5.3 Neuronal communication

Question 1

What are sensory receptors?

Answer 1

• SENSORY RECEPTORS are cells or sensory nerve endings that responded to a stimulus in the internal or external of an organism and can create action potentials.

Question 2

What are transducers?

Answer 2

• TRANSDUCERS are cells that convert one form of energy into another.

Question 3

What can different types of a transducer in a detect?

Answer 3

Each type of transducer is adapted to detect changes in a particular form of energy, a stimulus, where they then respond by creating a signal in the form of electrical energy called a nerve impulse.

Question 4

Name several stimuli, receptors and the energy types involved.

Answer 4

Stimulus, sensory and receptor, energy type

• Light intensity, rods and cones in the retina and light
• Temperature, temperature receptors in the skin and hypothalamus and heat
• Pressure on skin, pacinian corpuscles in the skin and movement
• Sound, vibration receptors in the cochlea of the ear, movement
• Movement, hair cells in the inner ear, movement
• Length of muscles, muscle spindles in skeletal muscles, movement
• Chemicals in the air, olfactory cells in epithelium lining the nose and chemical
• Chemicals in food, chemical receptors in taste buds on the tongueand chemical

Question 5

What are pacinian corpsicles?

Answer 5

PANCINIAN CORPUSCLES are pressure sensors found in the skin.

Question 6

How do pacinian corpsicles respond to pressure?

Answer 6

I. The corpuscle is an oval-shaped structure that consists of a series of concentric rings of connective tissue wrapped around the end of a nerve cell.

II. When pressure changes on the skin changes this deform the rings of connective tissue, which push against the nerve ending.

III. The corpuscle is sensitive only to changes in pressure that deform the rings of connective tissues, therefore when pressure is constant they stop responding.

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Question 7

How do the cells associated with the nervous system control their charge?

Answer 7

I. Cells associated with the nervous system have specialised gated channel proteins such as sodium channels and potassium channels which are specific to sodium ions (Naᐩ) and potassium ions (Kᐩ) respectively.

II. The membranes also contain sodium/potassium pumps which actively pump three sodium ions out of the cell for every two potassium they pump in.

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Question 8

How is the resting potential of neurones of cells associated with the nervous system maintained?

Answer 8

I. The membrane is more permeable to potassium ions so some leak out of the cell; the membrane is less permeable to the sodium ions so few are able to leak into the cell.

II. When the channel proteins are closed, the sodium/potassium ion pump works to create a concentration gradient.

III. The concentration of sodium outside the cell compared to the inside is high; the concentration of potassium outside the cell compared to the inside is low.

IV. The result of this ionic movement is a potential gradient across the cell membrane so that the outside is more negatively charged than the inside which is described as being polarised.

V. The negative potential is enhanced by the presence of negatively charged anions inside the cell.

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Question 9

How is a nerve impulse created?

Answer 9

A nerve impulse is created by altering the permeability of the nerve cell membrane to sodium ions by opening the sodium ion channels.

Question 10

How is a nerve impulse triggered?

Answer 10

A nerve impulse can occur as the sodium channels are sensitive to small movements of the membrane, so when the membrane is deformed by the changing pressure the sodium channels open to allow sodium ions to diffuse into the cell, down their concentration gradient, producing a generator potential (aka, a receptor potential).

If enough gates are opened and enough sodium ions enter the cell, the potential difference across the cell membrane changes significantly and will initiate an impulse or action potential.

Question 11

What are the are many types of neurones?

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Answer 11

There are many types of neurone, three of which are:

1. Sensory neurones: they carry an action potential from the sensory receptor to the CNS,
2. Relay neurones: they join sensory neurones to motor neurones,
3. Motor neurones: they carry an action potential from the CNS to the effector.

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Question 12

What is the response of a neurone to a stimulus?

Answer 12

Function of neurones

I. A stimulus is detected and its energy is converted to a depolarization of the receptor cell membrane.

II. The impulse has then got to be transmitted to other parts of the body along neurones as an action potential: a rapid depolarization of the membrane caused by the influx of sodium ions.

