Chapter 12 Somatic and Special Senses

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Chapter 12Somatic and Special Senses

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

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Receptor Types

• Chemoreceptors
• respond to changes in chemical concentrations

• Pain receptors
• respond to tissue damage

• Thermoreceptors
• respond to changes in temperature

• Mechanoreceptors
• respond to mechanical forces

• Photoreceptors
• respond to light

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Sensory Impulses

• stimulation of receptor causes local change in
  its membrane
• 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 actin potential
• peripheral nerves transmit impulses to CNS

• Sensation
• feeling that occurs when brain interprets
  sensory impulse

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Sensory Adaptation

• adjustment of sensory receptors from continuous
  stimulation
• stronger stimulus required to activate receptors
• smell receptors undergo sensory adaptation

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Somatic Senses

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

• three groups
• exteroceptive senses senses associated with
  body surface touch, pressure, temperature, pain
• proprioceptive senses senses associated with
  changes in muscles and tendons
• visceroceptive senses senses associated with
  changes in viscera

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Touch and Pressure Senses

• Free nerve endings
• common in epithelial tissues
• detect touch and pressure

• Meissners corpuscles
• abundant in hairless portions of skin
• detect light touch

• Pacinian corpuscles
• common in deeper subcutaneous tissues, tendons,
  and ligaments
• detect heavy pressure

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Touch and Pressure Senses
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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
• do not adapt

• Visceral Pain
• may exhibit referred pain
• not well localized

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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 Fibers

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

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

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Regulation of Pain Impulses

• Thalamus
• allows person to be aware of pain

• Pain Inhibiting Substances
• enkephalins
• serotonin
• endorphins

• Cerebral Cortex
• judges intensity of pain
• locates source or pain
• produces motor response to pain
• produces emotions to pain

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Stretch Receptors

• proprioceptors
• send information to CNS concerning lengths and
  tensions of muscles
• 2 main kinds of stretch receptors
• muscle spindles in skeletal muscles
• Golgi tendon organs in tendons

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Stretch Receptors
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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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