Structure of Human Brain:
The human brain is an enlarged, specialized frontal region of the spinal cord that lies on the cranial cavity of the skull. It is soft and delicate, white, flattened structure and weighs about 1200-1400 grams in man. It has 100 billion neurons, each can connect with as many as 25,000 other cells. It has a volume of 1440 c.c. It constitutes the major portion of the central nervous system. The brain is surrounded by three meninges- dura mater, arachnoid mater and pia mater. The human brain is divided into three major parts-
- Fore Brain (Prosencephalon).
- Mid Brain (Mesencephalon).
- Hind Brain (Rhombencephalon).
Fore Brain or Prosencephalon:
It is the largest part of the brain and forms anterior two-third of the brain and is formed of three parts- olfactory lobes, cerebral hemispheres and diencephalon.
Olfactory Lobes or Rhinencephalon:
They are a pair of solid club-shaped structures. Each olfactory lobe has an olfactory bulb and an olfactory tract. The olfactory lobe is covered dorsally by cerebral hemispheres. They are visible only ventrally. Each olfactory lobe has a cavity called Rhinocoel (olfactory ventricle or 1st ventricle). It is absent in human beings. The olfactory lobes control the sense of smell which are highly developed in Scoliodon (Shark, Dog Fish) and Dogs and very poor in case of man because man totally depends upon the sight in spite of moderate vision.
Cerebrum or Cerebral Hemisphere:
It constitutes 80% of the brain, thus it is the largest part of the brain. It consists of two large-sized cerebral hemispheres that are connected by a transverse band of nerve fibre, the corpus callosum. The left half of the brain controls the right side of the body and vice versa. The outer wall of the cerebrum is called cerebral cortex which makes the grey matter of it. It is thrown into folds to form the ridges and depressions called gyri and sulci respectively. The cerebral hemisphere is divided by a deep fissure (longitudinal fissure) of sulci into four distinct areas. These are called frontal, parietal, temporal and occipital lobes. The central sulcus demarks the frontal lobe from the parietal lobe, the parieto-occipital sulcus demarcates the parietal lobe from the occipital lobe and lateral sulcus or Sylvian fissure demarcates the frontal and parietal lobes from the temporal lobe.
The cerebral cortex is the outermost region of the cerebrum and is mainly made up of grey matter. It contains roughly 10 per cent of all neurons of brains. The central region of the cerebral cortex is called the cerebral medulla and it is formed of white matter. The cerebral cortex is the highest centre for many sensations and activities and has a number of separate areas.
- Motor area- lies in the frontal lobe and controls the voluntary movements.
- Premotor area- lies in the frontal lobe and controls involuntary movements.
- Association area- lies in the frontal lobe and controls association between various sensations and movements. It is associated with memory, learning, reasoning and intelligence.
- Somaesthetic area- lies in the parietal lobe and control general sensations like pain, touch, pressure, temperature.
- Visual area- lies in the lower region of the occipital lobe and controls the visual sensation.
- Auditory area- lies in the temporal lobe and centre for hearing, pitch, loudness and quality of sound is perceived.
- Olfactory area- lies deep in the temporal lobe and controls the sense of smell.
- Taste area- lies in the parietal lobe and controls gustation.
Functions of Cerebrum:
- It controls all the voluntary activities of the body.
- It perceives all the sensory information from the body.
- It transmits motor impulse to the skeletal muscle for the action.
- It is seat of memory, will, intelligency, reasoning and learning.
It is only visible from the inferior side of the brain. The cavity of the diencephalon is narrow, lined by ependyma named as 3rd ventricle. It communicates anteriorly with lateral ventricle through foramen of Monro and posteriorly with 4th ventricle by a passage iter.
Diencephalon has three parts- epithalamus, thalamus and hypothalamus.
Epithalamus constitutes the roof of the diencephalon and is non-nervous tissue. Its anterior part is vascularized and forms the anterior choroid plexus for the secretion of cerebrospinal fluid. Its posterior part forms the small stalk, the pineal stalk with a rounded body the pineal body on it. The pineal body is considered as a vestigial organ but may secrete melatonin hormone.
