Anatomy & Physiology I and II

Anatomy & Physiology Study Guide

©2018 Achieve Test Prep Page 185 of 367 Sensory information that arrives at the CNS is routed according to the location and nature of the stimulus. Neurons relay information from one point (the receptor) to another (a neuron at a specific site in the cerebral cortex) along the sensory pathways. A link called a labeled line forms a connection between the peripheral receptor and cortical neuron. Each labeled line consists of axons carrying information about one modality or type of stimulus (i.e., sound, touch, light, and pressure). The CNS interprets the modality entirely by the labeled line over which it arrives. All other characteristics of the stimulus—its strength, duration, and variation—are conveyed by the pattern and frequency of action potentials. Sensory coding translates complex sensory information into meaningful patterns of action potentials. Tonic receptors are sensory neurons that are always active. The frequency with which these receptors generate action potentials indicates the background level of stimulation. An increase or decrease in the stimulus changes the rate of action potential generation accordingly. Other receptors remain inactive; however, they activate for a short time in response to a change in the conditions they monitor. These phasic receptors send information about the stimulus regarding the rate and intensity of the change. Adaptation Adaptation occurs when there is a reduction in sensitivity in the presence of a constant stimulus. When the level of activity changes at the receptor, it is called peripheral adaptation. The receptor responds strongly at first, but, afterward, its activity gradually declines as the size of the generated potential gradually decreases. Characteristic of phasic receptors, this immediate response gives them an additional name: fast-adapting receptors. Peripheral adaptation reduces the quantity of information that reaches the CNS. Therefore, the brain is not constantly stimulated by the presence of shirt or pants on the body. The Detection of Stimuli Each receptor has a characteristic sensitivity. Specificity is determined by the receptor cell structure, from the presence of accessory cells, or from structures that shield the receptor cell from other stimuli. The dendrites of sensory neurons are the simplest receptors. Of note, these dendrites have branching tips that are not protected by accessory structures and, thus, are called free nerve endings . The area monitored by a single receptor cell is its receptive field . The CNS receives information from the receptive field when a sufficiently strong stimulus is present. It is acknowledged as “stimulus arriving at receptor X.” The smaller the receptive field, the greater the ability to localize the stimulus. The arriving stimulus can come in many forms. Regardless of the nature of the stimulus, sensory information arrives at the CNS in the form of action potentials. As noted, transduction is the translation of an arriving stimulus into an action potential by a sensory receptor. When a stimulus alters the transmembrane potential of the receptor cell, transduction begins. This change is the receptor potential and is either a graded hyperpolarization (more negative) or graded depolarization (more positive). The stronger the stimulus is, the larger the receptor potential that results. The Interpretation of Sensory Information