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Nervous System (Higher)
Sense Organs and Stimuli | Central Nervous System | Reflex Actions | The Eye
 
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 neurones (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.

 
Name of sense
  Sense organ  Stimulus
 Sight   Eye  Light
 Hearing  Ear  Sound
 Balance  Ear  Change in body position
 Taste  Tongue   Chemicals
 Smell   Nose  Chemicals
 Touch  Skin   Touch, pressure and temperature changes

 

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 neurones, surrounded by an insulating layer.

Information is passed from receptors to the brain along sensory neurones.  The brain then coordinates a response and sends a message along a motor neurone to an effector organ, which could be a muscle or a gland.
10.8_motor_neurone


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 neurones, a sensory neurone, relay neurone and motor neurone.

The gap between neurones is called a synapse.

When the impulse reaches the end of one neurone the nerve endings release a chemical that diffuses across the synapse to the next neurone, 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 neurone → coordinator → motor neurone → effector → response

tap below knee → in leg  → sensory neurone → spinal cord →  motor neurone → muscle in thigh → leg jerks straight
10.8_reflex_arc_V2


The Eye


Part of eye
 Structure and function
Retina The light-sensitive layer at the back of the eye; contains receptors which convert light into nerve impulses.
Lens Focuses light onto the retina.
Suspensory ligaments Hold the lens in place.
Ciliary muscles  Alter the shape of the lens during focusing.  Muscles contract to focus on near objects and relax to focus on far away objects.
Iris Coloured part of the eye that controls the amount of light entering the eye by changing the size of the pupil.
Pupil Hole which allows light to enter the eye.
Sclera Tough, white, outer layer of the eye.
Cornea Front part of the sclera which is transparent to let light into the eye.  It is curved so helps to focus light onto the retina.
Optic nerve  Contains sensory neurones that carry impulses from the retina to the brain.

 
10.8_eye_V2