Somatic and Special Senses

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Chapter 12

• Somatic and Special Senses

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RECEPTORS AND SENSATIONS
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Senses

• Sensory Receptors
• specialized cells or multicellular structures
  that collect information from the environment
• stimulate neurons to send impulses along sensory
  fibers to the brain

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Receptor Types

• specialized structures at the end of peripheral
  nerves that respond to stimuli
• can be classified according to their location in
  the body, stimulus type and structure

• Thermoreceptors are sensitive to temperature
  change
• Mechanoreceptors respond to a change in pressure
• (i.e. touch, pressure, vibrations, stretch)
• Photoreceptors (in retina of eye) respond to
  changes in light

• Chemoreceptors respond to changes in chemical
  concentrations
• Nociceptors respond to extreme (harmful) stimuli
  by producing
• the sensation of pain (i.e. all types under
  extreme stimuli

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Sensory Impulse

• All senses work in basically the same fashion
• Special sensory receptors collect information
  from the environment and stimulate neurons to
  send a message to the brain
• stimulation of receptor causes local change in
  its receptor potential
• a graded electrical current is generated that
  reflects intensity of stimulation
• if receptor is part of a neuron, the membrane
  potential may generate an action potential
• if receptor is not part of a neuron, the
  receptor potential must be transferred to a
  neuron to trigger an action potential
• peripheral nerves transmit impulses to CNS where
  they are analyzed and interpreted in the brain

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Sensations

• Sensation the conscious or unconscious awareness
  of external or internal stimuli
• Perception the conscious awareness and
  interpretation of sensations
• Projectionprocess in which the brain projects
  the sensation back to the apparent source
• it allows a person to pinpoint the region of
  stimulation

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Sensory Adaptation

• involves a decreased response to a particular
  stimulus from the receptors (peripheral
  adaptations) or along the CNS pathways leading to
  the cerebral cortex (central adaptation)
• sensory impulses become less frequent and may
  cease
• stronger stimulus is required to trigger
  impulses
• All sensory receptors, except nociceptors, adapt
  to continuous stimuli (i.e. undergo sensory
  adaptation)
• i.e. when you first put a band-aid on you feel it
  but soon dont notice it at all

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SOMATIC SENSES

• Receptors associated with skin, muscles, joints,
  and viscera provide somatic senses.

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Three groups

• Exteroceptive Senses
• detect changes at the body's surface
• touch
• pressure
• temperature
• Proprioceptive Senses
• detect changes in muscles, tendons, and body
  position
• Visceroceptive Senses
• detect changes in viscera
• only pain will be discussed here

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Three types of receptors

• free nerve endings (naked dendritic)
• in epithelium, CT
• Meissner's Corpuscles are encapsulated dendritic
  endings
• abundant in hairless portions of skin lips
• detect fine touch distinguish between two
  points on the skin
• Pacinian Corpuscles are also encapsulated
  dendritic endings
• common in deeper subcutaneous tissues, tendons,
  and ligaments
• detect heavy pressure and vibrations

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Temperature Senses

• Two types that respond to temperature change
• Heat receptors
• sensitive to temps above 25oC (77oF)
• unresponsive at temps above 45oC (113oF)
• Pain receptors are also triggered as this
  temperature approaches producing a burning
  sensation
• Cold receptors
• sensitive to temps between 10oC (50oF) and 20oC
  (68oF)
• below 10oC, pain receptors are triggered
  producing a freezing sensation
• Both undergo rapid sensory adaptation

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Sense of Pain

• Free nerve endings are the receptors that detect
  pain
• They are widely distributed throughout the skin
  and internal tissues, with the exception of the
  nervous tissue of the brain
• Pain Receptors (Nociceptors)
• function is protection against further tissue
  damage
• many stimuli may trigger them (i.e. temperature,
  pressure, chemicals)
• generally do not adapt to continual stimuli

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Sense of Pain cont.

• Visceral Pain
• only visceral receptors that produce sensations
• stretch receptors are stimulated by pressure
  and/or a decrease in oxygen levels
• may feel as if its coming from another area of
  the body referred pain
• may occur due to sensory impulses from two
  regions following a common nerve pathway to brain
  

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Sense of PAIN cont.

