The main organs of the nervous system are: Brain, Spinal cord and Nerves. The sense organs like eyes, ears, tongue, nose and skin can be considered to be other organs of the nervous system because they help in the functioning of the nervous system. The main organs of the nervous system.
The brain is located inside the skull of our head. The spinal cord is a very thick nerve which runs inside the cavity of backbone in our body. The upper end of spinal cord is attached to the brain. The nerves are a kind of wires which are distributed all over our body. The brain and spinal cord are connected to all the sense organs and other parts of our body by millions of nerves.
There are mainly two types of nerves in our body: cranial nerves and spinal nerves. The cranial nerves connect all the parts in the head directly to brain. The spinal nerves connect all the remaining parts of the body (like muscles and skin, etc.) to the spinal cord. There is also a third type of nerves called visceral nerves.
Most of the visceral nerves connect the internal organs of the body to spinal cord though some also connect to brain (Visceral nerves have not been shown in Figure 28 to keep the diagram simple and avoid confusion).
The cranial nerves, spinal nerves and visceral nerves are also of two types : sensory nerves and motor nerves. The nerves which carry messages from the body parts to the brain (or spinal cord) are called sensory nerves. And the nerves which carry message from the brain (or spinal cord) to the body parts for action are called motor nerves.
This is how the nervous system works:
When the sense organ (like eyes, ears, tongue, nose, or skin) in our body is affected, it sends the message to the brain in the form of electrical impulses (called nerve impulses) through the sensory neurons. The brain analyses this message and decides the action to be taken.
The brain then sends out instructions to the muscles of the concerned body part (for taking necessary action) through motor nerves. The concerned body part then acts according to the instructions sent by the brain. Please note that in the processing of complicated responses (which require thinking) both, the brain and spinal cord are involved, but in the simple responses (which do not require thinking), the spinal cord alone is involved.
The nervous system can be divided into two main parts:
1. Central nervous system (consisting of brain and spinal cord), and
2.Peripheral nervous system (consisting of all the nerves of the body like cranial nerves, spinal nerves and visceral nerves).
The peripheral nervous system can be further divided into two parts:
(i) Voluntary nervous system (which is under voluntary control from the brain), and
(ii) Autonomic nervous system (which operates automatically or involuntarily).
All the nerves of the body together make up the peripheral nervous system (PNS). They all enter or leave the central nervous system. The three types of nerves which make up the peripheral nervous system are spinal nerves, cranial nerves and visceral nerves. Spinal nerves arise from the spinal cord along most of the length of the spinal cord and spread throughout the body (except the head).
They all carry both sensory and motor neurons and are described as mixed nerves. Cranial nerves arise from the brain and spread throughout the head. They also carry both sensory and motor neurons.
The visceral nerves are a special kind of nerves which mostly arise from the spinal cord (though some also arise from the brain). They are connected to the internal organs of the body. Visceral nerves also carry both sensory and motor neurons.