Glossary: Coordination and Response in Humans

This chapter's terms in context

Chapter 12 rests on five terms spanning both of the body's coordination systems, the fast nervous system and the slower endocrine system: stimulus, neuron, synapse, reflex action and hormone. A stimulus is any change in the internal or external environment that is detected by a receptor and that triggers a response in an organism, light, heat, sound, touch, and changes in blood glucose concentration are all examples, and a stimulus by itself does nothing until a receptor and coordinating system act on it.

A neuron (nerve cell) is the basic functional unit of the nervous system, structurally adapted to carry electrical impulses rapidly over long distances: it has a long fibre called an axon, often insulated by a fatty myelin sheath that speeds up impulse conduction, and branching dendrites that receive impulses from other neurons. The three main types, sensory neurons, which carry impulses from a receptor to the central nervous system; relay neurons, which connect neurons within the central nervous system; and motor neurons, which carry impulses from the central nervous system to an effector such as a muscle or gland, form a pathway students are frequently asked to sequence correctly, and naming them in the wrong order is one of the chapter's most common errors.

A synapse is the small gap between the end of one neuron and the next, and impulses cross it not electrically but chemically: the arrival of an electrical impulse triggers the release of a chemical called a neurotransmitter from the first neuron, which diffuses across the gap and binds to receptors on the second neuron, triggering a new electrical impulse there. This chemical step at the synapse is precisely why impulses travel in one direction only through a neural pathway, and it is also the point where many drugs and toxins act, a detail examiners sometimes weave into an applied question.

A reflex action is a rapid, automatic and involuntary response to a stimulus that does not require conscious thought, coordinated by a simple pathway called a reflex arc: receptor, sensory neuron, relay neuron in the spinal cord, motor neuron, and effector, such as the knee-jerk reflex or the rapid withdrawal of a hand from a hot object. Because a reflex arc bypasses conscious processing in the brain, reflex actions are faster than voluntary responses, and a key exam point is explaining why speed, not thought, is the reflex's defining survival advantage.

A hormone, by contrast, is a chemical messenger produced by an endocrine gland and carried in the bloodstream to a target organ, where it produces a specific effect, insulin regulating blood glucose concentration is the example most often tested. Comparing nervous and hormonal coordination directly, in terms of speed, duration and the way the message travels (electrical impulse along a neuron versus chemical messenger in blood), is one of the most frequently asked comparison questions built from this single chapter.

Paper 2 questions on this chapter often supply an everyday scenario, such as a hand touching a hot pan or an unexpected loud noise, and ask a candidate to describe the resulting reflex arc in the correct order using named structures rather than describing the response in everyday language alone. A frequent scoring gap is naming the correct structures but placing them in the wrong sequence, particularly reversing the position of the relay neuron and the motor neuron, or forgetting that the sensory neuron carries the impulse towards the spinal cord rather than away from it. A second common gap is explaining hormonal coordination using nervous-system vocabulary, such as describing a hormone as travelling along a neuron; keeping the two systems' vocabulary strictly separate, impulse and neuron for nervous coordination, versus gland and bloodstream for hormonal coordination, avoids this cross-contamination.