The nervous system is a complex network of nerves and cells that carry messages to and from the brain and spinal cord to various parts of the body. It consists of large number of neurons that conduct the impulse in different parts of the body in the form of electrical signals i.e. nerve impulses. Their actions are rapid and localized. The nervous system detects environmental changes that impact the body, then works in tandem with the endocrine system to respond to such events.

The nervous system of human is divided into 3 categories :

1. Central nervous system (CNS)
2. Peripheral nervous system (PNS)
3. Autonomic nervous system (ANS)

1. CNS : The CNS consists of hollow structure of neural tissue situated along the mid dorsal axis of body.The neurons are supported by neuro-glial cells they can replicate and are non-excitable.It consists of brain and spinal cord.

Brain

The brain is made up of different parts. These include the cerebrum, the cerebellum, the thalamus, the hypothalamus and the brainstem.

The cerebrum is the largest part of the brain. It controls intelligence, memory, personality, emotion, speech, and ability to feel and move. It is divided into left and right hemispheres, linked by a band of nerve fibres in the centre of the brain called the corpus callosum.

Each hemisphere is divided into 4 lobes, or sections, which are all connected.

• The frontal lobes control movement, speech and some of the functions of the mind like behaviour, mood, memory and organization.
• The temporal lobes play an important part in memory, hearing, speech and language.
• The parietal lobes play an important part in taste, touch, temperature and pain, and also in the understanding of numbers, awareness of the body and feeling of space.
• The occipital lobes are vital for being able to see clearly.

Deep inside the brain are the thalamus and the hypothalamus. The thalamus moves information to and from the lobes, and controls movements and memory. The hypothalamus controls appetite, thirst and body temperature, and produces hormones that control the release of other hormones in the pituitary gland.

Spinal Cord

It is the cylindrical and elongated part of CNS lying inside the vertebral canal of the vertebral column. It is surrounded by the meninges and the cerebro spinal fluid (CSF). The PNS consists of nerves and ganglia, which lie outside the brain and the spinal cord. The main function of the PNS is to connect the CNS to the limbs and organs, essentially serving as a relay between the brain and spinal cord and the rest of the body. Unlike the CNS, the PNS is not protected by the vertebral column and skull, or by the blood–brain barrier, which leaves it exposed to toxins.

2. PNS

The peripheral nervous system can be divided into the somatic nervous system and the autonomic nervous system. In the somatic nervous system, the cranial nerves are part of the PNS with the exception of the optic nerve (cranial nerve II), along with the retina. The second cranial nerve is not a true peripheral nerve but a tract of the diencephalon. Cranial nerve ganglia, as with all ganglia, are part of the PNS. The autonomic nervous system exerts involuntary control over smooth muscle and glands. The connection between CNS and organs allows the system to be in two different functional states: sympathetic and parasympathetic.

3. ANS

Some part of our body perform the functions automatically under the control of special kind of nervous system called the autonomic nervous system (ANS). The function(control) of special kind of nervous sytem is initiated in the brain. It mainly controls the functions of visceral organs such as heart, blood vessels, iris, cilliary mucsles, many thoracic and abdominal organs and the  glands of the body. They are mainly motor nerves arising from the CNS. They travel to the effector organs along with the peripheral nerves.

The autonomic nervous system is divided into the sympathetic nervous system and parasympathetic nervous system.

Differences between sympathetic and parasympathetic nervous system.

Sympathetic nervous system	Parasympathetic nervous system
Involved in the fight or flight response.	Involved in maintaining homeostasis and also, permits the rest and digest response.
The sympathetic system prepares the body for any potential danger.	The parasympathetic system aims to bring the body to a state of calm.
Sympathetic system has shorter neuron pathways, hence a faster response time.	Has comparatively longer neuron pathways, hence a slower response time.
Increases heartbeat, muscles tense up.	Reduces heartbeat, muscles relaxes.
The pupil dilates to let in more light.	The pupil contracts.
Saliva secretion is inhibited.	Saliva secretion increases, and digestion increases.

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