5.3 - C - Neuronal Communication

Question 1

What are sensory receptors?

What is a stimulus?

Answer 1

Specialised cells that detect stimuli in the internal or external environment of an organism. They are transducers as they convert one type of energy to another.
A change in energy levels in the environment that causes voltage gated Na+ channels to open and Na+ to diffuse into receptor.

Question 2

List the steps in getting stimulus to response

Answer 2

stimulus ‐‐> receptor ‐‐> cell signalling ‐‐> CNS ‐‐> cell signalling ‐‐> response

Question 3

What are pacinian corpuscles?
Describe their structure
Explain how they work

Answer 3

Pressure sensors found in the skin.
Oval-shaped that consists of concentric rings of connective tissue wrapped around the end of a nerve cell.
Pressure on the skin causes the connective tissue to deform which causes sodium ion channels to distort and open, sodium ions diffuse in to the axon and produce an action potential.

Question 4

What do sensory neurones do?

Describe their structure

Answer 4

Carry action potential from a sensory receptor to CNS.

Long dendron, short axon.

Question 5

What do relay neurones do?

Describe their structure

Answer 5

Connects sensory and motor neurones in CNS.

Short dendrites, no dendron, short axon.

Question 6

What do motor neurones do?

Describe their structure

Answer 6

Carries action potential from CNS to effector.

Short dendron, long axon.

Question 7

Describe resting potential

Answer 7

The potential difference across the membrane while the neurone is at rest, about ‐60mV ‐‐> ‐70mV. This means there is no stimulus.
• sodium/potassium ion pumps [3 out/2 in]
• K+ leakage
• membrane polarised

Question 8

What are generator potentials?

Answer 8

Small depolarisation in receptor that don’t reach threshold potential

Question 9

Define depolarisation

Answer 9

Loss of polarisation due to entry of Na+

Question 10

What is threshold potential?

Answer 10

‐50mV

Caused by big enough stimulus so enough Na+ enter to create an action potential

Question 11

What is an action potential?

Answer 11

A brief reversal of the potential across the membrane of a neurone causing a peak +40mV compared to the resting potential of -60mV.
Transmitted along neurone
Action potentials are ‘all or nothing’ either +40mV or no action potential at all

Question 12

Define repolarisation

Answer 12

Where the membrane becomes polarised again due to loss of K+ through v‐g channels

Question 13

Define hyperpolarisation

Answer 13

Where too many K+ diffuse out ‐ potential difference under.

-70mV

Question 14

Describe and explain how an action potential is transmitted in a myelinated neurone

Answer 14

Stimuli cause the opening of sodium channels at a point on the neurone. This causes local currents ‐ diffusion of ions along the neurone. Stimulus causes Na+ channels to open ‐ Na+ diffuse into neurone causing action potential to be generated. This disrupts the resting potential ion balance. T Stimulus causes Na+ channels to open ‐ Na+ diffuse into neurone causing action potential to be generated. This disrupts the resting potential ion balance. This will continue along the neurone ‐ this is how an action potential is transmitted.

Question 15

Explain how voltage gated sodium channels work

Answer 15

As the sodium ions diffuse along the membrane, their presence reduces the p.d across the membrane and causes the voltage‐gated sodium ion channels to open.
This causes more sodium ions to enter the membrane ‐ an example of
positive feedback.

Question 16

What size is an action potential?

Answer 16

+40mV

Question 17

What determines whether an action potential is reached?

Answer 17

Whether of not the threshold potential is met

Question 18

If the action potential can be no bigger than +40mV, how does the CNS know the strength of a stimulus?

Answer 18

The strength of a stimulus is communicated with the frequency of the action potentials. Stronger stimulus = more Na+ enter = higher
frequency of action potentials.

Question 19

What is the refractory period?

What is its importance in nervous impulse conduction?

Answer 19

Follows an action potential in an area along the neurone.
In the absolute refractory period no impulse can be generated.
In the relative refractory period an impulse can only be generated if the stimulus is more intense than the normal threshold value.
The sodium v‐g channels close to stop another impulse being generated.
This happens because the resting potential needs to be restored
(redistribute sodium and potassium ions).
It also ensures impulses are separated and that they pass in one direction only along the axon.

Question 20

What is a myelinated sheath made out of?

Answer 20

A series of Schwann cells

Question 21

What are the gaps between Schwann cells called?

Answer 21

Nodes of Ranvier

Question 22

Describe what a myelinated sheath does.

What does this cause?

Answer 22

The myelin sheath acts as an electrical insulator. It is impermeable to Na+ and K+ ions as there are no ion channels in the myelinated regions. The movementsof ions that cause action potentials (depolarisation) can only occur at the nodes of Ranvier. This makes the local currents longer as the sodium ions diffuse (jump) from one node of Ranvier to another ‐ this is called saltatory conduction ‐ this speeds up transmission.

Question 23

Explain how an action potential travels down non-myelinated neurones

Answer 23

More than one neurone is loosely wrapped in one Schwann cell.
The action potential moves along the neurone as a wave instead of jumping from node to node (no nodes).

