Neuronal Communication

Question 1

Describe the mechanoreceptor.

Answer 1

1) Stimulus - pressure and movement.
2) Example - Pacinian corpuscle.
3) Example of sense organ - skin.

Question 2

Describe the chemoreceptor.

Answer 2

1) Stimulus - chemicals.
2) Example of receptor - olfactory receptor (detects smells).
3) Example of sense organ - nose.

Question 3

Describe the thermoreceptor.

Answer 3

1) Stimulus - heat.
2) Example of receptor - end-bulbs of Krause.
3) Example of sense organ - tongue.

Question 4

Describe the photoreceptors.

Answer 4

1) Stimulus - light.
2) Example of receptor - cone cell (detects different wavelengths).
3) Example of sense organ - eye.

Question 5

Name 4 sensory receptors.

Answer 5

• Mechanoreceptors.
• Chemoreceptors.
• Thermoreceptors.
• Photoreceptors.

Question 6

Define the term “sensory receptor”.

Answer 6

Specialised cell which detects a stimulus.

Question 7

Define the term “transducer”.

Answer 7

A device that converts one type of energy or signal into another. I.e. in sensory receptors they convert a stimulus into a nerve impulse.

Question 8

Define the term “stimulus”.

Answer 8

Detectable change in the internal or external environment of an organism, which is detected by the nervous system and can cause a response.

Question 9

State 3 characteristics of sensory receptors and for each explain why they are important.

Answer 9

1) They are specific to a single type of stimulus .
2) They act as a transducer - they convert a stimulus into a nerve impulse.
3) Sensitive.

Acting as a transducer is important as it means that information can be passed round the nervous system and eventually an effector, to initiate a response.

Specific because you do not want stimulus to trigger multiple receptors, because the initiated may not be suitable to bringing the conditions back to the norm.

Question 10

Draw and label a diagram showing the structure of a Pacinian corpuscle.

Answer 10

See pp.348

• The end of the sensory neurone is found within the centre of corpuscle, surrounded by layers of connective tissue.
• A blood capillary runs around the outside of the connective tissue, followed by a layer called a capsule.
• Within the membrane of the neurone there are Na+ channels which are responsible for transporting Na+ ions across the membrane.
• The neurone ending in a Pacinian Corpuscle has a special type of Na+ channel called a stretch-mediated Na+ channel. When these channels change shape (i.e. they stretch) their permeability to Na+ changes too.

Question 11

Explain how a Pacinian corpuscle converts mechanical pressure into a nerve impulse. Give the process.

Answer 11

1) In its normal state (resting state), the stretch-mediated Na+ channels in the sensory neurones membrane are too narrow to allow Na+ to pass through them. Corpuscle is at resting potential.
2) When pressure is applied to the Pacinian corpuscle, the corpuscle changes shape. This causes the membrane surrounding its neurone to stretch.
3) When the membrane stretches, the Na+ channels widen. Na+ now diffuse into the neurone.
4) The influx of positive Na+ changes the potential of the membrane. It becomes depolarised. This results in a generator potential.
5) In turn, the generator potential creates an action potential (a nerve impulse) that passes along the sensory neurone.
6) The action potential will then be transmitted along neurones to CNS.

Question 12

Outline the steps in a stimulus-response pathway and identify the role of the sensory, relay, and motor neurones in this pathway.

Answer 12

Known as the reflex arc.

1) Receptor - detects stimulus and creates an action potential in the sensory neurone.
2) Sensory neurone - carries impulse to spinal cord.
3) Relay neurone - connects sensory neurone to motor neurone within the spinal cord or brain.
4) Motor neurone - carries impulse to the effector to carry out the appropriate response.

Question 13

Draw and label a diagram of a motor neurone.

Answer 13

• Have a one long axon and many short dendrites. No dendrons.
• Cell body at the end of the axon on the left.
• Nucleus in the centre of the cell body.
• The unmyelinated gaps are called nodes of ranvier.

Question 14

Draw and label a diagram of a relay neurone.

Answer 14

• Many short axons and dendrons which branch off into dendrites.
• Cell body and nucleus are in the centre with dendrons branching off it.
• Myelin sheath surrounding the axons.

