nerve transmission

Question 1

what are action potentials

Answer 1

nerve impulses

Question 2

what do nerve cells require

Answer 2

a membrane potential

Question 3

whats a membrane potential

Answer 3

electrical charge difference across cell membrane – like a battery

Question 4

what are ion channels

Answer 4

allow ions to move by diffusion. These generate the membrane potential. They can be passive transport

Question 5

If no action potential then resting cell has resting membrane potential of what

Answer 5

minus 70mv

Question 6

explain Intra/extra cellular ion concentrations and The resting membrane potential

Answer 6

When membrane resting, so nerve not responding to a stimulus.

theres a particular distribution of high levels of sodium ions in extracellular environment and a high level of potassium ions in intracellular environment

When resting, more potassium ions leave cell than sodium ions entering cell, due to distribution of ions and charge across cell membrane

Sodium potassium transporter pump actively pumps these ions across membrane by ATP breakdown so actively

Question 7

what channels do we find at resting membrane potential

Answer 7

leakage channels

Question 8

explain leakage channels

Answer 8

Leakage channels = channels that assist membrane permeability to certain ions, but always work in maintaining resting membrane potential

• cytosol high in K+ & interstitial fluid high in Na+ (sodium –potassium pumps)

Leakage lets K+ through easily and Na+ poorly
- inside is negative relative to outside

Actual RMP depends on the relative leakage channel numbers

Question 9

explain action potential

Answer 9

Series of active events, that take the action along a nerve

during the events: channels actively open & close

Some initial event is required to reach a voltage threshold (~ - 55 mv)

this is a Stimulus
- any event bringing membrane to threshold

Question 10

what is the voltage threshold in mv

Answer 10

minus 55mv

Question 11

After the arrival of a stimulus, and once threshold reached, the action potential goes into what

Answer 11

depolarizing phase
and then action potential moves to a repolarizing phase

Question 12

explain depolarizing phase

Answer 12

membrane potential rises and becomes positive. So polarized membrane lost

Question 13

explain Repolarizing phase

Answer 13

potential restored to resting value

may overshoot = hyperpolarizing phase

then recovery to rest. Resting value is around minus 70mvs

Question 14

what happens If Stimulus is strong enough to reach threshold

Answer 14

1. Na+ channels open
   - Na+ ions enter the cell from extracellular fluid
   - depolarization of membrane occurs (positive membrane potential)
2. K+ channels then open
   - K+ leave the cell
   - repolarization (negative membrane potential)
3. Sodium/potassium pump
   restores original ion concentrations

Question 15

With hyperpolarization the stimulus allows potassium channels to what

Answer 15

open, hence internal environment becomes more negative as those ions leave

Question 16

an action potential can lead to the opening of the sodium channels leading to a degree of depolarization but isnt always what

Answer 16

sufficient enough to reach the threshold

Question 17

explain what happens in the full generation of an action potential, (on a graph)

Answer 17

Theres opening of sodium channels but in this case the stimulus is strong enough to allow more sodium channels to open. The net result is a movement of membrane potential from negative to positive, and action potential is generated

After peak achieved, cell repolarizes as sodium channels close and potassium channels open, going into a hyperpolarized phase where the membrane potential goes slightly lower than resting. This is then normalized with activity of sodium potassium pumps

Question 18

If threshold is reached then the action potentials are always what

Answer 18

the same size

Question 19

Stimulus must be sufficient enough for what to start

Answer 19

threshold to start

Question 20

After one AP there is a short period before next can be triggered
what is this called

Answer 20

refractory period – put in place to make sure that direction in which it travels is the same and impulse doesn’t come back on itself

Question 21

why does a strong stimulus creates a greater response

Answer 21

response because the frequency of action potentials is higher. Not to do with the size of the peak

Question 22

what substances can affect nervous transmission

Answer 22

caffein

alcohol

Local Anaesthetics –for treating pain etc

Question 23

explain hoe caffein affects nervous transmission

Answer 23

a stimulant found in coffee, tea, cocoa, cola etc.

acts by lowering the threshold level of the nerve

increases the possibility for a stimulus to result in a nerve potential

Question 24

explain how alcohol affects nervous transmission

Answer 24

acts as a nervous system depressant

increases the nerve threshold level, so may be minus 30mv compared to minus 55mv, so stimulus has to be stronger to reach the threshold levels

decreases the possibility of a stimulus to result in a nerve action potential. As takes more effort for nerve threshold to be achieved

Question 25

explain how Local Anaesthetics –for treating pain etc, affect nervous transmission

Answer 25

Generation of nerve action potentials can be prevented by local anaesthetics
such as novocaine and xylocaine.

