5.3.2: Structure and function of neurons

Question 1

what are impulses transmitted along?

Answer 1

Impulses are transmitted along neurones as an action potential.

Question 2

What is the action potential carried as?

Answer 2

A rapid depolarisation of the membrane caused by an influx of sodium ions.

Question 3

What are the three types of neurones?

Answer 3

• Motor neurones
• Sensory neurones
• Relay neurones

Question 4

What is the role of a motor neurone?

Answer 4

To carry the action potential from the central nervous system to an effector such as a muscle or a gland.

Question 5

What is the role of a sensory neurone?

Answer 5

To carry the action potential from a sensory receptor to the central nervous system.

Question 6

What is the role of a relay neurone?

Answer 6

To connect sensory and motor neurones.

Question 7

Why do neurones have many gated ion channels on their plasma membranes?

Answer 7

They control the entry and exit of sodium, potassium and calcium ions.

Question 8

Why do neurones have many gated ion channels on their plasma membranes?

Answer 8

They control the entry and exit of sodium, potassium and calcium ions.

Question 9

What do neurones have to maintain across their plasma membranes?

Answer 9

A potential difference.

Question 10

What does the cell body in a neurone contain?

Answer 10

nucleus, many mitochondria and ribosomes.

Question 11

Why do neurones have dendrites?

Answer 11

To connect to other neurones and carry impulses towards the cell body.

Question 12

Where do axons carry impulses?

Answer 12

Away from the cell body.

Question 13

What are neurones surrounded by?

Answer 13

Neurones are surrounded by a fatty layer that insulates the cell from electrical activity in other nerve cells nearby.

Question 14

What is the fatty layer composed of?

Answer 14

Schwann cells closely associated with the neurone.

Question 15

Describe the positioning of a motor neurone.

Answer 15

Motor neurones have their cell body in the CNS and have a long axon that carries the action potential out to the effector.

Question 16

Describe the positioning of a sensory neurone.

Answer 16

Sensory neurones have a long dendron carrying the action potential from a sensory receptor to the cell body, which is positioned just outside the CNS.
Then they have a short axon carrying the action potential into the CNS.

Question 17

Describe the positioning of a relay neurone.

Answer 17

Relay neurones connect the sensory and motor neurones.
They have many short dendrites and a short axon.
The number of dendrites and the number of divisions of the axon is variable.

Question 18

What proportion of peripheral neurones in a vertebrate are myelinated neurones?

Answer 18

One third

Question 19

What is a myelinated neurone?

Answer 19

Neurones that are insulated by an individual myelin sheath.

Question 20

Most sensory and motor neurones are associated with many Schwann cells. What do these cells make up?

Answer 20

They are tightly wrapped round the neurone so that the sheath actually consists of several layers of membrane and thin cytoplasm from the Schwann cells.

Question 21

What occurs at intervals around 1 to 3 mm along the neurone?

Answer 21

Gaps in the myelin sheath called nodes of Ranvier.

Question 22

Why can movement of ions across the membrane only occur at the nodes of Ranvier and why is this useful?

Answer 22

Because the myelin sheath is wrapped tightly around the neurone which prevents the movement of neurones across the neurone membranes.

This means that the impulse or action potential jumps from one node to the next node . This makes conduction more rapid.

Question 23

Describe how non-myelinated neurones are also associated with Schwann cells.

Answer 23

Several neurones are enshrouded in one loosely wrapped Schwann cell.

Question 24

How does the structure of non-myelinated neurones affect how the action potential moves along the neurone?

Answer 24

The action potential moves along the neurone in a wave rather than jumping from node to node.

Question 25

What are the advantages of myelination?

Answer 25

• Speed of transmission of action potentials.

- Carry impulses over longer distances.

Question 26

Why is faster transmission of action potentials important?

Answer 26

neurones carry action potentials over a long distance, this increased transmission means that the action potential reaches the neurone quickly and there is a more rapid response to a stimulus.

Question 27

Why is an increased speed of transmission not as important in non-myelinated neurones?

Answer 27

They tend to be shorter and carry action potentials over a shorter distance. They are often used in coordinating body functions such as breathing, and the action of the digestive system.