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Chapter 12 Nervous System III - Senses

General Senses •  receptors that are widely distributed throughout the body •  skin, various organs and joints

Special Senses •  specialized receptors confied to structures in the head •  eyes and ears

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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

Sensation •  a feeling that occurs when brain becomes aware of sensory impulse

Perception •  a person’s view of the stimulus; the way the brain interprets the information

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Receptor Types

Chemoreceptors •  respond to changes in chemical concentrations

Pain receptors (Nociceptors) •  respond to tissue damage

Thermoreceptors •  respond to changes in temperature

Mechanoreceptors •  respond to mechanical forces

Photoreceptors •  respond to light 4

Sensations

Sensation occurs when the brain becomes aware of sensory

impulses Perception

occurs when the brain interprets sensory impulses Projection process 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

•  ability to ignore unimportant stimuli

•  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

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General Senses

•  senses associated with skin, muscles, joints, and viscera

•  three groups

•  exteroceptive senses – senses associated with body surface; touch, pressure, temperature, pain •  visceroceptive senses – senses associated with changes in viscera; blood pressure stretching blood vessels, ingesting a meal

•  proprioceptive senses – senses associated with changes in muscles and tendons

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Touch and Pressure Senses

Free nerve endings • common in epithelial tissues •  simplest receptors •  sense itching

Meissner’s corpuscles • Small, oval masses of flattened connective tissue cells in connective tissue cells in a connective tissue sheath with 2 or more sensory nerve fibers branching into each corpusule •  abundant in hairless portions of skin; lips •  detect fine touch; distinguish between two points on the skin

Pacinian corpuscles • Relatively large, ellipsoidal structures composed of connective tissue fivers and cells • common in deeper subcutaneous tissues, tendons, and ligaments •  detect heavy pressure and vibrations 8

Touch and Pressure Receptors

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Temperature Senses

Warm receptors •  sensitive to temperatures above 25oC (77o F) •  unresponsive to temperature above 45oC (113oF)

Cold receptors •  sensitive to temperature between 10oC (50oF) and 20oC (68oF)

Pain receptors •  respond to temperatures below 10oC •  respond to temperatures above 45oC

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Sense of Pain

•  free nerve endings •  widely distributed •  nervous tissue of brain lacks pain receptors •  stimulated by tissue damage, chemical, mechanical forces, or extremes in temperature •  adapt very little, if at all

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Visceral Pain

•  pain receptors are the only receptors in viscera whose stimulation produces sensations •  pain receptors respond differently to stimulation •  not well localized

•  may feel as if coming from some other part of the body •  known as referred pain

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Referred Pain •  may occur due to sensory impulses from two regions following a common nerve pathway to brain

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Pain Nerve Pathways

Acute pain fibers •  A-delta fibers • thin, myelinated •  conduct impulses rapidly •  associated with sharp pain •  well localized

Chronic pain fibers •  C fibers • thin, unmyelinated •  conduct impulses more slowly •  associated with dull, aching pain •  difficult to pinpoint

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Regulation of Pain Impulses

Thalamus •  allows person to be aware of pain

Cerebral Cortex •  judges intensity of pain •  locates source of pain •  produces emotional and motor responses to pain

Pain Inhibiting Substances •  enkephalins •  serotonin •  endorphins

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Proprioceptors

•  mechanoreceptors-respond to mechanical stress on tissues •  send information to spinal cord and CNS about body position and length and tension of muscles •  Main kinds of proprioreceptors

•  Pacinian corpuscles – in joints •  muscle spindles – in skeletal muscles* •  Golgi tendon organs – in tendons*

*stretch receptors-respond to stretching in a tissue 16

Visceral Senses

Receptors are found on internal organs Free nerve endings Lamellated corpuscles

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Special Senses

•  sensory receptors are within large, complex sensory organs in the head •  smell in olfactory organs •  taste in taste buds •  hearing and equilibrium in ears •  sight in eyes

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Sense of Smell

Olfactory Receptors •  chemoreceptors •  respond to chemicals dissolved in liquids

Olfactory Organs •  contain olfactory receptors and supporting epithelial cells •  cover parts of nasal cavity, superior nasal conchae, and a portion of the nasal septum

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Olfactory Nerve Pathways

Once olfactory receptors are stimulated, nerve impulses travel through

•  olfactory nerves olfactory bulbs olfactory tracts limbic system (for emotions) and olfactory cortex (for interpretation)

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Olfactory Stimulation

Olfactory Code •  hypothesis •  odor that is stimulated by a distinct set of receptor cells and its associated receptor proteins

•  olfactory organs located high in the nasal cavity above the usual pathway of inhaled air

•  olfactory receptors undergo sensory adaptation rapidly

•  sense of smell drops by 50% within a second after stimulation

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Sense of Taste

Taste Buds •  organs of taste •  located on papillae of tongue, roof of mouth, linings of cheeks and walls of pharynx

Taste Receptors •  chemoreceptors •  taste cells – modified epithelial cells that function as receptors •  taste hairs –microvilli that protrude from taste cells; sensitive parts of taste cells

