Nervous System

THE NERVOUS SYSTEM The nervous system is organised such that there is a central nervous system, which receives information through the sensory system and then executes actions through the motor system. Central Nervous System (CNS) In humans, the CNS consists of the brain and the spinal cord. It functions as the central processing unit, where it receives information, integrates it, makes a decision about what to do and then delivers a command which may be either a positive or a negative response. The CNS receives information from the five special senses (sight, sound, smell, taste, and touch) and the special sense organs (eyes, ears, nose, and tongue) are strategically located close to the brain so that a constant volley of impulses can feed the brain. The ‘organ’ responsible for touch is the skin, and the skin has a very rich supply of sensory neurones in order to perceive even the slightest touch. The CNS executes commands via motor neurones that form the peripheral nervous system. The peripheral nervous system consists of 12 pairs of Cranial nerves (from the brain) and 29 pairs of Spinal nerves. The Neurone These cells are highly specialized and are described as being excitable. However, they are incapable of cell division and damage to nerve cells is usually permanent. The structure of the neurone shows remarkable specialisation:

There are 3 types of neurones: 1. The sensory neurone

2.

The intermediate neurone (relay fibre)

3.

The motor neurone

Their relative lengths of dendrons and axons can differentiate the three neurones. (The difference between the two being that the dendron carries the impulse towards the cell body and the axon carries the impulse away from the cell body) In addition, the position of the cell body can be used to identify the neurons.

The Organisation of the Nervous System STIMULUS

RECEPTOR

SENSORY NEURONE

CENTRAL NERVOUS SYSTEM (RELAY FIBRE) RESPONSE

EFFECTOR

MOTOR NEURONE

Any change in the external or internal environment (stimulus) can bring about a response according to the flow diagram depicted above. Types of stimuli would include not only the special senses but also touch, heat, cold, pain, pressure and pleasure (from the skin). Motor neurones will affect only two types of effectors namely muscle or glands, bringing about a response. The Nervous Impulse The main property or nerves and muscles that make them different to other cells are their excitability. What this means is that they are able to respond to a stimulus. This is due to the fact, that in the resting state their cell membrane is in a polarised state. The inside is negative relative to the outside bringing about a measurable potential difference of -70 mV, referred to as the Resting Membrane Potential (RMP). When a stimulus of sufficient strength is applied there is a change in the RMP, bringing about a reversal of charge such that the cell depolarises (The Membrane Potential becomes +40mV). This reversal of charge will stimulate neighbouring segments to depolarise resulting in a wave of depolarisation, which is effectively a nervous impulse. The Synapse This is the junction between 2 or more neurons. Unlike nervous transmission which is electrical, synaptic transmission is due to the release of a chemical, a neurotransmitter (eg. acetylcholine). • The impulse arrives at the synapse • Calcium ions move in • Chemical released by vesicles • Chemical diffuses across and binds with receptors • Sufficient binding causes the generation of an impulse in next neurone • Impulse travels along the next neurone Synapses ensure that the nervous impulse travels in only one direction. In the brain, they are believed to control memory, learning and intelligence.