It is a region present at the centre of the forebrain. All sensory information that reaches the cerebral hemispheres first passes through the thalamus. Its function is to interpret sensory and motor signals and then channel them to the appropriate cerebral cortex region.
It is the floor of the diencephalon, consists of masses of grey matter scattered in the white matter called hypothalamic nuclei. It is visible ventrally. It controls the involuntary functions like hunger, thirst, sleep, fatigue, sweating, sexual desire, water balance, blood pressure, anger, fear, temperature regulation etc. In association with the pituitary gland, it secretes neurohormones called hypothalamic hormones and controls the release of hormones from the anterior pituitary. It also controls the synthesis and release of posterior pituitary hormones. Hypothalamus also gives a nervous process called infundibulum which meets a pharyngeal outgrowth called hypophysis. Both collectively form master gland called the pituitary body. In front of the hypothalamus, there is a cross of two optic nerves called optic chiasma. Behind the hypothalamus, there is one pair of small, rounded, nipple-like bodies called mammillary bodies or corpora mammillaria.
Mid Brain or Mesencephalon:
It is found on the dorsal side and consists of two parts- optic lobes and cerebral peduncles. It is completely enveloped by cerebrum and contains many groups of nerve cells scattered in the white matter. Optic lobes are four in number (optic quadrigemina) consisting of a pair of upper superior colliculi and a pair of lower inferior colliculi. The superior and inferior colliculi of each side is known as corpora bigemina. The optic lobes are the centre for visual sensation and auditory stimuli. The cerebral peduncles or crura cerebri are a pair of thick bundles of longitudinal nerve fibres lying on the base of mid-brain. They connect forebrain with hindbrain. It is involved in controlling muscle tone and modifying some motor activities. Also relaying the impulses in the brain like sensory as well as motor.
Hind Brain or Rhombencephalon:
Hindbrain consists of three regions- Cerebellum, Medulla Oblongata and Pons Varoli.
Cerebellum or Little Brain:
It is the second largest part of the human brain which consists of two lateral cerebellar hemispheres and central worm-shaped part, the Vermis. It forms about 1/8th of the brain mass. It lies below the posterior position of cerebral hemispheres and above the medulla. The cerebellum is solid. A cross-section of the hemispheres shows a branching tree-like network of white matter known as Arbor Vitae. It is surrounded by a sheath of grey matter- Cerebellar Cortex. The later is greatly folded and externally shows the presence of gyri. The cerebellum helps to maintain the equilibrium of the body, moderation of voluntary movements and maintenance of muscle toning.
It is an oval mass present in front of cerebellum below the midbrain and above the medulla oblongata. It generally consists of nerve fibres which form a bridge between the two hemispheres of the cerebellum hence the name Pons (Pons-bridge). Its white matter tract also connects the medulla with higher brain centres and the spinal cord. Pons Varoli acts as a relay centre amongst different parts and sides of the brain. It also has a respiratory centre named as pneumotaxic centre or shutting centre.
Medulla Oblongata or Myelencephalon:
It is posterior most part of the brain. It is a pyriform or conical region which extends from the pons varoli above and is continuous with the spinal cord below. Medulla encloses a cavity called the fourth ventricle or Myeocoel which is continuous with the central canal of the spinal cord. The root of medulla oblongata is thin, non-nervous formed of epithelial tissue which forms the folds. These folds are called posterior choroid plexus. Below the plexus, the roof has three openings- a pair of lateral aperture (foramina of Luschka) and a pair of median aperture (foramen of Magendie). These apertures permit the exit of cerebrospinal fluid into space around the brain. Medulla Oblongata controls involuntary functions of the body through centres like cardiac centres (heartbeat); respiratory centres (respiration); vasomotor centres (a contraction of blood vessels); salivary centres (secretion of saliva) etc. It also controls coughing, sneezing, vomiting, urinating, defaecation, gut peristalsis, swallowing of food, etc.
Midbrain, pons varoli and medulla oblongata collectively form the brain stem. It connects forebrain with the spinal cord.
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