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Question 13

Neurones all have a similar basic structure that enables them to transmit the action potential as they are specialised cells:

What makes neurones well adapted to their function? (there are many adaptations)

Answer 13

1. Very long so they can transmit the action potential over a long distance
2. Plasma membrane has many gated ion channels to control the entry/exit of Naᐩ, Kᐩ and Ca²ᐩ
3. Naᐩ/Kᐩ pumps use ATP to actively transport Naᐩ out of the cell and Kᐩ ions into the cell
4. Maintenance of a potential difference across the plasma membrane
5. Cell body containing nucleus, many mitochondria and ribosomes
6. Numerous dendrites connected to other neurones to carry impulses towards the cell body
7. Axon that carries impulses away from the cell body
8. Surrounded by a fatty myelin sheath, comprised of Schwann cells, for insulation from electrical activity in other nerve cells nearby.

Question 14

Sketch the three neurones.

Answer 14

Question 15

What are the differences between the three different neurones?

Answer 15

Sensory

• Conducts action potential from sensory receptor to CNS
• Cell body just outside the CNS
• Long dendron
• Short axon

Relay

• Conducts action potential between sensory and motor neurones in coordinated pathways
• Cell body inside the CNS
• Variable number of short dendrites
• Short axon with a variable number of divisions

Motor

• Conduct action potential from the CNS to an effector
• Cell body inside the CNS
• Short dendrites
• Long axon

Question 16

Define a myelinated neurone.

Answer 16

A MYELINATED NEURONE has an individual layer of myelin around it.

Question 17

Define a non-myelinated neurone.

Answer 17

A NON-MYELINATED NEURONE has no individual layer of myelin around it.

Question 18

Define a non-myelinated neurone.

Answer 18

A NON-MYELINATED NEURONE has no individual layer of myelin around it.

Question 19

How many of the neurones within organisms are myelinated?

Answer 19

Around one-third of peripheral neurones in vertebrates are myelinated neurones; the remainder of the peripheral neurones and the neurones found in the CNS are non-myelinated neurones.

Question 20

How does a myelinated sheath increase the rate of conduction?

Answer 20

In myelinated neurones:

1. The myelin sheath is made of several layers of membrane and thin cytoplasm from Schwann cells which are tightly wrapped around the neurone
2. At intervals of 1 – 3 mm along the neurone are gaps in the myelin sheath creating nodes of Ranvier which are 2 – 3 µm long.
3. The tightly wrapped sheath prevents the movement of ions across the neurone membranes so it can only happen at the nodes of Ranvier which means that the action potential jumps from one node to the next, giving a very rapid conduction.

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Question 21

What causes a slower conduction in a non-myelinated neurone?

Answer 21

In non-myelinated neurones:

1. Several neurones may be enshrouded in one loosely wrapped Schwann cell
2. This causes the action potential to move along the neurones in a wave
3. This causes a slower conduction.

Question 22

Why is the increased speed of conduction in myelinated neurones an advantage?

Answer 22

1. Can transmit an action potential much more quickly than non-myelinated neurones at a typical speed of 100 – 200 m sᐨ¹, compared to 2 – 20 m sᐨ¹.
2. Carry action potentials over a long distance (from the sensory neurones to the CNS, from the CNS to the effectors) so the longest neurone in a human can be around 1 m in length. The increased speed of the transmission means that the action potential reaches the end of the neurone more quickly to produce a more rapid response to the stimulus.

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Question 23

Sketch a myelinated and non-myelinated neurone.

Answer 23

NOTE; non-myelinated neurones tend to be shorter and carry action potentials over only a short distances, therefore, the increased speed of the transmission is not so important anyway.

For example, they coordinate bodily functions like breathing and the action of the digestive system.

Question 24

Define the resting potential.

Answer 24

RESTING POTENTIAL is the potential difference across the membrane while the neurone is at rest.

Question 25

When a neurone is not transmitting an action potential it is at rest: however, it is actively pumping ions across its plasma membrane: how is the resting potential and potential charge difference across the membrane retained?

Answer 25

1. Sodium/potassium ion pumps use ATP to pump three sodium ions out of the cell for every two potassiums that are pumped in.
2. The gated sodium channels are closed whereas some of the potassium ion channels are open, making the plasma membrane more permeable to potassium ions than sodium ions so potassium ions tend to diffuse out.
3. The intracellular environment contains large organic anions making inside the cell negative, causing the cell membrane to be polarised.
4. The potential difference across the cell membrane is –60 mV.
5. This is called the resting potential.

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Question 26

How does the presence of a myelinated sheath affect the exchange of ions in neurones?

Answer 26

In myelinated neurones, the ion exchanges occur only at the nodes of Ranvier.

Question 27

Define an action potential.

Answer 27

ACTION POTENTIAL is a brief reversal of the potential across the membrane of a neurone causing a peak of +40 mV compared to the resting potential of –60 mV.​

Question 28

Define positive feedback.

