Neurophysiology Flashcards
What is the CNS composed of?
Brain and spinal cord.
What is the peripheral nervous system composed of?
Cranial and spinal nerves.
What are the 4 specialized sub-regions of all neurons?
Input, integration, conduction, output.
- Input: neurons.
- Integration: Cell body
- Conduction: Axon
- Output: terminals.
Inward current
Cations flow INTO the cell (negative) or anions flow OUT
Outward current
Cations flow OUT of the cell (positive) or anions flow IN
What is the cellular Vr?
Negative due to differences in ion concentrations and permeability.
What ion is permeable to the cell at rest? How does this ion move through the cell?
Potassium K (via leakage channels)
How are concentration gradients maintained at Vr?
Active transport of potassium and sodium.
What do electrical signals result from? (in terms of the cell)
Ion fluxes through channels, which change the Vm.
What two ways are channels activated?
- Ligand binding.
2. Changes in Vm (voltage induced)
How are channels deactivated?
Maintained depolarization (prolonged)
Describe the channel inactivation and activation of sodium voltage channels?
- at Vr, the channel is closed, but the inactivation gate is open (as the channel has not been inactivated).
- The cell depolarizes and the change in Vm causes the channel to open; both gates are open so sodium flows into the cell.
- After prolonged depolarization, the inactivation gate closes; the activation gate is open but no ions can pass through.
To reset, the activation gate closes, allowing the inactivation gate to open once Vm reaches Vr.
Channelopathies (what are examples of consequences?)
Diseases arise from mutations in genes that code for ion channel subunits.
ie)
Channel remains open too long (slow inactivation)
Cell becomes hyperexcitable
Uncontrolled excitation can cause brain seizures
Dravet’s syndrome
Mutation of the alpha-subunit gene for sodium channel; severe myoclonic epilepsy of infancy.
What is the electrical driving force on ions provided by?
The membrane potential.
Conductance
The ease with which ions flow across the membrane (siemens), Always positive.
Resistance
Inverse of conductance (how difficult it is for ions to flow) in Ohms.
What is equilibrium potential?
The membrane potential such that the current of a specific ion is 0.
When is the net current zero for a specific ion? (I=0)
When membrane potential is equal to equilibrium potential.
Both forces are equal and opposite (conc vs electrical)
What is the Ohm’s law we will use to find ionic current?
If I = V/R, then I = g(V), such that V is the net sum of (Vm-E)
What will the membrane voltage always move towards?
The E of the ion with the increasing conductance (much like electrons on a wire, they will take the path of the least resistance)
What is Nernst Equation?
E = 58/z log(X0/Xi)
z= valence of ion X0= concentration outside cell Xi= concentration inside cell.
What does the Nernst equation determine?
Which ion is contributing to a measured membrane current.
At resting potential, what is the membrane potential closest to? why?
Ek, the equilibrium potential of potassium because the membrane is highly permeable to potassium, not sodium.
Which scientists worked with squid axons? What did they discover?
Hodgkin and Huxley, defined the ionic basis of action potentials.
What is a graded potential? (Or Passive potential)
Can be depolarizing or polarizing and varies with strength of stimulus. It is initiated by a sensory or chemical stimulus.
Describe passive current flow
Passive conductance decays over time as ion flow reaches greater resistance. The resistance depends on the shape and size of the vessel. (smaller axons and dendrites have more resistance, like drinking a milkshake out of a coffee stir-stick.)
What is nerve action potential (AP)?
Rapid, short-lasting (transient) change in membrane potential that results in a brief reversal of polarity. Vi moves from a negative value to a positive value.
What is a spiking neuron?
A neuron that is firing APs
Threshold
Vm at which the AP is initiated, which is always more positive than Vrest.
Describe depolarization
Vrest reaches Vm, passing the threshold and opening voltage-dependant sodium channels.
Sodium channels then begin to all slowly close, and potassium channels begin to open to re-polarize.
What drug blocks sodium current?
TTX: Tetrodoxin
What drug blocks voltage-gated potassium channels?
TEA: Tetraethylammonium
How does conductance change during AP?
