3. Principles of Neural Transmission Flashcards
What are the 3 types of neuron?
Multipolar (standard), Bipolar and Unipolar
Define a multipolar neuron
Many dendrites, one axon
Define a bipolar neuron
One dendrite at one end and one axon at the other
Define a unipolar neuron
One branch leaves the cell body and spreads in 2 directions
What is are all neurons common function?
Receive, process, transmit and output info
What are the common structures of the neuron?
Dendrites, cell body, axon hillock, axon, axon terminals and myelin sheath
What is the axon hillock?
The attachment from the cell body to the axon - determines if the signal will go any further down the axon (if the transmission is strong enough)
A nucleus within an atom consists of…
Protons and neutrons
What orbits the nucleus in an atom?
Electrons
Atoms are held together by what type of force?
Electrostatic force (opposites attract)
What are ions?
Atoms that have lost or gained one or more electrons
What are cations?
Positively charged ions
What are anions?
Negatively charged ions
Salts are made of what?
Ion
Why are salts able to dissolve?
Because the ions are able to separate and move freely
What are the 3 cations crucial for neural signalling?
Sodium, Potassium and Calcium
What does sodium do?
Generating action potentials
What does potassium do?
Maintains resting potential
What is calcium responsible for?
Synaptic transmission
What are the 2 anions involved in neural signalling?
Chloride ions and proteins
What do chloride ions do?
Suppress action potentials
What do proteins do?
Maintaining resting potential
Where is sodium found in the body?
Everywhere, mainly outside neurons (extracellular)
Where is potassium found?
Mainly inside neurons (intracellular)
Where is calcium found?
Almost exclusively extracellular
Are chloride ions intra/extracellular?
Mostly extracellular
Are proteins intra/extracellular?
Mostly intracellular
Define diffusion
The movement of particles from regions of high concentration to low concentration
What is diffusion caused by?
The random movement of particles
The speed of diffusion depends on what?
Temp, size of particles and how difficult it is for particles to travel through the liquid (viscosity)
Define fully permeable
Allows all particles through
Define semi/selectively permeable
Only allows certain particles through
Neuronal cell membranes contain ‘pores’ made up of large proteins that allow certain ions to pass through, these are called…
Ion channels
Ion channels are specific to how many ions?
1
Very large proteins are usually up of several identical parts, which are called what?
Subunits
Ion channels are made up of ___ ___ that come together and are held together by their shape
Smaller proteins
What are ion channels critical for?
For cells to communicate with the ‘outside’
Neurons have an electrical potential across their membranes, this potential is a result of:
- Differences in ionic concentrations between the inside and outside of the neuron
- Ion channels in the neuronal cell membrane that only allow certain ions to pass in and out of the neuron
How can the membrane potential be measured?
Using tiny electrodes
Define an electrical potential
The strength of the electric field which surrounds charged particles
When neurons are at ‘rest’, the charge is what?
Slightly more negative than the outside
The slightly negative potential is known as…?
The resting membrane potential
What is the resting potential in volts?
Approx -70 millivolts (mV)
What type of permeability does the cell membrane have?
Semipermeable
Which ion channels are usually closed?
All of them apart from potassium
When neurons are at rest, which ions can still freely move across the cell membrane?
The potassium
Due to the concentration of potassium ions being higher inside the cell than outside, what happens?
Some of the potassium ions leave the cell by diffusion
Because potassium ions are the only ones which can move freely, what does this mean for the charge of the neuron?
It is slightly more negative as the positive potassium ions continue to leave
What are the 2 opposing forces that determines the resting potential?
- Concentration difference
2. Electrostatic force
Describe how the concentration difference determines the resting potential
It drives potassium out of the cell making the inside more negative (because there is a lower concentration of K outside the cell)
Describe how the electrostatic force determines the resting potential
Because opposites attract, potassium is drawn back into the negative inside of the cell
When the two forces are balanced, an equal number of K ions leave and enter the cell, this is called ___
The equilibrium potential (the resting potential)
When the concentration is balanced, why is there a slightly negative resting potential?
Because there are more proteins than potassium in the cell
Why is there more potassium inside the cell in the first place?
Because of the sodium-potassium pump
What is the sodium-potassium pump?
An ion channel that pumps sodium out of the cell and potassium into the cell
Why does the sodium-potassium pump require energy?
Because it works against the concentration gradient
Where does the sodium-potassium pump get energy from?
ATP
What quantity of sodium and potassium gets pushed in/out of the cell by the Na/K pump?
The Na/K pumps 3 Na out for every 2 K in
Define hyperpolarization
Increase in resting potential (making the membrane more negative)
Define depolarization
Decrease in resting potential (making the membrane less negative)
The membrane potential is the sum of…
The depolarization and hyperpolarization
What causes an action potential (AP) to occur?
If depolarisation exceeds a threshold
Describe the 3 main features of an AP
- Sudden & brief (1-2ms)
- Momentarily reverse the membrane potential (from -70mV to +40mV)
- Repolarizes quickly (back to normal)
What determines the intensity of AP’s?
The frequency that they occur
AP’s rely on which type of ion channels, what are they called?
Voltage-gated ion channels
What are voltage gated ion channels?
A complex cascade of opening and closing voltage-gated ion channels that govern the influx and outflow of Na and K ions
The voltage-gated channels are ___ at resting potential?