Answer 28

POSITIVE FEEDBACK is a mechanism that increases a change taking the system further away from the optimum.

Question 29

While the neurone is at rest it maintains a concentration gradient of sodium ions across its plasma membrane: the concentration of sodium ions outside is higher than inside; the concentration of potassium ions is higher than outside.

How is a generator potential created?

Answer 29

I. If some of the sodium ion channels are opened, then sodium ions will quickly diffuse down their concentration gradient into the cell from the surrounding tissue fluid causing a depolarization of the membrane.

II. In the generator region of a neurone, the gated sodium channels are opened by the action of the synapse. When a few sodium ions are allowed into the cell a small depolarization will occur known as the generator potential.

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Question 30

How does a generator potential become an action potential?

Answer 30

I. Most of the sodium ion channels in a neurone are opened by changes in the potential difference across the membrane so they are called voltage-gated channels. When there are sufficient generator potentials to reach a threshold potential they cause the voltage-gated sodium channels to open. This is an example of positive feedback – a small depolarization of the membrane causing a change that increases the depolarization further.

II. More gated channels are opened so that more sodium ions flood in causing larger depolarization which produces an action potential which is self-perpetuating – once it starts at one point in the neurone, it will continue along to the end of the neurone.

Question 31

What does the term ‘All-or-nothing’ refer to when describing a nerve impulse?

Answer 31

‘All-or-nothing’ refers to the fact that all nerve impulses are identical as all action potentials are the same magnitude of +40 mV where a stronger stimulus is transmitted as more frequent action potentials, not as large potentials.

Question 32

Stages of action potential:

Describe stage 1.

Answer 32

1. The membrane starts in its polarised resting state where the inside of the cell is –60 mV, compared to the outside: there is a higher concentration of potassium ions inside the cell than outside; there is a higher concentration of sodium ions outside the cell than inside.

Question 33

Stages of action potential:

Describe stage 2 and 3.

Answer 33

2. Some sodium ion channels open causing some sodium ions to diffuse into the cell.
3. This causes the membrane to depolarise: it becomes less negative with respect to the outside and reaches the threshold value of –50 mV.

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Question 34

Stages of action potential:

Describe stage 4 and 5.

Answer 34

4. Positive feedback causes nearby voltage-gated sodium ion channels to open causing an influx of sodium ions into the cell: as more sodium ions enter, the inside of the cell becomes positively charged compared to the outside.
5. The potential difference across the membrane reaches +40 mV: the inside of the cell is positively charged compared with the outside

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Question 35

Stages of action potential:

Describe stage 6 and 7.

Answer 35

6. The sodium ion channels close and the potassium channels open.
7. Potassium ions diffuse out of the cell bringing the potential difference back to negative inside compared with the outside during repolarization.

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Question 36

Stages of action potential:

Describe stage 8 and 9.

Answer 36

8. The potential difference overshoots slightly, making the cell hyperpolarised.
9. The original potential difference is restored so that the cell returns to its resting state.

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Question 37

What are some neurone related medical conditions?

Answer 37

Autoimmune diseases such as multiple sclerosis are caused by the demyelination of the motor neurones – the neurones lose their myelin sheath and are unable to conduct impulses properly.

Question 38

Describe the process of returning a neurone to its resting potential after an impulse.

Answer 38

I. After an action potential, the sodium and potassium ions are in the wrong places so the concentrations of these ions inside and outside the cell must be restored by the action of the sodium/potassium ion pumps.

II. For a short time after each action potential, it is impossible to stimulate the cell membrane to reach another action potential – this is known as the refractory period.

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Question 39

Why is the refractory period and hyperpolarization important?

Answer 39

It allows the cell to recover after an action potential and ensures that the action potentials are transmitted in only one direction.

Question 40

In the transmission of a nerve impulse what causes the concentration of sodium ions to increase?

Answer 40

1. When the action potential occurs the sodium ion channels open at that point in the neurone
2. The open sodium ion channels allow sodium ions to diffuse across the membrane from the region of higher concentration outside the neurone into the neurone so the concentration of sodium ions inside the neurone increases at that point

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Question 41

How does the concentration of sodium ions inside the neurone increasing at a point, cause the voltage-gated sodium channels to open?

Answer 41

3. Sodium ions continue to diffuse sideways along the neurone, away from the region of increase concentration, down their concentration gradient creating a local current – the movement of charge particles.
4. The local current causes a slight depolarization further along the neurone which affects the voltage-gated sodium ion channels, causing them to open.

