Foundation neuro

Question 1

Define action potential

Answer 1

depolarisation opens sodium, potassium or calcium channels gated by membrane voltage leading to temporary reversal of membrane polarity (inside positive)

Question 2

What is a theshold stimulus

Answer 2

the minimum required inward Na current to overcome the K+ outward current and meet the depolarisation level required to initiate an action potential (10-15mV)

Question 3

What is a threshold potential

Answer 3

◦ Threshold potential is the transmembrane potential which must be achieved to produce depolarisation of the membrane, ~55 mV

Question 4

What does the all or none ffect mean with reference to action potentials? How is information transmitted then?

Answer 4

• All-or-none effect is the finding that a subthreshold stimulus will produce no response, whereas all suprathreshold stimuli will produce an identical and maximal response.
• INformation signal depends on frequency not size of action potentials

Question 5

What are the 3 states of Na channels?

Answer 5

Resting
Inactive
Open

Question 6

Resting Na ionic gates refers to?

Answer 6

activation (m) gate closed, activation gate (h) open thus channel closed

Question 7

Depolarisation occurs when what confirmational change in the sodium channel happens?

Answer 7

m gate opens allowing Na entry; h gate begins to close slowly allowing brief period of entry (~<1-20 milliseconds) - sodium moves down electrochemical gradient and thus the cell moves towards the sodium equilibrium potential

Question 8

Why is there a time limited opening of Na channels

Answer 8

m gate opens allowing Na entry; h gate begins to close slowly allowing brief period of entry (~<1-20 milliseconds) - sodium moves down electrochemical gradient and thus the cell moves towards the sodium equilibrium potential

Question 9

Once a sodium channel is closed what state is it in?

Answer 9

‣ Inactivated - h gate closed, persists through repolarisation, returns to resting state when repolarisation complete due to negative membrane potential. Full recovery of the (h) gates takes 100ms longer or even longer after membrane potential restored
* It is notable that this slightly prolonged opening of the h gates contributes to an absolute refractory period
* The relative refractory period extends for 10-15msec longer due to hyperpolarisation depicted below

Question 10

Why is the relative refractory period slightly longer than the absolute refractory period

Answer 10

Slight hyperpolarisation occurs in repolarisation after which sodium gates may be open BUT a suprathreshold stimulus needed to overcome this slight hyperpolariasation
‣ Inactivated - h gate closed, persists through repolarisation, returns to resting state when repolarisation complete due to negative membrane potential. Full recovery of the (h) gates takes 100ms longer or even longer after membrane potential restored
* It is notable that this slightly prolonged opening of the h gates contributes to an absolute refractory period
* The relative refractory period extends for 10-15msec longer due to hyperpolarisation depicted below

Question 11

How to calcium concentration affect muscles and nerves?

Answer 11

Hypocalcaemia increases nerve and muscle excitability

Question 12

What effect does myelin have? What cell produces myelin

Answer 12

• Support cells called glia in the brain and spinal cord (Schwann cells) wrap cell membranes around axons acting as an insulator and increased resistance, reduces the capacitance of the membrane allowing action potentials to only really fire at nodes
  ◦ Permits saltatory conduction with propogation from one node to another by local circuits

Question 13

What is conductance

Answer 13

The ease at which ions move across the membrane (opposite of resistance)

Question 14

What is capacitance?

Answer 14

• Capacitance = the ability of the membrane to store electrical charge; and the capacitance fo the a nerve fibre is due to the two layered lipid matrix
  ‣ Amount of charge is proportional to surface area I.e. large diameter holds more charge
  ‣ Additionally capacitance varies with axon diametre - thicker axon means more capacitance, but axial resistance falls with increasing thickness

Question 15

Length or space constant is?

