The Reflex Arc (SQA National 5 Biology): Revision Note

The reflex arc

• Simple reflexes allow rapid, automatic responses to stimuli, the purpose of which is to protect an organism from harm, e.g.:

  • the pupil reflex prevents bright light from damaging the retina

  • the coughing reflex prevents obstructions from entering the airways

• Reflex nerve pathways are known as reflex arcs

  • Reflex arcs do not involve conscious parts of the brain, meaning that reflexes are faster than any other type of nervous response

The structure of a reflex arc

• In a reflex arc, a stimulus brings about a response when a nerve impulse is sent as follows:

receptor → sensory neuron → inter neuron → motor neuron → effector

The synapse

• Neurons never touch each other

• The junctions (gaps) in between them are called synapses

• The electrical impulse travels along the first neuron and triggers the release of chemicals from small sacs at the nerve ending

• The chemicals diffuse across the synaptic gap and bind with receptor molecules on the membrane of the second neuron

• This initiates the continuation of the electrical impulse in the second neuron