The nervous system is composed of two major categories or types of cells: the neurons and glial cells.

Neurons

The nervous system is composed of a specific types of cell known as the neuron (“neurone” or “nerve cell”). The nervous system of all animals is made up of extremely specialised cells known as neurons which can detect, receive and transfer various kinds of stimuli.
Neurons are different from other cells in many ways, but their major basic function is that they interact with other cells through synapses, which are membrane-to-membrane junctions that contain molecular mechanism that permits speedy transmission of signals, both electrical and chemical.
Numerous types of neuron have an axon, a protoplasmic projection that can expand to far-away parts of the body and make thousands of synaptic contacts. Axons regularly move through the body in bundles known as nerves.
Even in the nervous system of a single species like humans, hundreds of various types of neurons are present, with a broad variety of morphologies and roles.
These consist of sensory neurons that transform physical stimuli like light and sound into neural signals, and motor neurons that transform neural signals into activation of muscles or glands; conversely in a lot of species the great majority of neurons obtain all their contribution from other neurons and transfer their output to other neurons.

Glial cells

Glial cells are non-neuronal cells that offer support and nutrition, sustain homeostasis, form myelin, and take part in the transmission of signal in the nervous system. In the human brain, it is projected that the total number of glia approximately equals the number of neurons, even though the proportions differ in various portions of the brain.
Among the majorly crucial role played by glial cells are to support neurons and clutch them in place; to provide nutrients to neurons; to electrically insulate neurons; to annihilate pathogens and get rid of dead neurons; and to make available guidance cues directing the axons of neurons to their targets.
A very crucial type of glial cell oligodendrocytesin the central nervous system and Schwann cells in the peripheral nervous system produces layers of a fatty substance known as myelin that enfolds around axons and makes electrical insulation which permits them to put out action potentials much more hastily and competently.
Diagram showing the major divisions of the vertebrate nervous system
The nervous system of vertebrates humans inclusive is divided into the central nervous system(CNS) and the peripheral nervous system (PNS).
The (CNS) is the main division, and is made up of the brain and the spinal cord. The spinal canal possesses the spinal cord, whereas the head caries the brain.
The CNS is cased and shielded by the meninges, a three-layered system of membranes which include a tough, fibrous outer layer known as the dura mater. The brain is as well covered by the skull, and the spinal cord by the vertebral column.
The peripheral nervous system (PNS) is a combined name for all the nervous system structures that do not fall within the CNS.
The large majority of the axon bundles known as nerves are well thought-out to fit into the PNS, even while the cell bodies of the neurons to which they fit in exist in the brain or spinal cord. The PNS is somatic and visceral parts.
The somatic part is composed of the nerves that innervate the skin, joints, and muscles. The cell bodies of somatic sensory neurons are situated in dorsal root ganglia of the spinal cord.
The visceral part, as well known as the autonomic nervous system, possesses neurons that permeate the internal organs, blood vessels, and glands. The autonomic nervous system itself is composed of two parts: the sympathetic nervous system and the parasympathetic nervous system.
A few scientists as well include sensory neurons whose cell bodies is situated in the periphery for senses such as sense of hearing as part of the PNS; while others nonetheless omit them.
The vertebrate nervous system can as well be divided into areas known as grey matter and white matter. Grey matter which is merely grey in conserved tissue, and is better regarded as pink or light brown in living tissue possesses a high quantity of cell bodies of neurons.
White matter is mainly made up of myelinated axons, and takes its color from the myelin. White matter consists of all the nerves, and the majority of the interior of the brain and spinal cord.
Grey matter is created in clusters of neurons in the brain and spinal cord, and in cortical layers that line the surfaces of the brain and the spinal cord.
There is an anatomical rule which states that a cluster of neurons in the brain or spinal cord is known as a nucleus, while a cluster of neurons in the periphery is known as a ganglion. There are, nevertheless, a few exceptions to this rule, remarkably including the part of the forebrain known as the basal ganglia.