Question 15

Draw and label a diagram of a sensory neurone.

Answer 15

• They have one dendron (on the right), which carries the impulse to the cell body.
• The cell body is in the centre.
• They have an axon which carries the impulse away from the cell body.
• The dendron branches into dendrites
• The unmyelinated gaps are called nodes of ranvier.

Question 16

Define the term “dendrite”.

Answer 16

Dendrites are projections of a neurone that receive signals from other neurones. They conduct electrical messages to the neurone cell body for the cell to function

Question 17

Define the term “dendron”.

Answer 17

Dendrons are short extensions from the cell body . These extensions divide into smaller and smaller branches called dendrites. They are responsible for transmitting electrical impulses towards the cell body.

Question 18

Define the term “axon”.

Answer 18

Singular, elongated nerve fibres that transmit impulses away from the cell body. These fibres can be very long, for example, those that transmit impulses from the tips of toes and fingers to the spinal cord.
The fibre is cylindrical in shape and has a very narrow region of cytoplasm surrounded by a plasma membrane.

Question 19

Define the term cell body.

Answer 19

This contains the nucleus surrounded by the cytoplasm. Within the cytoplasm there are also large amounts of endoplasmic reticulum and mitochondria which are involved in the production of neurotransmitters.

Question 20

What are neurotransmitters?

Answer 20

Chemicals which are used to pass signals from one neurone to the next.

Question 21

Define the term myelinated sheath.

Answer 21

Membrane rich in lipid which surrounds the axon of some neurones, speeding up impulse transmission.

Question 22

Define the term “Schwann cell”.

Answer 22

Schwann cells form a lipid material called myelin, which wraps around the axon.

Question 23

Define the term node fo ranvier.

Answer 23

Nodes of Ranvier are the gaps between each adjacent Schwann cell. With the myelin sheath, they allow for a faster speed of transmission.

Question 24

Describe and explain the advantage of myelination.

Answer 24

• The myelin sheath is made of many layers of plasma membrane, Schwann cells produces these membranes by growing many times around the axon.
• The myelin sheath acts as an insulating layer and allows these myelinated neurones to conduct the electrical impulse at a much faster speed than unmyelinated neurones.
• Between each adjacent Schwann cell there is a small gap known as the node of Ranvier. This creates a gap in the myelin sheath.
• In myelinated neurones, the electrical impulses jump from one node to the next as it travels along the neurone.
• Called saltatory conduction and massively increases the speed of transmission.

Question 25

Explain why some neurones are myelinated and others are unmyelinated.

Answer 25

• When the stimulus is non-urgent (i.e. a dull pain or a small temperature change), the response does not have to be fast.
• When the stimulus is urgent myelination is necessary for a fast transmission to protect body from danger.

Question 26

Explain the role of sodium ions, potassium ions, organic anions, the sodium/potassium ion pump and potassium ion channels in establishing and maintaining the resting potential.

Answer 26

• Sodium/Potassium pump in the membrane of the axon is working constantly, actively transporting 2K+ inside the cell for every 3Na+ out.
• This creates a high concentration of Na+ outside the cell and a build of positive charge.
• K+ ion concentration builds up inside the cell but there are less positive ions inside compared to outside so the potential difference of the cell membrane with respect to the outside is negative (-70mv).
• This resting potential is maintained because the voltage-gated sodium ion channels are closed at this potential difference so sodium ions cannot diffuse back into the cell.
• The potassium ion channels are open, so some potassium ions diffuse out of the cell down their concentration gradient, but not many because of the positive charge outside of the cell that repels them.
• Potassium ions remain in higher conc inside the cell than out.

Question 27

Describe what resting potential in the axon is.

Answer 27

• When a neurone is not transmitting an impulse, the potential difference across its membrane is known as resting potential.
• In this state, the outside of the membrane is more positively charged than the inside of the axon.
• The membrane is said to be polarised as there is a potential difference across it.
• The potassium ion channels (mainly those that are not VG) are open, and the VG sodium ion channels are closed.
• Sodium/potassium pump is working. Pumping out 3 sodium ions for every 2 potassium ions in.