• Pain Nerve Pathways
• Acute pain
• occurs rapidly (0.1 sec)
• is not felt in deep tissues
• sharp, fast, pricking pain
• conducted on myelinated fibers
• ceases when stimulus is removed
• Chronic pain
• begins slowly and increases in intensity over a
  period of several seconds or minutes
• dull, aching, burning, throbbing pain
• can occur anywhere
• conducted on unmyelinated fibers
• may continue after stimulus is removed

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Stretch Receptors

• Stretch receptors are proprioceptors that send
  information to the spinal cord and brain
  concerning the length and tension of muscles

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SPECIAL SENSES

• SPECIAL SENSES are senses whose sensory receptors
  are located in large, complex organs in the head.
  
• The five special senses are vision, hearing,
  equilibrium, taste, and smell.

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OLFACTION

• Sense of Smell
• Organepithelial lining of nose

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Olfactory Receptors

• chemoreceptors that are located in the upper
  nasal cavity
• sensitive portion is cilia-like dendrites on
  bipolar neurons
• chemicals must be dissolved in solution to be
  detected
• undergo rapid sensory adaptation
• Olfactory Code
• hypothesis
• odor that is stimulated by a distinct set of
  receptor cells and its associated receptor
  proteins

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GUSTATION

• Sense of Taste
• Organ taste buds on tongue

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Taste Receptors

• chemoreceptors that are located in taste buds
• taste cells modified epithelial cells that
  function as receptors
• taste hairs microvilli that protrude from taste
  cellssensitive parts of taste cells
• Chemicals must be dissolved in saliva to be
  detected
• undergo rapid sensory adaptation

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Taste Sensations

• most taste buds are far posterior near the base
  of the tongue
• Four Primary Taste Sensations
• sweet stimulated by carbohydrates (tip of
  tongue)
• sour stimulated by acids (lateral tongue)
• salty stimulated by salts (perimeter of
  tongue)
• bitter stimulated by many organic compounds
  (posterior tongue)
• Taste varies from person to person

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Sense of Hearing

• OrganEar (Organ of Corti)

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Introduction

• The organ of hearing is the Organ of Corti, which
  is present in the cochlea of the inner ear
• The sensory receptors are called mechanoreceptors
• Once these mechanoreceptors are stimulated, the
  impulse travels on the cochlear branch of the
  vestibulocochlear (CN VIII) nerve, which leads to
  the primary auditory cortex (temporal cortex) of
  the cerebrum

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EAR STRUCTURE
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External Ear

• Auricle outer ear (cartilage)
• Function collection of sound waves
• External auditory meatus ear canal
• Function starts vibrations of sound waves and
  directs them toward tympanic membrane
• tympanic membrane
• vibrates in response to sound waves

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Middle Ear

• Function to amplify and concentrate sound
  waves.
• Tympanic cavity air-filled space behind
  eardrum separates outer from inner ear.
• Auditory ossicles 3 tiny bones in middle ear
• Malleus (hammer) is connected to tympanic
  membrane
• Incus (anvil) connects malleus to stapes
• Stapes (stirrup) connects incus to the
• Oval window the entrance to inner ear

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Middle Ear cont.

• Auditory (Eustachian) tube passageway which
  connects middle ear to nasopharynx (throat)
• Function to equalize pressure on both sides of
  the tympanic membrane, which is necessary for
  proper hearing.

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Inner Ear

• The inner ear consists of a complex system of
  intercommunicating chambers and tubes called a
  labyrinth. Actually, two labyrinths compose the
  inner ear
• Osseous labyrinth bony canal in temporal bone
• Perilymph fills the space between the osseous and
  membranous labyrinth
• Membranous labyrinth membrane within osseous
  labyrinth.
• Endolymph fills the membranous labyrinth.

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Inner Ear cont.

