Human Nervous System

"Do you have a brain?" We often ask this question to our friends who say something or act in a way of ignorance. "I am nervous". We say this in the fear of exams. "ouch" we scream and take off our hand when something sharp pricks it. Do you know that all this happens due to the nervous system, which has a wonderful networking system? Lets see some important things regarding this system.
The nervous system enables us to feel, think, move, hear, see etc. It controls body activities. It establishes co-ordination between different organs of the body. Nervous system is divided into 3 divisions, which inturn have their components.

You learn about central and peripheral nervous systems in this lesson.
Nervous system can sense the changes inside and outside the body. The nervous impulses are in the form of small electrical currents. Nervous system is made up of neurons and glial cells.
Neurons: These are the functional units which receive and process information and generate responses. Cyton, axon and dendrites are the different parts of a neuron.

Cyton or perikarya has a large neucleus and the cell body has nissile granules, axon otherwise called as nerve fibre is a projection from the cell body. The corners of the cell are drawn into a number of branched threads called dendrites. They form synapses with the dendrites or axons of other cells.
Synapse: It is the junctional place between two neurons. Dendrites of one axon lie in close association with dendrites of another axon.
Action Potential: The electric potential generated between axon and cell body is called action potential or nerve impulse.

Efferent (outgoing) Nerves: These carry impulses from brain to spinal cord or effector organs. They are also called motor nerves (movement of hand and legs).
Afferent (incoming) Nerves: These carry information from the sense organs to specific areas in brain and spinal cord. They are also called sensory nerves (ears, nose, tongue, eyes, skin etc).
Mixed Nerves: These have both motor and sensory nerve fibres.
Central Nervous System
Brain:It consists of two parts. The brain located in the skull and the spinal cord located in the vertebral column. It is a very soft tissue and is surrounded by a bony case called cranium, which protects the brain from injuries. The brain is covered by three membranes. A fluid called cerebro-spinal fluid flows in between the outer and middle membranes, which also protects it from injuries.
The weight of the average human brain triples between birth and adulthood. The final weight in an adult male is 1.4 kg and in female is 1.3 kg. It is about 3% of the body weight. It uses about 20% of the oxygen a man breathes, 20% of calories a man takes in and about 15% of body blood.

(i) Cerebrum: It is the largest part of the brain consisting of two hemispheres which control voluntary actions and are the seat of intelligence, memory association, imagination and will. The cerebral cortex consists of several ridges called 'gyri' and the grooves called 'sulci', which increase its surface to accommodate more number of neurons.
(ii) Cerebellum: The large mass with grey matter on the surface and white matter in the deeper layers. It controls all the voluntary movements of the body. It is responsible for the maintainance of equilibrium and posture of the body.
(iii) Medulla Oblongata: The lower most part of the brain which controls involuntary actions. It continues as the spinal cord in the vertebral column.
Spinal cord: It is the extention of medulla oblongata. It is concerned with reflex actions. From the spinal cord arise 31 pairs of spinal nerves. it acts as a relay station by receiving and sending information from body parts to brain and viceversa.

It consists of cranial nerves and spinal nerves.
Cranial nerves: 12 Pairs of these nerves take their origin from different areas of brain. Some of these nerves ae sensory. Some are motor and a few are mixed nerves. Branches of these nerves go to the retina, ear, nose, tongue, eye, muscles, face, neck and pharynx. Of these nerves, vagus is more important as it controls the rate of heart beat and secretions of pancreas.
Spinal nerves: 31 pairs of these nerves originate from spinal cord, which are mixed nerves.

The sensory fibres originate from the dorsal horn of the spinal cord, which carry information from sensory organs to spinal cord. The motor fibres take their origin from the ventral horn of the spinal cord, which supply information required for the movement of muscles.

Reflex Action and Reflex Arc:
Reflex Action: Simply called as reflexes are very important as they save us from painful and dangerous stimuli. They are fast, immediate; automatic and involuntary. These are 2 types.
Unconditional reflexes: These are inborn, not learnt and same in all individuals and animals.
Eg: Withdrawing our hand when we touch a hot object.
Conditional Reflexes: These are learnt by doing same actions many times.
Eg: standing in attention when hearing National Anthem.
Reflex Arc: It is the structural and functional unit that carries out the reflex actions. It consists of a receptor, a sensory nerve, an association neuron, motor nerve and an effector organ which carry the whole process of receiving information and generating electrical responses.