B10.2 The Structure and Function of the Human Nervous System

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: receptors and sense organs, the central nervous system, and sensory, relay and motor neurones. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Describe what the nervous system does and the pathway from stimulus to response
• Name the parts of the central nervous system
• Match sense organs to the stimuli their receptors detect
• Describe the three types of neurone and how their structure suits their function
• Calculate the speed of a nerve impulse

Key words

• Receptor: A cell or group of cells that detects a change and starts an electrical impulse.
• Neurone: A nerve cell, which carries electrical impulses.
• Nerve: A bundle of many neurones wrapped together.
• CNS: The central nervous system: the brain and spinal cord.
• Impulse: An electrical signal that travels along a neurone.
• Myelin sheath: A fatty layer around an axon that insulates it and speeds up impulses.

What the nervous system does

The nervous system lets you react to your surroundings and coordinates both voluntary and involuntary actions. Receptors detect a change and start electrical impulses, which travel along neurones to the central nervous system (CNS). The CNS coordinates the response and sends impulses to effectors: muscles that contract or glands that secrete hormones.

Worked example: a goalkeeper

Stimulus: a ball flying towards the goal. Receptors: light receptors in the eyes.
Impulses travel along sensory neurones to the brain, which decides what to do.
Impulses travel along motor neurones to the leg and arm muscles (effectors), which contract to make the save.

Answer: Eyes → sensory neurones → brain → motor neurones → muscles

Receptors and sense organs

Receptors are often grouped in sense organs. Each type of receptor responds to one kind of stimulus.

• Sight; Organ Eye; Receptors detect Light
• Hearing; Organ Ear; Receptors detect Sound
• Taste; Organ Tongue; Receptors detect Chemicals in food and drink
• Smell; Organ Nose; Receptors detect Chemicals in the air
• Touch; Organ Skin; Receptors detect Touch, pressure, temperature, pain

Receptors are not spread evenly in the skin. Fingertips and lips have the most, so they are the most sensitive; the soles of the feet and elbows have the fewest.

Exam tip: the stimulus for the nose and tongue is chemicals, not “smell” or “taste”.

Neurones

Sensory neurones carry impulses from receptors to the CNS. Relay neurones carry impulses within the CNS, from sensory to motor neurones. Motor neurones carry impulses from the CNS to effectors. Only relay neurones lie completely inside the CNS.

• Long axon; How it helps Carries impulses a long way, e.g. from the spinal cord to a toe.
• Dendrites; How it helps Many branches connect with lots of other neurones.
• Myelin sheath; How it helps Insulates the axon and speeds up the impulse (up to about 120 m/s).
• Nerve endings; How it helps Pass the impulse on to other neurones or to effectors.

Worked example: speed of an impulse

An impulse travels 1.1 m along a motor neurone in 0.010 s. Calculate its speed.
speed = distance ÷ time = 1.1 ÷ 0.010 = 110 m/s.

Answer: 110 m/s

Common mistakes

• Calling a neurone a “nerve”. A nerve is a bundle of neurones.
• Saying the brain is the whole CNS. It is the brain and spinal cord.
• Giving “seeing” as the stimulus for the eye. The stimulus is light.
• Mixing up sensory and motor neurones.

Revision questions

Try these, then check your answers below each one.

Question 1. Which two structures make up the central nervous system (CNS)? (Choose from: Brain and spinal cord, Brain and nerves, Spinal cord and muscles, Heart and brain.)

Answer: Brain and spinal cord

Working: The CNS is the brain and the spinal cord. Nerves connect it to the rest of the body.

Question 2. The diagram shows a motor neurone. Which letter labels the myelin sheath, which insulates the axon and speeds up the impulse? (Choose from: A, B, C, D, E.)

Answer: C

Working: C labels the myelin sheath (the fatty sections along the axon). A = dendrites, B = nucleus in the cell body, D = axon, E = nerve endings.

Question 3. An impulse travels 1.05 m along a motor neurone in 0.0084 s. Calculate the speed of the impulse.

Answer: 125 m/s

Working: 1.05 ÷ 0.0084 = 125 m/s.

Related lessons

• Previous lesson: B10.1 Principles of Homeostasis
• Next lesson: B10.3 Reflex Actions
• In this chapter: B10 RP Required Practical: Reaction Time

Continue in the full course

This lesson is section B10.2 (AGCSC1002) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.