CHAPTER 16 SENSE ORGANS Kenneth E. Muller, PhD, PT

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CHAPTER 16SENSE ORGANS

• Kenneth E. Muller, PhD, PT

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• Sensation- Conscious or subconscious awareness of
  external or internal stimuli.
• Perception- Conscious awareness interpretation
  of meaning of sensations.
• Sensory modality- Each type of sensation, touch,
  pain, vision, or hearing.
• Each sensory neuron carries information for 1
  modality only.
• Sensory modalities grouped into 2 classes
• General Senses
• Special Senses

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• General senses include
• Somatic senses-
• Tactile sensations such as touch, pressure,
  vibration
• Thermal sensations such as warmth cold
• Pain sensations
• Proprioceptive sensations
• Visceral sensations-
• Provide information about conditions within
  internal organs.
• Special senses-
• Include modalities of taste, smell, vision,
  hearing and balance.

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• Sensation
• Begins at a sensory receptor
• Can be either a specialized cell or dendrites of
  a sensory neuron.
• A given sensory receptor responds vigorously to
  one type of stimulus only.
• Stimulus- a change in the environment that can
  activate certain sensory receptors, but responds
  only weakly or not at all to other types of
  stimuli.
• Selectivity-
• The stimulus can be in one of 3 forms
• Electromagnetic- Light or heat
• Mechanical- Sound or pressure changes
• Chemical- Such as an increase in CO2.

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• 4 Events usually occur for a sensation to occur
• Stimulation of the sensory receptor.
• Transduction of the stimulus- Receptor converts
  the stimulus into a graded potential.
• Graded potential- A small deviation in the
  membrane potential that makes the cell more or
  -. Vary in amplitude depending on the strength
  of the stimulus. Useful for short-distance
  communication only.
• Generation of impulses- When a threshold is
  reached the impulse is propagated towards the
  CNS.
• Integration of sensory input- The sensory impulse
  is received and integrated by that particular
  region of the CNS.

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• Types of sensory receptors
• Free nerve endings- Bare dendrites with no
  structural specialization
• Pain, Thermal, Tickle, Itch some touch
  sensations.
• Encapsulated nerve endings- Dendrites enclosed in
  a connective tissue capsule with a distinctive
  structure. I.e- Pacinian corpulses
• Separate cells- Synapse with sensory neurons.
  I.e- special senses such as vision, hearing,
  smell, equlibrium, taste.
• Graded potentials can be
• Generator potentials- one or more impulses to a
  neuron
• Receptor potential- Neurotransmitter released

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• Sensory receptors can be grouped as
• Exteroceptors- At or near the external surface of
  the body.
• Interoceptors- In blood vessels or internal
  organs.
• Proprioceptors- Located in muscles, tendons,
  joints the middle ear
• Sensory receptors can be grouped by the type of
  stimulus they detect.
• Mechanoreceptors- detect pressure or stretching
• Thermoreceptors- detect changes in temperature.
• Nocioceptors- respond to physical or chemical
  damage to the tissues, pain.

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• Photoreceptors- detect light as it strikes the
  retina of the eye.
• Chemoreceptors- detect chemicals in the mouth,
  (taste) nose, (smell) and body fluids.
• Most sensory receptors can adapt to a stimulus
  that is maintained for a period of time.
• Receptors vary in how quickly they adapt.
• Rapidly adapting- pressure, touch, smell.
• Slowly adapting- Pain, body position, chemical
  composition of the blood.

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• Somatic sensations-
• Arise from stimulation of receptors embedded in
  skin or subcutaneous layer, in membranes of the
  mouth, vagina, anus, muscles, tendons, joints and
  the inner ear.
• Cutaneous sensations are somatic sensations that
  result from stimulating the skin. 4 types
• Tactile- touch, pressure, vibration, itch
  tickle. All are A fibers with the exception of
  itch tickle which are C fibers.
• Touch- Crude touch vs. discriminative touch.
  Meissner corpulses are receptors of
  discriminative touch located in dermis of
  hairless skin. Rapidly adapting, generate
  impulses mainly at the onset of touch. Eyelids,
  tip of tongue, lips, nipples, soles of feet, tip
  of penis, clitoris.

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• Hair root plexuses- also rapidly adapting.
• 2 types of slowly adapting touch receptors
• Type I, Merkels discs- discriminative touch,
  located in the fingertips, 25 of the receptors
  of the hands.
• Type II Ruffini corpuscles- Located deep in the
  dermis, 20 of the receptors of the hands.
  Located in ligaments, sensitive to stretching.
• Pressure Vibration-
• Pressure- sustained sensation felt over a larger
  area than touch.
• Pacinian corpuscles- widely distributed in the
  body, mostly in the subcutaneous layer, rapidly
  adapting.
• Sensation of vibration- rapidly repetitive
  sensory signals from tactile receptors.
  Receptors are the corpuscles of touch.

