Lecture 6 Physiology of Neurons Flashcards Preview

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Flashcards in Lecture 6 Physiology of Neurons Deck (49)
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1

what are electrical synapses like compared to chemical synapses

faster
bidirectional
much smaller gap -3.5nm
no plasticity (no learning)
no amplification
coupled via gap junctions

2

why is there no amplification in an electrical synapse

signal always weakened as transmitted from presynaptic to postsynaptic cell
(signal won't transmit if postsynaptic cell is too much bigger than presynaptic)

3

what can't excitatory presynaptic signal do in post synaptic cells

inhibit

4

what is spatial summation (of signals)

a neuron determines whether to fire based on amount of signals received from synapsing neurons
can then reach threshold

5

what is temporal summation of signals

input neuron is firing fast enough so that receiving neuron can add together signals (as can't recover from small depolarisations fast enough) and reach threshold

6

what are the steps of an action potential

threshold reached by stimulation
depolarisation
overshoot
repolarisation
after-hyperpolarisation with refractory period
resting state

7

what are the important voltages in an action potential

rest -70mV (near Ek)
threshold ~-50mV
Vm>0 overshoot (reaches ~+30mV)
inward rectifiers open -60mV (delayed rectifiers already open- repolarisation)

8

what does action potentials being 'all-or-none' mean?

carry no info about size of stimulus that stimulated them

9

how do neurons code the intensity of their synaptic input

firing frequency
different neurons for different strength stimuli

10

how does firing frequency represent the intensity of the activity

increasing threshold lowers firing freq and increasing excitatory synaptic activity increases it

11

how do lengthy synaptic currents affect firing frequency

when lengthy synaptic currents are small they create higher threshold potential due to accommodation than larger currents

12

what is accommodation

of Na+
inactivates duding the slower subthreshold depolarisation
first response stronger than later responses

13

how do different neurons code intensity

light touch receptors vs pain receptors - specific neuron for each stimuli

14

what is excitability

how easy it is to start nervous signalling
aka sensitivity in sensory cells, irritability in muscle or effector cells
risk of seizure or spasms if too much

15

how does increased threshold effect excitability

lowers it

16

what are excitability changes the basis of

psychotropic pharmacology - changes in threshold have profound health and behavioural effects

17

what are channels made up of

proteins
sometimes they conduct ions, sometimes they don't
have different conformational states

18

what controls voltage-gated channels

change states based on transmembrane voltage
open when membrane +ve so can conduct and increase permeability, closing when membrane depolarises

19

how are inward rectifiers affected by transmembrane voltages

inward rectifiers are the opposite of other channels
close when inside positive and open when negative

20

why is inactivated not the same as closed

both non conducting, inactivated is when channel stops conducting (after a delay) when membrane positive inside (closed is when negative)

21

what happens to the membrane when Na+ channels open

positive inside as Na+ high on outside than inside

22

what is the potential of the extracellular space

electrically joined in all cells so same voltage everywhere
extracellular fluid considered electrical ground

23

what happens to the membrane when K+ channels open

negative inside as K+ higher on inside than outside

24

what happens to the membrane when Ca2+ channels open

positive inside as Ca2+ higher on outside than inside
passively goes inward

25

how do ionic permeabilities affect voltage

increased permeability to K+ makes membrane negative, Na+ opposite

26

how is the voltage of the cell membrane determined

inter-related feedback loops
at rest Vm=~Ek
as conductance of K+ is greater than that of Na+ or Ca2+

27

what is lidocaine

local anaesthetic, applied topically

28

how does lidocaine work

raises the threshold so lowers excitability stopping local action potentials by blocking Na+ channels in pain neurons

29

what is Carbamazepine

anticonvulsant

30

how does carbamazepine work

inactivates sodium channels to raise AP threshold and lower excitability

31

what are examples of other Na+ channel blockers

antiarrythmic drugs (class 1 eg quinidine) work by lowering conduction velocity to extend refractory period
fugu fish poison (tetrotoxin (TTX))

32

how does Glibenclamide work

sulfonylurea to manage type 1 diabetes
increases excitability of pancreatic beta cells leading to increased insulin secretion (inhibit K+ channels)

33

what is chemical force

force on an ion
aka diffusional force
based on difference in concentration across a membrane

34

what is electrical force

force on an ion
based on membrane potential (Vm) which varies over time

35

what is the equilibrium potential

Ek aka reversal potential of K+
voltage when K+ in(electrical)=out(chemical) as electrochemical forces are in equilibrium
same for other ions

36

what is used to calculate equilibrium potential

Nernst Equation

37

how do equilibrium potentials differ

the more permeable the cell membrane is to K+ the more the membrane potential approaches the value of Ek

38

what are the equilibrium potentials of Ena, Ek, Eca, Ecl?

ENa = +60 mV
EK = -90 mV
ECa = +123 mV
ECl = -40 mV (in neurons –65 mV)

39

how do ion channels control voltage

open channels conducting with little competition - membrane potential to that ion's equilibrium potential
both open - halfway point between 2 equilibrium potentials

40

what are action potentials (compared to graded potentials)

stereotyped electrical signal
short-duration
in most neurons, skeletal and cardiomyocytes
a spike
all or none
require time to start due to conformational changes

41

what are graded potentials (compared to action potentials)

electrically localised - membrane potential
last a long time
much flatter in shape
conducted almost instantly
in receptor cells eg cones and rods
variable in duration and voltage

42

how is electricity conducted in axons

passive
all + = - at start, Na+ moves in, increasing flow and -ve charges
all charges move simultaneously in same direction
depolarisation jumps to NofR's - saltatory conduction

43

what makes saltatory conduction down an axon faster

large diameter as less resistance
myelinated

44

what are graded potentials issues with transmitting signals

changes in membrane potential don't propagate far via passive electrical forces and voltage diminishes further from source - needs help

45

why do voltage signals in graded potentials decrease further from the source

axon has a finite resistance

46

how do graded potentials transmit action potentials

transmit along length of axon, the AP re-amplifies the signal but slows down transmission at the nodes due to conformational changes

47

what is saltatory conduction

when action potential jumps from node to node for faster conduction velocity

48

what are typical conduction velocities

100 m/s for alpha motor fibres (myelinated, 15 um diam.)
1 m/s for C nociceptive fibres (unmyelinated, 0.2 - 1.5 um)

49

clinical uses for conduction velocity

Nerve conduction studies are used for evaluation of paraesthesias
numbness, tingling, burning
Evaluation of weakness of the arms and legs