Special Senses Flashcards

Special Senses

Receptors

• Can be simple, like the end of a dendrite, or complex, like the ear or eye.

• Receptors respond when a specific threshold is met (specificity).

• A specific stimulus will initiate the receptor.

Receptors (Continued)

• For receptor activation, the stimulus needs to be applied within the receptive field.

• Receptors on fingers are smaller, greater in number, and more receptive, which is why we use fingers to feel.

• Receptors on the forearm are larger and fewer in number, making this area less sensitive.

• Pain receptors respond to different stimuli and react differently than other sensory receptors.

Special Sense Receptors

• Provide important information about our environment.

• Each special sense originates at receptor cells.

• Two types of receptor cells:

  • Dendrites of specialized neurons (e.g., olfactory receptors).

    • Depolarized by contact with dissolved chemicals.

    • Depolarization is called a generator potential.

  • Specialized cells with unexcitable membranes.

    • Form synapses with processes of sensory neurons.

    • Upon depolarization, the receptor cell membrane undergoes graded depolarization.

    • This triggers the release of chemical transmitters at the synapse that depolarize sensory neurons, inducing a generator potential.

    • Action potentials are propagated to the CNS.

    • Slight synaptic delay allows modification of receptor cell sensitivity by presynaptic inhibition/facilitation.

    • Examples: taste, vision, equilibrium, hearing.

Olfaction

• Sense of smell.

• The nasal cavity contains olfactory organs, paired on either side of the nasal septum.

• Location of olfactory sensory neurons (receptors) responsible for olfaction.

• Odorants (air-borne particles) stimulate the receptors.

• Signals from receptors are sent to the olfactory cortex of the CNS for interpretation.

Olfactory Organs

• Paired structures that provide olfaction.

• Located in the nasal cavity on either side of the nasal septum.

• Cover the:

  • Inferior surface of the cribriform plate.

  • Superior portion of the perpendicular plate.

  • Superior nasal conchae of the ethmoid.

• Composed of two layers:

  • Olfactory epithelium containing:

    • Olfactory sensory neurons (receptor cells).

      • Each cell forms a dendritic bulb.

      • Dendrites of the bulb project past the epithelial surface into the surrounding mucus, increasing the binding surface area for odorants.

    • Supporting cells.

    • Basal epithelial cells (stem cells).

      • Constantly produce new receptor cells.

      • One of the few examples of neuronal replacement.

  • Lamina propria:

    • Underlying tissue; part of the mucous membrane.

    • Consists of:

      • Areolar tissue.

      • Numerous blood vessels.

      • Nerves.

      • Olfactory glands.

        • Secretions absorb water and form thick, pigmented mucus.

Olfactory Pathway

• Sensory neurons in the olfactory organ are stimulated by chemicals in the air.

• Olfactory sensory neuron axons are collected into 20 or more bundles (Olfactory Nerve, CNI) and leave the epithelium by penetrating the cribriform plate of the ethmoid bone.

• The first synapse occurs in the olfactory bulb, located just superior to the cribriform plate.

• Axons leaving the bulb travel along the olfactory tract to the olfactory cortex (in the temporal lobe), hypothalamus, and portions of the limbic system.

• The extended distribution explains why smells can produce profound emotional and behavioral responses.

Olfaction

• Odorants:

  • Dissolved chemicals that stimulate olfactory neurons.

  • Interact with membrane receptors called odorant-binding proteins on the membrane surface.

  • Generally small organic molecules.

  • The strongest smells are associated with molecules of either high water or high lipid solubilities.

  • As few as four odorant molecules can activate an olfactory receptor cell.

Olfaction - Olfactory Reception Process

• Occurs on the dendrites of olfactory receptor cells.

• Odorant binds to its receptor protein (G-coupled protein receptor).

• Activates adenylate cyclase.

• Adenylate cyclase is an enzyme that converts ATP to cyclic AMP (cAMP).

• cAMP opens sodium channels in the plasma membrane, beginning depolarization.

• If the depolarization is sufficient, an action potential is triggered and information is relayed to the CNS.

Interesting Smells

• You are able to distinguish between 2000-4000 different smells

• As the stimulus is passed to the olfactory center of the cerebral cortex, the impulse passes through the limbic system.

