Neurophysiology: Membrane Potential and Nervous System Organization

Neurophysiology Fundamentals

Overview

This study guide covers essential topics in neurophysiology, focusing on the resting membrane potential, the organization of the nervous system, and the structure and function of neurons. These concepts are foundational for understanding how the nervous system coordinates bodily functions and transmits signals.

Resting Membrane Potential

Definition and Importance

The resting membrane potential is the electrical potential difference across the cell membrane when a neuron is not actively transmitting a signal. It is crucial for the excitability of neurons and muscle cells.

• Membrane potential refers to the electrical disequilibrium between the extracellular fluid (ECF) and intracellular fluid (ICF).

• This difference is created by the selective permeability of the cell membrane to various ions and the activity of ion pumps.

• All living cells have a membrane potential, but excitable cells (neurons, muscle) typically range from -40 to -90 mV.

Generation of Membrane Potential

• Membrane potential arises from the combination of electrical gradients and concentration gradients of ions across the membrane.

• Key ions involved: Na+, K+, Cl-, and Ca2+.

• Resting cells are permeable to both Na+ and K+, but the membrane is about 40 times more permeable to K+.

• The sodium-potassium pump (Na+/K+ ATPase) maintains concentration gradients by pumping 3 Na+ out and 2 K+ in, generating a negative intracellular environment.

Equilibrium Potential

The equilibrium potential for an ion is the membrane potential at which the electrical and chemical driving forces are balanced, resulting in no net movement of that ion.

• Calculated using the Nernst equation:

$E = \frac{RT}{zF} \ln \left( \frac{[\text{ion}]_{\text{outside}}}{[\text{ion}]_{\text{inside}}} \right)$

• R: Gas constant

• T: Temperature in Kelvin

• z: Valence (charge) of the ion

• F: Faraday constant

Disturbances of Membrane Potential

Changes in ion concentration or membrane permeability can alter the membrane potential, affecting cell excitability.

• Hyperkalemia: Increased extracellular K+ concentration (>6 mM) depolarizes the cell.

• Hypokalemia: Decreased extracellular K+ concentration (<3.5 mM) hyperpolarizes the cell.

• Opening gated channels for Na+ or Ca2+ causes depolarization (membrane potential becomes less negative).

• Opening gated channels for K+ or Cl- causes hyperpolarization (membrane potential becomes more negative).

Organization of the Nervous System

Main Divisions

The nervous system is organized into two major divisions, each with specialized functions and structures.

• Central Nervous System (CNS): Consists of the brain and spinal cord. Responsible for processing and integrating information.

• Peripheral Nervous System (PNS): Includes all nerve tissue outside the CNS: cranial nerves, spinal nerves, ganglia, plexuses, and sensory receptors.

PNS Subdivisions

• Afferent (sensory) division: Carries information from sensory receptors to the CNS.

• Efferent (motor) division: Transmits commands from the CNS to effectors (muscles and glands).

• Autonomic nervous system: Controls involuntary functions and is further divided into sympathetic and parasympathetic branches.

Neurons: Structure and Function

Neuron Anatomy

Neurons are the basic signaling units of the nervous system, specialized for the transmission of electrical and chemical signals.

• Cell body (soma): Contains the nucleus and is the metabolic center.

• Dendrites: Receive incoming signals from other neurons.

• Axon: Transmits outgoing signals from the cell body to target cells.

• Axon terminals: Release neurotransmitters to communicate with other cells.

Neuron Types

• Sensory (afferent) neurons: Transmit information about stimuli (temperature, pressure, light) to the CNS.

• Interneurons: Complex branching neurons that facilitate communication within the CNS.

• Motor (efferent) neurons: Control skeletal muscles and internal organs.

• Autonomic neurons: Influence involuntary functions in organs via sympathetic and parasympathetic branches.

Neural Networks

• Nerves: Bundles of peripheral neurons that transmit sensory and motor information.

• Neural networks coordinate voluntary and involuntary actions and transmit signals throughout the body.

Table: Effects of Extracellular Potassium on Membrane Potential

This table summarizes how changes in extracellular potassium concentration affect the membrane potential and cell excitability.

Condition	[K+] in ECF	Effect on Membrane Potential	Cell Excitability
Normal (Normokalemia)	3.5-5 mM	Resting membrane potential maintained	Normal
Hyperkalemia	>6 mM	Depolarization	Increased
Hypokalemia	<3.5 mM	Hyperpolarization	Decreased

Additional info:

• Neuroglia (glial cells) are support cells in the nervous system, providing structural and metabolic support to neurons.

• Depolarization and hyperpolarization are key processes in the generation and propagation of action potentials.