L6 Cell Physiology of Neurones Flashcards
What is an electrical synapse?
Faster Bidirectional Smaller gap (nearly touching) No plasticity (no learning) No amplification Always excitatory Used for defensive reflexes, retina and brain
What are problems with the fact electrical synapsed cannot amplify?
Signals are weakened when they are transmitted, signal will not transmit if the post (lots of charge) is much bigger than the presynaptic (only small signal) as it can’t depolarise to make an action potential
What is spatial summation?
Neurones determine whether to fire based on all the signals received from other synapsing neurones (convergent signalling). Small depolarisations can then reach threshold.
What is temporal summation?
A high frequency of signals plus a long recovery from depolarisation means a neurone is still slightly depolarised and then easier to then reach the threshold.
What happens during initial depolarisation (AP)?
Cell at rest (-70mv), inward rectifier K+ channels are open.
Stimuli (nearby cell depolarising or a synaptic transmission) then slightly depolarises the cell.
How does positive feedback amplify depolarisation (AP)?
Initial depol opens a few Na+ channels, Na+ permeability increases and Vm becomes more positive, increasing permeability (FB loop)
Once above the threshold of -50mv the cell is committed to the AP
What happens during repolarisation (AP)?
Voltage in the cell becomes more negative
Delayed action events occur: Na+ channels are inactivated and delayed rectifier K+ channels open
What happens during the refectory period (AP)?
Neurone is incapable of reinitiating an AP, membrane is returned to resting state
Follows after-hyperpolarisation
What happens during after-hyperpolarisation (AP)?
Voltage goes more negative that at rest, once below -60mv delayed rectifier K+ channels open and remain until the next depolarisation
Membrane moves closer to Ek (-90mv) as K+ permeability is high
How does firing frequency code for intensity?
It represents intensity of activity, increasing excitatory synaptic activity increases firing frequency.
What causes accommodation?
Multiple small synaptic currents cause a higher threshold potential making the cell harder to depolarise and trigger an AP.
What is accommodation?
The reduction of Na+ current due to multiple below threshold synaptic transmissions.
How do neurones code for intensity?
Firing frequency.
Different neurones for different strength stimuli (light touch receptors vs pain receptors).
What does excitability mean?
How easy to start nervous signalling, also called sensitivity in sensory cells and irritability in muscle or effector cells.
Increased threshold lowers excitability.
Highly excitable systems are at risk of seizure.
Why are changes in excitability important?
Basis of psychotropic pharmacology.
Changes in threshold have profound health and behavioural effects.
What is the difference between closed and inactivated channels?
Both are non conducting but inactivated are when a channels stops conducting when the membrane is positive inside.
A closed channel stops conducting when the membrane is negative.
How is membrane voltage described?
From the intracellular face with respect the extracellular face (inside the cell).
What is the extracellular space’s voltage?
It is the same everywhere.
Extracellular fluid is considered the electrical ground.
What happens when Ca2+ channels open?
The membrane becomes positive inside as there is 10,000x more Ca2+ outside the cell than in so Ca2+ passively moves in.
How do ionic permeabilises control voltage?
Increased K+ permeability makes the membrane more negative, Na+ more negative.
At rest Vm= Ek as conductance of K+ outstrips Na+ or Ca2+.
What is Lidocaine?
Local anaesthetic applied topically.
What is Carbamazepine?
Anticonvulsant (seizure prevention in epilepsy).
How does Lidocaine work (physiologically)?
Raises the threshold and lowers excitability which stops APs locally but blocking some Na+ channels in pain neurones.
How does Carbamazepine work (physiologically)?
Raises the threshold and lowers excitability.
What are other Na+ channel blocking drugs?
Antiarrhthymics (Class 1) ie quinidine.
Lower conduction velocity extending the refectory period.
What is a Na+ channel blocking poison?
Fugu fish (tetrodotoxin (TTX)) Blocks a large number of Na2+ channels.
What is chemical force?
Diffusional force.
Difference in concentration across a membrane.
Positive for K+ (moving out).
Negative for Na+ (moving in).
What is electrical force?
Membrane potential basis, varies over time.
Based on matching charges on either side of a membrane, does not take concentration into account.
No unpaired charges= no force.
What is the equilibrium potential?
Reversal potential.
Voltage of ion flowing matches that of the ion flowing out.
Chemical forces match electrical forces (one pushing in and the other out).
What equation is used to calculate the equilibrium potential?
Nernst equation.
What is the equilibrium potential of Na+?
+60mv
What is the equilibrium potential of K+?
-90mv.
What is the equilibrium potential of Ca2+?
+123mv.
What is the equilibrium potential of Cl- in epithelial cells?
-40mv.
What is the equilibrium potential of Cl- in neurones?
-65mv.
What is the equilibrium potential if both Na+ and K+ channels are equally permeable?
An average of both of their equilibrium potentials as the EP matches the EP of the most permeable ion (ie in neurone cells this is K+ and the EP is -70mv).
What defines an action potential?
Stereotyped electrical signal
Short duration
In most neurones, skeletal and cardiomyocytes.
A spike.
All or nonE
Require time to start because of conformational changes.
What is a graded potential?
Ordinary change in electrical potential, transmembrane electrical changes that do not have an AP.
They happen when ions cross membranes.
Where do you find graded potentials?
Everywhere but mostly receptor cells (rod and cone cells).
What defines a graded potential?
Electrically localised (detected only by nearby cells), last a long time, flatter in shape and are conducted almost instantly but lake longer to return to normal.
What sizes of graded potential are there?
3 sizes, varying in duration and size.
What is saltatory conduction?
AP is passively conducted down the axon by hopping between the nodes of ranvier, this occurs through summation and graded potentials.
Overall leaving a positive net charge (depolarising) as the charge moves along.
Why is saltatory conduction required to conduct voltage?
Voltage signals decrease further from the source due to natural resistance of the axon.
What do graded potentials transmit?
Action potentials along the length of the action as a way to re-amplify signal.
What is the net effect of saltatory conduction?
Faster conduction velocity, initiating an AP at the node is slower as ion channels must be conformationally changed (quicker than initiating an AP in each cell).
What increases conduction velocity?
Myelin sheath and large diametre (lower resistance).
What is the conduction velocity of a myelinated fibre compared to unmyelinated?
100m/s for alpha motor fibres (myelinated)
1m/s for C nociceptive fibres (unmyelinated)
How can nerve conduction studies be useful?
Evaluation of paraesthesias (numbness, tingling, burning) and weakness in the arms and legs.
