5.1.3 Neuronal communication

Question 1

what is a sensory receptor?

Answer 1

A cell that detects a specific stimulus and converts the stimulus energy into the electrical energy of a generator potential

Question 2

what is a Photoreceptors?

Answer 2

sensory receptors which are sensitive to light – found in the retina of the eye

Question 3

what is a Pressure receptors?

Answer 3

sensory receptors which are sensitive to pressure – found in the skin

Question 4

what is a Thermoreceptors?

Answer 4

sensory receptors which aresensitive to temperature – found in the skin and within the hypothalamus

Question 5

what is a Chemoreceptors?

Answer 5

sensory receptors which are sensitive to the concentration of a specific chemical stimulus – found on the tongue, in the nose and in the wall of the carotid artery

Question 6

what is a Baroreceptors?

Answer 6

sensory receptors which are sensitive to blood pressure – found in the walls of certain arteries

Question 7

what is a Osmoreceptors?

Answer 7

sensory receptors which are sensitive to plasma salt ion concentration or water potential – found within the hypothalamus

Question 8

what will a generator potential trigger in a sensory neurone?

Answer 8

An action potential

Question 9

how would you describe Photoreceptors, thermoreceptors, Chemoreceptors, etc?

Answer 9

highly specific

Question 10

Give an example of a sensory receptor

Answer 10

The Pacinian corpuscle

Question 11

what is the Pacinian corpuscle?

Answer 11

sensory receptors present in the skin which are sensitive to pressure

Question 12

what are the pacinian corpsucle reffered to as?

Answer 12

mechanoreceptor

Question 13

what is a mechanoreceptor

Answer 13

the general term of any receptor sensitive to
movement or pressure changes

Question 14

how does the pacinian corpscle work?

Answer 14

Pacinian corpuscles convert the stimulus energy (pressure) into the electrical energy of a generator potential, which may trigger an action potential in a sensory neurone

Question 15

what is the pacinain corpsucel made up of?

Answer 15

a sensory neurone ending at its core, surrounded by layers of tissue that have gel between them.

Question 16

what is inside the plamsa membrane of the sensory neurone of the pacinian corpuslce?

Answer 16

stretch‐mediated sodium ion channels

Question 17

how do stretch‐mediated sodium ion channels work?

Answer 17

When the membrane is deformed (pushed into a different shape), theseretch‐mediated sodium ion channels change 3D shape into an open state. Hence the membrane becomes more permeable to sodium ions in response to the stimulus of pressure.

Question 18

What is this? Can you label it?

Answer 18

The pacinian Corpuscle

Question 19

how does the pacinian corpscle work? (mechanism) 7 steps

Answer 19

1. In the resting state (which occurs when the stimulus is absent):
   o The stretch‐mediated sodium ion channels in the plasma membrane of the sensory neurone (within the Pacinian corpuscle) are closed, i.e. impermeable to sodium ions;
   o There is however a resting potential of ‐70mV across the plasma membrane,such that the sensory neurone is negative inside – this has been established by the ongoing activity of sodium/potassium pumps;
2. Pressure (the stimulus) is applied to the layers of tissue that make up most of the corpuscle’s structure, deforming the shape of the corpuscle;
3. This causes deformation of the membrane of the sensory neurone within the corpuscle;
4. The consequence is that the stretch‐mediated sodium ion channels in the membrane of the sensory neurone undergo a change in 3D shape, such that they open.
5. The membrane is now more permeable to sodium ions, hence sodium ions now move by facilitated diffusion into the neurone, down their electrochemical gradient;
6. This depolarises the membrane, causing the membrane potential to become less negative: this is the generator potential;
7. If the extent of depolarisation of the generator potential exceeds a threshold, an action potential is triggered, which is transmitted by the dendron (and later the axon) of the sensory neurone, towards the central nervous system

Question 20

what is a neurone?

Answer 20

an individual specialised cell within the nervous system, capable of transmitting action potential

Question 21

what is a nerve?

Answer 21

a bundle of thousands of parallel neurones, surrounded by a protective sheath of connective tissue

Question 22

what is the conective tissue in a nerve called?

Answer 22

the perineurium

Question 23

what is a sensory neurone needed for?

Answer 23

Sensory neurones are needed to pass action potentials from receptors to the CNS; a sensory nerve is a nerve containing thousands of parallel sensory neurones

Question 24

what is a Motor neurones needed for?

