5.1.3 Neuronal Communication

Question 1

What are energy transducers?

Answer 1

• a cell that converts one form of energy into another
• each transducer is adapted to detect changes in a particular form of energy
• e.g. to an electrical impulse
• most sensory receptors are energy transducers

Question 2

What is a nerve impulse?

Answer 2

• when sensory receptros respond to a stimulius by creating a signal in the form of electrical energy

Question 3

Which sensory receptors detect a change in light intensity and what is the energy change involved?

Answer 3

• rods and cones in the retina
• light to electrical

Question 4

Which sensory receptors detect a change in temperature and what is the energy change involved?

Answer 4

• temperature receptors in the skin and the hypothalamus
• heat to electrical

Question 5

Which sensory receptors detect a change in pressure on the skin and what is the energy change involved?

Answer 5

• pacinian corpuscles in the skin
• movement to electrical

Question 6

Which sensory receptors detect a change in sound and what is the energy change involved?

Answer 6

• vibration receptors in the cochlea of the ear
• movement to electrical

Question 7

Which sensory receptors detect a change in movement and what is the energy change involved?

Answer 7

• hair cells in inner ear
• movement to electrical

Question 8

Which sensory receptors detect a change in length of muscle and what is the energy change involved?

Answer 8

• muscle spindles in skeletal muscles
• movement to electrical

Question 9

Which sensory receptors detect chemicals in the air and what is the energy change involved?

Answer 9

• olfactory cells in the epithelium lining the nose
• these receptors detect the presence of a chemical and create an electrical nerve impulse

Question 10

Which sensory receptors detect chemicals in food and what is the energy change involved?

Answer 10

• chemical receptors in taste buds on tongue
• detect the presence of a chemical and create an electrical nerve impulse

Question 11

What is a Pacinian corpuscle?

Answer 11

• a pressure sensor that detects changes in pressure on the skin

Question 12

Describe the structure of the Pacinian corpuscle and so, how it works

Answer 12

• oval-shaped structure
• consisting of series of concentric rings of connective tissue wrapped around the end of a nerve cell
• when the pressure on the skin changes, this deforms the rings of connective tissue, which push against the nerve ending
• corpuscle is sensitive only to change in pressure that deform the rings oof connective tissue
• so, when pressure is constant, they stop responding

Question 13

How is a generator potential produced?

Answer 13

• since sodium channels are sensitive to small movements of the membrane, when it is deformed by the changing pressure, the sodium channels open
• sodium ions diffuse into the cell, producing a generator potential

Question 14

Describe the action of sodium/potassium pumps on cell membranes

Answer 14

• the sodium/potassium pumps actively pump sodium ions out of the cell and potassium ions into the cell
• three sodium ions are pumped out for every two potassium ions pumped in
• when the channel proteins are all closed, the sodium-potassium pumps work to create a concentration gradient
• the concentration of sodium ions outside the cell increases, while the concentration of potassium ions inside the cell increases
• the membrane is more permeable to potassium ions, so some of these leak out of the cell
• the membrane is less permeable to sodium ions, so few of these are able to leak in
• this creates a potential gradient across a cell membrane
• the cell is negatively charged inside compared to outside

Question 15

When a cell is inactive, what is the cell membrane said to be and what does this mean?

Answer 15

• cell membrane is polarised
• it is negatively charged inside compared with the outside

Question 16

How is a nerve impulse created?

Answer 16

• by altering the permeability of the nerve cell membrane to sodium ions
• this is achieved by opening the sodium ion channels
• as the sodium ion channels open, the membrane permeability is increased
• sodium ions can move across the membrane down their concentration gradient into the cell
• the movement of ions across the membrane creates a change in potential difference across the membrane
• the inside of the cell becomes less negative than usual
• this is called depolarisation
• the change in potential across a receptor membrane is often called a generator potential

Question 17

What is an action potential?

Answer 17

• when enough gated channels are opened and enough sodium ions enter the cell, the potential difference across the cell membrane changes significantly to generate an impulse

Question 18

What are the three main types of neurone?

