Neurones and nervous coordination

Question 1

What six things is a mammalian (motor) neurone made up of

Answer 1

• Cell body
• Dendrons
• Axon
• Schwann cells
• myelin sheath
• Nodes of ranvier

Question 2

What are neurones

Answer 2

Specialised cells adapted to rapidly carry electrochemical changes (nerve impulses) from one part of the body to another.

Question 3

What is the function of the cell body of a neurone

Answer 3

• The cell body contains all of the usual cell organelles, including a nucleus and large amounts of rough endoplasmic reticulum.
• This is associated with the production of proteins and neurotransmitters.

Question 4

What is the function of the dendrons in a neurone

Answer 4

Dendrons are extensions of the cell body which subdivide into smaller branched fibres called dendrites, that carry nerve impulses towards the cell body.

Question 5

What is the function of the axon of a neurone

Answer 5

The axon is a single long fibre that carries nerve impulses away from the cell body.

Question 6

What are the Schwann cells of a neurone and what is their function

Answer 6

• Schwann cells are cells that surround the axon, protecting it and providing electrical insulation.
• They also carry out phagocytosis 9the removal of cell debris) and play a part in nerve regeneration.
• Schwann cells wrap themselves around the axon many times, so that layers of their membranes build up around it.

Question 7

What is the myelin sheath of a neurone and what is its function

Answer 7

• The myelin sheath forms a covering to the axon and is made up of the membranes of the Schwann cells.
• These membranes are rich in the lipid myelin.
• Neurones with a myelin sheath are known as myelinated neurones.

Question 8

What are the nodes of Ranvier of a neurone

Answer 8

• The nodes of Ranvier are contractions between adjacent Schwann cells where there is no myelin sheath.
• The constrictions are 2-3um long and occur every 1-3mm in humans.

Question 9

Describe the function of a sensory neurone and how this relates to its structure

Answer 9

• Sensory neurones transmit nerve impulses from a receptor to an intermediate or motor neurone.
• They have one dendron that is often very long.
• It carries the impulse towards the cell body and one axon carries this impulse away from the cell body.

Question 10

Describe the function of a motor neurone and how this relates to its structure

Answer 10

• Motor neurones transmit nerve impulses from an intermediate or relay neurone to an effector such as a gland or muscle.
• Motor neurones have a long axon and many short dendrites.

Question 11

Describe the function of intermediate/relay neurones and how this relates to their structure

Answer 11

• Intermediate or relay neurones transmit impulses between neurones.
• They have numerous, short processes.

Question 12

What is the definition of a nerve impulse

Answer 12

• A nerve impulse is a self-propagating wave of electrical activity that travels along the axon membrane.
• It is the temporary reversal of the electrical potential difference across the axon membrane.
• This reversal is between two states, called the resting potential and the action potential.

Question 13

Describe the ways in which the movement of ions (Na+ and K+) across the axon membrane is controlled

Answer 13

• The phospholipid bilayer do the axon plasma membrane prevents sodium and potassium ions diffusing across it.
• Channel proteins span the phospholipid membrane which have ion channels with gates that can be opened or closed so that sodium and potassium ions can only travel through them at specific times.
• Some channels remain opened at all times so that sodium and potassium ions move unhindered through them by facilitated diffusion.
• Some carrier proteins actively transport potassium ions into the axon and sodium ions out of the axon- this is the sodium-potassium pump.

Question 14

Explain what the resting potential of an axon is

Answer 14

• The inside of an axon is negatively charged relative to the outside- this is called the resting potential.
• This ranges from 50 to 90 millivolts (mV) but is usually 65 mV humans

Question 15

What is the resting potential of an axon in humans

Answer 15

-65 mV

Question 16

Describe how the resting potential of an axon is established and maintained

Answer 16

• Sodium ions are actively transported out of the axon by sodium-potassium pumps.
• Potassium ions are actively transported into the axon by the sodium potassium pumps.
• The active transport of sodium ions is greater than that of potassium ions- 3 sodium ions move out for every two potassium ions that move in.
• Although both sodium and potassium ions are positive, the outward movement of sodium ions being greater than the inward movement of potassium ions means that there are more sodium ions in the tissue fluid surrounding the axon that in the cytoplasm.
• There are also more potassium ions in the cytoplasm than in the tissue fluid, thus creating an electrochemical gradient.
• The sodium ions begin to diffuse back naturally into the axon while the potassium ions begin to diffuse back out of the axon.
• However, most of the gates in the channels that allow the potassium ions to move through are open, while most of the gates in the channels that allow the sodium ions to move through are closed.

