The Reflex Arc

• An involuntary (or reflex) response does not involve the conscious part of the brain as the coordinator of the reaction.
• Awareness of a response having happened occurs after the response has been carried out.
• Responses are therefore automatic and rapid – this helps to minimise damage to the body.
• The stimulus is detected by a (pain/pressure/touch) receptor in the skin.
• A sensory neurone sends electrical impulses to the spinal cord (the coordinator).
• An electrical impulse is passed to a relay neurone in the spinal cord.
• A relay neurone synapses with a motor neurone.
• A motor neurone carries an impulse to a muscle in the leg (the effector).
• The muscle will contract and pull the foot up and away from the sharp object (the response) when stimulated by the motor neurone.
• There are three main types of neurone in a reflex arc: sensory, relay and motor.
• Sensory neurones carry impulses from sense organs to the CNS (brain or spinal cord).
• Relay neurones are found inside the CNS and connect sensory and motor neurones.
• Motor neurones carry impulses from the CNS to effectors (muscles or glands).
• Sensory neurones are long and have a cell body branching off the middle of the axon.
• Relay neurones are short and have a small cell body at one end with many dendrites branching off it.
• Motor neurones are long and have a large cell body at one end with long dendrites branching off it.
• Neurones never touch each other, they are separated by junctions (gaps) called synapses.
• Synaptic junctions are incredibly small - around 10nm in size - and electrical impulses cannot cross them.
• In a reflex arc, there are synapses between the sensory and relay neurones, and the relay and motor neurones.
• Chemicals called neurotransmitters (such as dopamine and serotonin) are released into the synaptic cleft and diffuse across it (down a concentration gradient).
• The electrical impulse travels along the first axon.
• When an electrical impulse arrives at the end of the axon on the presynaptic neurone, chemical messengers called neurotransmitters are released from vesicles.
• The neurotransmitters diffuse across the synaptic gap and bind with receptor molecules on the membrane of the second neurone (known as the postsynaptic membrane).
• This stimulates the second neurone to generate an electrical impulse that travels down the second axon.
• The neurotransmitters are then destroyed or recycled to prevent continued stimulation of the second neurone which would cause repeated impulses to be sent.
• Synapses ensure that impulses only travel in one direction, avoiding confusion within the nervous system if impulses were travelling in both directions.
• As this is the only part of the nervous system where messages are chemical as opposed to electrical, it is the only place where drugs can act to affect the nervous system.