Potentials and gating Flashcards
Action potential
a nerve cell action potential is a reversal of trans-membrane voltage that is completed in 2-3ms
ion channels
regulators of changes in membrane permeability. changes in permeability to specific ions are due to the open or closed status of specific ion channels, opening/closing = gating
movement of ions causes…
current flow and change in transmembrane voltage
Ion channel gating:
chemical
located on the cell body and dendrites of a neuron, where the neuron receives chemical signals
Ion channel gating:
voltage
axon and axon terminals
Ion channel gating:
mechanical
exists?
local potentials (4 facts)
- opening of ion channels in region of receipt of chemical signal results in a local potential
- release of the chemical from the nerve terminal causes chemical signals to interact with the next cell in the chain (post-synaptic cell) which causes a change in voltage in the post-synaptic cell
- can be inhibitory or excitatory
- not actively propagated
- magnitude of the local potential decreases with distance away from the site of it’s initiation
Summation
the effect of local potentials on cell membrane potential is summed over both time (temporal summation) and space (spatial summation)
Summation process:
Facilitation
- membrane potential sits at -70mV (resting potential)
- chemical stimulus open sodium ion channels
- depolarisation occurs and the membrane potential raises to around -60mV
- repolarisation occurs = stimulus is removed and the excess sodium ions are transported out of the cytosol. membrane potential returns to -70mV
Summation process:
Inhibited
- chemical stimulus opens potassium ion channels
- hyperpolarisation occurs and the membrane potential lowers to around -80mV
- chemical stimulus is removed and the membrane potential raises back up to the resting potential (-70mV)
Summation process:
summation
Both stimuli applied at the same time, summation occurs, stimuli removed
Initial segment:
axon hillock
- the point where the axon joins the neuron cell body
- where an action potential is generated
- high density of voltage-gated Na channels
For an action potential to be generated:
the net voltage change at the axon hillock (summed local potentials) must exceed a minimal depolarisation (typically ~10mV), the threshold
Threshold
depolarising local potentials may result in opening of voltage-gated Na channels. Na channel opening drives further depolarisation. If sufficient Na channels open (at the initial segment) the depolarisation reaches a point at which large numbers of channels open resulting in a sudden large increase in Na influx - threshold.
Action potentials: refractory periods
Absolute refractory period
no matter how large the stimulus, another AP cannot be generated
Action potentials: refractory periods
Relative refractory period
an AP can be generated, but only in response to a very large stimulus
Propagation occurs in…
Unmyelinated axons
myelinated axons
Propagation:
stage 1
As an action potential develops at the initial segment, the membrane potential at this site depolarises to +30mV
Propagation:
stage 2
As the sodium ions entering at spreadt away from the open voltage-gated channels, a graded depolarisation quickly brings the membrane in segment 2 to threshold
Propagation:
stage 3
An action potential develops in segment 2. the initial segment begins repolarisation (and is now refractory)
Propagation:
stage 4
As the sodium ions entering at segment 2 spread laterally, a graded depolarisation quickly brings the membrane in segment 3 to threshold. The action potential can only move forward, not backward, because the membrane at the initial segment is in the absolute refractory period of repolarisation
Conduction velocity (4 facts)
- AP’s are transmitted along unmyelinated axons relatively slowly - approximately 1-5 m/s
- Given our large bodies and need for long axons (e.g. distance from lower motoneuron in spinal cord to muscles in foot is often > 1m), slow AP conduction velocity is not adequate for all our needs
- Nature has evolved an efficient way of dramatically increasing AP conduction velocity along many of our axons - myelin
- in myelinated axons, AP conduction velocity is typically in the range of 20-100 m/s
Synaptic transmission:
step 1
Action potential triggers the opening of voltage-gated calcium channels
Synaptic transmission:
step 2
Calcium ions diffuse into the axon terminal and trigger synaptic vesicles to release ACh by exocytosis
Synaptic transmission:
step 3
ACh diffuses across synaptic cleft, binds to ACh-gated sodium ion channels, and produces a graded depolarisation
Synaptic transmission:
step 4
Depolarisation ends as ACh is broken down into acetate and choline by AChE (enzyme)
Synaptic transmission:
step 5
The axon terminal reabsorbs choline from the synaptic cleft and uses it to synthesize new molecules of ACh
Electrical Synapse
Occur when pre- and postsynaptic cell membranes sit very close together and are joined by gap junctions. An electrical event in the presynaptic cell leads to an electrical event in the postsynaptic cell without the involvement of chemical signals. Cannot produce an AP in the postsynaptic cell.
EPSP
Excitatory post synaptic potential
IPSP
Inhibitory post synaptic potential
Postsynaptic potential summation:
Temporal
Summation of potentials over time.
An AP arrives, causes a postsynaptic response that doesn’t reach threshold but before it decays away another signal arrives and brings the cell to threshold and the cell fires an AP in the postsynaptic cell.
Postsynaptic potential summation:
Spatial
Summation of potentials in a certain space.
When two AP’s arrive at the same time anad are close enough, their voltage adds together and is therefore closer to threshold, increasing the likelihood of another AP being fired in the postsynaptic cell. If the combined voltage reaches threshold, an AP will be fired.
Nerve-nerve junctions (4 facts)
- Synapses are tiny, each synapse may be one of thousands on the post-synaptic cell
- AP in an individual neuron will rarely bring the post-synaptic cell to threshold
- Inputs may be excitatory or inhibitory (EPSP’s & IPSP’s), many transmitters used
- AP may result at axon hillock if depolarisation is strong enough at that point
Nerve-muscular junctions (4 facts)
- Synapses are huge, each muscle fibre receives input from only one neuron at one site
- AP in a neuron is very likely to bring muscle fibre to threshold
- Only excitatory inputs (no inhibition or IPSP’s), only ACh used
- AP initiated at nerve-muscle junction
