dendrites of a Golgi cell synapse with mossy fiber. The synaptic area of these cells is called the glomerulus and it is encapsulated by the processes of glial cells.

Afferent Fibers to Cerebellar Cortex

The cerebellar cortex receives afferent signals from other parts of brain through two types of nerve fibers.

1. Climbing fibers

2. Mossy fibers

Climbing fibers: they arise from the neurons of inferior olivary nucleus situated in medulla and reach the cerebellum via olivocerebellar tract. The inferior olivary nucleus relays the output signals from motor areas of cerebral cortex and the proprioceptive signals from different parts of the body to the cerebellar cortex via climbing fibers. The proprioceptive impulses from different parts of the body reach the inferior olivary nucleus through spinal cord and vestibular system.

After reaching the cerebellum, the climbing fibers ascend into the molecular layer and terminate on the dendrites of Purkinje cells. While passing through cerebellum, the olivocerebellar tract sends collaterals to cerebellar nuclei. Because of this, the impulses from cerebral cortex and proprioceptors of the body are conveyed to not only the cerebellar cortex but also the cerebellar nuclei by the climbing fibers. Each climbing fiber innervates one single Purkinje cell.

Mossy fibers: Unlike climbing fibers, the mossy fibers have many sources of origin namely motor areas of cerebral cortex, pons, medulla and spinal cord. Fibers arising from all these areas send collaterals to the cerebellar nuclei before reaching the cerebellar cortex. So, like climbing fibers, the mossy fibers also convey afferent impulses to both cerebellar nuclei and cerebellar cortex. Some of the mossy fibers arise from cerebellar nuclei. After reaching the granular layer of cerebellar cortex, the mossy fiber divides into many terminals. Each terminal enters the glomerulus and ends in a large expanded structure that forms the central portion of the glomerulus. The dendrites of granule cells and the axon and dendrites of Golgi cells synapse on the mossy fiber giving a thick bushy appearance. The word “mossy” refers to the appearance of a plant called moss, which grows into dense clumps, and hence, the name mossy fibers is given to these fibers.

Neuronal Activity in Cerebellar Cortex and Nuclei

The functions of cerebellum are executed mainly by the impulses discharged from the cerebellar nuclei. However, the cerebellar cortex controls the discharge from the nucleus constantly via the fibers of Purkinje cells. This is done in accordance with the signals received by the cerebellar cortex from different parts of brain and body via climbing and mossy fibers. The entire process involves a series of neuronal activity as mentioned in Table .