Somatic and Special Senses

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Chapter 10

• Somatic and Special Senses

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10.1 Introduction

• Our senses are not only necessary for us to enjoy
  life, but to survive.
• Sensory receptors detect changes in the
  environment and stimulate neurons to send nerve
  impulses to the brain.
• Sensory receptors vary greatly, but fall into two
  major categories.

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2 major categories

• Receptors associated with somatic senses.
• Receptors associated with specialized sensory
  organs.

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Receptors assoc. w/ somatic senses

• Receptors associated with the somatic senses of
  touch, pressure, temperature, and pain form one
  group.
• These receptors are widely distributed throughout
  the skin, muscles, joints and deeper tissues.
• They are structurally simple.

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Receptors associated with specialized sensory
organs

• Special senses of smell, taste, hearing,
  equilibrium, and vision.

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10.2 Receptors and Sensations

• Each receptor is more sensitive to a specific
  kind of environmental change but is less
  sensitive to others.
• This selective response distinguishes the senses.

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Type of Receptors

• Chemoreceptors- are stimulated by changes in the
  chemical concentration of substances. (Taste and
  smell)
• Pain receptors- by tissue damage.
• Thermoreceptors- by changes in temperature.
• Mechanoreceptors- by changes in pressure or
  movement.
• Photoreceptors- by light energy.

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Sensations

• are feelings that occur when the brain interprets
  sensory impulses.
• All nerve impulses that travel away from sensory
  receptors into the central nervous system are
  alike, the resulting sensation depends on which
  region of the brain is stimulated.

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Projection

• Because the brain projects the sensation back to
  its apparent source.
• Projection allows a person to pinpoint the
  location of stimulation thus the eyes seem to
  see, and the ears seem to hear.

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Sensory adaptation

• When sensory receptors are continuously
  stimulated, many of them undergo an adjustment
  called sensory adaptation.
• As receptors adapt, impulses leave them at
  decreasing rates, until finally these receptors
  may stop sending signals.
• Example a person walks into a room with a strong
  smell the longer the person is in the room the
  less noticeable the smell is.

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10.3 Somatic Senses

• Receptors associated with the skin, muscles,
  joints, and viscera make up the somatic senses.

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Pressure Touch Figure 10.1

• Three types of receptors detect touch and
  pressure.
• Free ends of sensory nerve fibers in the
  epithelial tissues are associated with touch and
  pressure.
• Meissners corpuscles are flattened connective
  tissue sheaths surrounding two or more nerve .
  Meissners fibers and are abundant in hairless
  areas that are very sensitive to touch, like the
  lips.
• Pacinian corpuscles are large structures of
  connective tissue and cells that resemble the
  layers of an onion. They function to detect
  deep pressure.

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Skin Cross Section
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Temperature Senses

• Temperature receptors include two groups of free
  nerve endings Warm receptors and cold receptors
  which both work best within a range of
  temperatures.
• Both types of receptors adapt quickly.
• Temperatures near 45 oC stimulate pain receptors
  temperatures below 10 oC also stimulate pain
  receptors and produce a freezing sensation.

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Pain

• Pain receptors consist of free nerve endings
  that are distributed throughout the skin and
  internal tissues, except in the nervous tissue of
  the brain, which has no pain receptors.
• Pain receptors adapt poorly.
• Visceral pain receptors are the only receptors in
  the organs that produce sensations. Stretching or
  spasm of smooth muscle.
• Referred pain occurs because of the common nerve
  pathways leading from skin and internal organs.
  An example would be a heart attack being felt as
  pain in the arm or as heartburn.

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Acute Pain

• Acute pain fibers are relatively thin, myelinated
  nerve fibers.
• They conduct nerve impulses rapidly and are
  associated with the sensation of sharp
  short-duration pain.
• Originates from a restricted area of the skin and
  seldom continues after the pain-producing
  stimulus stops.

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Chronic Pain

• Chronic pain fibers are thin, unmyelinated nerve
  fibers.
• They conduct impulses slowly and produce dull,
  aching pain that may be diffused and difficult to
  pinpoint.

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Regulation of Pain

• A person becomes aware of pain when impulses
  reach the thalamus in the brain, but the cerebral
  cortex judges the intensity and location of the
  pain.
• Other areas of the brain regulate the flow of
  pain impulses from the spinal cord and can
  trigger the release of chemicals called
  neuropeptides which inhibit the release of pain
  impulses in the spinal cord.
• Other chemicals called endorphins released in the
  brain provide natural pain control.

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10.4 Special Senses

• Special senses are those whose sensory receptors
  are within large, complex organs in the head.
• Smell Olfactory organs
• Taste Taste buds
• Hearing, equilibrium ears
• Sight -- eyes

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10.5 Sense of Smell

• The sense of smell is associated with complex
  sensory structures in the upper region of the
  nasal cavity. (Superior nasal concha)
• We usually smell food at the same time we taste
  it.
• Chemoreceptors

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Olfactory Organs

• Contain the olfactory receptors, are
  yellowish-brown masses that cover the upper parts
  of the nasal cavity, the superior nasal conchae,
  and a portion of the nasal septum.
• Olfactory receptors cells are bipolar neurons
  surrounded by columnar epilthelial cells.
• Olfactory impulses are interpreted in the
  temporal lobe of the brain.
• Anosmia is the partial or complete loss of smell.

