5.3 Neuronal Communication

Question 1

What are sensory receptors?

Answer 1

detect stimuli and are energy transducers, converting one form of energy to electrical energy as a nerve impulse

Question 2

What receptors respond to changes in pressure? How? What are some features?

Answer 2

Pacinian Corpuscles

• when pressure on skin changes, it deforms the ring. Sodium channels open, allowing them to diffuse into the cell and produce a generator potential.
• the movement across the membrane creates a change in potential difference.
• if a large enough stimulus is detected and enough gates are opened, the potential difference changes significantly = depolarisation and an action potential.
• rings of connective tissue around end of nerve cell
• fibroblasts producing connective tissue

Question 3

What is the function of the motor neurone?

Answer 3

carry action potentials from CNS to an effector

Question 4

What is the structure of a motor neurone?

Answer 4

• cell body with dendrites
• axon hillock
• very long axon with myelin sheath
• synaptic endings/ axon terminals

Question 5

What is the function of the relay neurone?

Answer 5

connect sensory and motor neurones

Question 6

What is the structure of a relay neurone?

Answer 6

• cell body with dendrites
• VERY short axon, with synaptic endings

Question 7

What is the structure and function of sensory neurones?

Answer 7

• carry APS from receptors to CNS
• may have dendrites to sensory receptor
• dendron with myelin sheath
• cell body
• axon with myelin sheath
• synaptic endings

Question 8

What are similarities in the structures of all the neurones?

Answer 8

• plasma membrane has many gated ion channels and Na+/K+ pumps
• cell body with nucleus, mitochondria and ribosomes
• ## dendrites carry impulses towards body, and axons away

Question 9

What are the differences between the neurones?

Answer 9

• motor: cell body in CNS and has LONG axon to effector, sensory has LONG dendron to cell body which is outside of CNS. Short axon into CNS
• relay: short dendrities + short axon. CNS
• third of peripheral nervous system neurones MYELINATED. Rest in PNS, and the CNS, ARE NOT.
• sensory and motor contain nodes of ranvier, relay dont

Question 10

What does myelination look like on a neurone? What is the purpose?

Answer 10

• Schwann cells that make up fatty sheath. Cytoplasm wrapped around axon.
• Gaps are nodes of Ranvier. The movement of ions can only occur here
• Jumping of impulse = rapid = increased speed of transmission over long distances

Question 11

What occurs in non myelinated neurones? What is the purpose?

Answer 11

• AP moves along in a wave
• Schwann cells loosely wrapped around multiple axons instead
• Shorter distances, coordinating functions where speed isn’t so important

Question 12

What is happening when neurones are at rest?

Answer 12

• Gated sodium ion channels are kept closed
• ATP is used to actively pump 3 Na+ ions out for every 2K+ ions inside
• Some potassium channels are open meaning the membrane is more permeable to them, and they tend to diffuse out.
• The cell cytoplasm also contains many large anions
• Membrane is POLARISED - more negative compared to outside = resting potential at about -60mV

Question 13

What are the stages of an action potential?

Answer 13

1. Resting potential
2. A few sodium ion channels open (due to a stimulus in receptor cells, or diffusion in neurones) and some sodium ions diffuse into the cell
3. Membrane reaches about -50mV - the THRESHOLD
4. Positive feedback causes nearby voltage gated sodium ion channels to open and loads of ions flood in = DEPOLARISATION
5. Potential difference reaches +40mV, where the sodium ion channels now close and potassium channels open.
6. Potassium ions diffuse OUT of the cell bringing the potential difference back to negative = REPOLARISATION
7. The potential difference overshoots slightly = HYPERPOLARISATION
8. K+ ion channels close. Original resting potential is restored via the pump.

Question 14

What is the refractory period?

Answer 14

after each AP it is impossible to reach another for some time -> allows the cell to recover and ensures APs are transmitted in one direction.

Question 15

How are local currents formed?

Answer 15

1. Sodium ion channel opens during depolarisation allowing Na+ to diffuse into the neurone
2. Localised increase in conc of Na+ inside the neurone -> AP
3. Sodium ions diffuse sideways along the axon/dendron away to a region of lower concentration.
4. Causes slight depolarisation of the membrane further down and causes opening of Na+ channels further down the neurone at a node of Ranvier.
5. The open channels allow rapid influx of sodium ions in, causing a full depolarisation along the neurone = another AP.

Question 16

What is saltatory conduction? What are the advantages?