The Human Nervous System:-

The human nervous system is divided into two parts:
(i) Central Nervous System (CNS)
(ii) Peripheral Nervous System (PNS)
The CNS is composed of the brain and the spinal cord and is the location for information processing and control. The PNS is composed of all the nerves of the body that are connected with the CNS – the brain and spinal cord. The nerve fibres of the PNS are divided into two:
(a) Afferent Fibres
(b) Efferent Fibres
The afferent nerve fibres transmit impulses from tissues/organs to the CNS and the efferent fibres transmit regulatory impulses from the CNS to the particular peripheral tissues/organs.
The PNS is classified into two:
The somatic nervous system and autonomic nervous system
The somatic nervous system conveys impulses from the CNS to skeletal muscles whereas the autonomic nervous system conveys impulses from the CNS to the involuntary organs and smooth muscles of the body. The autonomic nervous system is again classified into sympathetic nervous system and parasympathetic nervous system.
The neuron as structural and functional unit of nervous system
A neuron is a tiny structure made up of three main parts, specifically, cell body, dendrites and axon.
Cell Body: The cell body possesses the cytoplasm with characteristic cell organelles and definite granular bodies known as Nissl’s granules.
Dendrites: Dendrites are short fibres that branch repetitively and protrude out of the cell body. It is as well made up of Nissl’s granules and is known as dendrites. These fibres transmit impulses towards the cell body.
Axon: The axon is an enlongated fibre, which is subdivided repeatedly at the distal end. Every branch ends like a bulb-like structure known as synaptic knob which contain synaptic vesicles bearing chemicals refered to as neurotransmitters. The axons convey nerve impulses away from the cell body to a synapse or to a neuro-muscular junction.
In respect of the number of axon and dendrites, the neurons are separated into three types:
(a) Multipolar: A neuron that contains one axon and two or more dendrites; located in the cerebral cortex,
(b) Bipolar: A neuron that contains one axon and one dendrite, located in the retina of eye and
(c) Unipolar : A neuron with cell body and one axon only; usually visible in the embryonic stage).
The axon is made up of two types, namely, myelinated and nonmyelinated axon. The myelinated nerve fibres are covered with Schwann cells, which develop into a myelin sheath about the axon.
The gaps connecting two adjacent myelin sheaths are known as nodes of Ranvier. Myelinated nerve fibres are located in the spinal and cranial nerves.
Unmyelinated nerve fibre is covered by a Schwann cell that does not develop into a myelin sheath about the axon, and is frequently created in autonomous and the somatic nervous systems.
The human central nervous system
The brain is the central information processing organ of our body, and functions as the ‘command and control system’.
It regulates the voluntary movements, balance of the body, performance of vital involuntary organs like the lungs, heart, kidneys, etc., thermoregulation, hunger and thirst, circadian (24-hour) rhythms of our body, actions of quite a lot of endocrine glands and human actions.
It is as well the site for processing of vision, hearing, speech, memory, intelligence, emotions and thoughts.
The human brain is well sheltered by the skull. Inside the skull, the brain is enveloped and shielded by cranial meninges made up of an outer layer called dura mater, a very thin middle layer known as arachnoid and an inner layer touches the brain tissue known as the pia mater. The brain can be classified into three major parts:
(i) The Forebrain,
(ii)The Midbrain, and
(iii) The Hindbrain

Forebrain

The forebrain is made up of the cerebrum, thalamus and hypothalamus. Cerebrum consists of the greater part of the human brain. A profound cleft splits the cerebrum longitudinally into two halves. These two halves are known as the left and right cerebral hemispheres.
The hemispheres are linked by a tract of nerve fibres known as corpus callosum.
The layer of cells which envelops the cerebral hemisphere is known as the cerebral cortex and is made up of high up folds. The cerebral cortex is known as the grey matter as a result of its greyish appearance. The neuron cell bodies are intense here resulting to that colour.
The cerebral cortex is composed of motor areas, sensory areas and large regions that are not classified as sensory or motor in function. These regions are known as the association areas. They are responsible for intricate functions such as intersensory associations, memory and communication.
Fibres of the ducts are covered with the myelin sheath, which makes up the inner part of cerebral hemisphere. They offer an opaque white look to the layer and, therefore are known as the white matter.
The cerebrum wraps just about a structure known as thalamus, which is the main coordinating centre for sensory and motor signals. An additional highly essential part of the brain –the hypothalamus is found at the base of the thalamus.
The hypothalamus has some centres which regulate the body temperature, desire for eating and drinking. It as well possesses many groups of neurosecretory cells, which secrete hormonesknown as the hypothalamic hormones.

The Midbrain

The midbrain lies between the thalamus/hypothalamus of the forebrain and pons of the hindbrain. A canal known as the cerebral aqueduct passes via the midbrain. The dorsal part of the midbrain made up of mainly four round protrusions or lobes known as corpora quadrigemina. Midbrain and hindbrain together act as the stem of the brain.

Hindbrain

The hindbrain is made up of the pons, cerebellum and medulla which as well known as the medulla oblongata. Pons is made up of fibre ducts that can be integrated to different parts of the brain.
Cerebellum has extremely complicated surface which enable it to provide the extra space needed for a lot of neurons. The medulla of the brain is linked to the spinal cord. The medulla contains centres which control respiration, cardiovascular reflexes and gastric secretions.