HSS 390 Ch. 16 Special Senses

Sensory receptors

Anatomical structures specialized to detect a stimulus that act as transducers; may be simple in structure or a complex sense organ

Sense organ

Nerve tissue surrounded by other tissues that enhance response to a certain type of stimulus (e.g., the eye and cochlea)

Transduction

Process by which a stimulus energy is converted into electrical energy (an action potential); receptors will have a resting membrane potential (RMP) and stimulus will open a modality-gated ion channel specific to the receptor

• Ion flow → AP → conducted toward CNS for perception & interpretation

Chemoreceptors

Detect chemicals dissolved in fluid (e.g., smell or taste of food; oxygen levels in blood)

Thermoreceptors

Detect changes in temperature (thermo- - heat; e.g., receptors in skin, hypothalamus)

Photoreceptors

Detect changes in light (photo- = light; e.g., in the retina of eye [vision])

Mechanoreceptors

Detect physical/mechanical distortion of cell membrane (e.g., touch, pressure, vibration, stretch receptors; hearing & equilibrium)

Nociceptors

Detect pain stimuli (noci- = pain; e.g., chemical, heat, or mechanical damage to tissues)

General sensory receptors

Simple structures distributed throughout the body

• Somatic sensory receptors — monitor touch, pain, pressure, vibration, temperature, & proprioception

• Visceral sensory receptors — found in tissue of organs; monitor stretch, chemical environment, temperature, & pain

Special sensory receptors

Specialized sensory receptors in complex sense organs of the head; detect 5 special senses — olfaction, gustation, vision, hearing, & equilibrium

Olfaction

Detection of odorants in the air (olfact- = smell), chemicals that, when dissolved in nasal mucus, are detected by chemoreceptors

Olfactory epithelium/mucosa

The sensory receptor organ for olfaction located in the roof of the nasal cavity; contains…

• 1st order neurons of smell: 10-20 million olfactory receptor cells (neurons) that detect odorants

• Basal cells: act. as regenerating stem cells; replace olfactory receptor cells every 40-60 days

• Supporting cells: act as glial cells to support olfactory receptor cells

1st order neurons of smell

Olfactory receptor cells (neurons) that detect odorants

• Unique neurons — still undergo mitosis & are exposed to the environment

• One end has cilia (“olfactory hairs”) with chemoreceptors embedded in mucus

• Other end is the axon; axons collectively travel upwards as olfactory nerve (CN I)

2nd order neurons of smell

Located in olfactory bulb; their axons form the olfactory tract

Transduction of olfaction

Odorant dissolves in mucus → stimulates chemoreceptor on olfactory “hairs” (cilia) of olfactory receptor cell (1st order neuron)

• G-protein in receptor cell activates adenylate cyclase, converts ATP to cAMP

• cAMP opens ion channels for Na⁺ and Ca²⁺ (move INTO cell)

• Depolarization → AP in axons of CN I → conducted to 2nd order neuron in olfactory bulb

Pathway of olfaction

1. Olfactory nerve (CN I — 1st order neuron)

2. Olfactory bulb (2nd order neurons; axons from olfactory tract)

3. Primary olfactory cortex (in temporal lobe)

4. Hypothalamus (salivation, hunger, increased stomach acid, gagging, vomiting) OR amygdala/hippocampus of limbic system (emotion/memory related to smells)

*Does NOT involve the thalamus; olfactory bulb acts as the thalamus (“router”)

Gustation

Detection of tastants, molecules/ions in food/drink (gust- = taste); when dissolved in saliva, are detected by chemoreceptors

Taste buds

The sensory receptor organ for taste located on the tongue; contain…

• Taste/gustatory cells: modified epithelial cells that behave like neurons (but are NOT true neurons); detect tastants

• Basal cells: act as regenerating stem cells; replace gustatory cells

• Supporting cells: act as glial cells to support gustatory cells

Gustatory cells

Modified epithelial cells that behave like neurons and detect tastants; have microvilli (“taste hairs”) that contain chemoreceptors

• Release neurotransmitters onto sensory nerve fibers at their base

• Sensory nerves for taste belong to CN VII (ant. 2/3 of tongue) & CN IX (post. 1/3 of tongue)

