P1 Neuronal Communication

Question 1

What are neurones, and what types are there?

Answer 1

• Specialised animal cells that pass on nerve impulses.
• The 3 types of neurone are sensory, motor and relay neurone.

Question 2

Describe the structure and function of a sensory neurone.

Answer 2

• Pass impulses from receptors to the CNS.
• They are myelinated, the cell body is in the middle, dendrites at one end and an axon terminal at the other end.

Question 3

Describe the structure and function of a motor neurone.

Answer 3

• Pass impulses from the CNS to muscles/glands (effectors).
• They are myelinated, cell body at the end and is surrounded by dendrites, with an axon terminal at the opposite end.

Question 4

Describe the structure and function of a relay neurone.

Answer 4

• Pass impulses between sensory and motor neurones (found in the CNS).
• They are non myelinated, have their cell body at the end and contain lots of dendrites.

Question 5

What direction does charge move in an axon?

Answer 5

• From the receptor, towards the effector.
• The movement of charge is unidirectional.

Question 6

Which part of the neurone does the charge move through, and what is the charge called?

Answer 6

The charge is called a nerve impulse and it moves through the cytoplasm of neurones.

Question 7

What is the distribution of charge across an axon at rest, and how does a nerve impulse travel down an axon?

Answer 7

• At rest, there is a more positive charge on the outside of the axon compared to the inside, due to an overall greater concentration of ions on the outside.
• Sodium ions have a higher concentration on the outside of the axon, and potassium ions have a higher concentration on the inside of the axon.
• Sodium ions diffuse through a sodium ion channel (always open), down the electrochemical gradient, to the inside of the axon, while potassium ions diffuse through a potassium ion channel, down the electrochemical gradient, to the outside of the axon.
• The overall charge inside the axon becomes more positive than the outside. As like charges repel, potassium ions are repelled out of the axon down the electrochemical gradient and the charge inside the axon becomes less positive.

Question 8

What is the role of the sodium-potassium ion pump?

Answer 8

• The axon membrane contains a sodium-potassium ion pump, which is a co-transport carrier protein that can actively transport sodium and potassium ions.
• 3 sodium ions and a phosphate ion (from the hydrolysis of ATP) bind, triggering the carrier protein to change shape and force the 3 sodium ions across the membrane.
• This change in shape allow 2 potassium ions to bind, and then the phosphate ion is released, causing the protein to revert to it’s original shape, releasing 2 potassium ions across the membrane.
• This allows a neurone to maintain a higher concentration of ions outside the axon, and a lower concentration inside the axon.

Question 9

What is the resting potential?

Answer 9

• When there is no stimulus, the neurone maintains a less positive charge on the inside of the axon. It is maintained by the sodium-potassium ion pump.
• This means it is able to response to a nervous impulse that reaches the axon.

Question 10

What is the role of voltage-gated ion channels?

Answer 10

• At resting potential, voltage-gated ion channels are closed.
• When a stimulus triggers a nervous impulse, the inside of the axon becomes more positively charged than the outside of the axon (in one specific place, as a nerve impulse is a movement of positive charge).
• This nerve impulse causes the nearby axon to become slightly more positively charged, which causes voltage-gated sodium ion channels to open (to create an action potential).
• A reduce in the charge in the axon, causes voltage-gated potassium ion channels to open (to return to resting potential).
• Voltage-gated ion channels open/close by changing their tertiary structure.

Question 11

How does an action potential establish?

Answer 11

• A nerve impulse arrives at one end of an axon, causing the nearby section of axon to become slightly more positively charged.
• Therefore some voltage-gated sodium ion channels open, allowing some sodium ions to diffuse into the axon, so the overall charge inside the axon becomes more positive, so even more voltage-gated sodium ion channels open (more sodium ions diffuse into the axon).
• Once the overall charge inside the axon has reached it’s maximum positive charge, it has reached it’s action potential (an example of positive feedback).
• An action potential is very localised, it happens at one point of the axon and then travels as a wave of depolarisation.

Question 12

What is the all-or-nothing principle?

Answer 12

• An action potential is only triggered if enough sodium ions enter the axon (this is called the threshold for an action potential).
• If not enough sodium ions enter the axon, the threshold is not met and no action potential is triggered.

Question 13

How is a resting potential reestablished?

