Membrane Potential and Organization of the Nervous System: Study Notes for Anatomy & Physiology

Resting Membrane Potential

Definition and Measurement

The resting membrane potential (RMP) is the electrical potential difference across the plasma membrane of a cell at rest. It is typically measured using electrodes, with one placed inside the cell and another in the extracellular fluid.

• Typical RMP value: -70 mV in neurons

• Measurement: A voltmeter records the difference between the inside and outside of the cell.

• Graphical representation: Shows phases of depolarization, repolarization, and hyperpolarization.

Depolarization refers to a decrease in membrane potential (less negative), while hyperpolarization is an increase (more negative). Repolarization is the return to RMP after depolarization.

Ion Channel Proteins and Membrane Permeability

Types of Ion Channels

Ion channels are proteins that form open, water-filled passageways for ions to cross the cell membrane. They are essential for generating and propagating electrical signals in neurons.

• Open channels:

  • Leak channels

  • Pores (e.g., water pores)

• Gated channels:

  • Chemically gated channels

  • Voltage-gated channels

  • Mechanically gated channels

Channels are named for the primary ion that passes through them (e.g., Na+ channel). Gated channels control ion permeability, and their threshold voltage, activation, and inactivation rates vary by type.

Ion Concentrations and Equilibrium Potentials

Major Ions and Their Gradients

The distribution of ions across the cell membrane determines the equilibrium potential for each ion, calculated using the Nernst equation:

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

Ion	Extracellular Fluid (mM)	Intracellular Fluid (mM)	Eion at 37°C
K+	5 (normal: 3.5–5)	150	-90 mV
Na+	145 (normal: 135–145)	15	+60 mV
Cl-	108 (normal: 100–108)	10 (normal: 5–15)	-63 mV
Ca2+	1	0.0001	See Concept question 7

Electrical Signals in Neurons

Graded Potentials vs. Action Potentials

Neurons communicate using electrical signals, which are classified as graded potentials or action potentials.

• Graded Potentials:

  • Variable strength

  • Used for short-distance communication

  • Can be depolarizing or hyperpolarizing

  • Lose strength as they travel due to current leak and cytoplasmic resistance

• Action Potentials:

  • Very brief, large depolarizations

  • Rapid signaling over long distances

  • All-or-none response once threshold is reached

Trigger zone (axon hillock) is where action potentials are initiated if graded potentials reach threshold.

Organization of the Nervous System

Central and Peripheral Divisions

The nervous system is divided into the central nervous system (CNS) and peripheral nervous system (PNS).

• CNS: Brain and spinal cord; acts as the integrating center.

• PNS: Sensory (afferent) division and efferent division.

  • Sensory division: Transmits information to the CNS.

  • Efferent division: Somatic motor (skeletal muscle) and autonomic (smooth/cardiac muscle, glands) divisions.

  • Autonomic division: Sympathetic and parasympathetic branches.

  • Enteric nervous system: Network in digestive tract, can function autonomously.

Sensory Receptors

Types and Functions

Sensory receptors detect changes in the environment and transmit signals to the nervous system.

• Vision (photoreceptors)

• Hearing (mechanoreceptors)

• Balance (mechanoreceptors)

• Taste (chemoreceptors)

• Smell (chemoreceptors)

• Skin (mechanoreceptors, thermoreceptors, nociceptors)

• Viscera (nociceptors for pain)

Receptors are specialized for different modalities, such as light, sound, chemicals, and temperature.

Graphical Representation of Membrane Potential Changes

Phases of Membrane Potential

Membrane potential changes are visualized in graphs showing depolarization, repolarization, and hyperpolarization.

• Depolarization: Membrane potential becomes less negative.

• Repolarization: Return to resting membrane potential.

• Hyperpolarization: Membrane potential becomes more negative than resting.

These phases are critical for understanding how neurons transmit signals.

Summary Table: Types of Ion Channels

Channel Type	Mechanism	Example
Leak Channel	Always open	K+ leak channel
Chemically Gated	Opens in response to ligand binding	Acetylcholine receptor
Voltage-Gated	Opens in response to changes in membrane potential	Na+ channel in axon
Mechanically Gated	Opens in response to mechanical force	Touch receptor in skin

Key Terms

• Resting membrane potential

• Depolarization

• Repolarization

• Hyperpolarization

• Graded potential

• Action potential

• Ion channel

• Central nervous system (CNS)

• Peripheral nervous system (PNS)

• Sensory receptor

Example: The action potential in a neuron is initiated when a graded potential depolarizes the membrane at the axon hillock to threshold, opening voltage-gated Na+ channels and causing a rapid influx of Na+.

Additional info: The notes above expand on the brief points and images provided, adding definitions, context, and examples for clarity and completeness.