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The Special Senses
Special Senses
• Include smell, taste, vision, hearing, and equilibrium
• More complex sensory structure
• More complex integration both peripherally and centrally
• More complex experience
Olfaction
• This and taste are chemical senses
– Molecules must be volatile, water or lipid soluble
– Must dissolve in mucus produced by olfactory glands
– Probably hundreds of primary odor sensations
• Olfactory information not only transmitted to cortex, but also to limbic system
– Explains some elicited emotional responses
• Olfactory epithelium composed of receptors, supporting and basal stem cells
located in small superior region of nasal cavity
– Supporting cells support, nourish, electrically insulate receptors
– Basal stem cells produce new receptors (which last about 1 month)
• Olfactory (Bowman’s) glands - mucus secretion
– These and supporting cells innervated by facial (VII) nerve - in presence of
certain odors, there is increased activity of lacrimal gland and nasal mucus glands Site of Olfaction (graphic) Olfactory Epithelium (graphic) Olfactory Epithelium (graphic) Olfactory Receptors
• 10-100 million receptors in a square inch space
• Bipolar first order neurons
• At distal end, olfactory hairs (cilia) extend from dendrites
• In presence of chemical molecule, generator potential created
– Activated receptor integral proteins (G-proteins) → activate adenylate
cylcase → ATP converted to cAMP → changed Na+^ permeability
• If AP produced, information carried to olfactory bulb
• Receptors adapt quickly (50% in 1 sec)
– Complete adaptation can occur (after a minute) - partly related to central
adaptation
• Sensitivity can be very high (low levels)
– Methyl mercaptan added to natural gas because of high sensitivity
(1/25x10 -9^ mg/ml) Olfactory Pathways
• Olfactory receptors/olfactory nerves (1st^ order) → olfactory bulb/tract (2nd^ order)
→ primary (lateral) olfactory areas (medial side of temporal lobe) → orbitofrontal area (Brodmann’s area - 11) or other limbic regions and hypothalamus
• Primary olfactory area encompasses part of limbic system including
amygdaloid body - awareness of odors
• Orbitofrontal area - odor discrimination
– Some lateralization in right lobe
• Limbic and hypothalamic projections contribute emotional or experiential
responses to odors Taste
• Sense of taste enhanced by sense of smell
– Only five specific taste sensations as opposed to hundreds for smell
– Include sour, sweet, bitter, salty & umami
• Olfactory sensation more acute than taste
Gustatory Receptors
• Taste buds primarily on tongue within circumvallate or fungiform papillae
– Circumvallate across posterior of tongue, fungiform across whole of surface
• 10,000 buds in youth, declining with age
• Taste buds composed of three types of epithelial cells organized in capsules
with taste pore
– Receptor cells - about 50 cells per capsule with gustatory hairs (microvilli)
– Supporting cells
– Basal cells - at base of capsule, produce supporting cells which then
differentiate to receptor cells (which last about 10 days)
• In presence of dissolved chemical molecule, receptor potential created which
leads to proportional release of neurotransmitter, stimulating 1 st^ order neurons
– 1st order neurons may innervate numerous receptors in more than one
taste bud
• Five important taste sensations not necessarily specific to receptor cells
– Some localization
- Tip of tongue especially sensitive to sweet and salty
- Back of tongue, bitter (toxic substances) - threshold lowest of four
- Lateral portions of tongue, sour - threshold second lowest of four
- Na+^ (salty) and H+^ (sour) enter through ion-specific channels, other tastants via G-protein effect
- Umami not known to be regional
• Adaptation occurs in 1-5 minutes
– Partly receptor and central adaptation
Taste Bud (graphic) Circumvallate Papillae (graphic) Taste Buds (graphic)
