Chapter 9 The Senses

CLASSIFICATION OF SENSE ORGANS General sense organs (Table 9-1) Often exist as individual cells or receptor units Widely distributed throughout the body Special sense organs (Table 9-2) Large and complex organs Localized grouping of specialized receptors

CLASSIFICATION OF SENSE ORGANS Classification by presence or absence of covering capsule Encapsulated Unencapsulated (“free” or “naked”) Classification by type of stimuli required to activate receptors Photoreceptors (light) Chemoreceptors (chemicals) Pain receptors (injury) Thermoreceptors (temperature change) Mechanoreceptors (movement or deforming of capsule) Proprioceptors (position of body parts or changes in muscle length or tension)

CONVERTING A STIMULUS INTO A SENSATION All sense organs have common functional characteristics All are able to detect a particular stimulus A stimulus is converted into a nerve impulse A nerve impulse is perceived as a sensation in the central nervous system

GENERAL SENSE ORGANS (Table 9-1) Distribution is widespread; single-cell receptors are common Examples (Figure 9-1, Table 9-1) Free nerve endings—pain, temperature, and crude touch Meissner’s corpuscles—fine touch and vibration Ruffini’s corpuscles—touch and pressure Pacinian corpuscles—pressure and vibration Krause’s end-bulbs—touch Golgi tendon receptors—proprioception Muscle spindles—proprioception

SPECIAL SENSE ORGANS The eye (Figure 9-2) Layers of eyeball Sclera—tough outer coat; “white” of eye; cornea is transparent part of sclera over iris Choroid—pigmented vascular layer prevents scattering of light; front part of this layer made of ciliary muscle and iris, the colored part of the eye; the pupil is the hole in the center of the iris; contraction of iris muscle dilates or constricts pupil Retina (Figure 9-4)—innermost layer of the eye; contains rods (receptors for night vision) and cones (receptors for day vision and color vision) Conjunctiva—mucous membrane covering the front surface of the sclera and also lines the eyelid; kept moist by tears found in the lacrimal gland Lens—transparent body behind the pupil; focuses light rays on the retina

SPECIAL SENSE ORGANS The eye (cont.) Eye fluids Visual pathway Aqueous humor—in the anterior chamber in front of the lens Vitreous humor—in the posterior chamber behind the lens Visual pathway Innermost layer of retina contains rods and cones Impulse travels from the rods and cones through the bipolar and ganglionic layers of retina (Figure 9-4) Nerve impulse leaves the eye through the optic nerve; the point of exit is free of receptors and is therefore called a blind spot Visual interpretation occurs in the visual cortex of the cerebrum

SPECIAL SENSE ORGANS The ear The ear functions in hearing and in equilibrium and balance—receptors called mechanoreceptors Divisions of the ear (Figure 9-5) External ear Auricle (pinna) External auditory canal Curving canal 2.5 cm (1 inch) in length Contains ceruminous glands Ends at the tympanic membrane