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• Itch Tickle-
• stimulation of free nerve endings by certain
  chemicals such as bradykinin stimulate an
  inflammatory response.
• Thermal sensations
• Thermal receptors- free nerve endings that
  perceive cold warmth.
• Cold receptors- located in the stratum basale of
  the epidermis, a fibers, medium diameter
• Warm receptors- located in the dermis, C fibers
• Both cold warm receptors adapt rapidly. Temps
  below 10C and 48C stimulate nocioceptors,
  producing pain sensations.

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• Pain sensations-
• Pain- serves as a protective mechanism, a
  symptom. Signals the presence of tissue damaging
  conditions.
• Nocioceptors are the receptors for pain.
• Conditions that elicit pain- spasm, distention,
  ischemia, dilation, prolonged contraction.
• Types of pain-
• Fast pain slow pain
• Fast pain occurs very rapidly- usually within 0.1
  seconds of the stimulus. Medium diameter a
  fibers. Fast pain is not felt in the deeper
  tissues of the body.
• Slow pain- begins 1 or more seconds after the
  stimulus. Gradually increases in intensity. C
  fibers, may be excruciating, throbbing, found in
  deeper tissues.

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• Superficial somatic pain- comes from receptors in
  skin.
• Deep somatic pain- receptors in the muscles,
  joints, tendons fascia.
• Visceral pain- receptors in internal organs.
• Referred pain-
• Phantom limb pain-
• Physiology vs Psychology

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• Ophthalmology- The study of the eye and its
  disorders.
• Otolaryngology- The science that deals with the
  other special senses.
• Olfaction- Sense of smell.
• A chemical sense. The impulse arises from the
  interaction of molecules and propagate to the
  limbic system. Certain odors can elicit a strong
  emotional response.
• 10-100 million receptors for olfaction.
• Olfactory receptors respond to chemical
  stimulation of an odorant molecule by producing a
  receptor potential.

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• Olfactory cells 3 types
• Olfactory receptors- 1st order neurons.
• Support cells- support and nutrition to receptors
• Basal stem cells- continually undergo cell
  division to produce new receptors uncommon as
  most neurons cannot replicate.
• We can recognize 10,000 different scents
  depending on brain activity and different
  combinations of olfactory receptors.
• Olfaction has a low threshold.
• Adaptation happens rapidly, 50 after the 1st
  second.
• Pathway goes via CN I to the olfactory tract
  through the limbic system. (Emotions)

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• Gustation- The sense of taste.
• Only 4 classes of stimuli can be distinguished
• Sour, sweet, bitter, salty
• All other tastes are a combination of these 4
• Odors from food pass upward to stimulate
  olfactory receptors.
• When you have a cold you say it doesnt taste
  good.
• Nearly 10,000 taste buds, nearly all on the
  tongue.
• Some are also found on the soft palette.
• Taste buds are found on the elevations on the
  tongue called papillae which give the tongue its
  rough appearance.

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• Once a chemical is dissolved in saliva it makes
  contact with a receptor and a receptor potential
  is generated.
• Receptors in the tip of the tongue are more
  sensitive to sweet and salty, posterior tongue
  are highly sensitive to bitter, lateral tongue
  highly sensitive to sour.
• Poisonous substances tend to have a low
  thresholds.
• Complete adaptation to a specific taste can
  develop after 1-5 minutes of continuous
  stimulation.
• CN VII- anterior 2/3 of tongue, IX post 1/3, X
  throat and epiglottis.

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• Vision- More than 1/2 the sensory receptors in
  the body are located in the eyes.
• Accessory Structures of the eye-
• Eyelids- shade the eyes, protection, spread
  lubicant
• Eyelashes- Protection
• Eyebrows- Protection from perspiration.
• Lacrimal (tearing) apparatus- produces and drain
  lacrimal fluid. A watery solution containing
  salts and lysosome. If an irritating substance
  contacts the conjunctiva the glands will over
  secrete. Human emotion, a parasympathetic
  response.
• Extrinsic eye muscles- Superior, inferior rectus,
  lateral medial rectus, superior inf oblique.
• SO4, LR6, AR3

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• The wall of the eyeball has 3 layers.
• Fibrous tunic- the superficial coat of the
  eyeball. Avascular, contains the cornea which
  covers the iris. Sclera- the white of the eye,
  covers all the eyeball except the cornea, gives
  shape to the eyeball.
• Vascular tunic- Middle layer of the eyeball.
  Contains 3 parts
• Choroid- highly vascularized, provides nutrients
• Ciliary body- contains the ciliary muscle which
  alters the shape of the lens. Near vs far
  vision.
• Iris- Colored portion of eyeball, regulates the
  amount of light that enters through the pupil.
• Retina- inner coat of the eyeball

