THE CENTRAL NERVOUS SYSTEM BRAINSTEM,CEREBELLUM, DIENCEPHALON-Dr. Kibe INTRODUCTION The brain is one of the largest organs in adults. It consists approximately 100 billion neurons and 900 billion glia . And it weighs about 1.4 kg in adults Neurons of the brain undergo mitotic cell division only during the prenatal period and the first few months of postnatal life. No increase in number after that. Malnutrition during the crucial prenatal months of neuron multiplication is reported to hinder the process and result in fewer brain cells. The brain attains full size by about the eighteenth year but grows rapidly only during the first 9 years or so. BRAINSTEM • Three divisions of the brain make up the brainstem . • The medulla oblongata forms the lowest part of the brainstem, • The midbrain forms the uppermost part, • The pons lies between them, that is, above the medulla and below the midbrain. BRAIN STEM AND DIENCEPHALON Medulla Oblongata • The medulla oblongata is the part of the brain that attaches to the spinal cord. • It measures only a few centimeters (about 1 inch) in length and is separated from the pons above by a horizontal groove. • It is composed of white matter and a network of gray and white matter called the reticular formation • The pyramids are two bulges of white matter located on the ventral surface of the medulla. Fibers of the so-called pyramidal tracts form the pyramids. • The olive of the medulla is an oval projection appearing one on each side of the ventral surface of the medulla, lateral to the pyramids. • Located in the medulla's reticular formation are various nuclei , or clusters of neuron cell bodies. • Some nuclei are called control centers —for example, the cardiac, respiratory, and vasomotor control centers . Vital Centers in the medulla • The cardiac, vasomotor, and respiratory control centers are essential for survival and they are therefore called the vital centers. • They serve as the centers for various reflexes controlling heart action, blood vessel diameter, and respiration . • The medulla contains these centers and it is the most vital part of the entire brain • Blows at the base of the skull and bulbar poliomyelitis, for example, cause death if they interrupt impulse conduction in the vital respiratory centers. Pons • Just above the medulla lies the pons, composed, like the medulla, of white matter and reticular formation. • Fibers that run transversely across the pons and through the middle cerebellar peduncles into the cerebellum make up the external white matter of the pons and give it its arching, bridgelike appearance. Midbrain • The midbrain (mesencephalon ) is appropriately named. • It forms the midsection of the brain, because it lies above the pons and below the cerebrum. • Both white matter (tracts) and reticular formation compose the midbrain. • Extending divergently through it are two ropelike masses of white matter named cerebral peduncles • Tracts in the peduncles conduct impulses between the midbrain and cerebrum. • In addition to the cerebral peduncles, another landmark of the midbrain is the corpora quadrigemina (“body of fourfold twins”). • The corpora quadrigemina are two inferior colliculi and two superior colliculi . • Certain auditory centers are located in the inferior colliculus. • The superior colliculus contains visual centers . • Two other midbrain structures are the red nucleus and the substantia nigra (black matter due to black pigment in the cells). • Each of these consists of clusters of cell bodies of neurons involved in muscular control . Functions of the Brainstem • The brainstem performs sensory, motor, and reflex functions . • The spinothalamic tracts are important sensory tracts that pass through the brainstem on their way to the thalamus in the diencephalon. • The fasciculi cuneatus and gracilis and the spinoreticular tracts are sensory tracts whose axons terminate in the gray matter of the brainstem. • Corticospinal and reticulospinal tracts are two of the major tracts present in the white matter of the brainstem. • Nuclei in the medulla contain a number of reflex centers . • Vital centers are the cardiac, vasomotor, and respiratory centers. • Other centers present in the medulla are for various nonvital reflexes such as vomiting, coughing, sneezing, hiccupping, and swallowing. • Pons - contains centers for reflexes mediated by the fifth, sixth, seventh, and eighth cranial nerves. • In addition, the pons contains the pneumotaxic centers that help regulate respiration. • The midbrain , contains reflex centers for certain cranial nerve reflexes , for example, pupillary reflexes and eye movements, mediated by the third and fourth cranial nerves , respectively. CEREBELLUM The cerebellum (Little brain) is located just below the posterior