75-80% of taste comes from smell

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Taste Receptors

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Taste Sensations

Five Primary Taste Sensations •  sweet – stimulated by carbohydrates •  sour – stimulated by acids •  salty – stimulated by salts •  bitter – stimulated by many organic compounds •  umani - stimulated by certain amino acids

Spicy foods activate pain receptors

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Taste Nerve Pathways

Sensory impulses from taste receptors travel along •  cranial nerves to •  medulla oblongata to •  thalamus to •  gustatory cortex (for interpretation)

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Hearing

Ear – organ of hearing

Three Sections •  External •  Middle •  Inner

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External Ear

•  auricle •  collects sounds waves

•  external auditory meatus •  lined with ceruminous glands •  carries sound to tympanic membrane •  terminates with tympanic membrane

•  tympanic membrane •  vibrates in response to sound waves •  tympanic reflex-two small skeletal muscles protect the ear from loud noices

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Middle Ear

•  tympanic cavity •  air-filled space in temporal bone •  auditory ossicles

•  vibrate in response to tympanic membrane •  malleus, incus, and stapes

•  oval window •  opening in wall of tympanic cavity •  stapes vibrates against it to move fluids in inner ear

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Auditory Tube

•  eustachian tube •  connects middle ear to throat •  helps maintain equal pressure on both sides of tympanic membrane •  usually closed by valve-like flaps in throat

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Inner Ear

Three Parts of Labyrinths •  cochlea

•  functions in hearing •  semicircular canals

•  functions in equilibrium

•  vestibule •  functions in equilibrium

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Equilibrium

Static Equilibrium •  vestibule (utricle and saccule) •  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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Sight

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

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Eyelid

•  palpebra •  composed of four layers

•  skin •  muscle •  connective tissue •  conjunctiva

•  orbicularis oculi - closes •  levator palperbrae superioris – opens •  tarsal glands – secrete oil onto eyelashes •  conjunctiva – mucous membrane; lines eyelid and covers portion of eyeball

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Lacrimal Apparatus

•  lacrimal gland •  lateral to eye •  secretes tears

•  canaliculi •  collect tears

•  lacrimal sac •  collects from canaliculi

•  nasolacrimal duct •  collects from lacrimal sac •  empties tears into nasal cavity

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Extrinsic Eye Muscles

Superior rectus •  rotates eye up and medially

Inferior rectus •  rotates eye down and medially

Medial rectus •  rotates eye medially

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Extrinsic Eye Muscles

Lateral rectus •  rotates eye laterally

Superior oblique •  rotates eye down and laterally

Inferior oblique •  rotates eye up and laterally

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Structure of the Eye

•  hollow •  spherical •  wall has 3 layers

•  outer fibrous tunic •  middle vascular tunic •  inner nervous tunic

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Outer Tunic

Cornea •  anterior portion •  transparent •  light transmission •  light refraction

Sclera •  posterior portion •  opaque, white •  protection

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Middle Tunic Iris

•  anterior portion •  pigmented •  controls light intensity by controlling the size of the pupil

Ciliary body •  anterior portion •  pigmented •  holds lens •  moves lens for focusing

Choroid coat •  provides blood supply •  pigments absorb extra light

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Anterior Portion of Eye •  filled with aqueous humor

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Lens

•  transparent •  biconvex •  lies behind iris •  largely composed of lens fibers •  elastic •  held in place by suspensory ligaments of ciliary body

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Ciliary Body •  forms internal ring around front of eye •  ciliary processes – radiating folds •  ciliary muscles – contract and relax to move lens

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Accommodation •  changing of lens shape to view objects

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Inner Tunic

•  retina •  contains visual receptors •  continuous with optic nerve •  ends just behind margin of the ciliary body •  composed of several layers •  macula lutea – yellowish spot in retina •  fovea centralis – center of macula lutea; produces sharpest vision •  optic disc – blind spot; contains no visual receptors •  vitreous humor – thick gel that holds retina flat against choroid coat

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Posterior Cavity

•  contains vitreous humor – thick gel that holds retina flat against choroid coat

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Light Refraction Refraction

•  bending of light •  occurs when light waves pass at an oblique angle into mediums of different densities

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Types of Lenses

Convex lenses cause light waves to converge

Concave lenses cause light waves to diverge

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Focusing On Retina •  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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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 •  produce outlines of objects

Cones •  short, blunt projections •  contain light sensitive pigments called erythrolabe, chlorolabe, and cyanolabe •  provide vision in bright light •  produce sharp images •  produce color vision • 3 types: red, green and blue

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

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Life-Span Changes

Age related hearing loss due to •  damage of hair cells in organ of Corti •  degeneration of nerve pathways to the brain •  tinnitus

Age-related visual problems include •  dry eyes •  floaters (crystals in vitreous humor) •  loss of elasticity of lens •  glaucoma •  cataracts •  macular degeneration

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Clinical Application Refraction Disorders

•  concave lens corrects nearsightedness

•  convex lens corrects farsightedness