Answer 18

Motor neurones:

• carry an action potential from the central nervous system to an effector such as muscle or gland

Sensory neurones:

• carry the action potential from a sensory receptor to the CNS

Relay neurones:

• connect sensory and motor neurones

Question 19

Describe the structure of neurones

Answer 19

• many are very long so that they can transmit the action potential over a long distance
• plasma membrane has many gated ion channels that control the entry or exit of sodium, potassium or calcium ions
• sodium-potassium pumps use ATP to actively transport sodium ions out of the cell and potassium into the cell
• neurones maintain a potential difference across their plasma membrane
• a cell body contains the nucleus, many mitochondria and ribosomes
• numerous dendrites connect to other neurones. dendrites carry impulses towards the cell body
• an axon carries impulses away from the cell body
• neurones are surrounded by a fatty layer that insulates the cell from electrical activity in other nerve cells nearby. this is composed of Schwann cells

Question 20

What differentiates motor neurones?

Answer 20

• Motor neurones have their cell body in the CNS and have a long axon that carries the action potential out to the effector

Question 21

What differentiates sensory neurones?

Answer 21

• sensory neurones have a long dendron carrying the action potential from a sensory receptor to the cell body, which is positioned just outside the CNS
• they then have a short axon carrying the action potential into the CNS

Question 22

What differentiates relay neurones?

Answer 22

• relay neurones connect the sensory and motor neurones together
• they have many short dendrites and a short axon
• the number of dendrites and number of divisions of the axon is variable
• relay neurones are an essential part of the nervous system, which conduct impulses in coordinated pathways

Question 23

Which neurone is this?

Answer 23

• motor neurone

Question 24

Which neurone is this?

Answer 24

• sensory neurone

Question 25

Which neurone is this?

Answer 25

• relay neurone

Question 26

Which neurones are unmyelinated?

Answer 26

• 2/3 of peripheral neurones are unmyelinated
• neurones found in the CNS are not myelinated

Question 27

Describe the structure of myelinated neurones

Answer 27

• Schwann cells make up the myelin sheath
• they are wrapped tightly around the neurone so the sheath consists of several layers of membrane and thin cytoplasm from the Schwann cell
• at intervals of 1-3mm along the neurone are gaps in the myelin sheath called the nodes of Ranvier
• each node is very short (about 2-3µm long)

Question 28

How does the action potential move in myelinated neurones?

Answer 28

• the movement of ions across the neurone membranes are prevented by the myelin sheath
• so, movement of ions across the membrane can only occur at the nodes of Ranvier
• this means that the action potential, jumps from one node to the next, making conduction much more rapid

Question 29

Describe non-myelinated neurones

Answer 29

• several neurones may be enshrouded in one loosely wrapped Schwann cell
• the action potential moves along the neurone in a wave rather than jumping from node to node

Question 30

What are the advantages of myelination?

Answer 30

• action potential can be transmitted much more quickly
• myelinated neurones have a typical speed of transmission of 100-120ms^-1
• non-myelinated: 2-20ms^-1
• they can carry action potentials over long distances
• increased speed of transmission means that the action potential reaches the end of the neurone more quickly, so more rapid response to a stimulus
• non-myelinated neurones tend to be short, so can only carry action potentials over short distances (usually used in breathing etc so speed is not important)

Question 31

Describe what is occurring when the neurones are at rest

Answer 31

• it is actively pumping ions across its plasma membrane
• sodium/potassium ion pumps use ATP to pump three sodium ions out of the cell for every potassium ions pumped in
• the gated sodium ion channels are closed
• however, some potassium ion channels are open, so the plasma membrane is more permeable to potassium ions than sodium ions
• potassium ions tend to diffuse out of the cell
• the cell cytoplasm also contains large organic anions
• the interior of the cell is maintained at a negative potential compared to the outside
• the cell membrane is polarised at a p.d. of about -60mV
• this is resting potential
• in myelinated neurones, ion exchanges only occur at the nodes of Ranvier