portion of the cerebrum and is partially covered by it. A transverse fissure separates the cerebellum from the cerebrum. The cerebellum is the second largest part of the brain but has more neurons than all the other parts of the nervous system combined ! Thus the cerebellum has a lot of “computing power” compared with other parts of the brain. Fibers of the longer tracts enter or leave the cerebellum by way of its three pairs of peduncles 1. Inferior cerebellar peduncles : composed chiefly of tracts into the cerebellum from the medulla and cord (notably spinocerebellar, vestibulocerebellar, and reticulocerebellar tracts) 2. Middle cerebellar peduncles : composed almost entirely of tracts into the cerebellum from the pons , that is, pontocerebellar tracts 3. Superior cerebellar peduncles : composed principally of tracts from dentate nuclei in the cerebellum through the red nucleus of the midbrain to the thalamus The dentate nuclei is an important pair of cerebellar nuclei which lies in the each cerebellar hemisphere. Tracts connect these nuclei with the thalamus and with motor areas of the cerebral cortex . By means of these tracts, cerebellar impulses influence the motor cortex . Impulses in other tracts enable the motor cortex to influence the cerebellum . Functions of the cerebellum 1. Cerebellum compares the motor commands of the cerebrum with the information coming from proprioceptors in the muscle; impulses travel from the cerebellum to both the cerebrum and muscles to coordinate movements to produce the intended action 2. General functions a. Acts with cerebral cortex to produce skilled movements by coordinating the activities of groups of muscles b. Controls skeletal muscles to maintain balance c. Controls posture; operates at subconscious level to smooth movements and make movements efficient and coordinated d. Processes sensory information; complements and assists various functions of the cerebrum Diencephalon The diencephalon (between brain) is the part of the brain located between the cerebrum and the midbrain (mesencephalon). Although the diencephalon consists of several structures located around the third ventricle, the main ones are the thalamus and hypothalamus . The diencephalon also includes the optic chiasma , the pineal gland , and several other small but important structures. Thalamus The thalamus is a dumbbell-shaped mass of gray matter made up of many nuclei . Each lateral mass of the thalamus forms one lateral wall of the third ventricle . Extending through the third ventricle, and thus joining the two lateral masses of the thalamus, is the intermediate mass . Two important groups of nuclei that make up the thalamus are the geniculate bodies , located in the posterior region of each lateral mass. The geniculate bodies play a role in processing auditory and visual inpu t. FUNCTIONS OF THE THALAMUS 1. Plays 2 parts in the mechanism responsible for sensations a. Impulses from appropriate receptors, on reaching the thalamus, produce conscious recognition of the crude, less critical sensations of pain, temperature, and touch b. Neurons whose dendrites and cell bodies lie in certain nuclei of the thalamus relay all kinds of sensory impulses, except possibly olfactory, to the cerebrum 2. Plays a part in the mechanism responsible for emotions by associating sensory impulses with feelings of pleasantness and unpleasantness 3. Plays a part in the arousal or alerting mechanism 4. Plays a part in mechanisms that produce complex reflex movements EPITHALAMUS Various small nuclei just outside the thalamus and hypothalamus, collectively referred to as the epithalamus , are also included among the structures of the diencephalon. One of the most intriguing of the epithalamic structures is the pineal gland or pineal body HYPOTHALAMUS The hypothalamus is a small but functionally important area of the brain. It weighs little more than 7 g, yet it performs many functions of the greatest importance both for survival and for the enjoyment of life . It functions as a link between the psyche (mind) and the soma (body). It also links the nervous system to the endocrine system. Certain areas of the hypothalamus function as pleasure centers or reward centers for the primary drives such as eating, drinking, and sex . The hypothalamus a. Consists of several structures that lie beneath the thalamus b. Forms floor of the third ventricle and lower part of lateral walls c. Prominent structures found in the hypothalamus (1) Supraoptic nuclei —gray matter located just above and on either side of the optic chiasma (2) Paraventricular nuclei —located close to the wall of the third ventricle (3) Mamillary bodies —posterior part of hypothalamus, involved with olfactory sense d. Infundibulum —the stalk leading to the posterior lobe