The Reflex Arc

Sensory Neurones: Characterized by long axons with a cell body located on a side branch near the middle of the neurone's length. This structure allows for rapid transmission from peripheral receptors to the CNS.

Relay Neurones: These are short neurones found entirely within the CNS. They possess many dendrites to receive signals from sensory neurones and pass them to motor neurones.

Motor Neurones: These have a large cell body located at one end within the CNS, with long axons that extend out to reach effectors in the limbs or organs.

The Synapse: A microscopic gap, approximately $10\text{ nm}$ wide, between two neurones. Electrical impulses cannot jump this gap, so the signal must be converted into a chemical form.

Neurotransmitters: When an impulse reaches the end of an axon (the presynaptic terminal), it triggers the release of chemical messengers called neurotransmitters from vesicles.

Diffusion: These chemicals diffuse across the synaptic cleft down a concentration gradient. This is the slowest part of the reflex arc because chemical diffusion is slower than electrical conduction.

Receptor Binding: The neurotransmitters bind to specific, complementary receptors on the postsynaptic membrane of the next neurone, triggering a new electrical impulse.

Directionality: Synapses ensure that impulses only travel in one direction. This is because neurotransmitter vesicles are only present on the presynaptic side, and receptors are only present on the postsynaptic side.

Chemical vs. Electrical: While the impulse is electrical along the axon, it is chemical at the synapse. This makes the synapse the primary site where drugs or toxins can interfere with nervous system function.

Sequence Accuracy: Always double-check the order of the arc: Stimulus $\rightarrow$ Receptor $\rightarrow$ Sensory $\rightarrow$ Relay $\rightarrow$ Motor $\rightarrow$ Effector $\rightarrow$ Response. Swapping the sensory and motor neurones is a frequent error.

Synapse Mechanics: Remember that neurotransmitters move by diffusion, which is a passive process. If a question asks why a reflex arc is slower than a single neurone, the answer is the time taken for diffusion across synapses.

Effector Identification: Ensure you distinguish between the effector (the muscle or gland) and the response (the contraction or secretion). The effector is the 'part', the response is the 'action'.

Brain Involvement: Be careful with phrasing; the brain is not the coordinator of a reflex, but it does receive information about the event shortly after it occurs.