Physiology of neurons Flashcards
Describe electrical synapses
Faster Bidirectional Much smaller gap No plasticity No amplification Coupled via gap junctions Always excitatory
Where are electrical synapses used?
Used for defensive reflexes
Retina and brain
What is meant by no amplification in the electrical synapse?
Signal is always weakened as it is transmitted from pre-synaptic to post-synaptic cell
Signal will not transmit if post-synaptic cell is much bigger than pre-synaptic cells
Excitatory pre-synaptic signal cannot inhibit the post-synaptic cell
Define spatial summation
A neuron determines whether to fire based on the “add together” of all the tiny signals it is receiving from several other neurons synapsing on it (from both excitatory and inhibitory inputs). In this way small depolarisations (if there are many) can reach threshold
Define temporal summation
When the input neuron is firing fast enough so that the receiving neuron can “add together” the many tiny signals, ultimately reaching threshold.
This happens when the receiving neuron’s ability to recover from the tiny input (depolarisation) is slow enough that the next signal arrives while the receiving neuron has not yet recovered from the previous signal (i.e. it is still slightly depolarized)
Describe the action potential
At rest K+ that is going out of cell clamps the membrane potential negative (e.g. -70 mV)
An external factor (e.g. synaptic activity) causes the membrane to depolarise slightly. If the voltage reaches threshold, then
Na+ conductance shoots up, Na+ current goes into cell, membrane potential depolarises (voltage +)
With a time-delay, Na+ conduction diminishes (inactivation), K+ conductance increases, so K+ leaves cell, voltage returns to resting potential (i.e. the membrane repolarises)
Describe the initial depolarisation of the action potential
Starts at -70mV
Clamped by inward rectifier K+ channels, K+ flows out (dominant current)
Resting membrane potential is near Ek
Something causes the cell to become less negative. Depolarisation caused by a nearby cell depolarising
or synaptic transmission where a neurotransmitter opens a ligand-gated channel
Describe the depolarisation phase of the action potential
The initial depolarisation causes a few of the Na+ channels to open
Na+ permeability increases, Na+ current flows through channels into cell
The additional current of Na+ going into the cell, more depolarisation (ie the membrane potential moves closer to 0 mV)
This acts as a positive feedback loop
When the voltage goes above the threshold voltage (-50 mV), the cell is committed to an AP
APs are “all-or-none”.
The positive feedback of ↑ Na+ channel conductance and ↑ voltage continues until the membrane becomes quite positive (> +30 mV)
when Vm > 0, call this period the “overshoot”
Describe the repolarisation phase of the action potential
Repolarisation = the voltage becomes less positive (or more negative) inside the cell
Due to the passage of time, 2 delayed-action events occur
Na+ channel inactivation causes ↓ Na+ current going in
Delayed rectifier K+ channels open and ↑ K+ going out
These cause the membrane to be less positive and more negative inside
Define refractory period
period of time during which neuron is incapable of initiating an AP,
the 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
When does the refractory period occur?
Refractory period occurs mostly during after-hyperpolarization
Describe after hyperpolarization
After-hyperpolarisation (AHP) = at the end of an AP the voltage inside temporarily goes slightly more negative than at rest, followed by a return to the resting membrane potential
When the voltage goes below -60 mV, the inward rectifier K+ channels open again; they stay open until next depolarisation
These normally clamp the voltage toward EK, and are responsible for maintaining the resting membrane potential
During AHP: the ↑ K+ permeability and ↓ Na+ permeability and the membrane potential moves closer to EK
How do neurons code the intensity of their synaptic input?
Firing frequency represents the intensity of activity
Different neurons for different strength stimuli
How can firing frequency be increased?
Increasing excitatory synaptic activity
How can firing frequency be decreased?
Increasing threshold
When lengthy (>10 msec) synaptic currents are small, they create a higher threshold potential for action potential generation than larger currents do,
This is due to accommodation of Na+ current (which inactivates during the slower subthreshold depolarization)
Define excitability
How easy to start nervous signalling
What is the threshold?
voltage above which action potential fires
When do voltage gated channels open?
When the membrane becomes positive inside
When do channels close?
When membrane repolarizes
What is meant by inactivated?
Not the same thing as a closed channel
When a channel stops conducting when the membrane is positive inside
Why does the membrane become positive inside when Na+ channels are open?
Na+ is higher outside the cell than in
Why does the membrane become negative inside when K+ channels are open?
K+ ions travel from inside to outside
Due to the chemical gradient
Because [K+] is higher inside than outside the cell
This exit of K+ causes the membrane to become negative inside
Why does the membrane become positive inside when Ca2+ channels are open?
[Ca2+] is higher outside than inside
Calcium passively goes inward
At rest Vm is equal to what?
Ek because conductance of K+ is greater than conductance of Na+ or Ca 2+
What is lidocaine/lignocaine?
Local anaesthetic
Apply topically
Raises the threshold and thus lowers excitability which stops action potentials locally
Specifically blocks Na+ channels in the inactivated state in pain neurons
What is carbamazepine?
Anticonvulsant
Inactivates Na+ channels
Raises threshold and lowers excitability
How do antiarrhythmic drugs such as quinidine work?
Lowers conduction velocity which extends the refractory period
What two forces are present on each neuron?
The chemical force
The electrical force
What is the chemical force?
Also called diffusional force
Is based upon the difference in concentration across the membrane
What is the diffusional force?
This is based on Vm (the membrane potential, which varies over time)
Based on net charges
Where is K+ always high?
Inside the cell
What is an equilibrium potential?
Reversal potential of K+
Voltage where K+ flowing out = K+ flowing in because electrochemical forces on K+ are in equilibrium
This occurs when the diffusion (chemical) forces pushing K+ out of the cell equal the voltage (electrical) forces pushing K+ into the cell
What happens to membrane potential when a cell becomes more permeable to K+?
Approaches the value of Ek
What is the equilibrium potential of Na?
+ 60mV
What is the equilibrium potential of K?
- 90 mV
What is the equilibrium potential of Ca?
+ 123 mV
What is the equilibrium potential of Cl?
- 40 mV
What happens to membrane potential if equal permeability of Na+ and K+ occurs?
An average between the two equilibrium potentials
i.e. -15 mV
Describe action potential
A stereotyped electrical signal
Short duration
In most neurons, skeletal and cardiomyocytes
A spike
Always the same- all or none
Requires time to start because of conformational changes
Describe graded potentials
Decrease as they move along
Electrically localised
Last a long time
much Flatter in shape
Are conducted almost instantly
in receptor cells (eg rods & cones)
Variable in duration and voltage
Why do voltage signals diminish as you go farther from the source?
Axon has finite resistance
What is saltatory conduction?
When the action potential “jumps” from Node to Node
Net effect = faster conduction velocity
The electrotonic jumps between nodes are very fast.
Initiating an Action Potential at each node is slower
Conformational change of ion channels
When is the conduction velocity fastest?
Myelinated
Large diameter
What is the typical conduction velocity for alpha motor fibres?
100m/s
What is the typical conduction velocity for C noiceptive fibres?
1m/s
Give the clinical uses of conduction velocity
Nerve conduction studies are used for evaluation of paraesthesias
numbness, tingling, burning
Evaluation of weakness of the arms and legs
