The nervous system AQA BIOLOGY P2

define neurones

Specialised animal cells that pass on nerve impulses

Describe the structure of a non myelinated neurones

. Dendrites

. Cell body

. Axon

. Axon terminal

Describe the structure of a myelinated neurones

. Dendrites

. Cell body

. Myelin Sheath (made from schwann cells)

. Nodes of Ranvier

. Axon terminal

What do axon terminals form with neurones?

Synapses

Define what is meant by the term nerve impulse

A nerve impulse is a moving area of charge.

Which direction do nerve impulses travel?

Nerve impulses always travel in one direction, which are away from an receptor and towards an effector. This is known as unidirectionality.

How do the concentrations of sodium and potassium ions differ inside and outside the axon?

.There is a higher concentration of sodium ions outside the axon than inside the axon.

.There is a higher concentration of potassium ions inside the axon than outside the axon.

Why can sodium and potassium ions only cross the axon membrane through channel proteins, and not by simple diffusion?

. Sodium and potassium ions are hydrophilic.

. Sodium and potassium ions are charged.

. Sodium and potassium ions are insoluble in lipids.

what is the sodium-potassium pump is an example of?

Active transport

What ions does the sodium–potassium pump move into and out of the axon?

The sodium-potassium pump moves three Na+out of the axon, and two K+into the axon.

What is the purpose of the sodium-potassium pump?

The purpose of the sodium-potassium pump is to maintain a more positive charge outside the axon, and a less positive charge inside the axon.

Define what is meant by the term resting potential

Resting potential of a neurone is defined as when it is not exposed to a stimulus and a less positive charge is maintained inside the axon.

At resting potential, the inside of the axon is less positively charged than the outside. Why is this important for neurones?

It allows neurones to respond to any nerve impulses arriving at its axon. Axons of a neurone can only respond to a nerve impulse if the inside of the axon is less positive than the outside.

What happens to the charge inside the axon compared to the outside when a nerve impulse is triggered by a stimulus?

A nerve impulse triggered by a stimulus causes the inside of the axon to become more positivethan the outside of the axon.

what does the arrival of an nerve impulse is triggered by a stimulus, in the axon cause?

. A nerve impulse causes a small depolarisation inside the axon.

. This causes voltage gated sodium ion channels to open and allow sodium ions to diffuse across the axon membrane down their electrochemical gradient

. This causes the inside of the axon to become more positvely charged than outside of the axon

What is an action potential?

An action potential is when the inside of the axon of a neurone reaches its maximum positive charge.

How does the arrival of a nerve impulse trigger an action potential in an axon?

The positive charge of a nerve impulse arriving at the axon, leads to the opening of voltage-gated sodium channels, allowing sodium ions to enter the axon and reach threshold, triggering an action potential.

What is meant by the threshold in triggering an action potential?

. An action potential is only triggered if enough sodium ions enter the axon.

. In other words, an action potential is only triggered if the inside of the axon reaches a certain level of positive charge compared to the outside of the axon.

. This is known as threshold.

explain what is meant by the term ‘all or nothing principle’

An action potential either occurs fully or not at all.

describe what is meant by the term ‘depolarisation’

Depolarisation occurs when voltage-gated sodium channels in the axon membrane open. Sodium ions diffuse into the axon down their electrochemical gradient. This makes the inside of the axon more positive than outside of the axon.

describe what is meant by the term ‘repolarisation’

Repolarisation happens when voltage-gated potassium channels open, allowing potassium ions (K⁺) to leave the axon. This movement of K⁺ out of the axon causes the inside of the axon to become less positive.

describe what is meant by the term ‘hyperpolarisation’

hyperpolarisation is when the inside of the axon becomes more negative than its normal resting potential. This usually happens right after repolarisation, when potassium ions (K⁺) continue to leave the axon for a short time. Hyperpolarisation makes it less likely for the neuron to fire another action potential immediately.

describe what is meant by the term ‘Refractory period’

Neuron cannot fire another action potential immediately

describe what is meant by the term ‘speed of conductance’

The speed of the movement of charge

Factors affecting speed of conductance

. Axon diameter

. Temperature

. Myelin sheath

How does the diameter of an axon affect the speed of conductance?

The bigger the diameter of an axon, the faster the speed of conductance.

How does an increase in axon diameter affect the resistance inside the axon and the speed of conductance?

The bigger the axon diameter the lower the resistance inside the axon, resulting in faster speed of conductance.

