topic 6.5 - neurones and synapses

Question 1

Difference between nervous and endocrine system

Answer 1

endocrine system consists of glands that release hormones; nervous system consists of nerve cells called neurone

Question 2

What is the function of neurons?

Answer 2

to transmit electrical impulses

Question 3

draw a diagram of a standard neuron

Question 4

dendrite

Answer 4

short branched nerve fibres, for example those used to transmit impulses between neurone in one part of the brain or spinal cord

Question 5

axons

Answer 5

very elongated nerve fibres, for example those that transmit impulses from the fingers to the spinal cord

Question 6

how are nerve fibres adapted?

Answer 6

they are myelinated, which allows for saltatory conduction

Question 7

saltatory conduction

Answer 7

in myelinated nerve fibres the nerve impulse can jump from one node of ranvier to the next

Question 7

describe myelin

Answer 7

• consists of many layers of phospholipid bilayer
• deposited by Schwann cells, which grow round and round the nerve fibre

Question 7

node of ranvier

Answer 7

gap between the myelin deposited by adjacent Schwann cells.

Question 7

why is saltatory conduction faster than continuous transmission?

Answer 7

action potential to be conducted much faster and prevents the loss of the electrical signal through the cell membrane.

Question 8

define resting potential

Answer 8

the potential difference or voltage across the membrane of a neuron that is not transmitting a signal

Question 8

why is there a resting potential?

Answer 8

due to the imbalance of positive and negative charges across the membrane

Question 9

describe resting potential and how it is maintained

Answer 9

• sodium-potassium pumps transfer sodium and potassium ions across the membrane. Na+ ions pumped out and K+ ions pumped in. The number of ions pumped is unequal - when 3 Na+ ions are pumped out, only two K+ ions are pumped in, creating concentration gradients for both ions
• the membrane is about 50x more permeable to K+ ions than Na+ ions, so K+ ions leak back across the membrane faster than Na+ ions. As a result, the Na+ concentration gradient across the membrane is steeper than the K+ gradient, creating a charge imbalance
• there are proteins inside the nerve fibre that are negatively charged (organic anions) which increase the charge imbalance

Question 10

define an action potential

Answer 10

a rapid change in membrane potential, consisting of:
- depolarisation - a change from negative to positive
- repolarisation - a change back from positive to negative

Question 10

value of resting potential

Answer 10

-70mV

Question 11

describe depolarisation

Answer 11

due to the opening of sodium channels in the membrane, allowing Na+ ions to diffuse into the neuron down the concentration gradient. The entry of Na+ ions reverses the charge imbalance across the membrane, so the inside is positive relative to the outside. This raises the membrane potential to a positive value of about +30mV

Question 11

describe repolarisation

Answer 11

happens rapidly after depolarisation and is due to the closing of the sodium channel and the opening of potassium channels in the membrane. This allows K+ ions to diffuse out of the neuron, down their concentration gradient, which makes the inside of the cell negative again relative to the outside. The potassium channels remain open until the membrane has fallen to a potential close to -70mV

Question 12

why does it take a few milliseconds for the neuron to be able to transmit another nerve impulse?

Answer 12

the diffusion of potassium depolarises the neuron, but does not restore the resting potential as the concentration gradients of sodium and potassium ions have not yet been re-established.

Question 12

define a nerve impulse

Answer 12

action potentials propagated along the axons of neutrons

Question 13

why does the propagation of the action potential happen?

Answer 13

because the sodium ion movements that depolarise one part of the neutron trigger depolarisation in the neighbouring part of the neuron

Question 13

what is the function of the refractive period after a depolarisation?

