Sense Organs and Stimuli

The nervous system enables humans to detect changes in their surroundings and react to them.

The nervous system also sends electrical messages along neurons (nerve cells) in order to coordinate behaviour.

A stimulus is a change in the environment.

Special cells in the body called receptors detect stimuli.  The receptors are located in the sense organs.

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Central Nervous System

The central nervous system (CNS) is made up of the brain and spinal cord.

Branching out of the CNS are nerves.  Each nerve contains a bundle of neurons, surrounded by an insulating layer.

Information is passed from receptors to the brain along sensory neurons.  The brain then coordinates a response and sends a message along a motor neuron to an effector organ, which could be a muscle or a gland.

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Reflex Actions

Some responses to stimuli are automatic and happen very fast, for example, blinking or pulling away from a hot object.  These are called reflex actions and often protect the body from danger.

Reflex actions often only involve three neurons, a sensory neuron, relay neuron and motor neuron.

The gap between neurons is called a synapse.

When the impulse reaches the end of one neuron, the nerve endings release a chemical that diffuses across the synapse to the next neuron, where it triggers another impulse to be sent.

When the impulse reaches the effector, the muscle contracts.  If the effector is a gland, it responds by secreting a chemical.

For example, in the knee jerk reflex: 

stimulus  → receptor → sensory neuron→ coordinator → motor neuron→ effector → response

tap below knee → in leg  → sensory neuron→ spinal cord →  motor neuron→ muscle in thigh → leg jerks straight

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The Eye

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