Module 5.3 - Neuronal Communication

Question 1

(MA) How do receptors respond to pressure?

Answer 1

• receptors distorted
• sodium ion channels open
• sodium ions enter
• depolarisation of the membrane
• the threshold potential is met or exceeded
• action potential is generated

Question 2

(MA) Describe the resting potential stage of the graph of the generation of an action potential (horizontal).

Answer 2

• about -60mV to -70mV (read from graph)
• membrane is polarised
• when neurone is at rest (no stimulus)
• sodium/potassium ion pumps actively pump 3 Na out for every 2 K in to maintain resting potential
• K+ leak (diffuse) out of neurone

Question 3

(MA) Describe the depolarisation stage of the graph of the generation of an action potential (positive gradient).

Answer 3

• voltage gated sodium ion channels open + sodium ions diffuse into axon
• positive feedback: presence of sodium ions cause opening of more v-g sodium ion channels + so influx of more sodium ions
• potential difference increases from -65mV to +40mV (from graph)
• membrane is depolarised
• at +40mV, the sodium ion channels close (an action potential is transmitted along axon)

Question 4

(MA) Describe the repolarisation stage of the graph of the generation of an action potential (negative gradient).

Answer 4

• voltage gates potassium ion channels open
• potassium ions diffuse out of axon
• positive feedback: causes more potassium ion channels to open so more potassium ions diffuse out
• membrane is repolarised
• membrane becomes hyperpolarised
• potential difference falls from +40mV to -70mV

Question 5

(MA) What is meant by an ‘all or nothing response’?

Answer 5

if the stimulus is not strong enough the threshold value will not be reached and there will be no action potential generated

Question 6

(MA) How is an action potential generated?

Answer 6

• distortion of receptors produces a generator potential
• sodium ions diffuse into axon
• causes depolarisation of the membrane
• if the depolarisation exceeds the threshold value
• more voltage gated sodium ion channels will open causing more sodium ions to diffuse in
• action potential will be generated and transmitted down an axon

Question 7

(MA) How is an action potential transmitted down an axon in myelinated neurones?

Answer 7

• sodium ions diffuse along axon creating local currents
• due to the myelin sheath, the action potential moves by salutatory conduction where the action potential jumps from one node of Ranvier to the next
• presence of sodium ions cause voltage gated sodium ion channels to open so more sodium ions diffuse into axon (positive feedback)
• the transmission can only move in one direction due to the refractory period where voltage gates sodium ion channels behind the action potential to allow resting potential to be restored

Question 8

(MA) What is the difference between the structures of motor neurones and sensory neurones?

Answer 8

• sensory neurone: cell body not in CNS. Motor neurone’s is
• cell body of sensory neurone in middle of neurone, motor neurone’s at the end
• dendrites of sensory neurone are at the end of the axon but they connect directly to the cell body in the motor neurone
• sensory neurone has a shorter axon than the motor neurone
• Dendron present in sensory neurone but no Dendron in motor neurone

Question 9

(MA) What is the difference between the functions of motor neurones and sensory neurones?

Answer 9

• sensory neurone carries action potential from receptor to relay neurone in CNS
• motor neurone carries action potential from relay neurone in CNS to effector (muscle/gland)

Question 10

(MA) What is the structure of the myelin sheath?

Answer 10

• consists of Schwann cells wrapped around axon

- except at nodes of Ranvier where there are gaps in the myelination

Question 11

(MA) What is the function of the myelin sheath?

Answer 11

• act as an electrical insulator
• myelinated neurone conduction action potentials faster than non myelinated neurones
• depolarisation can only occur where sodium ion channels are present
• there are no v-g sodium ion channels in the myelinated regions so ion movement can only take place at the nodes
• causes salutatory conduction where the action potential jumps from one node of Ranvier to the next
• which means longer local currents (than in non-myelinated neurones)

Question 12

(MA) What is the refractory period and what is its importance?

Answer 12

• follows an action potential
• during which time an action potential cannot be generated (absolute refractory period)
• voltage gated sodium ion channels are closed
• allows resting potential to be restored in axon
• ensures impulses are separated
• ensures impulses on travel in one direction along axon

Question 13

(MA) What happens at the synapse?

Answer 13

• action potential arrives at presynaptic knob + causes v-g calcium ion channels to open
• Ca2+ ions diffuse into presynaptic knob
• Ca2+ causes vesicles containing acetylcholine (neurotransmitter) to move towards + fuse w/ presynaptic membrane
• ACh released into synaptic cleft by exocytosis
• ACh diffuses across synaptic cleft
• ACh binds to receptors on Na+ ion channels on postsynaptic membrane
• Na+ diffuse into postsynaptic membrane + generate new action potential
• ACh is hydrolysed by acetylcholinesterase

Question 14

(MA) What is the role of acetylcholinesterase?

Answer 14

• hydrolyses ACh into acetic acid (ethanoic acid) + choline
• unblocks receptors so allows Na+ channels to close on postsynaptic knob
• stops continuous production of action potentials in postsynaptic neurone
• allows repolarisation of postsynaptic membrane

Question 15

(MA) What is the role of the synapse?

Answer 15

-allows communication between neurones
-ensures transmission between neurones is only in 1 direction
>vesicles containing ACh are only found in presynaptic knob
>receptors for ACh only found on postsynaptic knob
-allows convergence: impulses from more than 1 neurone to pass into a single neurone + allows divergence: impulses from a single neurone to pass to more than 1 neurone
-filters out low level stimuli
-prevents overstimulation + fatigue of neurone
-allows many low level stimuli to be amplified