Signalling 2

Question 1

nerve impulse definition

Answer 1

wave of depolarisation of the membrane of the axon

Question 2

where are the ions distributed at resting potential

Answer 2

approx. 70mV
^[Na+] outside the cell
^[K+] inside the cell
^[Cl-] outside the cell

Question 3

process of action potential
DEPOLARISATION

Answer 3

1- Na+ channels open
2- Na+ flows into cell

Question 4

process of AP
REPOLARISATION

Answer 4

3- depolarisation causes K+ v.g. channels to open
4- Na+ channels close
5- K+ flows out of the cell

Question 5

process of AP
HYPERPOLARISATION

Answer 5

6- K+ channels are slow-closing, so the membrane overshoots and becomes hyperpolarised
~90mV

Question 6

process of AP
RETURN TO REST

Answer 6

Na+ channels reset
Na+/K+ pump

Question 7

what causes the wave of depolarisation to be unidirectional

Answer 7

refractory period
short period of time immediately following an AP when a new AP cannot be initiated in the same area of the membrane

Question 8

absolute refractory period

Answer 8

when Na+ channels are closed and cannot be activated

Question 9

relative refractory period

Answer 9

Na+ channels could reopen, but the membrane is hyperpolarised and so a larger stimulation is required to reach the threshold value (-55mV) for continuation of the AP

Question 10

AP stimulation

Answer 10

1- ligand binds to ligand-gated channels, causing them to open (or mechanically-gated)
2- Na+ moves down conc. gradient, through channels, into cell
3- number of Na+ moving in depends on strength of signal
4- if membrane potential = -55mV, v.g. Na+ channels open and there is an influx of Na+ (temporary deceleration of rate)

Question 11

how does an AP start

Answer 11

multiple inputs from multiple dendrites
- Excitatory Pre-Synaptic Potentials (EPSPs)
- Inhibitory Pre-Synaptic Potentials
(IPSPs)
- vary in magnitude (‘graded’)
summation of changes in membrane potential
- EPSPs increase chance of AP initiation; IPSPs decreases chance
if overall membrane potential reaches threshold voltage, the v.g. Na_+ channels open and AP is initiated

Question 12

summation

Answer 12

spatial - inputs from different areas - e.g. from different dendrites
temporal - input occurs multiple times from the same areas/ dendrite
- repeated inputs in a short time period

Question 13

how does summation affect APs

Answer 13

generated more or less FREQUENTLY

Question 14

reflex arc

Answer 14

stimulus -> receptor -> sensory neuron -> CNS -> relay neuron -> CNA -> motor neuron -> effector -> response

Question 15

NT general info

Answer 15

chemical signals
different NTs associated with different NS functions (e.g. memory, movement)
excitatory or inhibitory (promote or inhibit formation of AP in receiving neuron)
many drugs act on NT signalling

Question 16

NT chemical classification

Answer 16

amino-acids + derivatives
- e.g. glutamate - main excitatory NT in CNS
- e.g. GABA - main inhibitory NT in CNS
Catecholamines (monoamines)
- derived from Tyr
- e.g. dopamine, serotonin
Acetylcholine
- derived from choline
- NMJ
Peptides
- e.g. Substance P, endorphins

Question 17

what is the GABA-A receptor

Answer 17

an ion-channel-receptor (ionotropic)
Cl- ion channel

Question 18

GABA-A receptor process

Answer 18

GABA released from presynaptic neuron into synapse, which then binds to the GABA-A receptor on post-synaptic membrane
1- binding of ligand (GABA) to receptor
2- causes conformational change that opens ion channel (‘ligand-gated’)
3- Ion moves through channel along conc. gradient
4- Hyperpolarisation -> harder to reach threshold voltage for AP
Benzodiazepines, ethanol, some anaesthetics: potentiate GABA activation of GABA-AR
- more inhibition of APs - sedative action

Question 19

ACh at NMJ

Answer 19

nAChR = Na+ channel receptor
1- ACh binds
2- allows Na+ entry
3- Na+ influx (DEPOLARISATION) of muscle CM
4- depolarisation initiates contraction
Acetylcholinesterase in cleft removes ACh

Question 20

Myasthenia gravis (muscle weakness)

Answer 20

autoimmune antibodies attack ACh receptors at NMJ
treatment: AChE inhibitors used to enhance NMJ transmission

Question 21

signalling via GAP junctions

Answer 21

connexin proteins form connection with channel
ions carry charge from one cell to another
e.g. electrical synapses between neurons (most are chemical)
- cardiac myocytes

Question 22

cardiac myocytes

Answer 22

heartbeat: electrical impulse travelling across heart tissue causes contraction of cardiac muscle
specialised cells with neuron-like properties spread charge throughout hear (e.g. Purkinje fibres)