Neurophysiology: Membrane Potential and Organization of the Nervous System N1

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 voluntary and involuntary actions and transmits signals throughout the body.

Membrane Potential

Definition and Importance

The membrane potential refers to the electrical disequilibrium that exists between the extracellular fluid (ECF) and intracellular fluid (ICF) of a cell. This difference is crucial for the function of excitable cells such as neurons and muscle cells.

• Membrane potential difference: The voltage difference across the cell membrane.

• Electrical gradient: Created by the movement of ions across the membrane.

• Electrochemical gradient: Combination of electrical and concentration gradients.

Generation of Membrane Potential

Membrane potential arises due to selective permeability of the cell membrane to different ions, primarily sodium (Na+) and potassium (K+).

• When a leak channel for K+ is inserted, K+ moves out of the cell down its concentration gradient, creating an electrical gradient.

• As more K+ leaves, the inside of the cell becomes negatively charged, establishing a membrane potential.

Equilibrium Potential

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

• 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

Resting Membrane Potential

The resting membrane potential is the membrane potential of a cell when it is not actively transmitting signals. In excitable cells, this typically ranges from -40 to -90 mV.

• Primarily determined by K+ due to higher membrane permeability to K+ compared to Na+.

• Maintained by the sodium-potassium pump (Na+/K+ ATPase), which moves 3 Na+ out and 2 K+ in, generating a negative intracellular environment.

Disturbances of Membrane Potential

Changes in membrane potential can occur due to alterations in ion concentration gradients or membrane permeability.

• Hyperkalemia: Increased extracellular K+ concentration (>6 mM) leads to depolarization.

• Hypokalemia: Decreased extracellular K+ concentration (<3.5 mM) leads to hyperpolarization.

• Opening of gated channels can also change membrane potential by allowing ions to flow according to their gradients.

Organization of the Nervous System

Main Divisions

The nervous system is organized into two main divisions:

• Central Nervous System (CNS): Consists of the brain and spinal cord.

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

Functional Subdivisions

• Afferent (sensory) division: Carries information toward the CNS.

• Efferent (motor) division: Carries information away from the CNS to effectors (muscles and glands).

• Autonomic division: Controls involuntary functions and is further subdivided into sympathetic and parasympathetic branches.

Cells of the Nervous System

Neurons

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

• Structure:

  • Cell body (soma): Control center containing the nucleus.

  • Dendrites: Receive incoming signals from other cells.

  • Axon: Carries outgoing signals from the soma to target cells.

  • Axon terminals: Transmit signals to other neurons or effectors.

• Function:

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

  • Interneurons: Facilitate communication between neurons within the CNS.

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

Glial Cells

Glial cells are support cells in the nervous system that provide structural and metabolic support to neurons.

• Examples include astrocytes, oligodendrocytes, Schwann cells, and microglia.

Nerves

Nerves are bundles of peripheral neurons that transmit sensory and motor information between the CNS and the rest of the body.

• Sensory nerves: Carry information to the CNS.

• Motor nerves: Carry information from the CNS to effectors.

Table: Effects of Extracellular Potassium on Membrane Potential

Condition	[K+] in ECF	Effect on Membrane Potential
Normal (Normokalemia)	3.5-5 mM	Resting membrane potential maintained
Hyperkalemia	>6 mM	Depolarization (membrane potential becomes less negative)
Hypokalemia	<3.5 mM	Hyperpolarization (membrane potential becomes more negative)

Summary

• The membrane potential is a fundamental property of all living cells, especially excitable cells like neurons and muscle cells.

• It is determined by ion concentration gradients and membrane permeability, and is essential for signal transmission in the nervous system.

• The nervous system is organized into the CNS and PNS, with specialized cells (neurons and glia) and structures (nerves) for communication and control.

Additional info: Some details and terminology have been expanded for clarity and completeness based on standard Anatomy & Physiology curriculum.