Olfactory, Gustatory, and Visual Pathways - Chapter 14 Notes

The Special Senses

  • Taste, smell, sight, hearing, and balance
  • Touch — a large group of general senses
  • Special sensory receptors
    • Localized — confined to the head region
    • Receptors are distinct receptor cells
  • Special receptor cells
    • Are neuronlike epithelial cells or small peripheral neurons
    • Transfer sensory information to other neurons in afferent pathways

Taste (Gustation) and Smell (Olfaction)

  • The Chemical Senses: Taste and Smell
    • Taste — gustation
    • Smell — olfaction
    • Receptors are classified as chemoreceptors
    • Respond to chemicals
    • Food dissolved in saliva
    • Airborne chemicals that dissolve in fluids of the nasal mucosa

Taste (Gustation)

  • Taste receptors occur in taste buds
    • Most are found on the surface of the tongue
    • Located within tongue papillae
  • Papillae with taste buds
    • Fungiform papillae
    • Vallate papillae
    • Foliate papillae

Taste Buds

  • Collection of 50ext−−10050 ext{--}100 epithelial cells
  • Two major cell types
    • Gustatory epithelial cells
    • Basal epithelial cells
  • Contain long microvilli that extend through a taste pore to the surface of the epithelium
  • Cells in taste buds are replaced every 7ext−−107 ext{--}10 days

Gustatory Pathway and Taste Sensation

  • Five basic qualities of taste
    • Sweet, sour, salty, bitter, and umami
  • Umami is elicited by glutamate
  • The taste map is a myth — all taste modalities can be elicited from all areas containing taste buds

Gustatory Pathway

  • Taste information reaches the cerebral cortex primarily through the facial (VII) and glossopharyngeal (IX) nerves
  • Some taste information travels through the vagus nerve (X)
  • Sensory neurons synapse in the medulla
    • Located in the solitary nucleus
  • Impulses are transmitted to the thalamus and ultimately to the gustatory area of the cerebral cortex in the insula
  • Summary of the pathway
    • Taste receptors → cranial nerves VII, IX, X → solitary nucleus in the medulla → thalamus → gustatory cortex in the insula

Olfaction (Smell)

  • Olfactory receptors are part of the olfactory epithelium
  • Olfactory epithelium is pseudostratified columnar and contains three main cell types
    • Olfactory sensory neurons
    • Supporting epithelial cells
    • Olfactory stem cells

Olfactory Receptors and Receptive Structures

  • Cell bodies of olfactory sensory neurons are located in the olfactory epithelium
  • Each olfactory sensory neuron has an apical dendrite that projects to the epithelial surface
    • Ends in a knob from which olfactory cilia radiate
  • Olfactory cilia act as receptive structures for smell
  • Mucus captures and dissolves odor molecules

Olfactory Pathways to the Brain

  • Axons of olfactory sensory neurons gather into bundles called filaments of the olfactory nerve
  • Filaments penetrate the cribriform plate of the ethmoid bone and enter the olfactory bulbs
  • Olfactory bulbs synapse with mitral cells
  • Mitral cells transmit impulses along the olfactory tract to
    • The limbic system
    • The primary olfactory cortex

Disorders of the Chemical Senses

  • Anosmia — absence of the sense of smell
    • Due to injury, colds, allergies, or zinc deficiency
  • Uncinate fits — distortion of smells or olfactory hallucinations
    • Often result from irritation of olfactory pathways
    • Occurs after brain surgery or head trauma

The Eye and Vision

  • Vision is the dominant sense in humans
    • 70%70\% of all sensory receptors are in the eyes
    • 40%40\% of the cerebral cortex is involved in processing visual information
  • The anterior one-sixth of the eye’s surface is visible

Accessory Structures of the Eye (1 of 2)

  • Eyebrows — coarse hairs on the superciliary arches
  • Eyelids (palpebrae) — separated by the palpebral fissure
    • Tarsal plates — connective tissue within the eyelids
    • Tarsal glands — modified sebaceous glands

Accessory Structures of the Eye (2 of 2)

  • Conjunctiva — transparent mucous membrane
    • Palpebral conjunctiva
    • Bulbar conjunctiva
    • Conjunctival sac

Accessory Structures of the Eye

  • Lacrimal apparatus — keeps the surface of the eye moist
    • Lacrimal gland — produces lacrimal fluid
    • Lacrimal sac — fluid empties into nasal cavity

Extrinsic Eye Muscles

  • Six muscles that control movement of the eye
  • Originate in the walls of the orbit
  • Insert on the outer surface of the eyeball
  • Annular ring — origin of the four rectus muscles
  • The six extrinsic eye muscles are:
    • Lateral rectus and medial rectus
    • Superior rectus and inferior rectus
    • Superior oblique and inferior oblique