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Question 42

What does the opening of voltage-gated sodium channels allow?

Answer 42

The open channels allow a rapid influx of sodium ions causing a full depolarization, action potential, further along, the neurone – therefore, the action potential has moved along the neurone.

Question 43

Describe saltatory conduction using keywords.

Answer 43

I. In myelinated neurones, sodium and potassium ions cannot diffuse through the fatty myelin sheath layers.

II. The ionic movements that create action potentials can only occur at the nodes of Ranvier between the Schwann cells.

III. The local currents are therefore elongated so it appears that the action potential appears to jump from one node to the next – named saltatory conduction (Latin, meaning ‘to jump’).

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Question 44

What are the advantages of saltatory conduction?

Answer 44

I. The flow of sodium ions along the axon is much more rapid than the movement of an action potential involving an exchange of ions across the membrane.

II. Therefore a myelinated neurone will transmit the impulse much more quickly.

III. A myelinated neurone can conduct an action potential up to 120 m sᐨ¹.

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Question 45

How do we detect stimuli of different intensities?

Answer 45

• Due to the ‘all-or-nothing’ rule, the size of the action potential is unrelated to the intensity of the stimulus that caused it.
• However, we can still detect stimuli of different intensities, such as loud and quiet sounds, as the sensory region of the brain determines the intensity of the stimulus from the frequency of action potentials arriving: a higher frequency of action potentials means a more intense stimulus.

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Question 46

Describe the effect of a more intense stimulus on the neurone.

Answer 46

A more intense stimulus opens more sodium channels in the sensory receptor which produces more generator potentials, as a result, there is a more frequent action potential in the sensory neurone: therefore, more frequent action potentials entering the central nervous system.

Question 47

Define synapses.

Answer 47

SYNAPSES are the junction between two or more neurones, where one neurone can communicate with another neurone.

Question 48

Define a synaptic cleft.

Answer 48

A SYNAPTIC CLEFT is a small gap between neurones, approximately 20 nm wide.

Question 49

An action potential travels along a neurone as a series of ionic movements across the neurone membrane, however, it cannot cross the gap between neurones, the synaptic cleft. How is the impulse transmitted over the synaptic cleft?

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Answer 49

The action potential in the presynaptic neurone causes the release neurotransmitters that diffuse across the synaptic cleft (which is around 20 nm wide) and generates a new action potential in the post-synaptic neurone.

Question 50

Name a neurotransmitter and the matching synapse name.

Answer 50

Synapses that use acetylcholine as the neurotransmitter are called cholinergic synapses

Question 51

What adaptions does the pre-synaptic bulb have to carry out its function?

Answer 51

• Many mitochondria to carry out active processes
• Many smooth endoplasmic reticulum to package the neurotransmitter into vesicle
• Many vesicles of acetylcholine neurotransmitter to diffuse across the synaptic cleft
• Many voltage-gated calcium ion channels on the plasma membrane.

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Question 52

What adaptions does the post-synaptic membrane have to carry out its function?

Answer 52

Many specialised sodium ion channels, consisting of five polypeptide molecule, that have a receptor site that is complementary to acetylcholine.

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Question 53

In the transmission of an impulse across a synapse, state the steps involved in releasing the impulse into the synaptic cleft.

Answer 53

1. An action potential arrives at the synaptic bulb
2. The voltage-gated calcium ion channels open
3. Calcium ions diffuse into the synaptic bulb
4. The calcium ions cause the synaptic vesicles to move to and fuse with, the pre-synaptic membrane
5. Acetylcholine is released by exocytosis across the cleft

Question 54

In the transmission of an impulse across a synapse, state the steps involved in creating a new action potential from the neurotransmitter released into the synapse.

Answer 54

1. The acetylcholine molecules bind to the receptor site on the sodium ion channels in the post-synaptic membrane
2. The sodium channels open
3. Sodium ions diffuse across the post-synaptic membrane into post-synaptic neurone
4. An excitatory post-synaptic potential (EPSP) is created
5. If sufficient generator potentials combine then the potential across the post-synaptic membrane reaches the threshold potential and a new action potential is created in the post-synaptic neurone.

Question 55

What problem could acetylcholine cause if it was left in the synapse?

Answer 55

If acetylcholine is left in the synaptic cleft, it will continue to open the sodium ion channels in the post-synaptic membrane causing a constant action potential.

Question 56

What removes neurotransmitter from the synapse?

