Chapter 15: The Special Senses

J.F. Thompson, Ph.D. & J.R. Schiller, Ph.D. & G. Pitts, Ph.D.

The Five Special Senses:

Smell and taste: chemical senses (chemical transduction)

Sight: light sensation (light transduction)

Hearing: sound perception (mechanical transduction)

Equilibrium: static and dynamic balance (mechanical transduction)

Special Sensory Receptors

Distinct types of receptor cells are confined to the head region

Located within complex and discrete sensory organs (eyes and ears) or in distinct epithelial structures (taste buds and the olfactory epithelium)

The Chemical Senses: Taste and Smell

The receptors for taste (gustation) and smell (olfaction) are chemoreceptorschemoreceptors (respond to chemicals in an aqueous solution)

Chemoreception involves chemically gated ion channels that bind to odorant or food molecules

Taste

Location of Taste Buds

Located mostly on papillae of tongue

Two of the types of papillae: fungiform circumvallate

Taste Buds

Each papilla contains numerous taste buds

Each taste bud contains many gustatory cells

The microvilli of gustatory cells have chemoreceptors for tastes

The Five Basic Tastes

Sweet: sugars, alcohols, some amino acids, lead salts

Sour: H+ ions in acids

Salty: Na+ and other metal ions

Bitter: many substances including quinine, nicotine, caffeine, morphine, strychnine, aspirin

Umami: the amino acid glutamate (“beef” taste)

Taste Transduction Incompletely understood

A direct influx of various ions (Na+, H+) or the binding of other molecules which leads to depolarization of the receptor cell

Depolarization of the receptor cell causes it to release neurotransmitter that stimulates nerve impulses in the sensory neurons of gustatory nerves

Sensory Pathways for Taste

Afferent impulses of taste stimulate many reflexes which promote digestion (increased salivation, and gastrointestinal motility and secretion)

“Bad” taste sensations can elicit gagging or vomiting reflexes

Smell

Location of Olfactory (Odor) Receptors

Odor Receptors

Bipolar neurons

Collectively constitute cranial nerve I

Unusual in that they regenerate (on a ~60 day replacement cycle)

Odors

Very complicated

Humans can distinguish thousands

More than a thousand different odorant-binding receptor molecules have been identified

Different combinations of specific molecule-receptor interactions produce different odor perceptions

Transduction of Smell

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Binding of an odorant molecule to a specific receptor activates a G-protein and then a second messenger (cAMP)

cAMP causes gated Na+ and Ca2+ channels to open, leading to depolarization

Olfactory Pathway

One path leads from the olfactory bulbs via the olfactory tracts to the olfactory cortex where smells are consciously interpreted and identified

Another path leads from the olfactory bulbs via the olfactory tracts to the thalamus and limbic system where smells elicit emotional responses

Smells can also trigger sympathetic nervous system activation or stimulate digestive processes

Vision

Surface Anatomy of the Eye

Eyebrows divert sweat from the eyes and contribute to facial expressions

Eyelids (palpebrae) blink to protect the eye from foreign objects and lubricate their surface

Eyelashes detect and deter foreign objects

                                                                 

Conjunctiva A mucous membrane

lining the inside of the eyelids and the anterior surface of the eyes forms the conjunctival sac

between the eye and eyelid

Forms a closed space when the eyelids are closed

Conjunctivitis (“pinkeye”): inflammation of the conjunctival sac

The Lacrimal Apparatus

Lacrimal Apparatus: lacrimal gland lacrimal sac nasolacrimal duct

Rinses and lubricates the conjunctival sac

Drains to the nasal cavity where excess moisture is evaporated

Extrinsic Eye Muscles

Lateral, medial, superior, and inferior rectus muscles (recall, rectus = straight); superior and inferior oblique muscles

Internal Anatomy of the Eye--Tunics

Fibrous tunic: sclera & cornea

Vascular tunic: choroid layer

Sensory tunic: retina

Internal Anatomy of the Eye

Anterior Segment contains the Aqueous Humor Iris Ciliary Body Suspensory Ligament Lens

Posterior Segment contains the Vitreous Humor

Autonomic Regulation of the Iris

Pupil Constricts

Pupil Dilates

The Two Layers of the Retina Outer pigmented layer

has a single layer of pigmented cells, attached to the choroid tunic, which absorbs light to prevent light scattering inside

Inner neural layer has the photosensory cells and various kinds of interneurons in three layers

Neural Organization in the RetinaPhotoreceptors: rods

(for dim light) and cones (3 colors: blue, green and red, for bright light)

