|Human Brain and Neurone|
The human brain has the same general structure as the brains of other mammals, but is larger than any other in relation to body size. Large animals such as whales and elephants have larger brains in absolute terms, but when measured using the encephalization quotient which compensates for body size, the human brain is almost twice as large as the brain of the bottle nose dolphin, and three times as large as the brain of a chimpanzee. Much of the expansion comes from the part of the brain called the cerebral cortex, especially the frontal lobes, which are associated with executive functions such as self-control, planning, reasoning, and abstract thought. The portion of the cerebral cortex devoted to vision is also greatly enlarged in humans.
|The human brain has the same general structure as the brains of other mammals|
The human cerebral cortex is a thick layer of neural tissue that covers most of the brain. This layer is folded in a way that increases the amount of surface that can fit into the volume available. The pattern of folds is similar across individuals, although there are many small variations. The cortex is divided into four "lobes", called the frontal lobe, parietal lobe, temporal lobe, and occipital lobe. (Some classification systems also include a limbic lobe and treat the insular cortex as a lobe.) Within each lobe are numerous cortical areas, each associated with a particular function such as vision, motor control, language, etc. The left and right sides of the cortex are broadly similar in shape, and most cortical areas are replicated on both sides. Some areas, though, show strong lateralization, particularly areas that are involved in language. In most people, the left hemisphere is "dominant" for language, with the right hemisphere playing only a minor role. There are other functions, such as spatiotemporal reasoning, for which the right hemisphere is usually dominant.
|Different Parts of Neurone|
The nervous systems consists of vast number of cells called neurones,supported by a special type of connective tissue ,neuroglia. each neurone consists of a cell body and its processes,one axon and many dendrites.Neurone are reffered to as nerve cells.Bundles of axons bound together are called nerves. Neurones cannot divide,and for survival they need a continuous supply of oxygen and glucose.Unlike many other cells, neurones can synthesise chemical energy(ATP) only
from glucose.The physiological units of nervous system are nerve impulses or action potential, which are akin to tiny electrical charges.However, Unlike ordinary electrical wires, the neurones are actively involved in conducting nerve impulses In effect the initial strength of the impulse is maintained throughout the length of the neurone.Some neurones initiate nerve impulses are passed on and sometimes redirected.
PROPERTIES OF NEURONES:
Irritability is the ability to initiate nerve impulses in response to stimuli from:
· Outside the body e.g. touch light waves
· Inside the body e.g. a change in the concentration of carbon-di-oxide in the blood alters respiration; a thought
may result in voluntary movement.
Nerve cell vary considerably in size and shape but they are all too small to be seen by the naked eye.Cell bodies from the grey matter of the nervous system and are found at the periphery of the brain and in the centre of the spinal cord.Groups of cell bodies are called nuclei in the central nervous system and ganglia in the peripheral nervous system.An important exception is the basal ganglia(Nuclei) situated within the cerebrum.
AXONS AND DENDRITES:
Axons and dendrites are extensions of cell bodies and from the white matter of the nervous system.Axons are found deep in the brain and in groups called tracts,at the periphery of the spinal cord.They are reffered to as nerves or nerve fibres outside the brain and spinal cord.
Structure of axons:
The membrane of the axon is called the axolemma and it encloses the cytoplasmic extension of the cell body.Large axons and those of peripheral surrounded by myelin sheath. This consists of a series of schwann cell arranged along the length of the axon.There are tiny areas of exposed axolemma between adjacent schwann cells,called nodes of Ranvier,which assist the rapid transmission of nerve impulses in myelinated neurones. Some small fibres in the central nervous system are non-myelinated .The speed of transmission of nerve impulses is significantly slower in non-myelinated fibres.
This are the many short processes that receive and carry incoming impulses towards cell bodies.They have the same structure as axons but are ususally shorter and branching.In motor neurones they form the part of the synapes and in sensory neurone they form the sensory receptors that respond to specific stimuli.
|Different parts of Human brain|
This is the largest part of the brain and its occupies the anterior and middle cranial fossae.It is divided by a deep cleft,The longitudinal cerebral fissure, into right and left cerebral hemisphere each containing one of the lateral ventricles.Deep within the brain.The hemispheres are connected by a mass of the white matter called the corpus callosum.The falx cerebri is formed by the dura mater.It separates the 2 hemisphere and penetrates to the depth of the corpus callosum.The superficial part of the cerebrum is composed of nerve cell bodies or grey matter,forming the cerebral cortex and the deeper layers consist of nerve fibres or white matter.
The cerebral cortex shows many infolding or furrows of varying depth.The exposed areas of the folds are the gyri or convolutions and these are separated bu sulci.These convolutions greatly increased the surface area of the cerebrum.For descriptive purposes each hemisphere of the cerebrum is divided into lobes which take the names of the bones of the cranium under which they lie:
The boundaries of the lobes are marked by deep sulci.These are the central,lateral and parieto-occipital sulci.
|Different Parts of Cerebrum|
|Different Sensory Complex Response Control Area of Cerebrum|
|Different Sensory Complex Response Control Area of Cerebrum|
The cerebellum is situated behind the pons and immediately below the posterior cranial fossa.It is ovoid in shape and has two hemispheres, separated by a narrow median strip aclled vermis. Grey matter forms the surface of the cerebellum, and the white matter lies deeply.
· It co-ordinates activities associated with the maintenance of posture,balance and equilibrium.
· The cerebellum may also have a role in learning and language processing.
|Different Parts of Meninges|
The brain and spinal cord are completely surrounded by three layers of tissue, the meninges, lying between the skulland the brain, and between the vertebral foramina and the spinal cord.Named from outside inwards they are the:
· Dura mater
· Arachnoid mater
· Pia mater
The dura and arachnoid maters are separated by a potential space, the subdural space.The arachnoid and pia matersare separated by the subarachnoid space , containing cerebrospinal fluid.
The cerebral dura mater consists of two layers of dense fibrous tissue. The outer layer takes the place of the periosteum on the inner surface of the skull bones and the inner layer provides a protective covering for the brain.There is only a potential space between the two layers except where the inner layer sweeps inwards between the cerebral hemispheres to form falx cerebri; between the cerebellar hemispheres to form the falx cerebelli; and between the cerebrum and cerebellum to form the tentorium cerebelli. Venous blood from the brain drains into venous sinuses between the dura mater.
This is a layers 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,containing cerebrospinal fluid.The arachnoid passes through 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.
This is delicate layer of connective tissue containing many minute blood vessels. It adheres to the brain,completelycovering the convultions and and dipping into each fissure.It continues downwards surrounding the spinal cord.Beyonds the end of the cord it continues as the filum terminale,pierces the arachnoid tube and goes on, with the dura mater, to fuse with the periosteum of the coccyx.
|Different parts of Human brain|
THE 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.They communicate with the 3rd ventricle by interventricular foramina.
The 3rd ventricle is a cavity situated below the lateral ventricles between the two parts of the thalamus.It communicates with the 4th ventricle by a canal, the cerebral aqueduct.
THE FOURTH VENTRICLE:
The 4th ventricle is a diamond-shaped cavity situated below and behind the 3rd ventricle,between the cerebellum and pons .It is continuous below with the central canal of the spinal cord and communicates with the sub arachnoid space by foramina in its roof.Cerebrospinal fluid enters the sub arachnoid space through these openings and through the open distal end of the central canal of the spinal cord.