Chapter 16. Sensory Systems
Ayda Basgul Martin
Unit Outline
Part 1: Overall Organization of the Sensory Systems
Learning Objectives—Parts 1–3: Overall Organization of the Sensory Systems, Gustation, Olfaction
At the end of this part of this unit, you should be able to:
I. List the five main categories of sense receptors in the body based on the types of stimuli they respond to.
II. Describe the structure and function of cutaneous sensors and proprioceptors.
III. Describe the structure and function of the olfactory system.
IV. Describe the structure and function of the gustatory system.
A major role of sensory receptors is to help us learn about the environment around us, or about the state of our internal environment. Stimuli from varying sources, and of different types, are received and changed into the electrochemical signals of the nervous system. This occurs when a stimulus changes the cell membrane potential of a sensory cell. The stimulus can cause the sensory cell to produce an action potential, which is relayed into the central nervous system (CNS), where it is integrated with other sensory information—or sometimes higher cognitive functions—to become a conscious perception of that stimulus. The central integration in the CNS may then lead to a motor response.
Describing sensory function with the term sensation vs. perception is a deliberate distinction. Sensation is the activation of sensory receptor cells at the level of the stimulus. Perception is the central processing of sensory stimuli into a meaningful pattern. Perception is dependent on sensation, but not all sensations are perceived. Receptors are the cells or structures that detect sensations. A receptor cell is changed directly by a stimulus. A transmembrane protein receptor is a protein in the cell membrane that mediates a physiological change in a cell, most often through the opening of ion channels or changes in the cell signaling processes. Transmembrane receptors are activated by chemicals called ligands. For example, a molecule in food can serve as a ligand for taste receptors. Other transmembrane proteins, which are not accurately called receptors, are sensitive to mechanical or thermal changes. Physical changes in these proteins increase ion flow across the membrane and can generate an action potential or a graded potential in the sensory neurons.
Sensory Receptors
Stimuli in the environment activate specialized receptor cells in the peripheral nervous system. Different types of stimuli are sensed by different types of receptor cells. Receptor cells can be classified on the basis of three different criteria: cell type, position, and function. Receptors can be classified structurally on the basis of cell type and their position in relation to stimuli they sense. They can also be classified functionally on the basis of the transduction of stimuli, or how a mechanical stimulus, light, or chemical changes the cell membrane potential.
Structural Receptor Types: The cells tha