Question 28

Explain why a neurone is active when it is said to be resting.

Answer 28

Because it still carrying out active transport of ions in order to maintain the resting potential.

Question 29

Define the term “resting potential”.

Answer 29

The potential difference across the membrane of the axon of a neurone at rest (i.e. when an impulse is not being transmitted).

Question 30

Define the term “voltage-gated channel”.

Answer 30

Any ion channel that opens and closes in response to changes in electrical potential across the cell membrane in which the channel is situated

Question 31

Define the term “threshold potential”.

Answer 31

The membrane voltage that must be reached in an excitable cell (e.g., neurone or muscle cell) during a depolarisation in order to generate an action potential.

Question 32

Define the term “action potential”.

Answer 32

The change in the potential difference across a neurone membrane of the axon when stimulated. Approximately +40mv.

Question 33

Define the term “nerve impulse”.

Answer 33

The movement of action potential along a nerve fibre in response to a stimulus.

Question 34

Define the term “polarised”.

Answer 34

When there is a difference in millivolts inside the cell compared to outside the cell.

Question 35

Define the term “depolarisation”.

Answer 35

A neurone membrane is depolarised if a stimulus decreases its voltage from the resting potential in the direction of zero voltage.

Question 36

Define the term “repolarisation”.

Answer 36

The restoration of the difference charge between the inside and outside of the plasma membrane.

Question 37

Define term “hyperpolarisation”.

Answer 37

The change in the cell’s membrane potential that makes it more negative.

Question 38

Describe the process of depolarisation.

Answer 38

A stimulus is detected by a sensory receptor and this triggers depolarisation.

1) Neurone is at resting potential (not transmitting an impulse). Potassium ion channels are open and voltage-gated sodium ion channels are closed.
2) The energy from the stimulus triggers some VG sodium ion channels to open. Sodium ions diffuse into the cell down their electrochemical gradient. Makes the inside of the neurone less negative.
3) Potential difference of neurone reaches threshold potential, and this change in charge causes more VG sodium ion channels to open, allowing more sodium ions to diffuse in down the axon (positive feedback mechanism). This is depolarisation.

Question 39

Describe the process of repolarisation.

Answer 39

4) Following depolarisation, the potential difference reaches around +40mV. At this potential difference, the VG sodium ion channels close and the VG potassium ion channels open. Sodium ions can no longer enter the axon but the membrane is now more permeable to potassium ions.
5) Potassium ions diffuse out the axon down their electrochemical gradient. This reduces the charge, resulting in the inside of the axon becoming more negative than the outside.
6) Initially, lots of potassium ions diffuse out of the axon, resulting in the inside of the axon becoming more negative (relative to the outside) than in its normal resting state. This is hyperpolarisation. The VG potassium ion channels now close. The sodium-potassium pump (which continues working constantly) causes sodium ions to move out of the cell and potassium ions to move in (3 Na+ out for every 2 K+ in). The axon returns to its resting potential, now repolarised.

Question 40

Draw, label and annotate a graph of an action potential occurring over time to show the different stages of an action potential and what happens at each stage

Answer 40

See pp.350 or notes for a graph.
Action potential including refractory period takes places over 5milliseconds.
1) First stage is resting potential. Normally at about -70mV/ -65mV.
2) Stimulus triggers influx of sodium ions. Potential difference becomes less negative.
3) Reaches threshold potential.
4) Full depolarisation.
5) Repolarisation.
6) Hyperpolarisation.
7) Resting potential.

Question 41

Define the term “refractory period” and explain its importance for the conduction of the action potential.

Answer 41

A short time when the action cannot be excited/ stimulated again. During this time, the VG sodium ion channels remain closed, preventing the movement of sodium ions into the axon.
A refractory period is important because it makes sure action potentials as unidirectional (does not allow them to occur backwards as well as forwards). It also ensure action potentials do not overlap and occur as discrete impulses.

Question 42

Describe the role of the sodium/potassium pump after an action potential has occurred.

Answer 42

To restore resting potential.

Question 43

Define the term “local circuits”.

Question 44

Define the term “saltatory conduction”.