Question 17

What is the membrane of the axon said to be when at resting potential

Answer 17

Polarised

Question 18

Explain what an action potential is

Answer 18

• When a stimulus of sufficient size is detected by a receptor in the nervous system, its energy causes a temporary reversal of the charges either side of this part of the axon membrane.
• If the stimulus is great enough, the negative charge of -65 mV inside the membrane becomes a positive charge of around +40 mV.
• This is known as the action potential and this part of the axon membrane is said to be depolarised.

Question 19

What key property of the axon membrane allows depolarisation to occur

Answer 19

Depolarisation occurs because the channels in the axon membrane change shape, and hence open or close, depending on the voltage across the membrane (they are therefore called voltage-gated channels).

Question 20

Describe how an action potential is formed

Answer 20

• At resting potential some voltage-gated channels are open (those that are always open) but the sodium volatage-gated channels are closed.
• The energy of the stimulus causes some sodium voltage-gated channels in the axon membrane to open and therefore sodium ions diffuse into the axon through these channels along their electrochemical gradient.
• Being positively charged, they trigger a reversal in the potential difference across the membrane.
• As the sodium ions diffuse into the axon, more sodium channels open, causing an even greater influx of sodium ions by diffusion.
• Once the action potential of around +40 mV has been established, the voltage gates on the sodium ion channels close, thus preventing the further influx of sodium ions, and the voltage gates on the potassium ion channels begin to open.
• With some potassium voltage-gated now open, the electrical gradient that was preventing further outward movement of potassium ions is now reversed, causing more potassium ion channels to open.
• This means that yet more potassium ions diffuse out, starting repolarisation of the axon
• The outward diffusion of these potassium ions causes a temporary overshoot of the electrical gradient, with the inside of the axon being more negative (relative to the outside) than usual.
• This is hyperpolarisaiton and the period is called the refractory period.
• The closable gates on the potassium ion channels now close and the activities of the sodium-potassium pumps once again cause sodium ions to be pumped out and potassium ions in.
• The resting potential of -65 mV is re-established and the axon is said to be repolarised.

Question 21

What type of transport causes an action potential

Answer 21

Diffusion of sodium ions into the axon- this is a passive process

Question 22

What type of transport maintains the resting potential

Answer 22

Active transport- the sodium potassium pumps use active transport to maintain the resting potential which is an active process

Question 23

Summarise how an action potential travels along an axon

Answer 23

• The size of the action potential remains the same from one side of the neurone to the other.
• as one region of the axon produces an action potential and becomes depolarised, it acts as a stimulus for the depolarisation of the next region of the axon.
• An action potential is a travelling wave of depolarisation.
• The previous region of the membrane returns to its resting potential as the action potential travels.

Question 24

Describe how an action potential travels along an unmyelinated axon

Answer 24

• At resting potential the concentration of sodium ions outside the axon membrane is high relative to the inside, whereas that of the potassium ions is high inside the membrane relative to the outside.
• The overall concentration of positive ions is, however, greater on the outside, making this positive compared with the inside.
• The axon membrane is polarised.
• A stimulus causes a sudden influx of sodium ions and hence a reversal of charge on the axon membrane.
• This is the action potential and the membrane is depolarised.
• The localised electrical currents established by the influx of sodium ions cause the opening of sodium voltage-gated channels a little further along the axon.
• The resulting influx of sodium ions in this region causes depolarisation.
• Behind this new region of depolarisation, the sodium voltage-gated channels close and the potassium ones open.
• Potassium ions begin to leave the axon along their electrochemical gradient.
• So, once initiated, the depolarisation moves along the membrane.
• The action potential (depolarisation) is propagated in the same way further along the axon.
• The outward movement of the potassium ions has continued to the extent that the axon membrane behind the action potential has returned to its original charged state- it has been repolarised.
• Repolarisation of the axon allows sodium ions to be actively transported out, once again returning the axon to its resting potential in readiness for a new stimulus if it comes.

Question 25

Describe how an action potential passes along as myelinated neurone

Answer 25

• In myelinated axons, the fatty myelin sheath around the axon acts as an electrical insulator, preventing action potentials from forming.
• At intervals of 1-3mm there are breaks in this myelin insulation called nodes of Ranvier where action potentials occur.
• The localised circuits therefore arise between adjacent nodes of Ranvier and the action potentials ‘jump’ from node to node.
• This is saltatory conduction.

