3.6.2 Nervous coordination

Question 1

Describe how the resting potential of a neurone is reached

Answer 1

• the sodium potassium pump pumps 3 sodium ions out and 2 potassium ions in
• the membrane is impermeable to sodium ions (because voltage gated sodium ion channels are closed)
• potassium ions can leave because the membrane is more permeable to potassium ions (through leak channels, voltage gated potassium ion channels are closed)
• overall the potential difference across the membrane is -70mV
• the neurone is polarised

Question 2

What are the 6 stages of action potential and the potential difference of each?

Answer 2

• 1. Polarised -70mV
• 2. Depolarising -70mV to +40mV
• 3. Depolarised +40mV
• 4. Repolarising +40mV to -80mV
• 5. Hyper polarised (refractory period) -80mV
• 6. Polarised -70mV

Question 3

What is the refractory period?

Answer 3

• Can’t start another action potential because ions are in the wrong place
• (prevents pain becoming too intense)

Question 4

Describe depolarising stage (stage 2)

Answer 4

• sodium voltage gated ion channels open
• so sodium ions diffuse in
• potassium voltage gated ion channels closed
• inside becomes more positive until +40mV is reached
• Na+ entering, K+ inside
• -70 to +40

Question 5

Describe the depolarised stage (stage 3)

Answer 5

• Na+ voltage gated ion channels close
• K+ voltage gated ion channels open
• Na+ stop entering
• K+ can now start to leave
• +40

Question 6

Describe the replolarising stage (stage 4)

Answer 6

• Na+ voltage gated ion channels close at +40mV
• K+ voltage gated ion channels open at +40 so K+ leave
• inside becomes more negative
• +40 to -80

Question 7

Describe the Hyper polarised stage (stage 5)

Answer 7

• inside is too negative (-80mV)
• K+ voltage gated ion channels close at this voltage
• Too many K+ have left
• refractory period
• -80mV

Question 8

Describe the all or nothing principal (pacinican corpuscle)

Answer 8

• Smaller stimulus, less stretch mediated sodium ion channels open, only a few sodium ions enter, smaller potential difference across membrane, smaller generator potential which does not reach the threshold, no action potential
• NO NERVE IMPULSE
• Larger stimulus, greater number of stretch mediated ion channels open, more sodium ions enter, greater potential difference across membrane, greater generator potential which reaches the threshold
• NERVE IMPULSE PRODUCED

Question 9

What is a generator potential?

Answer 9

• a change in potential difference in a sensory neurone due to a stimulus
• a generator potential will only produce an action potential if the depolarisation is above threshold

Question 10

Why is the speed of transmission of an impulse faster along a myelinated axon than a non-myelinated axon?

Answer 10

• in non-myelinated: depolarisation occurs along the whole length of an axon membrane
• myelination provides electrical insulation
• in myelinated: depolarisation only at nodes of Ranvier (gaps in membrane) - saltatory conduction occurs

Question 11

Why is the speed of neurone transmission faster the greater the axon diameter? ***check mark scheme

Answer 11

• axons with a smaller diameter have a larger surface area to volume ratio,
• so more ions leak out of the axon,
• making it more difficult for threshold to be reached and therefore an action potential to be generated

Question 12

Why does a high temperature increase the rate of neurone transmission? *** modify

Answer 12

• temperature affects rate of diffusion

Question 13

Describe what happens at a cholinergic synapse ***

Answer 13

• an action potential arriving at the end of presynaptic neurone membrane causes Ca2+ to diffuse in
• causing the synaptic vesicles containing acetylcholine to fuse with the presynaptic membrane and release acetylcholine into the synaptic cleft by exocytosis
• acetylcholine attaches to receptors on post synaptic neurone membrane
• sodium ions enter leading to depolarisation
• acetylcholine (???) diffuses into post synaptic neurone causing depolarisation and if threshold is exceeded, an action potential is generated
• acetylcholine is broken down by acetylcholinesterase into choline and ethanoic acid
• which diffuse back and are reabsorbed into the presynaptic neurone. They are reformed into acetylcholine using energy from ATP hydrolysis

Question 14

What is meant by synapses being unidirectional?

Answer 14

• action potentials can only pass across a synapse from a presynaptic neurone to a postsynaptic neurone because…
• acetylcholine/neurotransmitter only released from end of presynaptic neurone
• acetylcholine receptors are only located on postsynapic neurone membrane

Question 15

Compare neuromuscular junction and cholinergic synapse

Answer 15

• both unidirectional
• NM only excitatory, CS excitatory or inhibitory
• NM connects motor neurone to muscles, CS connects 2 neurones
• NM end point for action potential, CS new action potential generated in next neurone
• NM acetylcholine bind to receptors on muscle fibre membrane, CS acetylcholine binds to receptors on post-synaptic membrane

Question 16

What happens at an inhibitory synapse?

Answer 16

• inhibitory neurotransmitter binding causes chloride ions to enter post-synaptic neurone and potassium ions to leave
• causes hyperpolarisation of neurone
• less likely that a new action potential will be generated
• because more sodium ions entering are needed to depolarise neurone

Question 17

How does damage to the myelin sheath affect muscle contraction?

Answer 17

• action potential travels more slowly
• so delay in muscle contraction
• action potential leaks to adjacent neurones
• so wrong muscle fibres contract