How does an increase in axon diameter affect the maintenance of resting potential inside the axon and the speed of conductance?

The bigger the axon diameter the fewer the potassium ions will diffuse out of the axon, maintaining resting potential inside the axon.

How does the temperature of an axon affect the speed of conductance?

The greater the temperature, the faster the speed of conductance.

why does the temperature of an axon affect the speed of conductance?

. Higher temperatures, results in higher rates of diffusion of sodium ions in the axon and therefore faster speed of conductance.

. However at high rates of temperature - enzymes can denature - prevent respiration - no production of ATP - sodium potassium (active transport) cannot function - no speed of conductance.

What are myelin sheath formed by?

Schwann cells

what are breaks, roughly every 2 millimeters, in the myelin sheath called?

nodes of Ranvier

Where can action potentials be triggered in a myelinated axon?

nodes of ranvier

Why can only nodes of Ranvier trigger action potentials in a myelinated axon?

Only at nodes of ranvier is there voltage gated sodium channels.

What is the term used when an action potential jumps from one node of Ranvier to the next?

saltatory conduction

What does the refractory period enable?

. enables unidirectionality

. enables discrete impulses

What is meant by the term synapse?

A junction that transfers an action potential between two neurons or a neuron and muscle fibre

What is meant by the term temporal summation?

When one single presynaptic neuron releases neurotransmitters repeatedly over a short period

What is meant by the term spatial summation?

When many presynaptic neurons release neurotransmitters all at once

What is meant by the phrase ‘synapses are unidirectional’?

Information travels one direction; from the presynaptic neuron towards the post synaptic neuron

What neurotransmitters do cholinergic synapses use?

Acetylcholine

describe synaptic transmission

1.action potential arrives at the presynaptic neuron and depolarises the presynaptic membrane

2.voltage gated calcium channels on presynaptic membrane open and allow the diffusion of calcium ions into the presynaptic neuron down a electrochemical gradient.

3.The influx of calcium ions causes the synaptic vesicle to move down the synaptic knob and fuse with the presynaptic membrane

4.Neurotransmitters (acetylcholine) are released into the synaptic cleft through exocytosis and diffuse towards the post synaptic neuron

5.Neurotransmitters (acetylcholine) bind to complimentary receptors on the post synaptic membrane

6.Sodium channels open, allowing the diffusion of sodium ions into the post synaptic neuron down an electrochemical gradient

7.The influx of sodium ion causes the post synaptic neuron to become depolarised

8.Once threshold is reached, an action potential is triggered

9.The enzyme Acetylcholinesterase hydrolyses acetylcholine into choline and ethanoic acid.

10.Choline and ethanoic acid are taken back into the presynaptic neurone, where they're recombined to form acetylcholine.

11.Acetylcholine is transported into synaptic vesicles using ATP.

What is meant by the term neuromuscular junction?

Synapses between a motor neuron and a muscle cell

describe an Action Potential Arriving at a Motor Neurone

1.Action potential arrives at the motor neuron (presynaptic neuron) which causes the presynaptic membrane to become depolarised

2.voltage gated calcium channels open and allow the diffusion of calcium ions into the presynaptic neuron

3.The influx of calcium ion causes synaptic vesicles to move down the synaptic knob and fuse with presynaptic membrane

4.Neurotransmitters are released into and diffuse down the synaptic cleft.

5.Neurotransmitters bind to complimentary receptors on the sodium channels causing the sodium channels to open

6.Sodium ions diffuse down their electrochemical gradient into the muscle cell and causes depolarisation of muscle cell, triggering an action potential. As a result, the muscle contracts.

7.Neurotransmitters are hydrolysed by an enzyme and the products diffuse into the presynaptic neuron to reform the neurotransmitter into the synaptic vesicles using ATP.

What is meant by the term excitatory neurotransmitters?

Excitatory neurotransmitters are neurotransmitters that trigger an action potential inside the postsynaptic neurone.

What is meant by the term inhibitory neurotransmitters?

Inhibitory neurotransmitters are neurotransmitters that make it harder to trigger an action potential inside the postsynaptic neurone.

What happens when neurotransmitters in inhibitory synapses bind to chloride channels on the postsynaptic membrane?

Neurotransmitters in inhibitory synapses bind to chloride channels on the postsynaptic membrane, causing them to open, which leads to a(n) hyperpolarisation of the postsynaptic membrane.