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The Parts and Functions of The Human Brain
The human brain is composed of about 100 billion multipolar neurons, which communicate with one another and with neurons in other parts of the nervous system. The brain lies within the cranial cavity of the skull.
PARTS OF THE BRAIN
The brain is divided into four major parts.
(1) The cerebrum
(2) Brain stem
(3) Diencephalon and
(4) The cerebellum.
The cerebrum is the largest part of the brain. It consist of two large masses known as the right and left cerebral hemisphere. The right and left cerebral hemisphere are connected by a deep bridge of nerve fibers called corpus callosum, while a layer of Dura mater separates them. The surface of the cerebrum has convolutions or ridges (gyri) separated by grooves, a shallow groove named sulcus and a deep groove named fissure. Several sulci divides each hemisphere into lobes, while the fissure is in two types, the longitudinal fissure, and transverse fissure. The longitudinal fissure separates the left from the right hemisphere, and the transverse fissure separates the cerebrum from the cerebellum.
THE LOBE OF THE CEREBRAL HEMISPHERE
The lobes of the cerebral hemisphere includes the following.
(1)Frontal lobe: this forms the anterior portion of the cerebral hemisphere, bordered posteriorly by a central sulcus, and inferiorly by a lateral sulcus.
(2) Parietal lobe: this lobe is posterior to the frontal lobe and is separated from it by the central sulcus.
(3) Temporal lobe: this lies below the frontal and parietal lobe, and it is separated from them by the lateral sulcus.
(4) Occipital lobe: the occipital lobe forms the posterior portion of the left and right cerebral hemisphere and is separated from the cerebellum by a shelf like extension of Dura matter.
(5)Insula: this is located deep within the lateral sulcus and parts of the fronter, parietal and temporal lobes cover it. The insula is separated from other lobes by a circular sulcus.
The outermost portion of the cerebrum is a thin layer of gray mater called the cerebral cortex. This layer covers the convolutions and dips into the sulci and fissure. Cerebral cortex are divided into motor, sensory, and association areas.
* Motor areas : motor area of the cerebral cortex lie in the frontal lobes, just in front of the central sulcus. The nervous tissue in this regions contains many large pyramidal cell are called upper motor neurons because of their location. The impulses from the pyramidal cell travel downward through the brainstem and into the spinal cord on the corticospinal tracts. Here they form synapse with lower motor neurons whose axons leave the spinal cord and reach the skeletal muscle fibers. Certain other regions of the frontal lobe affect motor function, for example a region called the motor speech area just anterior to the primary motor cortex. Superior to the lateral sulcus coordinates the complex muscular actions of the mouth, tongue, and larynx that makes speech possible. Above the motor speech area is a region called the frontal eye field, the motor cortex in this area controls voluntary movement of the eye and eyelids. Another region just in front of the primary motor area controls the muscular movement of the hands and finger that make skills such as writing.
*area Sensory: this is located in several lobes of the cerebrum interpret impulses that arrive from sensory receptors producing feelings or sensation, Such as sensations from all parts of the skin arise in the anterior portions of the parietal lobes along the central sulcus. The posterior parts of the occipital lobe affects vision, and the temporal lobe contain centers for hearing. The sense of smell arise from centers deep within the cerebrum while the sensory areas for taste are located near the bases of the central sulci along the lateral sulci. The centers in the right cerebral hemisphere interpret impulse originating from the left side of the body and vice versa.
*Association area: the association area connects with one another and with other brain structures. This area analyses, interpret sensory experiences, and oversee memory, reasoning, verbalizing, judgment, and emotion.
Beneath the cerebral cortex is a mass of white matter containing bundles of myelinated axons, connecting neuron cell bodies of the cortex with other parts of the nervous system, makes up the bulk of the cerebrum.
FUNCTIONS OF THE CEREBRUM
(1) The cerebrum provides a very high brain function
(2) It stores information comprising memory and uses it to reason.
(3) It also comprises of centers that interpret sensory impulse arriving from sense organs, and centers for initiating voluntary muscular movement.
VENTRICLES AND CEREBROSPINAL FLUID
Within the cerebral hemisphere and brain stem is a series of inter connected cavities called ventricles. This spaces are continues with the cerebral canal of the spinal cord. The ventricles contains cerebrospinal fluid. There are four ventricles in the brain.
(1) The lateral ventricle (first and second ventricles): this is the largest ventricle in the brain. They are located one each in the hemisphere.
(2) Third ventricle: this is narrow cavity along midline, superior to the hypothalamus.
(3) Fourth ventricle: this lies between the brain stem and cerebellum.
Cerebrospinal fluid is a clear colorless fluid found in sub arachnoid space. A sheath of ependymal cells found in the choroid plexuses produces them.
FUNCTION OF THE CEREBROSPINAL FLUID
(1) The cerebrospinal fluid provides shock-absorbing medium and buoys the brain inside the skull.
(2) It allows minor exchange of nutrient and waste.
(3) It provides optimal chemical environment for neuronal signaling.
