The human nervous system Notes

Oxford AQA IGCSE Biology: Revision notes

Key facts

  • The CNS (brain and spinal cord) coordinates responses; the PNS (nerves) connects it to the body.
  • A sensory neurone carries impulses to the CNS, a relay neurone connects within it, and a motor neurone carries them to an effector.
  • At a synapse, a chemical neurotransmitter carries the signal across the gap, so impulses go one way only.
  • A reflex arc is: stimulus, receptor, sensory neurone, relay neurone, motor neurone, effector, response.
  • The lens changes shape to focus on near or distant objects; the retina holds the light receptors.
  • Short sight is corrected with a concave lens; long sight with a convex lens.

Brain and spinal cord

The brain and spinal cord form the central nervous system (CNS); nerves running to the rest of the body form the peripheral nervous system (PNS).

The CNS processes information and coordinates responses. The PNS is made of sensory and motor nerves that connect the CNS to receptors and effectors.

Each part of the brain has its own job, shown below.

  1. 1

    Receptor

    Detects a stimulus

  2. 2

    Sensory neurone

    PNS: carries impulses to the CNS

  3. 3

    Brain or spinal cord

    CNS: processes and coordinates

  4. 4

    Motor neurone

    PNS: carries impulses away

  5. 5

    Effector

    Muscle or gland responds

A signal passing through the nervous system.

CNS

  • Brain and spinal cord
  • Processes information
  • Coordinates responses

PNS

  • Sensory and motor nerves
  • Connect CNS to the body
  • Carry signals both ways

Cerebral cortex

  • Voluntary actions
  • Memory, language, consciousness

Cerebellum

  • Coordinates muscles
  • Balance and posture

Medulla

  • Heart rate
  • Breathing rate

Hypothalamus

  • Body temperature
  • Water content

Which part of the brain controls breathing rate?

Neurones and synapses

Neurones carry electrical impulses, but at a synapse the signal is passed on chemically, which is why it can only travel in one direction.

There are three types of neurone: sensory, relay and motor. Each has a cell body with a nucleus and a long axon.

A synapse is the gap between two neurones. Impulses cannot jump it, so a chemical neurotransmitter crosses instead.

Sensory

  • Receptor to CNS
  • Carries impulse in

Relay

  • Inside the CNS
  • Links sensory to motor

Motor

  • CNS to effector
  • Muscle or gland responds
  1. 1

    Impulse arrives

    At the end of the first neurone

  2. 2

    Release

    Neurotransmitter enters the gap

  3. 3

    Binding

    It attaches to receptors on the next neurone

  4. 4

    New impulse

    Starts in the next neurone

Crossing a synapse

Why can impulses only travel one way across a synapse?

Reflex arcs

A reflex arc is the pathway of a fast, automatic response that does not wait for the brain to decide.

A reflex arc protects the body from danger: the response is rapid, automatic and does not involve conscious thought. Examples are pulling a hand from a flame and the pupil narrowing in bright light.

The impulse passes through the spinal cord, not the brain.

  1. 1

    Stimulus

    Detected by a receptor

  2. 2

    Sensory neurone

    Impulse to the spinal cord

  3. 3

    Relay neurone

    Passes the signal on

  4. 4

    Motor neurone

    Impulse to the effector

  5. 5

    Effector

    Muscle or gland gives the response

The reflex arc

Worked example

A hand touches a hot plate. Describe the pathway.

Which neurone carries the impulse to the effector?

The eye and focusing

The eye focuses light on the retina by changing the shape of the lens, controlled by the ciliary muscles and suspensory ligaments.

Light is refracted by the cornea and lens onto the retina, which has rods (dim light) and cones (colour). The optic nerve carries impulses to the brain.

Accommodation is the lens changing shape to focus. The iris makes the pupil smaller in bright light and larger in dim light.

Labelled cross-section of the human eye
Notice the cornea and lens that refract light, the ciliary muscles and suspensory ligaments that change the lens, and the retina where the image forms.

Near object

Ciliary muscles:
Contract
Suspensory ligaments:
Slacken
Lens:
Thicker, refracts more

Distant object

Ciliary muscles:
Relax
Suspensory ligaments:
Pulled tight
Lens:
Thinner, refracts less

Cornea

  • Refracts light

Iris

  • Controls pupil size

Lens

  • Focuses light on retina

Retina

  • Rods and cones; optic nerve to brain

To focus on a near object, the ciliary muscles...

Defects of vision

Short sight and long sight happen when light is not focused on the retina, and a lens in front of the eye corrects it.

In short-sightedness, distant objects are blurred because light focuses in front of the retina. A concave (diverging) lens spreads the rays out.

In long-sightedness, near objects are blurred because light focuses behind the retina. A convex (converging) lens brings the rays together.

retinalensF (focus)
Short sight: rays from a distant object meet at F in front of the retina, so the image on the retina is blurred (illustrative).

Short sight

Cause:
Light focuses in front of retina
Problem:
Distant objects blurred
Correction:
Concave lens

Long sight

Cause:
Light focuses behind retina
Problem:
Near objects blurred
Correction:
Convex lens

A person cannot see near objects clearly. They need a...

Try an exam question

Describe how the eye focuses on a near object.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.