The Brain-
• Brain is the part of CNS. Brain maintains homeostasis by receiving sensory input, integrating new and stored information, making decisions, and causing motor activities. We will organize the study of brain as coverings of the brain, parts of brain CSF and cranial nerves.
BRAIN COVERINGS-
• The brain is completely surrounded by three layers of tissue, known as meninges. These layers are present between the skull and the brain. These are-
• Dura mater- This layer lies closest to the bone of the skull and is a double layer of tough, fibrous, connective tissue. The outer layer is called the periosteal layer and the meningeal layer lies towards the brain.
Three extensions of the dura mater separate parts of the brain.
(1) The falx cerebri separates the two hemispheres (sides) of the cerebrum.
(2) The falx cerebelli separates the two hemispheres of the cerebellum.
(3) The tentorium cere-belli separates the cerebrum from thecerebellum.
• Arachnoid mater- This is a layer of fibrous tissue that lies between the dura and pia maters. It is separated from the dura mater by the subdural space, and from the pia mater by the subarachnoid space. The arachnoid mater passes over the convolutions of the brain and accompanies the inner layer of dura mater in the formation of the falx cerebri, tentorium cerebelli and falx cerebelli.
• Pia mater- This is a delicate layer of connective tissue containing many minute blood vessels. It adheres to the brain, completely covering the convolutions and dipping into each fissure.
PARTS OF BRAIN-
• Brain has four main parts –
• Cerebrum
• Cerebellum
• Diencephalon
• Brain stem
CEREBRUM-
• This is the largest part of the brain. It is anterior and upper part of brain. It is divided by a deep cleft, the longitudinal cerebral fissure, into right and left cerebral hemispheres. The cerebral cortex shows many infoldings or furrows of varying depth. The exposed areas of the folds are the gyri (convolutions) and these are separated by sulci (fissures). These convolutions greatly increase the surface area of the cerebrum.
• Each cerebral hemisphere can be further subdivided into four lobes. The lobes are named after the bones that cover them: frontal, parietal, temporal, and occipital lobes
• The superficial (peripheral) part of the cerebrum is composed of nerve cell bodies or grey matter, forming the cerebral cortex. Within the cerebrum the lobes are connected by masses of nerve fibers, or tracts, which make up the white matter of the brain. These tracts are
• 1. Association tracts contain axons that conduct nerve impulses between gyri in the same hemisphere.
• 2. Commissural tracts contain axons that conduct nerve impulses from gyri in one cerebral hemisphere to corresponding gyri in the other cerebral hemisphere. Three important groups of commissural tracts are the corpus callosum, anterior commissure, and posterior commissure.
• 3. Projection tracts contain axons that conduct nerve impulses from the cerebrum to lower parts of the CNS (thalamus, brain stem, or spinal cord) or from lower parts of the CNS to the cerebrum. An example is the internal capsule
• BASAL GANGLIA- - Deep within each cerebral hemisphere are three nuclei (masses of gray matter) that are collectively termed the basal ganglia. They act as relay stations with connections to many parts of the brain including motor areas of the cerebral cortex and thalamus. Their functions include initiation and fine control of complex movement and learned coordinated activities, such as posture and walking.
• LIMBIC SYSTEM- limbic system is a group of structures which encircles the upper part of the brain stem and the corpus callosum. The limbic system is sometimes called the “emotional brain” because it plays a primary role in a range of emotions, including pleasure, pain, docility, affection, fear, and anger. It also is involved in olfaction (smell) and memory
• AREAS OF BRAIN- There are many areas in the cerebrum which are associated with one or more functions. The main functional areas of the cerebral cortex are sensory areas, motor areas and association areas.
• 01.- Sensory areas- Sensory areas includes the somato sensory area, auditory (hearing) area, Olfactory (taste ) area and visual area.
• 02.- Motor areas- Motor areas includes the The primary motor area which lies in the frontal lobe. There are two neurons involved in the pathway to skeletal muscle. The first, the upper motor neuron, descends from the motor cortex through the internal capsule to the medulla oblongata. Here it crosses to the opposite side and descends in the spinal cord. At the appropriate level in the spinal cord it synapses with a second neurone (the lower motor neuron).
• The lower motor neuron leaves the spinal cord and travels to the target muscle of our body. It terminates at the motor end plate of a muscle fiber. Crossing of neurons means that the motor area of the right hemisphere of the cerebrum controls voluntary muscle movement on the left side of the body and vice versa. Damage to either of these neurons may result in paralysis.
