The excitable cell Flashcards
What is Eion?
- The equilibrium potential of a particular ion
- The membrane potential that would be achieved if the membrane was selectively permeable to that ion
How is resistance affected by cross sectional area?
Resistance is inversely proportional to CSA
R = 1/ CSA
What is temporal summation?
- Multiple inputs from the same neuron in a very short space of time
- Increases the chance of an action potential occurring
What happens during to falling phase of an action potential?
- Voltage gated K+ channels open
- K+ out of membrane via electrochemical gradient
- Na+ voltage gated channels close
- Vm –> Ek
What is IDF?
- Ionic driving force
- Rate at which ions travel from one side of the membrane to the other
- Proportional to Vm - Eion
What is the ‘space constant’?
- Distance from the site of depolaristion where the strength of depolarisaion is 37%
- Is proportional to membrane resistance / internal resistance
Where are Na+ channels localised?
- More concentrated at the nodes of Ranvier
How does the Na+ channel become inactivated?
~1ms after activation
- Globular portion swings up and occludes the pore
- Must be ‘reset’ to resting potential -65mV before another action potential can occur
What is undershoot?
Vm lower than normal
- Vm ~ Ek
- Due to delayed and rectifying voltage gated K+ channel
- Harder to fire action potential again
What is conduction velocity?
The speed of action potential propagation
Describe the properties of graded potentials
- LOCALISED changes in the membrane potential
- Any variable size
- Coded for by amplitude, which varies depending upon the stimulus
- Usually occur in the dendrites
- Pass both ways along the axon, fading from the site of origin
What is the Eion of K+?
-80mV
What 2 things is conduction velocity affected by?
- Diameter
- Myelination
What is the ‘equilibrium potential’?
- When diffusion = electrical force
- No net movement of ions
What is needed for a functioning nervous system?
- A negative membrane potential (-65mV)
- Negative inside (less Na/more K)
- Positive outside (more Na/less K)
Describe the properties of action potentials
- All or nothing
- Has ‘spikes’ which are FIXED in size and duration
- Doesn’t change when propagated
- Travels down the axon of a neuron
- In one direction
- Coded for by frequency
What does the Goldmann equation calculate?
- True Vm
Vm is not the same as the Eion of K+ as the membrane is selectively permeable to other ions, which change the Vm
What is the Eion of Na+?
+62mV
What can’t action potentials occur in dendrites?
Not a high enough number of Na+ channels
What are the costs of myelination?
- Metabolic cost
- Volume cost
What is the structure of the Na+ voltage gated channel?
- 4 domains, each with 6 transmembrane domains
- S4 domain has a voltage sensor, where a change in voltage causes a confirmational change activating the channel
What is the threshold of Na+ channels opening?
-40mV
What is the Nerst equation?
Eion = 61.54 x log ([K+]o / [K+]i)
What threshold is reached, what happens to the permeability of Na+ and K+
- They are switched
- Condunctance of Na+ is higher than K+
What 2 things does the movement of ions through their channels depend upon?
- Concentration gradient
- Difference in electrical potential
What does myelination acheive?
- Increases the resistance of the membrane, preventing current loss
- Increases space constant
What is spatial summation?
- Multiple potentials are received onto the post synaptic membrane at the same time
- From different areas of the body
- Increases chance of action potential occurring at higher EPSP felt
What is the Goldmann equation?
Vm = 61.54 x log ( (Pk[K+]o + Pna [Na+]o) / (Pk[K+]i + Pna[Na+]i) )
Where P is the permeability of that ion
Where do action potentials occur?
- At the axon initial segment (where the highest density of Na+ channels are)
- In pyrimidal, cells this is at the cell body
- In sensory neurons, this is at the nerve ending
What is an electrical synapse?
- Gap junctions
- Excitation can spread from one neuron to the other without a neurotransmitter
- Works both ways
- Rapid
- Rare
What happens during to rising phase of an action potential?
Vm –> Ena
- Rapid depolarisation
What does the Nerst equation calculate?
Eion