• The inner ear labyrinth can further be divided
  into three regions (cochlea, vestibule
  semi-circular canals)
• each with a specific function
• Cochlea snail shaped portion
• Function sense of hearing
• Semi-circular canals three rings
• Function dynamic equilibrium
• Vestibule area between cochlea and
  semi-circular canals
• Function static equilibrium

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The Choclea

• divided into two compartments
• Scala vestibuli upper compartment which extends
  from oval window to apex
• Scala tympani lower compartment which extends
  from apex to round window
• Both compartments are filled with perilymph
• Between the two bony compartments, we find the
  membranous labyrinth cochlear duct
• The cochlear duct is filled with endolymph

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Cochlea cont.

• There are membranes that separate the cochlear
  duct from the bony compartments
• Vestibular membrane separates the cochlear duct
  from the scala vestibuli
• Basilar membrane separates the cochlear duct from
  the scala tympani

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Organ of Corti

• The mechanoreceptors responsible for the sense of
  hearing are contained in the Organ of Corti
  16,000 hearing receptor cells located on the
  basilar membrane.
• The receptor cells are called "hair cells"
• The hair cells are covered by the tectorial
  membrane, which lies over them like a roof
• different frequencies of vibration move different
  parts of basilar membrane
• particular sound frequencies cause hairs of
  receptor cells to bend
• nerve impulse generated

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Sound through the ear

• Auditory Nerve Pathways

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First

• Sound waves arrive at the tympanic membrane

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Second

• Movement of the tympanic membrane causes
  displacement of the auditory ossicles.

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Third

• Movement of the stapes at the oval window
  establishes pressure waves in the perilymph of
  the vestibular duct.

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Fourth

• The pressure waves distort the basilar membrane
  on their way to the round window of the tympanic
  duct.

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Fifth

• Vibration of the basilar membrane causes
  vibration of hair cells against the tectorial
  membrane

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Sixth

• Information about the region and the intensity of
  stimulation is relayed to the CNS over the
  cochlear branch of cranial nerve VIII.
• thalamus for direction to the
• primary auditory cortex (temporal lobes) of
  cerebrum for interpretation

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SENSE OF EQUILIBRIUM

• Organs vestibule, utricle, saccule,
  semi-circular canals

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Equilibrium

• Static Equilibrium
• vestibule
• sense position of head when body is not moving
• Dynamic Equilibrium
• semicircular canals
• sense rotation and movement of head and body

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Static Equilibrium

• functions to sense the position of the head and
  help us maintain posture while motionless
• The vestibule of the inner ear contains the two
  membranous chambers responsible for static
  equilibrium
• The utricle communicates with the semi-circular
  canals
• The saccule communicates with the cochlear duct

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Static Equilibrium cont.

• Each of these chambers contains a macula organ
  of static equilibrium
• The macula is composed of "hair cells" that are
  in contact with a jelly-like fluid containing
  calcium carbonate crystals (otolith)
• When the head is moved, the gel sags due to
  gravity and the hair cells bend
• This triggers a sensory impulse

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Dynamic Equilibrium

• functions to prevent loss of balance during rapid
  head or body movement
• The three semi-circular canals contain the organ
  responsible for dynamic equilibrium.
• Each semi-circular canal ends in an enlargement
  called the ampulla
• Each ampulla houses a sensory organ for dynamic
  equilibrium called the crista ampullaris, which
  contains a patch of "hair cells" in a mass of
  gelatin
• When the head is moved, the gelatin stays put due
  to inertia causing the hair cells to bend.
• This triggers a sensory impulse

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Vision

• Organthe eye
• Visual Accessory Organs
• eyelids, lacrimal apparatus, extrinsic eye
  muscles

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Introduction

• The organ of vision is the retina of the eye
• The sensory receptors are called photoreceptor
• When photoreceptors are stimulated, impulses
  travel within the optic nerve (CN II) to the
  visual (occipital) cortex for interpretation

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Visual Accessory Organs

• Lacrimal gland tear secretion located on upper
  lateral surface
• Tears contain an enzyme called lysozyme, which
  functions as an anti-bacterial agent.
• Nasolacrimal duct duct which carries tears into
  nasal cavity (drainage)

• Eyelids protective shield for the eyeball.
• Conjunctiva inner lining of eyelid red
  portion around eye.
• Lacrimal apparatus tear secretion distribution

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Extrinsic Eye Muscles
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Structure of the Eye

• The eye is composed of three distinct layers or
  tunics
• The Outer Tunic (fibrous tunic)
• The Middle (vascular tunic)
• The inner (nervous tunic)