Answer 13

• In response to an action potential being triggered, voltage-gated sodium ion channels close (reducing the number of sodium ions entering the axon), and voltage-gated potassium ion channels begin to open, allowing some potassium ions to diffuse out of the axon.
• This makes the overall charge inside the axon less positive, so more voltage-gated ion channels open, and potassium ions diffuse out of the axon until it returns to it’s original level of charge.
• Voltage-gated potassium ion channels take a while to close, so even more potassium ions diffuse out, resulting in the overall charge in the axon becoming temporarily less positive than usual (hyperpolarisation).
• Voltage-gated potassium ion channels close, and sodium ions diffuse through sodium ion channels (down the electrochemical gradient), making the axon more positive.
• To reestablish a resting potential the sodium-potassium ion pump actively transports sodium ions out of the axon and potassium ions into the axon.

Question 14

What is depolarisation?

Answer 14

• When an action potential is being established, and the potential difference changes to a positive value (inside the axon becomes more positive than outside due to voltage-gated sodium ion channels opening).
• If enough sodium ions enter the axon, depolarisation results in an action potential.

Question 15

What is repolarisation?

Answer 15

• After the action potential, voltage-gated sodium ion channels close, and voltage-gated potassium ion channels begin to open, resulting in a change in potential difference from it’s maximum positive value, to a slightly negative value.

Question 16

How does an action potential move through an unmyelinated neurone?

Answer 16

• The influx of sodium ions into the axon (a positive potential difference inside the axon), triggers some sodium ions inside the axon to diffuse down an electrochemical gradient, along the axon. At the same time sodium ions outside of the axon diffuse in the opposite direction, down the electrochemical gradient.
• This movement of ions causes voltage-gated sodium ion channels further along the axon to open, meaning this part of the axon is depolarised and eventually causes an action potential.
• As the action potential moves down the neurone, the area behind resets to a resting potential. Voltage-gated sodium ion channels close and voltage-gated potassium ion channels open, so potassium ions diffuse out, it is depolarised (then hyper polarised), before returning to a resting potential.

Question 17

How does axon diameter affect the speed of conductance?

Answer 17

• The greater the axon diameter, the greater the speed of conductance:
  1. Axons with larger diameters have a lower resistance, so sodium ions can diffuse through at a faster rate.
  2. When axons have larger diameters, fewer potassium ions make contact with potassium ion channels, so fewer potassium ions diffuse out of the axon. When more potassium ions diffuse out of the axon, it becomes less positive and more difficult to maintain resting potential. Whereas when resting potentials are maintained, action potentials are triggered more quickly which increases speed of conductance.

Question 18

How does temperature affect speed of conductance?

Answer 18

• The higher the temperature, the greater the speed of conductance in the axon.
• At a higher temperature, there is a greater rate of diffusion, so sodium ions diffuse more quickly, resulting in a greater speed of conductance.
• However, if temperature is too high enzymes (eg. enzymes in respiration) will denature, meaning no ATP is generated, which affects the ability of the sodium-potassium ion pump to actively transport ions. Without the sodium-potassium ion pump, resting potential cannot be maintained and nerve impulses cannot be conducted.

Question 19

How does an action potential travel down a myelinated neurone?

Answer 19

• In a myelinated neurone, only Nodes of Ranvier contain voltage-gated sodium ion channels, therefore action potentials can only be triggered at the Nodes of Ranvier.
• Once an action potential is reached, sodium ions inside the axon diffuse across to the second Node of Ranvier (as there are no ion channels at the myelin sheath, so sodium ions can keep moving), depolarising the second Node of Ranvier, creating an action potential.
• Nodes of Ranvier also contain voltage-gated potassium ion channels, so the first node of Ranvier is repolarised, then hyper polarised and returns to resting potential.

Question 20

How does the myelin sheath affect the speed of conductance in myelinated neurones?

Answer 20

• The myelin sheath is made up of Schwann cells, which have a lipid cell membrane (phospholipid bilayer). Schwann cells are repeatedly wrapped to form the myelin sheath, so lipids are stacked, forming layers.
• Lipids don’t allow ions to pass through them, so ions cannot leave the axon via the myelin sheath. Therefore the myelin sheath acts like an electrical insulator.
• Therefore myelinated neurones only need to trigger action potentials at the nodes of Ranvier (as opposed to the whole axon), reducing the number of action potentials that need to be triggered, so myelinated neurones have a greater speed of conductance.

Question 21

What is the refractory period, and what is it’s function?