• Broca’s (motor speech) area- This is situated in the frontal lobe just above the lateral sulcus and controls the muscle movements needed for speech.
• 03. Association areas- These are connected to each other and other areas of the cerebral cortex by association tracts. They receive, coordinate and interpret impulses from the sensory and motor cortices permitting higher cognitive abilities such as learning and memory, perception and language, planning, and higher-order concept formation.
CEREBELLUM-
• The cerebellum lies below the posterior portion of the cerebrum. It is ovoid in shape and has two hemispheres. The superficial layer of the cerebellum, called the cerebellar cortex, consists of gray matter in a series of slender, parallel folds called folia ( leaves). Deep to the gray matter there are tracts of white matter called arbor vitae ( tree of life) that resemble branches of a tree.
• The cerebellum is concerned with the coordination of voluntary muscular movement, posture and balance. Cerebellar activity is not under voluntary control. The cerebellum controls and coordinates the movements of various groups of muscles ensuring smooth, even, precise actions. It coordinates activities associated with the maintenance of posture, balance and equilibrium.
DIENCEPHALON-
• This part of the brain connects the cerebrum and the midbrain. It consists of several structures such as the thalamus and hypothalamus, The pineal gland and the optic chiasma.
• Thalamus- This consists of two masses of grey and white matter situated within the cerebral hemispheres just below the corpus callosum, one on each side of the third ventricle
• Hypothalamus- The hypothalamus is a small but important structure which weighs around 7 g and consists of a number of nuclei. It is situated below and in front of the thalamus, immediately above the pituitary gland. The hypothalamus controls many body activities and is one of the major regulators of homeostasis. It controls and regulates the activities of ANS.
BRAIN STEM-
• Brain stem consists of –
• Mid Brain
• Pons
• Medulla oblongata
• Reticular formation
• Mid Brain-The midbrain is the area of the brain situated around the cerebral aqueduct between the cerebrum above and the pons below. It consists of nuclei and nerve fibers (tracts), which connect the cerebrum with lower parts of the brain and with the spinal cord. The anterior part of the midbrain contains paired bundles of axons known as the cerebral peduncles
• Pons-The pons is situated in front of the cerebellum, below the midbrain and above the medulla oblongata. It consists mainly of nerve fibers (white matter) that form a bridge between the two hemispheres of the cerebellum, and of fibers passing between the higher levels of the brain and the spinal cord. The anatomical structure of the pons differs from that of the cerebrum in that the cell bodies (grey matter) lie deeply and the nerve fibres are on the surface.
• Medulla Oblongata- The medulla oblongata extends from the pons above and is continuous with the spinal cord below. It is about 2.5 cm long and it lies just within the cranium above the foramen magnum. relay station for sensory nerves going to the cerebrum. The medulla contains autonomic centers such as the cardiac center, the respiratory center, the vasomotor center and the coughing, sneezing and vomiting center. The medulla is also the site of decussation of the nerve fibers.
• Reticular formation- The reticular formation is a collection of neurons in the core of the brain stem, surrounded by neural pathways that conduct ascending and descending nerve impulses between the brain and the spinal cord. The reticular formation is involved in coordination of skeletal muscle activity associated with voluntary motor movement and the maintenance of balance.
VENTRICLES OF BRAIN-
• The brain contains four irregular-shaped cavities, or ventricles, containing cerebrospinal fluid (CSF). They are:
• Right and Left lateral ventricles
• third ventricle
• fourth ventricle.
• Right and Left lateral ventricles- These cavities lie within the cerebral hemispheres, one on each side of the median plane just below the corpus callosum. They are separated from each other by a thin membrane, the septum lucidum, and are lined with ciliated epithelium.
• Third ventricle- The third ventricle is a narrow cavity along the midline superior to the hypothalamus and between the right and left halves of the thalamus. It communicates with the fourth ventricle by a canal, the cerebral aqueduct.