Bipolar cells are connecting interneurons

Ganglion cells’ axons become the Optic Nerve

Neural Organization in the Retina

Horizontal Cells enhance contrast (light versus dark boundaries) and help differentiate colors

Amacrine cells detect changes in the level of illumination

The Optic Disc

Axons of ganglion cells exit to form the optic nerve

Blood vessels enter to serve the retina by running on top of the neural layer

The location of the “blind spot” in our vision

Light must cross through the capillaries and the two layers of interneurons to reach the photoreceptors, the rods and cones

Micrograph of the Retina

Light

Opthalmoscope Image of the Retina The Macula Lutea (“yellow

spot”) is the center of the visual image

The Fovea Centralis is a central depression where light falls more directly on cones providing for the sharpest image discrimination

Light bouncing off RBCs’ hemoglobin causes “red eye” in flash photos

Ciliary process at the base of the iris produces aqueous humor

Scleral venous sinus returns aqueous humor to the blood stream

Glaucoma – any disturbance that increases aqueous humor volume and pressure which causes pain – ultimately the vitreous humor crushes the retina causing blindness

Circulation of the Aqueous Humor

Hearing

External Ear

Pinna (auricle): focuses sound waves on the tympanic membrane

Ceruminous glands guard the external auditory canal

Middle Ear & Auditory Tube

Three auditory ossicles (bones) serve as a lever system to transmit sound to the inner ear

Pharyngotympanic (auditory tube): connects to pharynx, allowing air pressure to equalize on both side of the tympanic membrane

Middle Ear Ossicles — (median view)

Malleus (hammer), incus (anvil) and stapes (stirrup) act to increase the vibratory force on the oval window

Tensor tympani and stapedius muscles control the tension of this lever system to prevent damage to the delicate tympanic and round window membranes

The Membranous Labyrinth

A series of tiny fluid-filled chambers in the temporal bone

Cochlea tranduces sound waves Semicircular canals and their ampullae transduce

balance and equilibrium The vestibule connects the two portions

The Cochlea – Two Coiled Tubes

Larger outer tube is folded but continuous (like a coiled letter “U”) – the scala vestibuli and scala tympani –contains perilymph fluid

Smaller inner tube is the scala media (cochlear duct) contains endolymph fluid

The Spiral Organ of Corti

Between the scala tympani and the scala media/cochlear duct is the complex receptor system: the spiral organ of Corti

Sensory Hair Cells stand on the basilar membrane and their processes are attached to the Tectorial Membrane

Wave Pulses in the Cochlea

Stapes moving at the oval window creates pulses of vibration in the perilymph of the scala vestibuli and scala tympani

Harmonic vibrations are created at right angles in the endolymph of the scala media which move the basilar membrane

Transduction of Sound Waves

Movement against the tectorial membrane stimulates the hair cells to send impulses to the auditory cortex

Round window moves to accommodate the vibrations initiated by the stapes

Base

Apex

Wave Pulses in the Cochlea Stapes moving at the oval window creates pulses

of vibration in the perilymph of the scala vestibuli and scala tympani

Harmonic vibrations are created at right angles in the endolymph of the scala media which move the basilar membrane

Transduction of Sound Waves

Resonance of Basilar Membrane

High notes are detected at the base of the cochlea

Low notes are detected at the apex

Due to differences in the width and flexibility of the basilar membrane

Base

Apex

Auditory Pathway Afferent impulses for

sounds are routed:

Vestibulocochlear Nerve VIII (cochlear branch)

Nuclei in the medulla oblongata where motor responses can turn the head to focus on sound sources

Primary Auditory Cortex in the temporal lobe for conscious interpretation

Balance and Coordination

Macula in the Saccule & Utricle

Chambers near the oval window filled with perilymph

CaCO3 otoliths (“ear stones”) slide over the surface lining cells in response to gravity

Static equilibrium tells the CNS “which way is up”

Macular Transduction Hair cells’ stereocilia move in response

to the sliding otoliths

To send impulses to the CNS for interpretation

Semicircular Canals

Three endolymph-filled tubes in the bony labyrinth

Each C-shaped loop is in a plane at right angles to the other two

Each has an expanded ampulla containing a sensory structure, the cupula

Ampullar Transduction Movement in the plane of one of the canals causes

endolymph to flow and bends the cupola

Hair cells’ stereocilia move in response to the movement

Dynamic equilibrium tells the CNS “which way is the head or body is moving”

Pathways of Balance and Orientation

Integration of sensory modalities: Sight Proprioception Static equilibrium Dynamic equilibrium

Output to skeletal muscles to position: Eyes Head and neck Trunk

Take a Tour of the Virtual Ear at: http://www.augie.edu/perry/ear/hearmech.htm

End Chapter 15