• This is why some smells can change a person's mood or trigger a memory about a loved one

• As we age, the receptor number declines, which is why it gets harder to smell and taste things

Gustation

• Sense of taste.

• Provides information about foods and liquids we eat and drink.

• Taste (gustatory) receptors

  • Most important ones are distributed over the superior surface of the tongue.

  • Also found in adjacent portions of the pharynx, larynx, and epiglottis.

  • Numbers decrease with age.

• Lingual papillae (papilla, a nipple-shaped mound):

  • Epithelial projections on the superior surface of the tongue.

Gustation - Types of Lingual Papillae

• Four types of lingual papillae:

  • Vallate papillae

    • Largest; up to 12; form an inverted “V” near the posterior margin of the tongue.

    • Shaped like the tip of a pencil eraser; surrounded by deep epithelial folds.

    • Each papilla contains up to 100 taste buds.

  • Foliate papillae

    • Found on the lateral margins of the posterior region of the tongue.

    • Contain taste buds.

  • Fungiform papillae

    • Shaped like small buttons within shallow depressions.

    • Each papilla contains about five taste buds.

    • Found on the anterior two-thirds of the superior surface of the tongue.

  • Filiform papillae

    • Provide friction that helps the tongue move objects around in the mouth.

    • Do not contain taste buds.

    • Found on the anterior two-thirds of the superior surface of the tongue.

Gustation - Taste Buds

• Sensory structures.

• An adult has about 5000 taste buds.

• Composed of:

  • Gustatory epithelial cell (taste receptor cell).

    • Extends slender microvilli (taste hairs) into surrounding fluids through a taste pore (narrow opening).

    • A typical cell survives about 10 days before being replaced.

    • About 40–100 receptor cells per taste bud.

  • Basal cells (stem cells).

    • Divide and mature to produce more gustatory epithelial cells.

Gustation - Taste Sensations

• Four primary sensations:

  • Sweet, salty, sour, bitter.

  • Taste buds in all portions of the tongue provide all four sensations.

  • Sensitivity to sensations may vary along the long axis of the tongue.

  • No difference in taste bud structure.

• Two other taste sensations:

  • Umami

    • Pleasant, savory taste.

    • Examples: beef broth, Parmesan cheese.

    • Detected by receptors sensitive to amino acids, small peptides, nucleotides.

    • Receptors present in vallate papillae.

  • Water receptors

    • Concentrated in the pharynx.

    • Information sent to the hypothalamus.

    • Affects water balance and blood volume regulation.

    • Prevents over-ingestion of water.

Gustation - Taste Receptor Sensitivity

• More sensitive to unpleasant stimuli.

• 100,000 times more sensitive to bitter and 1000 times more sensitive to sour (acids) compared to sweet and salty.

• May have survival value.

  • Toxic compounds are often bitter.

  • Acids can create chemical burns.

• Overall sensitivity declines with age.

  • The number of taste receptors declines.

  • The number of olfactory receptors declines.

Gustatory Reception

• Stimulated by dissolved chemicals.

• About 90 percent of receptor cells respond to two or more different taste stimuli.

• Different tastes involve different receptor mechanisms.

• Chemicals contacting taste hairs may:

  • Diffuse through plasma membrane leak channels.

  • Bind to receptor proteins of receptor cells.

  • Receptor cells adapt slowly, but central adaptation reduces sensitivity to new tastes.

Gustatory Reception - Two Types

• For salt and sour receptors:
  Sodium ions (salt) or hydrogen ions (sour) diffuse through Na^+ leak channels.
  The membrane and cell are depolarized, stimulating the release of neurotransmitters.

• For sweet, bitter, and umami receptors:
  Binding to sweet, bitter, or umami stimuli activates G-protein complexes called gustducins.
  Activation of a second messenger stimulates the release of neurotransmitters.

Gustatory Reception - Result of Neurotransmitter Release

• Stimulation of adjacent sensory neurons.

• Sensory neurons are depolarized.

• Leads to:

  • Generator potential.

  • Propagation of action potentials along the gustatory pathway to the CNS.

Gustatory Pathway

• Receptors (taste buds) respond to stimulation.

• Information is relayed on cranial nerves.

