Clase 12 Fisiología - Neurofisiología Sensitivo 3 (IG:@doctor.paiva)

Neurophysiology: Sensory Receptors and Signal Transduction

Introduction to Sensory Physiology

• The twelfth class of physiology focuses on sensory neurophysiology, discussing the types of sensory receptors and how stimuli are transduced into nerve impulses.

• Emphasis is placed on signal transmission intensity and the concepts of temporal and spatial summation related to sensory receptors.

Types of Sensory Receptors

• Various types of sensory receptors include mechanoreceptors (detect mechanical compression), thermoreceptors (detect temperature changes), nociceptors (pain receptors), electromagnetic receptors (detect light), and chemoreceptors (detect chemical changes).

• Mechanoreceptors like Pacinian corpuscles will be studied in detail; they respond to mechanical deformation.

Functionality of Thermo and Nociceptors

• Thermoreceptors are responsible for detecting cold and heat, while nociceptors sense tissue damage through chemical or physical alterations.

• Electromagnetic receptors, such as those in the retina, detect light; chemoreceptors monitor taste, smell, blood oxygen levels, osmolarity, and carbon dioxide concentration.

Information Transmission Mechanisms

• Different nerve fibers transmit various sensations like pain or pressure due to the "labeled line principle," which states that each nerve fiber connects to a specific central nervous system area.

• This principle explains why we perceive different stimuli distinctly based on their pathways to specific brain centers.

Transduction Process in Sensory Receptors

• All sensory receptors modify their membrane potential through receptor potentials triggered by stimuli affecting membrane permeability.

• Changes in membrane permeability lead to depolarization, resulting in action potentials that convey information throughout the body.

Mechanisms Behind Receptor Potentials

• Receptor excitation can occur via mechanical deformation, chemical interactions, temperature changes, or electromagnetic radiation—all leading to altered ion permeability.

• The fundamental cause behind these processes is a change in receptor membrane permeability that allows sodium ions to enter the cell.

Amplitude Relationships in Action Potentials

• The maximum amplitude for most receptor potentials is around 100 mV when sodium channels fully open; this correlates with action potential amplitudes being capped at 100 mV as well.

• Understanding thresholds for depolarization and repolarization is crucial for grasping how stimuli affect action potential generation during relative refractory periods.

Specific Case Study: Pacinian Corpuscles

Understanding Sensory Receptors and Adaptation

Mechanisms of Sensory Stimulation

• The process begins with any stimulus, such as pressure, causing deformation in a specific area, leading to changes in membrane permeability and sodium channel openings.

• This results in a unilateral action potential along myelinated fibers, indicating rapid conduction towards a pressure center. Initially, there is high frequency of sensory input that diminishes over time.

Pain Receptor Adaptation

• There is debate on whether pain receptors adapt; some sources claim they never do while others suggest slow adaptation occurs. However, it is generally accepted that pain receptors maintain sensitivity.

• When stimulated by pressure, the viscous liquid within certain receptors redistributes, ceasing action potential generation despite continued deformation.

Types of Receptors: Tonic vs. Phasic

• Tonic receptors adapt slowly and continuously inform the brain about body status relative to the environment (e.g., muscle tone).

• Examples include muscle spindles and vestibular maculae which provide ongoing feedback about body position.

Fast-Adapting Receptors

• Phasic receptors respond quickly but do not send continuous signals; they react to changes in stimuli (e.g., touch).

• An example includes how we initially feel clothing or jewelry but become unaware of them over time until the pressure is relieved.

Classification of Nerve Fibers

• Nerve fibers are classified into myelinated and unmyelinated types based on diameter and speed; larger diameters correlate with faster transmission rates.

• A-delta fibers transmit sharp pain quickly while C fibers carry slower chronic pain signals.

Signal Transmission and Spatial Summation

Signal Intensity Variation

• Different intensities of signals are transmitted through nerve fascicles; this involves understanding synaptic mechanisms at play.

Spatial Summation Explained

• Spatial summation refers to increasing signal intensity through progressively more active fibers.

Understanding Stimulus Intensity and Temporal Summation

Factors Influencing Stimulus Intensity

• The diameter of the stimulus plays a crucial role; larger diameters lead to greater damage in calados, indicating that intensity increases with both size and centrality of the stimulus.

• Two primary factors determine intensity: the size of the stimulus and its position relative to the center, where stimuli closer to the center yield higher intensity.

Temporal Summation in Response to Repeated Stimuli

• Temporal summation indicates that each repeated stimulus contributes to an increased threshold, leading to heightened overall intensity.

• A graphical representation shows time on one axis and impulse frequency on another; as more impulses from a single receptor are received, signal intensity rises significantly.