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10.6 Sense of Taste

• Taste buds are the special organ of taste.
• Taste buds are located primarily on the surface
  of the tongue but, are also scattered in the roof
  of the mouth and the walls of the throat.

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• Taste buds include a group of modified cells.
• Each Taste bud has 50 to 150 receptor cells.
• Receptor cells are replaced every three days.
• Tiny projections called taste hairs protrude from
  the outer ends of the taste cells and extend from
  the taste pore. Taste hairs are the most
  sensitive structure of the taste bud.

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Taste Sensations

• Sweet
• Sour
• Salty
• Bitter
• Are the 4 primary taste sensations.
• Saliva enhances the taste of food by dissolving
  the chemicals that cause taste.

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10.7 Sense of Hearing

• External ear consists of 2 parts.
• The outer, funnel like structure called the
  auricle.
• The S-shapted tube called the external auditory
  meatus.

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Cross Section of the Ear
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Middle Ear

• includes an air-filled space, an eardrum, and
  three small bones.
• Tympanic cavity is an air-filled space in the
  temporal bone.
• Eardrum is semitransparent membrane covered by a
  thin layer of skin on its outer surface and by
  mucous membrane on the inside.

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• 3. Auditory ossicles (bones) are the malleus,
  incus, and stapes. The three smallest bones in
  the body. The main function of the bone is to
  transmit and intensify the vibrations transmitted
  to the inner ear.
• 4. Oval Window- opening in the wall of the
  tympanic cavity. The ossicle (Bone) that sits in
  the oval window is the stapes.

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• 5. Auditory Tube connects the middle ear to the
  throat.
• The main function of the auditory tube is to
  equalize the pressure on both sides of the
  eardrum.

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Inner Ear

• The inner ear is a complex system of
  communicating chambers and tubes called the
  labyrinth.
• The labyrinth includes 3 Semicircular canals
  which provide a sense of equilibrium and a
  cochlea which functions in hearing.
• The apex of the cochlea to a membrane-covered
  opening in the wall of the inner ear called the
  round window.
• Hearing receptor cells are located in the organ
  of Corti.

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10.8 Sense if Equilibrium

• Static equilibrium senses the position of the
  head, maintaining stability, and posture when the
  head and body are still.
• Dynamic Equilibrium senses the motion of the head
  and aid in balancing the head and body during
  sudden movement. Semicircular canals are the
  organs that detect motion.

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10.9 Sense of Sight

• Eyelid has four layers. Skin, muscle, connective
  tissue, and conjunctiva.
• The skin of the eyelid is the thinnest skin of
  the body.
• The eyelids are moved by the orbicularis oculi
  muscle.
• Conjunctiva is a mucous membrane that lines the
  inner sufarces of the eyelids and folds back to
  cover the eyeball, except for the cornea.

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Figure 10.16 Table 10.2

• Lacrimal gland which secretes tears.
• 6 extrinsic muscles move the eye in a various
  directions.
• Superior Rectus move the eye upward.
• Inferior Rectus move the eye downward.
• Lateral Rectus move the eye outward.
• Medial Rectus move the eye inward.

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Structure of the Eye

• The eye is hollow.
• The eye has 3 distinct layers.
• Outer Tunic
• Middle Tunic
• Inner Tunic

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Outer Tunic

• Cornea which is the window of the eye and helps
  focus entering light rays.
• The transparency of the cornea is due to the
  small number of cells and the lack of blood
  vessels.
• Sclera white portion of the eye.
• Optic nerve

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Middle Tunic

• Choroid coat contains pigment-producing cells
  that help keep the inside of the eye dark.
• Ciliary body which is the thickest part of the
  middle tunic.
• Lens is clear, membrane like structure.
• Accommodation allows the shape of the lens to
  change as the eye focuses on a close object.

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Middle Tunic Continued..

• Iris colored portion of the eye.
• Aqueous humor watery fluid that helps the eye
  maintain shape.
• Pupil circular opening in the center of the iris.

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Inner Tunic

• Retina which contains the visual receptor cells
  (photoreceptors). It is a sheet of tissue in the
  back of the eye.
• Macula lutea is a yellowish spot on the retina.
• Fovea centrailis is a depression in the center of
  the macula lutea. This is the region that
  produces the sharpest vision.

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Inner Tunic Continued..

• Optic disc is on the retina it lacks receptor
  cells, it is commonly known as the blind spot.
• Vitreous humor supports the internal parts of the
  eye and helps maintain its shape.

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Visual Receptors

• Rods long, thin projections found in the retina.
  Colorless vision, in dim light.
• Cones short, blunt projects found in the retina.
  Detect color, provide sharp images.
• A Human eye has 125 million rods and 7 million
  cones.
• A Human eye has 150,000 cones in the fovea
  centralis and bird eye has 1 million cones.

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• Skin Cross Section. March 27, 2007.
  http//www.owlnet.rice.edu/psyc351/Images/Wolfe-F
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• Human Ear. March 27, 2007. http//images.google.
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• Eye. March 27, 2007. http//images.google.com/im
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