T1 L5:Physiology of Neurons

Question 1

give some features of electrical synapses

Answer 1

1- faster

2-bidirectional

3-much smaller gap=3.5nm

4- no plasticity

5- no amplification

Question 2

why can’t an electrical synapse be amplified?

Answer 2

1- Signal is always weakened as it is transmitted from pre-synaptic to post-synaptic cell

Question 3

what 2 thing can’t happen with the signal of an electrical synapse

Answer 3

1- Signal will not transmit if post-synaptic cell is much bigger than pre-synaptic cells

2- Excitatory pre-synaptic signal cannot inhibit the post-synaptic cell

Question 4

what is spatial summation

Answer 4

1- A neuron determines whether to fire based on the “add together” of all the tiny signals it is receiving from several other neurons synapsing on it (from both excitatory and inhibitory inputs). In this way small depolarisations (if there are many) can reach threshold
See diagram of convergent neuronal signalling

Question 5

what is temporal summation

Answer 5

When the input neuron is firing fast enough so that the receiving neuron can “add together” the many tiny signals, ultimately reaching threshold.
This happens when the receiving neuron’s ability to recover from the tiny input (depolarisation) is slow enough that the next signal arrives while the receiving neuron has not yet recovered from the previous signal (i.e. it is still slightly depolarized)

Question 6

look at AP outline in slide 7

Answer 6

• how was it

Question 7

describe an AP (4 marks)

Answer 7

At rest K+ that is going out of cell clamps the membrane potential negative (e.g. -70 mV)

An external factor (e.g. synaptic activity) causes the membrane to depolarise slightly. If the voltage reaches threshold, then

Na+ conductance shoots up, Na+ current goes into cell, membrane potential depolarises (voltage  +)

With a time-delay, Na+ conduction diminishes (inactivation), K+ conductance increases, so K+ leaves cell, voltage returns to resting potential (i.e. the membrane repolarises)

Question 8

describe how initially the neuron is depolarised

Answer 8

The cell starts at rest (-70 mV)

Inward rectifier K+ channels are open, K+ flowing out is the dominant current

Resting membrane potential is near EK

Something causes the cell to become less negative:

Depolarisation: inside the cell the voltage becomes less negative (or more positive)

Could be a nearby cell depolarising

Could be synaptic transmission where a neurotransmitter opens a ligand-gated channel

Question 9

what is the positive feedback loop of a depolarisation

Answer 9

-The initial depolarisation causes a few of the Na+ channels to open
Na+ permeability increases, Na+ current flows through channels into cell
The additional current of Na+ going into the cell  more depolarisation (ie the membrane potential moves closer to 0 mV)
This acts as a positive feedback loop
When the voltage goes above the threshold voltage (-50 mV), the cell is committed to an AP
APs are “all-or-none”.
The positive feedback of ↑ Na+ channel conductance and ↑ voltage continues until the membrane becomes quite positive (> +30 mV)
when Vm > 0, call this period the “overshoot”

Question 10

describe a refractory period

Answer 10

period of time during which neuron is incapable of reinitiating an AP,

the amount of time it takes for neuron’s membrane to be ready for a second stimulus once it returns to its resting state following an excitation

Refractory period occurs mostly during after-hyperpolarisation

Question 11

if APs are al or none (they carry no information about the size of the stimulus that elicited them
) how do they code for the intensity of their synaptic input

Answer 11

1- Firing frequency represents the intensity of activity:

• Increasing threshold lowers firing frequency (see figure)
• Increasing excitatory synaptic activity increases firing frequency
• When lengthy (>10 msec) synaptic currents are small, they create a higher threshold potential for action potential generation than larger currents do,
• This is due to accommodation of Na+ current (which inactivates during the slower subthreshold depolarization)

2-Different neurons for different strength stimuli:

-Light touch receptors vs. pain receptors (see sensory receptor lectures)

Question 12

what occurs with increased threshold

Answer 12

• a decrease in excitability

Question 13

what is threshold

Answer 13

• voltage above which action potential fires

Question 14

what is the basis for psychotropic pharmacology

Answer 14

• excitability changes

Question 15

describe why voltage-gated channels change states based on TM voltage

Answer 15

• These channels open when membrane becomes positive inside
• Channels in the open state can conduct = increased permeability
• Inward rectifiers are the opposite of other channels
• Channels close when membrane repolarises

Question 16

what’s the difference between an inactivated channel and a closed channel

Answer 16

Inactivated channel = when membrane is positive, channel tends to stop conducting.

