Lesson 17: Your Senses

Lesson 17: Your Senses

Senses • Special senses: taste, smell, touch, vision, hearing, balance • General senses:

• Somatic: tactile (touch, pressure, vibration); thermal (warm, cold); pain; proprioceptive (body position).

• Visceral: provides information about conditions within body fluids and internal organs.

Sensation • Definition: Conscious or subconscious awareness of the external

environment or internal conditions of the body. • To occur, four conditions must be met:

• A stimulus (change) in the environment must occur. • A sensory receptor must convert the stimulus to an electrical signal

which can become a nerve impulse. • The nerve impulse must be conducted along a neural pathway to

the brain. • A region of the brain must receive and integrate the nerve

impulse(s) into a sensation.

Perception • Definition: Mental

awareness of sensory stimulation.

• Occurs when the cerebral cortex interprets the meaning of sensations.

Sensory Adaptation • Definition: Phenomenon of a

sensation becoming less noticeable once it has been recognized by repeated stimulation

• Causes: • May result from sensory receptors

no longer sending impulses to the brain.

• May result from prioritization that the thalamus performs (its “gatekeeper function”). It passes only information of immediate importance to the cerebral cortex.

fMRI photo from NASA, thalamus pathways by Rami Tarawneh

Sensory Receptors • Exteroceptors: Sensory receptors that detect stimuli from outside the

body. • Interoceptors: Sensory receptors that receive stimuli from inside the

body—directly involved in homeostasis, regulated by negative feedback.

Functional Classification of Sensory Receptors

Mechanoreceptors Detect mechanical pressure; provide sensations of touch, pressure, vibrations, proprioception, and hearing and equilibrium; also monitor stretching of blood vessels and internal organs.

Thermoreceptors Detect changes in temperature

Nociceptors Pain receptors that respond to stimuli resulting from physical or chemical damage to tissue

Photoreceptors Detect light that strikes the retina of the eye

Chemoreceptors Detect chemicals in mouth (taste), nose (smell), and body fluids.

Osmoreceptors Sense the osmotic pressure of body fluids.

Receptors in Skin • Receptors sensitive to touch: Merkel’s

disks, Krause end bulbs, root hair plexus, Meissner corpuscles.

• Pressure: Pacinian corpuscles, Ruffini endings

• Pain: Free nerve endings • Temperature: Free nerve endings.

Courtesy of Clipart.com

• The sense of taste involves gustatory signals perceived by taste buds which can distinguish five primary tastes: • Sour • Sweet • Bitter • Salty • Umami (meaty or

savory) • Our brain utilizes olfactory,

tactile, and gustatory input in determining what something tastes like.

Sense of Taste


Sense of Smell • Smell is dependent on the olfactory

epithelium. • Some 10-100 million receptors for

sense of smell (olfaction) reside within the olfactory epithelium. • Individual receptors respond to

hundreds of different scents. • We recognize as many as 10,000

different odors. • Adaptation to odors occurs very

rapidly. • Sensitivity decreases by ~50% in

first second. • Some nerve impulses for smell and

taste travel to the limbic system. • Evoke emotional responses or flood

of memories.


Sense of Vision “Where there is no vision, the people perish.”

Proverbs 29:18

• Requires work in both eyes and brain.

• Much of the processing of stimuli occurs in the eyes before nerve impulses are sent to the brain.

• However, over 1/3 of the cerebral cortex is involved in processing visual information.


Anatomy of the Eye • Outermost Layer:

• Fibrous tunic • Contains sclera and cornea

• Sclera: • Posterior • White and fibrous collagen

tissue • Cornea:

• Anterior • Made up of transparent

collagen fibers • Easily transplanted • Refracts (bends) light rays

Anatomy of the Eye • Middle Layer:

• Vascular Tunic • Where blood vessels are

located • Contains choroid,

ciliary body, and iris • Choroid:

• Thin membrane that lines most of internal surface of sclera

• Contains many blood vessels

Anatomy of the Eye • Ciliary Body:

• Secretes aqueous humor • Houses ciliary muscle

• Smooth muscle • Alters shape of lens for

close-up or distant viewing

• Focusing: • Relaxation or contraction of

ciliary muscle alters the shape of the lens.

• When focusing on distant objects, muscle relaxes and lens flattens.

• When focusing on close objects, muscle contracts and lens is rounded.

Anatomy of the Eye • Lens

• Transparent structure. • Focuses light rays onto the retina.

• Pupil • Opening in center of iris through

which light enters. • Iris

• Donut-shaped, colored part of eye, color determined by amount of pigmentation.

• Has both circular and radial smooth muscle fibers.

• Determines size of opening of pupil.

• In bright light, pupil constricts as circular muscles of iris contract (parasympathetic).

• In dim light, pupil dilates as radial muscles of iris contract (sympathetic).

Anatomy of the Eye • Innermost Layer: Retina

• Lines the posterior ¾ of the eyeball

• Filled with clear, gelatinous material (vitreous humor)

• Beginning of visual pathway • Two layers:

• Neural layer • Pigmented layer

Anatomy of the Eye • Neural Layer:

• Multilayered outgrowth of brain, three distinct layers:

• Photoreceptor • Bipolar cell • Ganglion cell • Note that light must pass

through the ganglion & bipolar cell layers as well as the outer and inner synaptic layer zones before it interacts with the photoreceptor layer.