– Lacrimal sac with connection to nasal cavity via nasolacrimal duct
– Lacrimal fluid (tears) - about 1 ml/day - largely saline solution with lysozyme
(bactericidal) Accessory Structures of Eye (graphic) Lacrimal Apparatus (graphic) Anatomy of the Eye
• Three layers - fibrous tunic, vascular tunic, retina
• Fibrous tunic - outermost layer
– Cornea - transparent avascular anterior region
- Three tissue layers - nonkeratinized stratified squamous epithelium, collagenous connective tissue, simple squamous epithelium
- Transplants - avascular nature
– Sclera - white of the eye, primarily dense connective tissue, gives eye
shape Structures of the Eye (graphic) Vascular Tunic (Uvea)
• 2 nd^ layer has three regions
• Iris - composed of circular and radial smooth muscle, forms pupil, suspended
between cornea and lens by ciliary process
– Parasympathetic innervation controls circular muscles - constriction
– Sympathetic innervation controls radial muscles - dilation
• Ciliary body
– Ciliary processes - vascular, secrete aqueous humor, attachment for
suspensory ligaments
– Ciliary muscle - controls shape of lens
– Ciliary muscle and muscles of iris - intrinsic muscles
• Choroid - vacularized providing nutrient supply to retina, pigmented with
melanin Adjustment of the Pupil (graphic) Anterior Structures of the Eye (graphic) Retina
• Third layer is neural portion
• Optic disc - location of optic nerve and blood vessels, blind spot
• Remaining portion is sensory epithelium
– Underlying pigmented epithelium - melanin (light absorbance)
– Three neural layers - photoreceptive, bipolar cell, ganglion cell
- 6 million cones
- 120 million rods
– Central fovea - highest visual acuity, only cones, no overlying bipolar or
ganglion cell layer
– Other cell types that connect laterally - horizontal and amacrine cells
Opthalmoscopic View (graphic)
Retinal Layers (graphic) Retinal Layers (graphic) Lens
• Avascular, transparent, layered, proteinaceous structure (crystallins)
• Suspended behind iris by suspensory ligaments
• Cataracts
Interior Chambers
• Anterior cavity
– Filled with aqueous humor created by capillary filtration
- Produced at ciliary process, flows forward through pupil and reabsorbed at scleral venous sinus
- Replaced every 90 min - supports avascular lens and cornea
– Interocular pressure largely dependent on volume of aqueous humor
- When high, glaucoma - degeneration of retina results
• Posterior cavity - vitreous chamber
– Contains vitreous body - produced embryonically and not replaced
– Pushes retina against back of eye and maintains shape of eyeball
Image Formation
• Refraction of light by cornea and lens
– 75% by cornea
– Resulting image upside-down and backwards
• Lens responsible for accommodation to adjust for angle of incoming light
– When viewing something close, the ciliary muscle contracts pulling ciliary
process and choroid forward releasing tension of suspensory ligaments, lens thickens due to its elasticity
– Presbyopia - lens loses elasticity over time (age 40) and can’t focus on
close objects
• Iris adjusts with accommodation to keep light from penetrating periphery of
lens (close focus - closes pupil)
• Convergence – medial movement of two eyeballs
– required by binocular vision and dependent on distance of object
Refraction of Light and Accommodation (graphic) Sight Correction
• Emmetropic - normal vision
• Myopic - nearsighted (elongated eyeball or thick lens)
– Corrected by concave lens
• Hypermetropic (hyperopia) - farsighted (shortened eyeball or thin lens)
– Corrected by convex lens
• Astigmatism - abnormal shape
– Corrected by uneven grind of lens
Sight Abnormalities (graphic)
• Horizontal cells transmit inhibitory signals to adjacent regions - lateral inhibition
– Enhances contrast
• Ganglion cells excited by bipolar cells or via amacrine cells and produce APs
Visual Pathway
• Optic nerve → optic chiasm (only some fibers cross) → lateral geniculate
nucleus of thalamus → primary visual areas of occipital lobe of cortex (area 17)
