Flashcards in Lecture 21: Electrical Signalling In Nerves Deck (11):
1) Electrical signalling in excitable cells
-all cells have a resting membrane potential
-neurons and muscle cells are excitable cells
-excitable cells transmit electrical signals rapidly
-excitable cells transmit electrical signals by changing their membrane potential
Open/closing of ion channels-> change in number or type if ions moving into or out of the cell -> change in membrane potential
1) Electrical signalling in excitable cells
Leak and gated ion channels
-Leak channels are always open
-gated ion channels only open in response to a stimulus
-classes of gated ion channels are defined by their stimulus that controls their opening:
A) ligand-gated (chemical)
B) voltage-gated (Vm change)
C) mechanically gated (pressure, stretch)
Describing changes in membrane potential:
Changes in membrane potential (Vm) are describe relative to resting membrane potential.
Anything more positive than RMP is called depolarisation
Repolorisation is the membrane potential returning to the RMP
(Any part were it is coming back to resting membrane potential)
Hyper polarisation is when it is less than the resting membrane potential ie more negative
Electrical signalling in the nervous system:
Neurons have two main types of electrical signals, what are they?
The site of communication between two or more neurons is synapse. Can be electrical or chemical signal
Graded potentials: are input signals (generally on the dendrites from other neuron)
Action potential: are output signal which is generated in the axon hillock
-both types of signals involve opening or closing ion channels
Graded and action potentials have two types of current flow:
What are they and give some details for each?
1. Active current flow:
-flow of current down electrochemical gradient through gated ion channels
-voltage gated Na+ and K+ channels most important contributors to active current flow during AP
2. Passive current flows:
-often occurs after you have had some change due to active current flow.
-flow of current down electrochemical gradient through cytoplasm or leak channels
-active current flow through an ion channel sets up conditions for passive current flow: two regions of different charge (potential difference) connected by a conductor
Graded potentials in neurons:
Small potentials that occur in response to a stimulus acting on the cell
-usually occur in dendrites or soma
-ions moving through gated channels at the stimulus site produce a graded potential (change in Vm) generated a potential.
Ie sodium gated- as they move in the charge will become more positive which is called the graded potential.
-as the gated channels are only opened at the stimulus site, graded potentials can't be topped up' and therefor decay over short distances of time
Called graded because they change on a number of different factors for example how far away the stimulus is.
Spread of graded potentials by passive current flow
Why? Because graded potentials are spread entirely by passive current flow. We have a stimulus on neuron which opens an ion channel and you get active current flow through that ion channel to create a region of positive charge.
It has been cause by a transmitter binding to a receptor here, that transmitter is only being released in a certain region, so is only opening an ion channel here, the others remain close. So no active current flow through those ion channels. What that means is that we will get passive current flow of charge through to the rest of the neuron. But you loose current as that current moves away from the stimulus site. Some current will leak out through leaked channels.
-passive current flow is very fast but it decays within a short distance.
Properties of graded potentials:
-graded potential amplitude (amount?) depends on how big the stimulus is.
Ie of you get a stimulus of say Na were a large amount of molecules will bind to many receptors, so you will get sodium ions moving through all those different channels. The graded potential will therefore be big.
-graded potentials can add together (summary) ie if we get one bit of stimulus which opens a few ions channels and soon after another stimulus comes along and opens a few more ion channels, then you will get more sodium coming into cell thus greater graded potential.
-graded potentials are small approx 1mV
The transition of graded potentials to action potentials
Graded potential can trigger an action potential if that graded potential is big enough. (Must reach threshold)
1) stimulus opens your voltage gated sodium channels and sodium flows into the cell, producing a build up of positive charges and produces a graded potential.
2) depolarisation generated by graded potential opens voltage-gated sodium channels
3) which leads to sodium entry, more depolarisation
4) and opening of more voltage-gated sodium channels
What happens to the voltage gated sodium channels when threshold is reached? 3 things?
1) it has 2 different gates, activation gate and inactivation gate. Both these gates are triggered at threshold.
At threshold you get rapid opening of this sodium activation gate,
2) opening of voltage-gated potassium channels
3) you then get slow closure of this sodium inactivation gate (which functions to swing around and close the channel)
Even though all these three changes are initiated by the same event, they occur at different speeds.