• Two “zones”: • Outer synaptic • Inner synaptic

Photoreceptor Layer • Two types of cells highly specialized for detecting light rays:

• Rods: • Very sensitive to light • Allow for perception of shades of gray • 120 million per eye

• Cones: • Requires more light than rods • Permits color vision • Six million per eye • Concentrated at the central fovea

Photopigment • Rod and cone cells possess many disks that

have photopigment molecules, a substance that can absorb light and undergo a change in structure, embedded within them.

• Rods: • The photopigment in rods is

rhodopsin. • In the presence of light, rhodopsin is

broken down into opsin and retinal. • Cones: three different types of cones, each

with a different photopigment: • Each photopigment is composed of

retinal combined with a slightly different opsin molecule.

• Each is sensitive to a different color: blue, green, red.

• When stimulated by light of appropriate wavelength, the photopigments split into retinal and opsin.

Diagram by Madhero88

Generating a Nerve Impulse The splitting of the photopigment in the rods and cones of the retina activates an enzyme that initiates a series of reactions that leads to the generation of a nerve impulse in the optic nerve at the back of the eye.

Photo by David Hardman

Stereoscopic Vision • Once generated, the nerve impulses

travel along the optic nerve to the optic chiasm. • At the optic chiasm, crossing over

occurs. • ~1/2 of nerve fibers cross over to

the opposite side of the brain. • Fibers follow the optic tract into the

thalamus, then neural impulses travel along fibers to visual areas of the cerebral cortex.

• Each half of the brain receives input from both eyes; the brain integrates the impulses into a unified image: • It rights the image, originally

registered as inverted and reversed. • Processes visual area perceived by

each eye into a 3D image.

Photo by Rico Shen

Near- & Far-Sightedness • A normal (emmetropic) eye is capable

of refracting light rays from an object 20 feet away so that a clear image is focused on the central fovea of the retina. • The eye depends on its shape,

cornea, and lens to accomplish this. • In the near-sighted (myopic) eye,

the image is focused in front of the retina. • Use of a concave lens diverges the

light rays so that they have to travel further in the eyeball before being focused.

• In the far-sighted (hypermytropic) eye, the image is focused behind the retina. • Use of a convex lens converges the

light rays so that they have to travel less distance in the eyeball to be focused.


Presbyopia • Loss of lens elasticity that accompanies

aging. • Ability to focus close objects decreases. • “Reading” glasses required for close vision.


Blindness • Complete loss of cone vision = legally blind. • Loss of rod vision = difficulty seeing in dim

light. • Night Blindness

• Can result from prolonged deficiency of Vitamin A.

• Possesses below-normal amounts of rhodopsin.

• Color-blind: One or more photopigment is missing from cone cells. • Red-Green colorblindness is most

common. • Yellow-orange and/or green photopigment

is missing; the person cannot distinguish between red and green.

Illustration by E. W. Scripture, from Thinking, Feeling, Doing, 1895.

Anatomy of the Ear • Outer ear: extends from auricle

to tympanic membrane (ear drum).

• Middle ear: extends from ear drum to “windows.”

• Inner ear: extends from “windows” inward.

Outer Ear • Auricle (Pinna): Fleshy

trumpet-shaped external part of ear.

• External auditory canal: slightly S-shaped, air-filled structure, extends from opening of auricle to outer layer of tympanic membrane .

• Outer layer of tympanic membrane.

Middle Ear • Tympanic cavity: Air-filled

space within inner ear. The middle ear also includes the inner layer of tympanic membrane.

• Auditory Ossicles (bones): • Malleus (hammer) • Incus (anvil) • Stapes (stirrups)

• Auditory (eustachian) tube: Opening that extends from inner layer of the tympanic membrane to the nasopharynx of throat.

• Oval Window: small, membrane-covered, nearly transparent opening into inner ear.

• Round Window: similar to the oval window, only smaller.

Inner Ear • Outer Bony Labyrinth:

series of fluid-filled (perilymph) cavities (canals) surrounding the membranous labyrinth, including: • Cochlea • Vestibule • Semicircular canal

• Membranous Labyrinth: series of fluid-filled (endolymph) of sacs and tubes within the bony labyrinth,

• Connected to vestibulocochlear nerve.

Inner Ear: Cochlea • The cochlea has three fluid-filled

canals separated by membranes: • Vestibular canal • Cochlear canal • Tympanic canal

• It also has a tectorial membrane that is connected to hair cells embedded in the basilar membrane by microvilli.

• Hair cells communicate with the nerve endings of the cochlear nerve.

• The tectorial membrane, the hair cells, and the basilar membrane are components of the spiral organ or organ of Conti.

• All are involved in the generation of nerve impulses the cerebral cortex integrates into sound.

Diagram by Dick Lyon

Physiology of Hearing 1) The auricle directs sound waves into external auditory canal.

2) Sound waves strike the ear drum and cause it to vibrate. Loudness and pich depend on intensity and frequency of sound waves.

3) The ear drum, connected to the malleus, causes it to vibrate. Vibration moves to the incus and then to the stapes.

4) The movement of the stapes pushes the oval window in and out.

5) The movement of the oval window generates pressure waves in the perilymph of the cochlea.

6 & 7) Pressure waves in the perilymph push the vestibular membrane back and forth, creating pressure waves in the endolymph.

8) Pressure waves in the endolymph cause the basilar membrane to vibrate, thereby moving hair cells against the tectorial membrane.

9) Bending of hair cells releases neurotransmitters which travel to nerve endings and generate a nerve impulse.

10) The nerve impulse travels to the brain.


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Lesson 17: Your Senses