The Senses Nestor T. Hilvano, M.D., M.P.H. (Images Copyright Discover Biology, 5th ed., Singh-Cundy and Cain, Textbook, 2012.)

Learning Objectives State the functions of sensory pathways. Define sensory receptors and describe the 5 general categories of sensory receptors. Describe the human eye and its function. Compare the rods and cones of retina. Explain myopia, hyperopia, presbyopia and astigmatism. List the structure of ear in the order in which they participate in hearing. Explain how body position and movement are sense in the ear. Discuss how odor and taste receptors function.

Sensations All stimuli represent forms of energy Sensation involves converting energy into a change in the membrane potential of sensory receptors The brain interprets sensations, giving the perception of stimuli General senses (Somatic)- skin, muscles, joint, and viscera (internal organs) Special senses- sight (vision), smell (olfaction), taste (gustation), hearing (audition), and vestibular (balance/equilibrium)

Fig. 50-2 Functions of Sensory Pathways: sensory reception, transduction, transmission, and integration (brain perception) Weak receptor potential Action potentials Membrane potential (mV) –50 Membrane potential (mV) –70 Slight bend: weak stimulus –70 1 2 3 4 5 6 7 Brain perceives slight bend. Dendrites Stretch receptor Time (sec) 1 2 4 Axon 3 Brain Muscle Brain perceives large bend. Action potentials Large bend: strong stimulus Strong receptor potential Membrane potential (mV) Figure 50.2 A simple sensory pathway: Response of a crayfish stretch receptor to bending Membrane potential (mV) –50 –70 1 Reception –70 1 2 3 4 5 6 7 Time (sec) 2 Transduction 3 Transmission 4 Perception

Sensory Receptors: Transduce Stimuli into Action Potential Classification of Receptors: Pain (nociceptors) receptors – inflame/damage tissues Thermoreceptors – change in temp., hot or cold Mechanoreceptors – change in pressure or fluid movement; touch, pressure, stretching of muscles, motion, sound Chemoreceptors – change in odor and taste Electromagnetic receptors – electricity, magnetism, and light (photoreceptors) Receptors for General senses: Pain - free nerve endings Temperature –Ruffini (hot); Kraus End Bulb (cold) Crude (light) touch -Meissner’s corpuscle Proprioception (deep touch and pressure) -pacinian corpuscle Receptors for Special senses: Vision – rods and cones Audition – organ of corti Vestibular – maculae, cristae Olfaction- olfactory cell Gustation- taste bud

Figure 30.13 Some Mechanoreceptors in Human Skin Human skin contains many different types of neurons and sensory cells that provide detailed information about mechanical stimuli.

Eyeball - Vision Fibrous (outermost) layer = sclera and cornea Vascular (middle) layer = choroid, ciliary body, and iris *pupil – central opening * Light passes through a transparent lens that focuses images on the retina Nervous (innermost) layer = retina, photoreceptor layer (rods and cones) Rods - sensitive to low level of light; night vision Cones - detect colors of light; day vision _____ - center of sharpest vision (cones only) _____ - Blind spot (w/ nerve & b.v.) a. optic disc b. fovea centralis Aqueous humor filled the chamber in front of the lens Vitreous humor filled the chamber behind the lens

Figure 30.14 Major Features of the Human Eye A pupil adjusts the amount of light entering through the lens. The lens focuses images on the retina, where photoreceptor cells generate nerve impulses in sensory neurons that are closely associated with them.

Figure 30.17 Sensory Neurons in the Retina Send Visual Information to the Brain Layers of sensory neurons stacked above the rods and cones process visual information to some degree before sending it to the brain via the optic nerve.

Copyright © 2009 Pearson Education, Inc Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Visual Problems Passage of light: Light cornea aqueous humor pupil lens vitreous humor retina Nearsightedness (myopia)- Can’t see far distance; eyeball is longer; focus distant objects in front of retina; corrective lenses that are thinner in the middle Farsightedness (hyperopia)- Can’t see near distance; eyeball is short; focal point is behind the retina; corrective lenses that are thicker in the middle *presbyopia – hyperopia in old age, inelasticity of lens Astigmatism – blurred vision caused by misshapen lens or cornea; corrective asymetrical lens

Clinical Correlation ___ = clouding of lens aging, diabetes, smoking, and UV light ___ = death of retinal cells due to elevated pressure within the eye ___ = pink eye, results from damage or irritation of conjunctival surface. a. glaucoma b. conjunctivitis c. cataract

Ear: Hearing Mechanoreceptor (change in pressure due to sound waves) Outer ear = pinna and auditory canal * tympanic membrane Middle ear = contain ossicles (malleus, incus, stapes) * oval window Inner ear = cochlea (contains organ of corti) Auditory nerve Sound volume (loudness) ranging from 0 to 120 db. (long exposure to 90 db can cause hearing loss) Pitch (frequency) from 20 – 20,00 Hz. Sound at pinna auditory canal tympanic membrane middle ear oval window cochlea (organ of corti) send impulses to the brain (auditory cortex)

Semicircular canals (function in balance) Skull bones Oval window Figure 29.4B Semicircular canals (function in balance) Skull bones Oval window (behind the stirrup) Auditory nerve, to the brain Hammer Anvil Stirrup Eardrum Figure 29.4B The middle ear and the inner ear Cochlea Eustachian tube (connects to the pharynx) 14

Ear: Vestibular Sense: (Body Position and Balance) Inner ear – houses organs of balance a) Semicircular canals = crista ampullaris, receptor for changes in head’s rate of rotation b) Utricle and saccule – maculae, receptor for head position relative to the force of gravity and linear motion

Semicircular canals Flow of fluid Vestibular nerve Cupula Hairs Fig. 50-11 Semicircular canals Flow of fluid Vestibular nerve Cupula Hairs Hair cells Vestibule Figure 50.11 Organs of equilibrium in the inner ear Axons Utricle Body movement Saccule

Taste Receptors Chemoreceptors in taste buds 4 types a) sweet b) sour c) salty d) bitter A fifth taste perception called umami (elicited by glutamate) Taste perception varies with individuals and may be related to genetic traits

Figure 30.11 Taste in Humans Taste buds consist of groups of chemoreceptor cells and supporting cells. The receptor cells respond to chemicals that lodge in the tongue during eating.

Smell Receptors Chemoreceptors in nose (olfactory mucosa) – olfactory (bipolar) cells Integration of signals in the brain results in an odor perception Smell is tied with limbic system, evoking emotions and memories.

Figure 30.12 The Human Sense of Smell

Homework Define terms: rods, cones, presbyopia, myopia, hyperopia, astigmatism, cornea, lens, tympanic membrane, semicircular canals, utricle and saccule, cataract, glaucoma, conjunctivitis, optic disc, fovea centralis, macula, cristae ampullaris. Give the sensory receptors of the following: __ pain a. bipolar (olfactory) cells __ pressure and deep touch b. organ of corti __ taste c. free nerve endings __ olfaction d. pacinian corpuscle __ hearing e. taste bud Classify the receptors of the items listed below: __ change in pressure or fluid movement a. chemoreceptor __ change in light perception b. thermoreceptor __ change in smell of CO2 c. photoreceptor __ change in climate’s temperature d. mechanoreceptor