Anatomy of the Eyeball

  • Components of the eye — protect and support photoreceptors
  • Function — gather, focus, and process light into precise images
  • Anterior pole — most anterior part of the eye
  • Posterior pole — most posterior part of the eye
  • External walls — composed of three tunics
  • Internal cavity — contains fluids (humors)

The Fibrous Layer

  • Most external layer of the eyeball
  • Composed of two regions of connective tissue
    • Sclera — posterior 5/65/6 of the fibrous layer
    • White, opaque region
    • Provides shape and an anchor for eye muscles
    • Cornea — anterior 1/61/6 of the fibrous tunic
  • Scleral venous sinus — allows aqueous humor to drain

The Vascular Layer (Uvea) (1 of 2)

  • The middle coat of the eyeball — composed of choroid, ciliary body, and iris
  • Choroid — vascular, darkly pigmented membrane
    • Brown color from melanocytes
    • Prevents scattering of light rays within the eye
    • Choroid corresponds to the arachnoid and pia mater

The Vascular Layer (2 of 2)

  • Ciliary body — thickened ring of tissue that encircles the lens
  • Composed of: ciliary muscle
  • Ciliary processes — posterior surface of the ciliary body
  • Ciliary zonule (suspensory ligament) — attached around entire circumference of the lens

The Iris and the Pupil

  • Iris — visible colored part of the eye
  • Attached to the ciliary body
  • Composed of smooth muscle
  • Pupil — the round, central opening
    • Sphincter pupillae muscle
    • Dilator pupillae muscle
  • Pupillary light reflex — protective response of pupil constriction when a bright light is flashed in the eye

The Inner Layer (Retina)

  • Retina — deepest tunic
  • Composed of two layers
    • Pigmented layer — single layer of melanocytes
    • Neural layer — sheet of nervous tissue
  • Contains three main types of neurons
    • Photoreceptor cells
    • Bipolar cells
    • Ganglion cells

Photoreceptors (Rods and Cones) (1 of 3)

  • Two main types
    • Rod cells — more sensitive to light; allow vision in dim light
    • Cone cells — operate best in bright light; enable high acuity, color vision
  • Considered neurons

Photoreceptors (2 of 3)

  • Rods and cones have an inner and outer segment
  • Outer segments are receptor regions
    • Light-absorbing pigments are present
  • Light particles modify the visual pigment and generate a nerve impulse

Photoreceptors (3 of 3)

  • Photoreceptors are vulnerable to damage by light or heat
  • They cannot regenerate if destroyed
  • They continuously renew and replace their outer segments

Regional Specializations of the Retina

  • Ora serrata — neural layer ends at the posterior margin of the ciliary body
    • Pigmented layer covers ciliary body and posterior surface of the iris
  • Macula lutea — contains mostly cones
  • Fovea centralis — contains only cones; region of highest visual acuity
  • Optic disc — blind spot

Internal Chambers and Fluids (1 of 2)

  • The lens and ciliary zonules divide the eye
  • Posterior segment (cavity) — filled with vitreous humor
    • Clear, jellylike substance
    • Functions: transmits light, supports the posterior surface of the lens, helps maintain intraocular pressure

Internal Chambers and Fluids (2 of 2)

  • Anterior segment — divided into anterior and posterior chambers
    • Anterior chamber — between the cornea and iris
    • Posterior chamber — between the iris and lens
  • Anterior segment is filled with aqueous humor
    • Renewed continuously
    • Formed as a blood filtrate
    • Supplies nutrients to the lens and cornea

Visual Pathways

  • Most visual information travels to the cerebral cortex
    • The cerebral cortex is responsible for conscious seeing
  • Other pathways travel to nuclei in the midbrain and diencephalon

Visual Pathways to the Cortex (1 of 2)

  • Pathway begins at the retina
    • Light activates photoreceptors
    • Photoreceptors signal bipolar cells
    • Bipolar cells signal ganglion cells
    • Axons from ganglion cells exit the eye as the optic nerve

Visual Pathways to the Cortex (2 of 2)

  • Optic tracts send axons to the lateral geniculate nucleus of the thalamus
    • Synapse with thalamic neurons
    • Fibers of the optic radiation reach the primary visual cortex
  • Partial decussation of axons enables depth perception

Visual Pathways to Other Parts of the Brain

  • Some axons from the optic tracts branch to the midbrain
    • Superior colliculi
    • Pretectal nuclei
  • Other branches from the optic tracts branch to the suprachiasmatic nucleus