Answer 21

• The refractory period is when all voltage-gated sodium ion channels are closed, meaning an action potential cannot be triggered (hyperpolarisation causes the refractory period).
• The refractory period prevents the action potential from moving backwards, ensuring the nerve impulse is UNIDIRECTIONAL.
• The refractory period also ensures that the axons have a break between action potentials, meaning different impulses travelling down the same axon cannot merge together (DISCRETE impulses).

Question 22

How can strong stimuli and weak stimuli be differentiated (as they both reach action potential)?

Answer 22

• A weak stimuli triggers a small number of action potentials within a given period of time, whereas a strong stimuli triggers a large number of action potentials in the same period (a stronger stimuli has a greater frequency of action potentials).
• This occurs because stronger stimuli cause shorter refractory periods.

Question 23

What is a synapse?

Answer 23

• A junction that transfers an action potential between two neurones, or a neurone and a muscle fibre.

Question 24

What is a chemical synapse, and what structure does it have?

Answer 24

• Chemical synapses carry out the transfer of an action potential using neurotransmitters.
• It is made up of a presynaptic and a postsynaptic neurone, separated by a synaptic cleft.
• The synaptic knob (end of the presynaptic neurone) is filled with synaptic vesicles that store neurotransmitters, and has a large number of mitochondria.
• The pre-synaptic membrane contains voltage-gated sodium ion channels, and voltage-gated calcium ion channels.
• The post-synaptic membrane contains sodium ion channels with receptor sites that are complementary to the neurotransmitter.

Question 25

What additional features does a chemical synapse have?

Answer 25

• Synapses are unidirectional.
• The threshold for an action potential can be met in multiple ways:
  1. Temporal summation: a single presynaptic neurone releases neurotransmitters repeatedly over a short period.
  2. Spatial summation: many presynaptic neurones release neurotransmitters all at once.

Question 26

What are cholinergic synapses?

Answer 26

• Synapses that use the neurotransmitter acetylcholine.
• They are found in the CNS (between two neurones) and between neurones and muscle cells.

Question 27

What happens when an action potential arrives at presynaptic neurone?

Answer 27

1. The arriving action potential triggers the voltage-gated sodium ion channels to open and sodium ions enter the synaptic knob.
2. This leads to depolarisation, which causes the potential difference to reach it’s maximum positive value, triggering an action potential.
3. As a result of the action potential, voltage-gated calcium ion channels open and calcium ions diffuse into the synaptic knob.
4. The influx of calcium ions causes the synaptic vesicles (containing the neurotransmitter acetylcholine) to move down the synaptic knob, and fuse with the presynaptic membrane.
5. Synaptic vesicles release acetylcholine molecules into the synaptic cleft.

Question 28

What are the effects of acetylcholine on the postsynaptic neurone?

Answer 28

1. Acetylcholine molecules diffuse through the synaptic cleft until they reach the sodium ion channels on the postsynaptic membrane.
2. These sodium ion channels have two complementary receptor sites to acetylcholine, so two acetylcholine molecules bind to each sodium ion channel, triggering the sodium ion channel to change shape by changing it’s tertiary structure and opening.
3. Sodium ions then diffuse through the open sodium ion channel, into the postsynaptic neurone, down their electrochemical gradient.
4. This influx of sodium ions depolarises the postsynaptic neurone, more sodium ion channels open and more sodium ions diffuse in, causing an action potential.
   - Due to the narrow synaptic cleft, acetyl choline molecules have a short diffusion pathway, from the pre to the post synaptic neurone, increasing the rate of diffusion and allowing a rapid transmission of information from one neurone to the next.

Question 29

How does synaptic transmission end?

Answer 29

• The enzyme acetylcholinesterase is bound to the post-synaptic membrane.
• After an action potential is triggered at the post-synaptic membrane, acetylcholine molecules are pulled towards acetylcholinesterase, where they are broken down by a hydrolysis reaction.
• The sodium ion channels in the post-synaptic membrane then close, causing the action potential in the post-synaptic neurone to come to an end. This ensures the synapse is ready for the next synaptic transmission (for discrete impulses).

Question 30

How is acetylcholine recycled?