Question 24

Compare myelinated and non-myelinated neurones

Answer 24

M - Conduct a.p. much quicker because a.p can only occur at nodes (due to saltatory conduction).
N.M - Slower transmission.
M - Carry signals over long distances ‐ enables more rapid responses.
N.M - Carry signals over short distances ‐ used when a quick transmission is less necessary.
M - Found in peripheral nervous system carrying impulses from receptor to CNS/CNS to effector.
N.M - Found in the CNS and nerves controlling the organs with no conscious control e.g. lungs.

Question 25

What is a synaptic cleft?

Answer 25

The small gap in between 2 neurones. Action potentials can’t
get through this gap.

Question 26

What is a cholinergic synapse?

Answer 26

A synapse thar uses acetylcholine (ACh) as the neurotransmitter.

Question 27

What is a synaptic knob?

Answer 27

Swelling at the end of the presynaptic neurone.

Question 28

Acetylcholinesterase

Answer 28

An enzyme that hydrolyses acetylcholine into acetic acid and choline in the synaptic cleft.

Question 29

Give 3 reasons for hydrolysis

Answer 29

Stops continuous production of action potentials in the postsynaptic neurone.
Enables repolarisation of post synaptic membrane by unblocking receptors and stops Na+ channels from staying open.
Recycles the acetylcholine.

Question 30

What happens after the acetylcholine is hydrolysed?

Answer 30

Once the acetylcholine is hydrolysed, the acetic acid and choline
diffuse back into the presynaptic knob and are combined back into acetylcholine using ATP and then stored in vesicles.

Question 31

Define acclimatisation

Answer 31

Synapses fatigue and stop responding to stimuli. This helps to avoid overstimulation of effectors which could cause damage ‐ run out of neurotransmitter vesicles after repeated stimulation.

Question 32

How do synapses ensure one way transmission of impulses?

Answer 32

Presynaptic knobs have vesicles with acetylcholine.

Postsynaptic membranes are the only things to hold receptors for acetylcholine.

Question 33

1 action potential in the presynaptic neurone will not result in an action potential in the postsynaptic membrane.
Suggest why not and why this could be beneficial.

Answer 33

A low level stimulus may generate an action potential in the presynaptic neurone but it may not cause the release of enough acetylcholine vesicles to cause an action potential in the postsynaptic neurone (it will just cause a small generator potential ‐ known as a Excitatory post‐synaptic potential EPSP ‐ which won’t reach the threshold). This lets us filter out low level stimuli to avoid overstimulation.

Question 34

Suggest how an action potential could be generated in the postsynaptic neurone when only a low level stimulus is present

Answer 34

If a low level stimulus occurs again and again, several action potentials will be generated in the presynaptic neurone in a short time. These will cause more vesicles of acetylcholine to be released and the small generator potentials (several EPSPs) they cause in the postsynaptic neurone will combine to produce an action potential. Temporal summation is when several action potentials come from the same presynaptic neurone to create an action potential in the post synaptic neurone.

Question 35

When does spatial summation occur?

Answer 35

When action potentials arrive from converging presynaptic neurones cause a few vesicles each to be released into the same synapse causing an action potential in the postsynaptic neurone.

Question 36

What causes a generator potential?

What is it also known as?

Answer 36

Sodium ions moving into the cell. This happens by sodium channels being sensitive to small movements and deforming due to the pressure changes.
Generator potential.

Question 37

How many sodium ions are pumped for every how many potassium ions?

Answer 37

3 sodium out

2 potassium in

Question 38

Explain how and why neurones are specialised

Answer 38

Long so they can transmit the action potential over a long distance.
Cell surface membrane has many gated ion channels that control the entry or exit of sodium, potassium or calcium ions.
Sodium/potassium pumps use ATP atoms actively transport sodium ions out of the cell and potassium ions into the cell.
Neurones maintain a P.D. Across their cell surface membranes.
A cell body contains the nucleus, many mitochondria and ribosomes.
Numerous dendrites connect to other neurones. The dendrites carry impulses towards the cell body.
An axon carries impulses away from the body.
Neurones are surrounded by myelin sheaths.

Question 39

Where can movement of ions only occur on myelinated neurones?
Why?

Answer 39

Nodes of ranvier.
Because the myelin sheath is tightly wrapped around the neurone it prevents the movement of ions across the neurone membranes.

Question 40

Define positive feedback

Answer 40

A mechanism that increases a change taking the system further away from the optimum

Question 41

List the 9 stages of an action potential

Answer 41

The membrane starts in its resting state - polarised with the inside of the cell being -60mV compared to the outside. There is a higher concentration of sodium ions outside than inside and a higher concentration of potassium ions inside than inside.
Sodium ion channels open and some sodium ions diffuse into the cell.
The membrane depolarises - it becomes less negative with respect to the outside and reaches the threshold value of -50mV.
Positive feedback causes nearby V-G Na+ ion channels to open and many Na+ ions flood in. As more enter, the cell becomes positively charged inside compared with the outside.
The P.D. across the plasma membranes reaches +40mV. The inside of the cell is positive compared with the outside.
The sodium ion channels close and potassium channels open.
Potassium ions diffused out of the cell bringing the potential difference back to negative inside compared with the outside - this is called repolarisation.
The P.D. overshoots slightly making the cell hyperpolarised.
The original P.D. restored so that the cell returns to its resting state.