Transduction of gustation

Tastant dissolves in saliva → stimulates chemoreceptors on taste “hairs” of gustatory cells → depolarization/AP depends on “tastant” causing taste sensation…

• G-protein & cAMP = sweet, bitter, umami

• Opening of ion channels = salty, sour

Once depolarization, gustatory cell releases neurotransmitter at its base, which excites sensory nerve fibers

Sweet

Taste produced by organic carbohydrate molecules

• Depolarization through G-protein & cAMP

Bitter

Taste produced by organic molecules containing nitrogen, often with basic pH (e.g., unsweetened chocolate, nicotine, caffeine)

• Depolarization through G-protein & cAMP

Umami

Taste associated with protein molecules producing meaty flavor

• Depolarization through G-protein & cAMP

Salty

Taste produced by metal ions (e.g., Na⁺ and K⁺)

• Depolarization through opening of ion channels

Sour

Taste associated with acids (H+ ions) (e.g., vinegar)

• Depolarization through opening of ion channels

Pathway of gustation

1. Facial (CN VII) and glossopharyngeal (CN IX) nerves (formed by 1st order neurons found in ganglia)

2. Medulla oblongata (2nd order neurons)

THEN…

1. Thalamus (3rd order neurons)

2. Primary gustatory cortex (in insula)

OR…

• Hypothalamus (salivation. hunger, gagging, vomiting) OR amygdala/hippocampus of limbic system (emotion/memory related to food)

Vision

Perception of objects, color, and movement in the environment by means of the light they reflect/emit); light rays must enter the eye and be focused on the photoreceptors found in the retina

Optical components

Transparent structures that admit light rays and focus them on the retina, includes the cornea, aqueous humor, lens, & vitreous humor

Tunics of the eye

“Cloaks/coverings” forming the wall of the eyeball; includes the fibrous tunic, vascular tunic, & retina

Fibrous tunic

Outer, fibrous, avascular layer; consists of the sclera & cornea

Sclera

Part of the fibrous tunic of the eye; the dense, collagen-rich “white “ of eye (sclera- = tough, hard)

Cornea

Part of the fibrous tunic of the eye; the transparent region of modified sclera in front of eye that admits and refracts light

Vascular tunic

Middle vascular layer of the eye; consists of the choroid, ciliary body, & iris

Choroid

Part of the vascular tunic of the eye; rich in capillaries & melanocytes (melano- = dark, black)

• Capillaries nourish retina

• Melanin pigment absorbs light & prevents scattering of light rays

Ciliary body

Part of the vascular tunic of the eye; an extension of the choroid that forms a muscular ring around the lens

• Supports lens & iris

• Secretes aqueous humor

Iris

Part of the vascular tunic of the eye; the colored diaphragm controlling size of pupil (opening)

• Eye color is determined by the amount & type of melanin in melanocytes

Photopupillary reflex

Visceral reflex of the ANS in response to light (photo- = light); involves two sets of smooth muscle on the posterior iris

• Light → contraction of pupillary constrictor muscle (parasympathetic preganglionic axons travel with CN III)

  • Narrows focus of light onto the center of the les, where there is maximal curvature

• Low light (or sympathetic stimulation) → contraction of pupillary dilator muscle

Retina

Inner, neural tunic (layer); consists of pigmented and neural layers (contains photoreceptors) 

Pigmented layer of retina

Layer of the retina internal to the choroid that provides vitamin A and regenerates rhodopsin for photoreceptors; diffuses nutrients from choroid to neural layer

• Has melanin to attract light rays & prevent them from reflecting off retina

Neural layer of retina

Houses photoreceptors and other neurons associated with vision

Photoreceptor cells

NOT neurons — cells that absorb light using photopigments and initiate signal; include rods and cones

Rods

Highly sensitive photoreceptor cells that work well in reduced light and night vision; provide poor contrast in shades of gray

• Contain the photopigment (light-absorbing molecule) RHODOPSIN

Cones

Photoreceptors that function in color, clarity, and day vision; they are less sensitive and require brighter light

• Contain 3 types of photopigments (light-absorbing molecules) called PHOTOPSINS with sensitivity to light of different wavelengths (and therefore, color — red, green, & blue cones)

• Colorblindness results when a photopsin is lacking of deficient due to genetic mutation (X-linked → higher probability in males)