Answer 44

A form of nerve impulse conduction where the impulse travels from one node of ranvier to the next rather than travelling the whole length of the nerve fibre. Much faster.

Question 45

Describe and explain how an action potential is transmitted along an unmyelinated axon.

Answer 45

1) The initial stimulus causes a change in the sensory receptor which triggers an action potential in the sensory receptor, so the first region of the axon membrane is depolarised.
2) This acts as a stimulus for the depolarisation of the next region of the membrane. The process continues along the length of the axon, forming a wave of depolarisation.
3) Once sodium ions are inside the axon, they are attracted by the negative charge and the concentration gradient to diffuse further along inside the axon, triggering the depolarisation of the next section.

Question 46

Describe and explain how an action potential is transmitted along a myelinated axon.

Answer 46

1) Transfer of electrical impulses happens a lot faster because depolarisation can only occur at nodes of ranvier where no myelin is present in the membrane.
2) Longer localised circuits therefore arise between adjacent nodes. The action potential then ‘jumps’ from one node to another in a process called saltatory conduction.
3) This is much faster than a wave of depolarisation across the whole length of the axon membrane. Every time channels open and ions move it takes time, so reducing the number of places where this happens increases the speed of action potential transmission.
4) Long-term, saltatory conduction is also more energy efficient because repolarisation uses ATP in the sodium/potassium pump.

Question 47

Draw and annotate a graph of an action potential conducting down an unmyelinated axon. (Distance graph).

Answer 47

• Reverse of membrane potential to time graph.

- Hyperpolarisation first, then repolarisation, then depolarisation.

Question 48

Explain what is meant by the “all-or-nothing” response of neurones.

Answer 48

• A certain level of stimulus, the threshold value, always triggers a response.
• If this threshold value is reached an action potential will always be triggered.
• No matter how large the stimulus, the same sized action potential will always be triggered.
• If the threshold is not reached, no action potential will be triggered.

Question 49

Explain why the “all-or-nothing” response of neurones means that information must be transmitted by the frequency of impulse transmission.

Answer 49

• Size of stimulus will not effect the size of action potential that is triggered. Size of action potential will always be the same.
• This means it is the frequency of action potentials triggered that effects the initiated response.

Question 50

State the two other factors that effect the speed of transmission.

Answer 50

1) Axon diameter - the bigger the axon diameter, the faster the impulse is transmitted. This is because there is less resistance to the flow of ions in the cytoplasm, compared with those in a smaller axon.
2) Temperature - the higher the temperature, the faster the nerve impulse. This is because ions diffuse faster at higher temperatures. Maxes out at 40oC as higher temperatures cause proteins to be denatured.

Question 51

Define term “neurotransmitter”.

Answer 51

A chemical involved in communication across a synapse between adjacent neurones and a muscle cell.

Question 52

Define term “synapse”.

Answer 52

The junction (small gap) between two neurones or a neurone and an effector.

Question 53

Define term “synapse”.

Answer 53

The junction (small gap) between two neurones or a neurone and an effector.

Question 54

Define term “synaptic knob”.

Answer 54

The swollen end of the presynaptic neurone. It contains many mitochondria and large amount of endoplasmic reticulum to enable it to manufacture neurotransmitters.

Question 55

Define term “presynaptic membrane”.

Answer 55

The membrane of the neurone along which the impulse has arrived.

Question 56

Define term “cholinergic synapse”.

Answer 56

A receptor is cholinergic is uses acetylcholine as its neurotransmitter. Cholinergic synapse has these receptors .

Question 57

Define term “synaptic cleft”.

Answer 57

The gap which separates the axon of one neurone from the dendrite of the next neurone.

Question 58

Define term “postsynaptic membrane”.

Answer 58

The membrane of the neurone that receives the neurotransmitter.

Question 59

Define term “acetylcholine”.

Answer 59

An example of an excitatory neurotransmitter.

Question 60

Define term “acetylcholinesterase”.

Answer 60

An enzyme that causes rapid hydrolysis of acetylcholine. Its action serves to stop the excitation of a nerve after the transmission of an impulse.

Question 61

Define the term “synaptic vesicle”.