Answer 56

Acetylcholinesterase is an enzyme found in the synaptic cleft which hydrolyzes acetylcholine to ethanoic acid and choline to stop the transmission of signals.

Question 57

What happens to the component molecules of acetylcholine after it is broken down?

Answer 57

The ethanoic acid and choline are recycled when they re-enter the synaptic bulb by diffusion as they are recombined to acetylcholine, using ATP front the mitochondria, and then repackaged in vesicles for future use.

Question 58

The main role of synapses is to connect two neurones together so that a signal can be passed from one to the other. What could nerve junctions involve?

Answer 58

1. Several neurones from different places converging on one neurone
2. One neurone sending out signals that diverge to different effectors.

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Question 59

Why may summation be necessary for a signal to be passed on?

Answer 59

When one action potential passes down an axon to the synapse it will cause few vesicles of acetylcholine to diffuse across the cleft, resulting in only a small depolarization – this is an excitatory post-synaptic potential (EPSP).

It may take several EPSPs to reach the threshold voltage to cause an action potential as the defects of several EPSPs combine together to increase membrane depolarisation until it reaches the threshold – this combined effect is known as summation.

Question 60

What are the different types of summation?

Answer 60

1. Temporal summation: several action potentials in the same presynaptic neurone
2. Spatial summation: action potentials arriving from several different presynaptic neurones.

Question 61

Why may the effect of summation be compromised?

Answer 61

Some presynaptic neurones can produce inhibitory postsynaptic potentials (IPSPs) which can reduce the effects of summation and prevent an action potential in the postsynaptic neurone.

Question 62

Where may IPSPs be used in the body?

Answer 62

In many synapses in the brain, the EPSPs and IPSPs compete with each other and determine whether or not the postsynaptic membrane will generate an action potential. γ-aminobutyric acid (GABA) and glycine are common neurotransmitters involved in IPSPs. An IPSP can be achieved by opening chloride ions into the postsynaptic membrane neurone or by opening potassium ion channels that allow potassium ions out of the cell: in both cases, a temporary hyperpolarization is produced.

Question 63

Examples of how synapses control communication;

How could the nervous system respond to several very small stimuli?

Answer 63

When spatial summation occurs, action potentials from different part of the nervous system can contribute to generating an action potential in one postsynaptic neurone to create a particular response: this could be useful where several different stimuli are warning us of danger

Question 64

Examples of how synapses control communication

How could the nervous system filter out repeating very small stimuli from being received?

Answer 64

The continuation of several EPSPs could be prevented from producing an action potential by one IPSP.

Question 65

Examples of how synapses control communication;

How can the nervous system relay a message to multiple parts of the nervous system?

Answer 65

One presynaptic neurone might diverge to several postsynaptic neurones which can allow one action potential to be transmitted to several parts of the nervous system: this is useful in a reflex arc as one postsynaptic neurone elicits the response, while another informs the brain

Question 66

Examples of how synapses control communication

How do synapses ensure that action potentials are only transmitted in the right direction?

Answer 66

Synapses ensure that action potentials are transmitted in the correct direction as only the presynaptic bulb contains vesicles of acetylcholine, therefore if an action potential happens to start half way along a neurone and ends at the postsynaptic membrane, it will not cause a response in the next membrane

Question 67

Examples of how synapses control communication:

How can synapses filter out unwanted low-level signals?

Answer 67

Synapses can filter out unwanted low-level signals as if a low-level stimulus creates an action potential in the presynaptic neurone it is unlikely to pass across a synapse to the next neurone as there will not be enough vesicles of acetylcholine

Question 68

Examples of how synapses control communication:

How does the nervous system avoid overstimulation of an effector due to repeated stimulation, such as background noise?

Answer 68

After repeated stimulation a synapse may run out of vesicles containing the neurotransmitter so the synapse is said to be fatigued which means the nervous system will no longer respond to the stimulus and we ill become habituated to it – this explains why we get used to a smell or a background noise and avoids overstimulation of an effector which could cause damage.

Question 69

Examples of how synapses control communication:

How does the nervous system adapt to recognise a particular stimulus?

Answer 69

The creation and strengthening of specific pathways within the nervous system are thought to be the basis of conscious thought and memory. Synaptic membranes are adaptable, in particular, the postsynaptic membrane as it can be made more sensitive to acetylcholine by the addition of more receptors which means that a particular postsynaptic neurone is more likely to fire an action potential, creating a specific pathway in response to a stimulus