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The Outer Tunic

• Function protection
• Cornea transparent anterior portion
• Function helps focus (75) incoming light rays
• Sclera white posterior portion, which is
  continuous with eyeball except where the optic
  nerve and blood vessels pierce through it in the
  back of eye
• Functions
• protection
• attachment (of eye muscles)

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The Middle tunic (vascular)

• Function nourishment
• Choroid coat membrane joined loosely to sclera
  containing many blood vessels to nourish the
  tissues of the eye
• pigments absorb extra light
• Ciliary body anterior extension from choroid
  coat, which is composed of 2 parts
• Ciliary muscles which control the shape of the
  lens (i.e. Accommodation)
• Ciliary processes which are located on the
  periphery of the lens
• Suspensory ligaments extend from the ciliary
  processes on the lens to the ciliary muscles and
  function to hold the lens in place

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The Middle tunic cont.

• Iris colored ring around pupil
• thin diaphragm muscle
• lies between cornea and lens
• The iris separates the anterior cavity of the eye
  into an anterior chamber and posterior chamber
• The entire anterior cavity is filled with aqueous
  humor, which helps nourish the anterior portions
  of the eye, and maintains the shape of the
  anterior eye

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Aqueous Humor

• fluid in anterior cavity of eye
• secreted by epithelium on inner surface of the
  ciliary body
• provides nutrients
• maintains shape of anterior portion of eye

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The Inner tunic (nervous, sensory)

• Retina inner lining of the eyeball site of
  photoreceptors
• A picture of the retina can be taken with a
  camera attached to an ophthalmoscope
• The optic disc is the location on the retina
  where nerve fibers leave the eye join with the
  optic nerve
• the central artery vein also pass through this
  disk
• No photoreceptors are present in the area of the
  optic disk blind spot.
• The posterior cavity of the eye is occupied by
  the lens, ciliary body, and the retina
• The posterior cavity is filled with vitreous
  humor, which is a jelly-like fluid, which
  maintains the spherical shape of the eyeball

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Accommodation

• the process by which the lenschanges shape to
  focus on close objects
• The lens is responsible (with cornea) for
  focusing incoming light rays.
• If light rays are entering the eye from a distant
  object, the lens is flat.
• When we focus on a close object, the ciliary
  muscles contract, relaxing the suspensory
  ligaments. Accordingly, the lens thickens
  allowing us to focus.

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Figure 12.29
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Light Refraction

• Incoming light rays are refracted (bent) onto the
  retina due to the convex surface of both the
  cornea and the lens.
• Pathway of Light Through Eye
• 1. cornea
• 2. aqueous humor
• 3. lens
• 4. vitreous humor
• 5. photoreceptors in retina.
• Once the rods and/or cones are stimulated, a
  sensory impulse is carried

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Light Through the Eye

• as light enters eye, it is refracted by
• convex surface of cornea
• convex surface of lens
• image focused on retina is upside down and
  reversed from left to right

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Macula lutea yellow spot of mostly cones Fovea
centralis depression of ALL cones, sharpest
vision
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Visual Receptors

• Rods long, thin projections
• contain light sensitive pigment called
  rhodopsin
• hundred times more sensitive to light than cones
• provide vision in dim light
• produce colorless vision and outlines
• dark adapted all opsin and retinal is together,
  therefore rods are VERY sensitive, vision
  possible even in dark

• Conesshort/blunt
• projections
• contain light sensitive pigments called
  erythrolabe, chlorolabe, and cyanolabe
• provide vision in bright light
• produce sharp images
• produce color vision
• light adapted most opsin and retinal decomposes

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Visual Pigments

• Rhodopsin
• light-sensitive pigment in rods
• decomposes in presence of light
• triggers a complex series of reactions that
  initiate nerve impulses
• impulses travel along optic nerve

• Pigments on Cones
• each set contains different light-sensitive
  pigment
• each set is sensitive to different wavelengths
• color perceived depends on which sets of cones
  are stimulated
• red, green, or blue

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Stereoscopic Vision

• provides perception of distance and depth
• results from formation of two slightly different
  retinal images