The Special Senses

Olfaction

Olfactory organs

• Contain olfactory epithelium with olfactory receptors, supporting cells, basal cells

• Olfactory receptors are modified neurons

• Surfaces are coated with secretions from olfactory glands

• Olfactory reception involved detecting dissolved chemicals as they interact with odorant binding proteins

Olfaction

• Olfactory pathways

• No synapse in the thalamus for arriving information

• Olfactory discrimination

• Can distinguish thousands of chemical stimuli

• CNS interprets smells by pattern of receptor activity

• Olfactory receptor population shows considerable turnover

• Number of receptors declines with age

Gustation

Taste receptors

• Clustered in taste buds

• Associated with lingual papillae

Taste buds

• Contain basal cells which appear to be stem cells

• Gustatory cells extend taste hairs through a narrow taste pore

Gustatory pathways

• Taste buds are monitored by cranial nerves

• Synapse within the solitary nucleus of the medulla oblongata

• Then on to the thalamus and the primary sensory cortex

Gustatory discrimination

• Primary taste sensations

• Sweet, sour, salty, bitter

• Receptors also exist for umami and water

• Taste sensitivity shows significant individual differences, some of which are inherited

• The number of taste buds declines with age

Vision

Accessory structures of the eye

• Eyelids (palpebrae) separated by the palpebral fissue

• Eyelashes

• Tarsal glands

• Lacrimal apparatus

external structures of the eye

• Conjunctiva covers most of eye

• Cornea is transparent anterior portion

Lacrimal apparatus

• Secretions from the lacrimal gland contain lysozyme

• Tears form in the lacrimal glands, wash across the eye and collect in the lacrimal lake

• Pass through the lacrimal punctae, lacrimal canaliculi, lacrimal sac and nasolacrimal duct

The eye

• Three layers

• Outer fibrous tunic

• Sclera, cornea, limbus

• Middle vascular tunic

• Iris, ciliary body, choroid

• Inner nervous tunic

• Retina

internal structures of the eye

• Ciliary body

• Ciliary muscles and ciliary processes, which attach to suspensory ligaments of lens

• Retina

• Outer pigmented portion

• Inner neural part

• Rods and cones

The Special Senses

retina

• Retina contains rods and cones

• Cones densely packed at fovea (center of the macula lutea)

• Retinal pathway

• Photoreceptors to bipolar cells to ganglion cells, to the brain via the optic nerve

• Axons of ganglion cells converge at blind spot (optic disc)

• Horizontal cells and amacrine cells modify the signal passed along the retinal neurons

Eye anatomy

• Ciliary body and lens divide the anterior cavity of the eye into posterior (vitreous) cavity and anterior cavity

• Anterior cavity further divided

• anterior chamber in front of eye

• posterior chamber between the iris and the lens

Fluids in the eye

• Aqueous humor circulates within the eye

• diffuses through the walls of anterior chamber

• passes through canal of Schlemm

• re-enters circulation

• Vitreous humor fills the posterior cavity.

• Not recycled – permanent fluid

Lens

• Posterior to the cornea and forms anterior boundary of posterior cavity

• Posterior cavity contains vitreous humor

• Lens helps focus

• Light is refracted as it passes through lens

• Accommodation is the process by which the lens adjusts to focus images

• Normal visual acuity is 20/20

Visual physiology

• Rods – respond to almost any photon

• Cones – specific ranges of specificity

Photoreceptor structure

• Outer segment with membranous discs

• Narrow stalk connecting outer segment to inner segment

• Light absorption occurs in the visual pigments

• Derivatives of rhodopsin

Color sensitivity

• Integration of information from red, blue and green cones

• Colorblindness is the inability to detect certain colors

retinal adaptation

• Dark adapted – most visual pigments are fully receptive to stimulation

• Light adapted – pupil constricts and pigments bleached.

the visual pathway

• Large M-cells monitor rods

• Smaller more numerous P cells monitor cones

Seeing in stereo

• Vision from the field of view transfers from one side to the other while in transit

• Depth perception is obtained by comparing relative positions of objects from the two eyes

Visual circadian rhythm

• Input to suprachiasmic nucleus affects the function of the brainstem

• Circadian rhythm ties to day-night cycle, and affects metabolic rates

Equilibrium and Hearing

Both equilibrium and hearing are provided by receptors of the inner ear

Anatomy of the ear – External Ear

• Auricle or pinnae surrounds the ear

• External acoustic meatus ends on tympanic membrane

Middle ear

• Communicates with pharynx via pharyngotympanic membrane

• Middle ear encloses and protects the auditory ossicles

Inner ear

• Membranous labyrinth contains endolymph

• Bony labyrinth surrounds and protects membranous labyrinth

• Vestibule

• Semicircular canals

• Cochlea

Components of the inner ear

• Vestibule contains the utricle and saccule

• Semicircular canals contain the semicircular ducts

• Cochlea contains the cochlear duct

Windows

• Round window separates the perilymph from the air spaces of the middle ear

• Oval window connected to the base of the stapes

• Basic receptors of inner ear are hair cells

• Provide information about the direction and strength of stimuli

Equilibrium

• Anterior, posterior and lateral semicircular ducts are continuous with the utricle

• Each duct contains an ampulla with a gelatinous cupula and associated sensory receptor

• Saccule and utricle connected by a passageway continuous with the endolymphatic duct

• Terminates in the endolymphatic sac

• Saccule and utricle have hair cells clustered in maculae

• Cilia contact the otolith (statoconia)

Vestibular neural pathway

• Vestibular receptors activate sensory neurons of the vestibular ganglia

• Axons form the vestibular branch of cranial nerve VII

• Synapses within the vestibular nuclei

Hearing

• Cochlear duct lies between the vestibular duct and the tympanic duct

• Hair cells of the cochlear duct lie within the Organ of Corti

• Intensity is the energy content of a sound

• Measured in decibels

Pathway of sound

• Sound waves travel toward tympanic membrane, which vibrates

• Auditory ossicles conduct the vibration into the inner ear

• Tensor tympani and stapedius muscles contract to reduce the amount of movement when loud sounds arrive

• Movement at the oval window applies pressure to the perilymph of the cochlear duct

• Pressure waves distort basilar membrane

• Hair cells of the Organ of Corti are pushed against the tectoral membrane

Neural pathway

• Sensory neurons of hearing are located in the spiral ganglion of the cochlea

• Afferent fibers form the cochlear branch of cranial nerve VIII

• Synapse at the cochlear nucleus