• Visual field of each eye divided into central and peripheral halves
– Only fibers responsible for the lateral halves of visual field cross at optic
chiasm Bilateral Integration of Sight (graphic) Bilateral Integration of Sight (graphic) Hearing Structures
• Structurally divided into three regions - external, middle and inner ear
• External Ear
– Auricle (elastic cartilage) - helix and lobule
– External auditory meatus (temporal bone) - contains ceruminous glands
and hair
– Tympanic membrane (connective tissue overlaid by simple cuboidal
epithelium)
• Middle Ear
– Air filled and lined with epithelium
– Eustachian tube - connection with nasopharynx
– Auditory ossicles - malleus (against tympanic membrane), incus, stapes
(against oval window of cochlea)
- Tensor tympani and stapedius muscles - decrease ossicle vibration
• Inner Ear (labyrinth)
– Bony labyrinth contains perilymph (similar to CSF) surrounding
membranous labyrinth
– Posterior portion - vestibule containing utricle & saccule, and semicircular
canals and associated ampullae
– Anterior portion - cochlea (bony spiral shape and central modiolus)
Structures of the Ear (graphic) Structures of the Middle Ear Cochlea
• Three parallel chambers
– Scala vestibuli (ends at oval window) and connecting scala tympani (ends
at round window), filled with perilymph
– Cochlear duct (scala media) between and separated from others by
vestibular membrane and basilar membrane - filled with endolymph (high K+^ )
• Organ of Corti (spiral organ) - attached to basilar membrane
– Consists of epithelial supporting cells and 16K hair cells
– Hair cells have apical microvilli (stereocilia), synapse with 1st^ order neurons
– Gelatinous tectoral membrane overlays hair cells
– One row of inner hair cells (synapse with >90% of sensory neurons) and
three rows of outer hair cells (synapse with 90% of motor neurons) Spiral Organization of Channels (graphic) X-section of Channels (graphic) X-section of Channels (graphic) Organ of Corti (graphic) Physiology of Hearing
• Auricles aid in directing sound, particularly higher frequencies
• Sound vibrates tympanic membrane, vibrations transmitted to ossicles
• Ossicles amplify vibration (20X) to oval window
• Pressure waves in perilymph move up scala vestibuli, around helicotrema,
down scala tympani, to round window
• Both vestibular and basilar membrane vibrate, the basilar membrane moves
hair cells relative to tectorial membrane
• Spatial stimulation of hair cells dependent on frequency
– Basilar membrane narrower and stiffer at base, and vibrates in response to
higher frequencies (up to 20,000 Hz)
– Basilar membrane broader at apex, and vibrates in response to lower
frequencies (down to 20 Hz)
– Greatest frequency sensitivity 500-5000 Hz
• Number of hair cells stimulated dependent on intensity
• Transduction of signal at hair cell - mechanical bending of stereoocilia in one
direction (toward tallest) cause mechanically-gated K +^ channels (primarily) to open, causing depolarizing receptor potential (high K+^ in endolymph)
• Leads to voltage-gated Ca+^ channels in base of hair cell to open causing
release of neurotransmitter (glutamate?)
• Bending of stereocilia in opposite direction closes channels
Auditory Stimulation (graphic) Hearing Pathways
• Cochlear branch of vestibulocochlear nerve (VIII) → cochlear nuclei of medulla
→ partial crossing to superior olivary nuclei (also medulla) → inferior colliculus of midbrain from both cochlear nuclei & olivary nuclei → medial geniculate of thalamus → primary auditory area (areas 41 & 42 in temporal lobe) Vestibulocochlear Nerve (graphic) Physiol. of Static Equilibrium
• Measure relative to force of gravity (and linear acceleration)
• Primarily utricle and saccule
• Maculae (sensory epithelia) composed of hair cells and supporting cells
– Utricular and saccular maculae are perpendicular to one another
• Hair cells have stereocilia (microvilli) and one kinocilium (true cilia)
