The nervous system and stimuli

Animals increase their survival rate by responding to changes.These changes can be connected to their external environment, such as if it is too hot or too cold, the response to the external environment change would be to move.They also respond to changes in their internal environment, this ensures that the conditions are right for metabolic reaction to take place.Plants also increase their chances of survival by responding to changes in their environment.Any change in the external or internal environment is called a stimulus.

A stimulus is detected by a receptor, and an effector will produce a response.Receptors can be the sense organs, such as eyes, ears, nose, tongue and skin.  Groups of cells that detect external stimuli.  In the eye --> Rod and cone cells detect changes in light.Effectors are the cells that bring about a response to the stimuli.  They are likely to be muscle cells and cells found in glands, i.e. the pancreas.  Effectors respond in different ways, muscle cells for example will contract, whereas glands will secrete hormones.Receptors communicate with the effectors via the nervous system or the hormonal system or sometimes both.The nervous system is made up of all the neurones nerve cells.  The three main neurones are:  Sensory neurone, Relay neurone, Motor Neurone.

The Central Nervous System (CNS)The CNS is made up of the brain and spinal cord only.Receptors in a sense organ will detect a stimulus, they then sense electrical impulses along sensory neurones to the CNS.The CNS will then send a message to and effector, it does this by sending an electrical signal along a motor neurone, the effector will then act accordingly.The role of the CNS is to coordinate the response, to have a coordinated response, there will always need to be a stimulus, receptor and effector.By using high speed electrical impulses the nervous system can have very rapid responses.As quickly as the brain can be at deciding how to respond to stimuli, it at times is too slow, this is where the body will use and automatic reaction called a reflex.

ReflexesAn automatic response to certain stimuli is a reflex, and can help to reduce injury.For instance the pupil of the human eye responds to light,  if someone shines a bright light in the eye, the pupil automatically get smaller, this reduces the amount of light that gets into the eye and stops it becoming damaged.If the body gets a shock, the hormone adrenaline is automatically released, there is no waiting period to decide if you are in shock or not - it just happens.information in a reflex takes a route, this route is know as a reflex arc.

Reflex arc - neurones go through the spinal cord or unconscious part of the brain.When a receptor is subject to certain stimulus i.e. touching a naked flame, the receptor will send an impulse along a sensory neurone.When the impulse reaches the CNS it passes the message to a relay neurone, this is done across a synapse using chemicals.The relay neurone will pass the message to a motor neurone,  The message travels down the motor neurone to the effector, in the case of putting your hand in a flame, the effector is most likely to be a muscle.The muscle will then contract to pull your hand out of the way of the flame.  This process is so quick you don't even need to think about it!