Nervous System and Reflexes

Explained

From stimulus to response, in a fixed order

The nervous system carries information as electrical impulses along neurones. The central nervous system is the brain and the spinal cord, and everything else is the peripheral nervous system, made up of the nerves running to and from it.

Compared with hormones, nerve impulses are fast, short lived, and directed at one particular target rather than carried everywhere in the blood.

Three neurones, three jobs

• A sensory neurone carries impulses from a receptor to the central nervous system.
• A relay neurone carries impulses within the central nervous system, usually in the spinal cord.
• A motor neurone carries impulses from the central nervous system to an effector.

An effector is a muscle or a gland, because those are the only two things that can actually do something. A muscle contracts and a gland secretes.

The reflex arc

A reflex is a fast automatic response that protects the body and does not involve conscious thought. Pulling your hand from a hot object is the standard example.

The pathway is always in the same order: stimulus, receptor, sensory neurone, relay neurone, motor neurone, effector, response. Learning that sequence as a sentence is worth doing, because almost every question on this topic is asking you to reproduce part of it.

Notice what is missing. The impulse reaches the spinal cord and goes straight back out through the motor neurone without waiting for the brain. That is what makes it fast, and it is why your hand is already moving before you feel the pain. The brain is informed afterwards, which is why you do feel it.

Synapses

Neurones do not touch. There is a tiny gap between one and the next called a synapse, and an electrical impulse cannot jump it.

Instead the impulse arriving at the end of the first neurone causes a chemical, called a neurotransmitter, to be released into the gap. It diffuses across, binds to receptor molecules on the next neurone, and starts a new impulse there.

Two things follow from this design. It makes the impulse travel in one direction only, because only one side releases the chemical and only the other side has the receptors. And it slows the transmission slightly, because diffusion takes time, which is why a reflex with fewer synapses is faster.

Answering the standard questions

If asked to describe what happens when you touch something hot, name each stage in order and identify the structure involved at each one. The receptor is a temperature receptor in the skin, the coordinator is the spinal cord, the effector is a muscle in the arm, and the response is the arm contracting to pull the hand away.

If asked why the response is so quick, give the reason rather than the word fast. The pathway is short, it does not involve the brain, and it uses only two synapses.

If asked how an impulse crosses a synapse, three things are needed: a chemical is released, it diffuses across the gap, and it binds to receptors and triggers a new impulse. Leaving out the diffusion step is the usual way to drop a mark.