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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