OCR Biology A level - Neuronal communication

OCR Biology A level - Neuronal communication

What are the main structures of a neurone in a mammal?

• Axon

• Dendron - dendrites

• Cell body

Why are there large amounts of ER & mitochondria in the cell bodies of neurones?

For the synthesis of proteins and neurotransmitters.

Function of dendrons/dendrites?

• Receive signals from other neurons and transmit them to the soma(cell body).

What is the role of an axon?

To transmit impulses away from the cell body.

Sensory neuron.

• Transmits impulse from sensory receptor cell to relay/motor neuron/brain.

• One dendron, one axon

Relay neuron.

• Transmits impulse between neurons.

• Many short axons & dendrons.

Motor neuron.

• Transmits impulse from relay/sensory neuron to effector (eg. muscle/gland)

• One long axon, many short dendrites.

What constitutes a myelin sheath?

Many layers of plasma membrane created by schwann cells wrapping around the axon.

What are nodes of ranvier?

• Gaps between adjacent schwann cells

• Electrical impulses jump from node to node, meaning the speed of transmission is much quicker.

Stimulus for Mechanoreceptor?

Pressure & movement eg. pacinian corpuscle

Stimulus for chemoreceptor

chemicals

stimulus for thermoreceptor

heat

stimulus for photoreceptor

light

Describe sensory receptors role as a transducer

They convert the stimulus they detect which may be in the form of pressure, heat, light into an electric impulse (generator potential)

What and where are stretch mediated sodium channels found?

• In pacinian corpuscle, end of neuron

• When they change shape, their permeability to sodium ions changes

Describe the mechanism of how a pacinian corpuscle converts mechanical pressure into an electrical impulse.

1. In it’s resting state, the stretch mediated sodium channels are too narrow to allow the movement of sodium ions into the membrane. It has a resting potential.

2. When pressure is applied to the corpuscle , it changes shape, & the membrane of the neuron stretches, widening the SMSChannels.

3. Sodium ions now diffuse through the channels into the neuron, changing it’s p.d. to a generator potential. It has become depolarised.

4. The generator potential triggers an action potential to travel along the axon.

What is the usual value of the resting potential?

-70mV

How is the resting potential created and maintained?

1. The sodium potassium pump transports 3Na+ out of the axon membrane for every 2K+ it pumps in, so there is a greater concentration of sodium ions outside the membrane and a greater concentration of potassium ions inside the cytoplasm.

2. The ions diffuse down their electrochemical gradients through ion channels.

3. However, most of the sodium gated ion channels are closed, preventing the movement of Na+ back into the cytoplasm. K+ can diffuse through potassium ion channels out of the cytoplasm, so there is a greater concentration of positive charge outside the axon membrane than inside.

4. This creates the resting potential of -70mV.

How is an action potential transmitted?

1. Axon is at resting potential.

2. Energy of the stimulus triggers some voltage gates sodium ion channels to change shape and open.

3. This allows sodium ions to diffuse down their electrochemical gradient into the cytoplasm, further perpetuating the change in p.d., causing more voltage gated ion channels to open and more sodium ions to diffuse into axn (positive feedback)

4. When the p.d. across the axon membrane reaches about +40mV, the voltage gated sodium ion channels close, and potassium ion voltage gated channels open, allowing K+ to diffuse down their electrochemical gradient out of the membrane. This is known as repolarization as the inside of the membrane now becomes less positive.

5. Initially, lots of K+ diffuses out of the membrane, making the cytoplasm more negative relative to the outside of the membrane than it is at it’s resting potential (hyperpolarization).

6. Voltage gated potassium ion channels now close, and the sodium potassium pump moves 3:2 Na+:K+ across the membrane, restoring the resting potential.

What is the refractory period and why is it important?

It is a short period of time after an action potential has passed in which the axon cannot be excited again - sodium ion voltage gated channels remain closed, not allowing depolarisation to happen and hence not allowing another action potential to fire.

It is important because it ensures that impulses are discrete and don’t overlap, and also prevents the backwards propagation of an action potential.

What is saltatory conduction?

A process in which action potentials ‘jump’ from adjacent nodes of ranvier.

How does the strength of a stimulus affect the propagation of action potentials?

Stronger stimulus = higher frequency of action potentials