Anatomy

82

The Special Senses Objectives • Describe the sensory organs of smell, and olfaction. • Identify the accessory and internal structures of the eye, and explain their function. • Explain how light stimulates the production of nerve impulses and vision. • Describe the structures of the external and middle ear and explain how they function. • Describe the parts of the inner ear and their roles in equilibrium and hearing.

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

• Olfactory receptors are modified neurons • Olfaction detects 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 • Gustatory cells extend taste hairs through a narrow taste pore

Gustatory pathways • Taste buds are monitored by cranial nerves

• Synapse with the medulla oblongata, then 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

83

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 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

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)

84

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 – Composition is much like CSF

• 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

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.

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 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

85

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 auditory tube (Eustachian) • 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

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

• Each duct contains an ampulla with gelatinous cupula and sensory receptors • Saccule and utricle connected by a passageway between the endolymphatic duct

• Terminates in the endolymphatic sac • Saccule and utricle have hair cells clustered in maculae • Cilia contact the otolith (statoconia)

86

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