Answer 24

Motor neurones pass action potentials from the CNS to effectors (i.e. to muscles or glands); a motor nerve is a nerve containing thousands of parallel motor neurones.

Question 25

what is a mixed nerve?

Answer 25

A mixed nerve is a nerve containing both sensory and motor neurones, which are transmitting action potentials in opposite directions.

Question 26

what is the neuronal pathway?

Answer 26

A sequence of neurones

Question 27

what is the neuronal pathway from receptor to effector

Answer 27

Receptor

–

Sensory neurone

• (synapse)-

Relay neurone

-(synapse)-

motor neurone

–

effector

Question 28

what is the cell body of a neurone? 3 marks

Answer 28

contains the nucelus and majority of the cytoplasm

cytoplsma has lots of mitochondria - (producing ATP for synthesis of neurotransmitter molecules and for the sodium‐potassium pumps that maintain resting potential)

many ribosomes - (synthesising enzymes, ion channels and carrier proteins [sodium‐potassium pumps]

Question 29

what is the dendron of a neurone?

Answer 29

extensions of the plasma membrane and cytoplasm

transmitting action potentials TOWARDS the cell body

end of a dendron may branch into even thinner dendrites - (allows multiple receptors,
sensory neurones or relay neurones can form synapses with this neurone)

Question 30

what is a dendrite?

Answer 30

small branches coming from the Dendron of a neurone - (allows multiple receptors,
sensory neurones or relay neurones can form synapses with this neurone)

Question 31

what is the axon of a neurone?

Answer 31

single cylindrical extension of the plasma membrane and cytoplasm

transmitting action potentials AWAY from the cell body

Question 32

how to remember the position of the axon on a neurone?

Answer 32

Axon - Away

Question 33

What is the role of a sensory neurone?

Answer 33

To transmit action potentials from sensory receptors to the CNS, where the sensory neurone will synapsewith a relay neurone (or sometimes directly with a motor neurone).

Question 34

what is this? Label it.

Answer 34

sensory neurone

Question 35

what do these arrows represent?

Answer 35

the movement of an action potential along a sensory neurone

Question 36

what are the key features of a sensory neurone?

Answer 36

a single, long dendron

A cell body that is off to the side (in the middle)

a single, short axon

may be myelinated

Question 37

what is the role of a relay neurone?

Answer 37

To transmit impulses from sensory neurones to motor
neurones, or between other relay neurones in a neural network.

Question 38

where are relay neurones contained?

Answer 38

Only within the Cnetral Nervous System (Brain and Spinal cord)

Question 39

What is this? Label it.

Answer 39

Relay neurone

Question 40

what do these arrows represent?

Answer 40

the movement of an action potential along a relay neurone

Question 41

what are the key features of a relay neurone?

Answer 41

multiple, short dendrons

cell body located centrally - dendrons and axons radiating from it

multiple, short axons

usually not (fully) myelinated

Question 42

what is the role of a motor neurone?

Answer 42

To transmit impulses from a relay (or sensory) neurone to an
effector (muscle or gland), stimulating the effector to produce a response.

Question 43

what is this? label it

Answer 43

motor neurone

Question 44

what are the key features of a motor neurone?

Answer 44

multiple, short dendrites - connected to cell body (unmyelinated)

a cell body at one end

single, very long axon (myelinated) to a Neuromuscular junction

Question 45

what does it mean if a neurone is myelinated?

Answer 45

they are surrounded by a myelin sheath

Question 46

what is myelin? What is it made of?

Answer 46

myelin is made up of multiple layers of plasma membrane, forming when a Schwann cell wraps around the axon many times as it grows.

Question 47

what is between the schwann cells?

Answer 47

Nodes of Ranvier - narrow unmyelinated regions, where the axon is exposed

Question 48

what is the function of the nodes of ranvier?

Answer 48

During transmission of an action potential along the axon, only the nodes of Ranvier can depolarise, leading to saltatory (‘jumping’) conduction

Question 49

what is this? label it.

Answer 49

A meylinated axon

Question 50

What are the 3 main benefits of using a myelinated axon?