Question 26

What is saltatory conduction

Answer 26

An action potential ‘jumping’ between nodes of Ranvier in a myelinated neurone

Question 27

Why does an action potential travel faster along a myelinated neurone than an unmyelinated neurone with the same diameter

Answer 27

• The process of saltatory conduction in a myelinated neurone allows action potentials to ‘jump’ between adjacent nodes of Ranvier.
• The depolarisation only occurs at nodes of Ranvier in a myelinated neurone.
• In an unmyelinated neurone, the events of depolarisation have to take place all the way along an axon and this takes more time.

Question 28

What are the three key factors that determine how fast a nerve impulse travels

Answer 28

1) The myelin sheath
2) The diameter of the axon
3) Temperature

Question 29

Describe how the myelin sheath affects the speed of an action potential

Answer 29

• If a neurone has a myelin sheath, saltatory conduction takes place where the action potential jumps between adjacent nodes of Ranvier.
• This increases the speed of conductance from 30 ms^-1 in an unmyelinated neurone to 90 ms^-1 in a similiar myelinated one.

Question 30

Describe how the diameter of the axon affects the speed at which an action potential travels

Answer 30

• The greater the diameter of the axon, the greater the speed of conductance.
• This is due to less leakage of ions from a large axon.
• Leakage of ions makes membrane potentials harder to maintain.

Question 31

Describe how temperature affects the speed at which an action potential travels.

Answer 31

• Temperature affects the rate of diffusion of ions and therefore the higher the temperature, the faster the nerve impulse.
• The energy for active transport comes from respiration which is controlled by enzymes.
• Enzymes function more rapidly up to a certain temperatures above which the enzymes and the plasma membranes are denatured.
• At this point, impulses fail to be conducted at all.

Question 32

Why are nerve impulses described as all or nothing responses

Answer 32

• There is a certain level of stimulus- the threshold value- which triggers an action potential.
• Below the threshold value, no action potential is generated.

Question 33

How do organisms perceive the size of a stimulus

Answer 33

• By the number of impulses passing in a given time. The larger the stimulus, the more impulses pass in a given time.
• By having different neurones with different threshold values. The brain interprets the number and the type of neurones that pass impulses as a result of a given stimulus and thereby determines its size.

Question 34

What is the refractory period

Answer 34

• Once an action potential has been created in any region of an axon, there is a period afterwards when inward movement of sodium ions is prevented because the sodium voltage-gated channels are closed.
• During this time it is impossible for a further action potential to be produced.
• This is the refractory period.

Question 35

List the three purposes of the refractory period

Answer 35

1) It ensures that action potentials are propagated in one direction only.
2) It produces discrete impulses
3) It limits the number of action potentials.

Question 36

Explain how the refractory period ensures that action potentials are propagated in one direction only

Answer 36

• Action potentials can only pass from an active region to a resting region.
• This is because action potentials cannot be propagated in a region that is refractory, which means that they can only move in a forward direction.
• This prevents action potentials from spreading out in both directions.

Question 37

Explain how the refractory period ensures that only discrete impulses are produced

Answer 37

• Due to the refractory period, a new action potential cannot be formed immediately behind the first one.
• This ensures that action potentials are separated from one another.

Question 38

Explain how the refractory period limits the number of action potentials

Answer 38

• As action potentials are separated from one another this limits the number of action potentials that can pass along an axon in a given time.
• This limits the strength of a stimulus that can be detected.

Question 39

What is a synapse

Answer 39

The point where a neurone communicates with another neurone or an effector.

Question 40

How do synapses transmit information

Answer 40

Neurotransmitters

Question 41

Describe the structure of a synapse

Answer 41

• Neurones are separated by a small gap called the synaptic cleft
• The neurone that releases the neurotransmitter is called the presynaptic neurone.
• The axon of the presynaptic neurone ends in a swollen portion known as the synaptic knob which possesses many mitochondria and large amounts of endoplasmic reticulum.
• These are required in the manufacture of the neurotransmitter which takes place in the axon.
• The neurotransmitter is stored in synaptic vesicles.
• Once the neurotransmitter is released from the vesicles it diffuses across to the postsynaptic neurone which possesses specific receptor proteins on its membrane to receive it.

Question 42

Synapses have unidirectionality- what does this mean

Answer 42

Synapses can only pass information in one direction- from the presynaptic neurone to the post synaptic neurone.