FLOW OF CEREBROSPINAL FLUID
The cerebrospinal fluid in the lateral ventricle flow into the third ventricle through the interventricular foramina, then it flows from the third ventricle to the fourth through the cerebral aqueduct and from there escapes through the median aperture and lateral aperture to subarachnoid space and finally it is absorbed by arachnoid villi and released into the blood.
The brainstem is a bundle of nervous tissue that connects the cerebrum to the spinal cord. The brainstem includes the medulla oblongata, pons, and midbrain.
Medulla oblongata: This is superior to the spinal cords. It extends from the pons to the foramen magnum of the skull; its dorsal surface flattens to form the floor of the fourth ventricles. Its ventral surface is marked by the corticospinal tracts. The medulla oblongata controls some vital autonomic body function such as digestion, breathing, force, and rate of heartbeat. All sensory and motor tracts from the spinal cords also go through the medulla oblongata.
PARTS OF MEDULLA OBLONGATA
(I) pyramids: this is a protrusion formed by largest motor tracts.
(ii) Decussation of pyramid: this is where 90% of the motor tracts cross over, so that the left side of the brain is given control of the right side of the body and vice versa.
(iii) Olive: olive contains nuclei that relays impulses from propriocepters to cerebellum.
(iv) Cardiovascular centers: this part regulates rate and force of the heartbeat.
(v) Medullary rhythmicity area: this area adjusts basic rhythm of breathing.
five cranial nerves arise in the medulla oblongata, the includes the vestibulocochlear nerves(viii), glossopharyngeal nerves(ix), vagus nerves(x), accessary nerves(xi), and hypoglossal nerves(xii).
Pons: This is a rounded bulge on the underside of the brainstem, where is separate the midbrain from the medulla oblongata. The pons regulates some autonomic functions and a bridge that connects parts of the brain, like the left and right side of cerebrum. The pons includes the following parts:
(I) Pontine nuclei: this is where we get the connection of the cerebrum with the opposite hemisphere of cerebellum, in order to coordinate voluntary motor output.
ii) Pneumotaxic area and apneustic area: this help control breathing.
Four cranial nerve nuclei arise from pons, they includes the trigeminal nerves (v), abducens nerves (VI), facial nerves (vii), and vestibulocochlear nerves (viii).
Midbrain: this is a short section of the brainstem between the diencephalon and the pons. The midbrain coordinates movement, relays visual and auditory reflexes.
The midbrain includes the following parts:
I) cerebral peduncles: this are motor and sensory tracts, which go between the cerebrum and the midbrain.
ii) Cerebella peduncles: this is where the cerebellum connects to the midbrain.
iii) This is the posterior part of the midbrain it is composed of four colliculi, two on either side of the midbrain. The superior colliculi which reflex centers for visual activities and the inferior colliculi that relay impulses from ear to thalamus.
The cranial nerve that originates in the midbrain are the oculomotor nerves (iii), and trochlear nerves
This is where white matter (axon tracts) and gray matter (neuronal cell bodies) form netlike arrangement from superior portion of spinal cord to the inferior part of diencephalon. Its nerve fibers join centers of the hypothalamus, basal nuclei, cerebellum, and cerebrum with fibers in all major ascending and descending tracts. The ascending portion sends sensory axons directly to cerebrum or through thalamus. The descending portion regulate muscle tone. The reticular formation functions in consciousness, arousal, and attention.
The diencephalon is located between the cerebral hemispheres and above the midbrain. It surrounds the third ventricle and is composed largely of gray matter. The parts of diencephalon includes the thalamus, hypothalamus, and epithalamus.
THALAMUS: this is a central relay station for impulses except for the sense of smell, ascending from other parts of the nervous system to the central cortex. It possess nuclei for memory, awareness, and emotions.
The two halves of thalamus is connected by the inter-mediate mass.
HYPOTHALAMUS: this lie below the thalamus and forms the lower wall and floor of the third ventricle. It controls many body activities and regulate homeostasis.
EPITHALAMUS: this part is superior and posterior to the thalamus and includes pineal glands and habenular nuclei, which is involved in olfaction.
Circumventricular organ is also found in the diencephalon. It lie in the wall of the third ventricle and monitor changes in body chemistry.
The cerebellum is a large mass of tissue located below the occipital lobes of the cerebrum and posterior to the medulla oblongata and pons. It consist of two lateral hemisphere partially separated by a layer of Dura matter (falx cerebelli) and connected in the midline by a structure called the vermix.
PARTS OF CEREBELLUM
The cerebellum is composed of white matter with a thin layer of grey matter called the cerebellar cortex on its surface.
(1) Arbor vitae: this is a white matter on the inside of the cerebellum. the cerebellum also consist an anterior and posterior lobes from the back which helps govern subconscious aspect of skeletal muscle control, and also compose of the flocculonodular lobes which are more in the middle area of the cerebellum. They are involve in equilibrium and balance. The cerebellar peduncles is part of the cerebellum that connects it to the brain stem. The inferior cerebellar peduncle carry sensory information into the cerebellum, its middle part carries impulse for voluntary movement to the pons, while the superior end has more axons extending to the midbrain and thalamus.