5.3.4: Nerve impulses - transmission

Question 1

How is a local current created in the cytoplasm of the neurone?

Answer 1

• Sodium ion channels open at one particular point of the neurone.
• Balance of sodium and potassium ions is upset.
• Sodium ions allowed to flood into the neurone causing depolarisation, this causes a local current.

Question 2

What do local currents cause and how is this an example of positive feedback?

Answer 2

• Sodium ions begin to move along the neurone towards regions where their concentration is still lower.
• These local currents cause a slight depolarisation of the membrane and cause voltage-gated sodium ion channels further along the membrane to open.
• The open channels allow rapid influx of sodium ions causing a full depolarisation (action potential) further along the neurone.

Question 3

The action potential will continue to move in the same direction until it reaches the end of the neurone. Why will it not reverse direction?

Answer 3

Because the concentration of sodium ions behind the action potential is still high.

Question 4

What is the myelin sheath?

Answer 4

An insulating layer of fatty material, composed of Schwann cells wrapped tightly around the neurone.

Question 5

What are in between Schwann cells?

Answer 5

Small gaps- the nodes of Ranvier.

Question 6

Why can the ionic movements that create an action potential only occur at the nodes of Ranvier?

Answer 6

Sodium and potassium ions cannot diffuse through this fatty layer (myelin sheath).

Question 7

Describe saltatory conduction in nodes of Ranvier.

Answer 7

• Ionic diffusion can only occur at the nodes of Ranvier.
• This means that the local currents are elongated and sodium ions diffuse along the neurone from one node of Ranvier to the next.
• This means that the action potential appears to jump from one node to the next.

Question 8

What is the advantage of saltatory conduction?

Answer 8

• The myelin sheath means that action potentials can only occur at the nodes of Ranvier.
• Effectively, the action potential jumps from one node to the next.
• This speeds up the transmission of the action potential along the neurone.

Question 9

Why can a myelinated neurone transmit an impulse much quicker than a non-myelinated neurone?

Answer 9

-The flow of sodium ions along the axon in a local current is much more rapid than the movement of an action potential involving the exchange of ions across the membrane.

Question 10

What is the ‘all-or-nothing’ rule?

Answer 10

• All action potentials are the same intensity.

- Each one produces a depolarisation of +40mV.

Question 11

How can we detect stimuli of different intensities?

Answer 11

• Our brains determine the intensity of the stimulus from the frequency of action potentials arriving in the sensory region of the brain.
• A higher frequency of action potentials means a more intense stimulus.

Question 12

How does a stimulus with a higher intensity cause more frequent generator potentials?

Answer 12

• When a stimulus is at higher intensity, more sodium ion channels open in the sensory receptor.
• This produces more generator potentials.
• As a result, there are more frequent action potentials in the sensory neurone.
• As a result, there are more frequent action potentials entering the central nervous system.