Neurophysiology: Electrical Properties and Signal Transmission in Neurons

Neurophysiology: Electrical Properties and Signal Transmission in Neurons

Basic Electrical Concepts in Neurophysiology

Neurons are specialized cells that generate and transmit electrical impulses throughout the body. Understanding their function requires knowledge of fundamental electrical concepts as they apply to biological systems.

• Charge and Ions: Protons (positive charge) and electrons (negative charge) create electrical charge. Ions are atoms or molecules with a net charge due to unequal numbers of protons and electrons (e.g., Na+, Cl-).

• Electric Current: The flow of electrical charges, analogous to water flow in pipes.

• Voltage (Potential Difference): Separation of positive and negative charges creates a potential difference, measured in volts (V). This difference can perform work when charges flow as current.

• Resistance: Any factor that limits current flow, such as a lightbulb in a circuit or the cell membrane in neurons.

Transmembrane Potential and Resting Membrane Potential (RMP)

In neurons, positive and negative charges are separated across the cell membrane, creating the transmembrane potential. The inside of a resting neuron is more negative than the outside, typically at -70 mV.

• Typical RMP Values:

  • Neurons: -70 mV

  • Adipocytes: -40 mV

  • Skeletal muscle: -85 mV

  • Cardiac cells: -90 mV

• Ionic Distribution:

  • Primary intracellular cation: K+

  • Primary extracellular cation: Na+ (40x higher outside)

  • Ca2+ is also higher outside

  • Negatively charged proteins inside the cell contribute to the negative interior

• Membrane Permeability: The cell membrane is semipermeable, allowing selective ion movement through channels, thus maintaining the transmembrane potential.

Ion Movement: Gradients and Electrochemical Forces

Ions move across membranes due to chemical, electrical, and electrochemical gradients.

• Chemical Gradient: Ions move from high to low concentration (e.g., K+ leaks out of the cell).

• Electrical Gradient: Ions move toward the opposite charge (e.g., Na+ is attracted to the negative interior).

• Electrochemical Gradient: The sum of chemical and electrical forces acting on an ion across the membrane.

Establishing and Maintaining the Resting Membrane Potential

The resting membrane potential is established by the unequal distribution of ions and maintained by selective permeability and active transport mechanisms.

• K+ Leak Channels: K+ moves out of the cell, making the inside more negative. If only K+ is considered, the potential would be -90 mV.

• Na+ Leak: Na+ moves in more slowly; if only Na+ is considered, the potential would be +66 mV.

• Sodium-Potassium Pump (Na+-K+ ATPase): Actively transports 3 Na+ out and 2 K+ in, using 1 ATP per cycle, maintaining the -70 mV RMP.

• Energy Demand: The Na+-K+ pump uses about 70% of the nervous system's ATP, explaining the high oxygen and glucose requirements of nervous tissue.

Membrane Channels for Ion Movement

Different types of membrane channels regulate ion movement:

• Passive (Leak) Channels: Always open; ions move according to gradients (e.g., K+ leak channels).

• Chemically Regulated (Ligand-Gated) Channels: Open/close when specific chemicals (e.g., neurotransmitters) bind; found on dendrites and soma.

• Mechanically Regulated Channels: Open/close in response to mechanical forces (e.g., pressure, vibration); found on dendrites and soma.

• Voltage-Regulated Channels: Open/close in response to changes in membrane potential; found on axons and synaptic bulbs.

Local (Graded) Potentials

Local or graded potentials are small, localized changes in membrane potential caused by stimulation (e.g., neurotransmitter binding or mechanical force).

• Depolarization: Influx of Na+ makes the inside less negative (more positive).

• Characteristics:

  • Graded: Magnitude varies with stimulus strength.

  • Decremental: Signal weakens with distance from the stimulus site.

  • Reversible: If stimulation ceases, membrane returns to RMP.

  • Excitatory or Inhibitory: Excitatory depolarize, making action potentials more likely; inhibitory hyperpolarize, making them less likely.

• Repolarization: Na+-K+ ATPase restores RMP after depolarization.

Action Potentials

An action potential is a rapid, large change in membrane potential that propagates along the axon. It is generated when a local potential reaches a threshold at the axon hillock.

• Initiation: Local potential at axon hillock reaches threshold (-60 to -55 mV).

• Depolarization: Voltage-gated Na+ channels open; Na+ influx causes rapid rise to about +35 mV.

• Repolarization: Na+ channels inactivate; voltage-gated K+ channels open, K+ efflux restores negativity.

• Hyperpolarization: K+ channels remain open briefly, causing membrane potential to become slightly more negative than RMP.

• Characteristics:

  • All-or-None: If threshold is reached, action potential fires at full strength; otherwise, it does not occur.

  • Nondecremental: Action potentials do not weaken as they travel.

  • Irreversible: Once started, the action potential proceeds to completion.

Refractory Periods

After an action potential, the neuron is less excitable for a brief period.

• Absolute Refractory Period: No new action potential can be generated; Na+ channels are open or inactivated.

• Relative Refractory Period: A stronger-than-normal stimulus can initiate another action potential; K+ channels are still open.

Signal Propagation in Nerve Fibers

Action potentials propagate along axons by different mechanisms depending on myelination.

• Unmyelinated Fibers (Continuous Propagation):

  • Voltage-gated Na+ channels along the entire axon.

  • Action potentials stimulate adjacent regions in a chain reaction.

  • Propagation speed: ~6 ft/sec.

  • Signal moves forward due to refractory period behind the action potential.

• Myelinated Fibers (Saltatory Propagation):

  • Myelin insulates axon segments; voltage-gated channels concentrated at Nodes of Ranvier.

  • Action potentials "jump" from node to node (saltatory conduction).

  • Propagation speed: up to 120 m/sec.

  • Much faster and more efficient than continuous propagation.

Axon Diameter and Propagation Speed

The speed of action potential propagation depends on axon diameter and myelination.

Type	Diameter	Myelination	Speed	Function
Type A	4-20 µm	Myelinated	440 ft/sec	Sensory (position, balance, touch, pressure); motor to skeletal muscle
Type B	2-4 µm	Myelinated	60 ft/sec	Sensory (temperature, pain, general touch); autonomic motor
Type C	<2 µm	Unmyelinated	3 ft/sec	Sensory (temperature, pain, general touch); autonomic motor

• Myelination: Improves coordination and control; not fully developed until adolescence.

Clinical Note: Multiple Sclerosis (MS)

Multiple sclerosis is a neurological disorder characterized by patchy loss of myelin in the brain and spinal cord, replaced by scar tissue.

• Slows or blocks action potential propagation.

• Symptoms: muscle spasms, limb weakness, bladder dysfunction, sensory loss, visual disturbances.

Key Equations

• Nernst Equation (for equilibrium potential of an ion):

$E_{ion} = \frac{RT}{zF} \ln \left( \frac{[ion]_{out}}{[ion]_{in}} \right)$

• Goldman-Hodgkin-Katz Equation (for membrane potential considering multiple ions):

$V_m = \frac{RT}{F} \ln \left( \frac{P_{K^+}[K^+]_{out} + P_{Na^+}[Na^+]_{out} + P_{Cl^-}[Cl^-]_{in}}{P_{K^+}[K^+]_{in} + P_{Na^+}[Na^+]_{in} + P_{Cl^-}[Cl^-]_{out}} \right)$

Additional info: The Nernst and Goldman-Hodgkin-Katz equations are used to calculate equilibrium and membrane potentials, respectively, based on ion concentrations and permeabilities.