Question 43

What are the two types of summation

Answer 43

• Spatial summation
• Temporal summation

Question 44

Why is summation important

Answer 44

• Low frequency action potentials often lead to the release of insufficient concentrations of neurotransmitter to trigger a new action potential in the postsynaptic neurone.
• Summation allows them to do so by causing a rapid build up of neurotransmitter in the synapse.
• This occurs by spatial summation of temporal summation.

Question 45

What is spatial summation

Answer 45

• In spatial summation, a number of different presynaptic neurones together release enough neurotransmitter to exceed the threshold value of the postsynaptic neurone.
• Together they therefore trigger a new action potential.

Question 46

Describe temporal summation

Answer 46

• In temporal summation, a single presynaptic neurone releases neurotransmitter many times over a very short period.
• If the concentration of neurotransmitter exceeds the threshold value of the postsynaptic neurone, then a new action potential is triggered.

Question 47

What are inhibitory synapses

Answer 47

Synapses which make it less likely that a new action potential will be created on the postsynaptic neurone

Question 48

Describe how inhibitory synapses function

Answer 48

1) The presynaptic neurone releases a type of neurotransmitter that binds to chloride ion protein channels on the post synaptic neurone.
2) The neurotransmitter causes the chloride ion protein channels to open.
3) Chloride ions move into the post synaptic neurone by facilitated diffusion.
4) The binding of the neurotransmitter causes the opening of nearby potassium protein channels.
5) Potassium ions move out of the postsynaptic neurone into the synapse.
6) The combined effect of negatively charged chloride ions moving in and positively charged potassium ions moving out is to make the inside of the postsynaptic membrane more negative and the outside more positive.
7) The membrane potential increases to as much as -80 mV compared with the usual -65 mV at resting potential.
8) This is called hyperpolarisation and makes it less likely that a new action potential will be created because a larger influx of sodium ions is needed to produce one.

Question 49

What two key things does synapses acting as junctions allow

Answer 49

• A single impulse along one neurone to initiate new impulses in a number of different neurones at a synapse- this allows a single stimulus to create a number of simultaneous responses.
• A number of impulses to be combined at a synapse. This allows nerve impulses from receptors reacting to different stimuli to contribute to a single response.

Question 50

What are four key points that you need to understand the basic functioning of a synapse

Answer 50

1) A chemical (the neurotransmitter) is made only in the presynaptic neurone and not in the postsynaptic neurone.
2) The neurotransmitter is stored in synaptic vesicles. When an action potential reaches the synaptic knob the membranes of these vesicles fuse with the presynaptic membrane to release the neurotransmitter.
3) When released, the neurotransmitter diffuses across the synaptic cleft to bind to specific receptor proteins which are found only on the postsynaptic neurone.
4) The neurotransmitter binds with the receptor proteins and this leads to a new action potential in the postsynaptic neurone. Synapses that produce new action potentials in this way are called excitatory synapses.

Question 51

What is a cholinergic synapse

Answer 51

A synapse where the neurotransmitter is acetylcholine

Question 52

Describe the transmission of an action potential across a cholinergic synapse

Answer 52

1) The arrival of an action potential at the end of the presynaptic neurone causes calcium ion protein channels to open and calcium ions Ca 2+ enter the synaptic knob by facilitated diffusion.
2) The influx of calcium ions into the presynaptic neurone causes synaptic vesicles to fuse with the presynaptic membrane, releasing acetylcholine into the synaptic cleft.
3) Acetylcholine molecules diffuse across the narrow synaptic cleft easily because the diffusion pathway is short.
4) Acetylcholine then binds to receptor sites on sodium ion protein channels in the membrane of the postsynaptic neurone.
5) This causes the sodium ion protein channels to open, allowing sodium ions (Na+) to diffuse in rapidly along a concentration gradient.
6) The influx of sodium ions generates a new action potential in the postsynaptic neurone.
7) Acetylcholinesterase hydrolyses acetylcholine into choline and ethanoic acid which diffuse back across the synaptic cleft into the presynaptic neurone.
8) In addition to recycling the choline and ethanoic acid, the rapid breakdown of acetylcholine also prevents it from continuously generating a new action potential in the post-synaptic neurone, and so leads to discrete transfer of information across synapses.
9) ATP released by mitochondria is used to recombine choline and ethanoic acid into acetylcholine.
10) This is stored in synaptic vesicles for future use.
11) Sodium ion protein channels close in the absence of acetylcholine in the receptor sites.