Foundation neuro Flashcards
Define action potential
depolarisation opens sodium, potassium or calcium channels gated by membrane voltage leading to temporary reversal of membrane polarity (inside positive)
What is a theshold stimulus
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)
What is a threshold potential
◦ Threshold potential is the transmembrane potential which must be achieved to produce depolarisation of the membrane, ~55 mV
What does the all or none ffect mean with reference to action potentials? How is information transmitted then?
- 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
What are the 3 states of Na channels?
Resting
Inactive
Open
Resting Na ionic gates refers to?
activation (m) gate closed, activation gate (h) open thus channel closed
Depolarisation occurs when what confirmational change in the sodium channel happens?
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
Why is there a time limited opening of Na channels
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
Once a sodium channel is closed what state is it in?
‣ 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
Why is the relative refractory period slightly longer than the absolute refractory period
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
How to calcium concentration affect muscles and nerves?
Hypocalcaemia increases nerve and muscle excitability
What effect does myelin have? What cell produces myelin
- 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
What is conductance
The ease at which ions move across the membrane (opposite of resistance)
What is capacitance?
- 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
Length or space constant is?
- 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
Properties of unmyelinated nerve action potentials
- 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
Define a synapse
Anatomical junction where nerves communicate with other nerves, muscles or glands
How wide is a synapse
20-30nm
What are the two types fo synapse
Electrical
Chemical
What is the rate limiting step of neurotransmission
‣ Rate limiting step is presynaptic release of transmitter
Define resting membrane potential
- The resting membrane potential is the voltage (charge) difference between the intracellular and extracellular fluid, when the cell is at rest
What is the nerve resting membrane potential
-86mV
What is the nernst equation solved for K
-94mV
What does resting membrane potential depend on? What is the most important?
Selective ion permeability (the most important)
How does extracellular K influence RMP
- 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.
What is required for a Gibbs Donnan equilibrium 3
- Diffusion and generation fo electrical potential gradient
◦ Requires semipermeable membrane, impermeable proteins and freely permeable ions
What factors play into a Nernst potential
- 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})
Whaty does the Nernst potential calculate
- Calculates the potential difference of any ion if themembrane was permeable to it:
What is the aggregate equation for RMP accounting for all ions
GHK - Goldman Hodgkin Katz
How is calcium removed from cells?
◦ 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
What is the Nernst potential for Na
+64mV
What is the Nernst potential for Cl
-70mV
What factors delay axonal conduction
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
how fast does Depolarisation of a cell occur
1msec
Key charactersitics of excitation and conduction in nerves 4
- RMP
- Trigger for action potential - the theshold potential, all or none effect
- Physiology of an action potential
- Saltatory conduction or depolarisation wave
- Refractory period
Cirucmventricular organs
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