Closed
If you pass a current into the cell, the membrane begins to…
Depolarise (becomes less negative)
The channels begin to open when the membrane is…
Depolarised
What is the threshold for the voltage-gated channels to open?
-40mV
When the voltage gated channels, which ions diffuse from outside the cell to inside the cell as a result of the concentration gradient?
Sodium ions
When the sodium ions enter the cell, this further depolarises the cell which causes what to happen?
More channels to open
The increase in membrane potential goes from -70mV to what?
+40mV
Once the concentration of Na is balanced by the electrostatic forces, the voltage-gated channels become…
‘Inactivated’
What does it mean for the voltage-gated channel to be inactivated?
The channel will stop allowing sodium into the cell
At what voltage will the potassium channels open?
+30mV
When the potassium channels open, potassium will begin to leave the cell until when?
Until the potential reaches its resting potential again
When the original resting potential is reached, what happens to the sodium channels?
They return to their original closed state
Voltage-gated channels have a sensor ‘paddle’, describe this ‘paddle’
It is a protein structure which changes shape depending on membrane potential
When the ‘paddle’ changes shape, what happens in the voltage-gated channel?
They will open or close
What causes the ‘paddle’ to change shape?
The ‘paddles’ are pulled away/into the cell depending on the membrane potential - as a result of electrostatic forces
Describe the process of how the cell returns to its resting potential
When the Na channels close and become inactivated, the voltage-gated K channels open so that no more Na ions can enter by K ions begin to diffuse into the cell. This causes the membrane potential to become negative again
Why does the membrane potential briefly become hyper polarised?
Due to the K ions leaving the cell as a result of the concentration gradient
What is the refractory period?
The period immediately after an AP during which another AP cannot be elicited, due to the inactivation of the voltage-gated Na channels which remain inactive for a period of time long enough for the AP to move on
If an AP depolarises the neighbouring membrane region beyond the threshold, what will occur?
An AP in the neighbouring region and so on
What is the reason that AP’s can only spread in one direction?
Due to the refractory period
When the Na channels become inactivated they will remain closed regardless of membrane potential, what does this prevent?
Another AP from occurring
In the time that the channels are inactivated, where does the AP go?
It moves away from this area and therefore in one direction only
There is exposed membrane between the myelin sheath, what is this called?
Ranvier
Due to a myelinated axon, what does this mean in terms of the Ranvier nodes?
That ion flow only occurs at these nodes
Impulses leap from node to node, what is this called?
Saltatory conduction
What does it mean for a disease to be demyelinating?
It breaks down the myelin sheath
What does a demyelinating disease prevent?
Saltatory conduction
Why is fugu so toxic?
Because they contain tetrodotoxin
What does tetrodotoxin do?
Blocks the opening of voltage-gated Na channels and therefore APs cannot be generated
How do local anaesthetics work (what do they block)?
They block voltage-gated Na channels and therefore APs cannot be generated to signal pain to the brain
APs are transmitted from one neurone to another at synapses, what is the gap called?
The synaptic cleft
What are the 2 ways which signals can be transmitted across the synaptic cleft?
Chemically and electrically
Describe the 5 features of an electrical synapse
- Large channels which connect one cell to another
- Direct connection between two neurons
- Rely on special channel proteins - gap junctions
- Allows small molecules and ions to flow between cells
- Allow APs to pass from one neurone to the next
What are the special channel proteins in an electrical synapse called?
Connexin
Outline the 6 steps of how a chemical synapse works?
- AP arrives at the presynaptic membrane
- Voltage gated Ca2+ channels open, allowing Ca2+ into the cell
- Synaptic vesicles fuse with membrane and then rupture which releases neurotransmitter molecules
- Transmitter binds to postsynaptic receptors, opening ion channels
- If the channel is for Na+ - depolarisation occurs (EPSP)
- If channel is for CL - hyper polarisation occurs (IPSP)
- EPSP?IPSP propagated along membrane
What is an EPSP?
Excitatory postsynaptic potential
What is an IPSP?
Inhibitory postsynaptic potential
What are receptors?
Membrane proteins that bind neurotransmitters
Describe the lock and key principle
That each receptor type can only bind to a specific neurotransmitter
When a transmitter molecule binds to the receptor, how does it cause an ion channel to open?
By causing the receptor to change shape
What are the 2 types of receptor?
Ionotropic receptors and Metabotropic receptors
Define ionotropic receptors
The receptor itself is an ion channel (fast)
Define metabotropic receptors
The receptor causes a separate ion channel using G-protein coupling (slow)
When the ion channel is opened by the postsynaptic receptor, what does this result in?
Either depolarisation or hyperpolarisation
EPSPs and IPSPs will spread towards what in the postsynaptic neurone?
The axon hillock
Once the EPSPs/IPSPs reach the axon hillock, what determines whether an AP is elicited?
If the sum of the EPSPs/IPSPs reaches the threshold or not
If the sum of EPSPs/IPSPs reaches the threshold, what happens next?
The AP is elicited and travels along the axon to the axon terminal which causes release of neurotransmitter
EPSPs/IPSPs from different synapses can sum in __ and __
Time and space
Define spatial summation
Many axons converge on one neurone
Define temporal summation
Many EPSPs/IPSPs occur at the same time
