1.4d Nerve Impulse Transmission

Question 1

Resting membrane potential

Answer 1

The resting membrane potential is a state where there is no net flow of ions across the membrane.

Question 2

What does transmission of nerve impulse require?

Answer 2

The transmission of a nerve impulse requires changes in the membrane potential of the neurons plasma membrane.

Question 3

Acting potential

Answer 3

An action potential is a wave of electrical excitation along a neurons plasma membrane.

When action potential reaches the end of the neuron it causes vesicles containing neurotransmitters to fuse with the membrane - this releases neurotransmitters, which stimulate a response in a connecting cell.

Question 4

How do neurotransmitters make a response?

Answer 4

Neurotransmitters initiate a response by binding to their receptors at a synapse. Neurotransmitters receptors are ligand-gated ion channels.

Question 5

Depolarisation

Answer 5

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.
Depolarisation is a change in the membrane potential to a less negative value inside.

Question 6

Restoring the membrane potential

Answer 6

Binding of a neurotransmitter triggers the opening of a ligand-gated ion channels at a synapse. Ion movement occurs and there is a 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 the electrochemical gradient. This leads to a rapid and large change in the membrane potential. A short time after opening, the sodium channels become inactivated. Voltage gated potassium channels then open to allow potassium ions to move out of the cell to restore the resting membrane potential.

Question 7

Restoration of resting potential

Answer 7

Restoration of the resting membrane potential allows 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 8

Re-establishing resting membrane

Answer 8

Restoration of the resting membrane potential allows 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 9

Re-establishing ion concentration gradients

Answer 9

Following repolarisation the sodium and potassium ion concentration gradients are reduced. The sodium - potassium pump restores the sodium and potassium ions back to resting potential levels.

Question 10

Retina

Answer 10

The retina is the area within the eye that detects light and contains two types of photo receptors cells: rods and cones

Question 11

Rods

Answer 11

Function in dim light, but do not allow color perception. One type of opsin

Retinal-opsin complex is called rhodopsin.

Question 12

Cones

Answer 12

Responsible for color vision and only function in bright light.

Question 13

Animals

Answer 13

In animals the light sensitive molecule retinal is combined with a membrane protein, opsin to form the photoreceptors of the eye.

Question 14

Retinal and Rhodopsin function

Answer 14

Retinal absorbs a photon of light and rhosdopsin changes conformation to photoexcited rhodopsin.

Question 15

Photoexcited rhodopsin

Answer 15

A single photoexcited rhodopsin activates hundreds of molecules of G-protein. Each activated G-protein activates one molecule of PDE.

Question 16

PDE

Answer 16

Each active PDE molecule breaks down thousands of cGMP molecules per second. The reduction in cGMP concentration as a result of its hydrolysis affects the function of ion channels in the membrane of rod cells.

Question 17

How do rod cells work

Answer 17

A very high degree of amplification from a single photon of light results in sensitivity at low light intensities.