Conductance is greater for sodium, then greater for potassium.
What are leakage channels?
The basis of passive current flow; the pathway with the least resistance is through leakage channels once resistance increases down the length of an ion.
Refractory period
The time during which an axon is resistant to depolarization. Sodium channels are inactivated.
Where are sodium channels concentrated?
At the nodes of Ranvier.
What is the function of the myelin sheath?
To reduce current loss across the membrane (no leakage channels), acting as an electrical insulator.
Why is scorpion toxin relevant to APs?
This toxin binds to sodium channels to slow inactivation and produce a longer AP, causing paralysis and delayed response.
How do anesthetics work?
Block voltage-dependant sodium channels or hyperpolarize membranes using chlorine (Vm is SO negative that it would take god himself to reach threshold)
Multiple sclerosis
Loss of myelin sheath, slow spike conduction.
Bi-directional synapse
Electrical
Unidirectional synapse
Chemical
What is the first way of knowing that a synapse is chemical?
Vesicles.
Briefly describe the sequence of events that occur in transmission at a typical chemical synapse.
- NT is synthesized and packaged into vesicles.
- AP invades the terminal, causing calcium gates to open.
- Influx of calcium activates protein that drags vesicles to docking sites. Vesicles then fuse with membrane and dump NT into synapse.
- NT binds to post-cell receptor, which will open or close, causing an inhibitory or excitatory effect.
- NT are removed via glial cells or enzymatic degradation or diffusion
What occurs when glutamate is released from the PreSC?
Glutamate binds to receptors on PostSC, which activate a sodium channel and cause an EPSP.
What occurs when GABA is released from the PreSC?
GABA binds to PostSC, activating chlorine channels that cause the hyperpolarization of the post synaptic cell. (IPSP).
What was discovered about calcium current in an AP? How was this discovered?
Hodg and Hux used the squid axons to record calcium current with and without the use of a Calcium channel blocker. They found PSP present when the calcium channels were not blocked.
What are calcium chelators? How were they used in research?
Chelators do not impact the PreCS membrane potential, but bind to calcium to prevent them from activating the carrying proteins. Therefore NTs are not released and no PSP is present.
EPSPs _______ over long distances.
Decay.
“End-Plate”
Synapse between nerve and muscle: neuromuscular joint nmj
End-plate potential (EPP)
an EPSP at the nmj
Miniature endplate potential (MEPP)
Results from the release of a single vesicle.
Each quantum is _______ vesicle(s).
One.
Synaptic delay
Time between onset of of presynaptic spike and onset of EPSP.
What NT is released at the nmj? What does it do?
Acetylcholine, open channels that conduct both Na and K.
What breaks down Acetylcholine?
Acetylcholinesterase
What is the name for the receptor that acetylcholine binds to?
Nicotinic cholinergic receptor.
How do transmitter-gated channels and voltage-gated channels work together?
Acetylcholine receptors depolarize cell, which causes voltage-gated channels to open and further depolarize the cell (cause AP)
Synaptic Transmission Disorders
Myasthenia gravis: loss of voltage gated Ca channels; muscle weakness.
Botulism: impairs NT release at end-plate, muscle paralysis.
Reversal potentials
Membrane potential at which there is zero net current through a transmitter-gated channel
Equilibrum potential for a transmitter-gated channel permeant to more than one ion.
Describe how reversal potential is analogous to equilibrium potential at a synapse.
Equilibrium at a synapse is the equilibrium potential of sodium. The reversal potential is between sodium and potassium because both ions are present.
What is the relevance of the Goldman-Hodgkin-Katz equations?
States that the reversal potential is dependant on the relative conductances of the synaptic channel to the ions (also equilibrium potential for these ions).
What is the rule about Membrane voltage and reversal potential?
Vm will always move towards the reversal potential when the conductance increases.
How is synaptic strength measured?
Amplitude of EPSP
How is synaptic plasticity measured?
Changes in EPSP
What is Long-term Potentiation?
Form of synaptic plasticity that involves strengthening the synapse.
What is an example of synaptic plasticity at a mammalian central synapse?
LTP in the hippocampus.