Neuronal communication Flashcards
describe the role of ions + ion channels in the resting membrane potential
at resting potential the charge inside the neuron is more negative than the outside (inisde is -, outside is +)
Resting membrane potential describes the steady state of the cell, which is a dynamic process that is balanced by ion leakage and ion pumping.
RESTING POTENTIAL IS -70mV this maintained by 3Na+ ions being pumped out, 2Na+ ions being pumped in. Na+/K+ ATPase pump (process requires ATP to move ions through this channel because it’s moving them against a concentration gradient)
When the cell is at rest, the concentration of Na+ outside the cell is 10 times greater than the concentration inside. Also, the concentration of K+ inside the cell is greater than outside.
because
* more K+ passive leak channels than Na+ passive leak channels
* membrane 50 times more permeable to K+ than Na+
ALSO INSIDE IS NEGATIVE BECAUSE
* IMPERMEANT NEGATIVE PROTEINS IN CYTOPLASM aka Gibbs-Donnan equilibrium (effect): (The cytosol contains a high concentration of anions, in the form of phosphate ions and negatively charged proteins. Large anions are a component of the inner cell membrane, including specialized phospholipids and proteins associated with the inner leaflet of the membrane (leaflet is a term used for one side of the lipid bilayer membrane). The negative charge is localized in the large anions)
* PASSIVE IONIC FACILITATED DIFFUSION VIA ‘LEAK’ CHANNELS
describe the role of ions + ion channels in an action potential
1) Resting membrane potential= all gated Na+ and K+ channels closed. (Na+/K+ ATPase pump is maintaining -70mV)
2) DEPOLARISATION: Na+ voltage gated channels open (cuz stimulus hits threshold) so resting potential becomes more positive. Threshold is -55mV, Na+ wants to reach equilibrium which is +58mV (we don’t let it reach equilibrium as u will see in next step)
3) So, Na+ is flooding in cuz of step 2 + also cuz of -ve ions (impermeants) in cytoplasm also attract Na+, when the membrane potential reaches +30mV –> K+ voltage gated channels open= REPOLARISATION as +ve K+ is leaving
4) the K+ wants to reach equilibrium which is -93mV and cuz channels open slowly the K+ rush out fast= REFRACTORY PERIOD so membrane becomes HYPERPOLARISED (aka too negative -93mV)
5) Na+/K+ ATPase pump restores the membrane potential from this hyperpolarisation back to its resting membrane potential (-70mV)
definition of action potential= rapid changes in voltage across the membrane
Describe action potential propagation in unmyelinated axons vs myelinated axons
myelinated axon propagation of AP:
*myelin provides high resistance to ion flow across the membrane.
* resistance is lost at nodes of ranvier (uninsulated, low resistance gaps) =density of Na+ voltage-gated channels is high at nodes of ranvier (aka more channels at gaps/nodes)
*so the +ve charge after depolarisation just pushes the charge across because myelin keeps the ions in there. So AP ‘jumps’ (saltare = “to leap”) from node to node i.e. it doesn’t need ion exchange at myelinated bits, only ion exchange happening is at nodes of ranvier= this fast jumping of AP is energy efficient its called SALTATORY CONDUCTION
unmyelinated axon propagation of AP:
* passively spreading wave of depolarisation from point of stimulus
*when AP occurs at trigger site (axon hillock) +ve Na+ charges rush into cell via facilitated diffusion. Once Na+ inside= inside has +30mV +ve charge, so Na+ ions are attracted to -ve charge in segment around it so depolarisation wave will move to segment (2)
* membrane behind is in refractory period (-93mV) so depolarisation always moves away from where the stimulus began
=CONTINUOUS CONDUCTION
Explain neurotransmitter release at the pre-synaptic terminal
Describe neurotransmitter action at the post-synaptic neurone
explain how voltage sensitive Na+ channels work
Contains 2 gates
* Activation gate closed in resting state
* Inactivation gate open in resting state
* Activation gate open in response to depolarisation
* Opening very fast <0.1 ms
* Allows about 6000 Na+ ions into cell
* Inactivation gate closes in response to depolarisation
* Closing slow (around 1ms after depolarisation)
After repolarisation channels return to resting state
explain how voltage-sensitive K+ channels work
- Open in response to depolarisation, but open slightly slower than Na channels close
- They stay open during entire depolarisation
- Increase permeability of K+ to > resting permeability of K+
Produce repolarisation and undershoot (hyperpolarisation)
what is meant by the ‘threshold’ of an action potential
The action potential is an explosion of electrical activity that is created by a depolarizing current. This means that a stimulus is big enough to cause the resting potential to move toward 0 mV anything more positive than -55mV. When the depolarization reaches about -55 mV a neuron will fire an action potential. This is the threshold.
The sodium/potassium pump requires energy in the form of adenosine triphosphate (ATP), so it is also referred to as an _____. The concentration of Na+ is higher ___ the cell , and the concentration of K+ is ___ inside the cell
The sodium/potassium pump requires energy in the form of adenosine triphosphate (ATP), so it is also referred to as an ATPase. As was explained in the cell chapter, the concentration of Na+ is higher OUTSIDE the cell than inside, and the concentration of K+ is HIGHER inside the cell than outside
what do we mean by electrochemical exclusion of an voltage gated ion channel
Channels for cations (positive ions) will have negatively charged side chains in the pore. Channels for anions (negative ions) will have positively charged side chains in the pore. This is called electrochemical exclusion, meaning that the channel pore is charge-specific.
Some ion channels are selective for charge but not necessarily for size, and thus are called a nonspecific channel. These nonspecific channels allow cations—particularly Na+, K+, and Ca2+—to cross the membrane, but exclude anions, however these channels don’t discriminate on size
define the following:
1) ligand-gated channel
2) mechanically gated channel
3) voltage gated channel
ligand-gated channel (aka ionotropic receptors) opens because a signaling molecule, a ligand e.g. neurotransmitter,hormone latches onto it’s receptor
mechanically gated channel opens in response to a physical distortion/physical stretching of the cell membrane.
voltage-gated channel is a channel that opens/closes in response to changes in membrane potential
a weak stimulus tends to trigger ____ frequent action potentials
a weak stimulus tends to trigger LESS frequent action potentials
What are the advantages of saltatory conduction
In myelinated axons the AP jumps from one node of ranvier to the next node (saltatory conduction) because of myelination, it’s faster than the continuous conduction we see in unmyelinated axons. Saltatory conduction is also more energy efficient because we don’t need the Na+ moved by the pumps that require ATP at the myelinated sections, only ATP pumping is happening at nodes of ranvier (so less energy needed to move AP across in saltatory conduction)
what do we mean by membrane capcitance vs membrane resistance
Membrane capacitance is the amount of charge you have to move across the membrane to change the membrane potential a certain amount. (so myelination reduces the capacitance because it reduces the amount of ions lost so it maintains the charge making it easier for it to move aka low capacitance)
membrane resistance is dependent on the density of open channels. It is the membrane’s ability to pass current per unit membrane area. The lower the resistance the faster the rate of conduction as theres less opposing force for the ions to travel (n.b. the myelin
define exocytosis + endocytosis
Exocytosis (vesicle bursts open) is the fusion of secretory vesicles with the plasma membrane and results in the discharge of vesicle content into the extracellular space and the incorporation of new proteins and lipids into the plasma membrane.
Endocytosis (vesicle formation) is a cellular process in which substances are brought into the cell. The material to be internalized is surrounded by an area of cell membrane, which then buds off inside the cell to form a vesicle
