Electrical Signaling by Nerve Cells

Electrical Signaling by Nerve Cells

• Nervous system uses electrical and chemical signals for communication.
• Electrical signals travel along neuron surfaces, while chemical signals transfer between neurons.
• Intraneuronal flow is primarily electrical, and interneuonal information is primarily chemical.
• Neurons are electrically polarized, having a negative inside relative to the outside.
• Electrical signals are local alterations in the resting membrane potential.

Graded Potentials

• Graded potentials vary in duration and amplitude and happen in postsynaptic membranes and receptive areas of sensory receptors.
• Their intensity depends on synaptic input strength or stimulus intensity.
• They spread passively and dissipate within a millimeter.

Action Potentials

• Action potentials are large, brief signals that propagate actively along axons.

Chemical Synapses

• Chemical synapses involves neurotransmitter release from one neuron causing electrical changes in another.

Membrane Potential

• Cell membranes are lipid bilayers with embedded proteins critical for electrical properties.
• Inorganic ions are unequally distributed across the membrane.
• Lipid bilayer prevents diffusion of water and hydrophilic particles, maintaining ionic concentration.
• Membrane proteins allow selective permeability to ions, enabling charge distribution and voltage across the membrane.
• Resting membrane potential is most related to K+ concentration.

Ion Channels and Concentration Gradients

• Transmembrane proteins are a route for hydrophilic particles to cross the membrane.
• Some transmembrane proteins are molecular pumps that move particles against concentration gradients.
• Ion channels facilitate particle movement down concentration gradients and establish resting membrane potential.
• Ion channels are proteins that zigzag across the membrane, surrounding a pore.
• The dimensions of a pore and the charges on the protein segments lining it make a channel more or less selective for particular ions.
• Channels can exist in open or closed conformations, switching back and forth.
• The amount of time channels spend in each state can be influenced by membrane potential, binding substances, intracellular changes, mechanical deformation, or temperature changes.
• Number of open channels determines the number of ions that move across the membrane.
• Treatments that alter the likelihood of channels opening change a membrane's permeability to that ion.

Equilibrium Potentials

• Equilibrium requires the number of ions leaking out equals the number returning.
• A concentration gradient of an ion is counterbalanced by the membrane potential.

Nernst Equation

• Nernst equation expresses the equilibrium potential mathematically
• Vx = (RT/zF) * ln ([X]o/[X]i)
• Vx is the equilibrium potential for ion x - R is the gas constant - z is the valence of ion x
  • T is temperature in °K
  • F is Faraday's number
  • ₁ and [X]₂ are the extracellular and intracellular concentrations of ion x.
• For typical K+ concentrations at 37°C yields - VK = 62 log10([K+]o/[K+]i) = 62 log10(4/130) = -92 mV

Steady-State Potentials

• Real membranes are permeable to multiple ions (K+, Na+, Cl-), so the Nernst equation isn't enough
• Normal Na+ concentration gradient is opposite to K+ gradient.
• Adding Na+ permeability makes the neuronal interior less negative at a steady state.
• Na+/K+-ATPase pumps use ATP to move Na+ out and K+ in, maintaining ion gradients.
• Membrane potential of real neurons is a weighted average of equilibrium potentials, with the weighting for each ion as the membrane's relative permeability.
• Goldman-Hodgkin-Katz equation quantitatively expresses it:
  • Vm = 62 log10(PK[K+]o + PNa[Na+]o + PCl[Cl]i / PK[K+]i + PNa[Na+]i + PCl[Cl]o)
  • Vm is membrane potential, P is permeability, [] is concentration. Although initially intimidating, this is simply a combined series of Nernst equations with relative permeabilities added

Graded Changes in Membrane Potential

• Localized changes in ion channel probabilities cause membrane potential changes.
• Potential changes are graded because the changes in probability are graded.
• Neurons use localized changes in the probabilities of sets of ion channels being open or closed to cause membrane potential charges at specific sites.
• Increases or decreases in membrane permeability toward the equilibrium potential for that ion

Spread of Membrane Potential Changes

• Graded potentials spread as the result of electrical properties of cytoplasm and membrane.
• Ions moving through open channels constitute a current, which moves into and through the cytoplasm.
• Ionic current crosses the membrane to return to its starting point.
• Electrical circuits with resistors and capacitors change the time course of signals.

Action Potentials

• Neurons use action potentials to move signals effectively from one part of the cell to another.
• Action potentials are all-or-none signals propagated actively down an axon without losing amplitude.
• Action potentials use voltage-gated Na+ and K+ channels.
• Depolarization causes both types of channels to open at different time courses.
• Na+ channels open quickly, increasing Na+ permeability and causing more depolarization and approach VNa.
• Na+ channels spontaneously move to an inactivated state and cannot be made to open until the membrane potential returns to its resting level; repolarization of the membrane resets ( deinactivated) the Na+ channels.
• As Na+ channels inactivate, K+ permeability returns to dominance and the membrane potential moves toward VK. The added K+ permeability moves the membrane potential even closer to VK than usual.

Threshold and Trigger Zones

• Action potentials mean that a stimulus must surpass a critical level of depolarization.
• Zones in which graded potentials are produced usually have too few voltage-gated Na+ channels to produce action potentials at all. In most neurons, the axon's initial segment is thought to be the principal trigger zone

Refractory Periods

• Inactivation of voltage-gated Na+ channels terminates an action potential, but it has another important consequence
• For a brief period following the peak of an action potential, most Na+ channels at that site are inactivated, and so few are available that the membrane is uneatable
• The relative refractory period is a period of declining threshold, so the larger the level of background depolarization, the more frequently threshold is reached

Propagation of Action Potentials

• Action potentials are initiated at trigger zones.
• They begin to spread to neighboring areas of membrane and depolarize them to threshold, in turn causing an action potential in the next neighboring area.
• Under ordinary circumstances, this propagation is unidirectional, away from the cell body and toward the axon's terminals.
• In essence, the longer the length constant, the farther an action potential can "reach" down an axon before declining to a subthreshold value
• The faster the length constant:
  • Increase the diameter of the axon
  • Myelination, which allows relatively thin axons to conduct rapidly.

Peripheral Nervous System Axons

• Axons come in a range of sizes and speeds.
• Size and conduction velocity of the axons in spinal nerves correlate with function
  • Smallest diameter axons (unmyelinated or thinly myelinated fibers)
    • Mostly visceral afferents, efferents and pain, and temperature information
  • More heavily myelinated axons deal with skin, skeletal muscles, and joints.

Simple Neuronal Circuit

• Preceding elements combine to explain a simple the stretch reflex.