Answer 50

electrical insulation – the multiple layers of membrane (containing phospholipids) are an effective electrical insulator, decreasing the chance that action potentials will erroneously be transmitted directly from one axon to other axons that may lie in parallel with it within the same nerve;

Increased speed of transmission of action potentials, due to saltatory conduction – the depolarisation of the action potential ‘jumps’ from node of Ranvier to node of Ranvier, rather than being transmitted continuously as a wave of depolarisation; saltatory conduction is around 100x faster than continuous transmission, allowing more rapid responses to stimuli and hence increased chance of survival;

Less energy needed for transmission of action potentials – only the nodes of Ranvier needed to undergo repolarisation (involving active transport of Na+ and K+ ions) after the action potential has passed, so less ATP is needed.

Question 51

what must occur for an action potential to be submitted?

Answer 51

neurone must first be in a suitable resting state, more specifically there must be a resting potential across its plasma membrane

Question 52

what must happen to the resting potential after an action potential is transmitted?

Answer 52

the resting potential must be restored, so that the neurone can transmit another action potential when stimulated to do so.

Question 53

what is resting potential?

Answer 53

The resting potential is the potential difference (voltage) across the plasma membrane of a neurone, when it is not in the process of transmitting an action potential.

Question 54

what is the value of the resting potential? How can it be interpreted?

Answer 54

‐70mV

The minus sign here is indicating that the inside (cytoplasm) of the cell has an excess of
negative charge, whereas the outside (tissue fluid) has an excess of positive charge.

Question 55

how is resting potential formed? (basics)

Answer 55

Potential difference means there is an imbalance of charge across the membrane, i.e. an unequal distribution of charged particles (ions) between the cytoplasm on one side of the membrane and the tissue fluid on the other side.

Question 56

what are the two key mechanisms that contribute to establishing and maintaining the resting
potential of a neurone?

Answer 56

1. The sodium/potassium (Na+/K+) pump
2. Higher membrane permeability to K+ than to Na+

Question 57

describe the The sodium/potassium (Na+/K+) pump

Answer 57

Carrier proteins embedded in the plasma membrane

Active transport of ions using energy from ATP hydrolysis

3 Na+ ions out of the cell

2 K+ ions in to the cell.

net movement of positive charge out of the cell (3 cations out, 2 cations in)

inside of the cell now negative compared to the outside, i.e. the membrane is polarised.

(Both the Na+ ions and the K+ ions are being moved against their own concentration gradients, i.e. active transport from a region of lower to a region of higher concentration)

Question 58

What do the The sodium/potassium (Na+/K+) pump establish after active transport?

Answer 58

very steep concentration gradients for both ions are established:

the concentration of Na+ will now be much higher outside the cell than inside;

the concentration of K+ will now be much higher inside the cells than outside.

Question 59

Describe the Higher membrane permeability to K+ than to Na+

Answer 59

The phospholipid bilayer, has a hydrophobic core of fatty acid tails and is impermeable to ions

ion channels embedded in the bilayer

provide permeability to specific ions by allowing facilitated diffusion of ions down their concentration (or electrochemical) gradient [Only if the appropriate channels are present in an open state]

Some ion channels may be ‘open’, but many ion channels may be ‘gated’

Na+ ion channels:

Due to Na+/K+ pump, there is a higher conc of Na+ out the cell, ∴ Na+ move into the cell down its concentration gradient

HOWEVER, voltage‐gated Na+ ion channels are likely to be closed, ∴ plasma membrane has very low permeability to Na+

∴very little positive charge can actually move into the cell (despite the steep concentration gradient for Na+), preventing the dissipation (loss) of the inside‐negative resting potential.

K+ ion channels

due to Na+/K+ pump, there is a higher conc of K+ in the cell, ∴ K+ move out of the cell down its concentration gradient

Many K+ ion channels are open at this time, meaning the plasma membrane has
much higher permeability to K+ than it does to Na+

There will be a significant net loss of positive charge from the cell, as more
K+ ions can move out of the cell than Na+ can move in.

This results in net loss of positive charge from the neurone and hence an insidenegative
resting potential of ‐70mV.

Question 60

what does ‘gated’ mean in terms of ion channels?

Answer 60

The ion channels can either be in an open state or a closed state (with a ‘gate’ blocking the pore).

Question 61

what causes ‘gated’ channels to open and close?

Answer 61

several different triggers

for example:

voltage‐gated ion channels - only open if the membrane potential reaches a certain threshold

ligand‐gated ion channels - only open if a certain molecule binds to a complementary receptor site on the channel

Question 62

Draw the cell membrane to show how the resting potential is maintained

Answer 62

Question 63

what is depolarisation?