Question 42

Explain the 4 steps in the formation of local currents and the transmission of a nerve impulse

Answer 42

When an A.P. Occurs the Na+ ion channels open at that point in the neurone.
The open Na+ channels allow Na+ ions to diffuse across the membrane from the region of higher concentration outside the neurone into the neurone. The concentration of Na+ ions inside the neurone rises at the point where the Na+ channels are open.
Na+ ions continue to diffuse sideways along the neurone, away from the region of increased concentration. This movement of charged particles is a current called a local current.
The local current causes a slight depolarisation further along the neurone which affects the V-G Na+ ion channels, causing them to open. The open channels allow rapid influx of Na+ ions causing a full depolarisation (A.P.) further along the neurone. The A.P. has therefore moved along the neurone.

Question 43

What is saltatory conduction?

Answer 43

The myelin sheath acts as an electrical insulator. It is impermeable to Na+ and K+ ions as there are no ion channels in the myelinated regions. The movements of ions that cause action potentials (depolarisation) can only occur at the nodes of Ranvier. This makes the local currents longer as the sodium ions diffuse (jump) from one node of Ranvier to another.

Question 44

What is a cholinergic synapse?

Answer 44

A synapse that uses acetylcholine as its neurotransmitter

Question 45

What is a neurotransmitter?

Answer 45

A chemical used as a signalling molecule between 2 neurones in a synapse

Question 46

How wide is a synaptic cleft?

Answer 46

Approx 20nm

Question 47

Give 4 specialised features of the pre-synaptic bulb

Answer 47

Many mitochondria - indicating that an active process involving ATP occurs.
A large amount of SER, which packages the neurotransmitter into vesicles.
Large number of veciscles containing molecules of acetylcholine, the transmitter that will diffuse across the synaptic cleft.
A number of V-G Na+ channels on the cell surface membrane.

Question 48

Describe the channels on the post-synaptic membrane

Answer 48

Sodium ion channels that consist of 5 polypeptide molecules. 2 of these have a special receptor site that is specific to acetylcholine. The receptor sites have a shape that’s complementary to the shape of the acetylcholine molecule. When acetylcholine is present in the synaptic cleft it binds to the 2 receptor sites and causes the sodium ion channel to open.

Question 49

List the sequence of events in the transmission of a signal across the synaptic cleft.

Answer 49

An action potential arrives at the synaptic cleft.
The V-G Ca2+ ion channels open and the ions diffuse into the synaptic bulb. They then cause the synaptic vesicles to move to and fuse with the pre-synaptic membrane.
Acetylcholine is released by exocytosis. It diffuses across the cleft and binds to the receptor sites on the Na+ ion channels in the post-synaptic membrane. The Na+ channels open and Na+ ions diffuse across the post-synaptic membrane into the post-synaptic neurone.
A generator potential is created. If sufficient G.P’s combine then the potential across the post-synaptic membrane will reach the threshold potential. A new action potential is created in the post-synaptic cleft.

Question 50

What is a generator potential also known as?

Answer 50

Excitatory post-synaptic potential (EPSP)

Question 51

Where is acetylcholinerase found?
What does it do?
What happens to the products?

Answer 51

The synaptic cleft.
It hydrolysed the acetylcholine to ethanol acid (acetic acid) and choline. This stops the transmission of signals, so that the synapse does not continue to produce action potentials in the post-synaptic neurone.
The ethanoic acid and choline are recycled. They re-enter the synaptic bulb by diffusion and are recombined to acetylcholine using ATP from respiration in the mitochondria. The recycled acetylcholine is stored in synaptic vesicles for future use.

Question 52

When does summation occur?

Answer 52

When the effects of several excitatory post-synaptic potentials (EPSPs) are added together.

Question 53

What is the difference between temporal summation and spatial summation?

Answer 53

Temporal summation - a series of action potentials from the same pre-synaptic neurone causes an action potential.
Spatial summation - several pre-synaptic neurones may each contribute to producing an action potential in the post-synaptic neurone.

Question 54

What does IPSPs stand for?
Where are they made?
What do they do?

Answer 54

Inhibitory post-synaptic potentials.
Some pre-synaptic neurones can produce these.
They can reduce the effect of summation and prevent an action potential in the post-synaptic neurone.

Question 55

Never junctions may involve several neurones.

List several ways this enables synapses to control the communication passed along the nervous system.

Answer 55

Combo of EPSPs could be prevented from producing an action potential by one IPSP.
Spatial summation.
After time a synapse May run out of vesicles containing the neurotransmitter - becoming fatigued. The N.S. no longer makes a response - we are habituated to it. Avoids overstimulation.
Can filter out unwanted low-level signals.
Low-level A.Ps can be amplified by summation.