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, which are essential for communication within the nervous system.

• Neurons form intricate networks to transmit signals between different body parts.

• Each neuron consists of a cell body, dendrites (input ends), and a long axon (output end).

• The axon propagates electrical impulses away from the cell body.

Classification of Neurons:

• Sensory neurons: Receive stimuli from sensory organs and convey information about external and internal environments (e.g., heat, light, pressure).

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

• Interneurons: Transmit information between neurons.

Example: Sensory neurons detect muscle tension and transmit this information to motor neurons, which then adjust muscle activity accordingly.

Action Potential

An action potential is a rapid, transient change in the electrical potential across a neuron's membrane, allowing the transmission of nerve impulses. This process is fundamental to neural communication and muscle activation.

• Action potentials are produced when a neuron receives an appropriate stimulus (chemical, mechanical, or electrical).

• The electrical pulse is propagated along the axon, maintaining constant magnitude and duration regardless of stimulus intensity.

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

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

• The impulse is a sudden rise in potential inside the axon to about , followed by a rapid decrease to about , and a slow return to the resting state.

• The entire pulse passes a given point in a few milliseconds; fast axons can propagate pulses at speeds up to .

Example: In experiments, an externally applied voltage is commonly used to elicit action potentials in neurons.

Equation:

• Resting potential: typically around .

• Action potential: spike to , then drop to .

Axon as an Electric Cable

The axon can be modeled as an electrical cable, allowing the application of physical principles to understand signal propagation. This model uses resistors and capacitors to represent the axon's properties.

• Current flows both inside and outside the axon membrane.

• The axon is represented as a series of electrical circuits, with resistances and capacitances corresponding to biological structures.

Example: The cable model helps explain how voltage changes propagate along the axon and how signal strength diminishes with distance.

Properties of Sample Axons

Analysis of Axon Circuit

The axon circuit can be simplified using electrical circuit models. 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 as they travel along the axon without active regeneration.

Synaptic Transmission

Synaptic transmission is the process by which the action potential is transferred from one neuron to another or to a muscle cell. This can occur via electrical or chemical means.

• In vertebrates, transmission is usually chemical, involving neurotransmitters.

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

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

Example: Acetylcholine is a neurotransmitter released at neuromuscular junctions to stimulate muscle contraction.

Action Potentials in Muscles

Muscle fibers generate and propagate action potentials in a manner similar to neurons, enabling muscle contraction and movement.

• The shape of the action potential in muscle fibers is similar to that in neurons, but its duration is typically longer (about 20 milliseconds).

Example: The longer duration of muscle action potentials allows for sustained muscle contraction necessary for movement.

Additional info: These notes expand on the biophysical principles underlying neural and muscular electrical activity, integrating physical models and equations relevant to college-level physics and biophysics courses.