Answer 63

Depolarisation is the loss and eventual reversal of the potential difference across the membrane of a neurone

Question 64

how is an action potential generated?

Answer 64

when the extent of depolarisation of a membrane exceeds a threshold potential (‐50mV)

Question 65

what is the value of the threshold potential?

Answer 65

(‐50mV)

Question 66

what is the value of the maximum extent of depolarisation during an action potential?

Answer 66

+40mV

Question 67

how does the transmission or propagation of an action potential along a dendron or axon work?

Answer 67

there is a spreading of a wave of depolarisation down the plasma membrane of the neurone.

Question 68

how long is cell depolarisation?

Answer 68

very short‐lived: as soon as the maximum depolarisation of +40mV is reached, there is immediately a repolarisation process to reverse the balance of charge and restore the resting potential of ‐70mV once more.

Question 69

Draw a graph showing the changes in potential differences during an action potential

Answer 69

Question 70

Describe the mechanism for the generation of an action potential in a sensory neurone

Answer 70

1. There is an (inside‐negative) resting potential across the plasma membrane of the neurone of ‐70mV – this has been established by the action of the Na+/K+ pump and the higher membrane permeability to K+ than to Na+; there are many open K+ channels at this point but voltage‐gated Na+ channels are closed
2. The stimulus energy triggers the opening of some Na+ channels, increasing the permeability of the membrane to Na+: so, Na+ ions begin to move by facilitated diffusion into the neurone, down their electrochemical gradient – the process of depolarisation has begun and the membrane potential starts to become less negative, due to the net influx of cations
3. If the extent of depolarisation is sufficient (which corresponds to the original stimulus being strong enough to open enough Na+ channels), the threshold potential of ‐50mV will be exceeded: this triggers the opening of the voltage‐gated Na+ channels which were previously closed – the membrane is now even more permeable to Na+, so more Na+ ions move by facilitated diffusion into the neurone, causing further depolarisation; since the entry of some Na+ ions has triggered the entry of further Na+ ions, this step is an example of positive feedback
4. Due to the entry of so many Na+ ions, the depolarisation reaches its maximum extent, +40mV (inside‐positive) – at this point, the voltage‐gated Na+ channels close, but voltagegated K+ channels now open; this means that the plasma membrane of the neurone now has much higher permeability to K+ than to Na+;
5. K+ ions move out of the neurone by facilitated diffusion, down their electrochemical gradient, through the open voltage‐gated K+ channels – this represents a loss of positive charge from the cell and causes repolarisation of the membrane, back towards the insidenegative resting potential
6. So many K+ ions leave the cell that there is an overshoot into a membrane potential that is even more negative (e.g. ‐80mV) than the resting potential – this is called hyperpolarisation, which is short‐lived as the voltage‐gated K+ channels now close; however, during hyperpolarisation it is not possible for a new action potential to be initiated
7. Resting potential is now restored: the Na+/K+ pump restores the concentration gradients of Na+ and K+ so that it becomes possible for a new action potential to be generated if there is an appropriate stimulus.

Question 71

what must occur for the action potential to be transmitted down an axon?

Answer 71

the wave of depolarisation has to spread along the plasma membrane.

Once one region of the plasma membrane is depolarised (in response to a stimulus), this depolarisation itself acts as a stimulus for depolarisation of the next region of the membrane (as described in more detail below).

Question 72

what occurs during hyperpolarisation?

Answer 72

during the hyperpolarisation phase and for a short time afterwards, transmission of a new action potential is not yet possible – this is called the refractory period. During the refractory period, the voltage‐gated Na+ channels are not yet able to reopen; this means it will not be possible to achieve the extent of depolarisation (due to Na+ entry into the neurone) that needs to occur in an action potential.

Question 73

what is the refactory period?

Answer 73

during the hyperpolarisation phase and for a short time afterwards, transmission of a new action potential is not yet possible. The voltage-gated Na+ channels are not able to open yet

Question 74

why is the refactory period significant?

