Human Physiology - William F. Ganong 2002

Physiology of Nerve and Muscle Cells
Initiation of Impulses in Sense Organs
Senses

Modalities of Sensation

Because sensory receptors are specialized to respond to one specific form of energy, and the environment

contains many different forms of energy, it follows that there must be A wide variety of receptor types. While we learn in school that There are five senses, the inadequacy of this traditional view becomes immediately apparent when we list the major human sensory modalities and their respective receptors. The first 11 modalities in Table 5-1 represent conscious sensations. In addition, There is a vast array of sensory receptors that process information which never reaches consciousness. For example, Muscle spindles provide feedback on muscle length, while other receptors monitor variables such as ARTERIAL Blood PRESSURE, the Temperature of blood in the HEAD, and the pH of CEREBROSPINAL FLUID. The existence of further receptors of this kind is strongly suspected, and future research will undoubtedly expand this list of "unconscious sensations." Nevertheless, any catalog of sensations remains inherently limited and cannot be considered exhaustive. Rods and cones, for instance, respond maximally to light of different wavelengths, and there are distinct cone populations for each of the three primary colors. Four basic taste modalities are recognized—sweet, salty, sour, and bitter—each served by a distinct receptor type. We perceive different pitches primarily because various groups of Hair Cells within The Organ of Corti are maximally activated by sound waves of varying frequencies. Whether these diverse responses to light, taste, and sound should be classified as separate "senses" is largely a semantic question and, in this context, purely academic.

Classification of Sense Organs

Scientists have made numerous attempts to classify sensations, yet none has proven entirely comprehensive. Traditionally, the senses include Olfaction, Vision, Hearing, rotational and linear acceleration, and taste; cutaneous sensations are associated with receptors in the Skin, whereas visceral sensations relate to the perception of signals from the internal environment. Pain originating from visceral structures is generally categorized as visceral pain. Another classification system divides receptors into functional groups: telereceptors (distance receptors), which respond to events at a distance; exteroceptors, which monitor the immediate external environment; interoceptors, which monitor the internal environment; and proprioceptors, which provide information about body position in space at any given moment. However, the conscious component of Proprioception (the "body image") is synthesized from information originating not only from receptors within and around joints, but also from cutaneous tactile receptors and baroreceptors. Other specialized terms are also used on occasion. Because pain fibers possess connections that mediate strong withdrawal Reflexes (see Chapter 6), and because pain is triggered by potentially harmful or destructive stimuli, pain receptors are sometimes referred to as nociceptors. The term chemoreceptor is applied to receptors activated by changes in The chemical composition of their surrounding environment. These include taste and olfactory receptors, as well as visceroreceptors sensitive to fluctuations in plasma oxygen levels, pH, and osmotic pressure.

Class="center">Table 5-1. Major Sensory Modalities

¹ The first 11 are conscious sensations.

² See text.

Cutaneous Sense Organs

Cutaneous sensation is traditionally divided into four modalities: touch-pressure (where pressure represents sustained touch), cold, warmth, and pain. The skin contains a variety of sensory endings, including free nerve endings, expanded tip endings on sensory nerve fibers, and encapsulated endings. Expanded endings include Merkel disks and Ruffini endings (Fig. 5-1), while encapsulated endings comprise Pacinian corpuscles, Meissner's corpuscles, and Krause end bulbs. Ruffini endings and Pacinian corpuscles are also found in the deep fibrous layer of the dermis, which additionally harbors sensory nerve endings wrapped around hair follicles. However, neither expanded nor encapsulated endings are strictly necessary for cutaneous sensation; their distribution varies considerably across different body regions. It has been repeatedly demonstrated that all four cutaneous sensations can be elicited from areas that, upon histological examination, contain only free nerve endings. Where present, expanded or encapsulated endings function as mechanoreceptors responsive to tactile stimuli. Meissner's and Pacinian corpuscles act as rapidly adapting mechanoreceptors, whereas Merkel disks and Ruffini endings serve as slowly adapting mechanoreceptors. Hair follicle nerve endings mediate touch, with hair movements initiating tactile sensations. It is important to note that although cutaneous sensory receptors lack histological Specificity, they are physiologically specific. Consequently, each individual nerve ending signals one, and only one, cutaneous sensory modality.

Fig. 5-1. Sensory receptors in the skin. Ruffini corpuscles (A) and Merkel disks (B) represent expanded types of sensory nerve fiber endings. Meissner's corpuscles (C), Pacinian corpuscles (D), and Krause end bulbs (E) are encapsulated endings; F shows free nerve endings interspersed among tissue cells.