Electrobiology

Nervous System

The nervous system is a complex network of neurons that receives, processes, and transmits information throughout the body. Neurons are specialized cells responsible for the conduction of electrical impulses, enabling communication between different body parts.

• Neurons form intricate networks for information transfer.

• Electrical pulses are produced in response to appropriate stimuli.

• Pulses propagate along the neuron’s cabling structure (axon).

• Pulses are constant in magnitude and duration, independent of stimulus intensity.

Example: Sensory neurons in the skin detect pressure and transmit signals to the brain for processing.

Neuron Types and Structure

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

• Sensory neurons: Receive stimuli from sensory organs and monitor the body’s environment.

• Motor neurons: Carry messages that control muscle cells, based on sensory input and central nervous system signals.

• Interneurons: Transmit information between neurons, facilitating complex reflexes and processing.

Each neuron consists of a cell body, dendrites (input ends), and a long axon (output tail). The axon conducts impulses away from the cell body.

Example: A simple neural circuit involves a sensory neuron detecting heat, an interneuron processing the signal, and a motor neuron triggering muscle contraction.

Action Potential

Generation and Properties

An action potential is the rapid change in electrical potential across a neuron's membrane, enabling signal transmission. It is studied by measuring voltage changes in the axon relative to the surrounding fluid.

• The nerve impulse is elicited by a stimulus (chemical, mechanical, or electrical).

• Most experiments use externally applied voltage as the stimulus.

• An impulse is generated only if the stimulus exceeds a threshold value.

• The action potential is a sudden rise in axon potential to about .

• The potential then rapidly decreases to about and returns to the resting state.

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

• Fast axons can propagate pulses at speeds up to .

Example: The action potential in a neuron is essential for transmitting signals from the brain to muscles, enabling movement.

Axon as an Electric Cable

Electrical Model of the Axon

The axon can be modeled as an electrical cable, with current flowing both inside and outside the axon membrane. This model uses resistors and capacitors to represent the electrical properties of the axon.

• Current flows through the axon and its surrounding fluid.

• The axon is represented as a series of electrical circuits with resistances () and capacitances ().

Example: The cable model helps explain how electrical signals attenuate as they travel along the axon.

Properties of Sample Axons

The following table compares the properties of nonmyelinated and myelinated axons:

Analysis of Axon Circuit

The axon circuit can be simplified to a series of resistors and capacitors. When a steady voltage is applied at one point, the voltage decreases exponentially along the axon:

• The voltage at distance from the point of application is given by:

• Where is the length constant (about for typical axons).

• At , the voltage decreases to 37% of its initial value.

Example: This exponential decay explains why signals weaken over long distances in nonmyelinated axons.

Synaptic Transmission

Mechanism of Signal Transfer

Signals are transmitted from axons to other neurons or muscle cells via synaptic transmission. In vertebrates, this process is usually chemical.

• There is a gap (synapse) of between the nerve ending and the target cell.

• When the impulse reaches the synapse, a chemical substance is released, diffusing across the gap and stimulating the adjacent cell.

• In some cases, transmission is electrical, but chemical transmission is predominant in vertebrates.

Example: Neurotransmitters such as acetylcholine are released at the neuromuscular junction to trigger muscle contraction.

Action Potentials in Muscles

Muscle Fiber Impulses

Muscle fibers generate and propagate electrical impulses similarly to neurons. The action potential in muscle fibers has a similar shape but typically lasts longer (about 20 milliseconds).

• Electrical impulses in muscle fibers enable coordinated contraction and movement.

• Duration of muscle action potentials is longer than in neurons.

Example: The heart muscle relies on action potentials for rhythmic contractions.

Additional info: These notes expand on the biophysical principles underlying nerve and muscle impulse transmission, including cable theory and synaptic mechanisms, which are relevant to physics students studying biological applications of electricity and circuits.