Answer 13

prevents propagation of an action potential backwards along an axon

Question 14

describe how local currents, or propagation of the action potential, come about

Answer 14

1. depolarisation of part of the axon reduces the concentration of sodium ions outside the axon and increases it inside
2. depolarised part of the axon therefore has different sodium ion concentrations to the neighbouring part of the axon that has not yet depolarised
3. inside the axon sodium ions diffuse along inside the axon to the neighbouring part that is still polarised
4. outside the axon the sodium ions diffuse from the polarised part back to the part that has just depolarised

Question 15

state the effect of local currents

Answer 15

reduce the concentration gradient in the part of the neuron that has not yet depolarised. this makes the membrane potential rise from the resting potential of -70mV to about -50mV. sodium channels in the axon membrane are voltage-gated and open when a membrane potential of -50mV is reached (known as threshold potential). this causes depolarisation

Question 16

draw a diagram for local current

Answer 16

pg 323

Question 17

draw and describe an action potential peak

Answer 17

pg 324

Question 18

define a synapse

Answer 18

a junction between neurons and between neurone and receptor or effector cells

Question 19

when pre-synaptic neurons are depolarised, they release

Answer 19

a neurotransmitter into the synapse

Question 20

describe synaptic transmission

Answer 20

1. a nerve impulse is propagated along the pre-synaptic neuron until it reaches the end of the neuron and the pre-synaptic membrane 2. depolarization of the pre-synaptic membrane causes calcium ions (Ca2+) to diffuse through channels in the membrane into the neuron 3. influx of calcium causes vesicles containing neurotransmitter to move to the pre-synaptic membrane and fuse with it 4. neurotransmitter is released into the synaptic cleft by exocytosis 5. the neurotransmitter diffuses across the synaptic cleft and binds to receptors on the post-synaptic membrane 6. the binding of the neurotransmitter to the receptors causes adjacent sodium ion channels to open 7. sodium ions diffuse down their concentration gradient into the post-synaptic neuron, causing the post-synaptic membrane to reach the threshold potential 8. an action potential is triggered in the post-synaptic membrane and is propagated on along the neuron 9. the neurotransmitter is rapidly broken down and removed from the synaptic cleft

Question 21

function of acetylcholine

Answer 21

used as the neurotransmitter in many synapses

Question 22

how is acetylcholine produced

Answer 22

in the presynaptic neuron by combining choline, absorbed from the diet, with an acetyl group produced during aerobic respiration

Question 23

describe the use of acetylcholine as a neurotransmitter

Answer 23

- loaded into vesicles and then released into the synaptic cleft during synaptic transmission - receptors for acetylcholine in the post-synaptic membrane have a binding site to which acetylcholine will bind - acetylcholine only remains bound to receptor for a short time, during which only one action potential is initiated in the post-synaptic neuron - enzyme acetylcholinesterase is present in the cleft and rapidly breaks down acetylcholine down into choline and acetate - choline reabsorbed into pre-synaptic neuron, where it is converted back into active neurotransmitter by recombining it with an acetyl group

Question 24

what do neonicotinoids do?

Answer 24

insecticides: - they bind to the acetylcholine receptor in cholinergic synapses in the CNS of insects - acetylcholinesterase does not break them down, so binding is irreversible - receptors blocked so acetylcholine is unable to bind and synaptic transmission is prevented - results in insect paralysis and death

Question 25

give an advantage and disadvantage of neonicotinoid use

Answer 25

+ not highly toxic to humans and other mammals: greater proportion of synapses in CNS are cholinergic in insects than in mammals and neonicotinoids bind less strongly to acetylcholine receptors in mammals than insects - effects on honeybees and other beneficial insects

Question 26

a nerve impulse is only initiated if...

Answer 26

the threshold potential is reached

Question 27

nerve impulses follow an ---- why?

Answer 27

all or nothing principle; only at the threshold potential do voltage-gated sodium channels start to open, causing depolarisation

Question 28

what determines whether a nerve impulse travels to the neighbouring neuron? explain

Answer 28

the amount of neurotransmitter secreted following depolarisation of the pre-synaptic membrane may not be enough to cause the threshold potential to be reached in the post-synaptic membrane, which then does not depolarise.

Question 29

what happens when the post-synaptic membrane does not depolarise?

Answer 29

sodium ions that have entered the post-synaptic neuron are pumped out by sodium-potassium pumps and the post-synaptic membrane returns to the resting potential

Question 30

what mechanism can be used to process information from different sources in the body to help in decision-making?

Answer 30

- a typical post-synaptic neuron in the brain or spinal cord has synapses with many pre-synaptic neurone - it may be necessary for several of these to release neurotransmitter at the same time for the threshold potential to be reached and nerve impulse to be initiated