Dynamic Polarization
Difference in anatomy and function across regions. Anatomical: Dendrite/Axon. Functional: Reception/Transmission. Dynamic due to changing inputs and outputs over time.
Dendrites
Receptive surface of neurons with a distinctive shape matching their function. Receive, filter, and organize inputs. Contains ligand-gated cation channels which generate EPSPs.
Axon initial segment
Head of axon, where density of voltage-gated Na+ channels is highest so that action potentials can be most easily generated.
Axon
Conductive surface of neurons. Dense in voltage gated Na+ and K+ channels which create and regenerate APs. Myelin insulates the membrane and allows for saltatory conduction along the nodes of Ranvier.
Axon Terminal
Transmitters. Contain voltage gated Ca2+ channels which triggers release of synaptic vesicles, and chemical communication.
Synapse
Junction (20-50 nm) where chemical signals are released and received at the dendrite of another neuron.
Soma
Assembles and packages proteins, and receives some signals.
Cell Membrane
Hydrophobic center of bilayer prevents ions from passing through, creating the ability to maintain different electrical concentrations on either side. Acts as a capacitor, where ion channels act as parallel resistors.
Membrane potential
The difference in voltage across the plasma membrane. (-65 to -70 mV). Determined by a concentration gradient across the cell membrane (maintained mainly by Na+/K+ pump). And selective permeability determined by ion channels.
K+
140:3 I:O, Ev = -102
Na+
18:145 I:O, Ev= +56
Cl-
7:120 I:O, Ev= -76
Ca2+
100 nM:1.2 I:O, Ev= +125
GHK Equation
Vm=61mV⋅log10((PK[K+]out+PNa[Na+]out+PCl[Cl−]in)/(PK[K+]in+PNa[Na+]in+PCl[Cl−]out)). Shows how permeability effects membrane potential. Increased permeability of X causes movement towards Ex. In most axons, Cl- is ignored, and in glial cells only K+ matters
Sodium-Potassium Pump
Affects the ion concentration, maintaining high K+ inside and high Na+ outside. Only slightly affects charge.
Driving force
Vm - Ex. When positive cations leave the cell and anions enter. Opposite when negative.
Ohm’s Law
Current = driving force * conductance. Total current = the sum of currents of ions X.
Steady State
When an neuron is at rest, Im = 0. Thus, the driving forces * conductance must balance out.
Conductance
Inverse resistance, proportional to channel permeability. acts like a scalar multiple for the electrical driving forces
Length Constant
how far a change in membrane potential spreads along a dendrite or axon before it decays significantly (down to 37%). Equal to sqrt(rm/ri). If larger, voltage will spread farther. Affects spatial summation: If larger, more distant dendrites can affect AP generation more. Proportional to sqrt(diameter)
Rm
Membrane resistance, essentially, how “leaky” the membrane is. equal to specific Rm/circumference
Ri
Internal resistance. Resistance of the cytoplasm to the current flow. equal to specific Ri/area.
Capacitance (Cm)
ability for the membrane to store charge. larger capacitance increases the time it takes to de/repolarize. myelination decreases capacitance due to increasing distance between surfaces of the membrane
Membrane time constant
How quickly the membrane potential changes after a current is applied (time to change by 63% of final value). Equal to RmCm. Higher tau1 increases temporal summation.
