14.3 A Reflex Arc

DEFINITION: "The nerve pathway in the body taken by an action potential which leads to a rapid, involuntary response to a stimulus"

- Involves only 3 NEURONES: One of the neurones is called the spinal cord, and so this type of reflex is known as a spinal reflex
- The main stages of a spinal reflex:

Stimulus - heat from object
Receptor - thermoreceptors in the skin detect this which generates nerve impulses in the sensory neurone
Sensory neurone - passes nerve impulses to the spinal cord
Co-ordinator - (intermediate neurone) links the sensory neurone to the motor neurone in the spinal cord
Motor neurone - carries nerve impulses form the spinal cord to a muscle in the upper arm
Effector - the muscle in the upper arm which is stimulated to contract
Response - Pulling the hand away from the object

Why reflex actions are important:

As they are involuntary so that the brain in not involved in the decision making process (thereby faster), preventing it from being overloaded by stimuli which carry out the same response and can carry out more complex responses. (Some impulses are sent to the brain so that it is informed of what is happening and can override the reflex if necessary)
Protect the body from harm and are effective from birth and therefore do not have to be taught/learnt
Fast (because the neurone pathway is short with very few, typically one or two, synapses) which is very important in withdrawal reflexes

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