Bipolar cells

First-order neurons of vision; dendrites synapse with rods & cones and axons synapse with ganglion cells

Ganglion cells

Second-order neurons of vision located in a single layer next to the vitreous body; axons from the optic nerve (CN II)

Pathway of light to the retina

Ganglion layer → bipolar layer → photoreceptors → pigmented layer

Opthalmoscope

Tool that is used to examine the fundus (rear) of the eye, namely the retina (ops- = eye; scope- = examine, observe, see)

Optic disc

Area on the fundus where blood vessels enter/exit and axons of ganglion cells (CN II) exit; no photoreceptors = “blind spot”

Macula lutea

Patch of cells in the center of the fundus containing the fovea centralis (macula- = little spot; lutea- = yellow)

Fovea centralis

Area in the center of the macula lutea; high density of cones with almost no rods → point of highest visual clarity/detail

• Every other area of the retina contains primarily rods and functions well in low light

Refraction of light rays

Bending of light rays required to focus them on the fovea centralis; light rays are bent when they pass through the cornea and lens (shape of cornea does not change, but the lens curves as objects move closer to the eye)

Accomodation

The lens becomes increasingly curved/spherical as objects move closer to the eye; this bends light rays more & more to focus them on the fovea centralis

• Far = lens is Flat

• Close = lens is Curved

• Ability of lens to curve lessens with age due to decrease in elastic fibers

Photopigments

Light-absorbing molecules in rods & cones; consist of opsin protein + light-absorbing retinal (made from Vitamin A)

Light adaptation (adjusting from low light → bright conditions)

1. Pupils constrict due to photopupillary reflex

2. Retinal is initially “bent” in cis-retinal form

3. Light rays → retinal straightens into trans-retinal form

4. Retinal separates from opsin (“bleaching” of retinal) — photopsin is cones is quickly regenerated; cones take over

Dark adaptation (return of sensitivity to low light after bright light)

1. Pupils dilate due to low light

2. Bleached retinal must be regenerated to cis-retinal (“bent”) form for rods to function again in low light

3. Cis-retinal rejoins with opsin; rhodopsin now functions again — may take 20-30 mins. for rhodopsin to be fully regenerated

All rods and cones use _____ with bipolar cells.

Glutamate

Use of glutamate in vision

• All rods and cones use glutamate with bipolar cells

• Glutamate release is continuous in the dark; when light “bleaches” the retina, glutamate release stops

• Bipolar & ganglion neurons both have “on” and “off” versions that are excited or inhibted by glutamate; when light intensity changes, the shift in excitation (APs) is percieved as images

Pathway of vision (signal)

1. Bipolar cells/neurons (1st order neurons)

2. Optic nerve (CN II; axons of ganglion cells; 2nd order neurons)

3. Optic chaism (crossing half of the axons in each optic nerve)

4. Optic tracts to thalamus (3rd order neurons)

THEN…

• Midbrain (superior colliculi for visual reflexes/nucleus of CN III for photopupillary reflex)

OR…

• Primary visual cortex (occipital lobe)

Because of optic chaism, damage to one occipital lone will result in partial blindness in _____.

Each eye (both eyes)

Hearing

Perception of sound (waves) due to vibrations; relies on mechanoreceptors and signals are transported via vestibulocochlear nerve (CN VIII)

Equilibrium

Awareness and monitoring of head position; relies on mechanoreceptors and signals are transported via vestibulocochlear nerve (CN VIII)

• Combines visual and proprioception inputs

• Maintains coordination, balance, and orientation in 3-D space

Sections of ear

Outer, middle, and inner ear

• Outer & middle — transmit vibrations (sound detection)

• Inner ear — vibrations cause fluid pressure waves, which is converted to APs

Outer/external ear

Consists of the auricle and external acoustic meatus, which funnel sound waves inward to the tympanic membrane (vibrates when sound waves hit it)

Middle ear

An air-filled cavity containing the auditory ossicles and auditory tube

Auditory (Eustachian) tube

Passage extending from middle ear to nasopharynx; yawning, swallowing, & chewing allows air movement through the tube & equalizes pressure on either side of the tympatic membare

• Middle ear infections (otitis media) often result from infections spreading from the throat to the middle ear through the auditory tube

