20.1: Introduction to Special Senses
Sensory receptors play an integral part in comprehending our external and internal environments. They receive diverse stimuli, converting them into the nervous system's electrochemical signals. This conversion occurs as the stimulus alters the sensory neuron's cell membrane potential, instigating the generation of an action potential. This action potential is subsequently transmitted to the central nervous system (CNS), which integrates with other sensory data or higher cognitive functions. This leads to a conscious awareness of the original stimulus. The central integration may eventually induce a motor response.
Terminology such as 'sensation' or 'perception' is employed intentionally when delineating sensory function. Sensation refers to the initiation of sensory receptor cells concomitant with stimulus exposure. Conversely, perception involves the brain's interpretation of sensory stimuli into recognizable patterns. While sensation is a prerequisite for perception, it does not guarantee the latter. Structures or cells that discern sensations are known as receptors. The transformation of a receptor cell occurs directly in reaction to a stimulus. A transmembrane protein receptor within the cell membrane instigates a physiological alteration in a neuron. This typically transpires via ion channel opening or modifications to cell signaling pathways. Activation of the transmembrane receptors occurs due to chemicals named ligands. For instance, specific molecules present in food may act as ligands for taste receptors. Though not strictly defined as receptors, other transmembrane proteins respond to mechanical or thermal variations. These physical transformations in the proteins can amplify ion translocation across the membrane and trigger an action or a graded potential in sensory neurons.
Sensory Receptors and Perception of Environmental Stimuli
Environmental cues instigate the activation of specialized sensory cells within the peripheral nervous system. These sensory cells are diverse, each specifically attuned to varying forms of stimuli. These sensory cells can be categorized on three primary bases: cellular morphology, spatial location relative to the sensed stimuli, and functional characteristics. Structurally, sensory cells can be categorized by their distinct cellular type and their spatial positioning close to the stimuli they are equipped to perceive. From a functional standpoint, these cells can be classified by their unique ability to transduce stimuli - the process by which a physical stimulus, light, or chemical alteration translates into a change in the cell membrane potential.
Structural Receptor Types
Sensory information translation is principally performed by
- Neurons possessing non-encapsulated nerve endings, where dendritic extensions are integrated within the tissue receiving sensory input
- Neurons with encapsulated nerve termini, where sensory nerve endings are enveloped in a connective tissue layer, augmenting their sensitivity
- Specialized receptor cells are constructed with explicit structural components to interpret a specific stimulus category.
An instance of neurons with non-encapsulated nerve endings is the nociceptive and thermoreceptive neurons within the skin's dermis. Pacinian corpuscles, neurons with encapsulated nerve termini sensitive to mechanical pressure and touch, are also found in the skin’s dermis. Photoreceptors in the retina exemplify a specialized receptor explicitly designed to respond to light stimuli.
Receptors can alternatively be stratified based on their proximity to the stimulus source. Exteroceptors represent a class of receptors near an external environmental stimulus, exemplified by the somatosensory receptors within the dermal layers. Interoceptors, in contrast, interpret stimuli originating from the internal body organs and tissues. An illustration of this would be receptors that register fluctuations in blood pressure within the aortic or carotid sinus. On the other hand, proprioceptors are located adjacent to moving body segments, such as muscles, and are critical for interpreting positional changes in the tissues during movement.
Receptor Functionality Classification
Receptors can be further bifurcated based on their mechanism of converting stimuli into membrane potential alterations. Generally, stimuli can be classified into three broad categories.
Certain stimuli comprise ions and macromolecules that interact with receptor proteins on the cell membrane, instigating a change when these biochemical entities diffuse across the cell membrane.
Other stimuli correspond to changes in physical environmental conditions under which receptor cell membrane potentials are susceptible to alterations.
The remaining category of stimuli encompasses electromagnetic radiation, notably visible light, which can be sensed by the human eye. Interestingly, several organisms possess specialized receptors absent in humans, such as snakes' heat-detecting sensors, bees' sensors for ultraviolet light, or birds' receptors for magnetic fields during migration.
Cells that interpret signals can be further classified into various types, depending on the nature of the stimulus they transduce. Chemoreceptors, for instance, process chemical signals, enabling the recognition of the taste or odor of an entity. Osmoreceptors detect fluctuations in body fluid solute concentrations. Nociceptors primarily serve as a chemical sensing system, discerning the existence of chemical substances arising from tissue harm or similar intense stimuli, thereby perceiving pain. Physical stimuli, encompassing pressure, vibration, auditory sensations, and positional awareness (equilibrium), are discerned via a mechanoreceptor. Thermoreceptors are specialized receptors that detect temperature changes, with distinct types responsive to temperatures either higher (heat) or lower (cold) than the average body temperature.
Sensory Modalities
Interrogating the average person regarding the human senses results in enumerating the five fundamental senses—gustation, olfaction, tactician, audition, and vision. Nevertheless, such a list is not exhaustive. Crucially absent is equilibrioception, or our sense of balance. Furthermore, the broad category of tactician can be categorized into more specialized modalities, such as pressure, vibration, stretch, and position of hair follicles, all discerned by different mechanoreceptors. Additional neglected modalities encompass thermoception, the detection of temperature by thermoreceptors, and nociception, the perception of pain via nociceptors.
In physiology, sensory perception can be categorized into general or specific frameworks. General sensory perception is pervasive throughout the body, with receptor cells embedded within the structure of other organs. For instance, mechanoreceptors in the skin, muscles, or blood vessel walls exemplify this category. Such general senses typically contribute to touch, proprioception (body's spatial orientation), kinesthesia (motion awareness), or autonomic functions through visceral senses. In contrast, specialized senses are associated with specific organs like the eye, inner ear, tongue, or nose.
Every sense is designated as a sensory modality, a term that encapsulates the concept of information encoding and mirrors the idea of transduction. The primary sensory modalities can be cataloged based on their respective transduction mechanisms. Taste and smell fall under chemical senses, while touch, often denoted as a general sense, encompasses chemical sensation in the form of nociception or pain. Mechanoreceptors detect sensations like pressure, vibration, muscle stretch, and hair movement due to an external stimulus. Mechanoreceptors also enable hearing and balance, while photoreceptors facilitate vision.
The myriad of sensory modalities, potentially amounting to 17, necessitates the fragmentation of the five primary senses into more specific subcategories or submodalities. Each sensory modality corresponds to the sensation of a unique stimulus. For instance, somatosensation, the general sense of touch, can be subdivided into various submodalities such as light pressure, deep pressure, vibration, itch, pain, temperature, or hair movement.
