the central nervous system-3

Cerebellum

The cerebellum, the second-largest brain region, is located there. The cerebrum's occipital lobe is below it. The cerebellum is inferior to the posterior region of the cerebrum and posterior to the medulla and pons. It is made up of two lateral hemispheres and the vermis, a portion that is located close to the midline. The anterior, posterior, and flocculonodular lobes make up each hemisphere and are each divided by a deep fissure.

White matter makes up the majority of the inside of the cerebellum, whereas gray matter makes up the outer layer (cerebellar cortex). The medulla, pons, and midbrain are all connected to the cerebellum via a number of nerve fibers. The eyes, balance organs, and sensory receptors in the muscles, tendons, and joints all send sensory signals to the cerebellum. Movements of the skeletal muscles are coordinated by the anterior and posterior lobes. The flocculonodular lobe keeps the balance and equilibrium.

Functions of the Cerebellum

• Smooth, coordinated voluntary movements are created by and under the control of the cerebellum.
• It preserves equilibrium and balance.
• It keeps the muscles toned and coordinates the contraction of the numerous different muscles used in intricate movements.

Cerebrum

The biggest and topmost region of the brain is called the cerebrum. The cerebral cortex, an outer layer of gray matter, makes up this structure. The cortex's surface is heavily folded. The shallow grooves known as sulci divide the upward folds, or catted gyri. The cerebrum's longitudinal fissure, which separates it into the right and left hemispheres, is one of the deepest sulci, also known as fissures. With the exception of their lower midportions, which are connected by a structure known as the corpus callosum, these halves are nearly separate structures.

Each cerebral hemisphere is divided into four primary lobes, and each lobe is further divided into countless convolutions by two deep sulci. The frontal lobe, parietal lobe, temporal lobe, and occipital lobe are among the cranial bones that bear their names. Of all the lobes, the frontal lobe is the biggest. The majority of the interior of the cerebrum is made up of white matter, which is composed of bundles of neuronal fibers (tracts). The basal ganglia, a few islands of gray matter within this white matter, play a crucial role in producing automatic postures and movements.

The Lobes of the Brain

• Frontal Lobes: These are the largest of all the lobes and they form the anterior part of cerebral hemispheres. They are located anterior to the central sulci and superior to the lateral sulcus. They controls motor activity of the body.
• Parietal Lobes: These lobes are related to the internal aspects of posterior and superior part of the parietal bones. Each lobe is bounded anteriorly by the central sulcus, posteriorly by the superior part of the parieto-occipital sulcus and inferiorly by the imaginary line extending from the posterior ramus of the lateral sulcus. They are related to intellectual function and sensory function.
• Temporal Lobes: These lobes lie inferior to lateral sulci. They have hearing centers.
• Occipital Lobes: These lobes are relatively small and located posterior to the parieto-occipital sulci. These lobes are very important because they contain the visual area.

Functional Areas of the Cerebral Cortex

The cerebral cortex enables us to remember, understand, perceive, and communicate. It contains following kinds of functional areas:

• Primary Motor Area: The motor area is located in the precentral gyrus on the superolateral surface of the hemisphere and in the anterior part of the paracentral lobule on the medial surface. When these areas are stimulated electrically, movements occur in various parts of the body. Anatomically, these areas give origin to projection fibres that form the corticospinal and corticonuclear tracts. Specific regions within the area are responsible for movements in specific parts of the body. Stimulation of the paracentral lobule produces movement in the lower limbs. The trunk and upper limb are represented in the upper part of the precentral gyrus, while the face and head are represented in the lower part of the gyrus i.e. human body is represented in an upside down manner. Another feature of interest is that the area of cortex representing a part of the body is not proportional to the size of the part, but rather to intricacy of movements in the region. Thus, relatively large areas of cortex represents the hands and lips.
•  Premotor Area: The premotor area is located just anterior to the motor area. It occupies the posterior parts of the superior, middle and inferior frontal gyri. The part of the premotor area located in the superior and middle frontal gyri. Stimulation of the premotor area results in movements, but these are somewhat more intricate than those produced by stimulation of the motor area. The premotor area appears to be responsible for programming the intended movements and control of movements in progress.
• Motor Speech Area: The motor speech area of broca lies in the inferior frontal gyrus (Broadmann areas 44 and 45). Injury to this region results in inability to speak (aphasia), even though the muscles concerned are not paralyzed. These effects occur only if damage occurs in the left hemisphere in right-handed persons and in the right hemisphere in left-handed persons. In other words, motor control of speech is confined to one hemisphere, which controls the dominant upper limb. Apart from the motor speech area of Broca, there are two other areas concerned with control of speech. One of these is located in the temporal and parietal lobes (sensory speech area of Wernicke), while the other is located in the supplementary motor area.
• Sensory Area: The sensory area is located in the postcentral gyrus and is called the first somatosensory area. It also extends onto the medial surface of the hemisphere where it lies in the posterior part of the paracentral lobule. Responses can be recorded from the sensory area when individual parts of the body are stimulated. Sensation of pain, touch, itching, temperature, joint and muscle position are involved in the preception. They then identify the body region being stimulated. The amount of sensory cortex devoted to a particular body region is related to how sensitive that region is (that is, how many receptors it has), not to the size of the body region. In human, the lips, the digits and the tongue are the most sensitive body areas. Hence, a larger region of the somatosensory area receives impulses from those areas than from the thorax or abdomin.
• Visual Areas: The areas concerned with vision are located in the occipital lobe, mainly on the medial surface, both above and below the calcarine sulcus. It receives impulses from the retina of the eyes.
•  Auditory (Acoustic) Area: The acoustic area or the area for hearing is situated in the temporal lobe. It lies in that part of the superior temporal gyrus, which forms the inferior wall of the posterior ramus of the lateral sulcus. It receives and interpretes impulses of sound from the ear.
• Olfactory Areas: It is situated in the medial aspects of the temporal lobes. It receives and interprets impulse from the superficial nasal cavities.
• Frontal Eye Field: The frontal eye field lies in the middle frontal gyrus anterior to the precentral gyrus. Stimulation of this area causes both eyes to move to the opposite side. These are called conjugate movements.
•  Supplementary Motor Area: This lies on the medial surface of the cerebral hemisphere on the medial frontal gyrus.
• Gustatory (taste) Area: This taste area is located in the parietal area, just superior to posterior ramus of the lateral sulcus. It receives and interprets impulse from the tongue.

Cerebral White Matter

Beneath the gray matter of the cortex is the white matter of the cerebrum. It is made up of three kinds of tracts or bundles of fibers: association tracts, commissural tracts, and projection tracts.

Association tracts connect two functional areas of the cortex within the same hemisphere. Commissural tracts-connect two hemispheres. The largest commissure is corpus callosum. Projection tracts carry impulses between the cerebral cortex and other parts of the CNS. Example includes pyramidal tract.

Functions of the Cerebrum

• It provides the consciousness
• The cerebrum perceives the sensory stimuli from the sensory organs.
• The temporal lobe's auditory area interprets incoming nervous signals from the ear as very specific sounds.
• The visual area of the cortex in the occipital lobe helps to identify and understand specific images.
• The somatosensory area of the cerebral cortex also helps in the perception of smell and taste.

 Hemisphere Dominance

• Both (the right and the left) cerebral hemispheres function in receiving and analyzing sensory input and sending motor impulses to the opposite side of the body.
• The right cerebral hemisphere controls muscular activity in, and receives sensory input from the left half of the body.
• The left cerebral hemisphere controls muscles and receives input from the right half of the body.
• Most people exhibit hemisphere dominance for the language-related activities of speech, writing, and reading.
• The left hemisphere is dominant in 90% of the population, although some individuals have the right hemisphere as dominant, and others show equal dominance in both hemispheres.
•  The non-dominant hemisphere specializes in nonverbal functions and controls emotions and intuitive thinking
• The basal ganglia are masses of gray matter located deep within the cerebral hemispheres that relay motor impulses from the cerebrum and help to control motor activities by producing inhibitory dopamine.
• The Basal ganglia include three types of nuclei: the caudate nucleus, the putamen, and the globus pallidus.