Answer 15

• The decay of electronic potential per unit length along the axon is given by the length or space constant
  ◦ Defined as the distance from the stimulus to the point at which voltage falls to 36.7% of maximal potential

Question 16

Properties of unmyelinated nerve action potentials

Answer 16

• Moves in a wave of changing permeability —> local current of positive charge moving along the cell
• Refractory period prevents retrograde flow and restricts frequency
• The larger the nerve the faster the velocity as the reduced resistance to longitudinal current flow

Question 17

Define a synapse

Answer 17

Anatomical junction where nerves communicate with other nerves, muscles or glands

Question 18

How wide is a synapse

Answer 18

20-30nm

Question 19

What are the two types fo synapse

Answer 19

Electrical
Chemical

Question 20

What is the rate limiting step of neurotransmission

Answer 20

‣ Rate limiting step is presynaptic release of transmitter

Question 21

Define resting membrane potential

Answer 21

• The resting membrane potential is the voltage (charge) difference between the intracellular and extracellular fluid, when the cell is at rest

Question 22

What is the nerve resting membrane potential

Answer 22

-86mV

Question 23

What is the nernst equation solved for K

Answer 23

-94mV

Question 24

What does resting membrane potential depend on? What is the most important?

Answer 24

Selective ion permeability (the most important)

Question 25

How does extracellular K influence RMP

Answer 25

• K concentration is 30 fold increased in the cell and there is some degree of perameability so if K concentration is lower outside the cell (hypokalaemia) it results in increased K leak and hyperpolarisation.

Question 26

What is required for a Gibbs Donnan equilibrium 3

Answer 26

• Diffusion and generation fo electrical potential gradient
  ◦ Requires semipermeable membrane, impermeable proteins and freely permeable ions

Question 27

What factors play into a Nernst potential

Answer 27

• Where:
  ◦ E = equilibrium potential for the particular ion, and nernst potential
  ◦ R = universal gas constant
  ◦ T = temperature
  ◦ n = valence of the ion
  ◦ F = Faraday’s constant
  ◦ lnQ = ln ({ion concentration inside}/{ion concentration outside})

Question 28

Whaty does the Nernst potential calculate

Answer 28

• Calculates the potential difference of any ion if themembrane was permeable to it:

Question 29

What is the aggregate equation for RMP accounting for all ions

Answer 29

GHK - Goldman Hodgkin Katz

Question 30

How is calcium removed from cells?

Answer 30

◦ Membrane bound low internal calcium concentration pump: active transport Ca-ATPase activcated by calmodulin. Requires 1 x ATP for every calcium extruded
◦ Na/Ca exchanger - 1x calcium for 3 external sodium
‣ Electrogenic
‣ Active at higher intracellular calcium concentrations
‣ Secondary active transport

Question 31

What is the Nernst potential for Na

Answer 31

+64mV

Question 32

What is the Nernst potential for Cl

Answer 32

-70mV

Question 33

What factors delay axonal conduction

Answer 33

Factors which affect neuronal conduction include:
* Myelination: (myelinated fibres conduct faster) through saltatory conduction (reduced capacitance)
* Thickness of the fibre: the thicker the fibre, the faster the conduction due to reduced capacitance
* Properties of the membrane: capacitance and resistance (the lower they are, the faster the conduction)
* Properties of the extra-axonal environment (eg. electrolyte derangement): all decrease the velocity (and sometimes amplitude) of nerve conduction
◦ Hyponatremia - Na gradient is what drives depolarisation, rate of voltage change lower
◦ Hypocalcaemia - decreases amplitude of action potential, but velocity unchanged
◦ Hypermagnesemia
◦ Acidosis - alters Na flow through Na voltage gated channels
◦ Hypothermia

Question 34

how fast does Depolarisation of a cell occur

Answer 34

1msec

Question 35

Key charactersitics of excitation and conduction in nerves 4

Answer 35

1. RMP
2. Trigger for action potential - the theshold potential, all or none effect
3. Physiology of an action potential
4. Saltatory conduction or depolarisation wave
5. Refractory period

Question 36

Cirucmventricular organs

Answer 36

Any CNS structure outside the BBB

Sensoing structures e.g.
1. chemoreceptor trigger zone (identies toxins in systemic circulatiuon)
2. Hypothalamus - osmorecpeotrs
3. Subfornical organ - CVS and fluid balance
4. Organum vasculosum

Secreting substances
1. Pituitary
2. Pineal gland
3. Choroid plexus 0 CSF