NURS 207: Nervous System Basics

Nervous System - Resting Membrane Potential and Neuron

• NURS 207 (N01) course covers the Nervous System, focusing on resting membrane potential and neurons.
• Objectives: Understand the basics of resting membrane potential generation and the anatomy/physiology of a typical neuron.

Membrane Permeability

• Phospholipid bilayers in cell membranes are impermeable to charged molecules (e.g., Na+, K+, Cl-, Ca++).
• These molecules are also insoluble in the hydrophobic membrane core.
• Large water-soluble molecules (e.g., proteins, nucleic acids, sugars) also require channels to cross the membrane.
• Small uncharged polar molecules (e.g., CO2, O2, NH3, water, mostly with aquaporins) can cross freely.

Electrolyte Distribution

• Interstitial Fluid: Major electrolytes are Na+, Cl-, and HCO3-.
• Intracellular Fluid: Major electrolytes are K+, HPO42- (phosphate ion), and negatively charged proteins.
• The uneven distribution of electrolytes creates an electrochemical disequilibrium across the plasma membrane, crucial for generating the resting membrane potential.

Dominant Ions (Extracellular vs. Intracellular)

• Extracellular fluid: Primarily Na+ and Cl-.
• Intracellular fluid: Primarily K+ and negatively charged proteins.

Electrical Properties of the Cell Membrane

• Plasma membranes behave as ionic conductors, allowing the flow of ionic currents.
• Concentration gradients dictate ion flow direction across the membrane.
• Membranes function as capacitors, holding charges and generating a transmembrane potential. This transmembrane potential (voltage difference) is crucial between intra- and extracellular space.

Generation of Membrane Potential

• Equilibrium: When a cell and solution are at electrical and chemical equilibrium the cell membrane acts as an insulator to prevent free movement of ions between cells.
• Ion Concentration Gradients: The Na+-K+ ATPase creates concentration gradients with 3 Na+ ions pumped out and 2 K+ ions pumped in; this leads to a net intracellular negative charge.
• Relative Charge Scale: Setting the extracellular fluid to zero (ground) establishes a negative membrane potential (approx. -70 mV) in the cell.

K+ Leak Channels

• Plasma membranes have more K+ leak channels than Na+ leak channels.
• The concentration gradient drives K+ out of the cell.
• The accompanying negative ions (anions) inside the cell try to follow K+, but are largely trapped by the cell membrane (impermeable to large anions). This further contributes to the negative membrane potential.

Equilibrium Potential

• Equilibrium potential is achieved when the opposing forces of the concentration gradient and electrical gradient are balanced.
• This results in a zero net movement of ions.
• The resting membrane potential's value is close to the equilibrium potential for K+.

Resting Membrane Potential

• All living cells have a resting membrane potential.
• Chemical and electrical disequilibrium exists between intra- and extracellular fluids in a resting state (not stimulated).
• The resting potential is the voltage difference across the cell membrane at rest.
• Maintaining this potential requires ATP and the Na+/K+ pump.
• At rest, the cell is in a dynamic steady state but not at equilibrium. This state of dynamic steady state involve chemical and electrical disequilibrium.

Neuron Anatomy and Physiology

• Structure: Neurons are excitable cells that transmit information via electrical or chemical signals. They have specific parts, including the cell body, dendrites, and axon.

• Function: Signals flow from dendrites to cell bodies along axons. Neurons receive, process, and transmit information to other neurons or effector cells.

• Types of Neurons: Neurons are classified into sensory, interneurons, or motor neurons based on their specific functions. This classification is related to their structure.

• Classification by Structure: Multipolar, bipolar, and unipolar.

• Classification by Function: Sensory (afferent), interneurons (association), and motor (efferent) neurons.

Cell Body (Soma)

• Contains the nucleus and organelles.
• Occupies a small portion of the neuron's total volume.
• Proteins produced within the nucleus are transported to other parts of the neuron via cytoskeleton (microtubules).

Dendrites

• Thin branches that receive incoming signals from other neurons.
• Incoming signals are in form of graded potentials.
• Integration of signals occurs within the cell body.
• Dendritic spines enable an increased number of possible contact sites between neurons and their neighboring cells.
• Contain polyribosomes, capable of protein synthesis.

Axon

• Long, slender projection that transmits signals away from the cell body.
• Originates from the axon hillock (trigger zone).
• Electrical signals are transmitted down the axon and chemical signals at the presynaptic axon terminals (which is a type of region that contain neurotransmitters)
• Axons have branches (collateral axon terminals) along their length.
• Presynaptic terminals contains synaptic vesicles filled with neurotransmitters
• Retrograde transport through Axons is possible, with material moving back to the soma.

Axonal Transport

• Transport vesicles and organelles along microtubules within the axon.
• Requires motor proteins (e.g., kinesin, dynein).
• Enables bidirectional movement of materials along the axon.

Additional Information

• Important Concepts: Resting membrane potentials, graded potentials, action potentials, and the role of ion channels and pumps in maintaining these potentials.
• Nernst Equation: Used to calculate equilibrium potential for an ion based on concentration gradients and ion charges.
• Goldman-Hodgkin-Katz (GHK) Equation: Incorporates the permeabilities of multiple ions to estimate the membrane potential. Ions like K+, Na+, and Cl- in an excitable cell are relevant to measure and calculate electrical signals and membrane potentials.