Answer 74

prevents action potentials spreading backwards into regions of the membrane that have already undergone recent depolarisation;

prevents action potentials overlapping and merging with one another, making sure each action potential is a discrete signal

Question 75

describe the mechanism for the continuous propagation of an action potential down an axon for an UNMYELIANTED AXON

Answer 75

1. In the region of the membrane that is already depolarising, there is entry of Na+ ions into the cytoplasm of the neurone through open Na+ channels;
2. Due to the high Na+ ion concentration building up in the cytoplasm at this point, some of these Na+ ions diffuse through the cytoplasm into a region further along the neurone where the Na+ concentration is lower;
3. Meanwhile, outside the neurone (in the surrounding tissue fluid), there is a decreasing concentration of Na+ ions in the region where depolarisation is already occurring, since Na+ ions are moving from the tissue fluid into the neurone;
4. Hence, outside the neurone, Na+ ions from a region further along will diffuse in the tissue fluid towards the region of lower Na+ concentration (where depolarisation is already occurring);
5. These movements of Na+ ions in the cytoplasm and tissue fluid (in opposite directions) set up a ‘local circuit’ and cause depolarisation of the next region of membrane;
6. Once the extent of depolarisation is sufficient here to exceed the threshold potential of ‐50mV, voltage‐gated Na+ channels open;
7. This is a positive feedback step that allows facilitated diffusion of Na+ ions into the neurone in this region, increasing the depolarisation to reach its maximum of +40mV;
8. In this way, the action potential is propagated along the axon of the neurone, with the above sequence of events repeating continuously.

Question 76

what is this? label this diagram

Answer 76

A diagram showing how local circuits cause propagation of an action potential

Question 77

what is an action potential described as?

Answer 77

An action potential is an ‘all‐or‐nothing’ event

Question 78

what is an ‘all‐or‐nothing’ event?

Answer 78

the event (the action potential) either occurs (with a fixed
amplitude and predictable profile of changes in potential difference), or it does not occur at all.

Question 79

why is action potential described as an all or nothing event?

Answer 79

Due to threshold potential

If a stimulus is strong enough, it will trigger an action potential – if not, it will not.

An even stronger stimulus cannot trigger a higher amplitude action potential though, because all action potentials are the same amplitude.

Question 80

what causes the action potential to be an all or nothing event?

Answer 80

only if the extent of depolarisation exceed this will the positive feedback phenomenon of voltage‐gated Na+channels opening occur.

Question 81

how does an action potential transmit infomation abouit a stronger stimulus?

Answer 81

the frequency of action potentials will be higher if the stimulus is stronger, i.e. more action potentials are generated and transmitted in given time

Question 82

what does this graph represent?

Answer 82

Question 83

what is Saltatory conduction?

Answer 83

The movement of an action potential across a meylinated neurone, jumping from one node of Ranvier to another.

Question 84

which area in a myelinated axon can depolarise?

Answer 84

only the unmyelinated regions - NODES OF RANVIER

Question 85

what does depolarisation actually mean?

Answer 85

a change in potential difference from negative to postive across the membrane of a neurone

Question 86

What are the pro’s of salatatory conduction?

Answer 86

1. rate of transmission is much faster than in unmyelianted neurones. - action potential therefore appears to jump from node to node, giving more rapid transmission of the action potential (of around 100m s‐1) compared to the continuous propagation (at around 1m s‐1) [increasing adaptive vaule]
2. more energy efficient - only the nodes of Ranvier depolarise, only these regions need to be repolarised using Na+/K+ pumps, so overall there will be less ATP needed.

Question 87

describe the mechansism of an Action potential transmission in a myelinated neurone by saltatory conduction

Answer 87

1. There is depolarisation of the plasma membrane of the axon at an unmyelinated region (node of Ranvier);
2. A local circuit is established that reaches the next node of Ranvier, due the diffusion of Na+ ions away from the next node in the tissue fluid and towards the next node in the cytoplasm;
3. The next node begins to depolarise due to this Na+ ion movement and, once the threshold potential of ‐50mV has been reached, voltage‐gated Na+ channels open at this node;
4. Na+ ions enter the neurone causing further depolarisation of the node, corresponding to an action potential;
5. The process repeats, with Na+ ion movement in the tissue fluid and cytoplasm causing the next node to start depolarising;
6. The local circuits involved in depolarisation of each subsequent node are much longer that the local circuits that bring about in continuous transmission: the action potential jumps from node to node – because there only needs to be voltage‐gated Na+ channels opening at nodes and only Na+ ion entry into the neurone at nodes, the overall speed of transmission is much faster.

Question 88

what are the three main mechanisms in increasing the speed of an action potential?

