13.2 The Nervous System

Question 1

What are neurones?
What are the three main types?

Answer 1

The nervous system is made up of a complex of specialised nerve cells called neurones.
Role of the neurone is to transmit an electrical impulse rapidly around the body so that the organism can respond to changes in its internal and external environment.
There are three main types of neurone:
1) sensory neurones - transmit nerve impulses from receptors to the central nervous system (CNS) - the brain and spinal cord
2) motor neurones - transmit nerve impulses from the CNS to effectors
3) relay neurones - transmit nerve impulses between sensory neurones and motor neurones

Question 2

Describe the structure of a neurone

Answer 2

All neurones have a cell body with a nucleus.
The cell body has extensions that connect to other neurones - dendrites and dendrons carry nerve impulses towards the cell body (dendrites are smaller branches of a dendron), and axons carry nerve impulses away from the cell body.

Question 3

Describe the structure of a sensory neurone

Answer 3

Sensory neurones have short dendrites and one long dendron to carry nerve impulses from receptors cells to the cell body, and
one short axon that carries nerve impulses from the cell body to the CNS

Question 4

Describe the structure of motor neurones

Answer 4

• have many short dendrites that carry the nerve impulses from the CNS to the cell body
• one long axon that carries nerve impulses from the cell body to the effectors cells

Question 5

Describe the structure of relay neurones

Answer 5

• many short dendrites that carry nerve impulses from the sensory neurone to the cells body
• many short axons that carry nerve impulses away from the cell body and to the motor neurones

Question 6

Describe nervous communication pathway

Answer 6

• a stimulus is detected by receptor cells
• a nerve impulse is sent along a sensory neurone
• when a nerve impulse reaches the end of a neurone, chemicals called neurotransmitters take the information across the synapse to the next neurone
• the CNS process the information, then sends impulses along motor neurones to an effector

Question 7

Why are sensory neurones described as transducers?

Answer 7

Sensory neurones convert the energy of a stimulus into electrical energy (nerve impulses)
They act as transducers - something that converts one form of energy into another

Question 8

What is meant by the ‘resting potential’?

Answer 8

When a receptor is at its resting state (not being stimulated), theres a difference in charge between the inside and outside of the cells - the inside is negatively charged relative to the outside.
This means there’s a potential difference across the membrane.
The resting potential is generated by ion pumps and ion channels in the membrane.

Question 9

What is meant by the ‘generator potential’?

Answer 9

• when a stimulus is detected, the cell membrane is excited and become more permeable, allowing more ions to move in and out of the cell - altering the potential difference

The change in potential difference due to a stimulus is called the generator potential

A bigger stimulus excites the membrane more, causing a bigger movement of ions and bigger change in p.d. - so a bigger generator potential is produced

Question 10

Describe what is meant by the ‘action potential’

Answer 10

If the generator potential is big enough it’ll trigger an action potential (nerve impulse) along a neurone.
An action potential is only triggered if the generator potential reaches the threshold level.

If the stimulus is too weak the generator potential wont be big enough and wont reach the threshold level, so there’s no action potential

Question 11

Describe and explain the stimulation of a Pacinian corpuscle
What is a Pacinian corpuscle?

Answer 11

• Pacinian corpuscles are mechanoreceptors - they detect stimuli mechanical stimuli, e.g. pressure and vibrations
• they’re found in your skin
• they contain the end of a sensory neurone, called a sensory nerve ending
• the sensory nerve ending is wrapped in lots of layers of connective tissue called lamellae.
• when a Pacinian corpuscle is stimulated, e.g. by a tap on the arm, the lamellae are deformed and press on the sensory nerve ending
• this causes deformation of stretch-mediated sodium ion channels in the sensory neurone’ cell membrane
• the sodium ion channels open and sodium ion diffuse into the cell, creating a generator potential
• if the generator potential reaches the threshold, it triggers an action potential