• Fourth ventricle- The fourth ventricle is a diamond-shaped cavity situated below and behind the third ventricle, between the cerebellum and pons. It is continuous below with the central canal of the spinal cord and communicates with the subarachnoid space by foramina in its roof. Cerebrospinal fluid enters the subarachnoid space through these openings
CSF-
• Cerebrospinal fluid is secreted into each ventricle of the brain by choroid plexuses. These are vascular areas where there is a proliferation of blood vessels surrounded by ependymal cells in the lining of ventricle walls. CSF passes back into the blood through tiny diverticula of arachnoid mater, called arachnoid villi, which project into the venous sinuses
• CSF is secreted continuously at a rate of about 0.5 ml per minute, i.e. 720 ml per day. The volume remains fairly constant at about 150 ml, as absorption keeps pace with secretion. CSF pressure may be measured using a vertical tube attached to a lumbar puncture needle inserted into the subarachnoid space above or below the 4th lumbar vertebra. Normal CSF pressure is 10-30 mm of water.
FUNCTIONS OF CSF-
• 1. Mechanical protection. CSF serves as a shock-absorbing medium that protects the delicate tissues of the brain and spinal cord from jolts that would otherwise cause them to hit the bony walls of the cranial cavity and vertebral canal.
• 2. Chemical protection. CSF provides an optimal chemical environment for accurate neuronal signaling. Even slight changes in the ionic composition of CSF within the brain can seriously disrupt production of action potentials and postsynaptic potentials.
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3. Circulation- CSF continuously circulates and allows
exchange of nutrients and waste products between the blood and nervous tissue.
CRANIAL NERVES-
• The human body contains 12 pairs of cranial nerves that emerge from the brain and supply various structures, most of which are associated with the head and neck.Their names generally suggest their distribution or function and they are numbered using Roman numerals according to the order they connect to the brain, starting anteriorly.
• I. Olfactory nerves (sensory) - These are the nerves of the sense of smell. Their sensory receptors and fibres originate in the upper part of the mucous membrane of the nasal cavity.
• II. Optic nerves (sensory)-These are the nerves of the sense of sight. The fibers originate in the retina of the eyes and theycombine to form the optic nerves
• III. Oculomotor nerves (motor)-These nerves arise from nuclei near the cerebral aqueduct. They supply the four of the six extrinsic muscles, which move the eyeball.
• IV. Trochlear nerves (motor)-These nerves arise from nuclei near the cerebral aqueduct. They supply the superior oblique musclesof the eyes.
• V. Trigeminal nerves (mixed)-These nerves contain motor and sensory fibres and are among the largest of the cranial nerves. They are the main sensory nerves for the face and head and receive impulses of pain, temperature and touch. The motor fibres stimulate the muscles of mastication. It has three branches - The ophthalmic nerves, The maxillary nerves and the The mandibular nerves .
• VI. Abducent nerves (motor)-These nerves arise from nuclei lying under the floor of the fourth ventricle. They supply the lateral rectus muscles of the eyeballs.
• VII. Facial nerves (mixed)-These nerves are composed of both motor and sensory nerve fibres,The motor fibres supply the muscles of facial expression. The sensory fibres convey impulses from the taste buds for sense of taste.
• VIII. Vestibulocochlear (auditory) nerves (sensory)-These nerves are composed of two divisions, the vestibular nerves and cochlear nerves. The vestibular nerves arise from the semicircular canals of the inner ear and are associated with the maintenance of posture and balance. The cochlear nerves originate in the spiral organ (of Corti) in the inner ear and convey impulses of hearing .
• IX. Glossopharyngeal nerves (mixed)-The motor fibres arise from nuclei in the medulla oblongata and stimulate the muscles of the tongue and pharynx and the secretory cells of the parotid (salivary) glands. The sensory fibres convey impulses from the posterior third of the tongue, the tonsils and pharynx and from taste buds in the tongue. These nerves are essential for the swallowing and gag reflexes.
• X. Vagus nerves (mixed)- these are longest cranial nerves. They pass down through the neck into the thorax and the abdomen. These nerves form an important part of the parasympathetic nervous system. The motor fibres arise from nuclei in the medulla and supply the smooth muscle and secretory glands of the pharynx, larynx, trachea, bronchi, heart, carotid body, oesophagus, stomach, intestines, exocrine pancreas, gall bladder, bile ducts, spleen, kidneys, ureter and blood vessels in the thoracic and abdominal cavities. The sensory fibres convey impulses from the membranes lining the same structures to the brain.
• XI Accessory nerves (motor)- These nerves arise from nuclei in the medulla oblongata and supply the sterno-cleidomastoid and trapezius muscles.
• XII Hypoglossal nerves (motor)- These nerves arise from nuclei in the medulla oblongata. They supply the muscles of the tongue and muscles surrounding the hyoid bone and contribute to swallowing and speech.
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