  • Facial nerve (VII) innervates taste buds on the anterior 2/3 of the tongue, from the tip to the line of vallate papillae.

  • Glossopharyngeal nerve (IX) innervates vallate papillae and posterior 1/3 of the tongue.

  • Vagus nerve (X) innervates taste buds on the surface of the epiglottis.

• The sensory afferents carried by these three cranial nerves synapse in the solitary nucleus of the medulla oblongata.

• After another synapse in the thalamus, the information is projected to the appropriate portions of the gustatory cortex of the insula.

• The axons of the postsynaptic neurons cross over and enter the medial lemniscus of the medulla oblongata.

Gustatory Reception - Taste Perception

• Conscious perception of taste produced by processing at the primary sensory cortex.

• Information from taste buds is correlated with other sensory data:

  • Texture of food.

  • Taste-related sensations (“peppery” or “burning hot”).

  • The level of olfactory stimulation plays an important role.

  • Several thousand times more sensitive to “tastes” when the sense of smell is functioning.

Major Layers of Eye

• Fibrous layer

  • Cornea

  • Sclera

• Vascular layer

  • Choroid

  • Iris

  • Ciliary body

• Inner layer (the Retina)

  • Pigmented layer

  • Neural Layer

Inner Layer: The Retina

• Relies on blood and nourishment from the choroid layer

• On the posterior part of the eye

• Comprised of two separate parts

  • Pigmented layer next to choroid

    • Helps absorb the light rays

  • Neural layer which contains the photoreceptors
    Involved in the image formation
    Rods
    Cones

Photoreceptors

• Rods
  Responsible for seeing shades, shape and movement
  Very numerous
  Distributed around the edges of the retina

• Cones
  Responsible for seeing color and visual acuity

• Optic disc (blind spot)
  Area where optic nerve leaves the eye
  No photoreceptors, no image formation

Vision: Formation of The Visual Image

• Structures Involved:
  Cornea
  Aqueous Humor
  Lens
  Vitreous Humor
  Macula Lutea with Fovea Centralis
  Rods and Cones
  Optic nerve
  Visual Cortex in Cerebral Cortex

Physiology Involved in Vision

• Refraction of light
  Focusing on an object causes the light and image to bend as it passes through cornea, aqueous humor, lens and vitreous humor
  The bending ensures that the image is focused onto the Fovea Centralis

• Accommodation of Lens
  For looking at a close object, ciliary muscle contracted, lens rounded
  For looking at objects far away ciliary muscle relaxed, lens flattened

Photoreception

• Photoreceptors:
  Detect photons (basic units) of light
  Light energy also occurs as a wave
  Our visible spectrum of light is 400–700 nm

• Contain visual pigments that detect light
  Are derivatives of rhodopsin (pigment in rods)
  Consist of:
  Opsin (protein that determines the wavelength absorption of pigment)
  Retinal (pigment synthesized from vitamin A)

Photoreceptor Structure

• Pigmented epithelium
  Absorbs excess photons
  Those that are not absorbed by the visual pigments

• Outer segment
  Contains flattened, membranous plates or discs
  Contain visual pigment
  In cones: discs are plasma membrane infoldings and outer segment tapers to a blunt point
  In rods: each disc is a separate entity and outer segment forms elongated cylinder

• Inner segment
  Contains major organelles
  Responsible for all cell functions other than photoreception

• Each photoreceptor synapses with a bipolar cell

Photoreception

• Rods
  Contain pigment rhodopsin
  When light hits rhodopsin it breaks down, which stimulates the rods
  Rods all contain the same type of opsin
  Respond to blue-green wavelengths of light (perceived as white)

Photoreception - Color Vision

• Three types of cones
  Each with a different form of opsin, sensitive to a different range of wavelengths (ranges overlap)
  Blue cones (16 percent of all cones)
  Green cones (10 percent of all cones)
  Red cones (74 percent of all cones)

• Combined differential stimulation allows the brain to discern colors
  All stimulated equally = white

Photoreception - Steps

Resting state (in the dark)

• chemically gated sodium ion channels in the plasma membrane in the outer segment are kept open due to the presence of cGMP

• The inner segment continuously pumps sodium ions out of the cytosol

• This movement of ions is called dark current

• Keeps resting membrane potential about –40 mV

• The photoreceptor continually releases neurotransmitters across the synapse to bipolar cells