Answer 88

1. saltatory conduction due to myelination (see previous flashcards)
2. increased axon diameter - wider axon = resistance to diffusion of Na+ ions in the cytoplasm
3. increased temperature - Na+ and K+ = more kinetic energy = move more quickly by (facilitated) diffusion. [Only up to 40°C, after transmission slows due due to ion channel and Na+/K+ pump denaturation]

Question 89

what is a synapse?

Answer 89

the junction between two neurones

Question 90

what is the small gap between the pre and post synaptic neurones called?

Answer 90

Synaptic cleft

Question 91

what type of transmission occurs in the synaptic cleft?

Answer 91

chemical transmission

Question 92

why are synapses important?

Answer 92

1. Allos for transmission of impulses in one direction only
2. impulse from one neurone to pass to many other neurones, resulting in many simultaneous responses to a single stimulus
3. Multiple pre‐synaptic neurones may synapse with a single post‐synaptic neurone, allowing summation of impulses from multiple receptors to produce a single response

Question 93

how wide is the synaptic cleft?

Answer 93

20‐30nm wide

Question 94

what is the most common type of synapse in our nervouse system, and what is the name of the neurotransmitter which is used?

Answer 94

cholinergic synapse

acetylcholine molecules as the neurotransmitter

Question 95

Read and remember this 16 step mechanism for the cholinergic synapse LMAO

Answer 95

1. The arrival of an action potential at the synaptic knob of the pre‐synaptic neurone results in depolarisation of the plasma membrane of this neurone;
2. Voltage‐gated calcium ion channels in the plasma membrane open;
3. Calcium ions move by facilitated diffusion into the pre‐synaptic neurone through these open channels;
4. The calcium ions bind to synaptic vesicles in the cytoplasm, which contain the acetylcholine (ACh) neurotransmitter molecules;
5. This triggers the vesicles to move towards the pre‐synaptic membrane – the vesicles attach to microtubules (via motor proteins), which act as tracks directing the movement, using energy from ATP hydrolysis;
6. The vesicle membranes fuse with the pre‐synaptic membrane, resulting in the release of ACh molecules into the synaptic cleft by exocytosis;
7. The ACh molecules diffuse across the cleft;
8. The ACh molecules bind to glycoprotein receptors with binding sites of complementaryshape, located in the post‐synaptic membrane;
9. The binding of ACh to a receptor triggers a change in the 3D shape of the receptor, resulting in the opening of a sodium ion channel (called a neurotransmitter‐gated sodium ion channel) that is part of the same glycoprotein;
10. Sodium ions move by facilitated diffusion into the post‐synaptic neurone through these open channels, depolarising the post‐synaptic membrane11. If the threshold potential is exceeded (due to sufficient movement of positivelycharged sodium ions into the cell), more sodium ion channels open: these are voltagegated sodium ion channels;
11. Now, more sodium ions can enter the post‐synaptic neurone, as the membrane is now more permeable to sodium ions – this is a positive feedback step (as the entry of some sodium ions has triggered the entry of even more sodium ions);
12. The post‐synaptic membrane is further depolarised and a full‐strength action potential has been triggered;
13. The action potential is now transmitted via the membrane of the post‐synaptic neurone as far as the next synapse (or to a neuromuscular junction);
14. To prevent continuous inappropriate stimulation of the post‐synaptic neurone, there is an enzyme called acetylcholinesterase (ACE) that breaks down the acetylcholine that is attached to the receptors (resulting in the sodium ion channels returning to their closed state).
15. The breakdown products of ACh are choline and ethanoic acid: these are reabsorbed by the pre‐synaptic neurone and are recycled to produce new ACh molecules (in a process that requires energy from ATP).

Question 96

what is going on in each step of this diagram?

Answer 96

Question 97

what is Acetylcholine (Ach) an example of?

Answer 97

an excitatory neurotransmitter - triggers opening of sodium ion channels

This results in positively‐charged sodium ions entering the post‐synaptic neurone, leading to depolarisation of the membrane; an action potential will be the consequence if the threshold potential is exceeded

Question 98

what does an excitatory neurotransmitter do?

Answer 98

triggers opening of sodium ion channels

Question 99

give an example of an inhibitory neurotransmitter

Answer 99

GABA

Question 100

what does an inhibitory neurotransmitter do?

Answer 100

These cause chloride ion channels to open in the post‐synaptic membrane

The result is that negatively‐charged chloride ions enter the post‐synaptic neurone, leading to the hyperpolarisation of the membrane; an action potential is now very unlikely to occur because the membrane potential has become even more negative (further from the threshold).