Closed channel = when membrane is negative, channel tends to stop conducting.

Question 17

what does the voltage depend on

Answer 17

-

Question 18

at rest, Vm=

Answer 18

~Ek

Question 19

describe lidocaine

Answer 19

• local anaesthetic
• apply topically
• Raises the threshold
  ,And thus lowers excitability
  ,Which stops action potentials locally

-Lidocaine specifically blocks Na+ channels in the inactivated state

Question 20

describe carbamazepine

Answer 20

• anticonvulsant
• Carbamazepine inactivates sodium channels
• Raises AP threshold and Lowers excitability

-

Question 21

describe an antiarrhythmic drug

Answer 21

• quinidine
• works by lowering conduction velocity
• which extends the refractory period

Question 22

what’s the difference between the 2 forces on each ion

Answer 22

1- chemical force:

Also called diffusional force

Is based upon the difference in concentration across the membrane

E.g. If there is 10X as much Na+ outside than inside, the chemical force on Na+ channels is 60 mV directed into the cell

2- The electrical force
This is based on Vm (the membrane potential, which varies over time)

Question 23

look at slide 24

Answer 23

-how was it

Question 24

what is the Ek-

Answer 24

The Equilibrium Potential

Question 25

describe the Equilibrium Potential

Answer 25

EK is also called the reversal potential of K+.

• EK is the voltage where K+ flowing out = K+ flowing in because electrochemical forces on K+ are in equilibrium
• This occurs when the diffusion (chemical) forces pushing K+ out of the cell equal the voltage (electrical) forces pushing K+ into the cell

Question 26

give some common equilibrium potentials

Answer 26

ENa = +60 mV

EK = -90 mV

ECa = +123 mV

ECl = -40 mV (in neurons –65 mV)

Question 27

give a mnemonic for equilibrium potentials

Answer 27

A CAt is as easy as 1-2-3

A NAg retires early at 60.

A CLub for fighting Ali Baba’s 40 thieves

A King will die at 90.

Question 28

what is the difference between graded action potentials and action potentials

Answer 28

AP:
- A Stereotyped electrical signal

• Short-duration
• in most neurons, skeletal and cardiomyocytes
• Always the same — “All or none”
• Require time to start because of conformational changes

Electrically localised

Last a long time

much Flatter in shape

Are conducted almost instantly

in receptor cells (eg rods & cones)

variable -in duration and voltage

Question 29

describe a graded potential

Answer 29

• Changes in membrane potential do not propagate very far via passive electrical forces

Voltage signals diminish as you go farther from the source
This happens because the axon has a finite resistance

For mV changes of cells and APs, the change would be all but extinguished in 1 cm

Electrotonic conduction (graded potentials) transmits signal along the length of the axon.
The AP is a way for re-amplifying that signal

Question 30

describe saltatory conduction

Answer 30

• When the action potential “jumps” from Node to Node
• Net effect = faster conduction velocity
• The electrotonic jumps between nodes are very fast.

-Initiating an Action Potential at each node is slower
Conformational change of ion channels

Question 31

what factors affect conduction velocity and how

Answer 31

faster when:

• axon is myelinated
• larger diameter of axon

Question 32

what are the typical conduction velocities for alpha and c neurons

Answer 32

• Alpha motor fibres: 100 m/s for myelinated, 15 um diam

- for C nociceptive fibres :1 m/s for unmyelinated, 0.2 - 1.5 um