T1 L5 Physiology of neurons Flashcards
Describe an electrical synapse
Faster Bidirectional Smaller gap = 3.5nm No plasticity No amplification
What is spatial summation
Neuron determines whether to fire based on add-together of all the tiny signals from several neutrons synapsing on it.
What is temporal summation?
One input neuron is firing fast enough so the receiving neuron can add together the many tiny signals to reach threshold
What allows temporal summation to occur?
Receiving neuron must recover slowly from the previous signal
Briefly describe the 4 stages of action potential
1) At rest, K+ going out of the cell clamps the membrane negative
2) External factor causes the membrane to depolarise slightly
3) Na+ conductance shoots, Na+ current goes into cell, membrane depolarises
4) After a time delay, Na+ conduction diminishes, K+ conductance increases, K+ leaves cell & voltage returns to resting potential
What happens during initial depolarisation?
Inward rectifier K+ channels are open
Something causes cell to become less negative
State the 4 stages of the action potential
Initial depolarisation Positive feedback of depolarisation Repolarisation Refractory period After-hyperpolarisation
What can cause the cell to become less negative
A nearby cell depolarising
Synaptic transmission where a neurotransmitter opens a ligand-gated channel
What happens during positive feedback of depolarisation?
Initial depolarisation causes a few Na+ channels to open
Additional current of Na+ going into cell –> more depolarisation
Acts as a positive feedback loop
What happens during repolarisation?
Na+ channel inactivation –> decreased Na+ current going in
Delayed rectifier K+ channels open –> increases K+ going out
What is the refractory period?
The period of time where a neuron is unable to reinitiate an action potential
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
What is the definition of after-hyperpolarisation?
At the end of an action potential the voltage inside temporarily goes slightly more negative than at rest followed by a return to resting membrane potential
What happens during after hyperpolarisation?
Inward rectifier K+ open again
The K+ permeability increases
Decrease in Na+ permeability
Membrane potential moves closer to Ek
How do neutrons code the intensity of their synaptic input?
Firing frequency
Different neurons for different strength stimuli
Describe how firing frequency represents the intensity of activity
Increasing the threshold lowers firing frequency
Increasing excitatory synaptic activity increases firing frequency
Small, length synaptic currents create a higher threshold potential for AP generation than larger currents
What is excitability?
How easy it is to start nervous signalling
What does a high excitability increase the risk of?
Seizures or spasms
What happens to membrane potential when Na+ channels open?
The membrane becomes positive inside as Na+ concentration is higher outside the cell
What happens to membrane potential when K+ open?
The membrane becomes more negative inside
K+ travel from inside to outside
What happens to membrane potential when Ca2+ channels open?
Membrane becomes more positive inside
Calcium passively goes inward
What is lidocaine?
Lignocaine
Local anaesthetic
How does lidocaine work?
Blocks Na+ channels, which raises threshold & lowers excitability to locally stop action potentials
What is carbamazepine?
Anticonvulsant
How does carbamazepine work?
Inactivates sodium channels
Raises action potential threshold & lowers excitability
What are the 2 forces on an ion?
Chemical force
Electrical force
What is chemical force?
Also known as diffusional force
Based upon the difference in concentration across the membrane
What is electrical force?
Based on Vm
Affects every ion in that area
What is used to calculate 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 Ca?
+123mV
What is the equilibrium potential of Cl?
-40mV
What happens to the membrane potential if the membrane is equally permeable to Na+ & K+ ?
The membrane potentials will be the average between their equilibrium potentials
-15mV
Describe the properties of an action potential
Stereotyped electrical signal
Short duration
In most neurons, skeletal & cardiomyocytes
Spike
Always the same - ‘‘all or none’’
Require time to start because of conformational changes
Describe the properties of a graded potential
Electrically localised
Last a long time
Much flatter in shape
Conducted almost instantly
What is saltatory conduction?
When the action potential jumps from node to node
What is the net effect of saltatory conduction?
Faster conduction velocity
What is the typical conduction velocity for alpha motor fibres (myelinated, 15um diameter)?
100m/s
What is the typical conduction velocity for C nociceptive fibres (unmyelinated, 0.2-1.5um)?
1m/s
What are the clinical uses of conduction velocity?
Nerve conduction studies to evaluate paresthesias
Evaluation of weakness of the arms & legs
What conditions can conduction velocity diagnose?
Peripheral neuropathy Carpal tunnel syndrome Ulnar neuropathy Guillain-Barre syndrome Spinal disc herniation