Auditory ossicles

Three tiny bones of the middle ear; tympanic membrane is attached to malleus, which vibrates the incus, which vibrates the stapes (MIS)

• Stapes moves in and out of oval window, initiating pressure waves in the inner ear fluid

Inner ear

A bony labyrinth of spaces carved out of the temporal bone consisting of membrane-lined, fluid-filled tubes; three main regions — cochlea, vestibule. semicircular canals

Cochlea

Part of the inner ear responsible for hearing (cochlea- = snail shell); consists of three fluid-filled chambers separated by membranes

Cochlea duct

Middle chamber of the cochlea filled with endolymph and containing spiral organ; separated from…

• Scala vestibuli by vestibular membrane

• Scala tympani by basilar membrane 

Spiral organ

A sensory epithelium containing “hair cells” sitting on the basilar membrane; found within the cochlear duct contains mechanoreceptors for hearing

• Hairs cells are NOT neurons, but they release neurotransmitter onto dendrites of neurons associated with CN VIII

• Hair cells have microvilli (“stereocilia”) covered by a gelatinous tectorial membrane

• Primary/1st order neurons of CN VIII are in spiral ganglion

Inner hair cells

Located in a single row within the spiral organ & act as main mechanoreceptors; communicate with 90% of the dendrites in CN VIII

Outer hair cells

Located in three rows within the spiral organ; act to “tune” the inner hair cells and can tense the basilar membrane

Scala vestibuli

Superior chamber of the cochlea filled with perilymph; begins at oval window and spirals up to apex

Scala tympani

Inferior chamber of the cochlea filled with perilymph; begins at apex and spirals down to round window

Transduction of sound

• Inner hair cels contain K+ ion channels at their tips and tip link proteins that connect them

  • Surrounding endolymph is HIGH in K+

• Every time the basilar membrane moves up, hair cells and pushed into tectorial membrane & their tips are tilted

  • Physical distortion of tip links pulls open K+ ion channels (mechanically-gated channels)

  • K+ will diffuse INTO the hair cell and depolarize it

  • Hair cell releases neurotransmitter from its base, exciting the sensory neuron of CN VIII, leading to an AP

• When the basilar membrane moves down, the process reverses, ion channels close, and neurotransmitter release stops

Pathway of hearing

1. CN VIII  (cochlear branch of vestibulocochlear nerve; 1st order neurons in spiral ganglion)

2. Cochlear nucleus (in medulla oblongata; 2nd order neurons)

3. Midbrain (inferior colliculi for startle response & reflex turning of head to sound; 3rd order neurons)

4. Thalamus (4th order neurons)

5. Primary auditory cortex (located in temporal lobe)

**Excessive decussation/crossing of axons; rarely unilateral hearing loss if one cortex is damaged

Vestibular apparatus

Contains the mechanoreceptors for equilibrium; consists of the vestibule and semicircular ducts

Vestibule

Part of the vestibular apparatus that detects orientation when head is erect (“static equilibrium”) vs. tilted and linear acceleration (e.g., riding in an elevator or accelerating in a car)

• Consists of two chambers — saccule and utricle

• Receptors are found in macula

Macula

Within the utricle & saccule; patches of hair cells with stereocilia and one kinocilium projecting into gel called the otolithic membrane

• Otolithic membrane contains calcium carbonate crystals that give it weight and make it responsive to gravity/motion (e.g., head erect = minimal movement of otolithic membrane and minimal stimulation of hair cells)

• Hair cells are synapsing with the vestibular branch of VIII

Semicircular ducts

Part of the vestibular apparatus that detects rotational/angular acceleration (e.g., spinning in a chair or shaking your head “no”)

• 3 semicircular ducts are oriented in a different plane, to detect different planes of rotation

• Receptors are found in ampulla

Ampulla

A dilated sac within each semicircular duct; each ampulla has a mound of hair cells called the crista ampullaris

• Contains stereocilia and kinocilium embedded in a gel called the cupula

  • When head rotates, endolymph pushes against cupula, bending stereocilia

  • If stereocilia bend toward kinocilium, the hair cell depolarizes

  • If stereocilia bend away from kinocilium, the hair cell repolarizes

• Hair cells are synapsing with the vestibular branch of VIII

Pathway of equilibrium