Exposure to light

• The retinal molecule in rhodopsin changes shape in the process called opsin activation
  From bent 11-cis form to more linear 11-trans form

• Opsin activates transducin
  G protein bound to disc membrane

• Transducin activates enzyme phosphodiesterase (PDE)

• Phosphodiesterase breaks down cGMP

• Removing cGMP inactivates sodium channels

• The rate of sodium entry into the cytosol decreases

Active state

• The decrease in sodium entry reduces dark current

• Active transport of sodium out of the cell continues

• Membrane potential drops to –70 mV

• The hyperpolarization of the membrane decreases the rate of neurotransmitter release

• Decreasing rate signals bipolar cell that the photoreceptor has absorbed a photon

Steps in Photoreception Continued

• Rhodopsin cannot respond to another photon until the original shape of a retinal is regained

• Three-step process for regeneration of visual pigments:
  Bleaching
  Entire rhodopsin molecule is first broken into retinal and opsin
  Retinal Converted Back to Cis Shape
  Requires energy (ATP)
  Reassembly
  Opsin and retinal are reassembled as rhodopsin

Color Vision and Color Blindness

Color Vision:

• Made possible by the reflection of photons from one region of the visible spectrum while photons from other regions are absorbed.
  If all photons are reflected, the object appears white.
  If all photons are absorbed, the object appears black.
  Discrimination of those photons (therefore of color) happens through the integration of information signaled from all three types of cones.
  Red, Blue, and Green – if all three seen – normal color vision
  Wide ranging discrimination of colors is through the variation in stimulation of the above three cone types.
  Color blindness:
  Inability to distinguish certain colors
  Due to nonfunctioning or absent cones (one or more types)
  ie. Red-green color blindness: missing red cones
  Partial color blindness - Not uncommon

Structures of The Visual Pathway

• Begins at the photoreceptors (rods and cones)

• Ends at the visual cortex of the occipital lobes of the cerebrum

• Crosses at the optic chiasm

• Pathway:
  Photoreceptors in retina
  Optic nerve (CNII)
  Optic chiasm (where crossing over of half the information from one side to the other occurs)
  Optic tract
  Lateral geniculate body (part of the thalamus)
  Projection fibers from lateral geniculate to the visual cortex (in the occipital lobe)

Central Processing and Field of Vision

• Perception of the visual image

• Result of integration of data that reaches the visual cortex of the occipital lobes
  Slighly different for each eye

• Field of vision – combined regions that are visible by the right and left eyes

• Collateral fibers from the lateral geniculates or optic tracts send additional information to parts of the midbrain or hypothalamus
  Helps with reflexes (i.e. eye movement), processing for motor commands, and circadian rhythm

Hearing and Equilibrium

• Anatomy of the ear amplifies and protects.

• Sound waves vibrate the tympanic membrane and convert sound waves into mechanical movement.

• Auditory ossicles conduct vibrations to the internal ear.

• They function as a lever system, collecting force and focusing it on the oval window, resulting in considerable amplification.

• Contractions of the tensor tympani and stapedius muscles protect the tympanic membrane and ossicles from violent movement under very noisy conditions.

Labyrinths of the Internal Ear - Bony Labyrinth

• Shell of dense bone that surrounds and protects the membranous labyrinth.

• Filled with perilymph, a liquid closely resembling CSF, between the bony labyrinth and membranous labyrinth.

• Consists of three parts:
  Semicircular canals
  Vestibule
  Cochlea

Labyrinths of the Internal Ear - Membranous Labyrinth

• Collection of fluid-filled tubes and chambers that houses receptors for hearing and equilibrium and contains fluid called endolymph.

• Consists of three parts:
  Semicircular ducts (within the semicircular canals)
  Receptors are stimulated by the rotation of the head.
  Utricle and saccule (within the vestibule) They provide sensations of gravity and linear acceleration.
  Cochlear duct (within the cochlea)
  The cochlear duct is sandwiched between a pair of perilymph-filled chambers, resembles a snail shell, and its receptors are stimulated by sound.

