6. ET: neurons

Question 1

name the features of the neuron and their functions

Answer 1

dendrites - receives electrical signals
cell body - contains nucleus and organelles, passively conveys signals
axon hillock - trigger zone for action potentials
axon - propagates axon potentials
axon terminal - releases chemical signals

Question 2

what is the RMP’s value and the meaning of this

Answer 2

~ -70mV; charge inside the cell is ~70mV more negative than outside

Question 3

differences in intracellular and extracellular Na+ and K+ concentration in neurons

Answer 3

Na+ 10 fold greater outside, K+ 20 fold greater inside

Question 4

explain leak channels and relative permeability of each Na+ and K+

Answer 4

K+ has many leak channels mediating flow outwards, while Na+ has less leak channels that mediate flow inwards
at rest, P K+: P Na+ = 40:1

Question 5

what is the equilibrium potential

Answer 5

an intracellular potential at which the net flow of an ion is zero, according to its electrochemical gradient

Question 6

equilibrium potential K+

Answer 6

-80 mV

Question 7

equilibrium potential Na+

Answer 7

+60mV

Question 8

what is the nernst equation

Answer 8

calculates EP for a singular permeant ion based on concentrations
E(ion) = 61.5 x log (ion conc outside)/ (ion conc inside)

Question 9

what is the goldman equation

Answer 9

calculates RMP based on permeability of multiple permeant ions
RMP = 61.5mV log x (permeability ions outside)/(permeability ions inside)

Question 10

amplitude of action potential is: (voltage and features)

Answer 10

constant, doesn’t depend on intensity of stimulus ~100mV

Question 11

what is hyper polarisation

Answer 11

when the membrane potential becomes more negative

Question 12

what is depolarisation

Answer 12

when the membrane potential becomes more positive

Question 13

define an action potential

Answer 13

brief fluctuation in membrane potential caused by a transient opening of voltage-gated ion channels which spreads along an axon

Question 14

what is ‘threshold’ in neurons

Answer 14

~ -55 mV

Question 15

what occurs prior to an action potential

Answer 15

a slow graded depolarisation is evoked by a stimulus, which can be physical, chemical, and affects a ligand or mechanically gated ion channel

Question 16

name the three steps of an action potential

Answer 16

1. depolarisation
2. repolarisation
3. after hyperpolarisation

Question 17

describe the depolarisation stage of an AP with respect to changes in ion permeability and MP

Answer 17

if MP reaches threshold, a fast depolarisation to around +30mV occurs due to rapid opening of voltage-gated Na+ channels
which increases permeability of Na+ to 20:1 (Na+:K+)

Question 18

describe the repolarisation stage of an AP with respect to changes in ion permeability and MP

Answer 18

MP is becoming more negative
as Na+ channels close due to closing of inactivation gate (Na+) decreasing
and K+ channels open as delayed response to AP, P (K+) increasing

Question 19

describe the after hyperpolarisation stage of an AP with respect to changes in ion permeability and MP

Answer 19

MP briefly overshoots/becomes more depolarised/more negative
Na+ channels are back to resting state w activation gate closed
K+ channels are open
permeability = K+:Na+ 100:1

Question 20

what is the absolute refractory period

Answer 20

the period after an AP is generated (depolarisation and repolarisation) when a new AP cannot be fired, as voltage-gated Na+ channels are temporarily unexcitable

Question 21

what is the relative refractory period

Answer 21

the stage where an action potential can be generated, but stimulus must be bigger than usual due to MP being more negative than usual, as membrane is in after hyperpolarisation state

Question 22

why does influx of Na+ stop in an AP

Answer 22

1. the electrical gradient for influx decreases as more Na+ comes in, making the inside of neuron more positive
2. the channels deactivate quickly, due to the inactivation gate

Question 23

explain the structure and function of the two Na+ channel gates

Answer 23

1. activation gate -
   extracellular side
   closed at rest
   opens first and quickly upon depolarisation
   closes during AHP (starts closing in repolarisation)
2. inactivation gate:
   intracellular side
   open at rest
   closes second, starts in repolarisation
   reverts to resting state in AHP

Question 24

what ions does cathode attract, what charge aggregates at the membrane adjacent, what movement of ions occurs, what happens to MP

Answer 24

cations, positive charge, ions move from outside to inside, membrane depolarises

Question 25

what ions does anode attract, what charge aggregates at the membrane adjacent, what movement of ions occurs, what happens to MP

Answer 25

anions, negative charge, ions move outside to inside, membrane hyper polarises

Question 26

how are APs evoked internally/physiologically

Answer 26

APs generate at the axon hillock at the top of the axon and spread down the axon in one direction, as the Na+ channels behind them are in the refractory period

Question 27

what are the two paths for current to flow in external AP generation, and what are their differences?

Answer 27

outside the axon from anion to cation - path of least resistance, most current moves this way across the membrane, inside the axon - only this path changes RMP/generates AP

Question 28

diameter of myelinated axons vs myelinated axons

Answer 28

5-10 uM, ~1 uM

Question 29

what is passive spread of charge described as

Answer 29

the natural tendency for positive charges to move towards negative charges

Question 30

what happens on the outside of the axon

Answer 30

positive charge moves backward towards depolarised area, to balance/complete the circuit

Question 31

passive can occur in one or both directions, and typically lasts for how long?