These drugs prevent the opening of sodium gated channels in the nerve cell membrane
- pain messages are not carried from the site of injury back to the CNS. So feel no pain

Question 26

how is a nerve impulse conducted

Answer 26

Each section triggers the next by local currents

refractory period keeps it going the right direction

Question 27

explain the conduction of a nerve impulse in an Unmyelinated fibre

Answer 27

continuous conduction

Question 28

explain the conduction of a nerve impulse in a myelin fibre

Answer 28

saltatory conduction – action potential jumps from unmyelinated regions to myelinated regions. So its faster

can only be triggered at Nodes of Ranvier

Question 29

what fibres conduct nerve impulses fastest

Answer 29

Myelinated fibers are faster & move larger neurons faster

Question 30

explain synaptic transmission

Answer 30

Sequence of events at synapse

if action potential strong enough to reach synapse then a new series of events occurs

These are Triggered by voltage change of the action potential

if 2 neurons close to one another and communicating with one another:

• Sending neuron –
  the first is called, presynaptic
• Receiving neuron
  the second neurone is called postsynaptic
• Neurotransmitter
  chemical that carries signal across synaptic cleft

Between the pre and post synaptic neurons theres a synaptic cleft

Question 31

explain the presynaptic events

Answer 31

1. Action potential arrives at presynaptic end bulb
2. Opens voltage gated Ca2+ channels
   - Ca2+ flows into cell from synaptic cleft
   - increased Ca2+ concentration in presynaptic cell,
   - causing exocytosis of synaptic vesicles
3. Neurotransmitter (NT) released into cleft
   diffuse across to postsynaptic cell membrane

Question 32

explain the Postsynaptic Events

Answer 32

4. Once NT binding at postsynaptic receptors
5. Binding causes a Chemical trigger of ion channels, to open
6. May depolarize or hyperpolarize postsynaptic cell membrane, depending on which channels open, due to the nature of the neurotransmitter
7. If threshold reached at axon hillock
   - then postsynaptic cell action potential results, and the evets occur again but in post synaptic cell

Question 33

how is Termination of Synaptic Transmission done

Answer 33

by removing the neurotransmitter

Question 34

how is the neurotransmitter removed to allow termination of Synaptic Transmission

Answer 34

Diffusion

Destroyed by enzymes in cleft

Transported back into presynaptic cell

Destroyed by neuroglia

Question 35

name some neurotransmitters

Answer 35

Acetylcholine (Ach)
- common in PNS most common in peripheral nervous system
- may be stimulatory (by causing depolarization)or inhibitory (by causing hyperpolarization)

Amino Acids
- glutamate, aspartate, gamma aminobutyric acid (GABA), glycine

Modified amino acids
- norepinephrine (NE), dopamine (DA), serotonin (5-HT)

Neuropeptides
- endorphins

Nitric oxide (NO)

Question 36

name some chemicals affecting synaptic transmission

Answer 36

Botulinum toxin – a neurotoxin protein

Curare – plant extract

Nerve agents

Question 37

explain Botulinum toxins affect on synaptic transmission

Answer 37

its a neurotoxin protein

inhibits release of acetylcholine

inhibits muscle contraction (leading to paralysis)

One of Most acutely lethal toxins

Question 38

explain Curare affect on synaptic transmission

Answer 38

competes for acetylcholine receptor sites on muscle cells

muscle relaxant, so muscle function lost ( used in surgical procedures previously) in high doses it can lead to death

Question 39

explain nerve agents affect on synaptic transmission

Answer 39

inactivates acetylcholinesterase. By blocking activity of enzymes that break down acetylcholinesterase

leads to tetanus (paralysis)

Can lead to death even mins after exposure