The Nervous System

Doc Brown's Biology Revision Notes

Suitable for GCSE/IGCSE/O level Biology/Science courses or equivalent

This page will answer many questions e.g.

What do we mean by a sensory organ?

What are your five sense organs?

How are signals from sensory organs sent to the brain?

What is a synapse? What is a sensory neuron?



  • You should appreciate that the nervous system and hormones enable us to respond to external changes.

  • The nervous system and hormones also help us to control conditions inside our bodies.

  • a) You should know that the nervous system enables humans to react to their surroundings and coordinate their behaviour.

    • Any change in your surroundings eg temperature, visual, sound etc. is potentially a detectable stimulus to one of you sensory organs eg skin, eyes, ears etc. The stimulus might be chemical, light, pain, position, pressure, sound, temperature, touch etc.

    • You have five different sense organs ears, eyes, nose, skin and tongue which contain receptors (groups of cells) that are sensitive to particular stimuli.

    • In the receptor cells the stimulus input is converted into an electrical signal - an electrical impulse which is sent to the brain.

    • The reflex actions that can happen by virtue of our central nervous system help prevent injury from various sources in potentially dangerous situations.

  • b) Know that cells called receptors can detect stimuli (changes in the environment outside the organism).

    • Know that receptors and the stimuli they detect include:

      • Light receptor cells in the eyes that are sensitive to light, the light energy creates electrical signals that are sent to the brain for 'processing'.

      • Sound receptors in the ears that are sensitive to sound vibrations in the air

        • There are also balance receptors in the ears that are sensitive to changes in position and enable us to keep our balance.

      • The receptors on the tongue are sensitive to chemicals and enable us to taste, and therefore detect, a wide variety of different foods (bitter, salty, sour, sweet chemical stimuli etc.) or anything else in contact with the tongue - good or bad!

      • The receptors in the nose are also sensitive to chemicals and enable us to smell all sorts of different things which may be a pleasant or unpleasant experience.

      • The receptors in the skin that are sensitive to touch, pressure, pain and to temperature changes.

    • Note that knowledge and understanding of the structure and functions of sense organs such as the eye and the ear are not required here.

  • c) Know that light receptor cells, like most animal cells, have a nucleus, cytoplasm and cell membrane.

  • d) Know and understand that information in the form of an electrical signal, from receptors, passes along cells in nerves (neurones) to the brain through the central nervous system (spinal cord ==> brain)  and ...

    • ... the brain then coordinates the response,

    • ... reflex actions are automatic and rapid,

    • ... and often involve sensory, relay and motor neurones.

      • Note that knowledge of the functions of the cell components is not required here.

  • e) You should know and understand the role of receptors, sensory neurones, motor neurones, relay neurones, synapses and effectors in simple reflex actions.

    • Receptors - groups of cells that respond to a particular stimulus

    • Nerve cells are called neurons, elongated cells that carry electrical signals or impulses all around the body.

    • Sensory neurones - the nerve cells that transmit the electrical impulse signal from the receptors in the sense organs to the spinal chord and brain (central nervous system).

    • Relay neurones - the nerve cells that transmit the electrical signals from sensory neurones to the motor neurones.

    • Synapse - a connection between two neurones eg the junction between a sensory neurone and a relay neurone, it enables the electrical impulse signal to reach the spinal cord and brain (ie the central nervous system). Between the end of one neurone, and the start of another, chemicals are released in the gap that rapidly diffuse across the gap in the synapse, transferring the electrical signal.

    • Neurotransmitter - chemicals produced that transmit the electrical signal across a synapse gap between one neurone cell and another.

    • Myelin sheath - is a fatty electrically insulating tissue layer around the axon connections between neurones - the axon in the neurone cells carries the electrical signal - if there was no myelin insulation, the signal will be lost.

    • Motor neurones - the nerve cells that transmit the electrical signals through the central nervous system from the brain via the spinal cord to the effector cells of the muscles or glands from one neurone to another.

    • Effectors - the muscles or glands that respond in a variety of ways to the electrical signal from the brain.

    • Reflex actions are automatic responses to stimuli detected by the receptors in the organs of the body.

      • They are an important defence mechanism of our body eg

      • If in danger your body releases the hormone adrenaline to heighten your mental and physical response.

      • If the intensity of light impacting on your eye is too great, your pupil automatically gets smaller to allow less light. In a dimly lit room, the opposite response occurs and your pupil widens to let more light in.

      • If something hot touches your skin, on feeling pain you immediately try to recoil from the heat source eg on burning your hand, the muscles rapidly contract to take your hand away.

    • Know that in a simple reflex arc action from a receptor to an effector (via spinal cord and an unconscious part of the brain):

      • A stimulus detected by receptors (receptor cells) causes impulses from a receptor to pass along a sensory neurone (nerve cell) to the central nervous system.

      • At a nerve junction (synapse) between a sensory neurone and a relay neurone in the central nervous system, a chemical is released that causes an impulse to be transmitted by a relay neurone,

      • A chemical is then released at the synapse between a relay neurone and motor neurone in the central nervous system, causing impulses to be sent along by a motor neurone to the organ (the effector) that brings about the response (of the effector cells).

      • The effector is either a muscle or a gland, a muscle responds by contracting or a gland responds by releasing (secreting) chemical substances eg the central nervous systems decides what is to be done depending on what stimulus is received, so ...

        • Muscles in your arm may contract to withdraw your hand from a heat source or sharp point.

        • Glands may secrete a particular hormone in response to a particular stimulus eg adrenalin in a 'flight response' from a dangerous situation.

        • The pupils in  your eyes respond by decreasing/increasing in size if the light level is too high/low.

      • Summary of the reflex arc sequence via the central nervous system:

        • stimulus ==> receptor cell ==> sensory neurone ==synapse==> relay neurone in central nervous system =synapse==> motor neurone ==> effector ==> response

        • The reflex arc action is fast, no thinking involved, just a rapid automatic response on the part of your body!

      • Note on comparing nerve and hormone functions

        • Hormones effectively act as 'chemical messages' to trigger particular biochemical reactions and their effect is more general and relatively long-lasting compared to eg the nervous impulses and responses of reflex arc.

        • Compared to the hormone system of response and control in the body, nerve signals are electrical (not chemical), the nerves act very fast - a short burst of a nervous impulse for a short time, acting from one precise area to another in the body.

  • Any practical work and investigations you did should also be revised  (which should also be revised, helps in understanding 'how science works' and context examination questions):

    • reaction times measuring reaction times using metre rules, stop clocks or ICT,

    • using forehead thermometers before and after exercise,

    • demonstrating the speed of transmission along nerves by candidates standing in a semi-circle and holding hands and squeezing with eyes closed,

    • designing an investigation to measure the sensitivity of the skin,

    • demonstrating the knee jerk reaction,

    • investigation to measure the amount of sweat produced during exercise,



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