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• Retina lines the posterior 3/4 of the eyeball and
  is the beginning of the visual pathway.
• Blood vessels can be seen in the retina through
  an opthalmoscope. The only place in the
  body. Optic disc- Also known as the blind spot.
  Site where the optic nerve exits the eyeball.
  Optic nerve is bundled with central retinal
  artery vein.
• Retina consists of a pigment epithelium of
  melanin producing cells. Melanin absorbs stray
  light rays. Light sensitivity- lack of pigment
  epithelium.
• The neural portion of the retina is a
  multilayered outgrowth of the brain that
  processes visual data before transmitting data to
  the thalamus.

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• Photoreceptors- 2 types
• Rods- 120 million. Allow us to see in dim
  light.
• Cones- 6 million. Produce color vision. Most
  visual experiences are mediated by the cone
  system.
• Lens- Posterior to the pupil. Fine tunes
  focusing of light rays onto the retina to aid
  clear vision.
• Interior of the eyeball-Divided by the lens into
  2 cavities.
• Anterior cavity
• Anterior chamber- behind cornea and in front of
  the iris
• Posterior chamber- behind iris

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• Aqueous humor- watery fluid that is filtered to
  nourish lens and cornea.
• Intraocular pressure is maintained by aqueous
  humor.
• Vitreous chamber- posterior cavity that lies
  between lens and retina. Contains the vitreous
  body, a jellylike substance that contributes to
  the intraocular pressure. Does not undergo
  replacement. Formed during embryonic development.

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• Image formation-
• Refraction of light rays- bending. Images
  focused on the retina are inverted. Brain learns
  early in life to coordinate visual images.
• Accommodation- change in the shape of the lens.
  When eye is focusing on a close object the lens
  becomes more curved and refracts the rays more.
• Point of near vision- Presbyopia
• Refraction abnormalities-
• Emmetropic eye- Normal eye.
• Myopia- Nearsighted
• Hypermetropia- Farsighted
• Astigmatism- Cornea or lens has abnormal curve.
• Constriction or narrowing of the pupil.
• Autonomic reflex for light rays

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• Convergence- binocular vision- Both eyes focus on
  1 set of objects.
• Photoreceptors-
• Rods cones named for the appearance of the
  outer segment of the cell.
• Transduction of light energy into a receptor
  potential.
• Photopigments- Integral proteins in the plasma
  membrane of the outer part of the cell. 3
  different cones photopigments are present in the
  retina.
• Color blindness- Inability to distinguish between
  certain colors, an absence of 1 of the 3 cone
  photopigments. Most common is red-green color
  blindness.

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• Good vision depends on carotenoid rich vegetables
  such as carrots, spinach, broccoli, and yellow
  squash. Prolonged vitamin A deficiency can cause
  night blindness (nyctalopia)
• Light dark adaptation- visual system adjusts
  quickly.
• Brain pathway- Axons of CN II pass through the
  optic chiasm where some fibers cross to the other
  side and some remain on the same side. Primary
  visual cortex (area 17) Occipital lobe.

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• The Ear- 3 principle regions
• The outer ear- collects sound waves
• Contains the tympanic membrane.
• The middle ear- conveys sound vibration. Small
  air filled cavity in the temporal bone. Contains
  the 3 smallest ones in the body.
• Malleus
• Incus
• Stapes- fits into the round window.
• The inner ear- houses receptors for hearing and
  equilibrium. Also called the labyrinth. Houses
  a series of semicircular canals.
• Bony labyrinth which encloses a membranous
  labyrinth

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• Bony labyrinth-
• Contains the semicircular canals and the
  vestibule, both of which have receptors for
  equilibrium.
• Cochlea- Contains receptors for hearing.
• Lined with perilymph- similar to CSF
• 3 bony semicircular canals- each lies at right
  angles to the other 2.
• Hearing-
• Sound wave detected in outer ear
• Hits tympanic membrane- alternating high and low
  pressure vibration
• Contacts malleus to incus to stapes.
• Stapes pushes oval window in and out sets up
  fluid pressure in perilymph.
• Generates receptor potential in the cochlear
  fibers

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• CN VIII to cochlear nuclei in the medulla.
• Equilibrium-
• Static- maintenance of the position of the body
  relative to gravity.
• Dynamic- maintenance of body position in response
  to sudden movements.
• Receptor organs are called vestibular apparatus.
• Otolithic organs
• Utricle- small sacs in the labyrinth
• Saccule- small bag like sac.
• Contain a thickened region called the macula-
  receptors for static equilibrium and some dynamic
  equilibrium.

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