Question 32

Describe how the action potential is generated

Answer 32

• at rest, the concentration of sodium ions is higher outside than inside
• the concentration of potassium ions is higher inside than outside
• if some sodium ion channels are opened, the sodium ions will quickly diffuse down their concentration gradient into the cell from surrounding tissue fluid
• this causes depolarization of the membrane
• in the generator region of a neurone, the gated channels are opened by the action of the synapse
• when a few gated channels open, they allow a few sodium ions into the cell and produce a small depolarisation
• this is generator potential
• when more gated channels are opened, the generator potential are combined to produce a larger depolarisation
• if it reaches a particular magnitude, it will cause an action potential
• most sodium ion channels in a neurone are opened by changes in the potential difference
• they are called voltage-gated channels
• when there are sufficient generator potentials to reach the threshold potential, they cause the voltage-gated channels to open
• positive feedback
• the opening of voltage-gated sodium ion channels allow a large influx of sodium ions and the depolarisation reaches +40mV on the inside of the cell
• once this value is reached, the neurone will transmit the action potential
• the action potential is self-perpetuating
• once it starts, it will continue to the end of the neurone
• all action potentials are the same magnitude (+40mV)

Question 33

Describe the stages of an action potential

Answer 33

1. the membrane starts in its resting state
   - it is polarised with the inside of the cell being -60mV compared to the outside
   - there is a higher concentration of sodium outside than in and
   - a higher concentration of potassium ions inside than out
2. sodium ion channels open and some sodium ions diffuse into the cell
3. the membrane depolarises and reaches the threshold value of -50mV
4. positive feedback causes nearby voltage-gated sodium ion channels to open and many sodium ions flood in
   - as more sodium ions enter, the cell becomes positively charged inside compared with outside
5. the p.d. across the plasma membrane reaches +40mV
   - the inside of the cell is positive compared to outside
6. sodium ion channels close and potassium channels open
7. potassium ions diffuse out of the cell bringing the potential difference back to negative inside
   - this is repolarisation
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

Question 34

What is the refractory period?

Answer 34

• after an action potential, the sodium and potassium ions are in the wrong places
• the concentration of these ions must be restored by the action of the sodium/potassium ion pumps
• for a short time after each action potential, it is impossible to reach another action potential
• this is the refractory period and allows the cell to recover
• it also ensure the action potentials are transmitted in only one direction

Question 35

What are local currents?

Answer 35

• the opening of sodium ion channels at one particular point of the neurone upsets the balance of sodium and potassium ions set up by the action of sodium/potassium pumps
• when sodium ions are allowed to flood into the neurone causing depolarisation, this creates local currents in the cytoplasm of the neurone
• sodium ions begin to move along the neurone towards regions where their concentration is still lower
• these local currents cause a slight depolarisation of the membrane and cause sodium ion channels further along the membrane to open

Question 36

Describe the formation of local currents and the transmission of a nerve impulse

Answer 36

1. when an action potential occurs the sodium ion channels open at the 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
   - the concentration of sodium ions inside the neurone rises st the point where sodium ions channels are open
3. sodium ions continue to diffuse sideways along the neurone, away from the region of increased concentration
   - this movement of charged particles is local current
4. the local current causes a slight depolarisation further along the neurone which affects the voltage-gated sodium ion channels, causing them to open
   - the open channels allow a rapid influx of sodium ions causing an action potential further along the neurone
   - so the action potential has moved along the neurone

Question 37

How does the action potential continue to move in the same direction?

Answer 37

• it will not reverse direction because the concentration of sodium ions behind the action potential is still higher

Question 38

Describe saltatory conduction

Answer 38

• ionic movements that create an action potential along occur at the nodes of Ranvier
• the local currents are therefore elongated and sodium ions diffuse along the neurone from one node of Ranvier to the next
• so the action potential jumps from one node to the next

Question 39

What are the advantages of saltatory conduction?

Answer 39

• action potentials can only occur at the gaps between the Schwann cells
• this speeds up the transmission of the action potential along the neurone
• it can conduct an action potential at up to 120 m s ^-1

Question 40

How can different intensities of stimuli be detected?

Answer 40

• all action potentials are the same intensity (+40mV)
• a higher frequency of action potentials means a more intense stimulus
• more sodium channels are opened in the sensory receptor
• this produces more generator potentials
• so there are more frequent action potentials in the sensory neurone
• therefore, more frequent action potentials entering the central nervous system

Question 41

What is a synapse and a synaptic cleft?

Answer 41

• a synapse is a junction between two or more neurones, where one neurone can communicate with or signal to another neurone
• the synaptic cleft is the small gap between the two neurones
• it is approx. 20nm wide

Question 42

What are cholinergic synapses?