Answer 16

• the jumping of action potentials from one node to the next
• the local currents are elongated, and then the ionic movements occur at the Nodes
• speeds up transmission

Question 17

How is the intensity of a stimulus transmitted?

Answer 17

• all action potentials are the same intensity
• FREQUENCY of APs arriving at the sensory region of the brain.
• higher frequency = more intense stimulus

Question 18

What are cholinergic synapses?

Answer 18

• junction between two neurones -> synaptic cleft
• a synapse that uses acetylcholine as the neurotransmitter released from the pre-synaptic neurone which diffuses across the cleft to the post-synaptic neurone

Question 19

What specialised features does the pre-synaptic bulb contain?

Answer 19

• many mitochondria for ATP
• SER to package neurotransmitter into vesicles
• large numbers of vesicles with acetylcholine
• number of voltage gated calcium ion channels

Question 20

How is the post-synaptic membrane specialised?

Answer 20

• many sodium ion channels that respond to neurotransmitter, with 5 polypeptide molecules
• two of these have a special receptor site specific to acetylcholine, complementary
• binding causes Na+ ion channels to open

Question 21

Describe the transmission across a synapse process

Answer 21

1. AP arrives at synaptic bulb
2. Voltage gated Ca2+ ion channels open
3. Ca2+ ions diffuse into the synaptic bulb
4. They cause the vesicles to move to and fuse with the pre-synaptic membrane
5. Acetylcholine is released by exocytosis and diffuses across cleft
6. It binds to receptor sites on sodium ion channels on post-synaptic membrane and causes them to open
7. Sodium ions diffuse into the post-synaptic neurone
8. EPSP is created (excitatory post synaptic potential)
9. If threshold then an AP is generated, which passes down.

Question 22

What is the role of acetylcholinesterase?

Answer 22

• Found in synaptic cleft
• Hydrolyses the acetylcholine to ethanoic acid and choline, which are recycled.
• they reenter the bulb by diffusion and endocytosis and are recombined to acetylcholine using ATP
• Stops over-transmission of signals

Question 23

What is an excitatory post-synaptic potential vs an inhibitory post-synaptic potential?

Answer 23

• EPSP: the small depolarisation across the post-synaptic membrane, when a relatively small number of acetylcholine molecules diffuse and bind
• IPSP: reduce the effect of summation and prevent an AP in the post-synaptic neurone

Question 24

What is summation? Describe the two types

Answer 24

when the effects of several impulses are added together

several APs in same pre-synaptic neurone = TEMPORAL

AP’s arriving from several different pre-synaptic neurones = SPATIAL

Question 25

How is communication controlled at synapses?

Answer 25

- several pre-synaptic neurones might converge to one post-synaptic neurone -> could be useful when diff stimuli are warning us of danger - one pre-synaptic neurone might diverge to several post-synaptic neurones -> useful in reflex arc - synapses ensure APs go one way - synapses can filter out low level, unwanted signals, or summation can amplify them - after repeated stimulation a synapse might run out of neurotransmitter = habituation.

Question 26

The sensory neurone was stimulated electrically and the FURA-2 fluorescence in the synaptic bulb was measured. At the same time, an electrode recorded the membrane potential in a postsynaptic neurone. The results of this study are shown i Strength of electrical stimulation low medium high FURA-2 fluorescence in synaptic bulb low medium high Highest membrane potential in postsynaptic neurone (mV) –60 +40 +40 The intensity of FURA-2 fluorescence is proportional to the concentration of Ca2+ ions. The scientists concluded that changes in the concentration of Ca2+ ions in the presynaptic neurone caused an action potential in the postsynaptic neurone. Evaluate and explain the scientists’ conclusion. .

Answer 26

conclusion is valid because: 1 concentration of Ca2+ is proportional to strength of stimulus  2 Ca2+ change from low to , medium / high , causes increase in (membrane) potential  3 action potential in , presynaptic neurone / synaptic bulb leads to , opening of Ca2+ channels / entry of Ca2+ 4 Ca2+ , causes / AW , release of neurotransmitter  5 neurotransmitter causes , Na+ / sodium ion channels to open in (post-synaptic) neurone  6 if threshold is exceeded this causes action potential in (postsynaptic) neurone / depolarises (postsynaptic) membrane  conclusion may not be valid because: 7 changes in Ca2+ concentration may not be thecause of (postsynaptic) action potential 8 Ca2+ change from medium to high but no change in (membrane) potential 