Answer 31

both directions, and less than 1 uM

Question 32

speed of conduction in myelinated axons

Answer 32

20-100 m/s

Question 33

glial cells involved in myelination

Answer 33

oligodendrocytes CNS schwann cells PNS

Question 34

speed of conduction in myelinated axons

Answer 34

1 m/s

Question 35

how does myelination increase efficiency/speed of conduct

Answer 35

myelin is insulting, so prevents dissipation of passive current along the axon. therefore APs don’t need to be regenerated at every part of the membrane, rather are only generated at the nodes of ranvier.

Question 36

pattern of AP generation in myelinated vs unmyelinated axons

Answer 36

saltatory (in big leaps) vs continuous

Question 37

benefits and difficulties of myelination

Answer 37

less passive current loss, faster conduction, less energy required for maintenance of ion concentrations as Na+/K+ ATPase has to work less hard to regenerate concentrations but myelination requires space and energy

Question 38

what neurons are in the peripheral nervous system

Answer 38

afferent sensory neurons, and efferent motoneurons

Question 39

how is information about strength of stimulus conveyed by sensory neurons

Answer 39

strength/amplitude of the receptor potential, and frequency of APs

Question 40

what is the receptor potential

Answer 40

a graded depolarisation from non-voltage gated ion channels in response to chemical or mechanical stimulus, triggering an AP that spreads along the axon to the CNS

Question 41

what is the presynaptic terminal

Answer 41

the end of a transmitting axon, where the electrical signal (the action potential) is converted into a chemical signal (neurotransmitter release)

Question 42

describe synaptic transmission in 6 steps, starting with 1. AP travels down the...

Answer 42

1. ... presynaptic axon to the presynaptic terminal 2. AP triggers opening of Ca2+ channels 3. Ca2+ triggers release of neurotransmitters via exocytosis 4. neurotransmitters diffuse across the synaptic cleft and react with postsynaptic receptors 5. causing ligand-gated ion channels to open in postsynaptic neuron, causing a local depolarisation 6. AP is triggered in postsynaptic neuron

Question 43

what is a neuromuscular junction

Answer 43

the synapse where a motor neuron communicates with a muscle fibre

Question 44

what is the end plate potential (EPP)

Answer 44

the local depolarisation of a muscle fibre at the neuromuscular junction caused by release of Ach, and causing opening of non-selective cation channels, triggering an AP along the muscle fibre.

Question 45

what is an IPSP, incl ion channels involved and neurotransmitters

Answer 45

inhibitory postsynaptic potential: graded hyperpolarisation of the postsynaptic membrane, making it less excitable/likely to fire APs. caused by transient opening of K+ channels neurotransmitters usually GABA and glycine

Question 46

what is an EPSP, incl ion channels involved and neurotransmitters

Answer 46

excitatory postsynaptic potential: graded depolarisation of the postsynaptic membrane, making it more likely to fire APs. caused by transient opening of K+, Na+, sometimes Ca2+ neurotransmitters usually Ach and glutamate

Question 47

name and compare the two main classes of neurotransmitters

Answer 47

classical/small molecule: small act directly on channels in postsynaptic membranes action is fast (milliseconds) neuropeptides/neuromodulators: larger act indirectly/metabotropic modulatory action action is slower (seconds-mins_

Question 48

what is long term potentiation

Answer 48

increasing number of receptors/strength of synpase/sensitivity of neuron to stimulus caused by repeated use

Question 49

factors affecting speed of synaptic transmission

Answer 49

type of neurotransmitter transmitted type of neurotransmitter receptor/channel complex in postsynaptic membrane

Question 50

what is glutamate, what receptors does it have

Answer 50

classical neurotransmitter; primary excitatory neurotransmitter in CNS. has 3 receptors: AMPA - causes fast synaptic response in PS neuron NMDA - slow synaptic response in PS neuron Kainate - longer duration modulatory action

Question 51

what is long term depression

Answer 51

decrease in strength of synapse/number of receptors due to stimulation of a neuron frequently in certain way

Question 52

3 methods of neurotransmitter inactivation

Answer 52

1. diffusion of neurotransmitter away from synaptic cleft 2. enzymatic degradation in the synaptic cleft - e.g acetylcholine esterase 3. reuptake - involves specific neurotransmitter transporters in the presynaptic membrane. beneficial as doesn't require regeneration.

Question 53

what is the idea of 'integration of synaptic inputs'

Answer 53

each neuron forms thousands of synapses, all of which translate to very small PSPs at the axon hillock (0.1mV). neurons must integrate thousands of these through summation to fire an AP

Question 54

describe two types of summation for synaptic integration

Answer 54

temporal summation: neuron is stimulated with increased frequency in order to build up amplitude spatial summation: multiple presynaptic neurons stimulate one postsynaptic neuron in order to build up amplitude

Question 55

what is excitotoxicity

Answer 55

neurotoxic process in which excessive release of excitatory (glutamate) neurotransmitters/ excessive stimulation of the receptors leads to accumulation of Ca2+, activating enzymes leading to cell damage/death

Question 56

how does excitotoxicity damage the cell and cause cell death

Answer 56

mitochondrial dysfunction, oxidative stress from reactive oxidative species, apoptosis.