Biology - 1.4 - part 2 - nerve impulse

What is resting membrane potential?

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

What is the resting membrane potential typically between for neutrons?

-60 and -80 millivolts. (-70 mV)

What does the transmission of a nerve impulse require?

It requires changes in the membrane potential of the neuron’s plasma membrane.

What is an action potential?

A wave of electrical excitation along a neuron’s plasma membrane.

What is the value for the action potential?

-55 mV.

How do neurotransmitters initiate a response?

They initiate a response by binding to their receptors at a synapse.

What is nerve transmission?

A wave of depolarisation of the resting potential of a neuron.

Why is action potential explained as depolarisation?

In its normal state, the inside of a neuron cell is negative (-70mV)

What does depolarisation (and hence nerve transmission) result from

It is a wave of electrical excitation along a neuron’s plasma membrane

What is depolarisation a result of?

The entry of positive ions which trigger the opening of voltage-gated sodium channels, and so further depolarisation occurs.

What does the depolarisation of the plasma membrane trigger?

It triggers the opening of voltage-gated sodium channels and so further depolarisation occurs. (sodium ions enter the cell)

What restores the membrane potential again?

The sodium channels become inactivated and potassium channels are opened to restore the resting membrane potential. (potassium ions move out of the cell)

Explain the chain reaction that happens during depolarisation of the plasma membrane.

When one patch of membrane becomes depolarised, it cause the inside of the cell in that area to become positive. This affects the neighbouring membrane and causes voltage-gated sodium channels in the next patch to open and sodium ions to enter.

What allows a nerve impulse to travel along an axon?

The chain reaction/wave of depolarisation.

What happens when the action potential reaches the end of the neuron?

It causes vesicles containing neurotransmitter to fuse with the membrane (of the neuron/presynaptic membrane), the neurotransmitter is released and binds to receptors on the membrane of the next cell (post-synaptic membrane). This stimulates a response in the next cell.

What des the restoration of the resting membrane potential allow for?

The inactive voltage-gated sodium channels to return to a conformation that allows potassium to move out of the cell. This makes them ready to open again in response to depolarisation of the membrane (action potential).

What is re-established by the sodium-potassium pump?

Ion concentration gradients, it actively transports excess ions in and out of the cell.

What is the retina?

The area within the eye that detects light and contains photoreceptor cells.

What types of photoreceptor cells does the retina contain?

Rods and cones.

What are rod cells useful for?

Vision in areas of low light as they are sensitive to changes in light intensity. However, they do not allow colour perception.

What are cone cells useful for?

They are sensitive to specific colours (wavelengths) of light. They allow animals to have coloured vision and only function in bright light.

What do people with colourblindness lack?

A particular type of cone cell in their retina.

What is the light-sensitive molecule retinal combined with (in animals)?

A membrane protein called opsin.

What does the the retinal-opsin complex help to form?

The photoreceptors.

What is the retinal-opsin complex called in rod cells?

Rhodopsin.

What happens when retinal absorbs a photon of light?

Rhodopsin changes conformation to photoexcited rhodopsin.

What does photoexcited rhodopsin activate?

It activates a G-protein called transducin, which activates the enzyme phosphodiesterase (PDE).

What does phosphodiesterase (PDE) catalyse?

PDE catalyses the hydrolysis (break down) of a molecule called cyclic GMP (cGMP).

What does the hydrolysis (break down) of cGMP cause?

It causes the ion channels in the membrane of rod cells to close, which triggers nerve impulses in the neurone of the retina.

How many G proteins (transducin) does one photoexcited rhodopsin activate?

It activates hundreds of transducin molecules.

What amplifies the signal?

A cascade of proteins.

Why can rod cells respond to low intensities of light?

A very high degree of amplification.

How are different photoreceptor proteins formed in cone cells?

Different forms of opsin combine with retinal.

What is the difference between the different photoreceptors in cone cells?

They each have a different maximal sensitivity to a specific wavelength.

What are the wavelengths that the different photoreceptor cells have a maximal sensitivity to?

Red, green, blue or UV.