AQA Core Biology 1a (i) - Human Biology

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 AQA Core Biology 1a (i) - Human Biology.

The Nervous System.

This let's you react to what is going on around you.

Sense Organs Detect Stimuli.

stimulus is a change in your environment which you may or may not need to react to. You need to be constantly monitoring what's going on so you can respond if you need to.

1) You have five different sense organs:

  • Eyes.
  • Ears.
  • Nose. 
  • Tongue. 
  • Skin.

2) They all contain different receptors. Receptors are groups of cells which are sensitive to a stimulus. They change stimulus energy (e.g. light energy) into electrical impulses.

3) A stimulus can be light, sound, touch, pressure, chemical or a change in position or temperature.

The Five Sense Organs and the Receptors They Contain:

1) Eyes - Light Receptors. 

2) Ears - Sound and Balance Receptors.

3) Nose - Smell Receptors, (sensitive to chemical stimuli).

4) Tongue - Taste Receptors*.

5) Skin - Sensitive to Touch, Pressure and Temperature Change.

* A tongues taste receptors are sensitive to bitter, salt, sweet and sour, plus the taste of savory things like mono-sodium glutamate (MSG) - Chemical Stimuli.

The Central Nervous System Coordinates a Response.

1) The Central Nervous System (CNS) is where all the information from the sense organs is sent, and where reflexes and actions are coordinated.

The central nervous system consists of the brain and the spinal cord only.

2) Neurons (nerve cells) transmit the information (as electrical impulses) very quickly to and from the CNS.

3) "Instructions" from the CNS are sent to the effectors (muscles and glands), which respond accordingly.


Neuron (

Sensory Neurons:

The nerve cells that carry signals as electrical impulses from the receptors in the sense organs to the central nervous system.

Motor Neurons:


The nerve cells that carry signals to the effector muscles or glands.


Muscles and glands are known as effectors, they respond in different ways. Muscles contract in response to a nervous impulse, whereas glands secret hormones.


Your brain can respond to a stimulus pretty quickly. But sometimes, waiting for your brain to make a decision can be too slow, so you have reflexes.

Reflexes Help Prevent Injury.

1) Reflexes are automatic responses to certain stimuli. They reduce the chance of you being injured

2) For example, if someone shines a bright light into your eyes, your pupils automatically get smaller to let in less light. This stops it getting damaged.

3) Or if you get a shock your body releases the hormone adrenaline automatically, it doesn't wait to decide if you are shocked. 

4) The passage of information in a reflex (from receptor to effector) is called a reflex arc.

The Reflex Arc Goes Through the Central Nervous System.

1) The neurons in reflex arcs go through the spinal cord (or an unconscious part of the brain).

2) When a stimulus (e.g. a painful bee…


self-confessed worrier :/


really good! thanks! i was at a complete loss at how to revise :) thank so much!!!



thank you sooooo much!!! this was so helpful!!

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