Neurophysiology Flashcards
Neurons convey messages using
____________.
electrical signals
Ohm’s Law
I = V/R
How is the membrane lipid-bilayer also a diffusion bilayer?
• Impermeable to ions • Functions as a capacitor • Membrane potential: separation of the +ve and –ve charges • Ions flow through channels
Ion flow is dependent on _______________.
size & charge
Common characteristics of ion
channels
• Multiple states:
– Open
– Closed
• Selectivity: – Minimal selectivity: nature of amino acid residues lining its pore (anions vs cations) – Highly selective: size of the pore (e.g. Na+ >>>K+)
• Gating
Gating of ion channels:
• Channels flip between open and closed states • Factors can change this probability: – Voltage gated – Ligand gated – Thermally gated – Mechanically gated
Membrane potential
- Impermeable PM will not allow the ions to flow down their concentration gradient
- Negative and positive charges would be identical in each side
Extracellular concentrations:
Na: 140 mM
K: 4 mM
Cl: 120 mM
Ca: 2.5 mM
Intracellular concentrations:
Na: 15 mM
K: 130 mM
Cl: 5 mM
Ca: 0.0001 mM
Equilibrium potential:
the potential at which an ion is in electrochemical equilibrium
Resting membrane potential is heavily influenced, but not determined, by ____________.
K gradient
Factors that influence the net passive diffusion:
- Electrochemical gradient
– Chemical gradient
– Difference in electrical potential, (positively charged ions will tend to move toward regions of more negative potential) - Conductance of the channel
– At resting membrane potential, Na conductance is small
Electrical properties of neurons
- Passive (cable properties)
* Active
Cable properties of neurons
• Amplitude of the potential change decays exponentially as it moves away from its source • Change in Vm passively spreads in both directions along the axon/dendrite
Length constant:
- distance over which the potential falls by
1-(1/e) or 63% from its original value. - depends on the Rm (resistance of the membrane) and the Rl (longitudinal resistance).
Action potential (AP)
Actively propogates over long distances
Properties of an Action Potential:
- Threshold
- All-or-none response
- Latency
- Refractory Period
- Propagation
Refractory Periods:
- Absolute: no way to generate an AP
* Relative: requires higher stimulus and produces lower AP
Tetrodotoxin
- Blocking Na channels of the peripheral nerves causes weakness and numbness
- Larger doses can result in respiratory paralysis and death
- Example: puffer fish poison
How does the longitudinal resistance affect the length constant?
Increasing the diameter will decrease the longitudinal resistance, thus increasing the length constant
How does the membrane resistance affect the length constant?
Increasing the membrane resistance increases the length constant
What determines the speed of the AP?
Length constant
How can the length constant be increased?
- Increasing the diameter of the neuron
- Increasing the resistance of the axonal membrane
- insulating the axon - myelination
Properties of the length constant:
• Cable property:
Processes with larger diameter
(i.e. higher length constant)
• Active property (AP):
Increasing the length constant will increase the speed of AP propagation (faster).
Myelination saves _________, which allows for quicker propagation.
space
Demyelinating diseases
- Guillain-Barre syndrome
* Multiple sclerosis
Channelopathy
Defective ion channels
• Hyperkalemic periodic paralysis:
– Episodes of weakness and decreased muscle tone
(may follow exercise or K-rich food)
– During the attacks, the involved muscle fibers are
unable to fire APs (cells are depolarized by 30-40 mV)
– Mutation of the muscle voltage-gated Na
channels that prevents some % of them from being
completely inactive after depolarization
– Small but constant inward current…
inactivating normal Na channel…render muscle
inexcitable
Myotonia (muscle tone):
Having difficulty getting muscle to relax once they have contracted
What are the forms of myotonia?
• Two similar forms: Thomsen’s disease and
Becker’s disease
– The muscles of affected individuals relax unusually slowly following a sudden contraction
Myotonic goats
• “fainting” goats
• Cl channels abnormality:
– Normal skeletal muscles: relatively high Cl
permeability; resting membrane potential is influenced by the Cl concentration gradients.
– Mutation in the Cl channel results in decreased conductance….. reduces the amount of depolarizing current required to reach threshold
