Neurons

Definition and General Characteristics

Neurons, also known as nerve cells, are the structural units of the nervous system. They are highly specialized cells designed to conduct electrical impulses throughout the body, enabling communication within the nervous system.

• Large, highly specialized cells that conduct impulses

• Billions present in the human body

Special Functional Characteristics

• Excitability: Ability to respond to stimuli and convert it into a nerve impulse

• Extreme longevity: Neurons can live and function for a lifetime

• Amitotic: Most neurons do not divide after maturation

Special Structural Characteristics

• Cell Body: Contains the nucleus and metabolic machinery

• One or more processes: Extensions from the cell body, including dendrites and axons

• Dendrites: Receive incoming signals

• Axon: Transmits outgoing signals

Neuron Structure

Major Parts of a Neuron

Neurons are composed of two major parts: the cell body (soma) and processes (dendrites and axon).

• Cell Body (Soma): The biosynthetic and metabolic center of the neuron

• Processes:

  • Dendrites: Act as antennas to receive electrical information from other neurons

  • Axon: Transmits electrical impulses away from the cell body

Detailed Structure

• Dendrites:

  • Short, branching extensions

  • Receive electrical signals from other neurons

  • Transmit signals toward the cell body

• Axon:

  • Long, singular process

  • Begins at the axon hillock

  • Functionally, action potentials begin at the initial segment

  • Can be myelinated (covered with myelin sheath) or non-myelinated

  • Distal end contains axon terminals for synaptic transmission

Membrane Potentials

Definition and Importance

The membrane potential (MP) is the electrical charge difference across the plasma membrane of cells, measured in millivolts (mV). In neurons, this property is crucial for excitability and the transmission of nerve impulses.

• Resting membrane potential (RMP): The membrane potential while the cell is at rest

• For neurons, RMP = -70 mV

• RMP is always negative in all cells

• Membrane potential can rapidly change, allowing for excitability

Contributors to Resting Membrane Potential

The resting membrane potential is established by differences in ion concentrations across the plasma membrane, primarily sodium (Na+) and potassium (K+).

• Na+ concentrations: Greater outside the cell than inside

• K+ concentrations: Greater inside the cell than outside

Table: Ion Concentrations Across the Neuronal Membrane

Key Equations

The Nernst equation can be used to calculate the equilibrium potential for a particular ion:

Where:

• Eion: Equilibrium potential for the ion

• R: Universal gas constant

• T: Temperature in Kelvin

• z: Valence of the ion

• F: Faraday's constant

• [ion]outside: Ion concentration outside the cell

• [ion]inside: Ion concentration inside the cell

Summary Table: Neuron Structure and Function

Example: Application in the Nervous System

Motor neurons transmit signals from the central nervous system to muscles, enabling movement. Sensory neurons carry information from sensory receptors to the brain for processing.

Additional info: The notes above expand on the brief points and diagrams in the original slides, providing definitions, context, and relevant equations for college-level Anatomy & Physiology students.