Answer 61

Vesicles containing neurotransmitters. The vesicles fuse with the presynaptic membrane and release their contents into the synaptic cleft.

Question 62

Define the term “neurotransmitter receptor”.

Answer 62

Receptor molecules which the neurotransmitter binds to in the postsynaptic membrane.

Question 63

Draw, label and annotate a diagram to show the structures present in a cholinergic synapse.

Answer 63

See pp.355

1) presynaptic neurone.
2) synaptic vesicles.
3) Calcium ion channels on the presynaptic membrane.
4) synaptic cleft.
5) Sodium ion channels on postsynaptic membrane.

Question 64

Describe the structure of the sodium channels on the post synaptic membrane.

Answer 64

• Made up of 5 protein subunits.
• Has a receptor site, where acetylcholine (the neurotransmitter) binds.
• Na+ passes through channel.

Question 65

Describe the sequences of events that occur at a synapse that can result in an action potential being generated in the post-synaptic neurone.

Answer 65

1) The action potential reaches the end of the presynaptic neurone.
2) Depolarisation of the presynaptic membrane causes calcium ion channels to open.
3) Calcium ions diffuse into presynaptic knob.
4) This causes synaptic vesicles containing neurotransmitters to fuse with the presynaptic membrane. Neurotransmitter is released into the synaptic cleft by exocytosis.
5) Neurotransmitter diffuses across synaptic clef (very short distance) and binds with specific receptor molecule on the postsynaptic membrane.
6) This causes sodium ion channels to open.
7) Sodium ions diffuse into the postsynaptic neurone.
8) This triggers an action potential and the impulse is propagated along the post-synaptic neurone.

Question 66

Describe the role of acetylcholinesterase and explain how acetylcholine is recycled.

Answer 66

Acetylcholinesterase is an enzyme situated on the postsynaptic membrane which acts to break down any acetylcholine neurotransmitters left in the synaptic cleft after the excitation has passed.
Acetylcholine is hydrolysed to give choline and ethanoic acid.
These products are taken back to the presynaptic knob to be recycled into acetylcholine. This uses ATP from the mitochondria.
Ensures postsynaptic membrane is ready to receive another impulse.

Question 67

Explain why synapses are unidirectional.

Answer 67

• Impulses can only travel from presynaptic neurone to postsynaptic neurone because the neurotransmitter receptors are only present in the postsynaptic membrane.

Question 68

Define the term “summation”.

Answer 68

The build up of neurotransmitter in a synapse to sufficient levels to trigger an action potential.
(The amount of neurotransmitter released from single impulse is not always enough to trigger an action potential in the post-synaptic membrane because threshold level is not reached).

Question 69

Define the term “temporal summation”.

Answer 69

This occurs when a single presynaptic neurone releases neurotransmitter as a result of an action potential several times over a short period. This builds up in the synapse until the quantity is sufficient to trigger an action potential.

Question 70

Define the term “spatial summation”.

Answer 70

This occurs when a number of presynaptic neurones connect to one postsynaptic neurone. Each releases a neurotransmitter which builds up a high enough level in the synapse to trigger an action potential in the single postsynaptic neurone.

Question 71

State 3 roles of synapses and for each describe the importance of this role to the nervous system.

Answer 71

• Ensure impulses are unidirectional.
• Allow an impulse from one neurone to be transmitted to a number of neurones at multiple synapses. This results in a single stimulus creating a number of simultaneous responses.
• Alternatively, a number of neurones may feed in to the same synapse with a single post-synaptic neurone. This results in stimuli from different receptors interacting to produce a single result.

Question 72

Explain the difference between excitatory and inhibitory neurotransmitters.

Answer 72

1) Excitatory - these neurotransmitters result in the depolarisation of the postsynaptic neurone. If the threshold is reached in the postsynaptic membrane an action potential is triggered. Acetylcholine is an example of an excitatory neurotransmitter.
2) Inhibitory - these neurotransmitters result in the hyperpolarisation of the postsynaptic membrane. This prevents an action potential being triggered. Gamma-aminobutyric acid (GABA) is an example of an inhibitory neurotransmitter that is found in some synapses in the brain.