The nervous system is a complex network of nerves and cells that perform three overlying functions of sensory input, integration, and motor output. This process is generally the same even at a very primitive level of the nervous system.
- The sensory input is sensing the environment and changes in an organism and is carried out by sensory organs like eyes, ears, nose, tongue, and skin, some of them performing simultaneously.
- The integration involves the processing of information and is carried out by the central nervous system (CNS), which is composed of the brain and the spinal cord.
- Moto-neuron output is the conduction of signals from the integration centre, the central nervous system, and is carried out by a group of effector cells, the muscle cells or gland cells, which actually carry out body’s responses to external stimuli. Both sensory input and motor output signals are carried through nerves, which are long rope-like structures made from nerve cells. Nerve cells are of two types – neurons and glia.
Neurons are the cells which actually carry through signals, whereas glial cells provide supporting structures and maintenance of neuronal cells. Nerves, many times, are made from end to end connection between neurons, supported by the glial cells. The nerves that communicate sensory and motor signals between the central nervous system and the rest of the body are collectively referred to as the peripheral nervous system (PNS). Sensory inputs are received by receptor cells located in sensory organs. For examples, light receptor cells are located in eyes, or chemical receptor cells are located on the surface of the tongue.
Signals from these receptors are carried through sensory neurons of the peripheral nervous system into the central nervous system, and after processing, the instructions are communicated through the motor neurons of the peripheral nervous system to effector cells, such as muscles. Communication from the receptor cells to effector cells is carried in two forms – chemical and electrical. Since communication of information involves more than one cells, the communication is through special chemicals called neurotransmitters or a specialized form of an electrical signal called an action potential.
Neurons are the functional unit of the nervous system. A neuron consists of three major parts – a cell body that contains the nucleus, dendrites, which receive signals, and a long axon that carries the signal to the next cell. Length of neurons varies depending on their location. Neurons located in the central nervous system could be a few millimetres long, but some of the neurons in the peripheral nervous system could be more than a meter long. In a normal human body, there are about two billion neurons, approximately 1 billion in the brain, and another billion in the rest of the body. Glial cells are supporting cells that provide structural and functional support to the neurons and help them carry out their functions.
For example, Schwann cells provide a covering of the axons in the peripheral nervous system.
End of a neuron is known as synaptic terminal, which generally connects with either another neuron to continue the process of communication or to a muscle to trigger muscle action. Glial cells outnumber neurons by 10 to 50-folds.
Organization of Nervous System Structurally
The nervous system is organized into two parts – the central nervous system and the peripheral nervous system. The central nervous system is made of the brain and the spinal cord. The brain is divided into three parts – Forebrain, Midbrain, and Hindbrain. The Forebrain develops into two parts – the telencephalon which consists of the cerebrum or the cerebral hemispheres and includes cerebral cortex, white matter, and basal nuclei; and diencephalon which consists of thalamus, hypothalamus, and epithalamus. The Midbrain develops through mesencephalon into a part of the brainstem. The Hindbrain develops through two parts, the metencephalon, and myelencephalon. The metencephalon eventually develops into pons (part of the brainstem) and cerebellum. The myelencephalon derives into medulla oblongata, which is also part of the brain stem.
The spinal cord which is about 45 cm in length, starts from the brain stems and stretches to the lowest end of the backbone. It comprises a nerve bundle and is protected by a series of vertebrae which are divided into five regions – cervical, thoracic, lumbar, sacral, and coccygeal.
The spinal cord itself spans only about two-thirds of the vertebral column, but the rest of the space is filled with nerve fibers of spinal roots. Both the spinal cord and the brain contain fluid-filled spaces or cavities called cerebrospinal fluid (CSF) which contains nutrients, hormones, and the white blood cells. Additionally, the cerebrospinal fluid acts as the shock absorber cushioning the brain and provides a direct link across the blood-brain barrier for exchanging nutrients and other essential biomolecules.