Answer 42

• synapses that use acetylcholine as the neurotransmitter

Question 43

What is the pre-synaptic bulb and describe its features

Answer 43

• the pre-synaptic neurone ends in a swelling called the pre-synaptic bulb
• it has many mitochondria: indicating an active process needing ATP
• a large amount of smooth endoplasmic reticulum, which packages the neurotransmitter into vesicles
• large numbers of vesicles containing molecules of a chemical called acetylcholine, the transmitter that will diffuse across the synaptic cleft
• a number of voltage-gated calcium ion channels on the cell surface membrane

Question 44

Describe the post-synaptic membrane

Answer 44

• it contains specialised sodium ion channels that can respond to the neurotransmitter
• these channels consists of five polypeptide molecules
• two of these polypeptides have a specific receptor site to acetylcholine due to the complementary receptor site
• when acetylcholine is present in the synaptic cleft, it binds to the two receptor sites and causes the sodium ions channel to open

Question 45

Describe the sequence of transmission across the synapse

Answer 45

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 causes the synaptic vesicles to move to and fuse with the pre-synaptic membrane
5. acetylcholine is released by exocytosis
6. acetylcholine molecules diffuse across the cleft
7. acetylcholine molecules bind to the receptor sites on the sodium ion channels in the post-synaptic membrane
8. the sodium ion channels open
9. sodium ions diffuse across the post-synaptic membrane into the post-synaptic neurone
10. a generator potential or excitatory post-synaptic potential (EPSP) is created
11. if sufficient generator potentials combine then the potential across the post-synaptic membrane reaches the threshold potential
12. a new action potential is created in the post-synaptic neurone

Question 46

What is the role of acetylcholinesterase?

Answer 46

• if acetylcholine is left in the synaptic cleft, it will continue to open the sodium ion channels in the post-synaptic membrane and will continue to cause action potentials
• acetylcholinesterase hydrolyses the acetylcholine to ethanoic acid and choline
• this stops the transmission of signals
• 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 47

What are some more complex nerve junctions?

Answer 47

• it may involve several neurones
• e.g. several neurones from different places converging on one neurone or one neurone sending signals to several neurones

Question 48

What is excitatory post-synaptic potential (EPSP)?

Answer 48

• when one action potential passes down an axon to the synapse, it will cause a few vesicles to move to and fuse with the pre-synaptic membrane
• a relatively small number of acetylcholine molecules diffuse across the cleft produces a small depolarisation
• this is EPSP
• on its own, it is not sufficient to cause an action potential in the post-synaptic neurone

Question 49

What is summation?

Answer 49

• it occurs when the effects of several ESPSs are added together to increase membrane depolarisation and reach the threshold

Question 50

What is temporal summation?

Answer 50

• summation from several action potentials in the same pre-synaptic neurone

Question 51

What is spatial summation?

Answer 51

• summation from action potentials arriving from several different pre-synaptic neurones

Question 52

What are inhibitory post-synaptic potentials (IPSPs)?

Answer 52

• some pre-synaptic neurones can produce inhibitory post-synaptic potentials
• these reduce the effect of summation and prevent an action potential in the post-synaptic neurone

Question 53

How do synapses control the communication passed along the nervous system?

Answer 53

• several pre-synaptic neurones might converge on one post-synaptic neurone
• this can allow action potentials from different parts of the nervous system to contribute to generating an action potential in one post-synaptic neurone, creating a particular response
• this spatial summation could be useful for warning of danger
• the combination of several EPSPs could be prevented from producing an action potential by one IPSP
• one pre-synaptic neurone might diverge to several post-synaptic neurones
• useful for action potential to be transmitted to several parts of the nervous system
• useful for reflex arc
• synapses ensure action potentials are transmitted in the correct direction as only the pre-synaptic bulb contains vesicles of acetylcholine
• synapses filter out unwanted low-level signals as several vesicles f acetylcholine must be released to create an action potential in the post-synaptic neurone
• low-level action potentials can be amplified by summation
• habituation: after repeated stimulation, a synapse may run out of vesicles containing the neurotransmitter so the nervous system no longer responds to the stimulus
• creation and strengthening of specific pathways is the basis of conscious thought and memory
• synaptic membranes are adaptable
• the post-synaptic membrane can be made more sensitive to acetylcholine by adding more receptors
• so it is more likely to fire an action potential, creating a specific pathway in response to a stimulus