The Human Nervous System (AQA GCSE Combined Science: Synergy: Life & Environmental Sciences): Revision Note

Nervous System Function

• The human nervous system consists of:

  • Central nervous system (CNS) – the brain and spinal cord

    • The CNS is the coordinator that coordinates the response of effectors which may be muscles contracting or glands secreting hormones

  • Peripheral nervous system (PNS) – all of the nerves in the body

• The nervous system enables humans to react to their surroundings and to coordinate their behaviour

  • The receptors detect stimuli in the environment

  • Information is sent through the nervous system as electrical impulses – electrical signals that pass along nerve cells known as neurones

    • A bundle of neurones is known as a nerve

• The pathway through the nervous system is:

stimulus → receptor → coordinator → effector → response

Adaptations of the nervous system

• Neurones have a cell body (where the nucleus and main organelles are found) and cytoplasmic extensions from this body called axons and dendrites

• Some human neurones have axons over a metre in length (but only 1 - 4 micrometres wide)

• This is far more efficient than having multiple neurones to convey information from the CNS to effectors – less time is wasted transferring electrical impulses from one cell to another

• The axon is insulated by a fatty myelin sheath with small uninsulated sections along it (called nodes) which the impulse jumps along

The Reflex Arc

• A reflex action is an involuntary response that does not involve the conscious part of the brain as the coordinator of the reaction

• Awareness of a response having happened occurs after the response has been carried out

• Reflex actions are therefore automatic and rapid – this helps to minimise damage to the body

1. The pin (the stimulus) is detected by a (pain/pressure/touch) receptor in the skin

2. A sensory neurone sends electrical impulses to the spinal cord (the coordinator)

3. An electrical impulse is passed to a relay neurone in the spinal cord

4. A relay neurone synapses with a motor neurone

5. A motor neurone carries an impulse to a muscle in the leg (the effector)

6. The muscle will contract and pull the foot up and away from the sharp object (the response) when stimulated by the motor neurone

Synapses

• Neurones never touch each other, they are separated by junctions (gaps) called synapses

• Synaptic junctions are incredibly small - around 10nm in size - and electrical impulses cannot cross them

• In a reflex arc, there are synapses between the sensory and relay neurones, and the relay and motor neurones

• Chemicals called neurotransmitters (such as dopamine and serotonin) are released into the synaptic cleft and diffuse across it (down a concentration gradient)

Reaction times

• Reaction time is the time taken for a person to respond to a stimulus.

• Times vary from person to person

  • depending on how quickly the nervous system detects the stimulus, processes it in the brain or spinal cord, and sends a signal to the muscles

  • Typically, values can range from 0.3s to 0.9s

• These can be measured using methods such as the ruler drop test