Biophysics: Electrobiology, Nervous System, and Action Potentials

Electrobiology

Introduction

Electrobiology is a branch of biophysics that studies the electrical properties of biological cells and tissues. In the context of the nervous system, it focuses on how neurons generate and transmit electrical signals, known as action potentials, and how these signals propagate through neural circuits.

Nervous System

Structure and Function of Neurons

The nervous system is composed of a complex network of neurons that receive, process, and transmit information throughout the body. Neurons are specialized cells responsible for communication within the nervous system.

• Neurons form intricate networks to transmit information from one part of the body to another.

• When a neuron receives an appropriate stimulus, it produces electrical pulses called action potentials.

• The electrical pulse is propagated along the neuron's cablelike structure, the axon.

• Action potentials are constant in magnitude and duration, regardless of the intensity of the stimulus (all-or-none principle).

Types of Neurons

Neurons are classified into three main types based on their function:

• Sensory neurons: Receive stimuli from sensory organs and monitor both the external and internal environment. They convey information about factors such as heat, light, pressure, muscle tension, and odor.

• Motor neurons: Carry messages that control muscle contractions and glandular outputs. These messages are based on information from sensory neurons and the central nervous system (CNS).

• Interneurons: Transmit information between neurons, often within the CNS, facilitating communication and integration of signals.

Neuron Anatomy

• A typical neuron consists of a cell body (soma), dendrites (input ends), and a long axon (output tail).

• Dendrites receive incoming signals from other neurons.

• The axon conducts electrical impulses away from the cell body toward other neurons or effector cells.

Example: The classic structure of a motor neuron includes dendrites, a cell body, a myelinated axon, and synaptic terminals.

Action Potential

Generation and Measurement

An action potential is a rapid, transient change in the membrane potential of a neuron, allowing the transmission of electrical signals along the axon.

• To study nerve impulses, a probe is inserted into the axon to measure voltage changes relative to the surrounding fluid.

• The nerve impulse is elicited by a stimulus applied to the neuron or axon, which may be chemical, mechanical (pressure), or electrical (applied voltage).

• In laboratory experiments, the stimulus is often an externally applied voltage.

Action Potential Characteristics

• An action potential is produced only if the stimulus exceeds a certain threshold value.

• Once generated, the impulse propagates down the axon from the point of stimulation.

• The action potential involves a sudden rise in the axon's internal potential to about +30 mV, followed by a rapid decrease to about -90 mV, and then a slow return to the resting state.

• The entire pulse passes a given point in a few milliseconds.

• Fast-acting axons can propagate pulses at speeds up to several tens of meters per second.

Example: The classic action potential graph shows a sharp depolarization (rise to +30 mV), repolarization (drop to -90 mV), and return to the resting potential (around -70 mV).

Action Potential Phases

• Resting potential: The stable, negative membrane potential of a neuron at rest (typically around -70 mV).

• Depolarization: Rapid influx of sodium ions ($Na^+$) causes the membrane potential to become positive.

• Repolarization: Efflux of potassium ions ($K^+$) restores the negative membrane potential.

• Hyperpolarization: The membrane potential temporarily becomes more negative than the resting potential.

Key Equations

• The voltage along the axon decreases exponentially with distance from the point of application:

$V(x) = V_0 e^{-x/\lambda}$

• Where $V(x)$ is the voltage at distance $x$, $V_0$ is the initial voltage, and $\lambda$ is the length constant (about 0.8 mm in the example).

Axon Circuit

Electrical Properties of the Axon

The axon can be modeled as an electrical cable, with properties such as resistance and capacitance per unit length. This model helps explain how electrical signals attenuate as they travel along the axon.

• The axon circuit is simplified as a combination of resistors and capacitors, representing the membrane and internal/external fluids.

• If a steady voltage $V_0$ is applied at one point, the voltage decreases exponentially along the axon, as described above.

Transmission at Synapses

• Signals are transmitted from one neuron to another or to muscle cells at specialized junctions called synapses.

• In some cases, action potentials are transmitted by direct electrical conduction, but in vertebrates, chemical transmission is more common.

• The synaptic gap is typically about $10^{-8}$ m wide.

• When the impulse reaches the synapse, a chemical neurotransmitter is released, diffuses across the gap, and stimulates the adjacent cell.

Muscle Fibers and Action Potentials

• Muscle fibers also produce and propagate action potentials, similar in shape to those in neurons but usually of longer duration (about 20 ms).

Summary Table: Types of Neurons and Their Functions

Type of Neuron	Function	Example
Sensory Neuron	Receives stimuli from environment; transmits sensory information to CNS	Touch, temperature, pain receptors
Motor Neuron	Transmits signals from CNS to muscles/glands	Stimulates muscle contraction
Interneuron	Connects neurons within CNS; processes and integrates information	Spinal cord interneurons

Additional info: The notes are based on introductory biophysics and neurobiology concepts, suitable for college-level physics or biophysics courses. The cable model and exponential voltage decay are foundational in understanding signal propagation in excitable tissues.