15.4 - Speed Of The Nerve Impulse

Question 1

What factors affect the speed at which action potentials travel along an axon?

Answer 1

1) The myelin sheath
2) Axon diameter
3)Temperature

Question 2

How does the myelin sheath affect the speed at which an action potential travels

Answer 2

• The myelin sheath acts as an electrical insulator, preventing action potentials from forming continuously along the axon.
• Instead, they jump from one node of Ranvier to another, a process called saltatory conduction.
• This speeds up transmission from around 30 m/s in unmyelinated neurons to as much as 90 m/s in myelinated neurons.

Question 3

How does the diameter of the axon affect the speed at which an action potential travels

Answer 3

• Larger axons conduct impulses faster.
• This is because larger axons experience less leakage of ions, which makes it easier to maintain membrane potentials and speeds up impulse transmission.

Question 4

How does temp affect the speed at which an action potential travels

Answer 4

• Higher temperatures increase the rate of ion diffusion, speeding up the nerve impulse.
• This happens because the enzymes involved in active transport, like the sodium-potassium pump, work faster at higher temperatures.
• However, if the temperature gets too high, these enzymes and membrane proteins become denatured, preventing nerve impulses from being conducted.
• Cold-blooded (ectothermic) animals are particularly affected by environmental temperature changes, as their body temperature fluctuates with their surroundings.

Question 5

What is the all-or-nothing principle in nerve impulses?

Answer 5

The all-or-nothing principle describes how a nerve impulse is generated based on a threshold stimulus:

• If a stimulus is below the threshold value, no action potential is generated, meaning no nerve impulse occurs—this is the “nothing” part.
• If the stimulus is equal to or above the threshold value, an action potential is generated, leading to a nerve impulse being transmitted—this is the “all” part.
• Regardless of the strength of the stimulus, all action potentials are of the same size, so the strength of a stimulus does not affect the size of the action potential.

Question 6

If action potentials are all the same size, how does the body perceive the strength of a stimulus?

Answer 6

1) The frequency of action potentials
- Stronger stimuli generate a greater frequency of action potentials in a given period.
- The more intense the stimulus, the more action potentials produced in a short time.
2) Activation of different types of neurones
- Different neurons have different threshold values.
- Some neurons may respond to weak stimuli, while others only respond to stronger stimuli.
- The brain interprets the number and type of neurons firing to assess the strength of a stimulus.

Question 7

What is the refractory period in nerve impulses?

Answer 7

• The refractory period is the time after an action potential has been generated during which it is impossible to initiate another action potential.
• This occurs because the sodium voltage-gated channels are closed, preventing the inward movement of sodium ions.

Question 8

What does the refractory period ensure that

Answer 8

1) Action potentials are propagated in one direction only.
2) Discrete impulses are produced, meaning each action potential is separated from the next.
3) The number of action potentials generated in a given time is limited, controlling the strength of stimuli that can be detected.

Question 9

How does the refractory period ensure that action potentials are propagated in one direction only?

Answer 9

• The refractory period prevents action potentials from moving backwards along the axon because the region of the axon behind the current action potential is in a refractory state and cannot generate another action potential.
• As a result, action potentials can only move forward, from an active region to a resting region, ensuring unidirectional propagation.

Question 10

How does the refractory period ensure that action potentials are discrete?

Answer 10

• The refractory period ensures that each action potential is separated from the next.
• This is because a new action potential cannot form immediately behind the current one, leading to discrete impulses rather than a continuous wave of depolarisation.
  -This ensures that each signal is distinct and clearly identifiable by the nervous system.

Question 11

How does the refractory period limit the number of action potentials that can be generated in a given time?

Answer 11

• The refractory period limits the number of action potentials because a new action potential cannot be generated immediately after the previous one.
• This separation restricts how many impulses can travel along the axon in a given period.
• As a result, it helps regulate the frequency of nerve impulses and controls the strength of stimulus that can be detected by the nervous system.