Neurotransmission Flashcards
What is resting membrane potential?
-70mV
The inside of the cell is more negative than the outside of the cell
Define electrostatic pressure.
The attraction of negatively charged ions to positively charged ion (and visa versa)
What ions are in high concentration outside the cell?
Sodium (Na+) and chlorine (Cl-)
What is the protein pump and its function?
It is driven by ATP
Moves 3Na+ out of the cell and brings 2K+ into the cell
What is diffusion?
It is a passive process (needs no ATP)
During resting membrane potential how does Cl- move in/out of the cell?
Cl- moves out by diffusion and is kept in by electrostatic pressure
How does Na+ move into the cell?
By diffusion and electrostatic pressure
Describe Na+ entering the cell and in what amount.
The membrane is only slightly permeable to Na+ so it allows a smaller amount of Na+ into the cell
It enters by diffusion and electrostatic pressure
Describe the passive mechanism that maintain charges in/outside the cell
A high negative charge is maintained in the cell as the membrane is only slightly permeable to Na+ meaning small amounts of Na+ enter the cell
A high positive charge is kept outside the cell as the membrane is very permeable to K+ so a lot of K+ leaves the cell
How does K+ move in/out of the cell?
Moves out by diffusion and is kept in by electrostatic pressure
What does an inhibitory post-synaptic potential do (IPSP)? Where would you find one in the body?
Ions flow through the membrane causing it to become more negative - when the channel opens Cl- flows inwards and K+ flows outwards. It results in hyper-polarisation
All the channels in an inhibitory synapse cause an IPSP
On an inhibitory synapse
What does an excitatory post-synaptic potential do (EPSP)? Where would you find one in the body?
Ions flow through the membrane causing it to become more positive
On an inhibitory synapse
What are chemically-gated ion channels? Where are they located?
The channel opens or closes in response to chemicals like neurotransmitters and allow the selective flow of ions through (e.g. Na+, K+, Cl-)
The post-synaptic membrane of the soma and dendrites of neurons
What are voltage-gated ion channels? Where are they located?
The channel opens or closes in response to action potentials and synaptic activity. In response to action potentials they allow the selective flow of Na+ and K+ and in response to synaptic activity they allow the selective flow of Ca2+
The post-synaptic membrane of the axon hillock of neurons
What happens when the neurotransmitter binds to the receptor on the ion channel of the post-synaptic membrane?
Na+ ions are allowed to flow into the open channel rapidly by diffusion and by electrostatic pressure (as the ions are from high to low conc. of Na+ and are moving form an area of relative positive charge to relative negative charge)
Define graded potential
The (chemically-gated) channel is only open for a discrete period of time before it snaps shut so only a finite amount of Na+ can move into the cell
What stops the flow of Na+ through the ion channel?
Enzymes on the synaptic cleft are attached to the channel that degrade the neurotransmitter which closes the channel and stops the flow of Na+ into the cell
What movement of ions does an EPSP cause?
What does this cause?
What is the end result of an EPSP?
Na+ inwards (rapidly). K+ outwards
The inside of the membrane to become less negative/more positive, reduction in membrane potential (-70mV to -50mV)
Depolarisation
What movement of ions does an IPSP cause?
What does this cause?
What is the end result of an IPSP?
Cl- inwards. K+ outwards
More negative charge inside the membrane i.e. an increase in membrane potential (-70mv to -80mV)
Hyper-polarisation
What is spatial summation?
The number of channels opening across a space sums up to have a greater effect
How many channels are open
What is temporal summation?
The summing of graded potentials across a membrane because as each channel opens more ions flow in
How frequency the channels are opening
What determines graded potential?
Spatial and temporal summation
What determines the overall membrane potential at the axon hillock?
The balance/relative quantity of IPSP and EPSPs
What are the 4 phases/stages of action potential generation?
Resting state
Depolarising phase
Re-polarising phase
Hyperpolarisation
What is the absolute refractory period?
The time during which it is impossible to generate another action potential as the Na+ channels are already open
What is the relative refractory period?
