Unit 1 Part 4d (Nerve Impulse Transmission)

Question 1

(I) What is the resting membrane potential?

Answer 1

A state where there is no net flow of ions across the membrane

Question 2

(I) What does transmission of a nerve impulse require?

Answer 2

Changes in the membrane potential of the neuron’s plasma membrane

Question 3

(I) What is action potential?

Answer 3

Wave of electrical excitation along a neuron’s plasma membrane

Question 4

(I) How do neurotransmitters initiate a response?

Answer 4

By binding to their receptors - which are ligand-gated ion channels - at a synapse

Question 5

(I) What is depolarisation?

Answer 5

Change in the membrane potential to a less negative value inside

Question 6

(I) Explain the depolarisation of the plasma membrane

Answer 6

Depolarisation of the plasma membrane as a result of the entry of positive ions triggers the opening of voltage-gated sodium channels, and further depolarisation occurs

Question 7

(I) Explain the process by which the inactivation of sodium channels and the opening of potassium channels restores the resting membrane potential

Answer 7

1) Binding of a neurotransmitter triggers the opening of ligand-gated ion channels at a synapse.
2) Ion movement occurs and there is depolarisation of the plasma membrane. If sufficient ion movement occurs, and the membrane is depolarised beyond a threshold value, the opening of voltage-gated sodium channels is triggered and sodium ions enter the cell down their electrochemical gradient
3) This leads to a rapid and large change in the membrane potential
4) A short time after opening, the sodium channels become inactivated.
5) Voltage-gated potassium channels then open to allow potassium ions to move out of the cell to restore the resting membrane potential.

Question 8

(I) What does depolarisation of a patch of membrane cause?

Answer 8

Neighbouring regions of membrane to depolarise and go through the same cycle, as adjacent voltage-gated sodium channels are opened

Question 9

(I) What happens when the action potential reaches the end of the neuron?

Answer 9

It causes vesicles containing neurotransmitter to fuse with the membrane.
This releases neurotransmitter which stimulates a response in a connecting cell.

Question 10

(I) What does restoration of the resting membrane potential allow?

Answer 10

The inactive voltage-gated sodium channels to return to a conformation that allows them to open again in response to depolarisation of the membrane

Question 11

(I) What happens to sodium and potassium ion concentration gradients following repolarisation?

Answer 11

They are reduced. The sodium potassium pump restores the sodium and potassium ions back to resting potential levels.

Question 12

(II) What is the retina?

Answer 12

An area within the eye that detects light and contains two types of photoreceptor cells: rods and cones

Question 13

(II) Describe rods

Answer 13

Function in low light but do not allow colour perception

Question 14

(II) Describe cones

Answer 14

Responsible for colour vision and only function in bright lights

Question 15

(II) How do cones work?

Answer 15

Different forms of opsin combine with retinal to give different photoreceptor proteins, each with a maxima, sensitivity to specific wavelengths: red, green, blue or UV.

Question 16

(II) Describe the combination of opsin and retinal

Answer 16

In animals, the light sensitive molecule retina, is combined with a membrane protein - opsin - to form the photoreceptors of the eye, primarily rhodopsin

Question 17

(II) Describe the action of rhodopsin

Answer 17

1. Retinal absorbs a photon of light and rhodopsin changes conformation to photo excited rhodopsin
2. A single photo excited rhodopsin activates hundreds of molecules of G protein called transduction
3. Each activated transducin activates one molecule of the enzyme PDE.
4. Each active PDE catalyses the breakdown of a molecule called cyclic GMP from many ligand-gated Na channels and they close.
5. Na channels close halting inward leakage of the ions. Membrane potential increases and a nerve impulse is generated in neurons in the retina.

Question 18

(II) What allows rods to respond to low intensities of light?

Answer 18

A very high degree of amplification