The central nervous system is the most complex and delicate of all the body systems.
It is a coordination network that controls every thought, movement and internal process of the body.
At its centre is the brain which contains billions of nerve cells which send out and receive nerve impulses. The spinal cord, the second part of the central nervous system is a thick bundle of nerves that runs down the centre of the spine and connects the brain to the rest of the body.
The brain weighs 2% of the body’s weight but uses 20% of its energy.
The spinal cord extends from the bottom of the skull down through the first 19 vertebrae of the back. There are 31 pairs of nerves that branch from the spinal cord. Each pair of nerves is linked to that part of the body closest to the branch point.
The delicate spinal cord is protected by the spinal column. Each vertebra has a cavity which the spinal cord passes through. Spinal nerves emerge from between each vertebra.
Outside the central nervous system is the peripheral nervous system, a branching network of nerves that reach every part of the body and transmit signals between the body and the central nervous system.
The billions of linked nerve cells that form the nervous system are called neurones. They carry electrical signals or nerve impulses, at high speeds. Spinal nerves carry sensory impulses to the spinal cord and brain and motor nerve impulses from the brain to the rest of the body.
Sensory neurones carry nerve impulses from sensors to the central nervous system and motor neurones relay signals from the central nervous system that make muscles contract. The central nervous system sends out messages to 640 muscles around the body.
The brain can be divided into three main structures; the cerebrum, the cerebellum and the brain stem.
The cerebrum forms the bulk of the brain and is separated into right and left halves or hemispheres. This part of the brain is responsible for many functions including speech, memory and, consciousness and logical and emotional thought.
The areas of the cerebrum that receive nerve impulses from receptors are called sensory areas while those standing out impulses are called motor areas.
The cerebral cortex or surface of the cerebrum contains localised areas concerned with specific functions but also has a number of cranial nerves which are connected directly to the organs such as the eyes and the ears (optic nerve and auditory nerve).
The cerebellum is the region of the brain that controls balance and muscular coordination. It is located at the back of the brain and allows precise control during activities such as walking and running.
The brain stem links the brain to the spinal cord and is responsible for regulating the body’s crucial life support mechanisms such as heart rate, blood pressure and breathing.
Nerve cells that originate and process electrical impulses are called neurones. Neurones are cells which process, transmit and receive nerve impulses (electrical signals). They are connected to other neurones or to receptors in muscles, organs or glands via synapses. There are billions of neurones in the human body, constantly sending and receiving messages to and from the brain and the spinal cord.
Sensory neurones conduct nerve impulses from receptors such as eyes, nose, ears to the central nervous system. Motor neurones conduct nerve impulses from the central nervous system to effectors such as muscles and endocrine glands.
A neurone consists of three parts; a cell body; fibres that carry nerve impulses into cell bodies (known as dendrons in motor neurones) and axons which carry nerve impulses from cell bodies. Neighbouring neurones are separated by a gap called synapse. The synapse is the connection between an axon and dendrite or target cell.
Arrival of a nerve impulse at an axon terminal (axon is a nerve fibre which carries nerve signals away from the neuron cell body), stimulates the release of neurotransmitters, these travel to receptors in the dendrite (a dendrite carries signals toward the neuron cell body) or target cell and trigger a second impulse. The process continues until the signal reaches its destination.
There is also a synapse where motor neurones and muscle fibres meet.
When a neurone is stimulated, by heat, cold, touch, sound vibrations or some other message, each generates a tiny electrical pulse and a changing chemical levels. This electricity and chemical change travels the full length of the neurone to the synapse and then across the gap to the next cell.
The process continues from the receptor nerve cells in a part of the body such as the fingertips, all the way to the spinal cord.
From there, the message travels along a complex system of cells to the brain. The sensory area of the brain processes and interprets the message, then sends back a response from a motor area through a series of motor neurones which work in the same way but connect to muscle fibres.
It sounds complex but the nerve impulse takes just one 100th of a second to travel from your big toe to your spinal cord.
