The Nervous System

Enables humans to react to their surroundings and coordinate their behaviour

Electrical impulses is information sent through the nervous system

  • These are known as neurones if sent along nerve cells

A nerve is a bundle of neurones.

Key Terms

Stimulus: A detectable change in the internal or external environment of an organism

Receptor: A cell or organ which detects a stimulus

Sensory Neurone: Carries nerve impulses from receptors to CNS

Motor Neurone: Carries nerve impulses from CNS to effectors

Relay Neurone: Receive signals from sensory neurons and pass them on to motor neurons

Voluntary Response: Produces a conscious response to a stimulus

Reflex Response: Produces an automatic response to a stimulus

Synapse: Gap between two neurones

Axon: The fibre that transmits electrical impulses away from the cell body

The three types of neurone

Central Nervous System (CNS)

The CNS consists of the brain and spinal chord

Coordinates the response by directing the electrical impulses to the correct effector

The CNS consists of the brain and spinal cord, acting as the body's main control and processing centre

The effector is either a muscle contraction or secretion from a gland

The Peripheral Nervous System (PNS) comprises all nerves outside the CNS that connect it to limbs and organs

The PNS is made up of all the nerves in the body (from the CNS)

The process:

  • A stimulus is detected

  • The receptor converts the stimulus into an electrical impulse

  • This is sent to the relay neurone in the CNS through a sensory neurone

  • The CNS coordinates a response and sends that electrical impulse along the motor neurone to the effector

  • The effector initiates a response

What Makes Up a Neurone?

Nerve Cell

An axon - the fibre that transmits electrical impulses away from the cell body

This is adapted to ensure fast transmission via:

  • a myelin sheath - a fatty layer which insulates the axon

  • long length (over a metre)

A dendrite - branched endings that create links with other neurones

A synapse - the gap between two neurones which blocks electrical impulses from travelling across

A synapse

When an impulse arrives on the presynaptic neurone, neurotransmitters diffuse across the gap and bind with the receptors on the postsynaptic membrane.

This stimulates the postsynaptic neurone to generate an electrical impulse, which then travels along the axon.

The neurotransmitters are then destroyed to prevent continued stimulation of the second neurone.

Reflex Arcs

An involuntary response that doesn’t involve the brain as a coordinator

Responses are rapid to protect the body from harm

Process of a reflex arc:

  • A stimulus is detected by a receptor (pain/pressure)

  • The sensory neurone sends electrical impulses to the spinal cord, then the relay neurone

  • A relay neurone synapses with a motor neurone, which transports the impulses to the muscle in the effector (e.g. an arm/leg)

  • The effector initiates a response

The Eye

The eye is a sense organ

It contains two receptor cells: rod cells (sensitive to light) and cone cells (sensitive to colour)

eye-structure

Functions:

  • Cornea - Refracts light as it enters the eye

  • Iris - Controls how much light enters the eye

  • Lens - Changes shape to focus light onto the retina

  • Retina - Converts the light into electrical impulses

  • Pupil - Hole that allows light to enter the eye

  • Optic Nerve - Carries the electrical impulses away from the eye to the brain

When the eye focuses on a distant object:

  • Ciliary muscle relaxes

  • Suspensory ligaments contract

  • The lens is pulled thinner, so light is refracted less

When the eye focuses on a near object:

  • Ciliary muscle contracts

  • Suspensory ligaments slacken

  • The lens becomes fatter so light is refracted more

In dim light, your pupil dilates to allow more light into the eye.

In bright light, your pupil constricts to prevent too much light entering the eye & damaging the retina.

Both of this happens due to your radial muscles contracting/a reflex action.

Eye Defects

Short sightedness/myopia:

  • Cannot see distant objects

  • Focal point is before the retina

  • Treated with a concave lens

Long sightedness/hyperopia:

  • Cannot see close up objects

  • Focal point behind the retina

  • Treated with a convex lens

Colour blindness:

  • Cones in the retina are absent/don’t work

  • Genetically inherited

  • Tinted lenses emphasise colours more - no treatment

The Brain

The brain is part of our CNS - along with the spinal cord

There are five structures in the brain:

  • Cerebrum - Responsible for memory, intelligence, language, and consciousness (personality)

  • Medulla - Controls unconscious activities e.g. heart and breathing rate

  • Hypothalamus - Regulates body temperature and produces hormones that control the pituitary gland

  • Cerebellum - Balance, muscles, and coordination

  • Pituitary Gland - Produces hormones, e.g. ones used in the menstrual cycle

Structures of the Brain

It is extremely difficult to study the brain because:

  1. The brain is complex and delicate

  2. Ethical considerations (people with brain damage unable to provide informed consent)

  3. Interpreting the results of MRI scans can be complicated, since patterns are not yet understood

There are many methods of studying the brain however, including MRI scans, studying the brains of dead people, and case studies of people with abnormal brain functions.