Receptors For Hearing

• Cochlear duct
  Long, coiled tube filled with endolymph
  Lies between a pair of perilymphatic chambers

• Scala vestibuli (vestibular duct)
  Separated from cochlear duct by vestibular membrane

• Scala tympani (tympanic duct)
  Separated cochlear duct by basilar membrane
  Both ducts interconnect at tip of cochlear spiral creating one long chamber
  Begins at oval window (base of scala vestibuli)
  Ends at the round window (base of scala tympani)

• Hair cells for hearing located in the organ of Corti (spiral organ) on the basilar membrane

Receptors for Hearing - Organ of Corti

• Hair cells are arranged in longitudinal rows that lack kinocilia.

• Stereocilia are in contact with the overlying tectorial membrane and embedded in the basilar membrane.

• Sound waves cause pressure waves within perilymph, vibrating the basilar membrane.

• Stereocilia press into the tectorial membrane and are distorted.

• More movement = more hair cells are stimulated.

• Sensory neurons relay the message through the spiral ganglion and cochlear branch of vestibulocochlear nerve (VIII).

Physiology of Hearing

• Sound
  Waves of pressure conducted through a medium such as air or water
  In air, pressure waves create alternating areas of compressed and separated air molecules

• Wavelength of sound
  Distance between two adjacent wave crests (peaks) or between two adjacent troughs

• Hearing is the perception of sound

Hearing pathway to the CNS

• Stimulation of hair cells activates sensory neurons.

• To ipsilateral auditory cortex

• Cochlea
  Low-frequency sounds
  High-frequency sounds

• Vestibular branch
  Information carried on the cochlear branch of the vestibulocochlear nerve (VIII)

• Vestibulocochlear nerve (VIII)
  To reticular formation and motor nuclei of cranial nerves

• Superior olivary nucleus

• Inferior colliculus coordinates responses to acoustic stimuli

• Auditory sensations synapse in the medial geniculate nucleus of the thalamus

• Projection fibers carry information to specific locations in the auditory cortex of the temporal lobe

• Cochlear nuclei of the medulla oblongata relay information to ponds and midbrain
  Motor output to the spinal cord through the tectospinal tracts

• Primary pathway

• Secondary pathway

• Motor output

Receptors For Equilibrium - Semicircular Ducts

• Three ducts (anterior, posterior, lateral) are continuous with the utricle and filled with endolymph.

• Each contains an enlarged region (ampulla).

• The area housing receptors in that region is the crista ampullaris (ampullary crest).

• Kinocilia and stereocilia of hair cells are embedded within the cupula.

• A flexible, elastic, gelatinous structure extends the width of the ampulla.

Receptors for Equilibrium - Semicircular Ducts Cont.

• Head rotating in the plane of one duct moves endolymph, pushing the cupula to the side, causing distortion of hair cells.

• Movement one way causes stimulation, and opposite movement causes inhibition.

• The cupula rebounds to the normal position when endolymph stops moving.

• Horizontal rotation (“no”) stimulates the lateral duct receptors.

• Nodding (“yes”) stimulates the anterior duct receptors.

• Tilting the head to the side stimulates the posterior duct receptors.

Receptors for Equilibrium - Utricle and Saccule

• Provide equilibrium sensations, whether the body is stationary or moving.

• Connected by a slender passageway that is continuous with the endolymphatic duct and ending in the endolymphatic sac.

• the sac projects into the subarachnoid space

• After being secreted in the cochlear duct, endolymph returns to general circulation at the endolymphatic sac

Receptors For Equilibrium - Utricle and Saccule Cont.

• Contain hair cells clustered in maculae (macula, spot).

• Hair cells of the utricle project vertically, and hair cells of the saccule project laterally.

• Hair cell processes are embedded in a gelatinous mass called the otolithic membrane.

• the surface contains densely packed calcium carbonate crystals called otoliths (“ear stones”).

Receptors for Equilibrium - Utricle and Saccule Cont.

• Changes in the position of the head cause distortion of hair cell processes in the maculae, sending signals to the brain.

• With head in upright position, otoliths sit on top of the otolithic membrane in the utricle.

• Weight presses on the surface, compressing hair cells, but not bending them.

• With a tilted position or with linear movement, gravity pulls on otoliths, shifting them to the side.

• Movement distorts hair cell processes, stimulating macular receptors.