Answer 30

• Acetylcholinesterase bread down acetylcholine into ethanoic acid and choline.
• Ethanoic acid and choline diffuse back across the synaptic cleft, towards the presynaptic neurone.
• Choline is charged, so it passes into the presynaptic neurone by facilitated diffusion, which requires a transport protein. Ethanoic acid passes into the presynaptic neurone by simple diffusion.
• Once inside the synaptic knob , ethanoic acid and choline are bought back together by an enzyme to form acetylcholine.
• ATP is then used to actively transport acetylcholine back to the synaptic vesicles (presynaptic knob contains lots of mitochondria, so acetylcholine can be quickly transported ready for the next synaptic transmission).

Question 31

What is the structure of a neuromuscular junction?

Answer 31

• The presynaptic (motor) neurone, has the same structure as a cholinergic synapse, containing voltage-gated sodium and calcium ion channels, synaptic vesicles containing neurotransmitters, and lots of mitochondria.
• The muscle also contains a postsynaptic membrane that has sodium ion channels that can bind to two neurotransmitters.
• When an action potential arrives at a motor neurone, the same sequence of events happens as at a cholinergic synapse, resulting in neurotransmitters being released into the synaptic cleft.

Question 32

What is the effect of neurotransmitters on muscle cells?

Answer 32

• Neurotransmitters diffuse through the synaptic cleft and two neurotransmitter molecules bind to each sodium ion channel (by complementary receptors) on the post-synaptic membrane, and the sodium ion channel opens.
• Sodium ions diffuse Ito the muscle cell, down their electrochemical gradient, causing depolarisation, which leads to muscle contraction.
• To end synaptic transmission, neurotransmitter molecules are broken down by an enzyme via a hydrolysis reaction. These products then diffuse back to the presynaptic membrane and are recycled to reform the neurotransmitter, which is actively transported back to the synaptic vesicle.

Question 33

What is an excitatory neurotransmitter?

Answer 33

A neurotransmitter that leads to depolarisation of the postsynaptic neurone, triggering an action potential inside the postsynaptic neurone.

Question 34

What is an inhibitory neurotransmitter?

Answer 34

A neurotransmitter that prevent depolarisation of the postsynaptic neurone, preventing an action potential from being triggered in the postsynaptic neurone.

Question 35

How does an inhibitory synapse work?

Answer 35

1. Once neurotransmitters diffuse across the synaptic cleft to the postsynaptic membrane, they bind to complementary receptor sites on chloride ion channels.
2. This triggers the chloride ion channels to open, allowing chloride ions to diffuse in, down their electrochemical gradient.
3. This influx of chloride ions causes the postsynaptic neurone to become hyper polarised, therefore more sodium ions are needed for the potential difference to reach it’s maximum positive value to trigger an action potential.
4. Therefore it is much harder to trigger an action potential.

• Cholinergic synapses can be inhibitory or excitatory, whereas neuromuscular junctions are always excitatory (resulting in muscle contraction).

Question 36

How to answer exam questions about the effects of drugs on a synapse?

Answer 36

1. Identify the main target structure.
2. Recall the role of the target structure during synaptic transmission, and the steps that follow.
3. Identify the effect of drugs on the target structure.
4. Identify the effect of drugs on the whole synaptic transmission.

Question 37

What are sensory receptors and what are their functions?

Answer 37

• Sensory receptors are specialised cells that detect a stimulus.
• They only respond to a specific stimulus, and they act as transducers (convert the stimulus into a nervous impulse).

Question 38

What is the pacinian corpuscle, and what is it’s function?

Answer 38

• A receptor found deep in the skin of animals. It has a sensory neurone at it’s centre and is surrounded by layers of tissue called lamellae.
• It only responds to mechanical pressure from the environment, eg. something touching the skin.
• Inside the sensory neurone’s membrane, there are stretch mediated sodium ion channels (transport proteins that open when the membrane is stretched, when the skin is under mechanical pressure).
• Therefore sodium ions enter and depolarise the sensory neurone (called a generator potential). When multiple generator potentials build up, the threshold value is reached and an action potential is triggered in the axon of the sensory neurone, and passed on to the CNS.

Question 39

What is temporal summation?

Answer 39

• Multiple nerve impulses from a single pre-synaptic neurone occur in succession.
• This increases the concentration of neurotransmitters in the synaptic cleft, therefore increasing the likelihood of reaching the threshold to trigger an action potential.

Question 40

What is spatial summation?

Answer 40

• Multiple pre-synaptic neurones connect to the same post-synaptic neurone.
• The combination of inhibitory and excitatory neurones are ‘added together’ to determine whether an action potential is triggered.