The time during which it is possible to generate another action potential as the Na+ channels are starting to shut (re-polarisation is occurring)
A greater stimulus would be needed to generate this AP
What does self propagating mean?
Once depolarisation occurs in one channel it causes depolarisation in adjacent channels so that every channel in that area on the axon hillock is open and allowing Na+ in. This means that action potentials are self propagating
Outline what occurs in the resting state during action potential (AP) generation.
All gated K+ and Na+ channels are closed (Na+ activation gate is closed and the inactivation gate is open). The K+ channel opens/closes more slowly than the Na+ channel
Outline what occurs in the depolarising phase during action potential (AP) generation.
The Na+ activation gate opens and Na+ flows rapidly into the cell due to its diffusion gradient and due to electrostatic pressure.
K+ channels remain closed
Outline what occurs in the re-polarising phase during action potential (AP) generation.
Na+ channels close when high levels of depolarisation area reached (the inactivation gate closes)
K+ gate opens allowing a gradual flow of K+ ions out of the cell
Outline what occurs in the hyper-polarisation phase during action potential (AP) generation.
K+ channels slowly close and some K+ ions leak out which causes hyper-polarisation. Na+ gates are fully closed
What type of axons are involved in continuous conduction?
Unmyelinated (no myelin sheath around their membrane) axons are propagated
What type of axons are involved in saltatory conduction?
Only axons that are myelinated with Nodes of Ranvier are propagated
Outline what occurs in continuous conduction? (3)
- Voltage-gated channels along the membrane of un-myelinated axons allow large amounts of Na+ into the axon which causes an action potential due to the high positive charge
- Na+ will move along the membrane of the axon following its diffusion and electrostatic force causing adjacent voltage-gated channels to open
- These channels allow more Na+ into the axon which propagates the process along to the axon terminal
Outline what occurs in saltatory conduction? (3)
- All nodes of Ranvier have voltage-gated Na+ and K+ channels in high density.
- When an AP is generated upwards from a Node of Raniver the AP causes an influx of Na+ into the axon
- Na+ causes a depolarisation of the membrane at the Node of Raniver causing Na+ channels to open
- Na+ follows its diffusion and electrostatic force along the membrane moving from high to low concentration
- As the concentration of Na+ is so high near the node of Ranvier the velocity of Na+ along the axon will also be high
What has to happen for the neurotransmitter to be released into the synaptic cleft?
The action potential is propagated along the axon either by continuous or saltatory conduction
When the AP arrives at the axon terminal they open the Ca+ gated channels (rather than voltage-gated channels) which cause synaptic vesicles to release the neurotransmitter into the synaptic cleft
What happens once the neurotransmitter is released into the synaptic cleft?
it binds to chemically-gated channels causing either an EPSP or an IPSP
The neurotransmitter is reabsorbed into the axon terminal or is broken down by synaptic enzymes to ensure the EPSP or the IPSP is time-limited
Where do sensory receptors generate their action potential?
Periphery - in the axon terminal
They do not generate the AP at the axon hillock like other neurons
What type of channels does mechanoreceptors have?
Voltage-gated (Na+, K+)
What happens when there is mechanical pressure on the nerve membrane?
- The nerve membrane becomes deformed, increase in membrane permeability causing Na+ to flow into the cell rapidly which causes membrane depolarisation.
- If the membrane depolarises by more than -50mV the voltage-gated channels open causing an influx of Na+ and an action potential to be generated.
Where does the action potential go in a mechanoreceptor after being generated?
It travels up the axon through the dorsal root into the spinal cord and on to the brain
What determines the frequency of action potential?
The level of mechanical perturbation on the receptor - axon membrane increases its permeability, increased Na+ flowing in, increased depolarisation which means a higher frequency of action potentials
Describe a pseudo-unipolar cell
What kind of neuron is it?
Has a cell body with 1 axon emanating from it and splits at a T-junction into 2 axons; 1 ascending to the CNS and 1 descending down the periphery
Sensory neuron
