Chapter 17: The Special Senses

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Chapter 17 The Special Senses
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General Senses

• temperature
• pain
• touch
• pressure
• vibration
• proprioception

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• General Sense Receptors
• Distributed throughout body
• Info is sent to primary sensory cortex
• Free Nerve Endings simplest receptors
• Branching tips of dendrites
• Not protected by accessory structures

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Special Senses

• Olfaction
• Gustation
• Vision
• Equilibrium
• Hearing

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• Special Sensory Receptors
• located in sense organs such as the eye or ear
• Are protected by surrounding tissues
• Composed of specialized sensory receptor cells
  that synapse with sensory neurons
• Info is distributed to specific areas of cerebral
  cortex

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• Receptor Specificity
• Each receptor has a characteristic sensitivity
• Receptive Field
• Area monitored by a single receptor cell
• larger the receptive field, the more difficult it
  is to localize a stimulus

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Arriving Stimulus

• Take many forms
• physical force (such as pressure)
• dissolved chemical
• sound
• light

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• Labeled lines
• Links between receptor and CNS
• All action potentials received along a line are
  perceived as the same type of stimulus

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• Tonic Receptors
• always active
• Monitor background stimuli
• Phasic Receptors
• normally inactive
• Become active for a short time whenever a change
  occurs
• Provide information about the intensity and rate
  of change of a stimulus

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• Adaptation
• Reduction in sensitivity of a constant stimulus
• Your nervous system quickly adapts to stimuli
  that are painless and constant
• Peripheral vs. central

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3 Types of General Receptors

• Simplest classification scheme
• Exteroceptors
• Proprioceptors
• Interoceptors

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Detailed Classification System

• Divides the receptors by the nature of the
  stimulus that excites them
• nociceptors
• thermoreceptors
• mechanoreceptors
• chemoreceptors

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Nociceptors

• Common in
• superficial portions of the skin around the
  walls of blood vessels joint capsules and within
  the periostea of bones
• Tonic with no significant adaptation
• Free nerve endings with large receptive fields
• Sensitive to
• extremes of temperature
• mechanical damage
• dissolved chemicals

Figure 152
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Thermoreceptors

• Phasic do display adaptation
• free nerve endings located in
• the dermis skeletal muscles the liver and the
  hypothalamus
• Conducted along the same pathways that carry pain
  sensations

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Mechanoreceptors

• Sensitive to stimuli that distort their cell
  membranes
• Tonic with no significant adaptation
• Contain mechanically regulated ion channels
• 3 Classes of Mechanoreceptors
• Tactile receptors
• Baroreceptors
• Proprioceptors

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• Tactile receptors
• sensitive to touch, pressure, and vibration
• Free nerve endings
• Merkel Discs Meissners corpuscles Pacinian
  corpuscles and Ruffini corpuscles

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• Baroreceptors
• detect pressure changes in the walls of blood
  vessels and in portions of the digestive,
  reproductive, and urinary tracts
• Free nerve endings
• Proprioceptors
• monitor the positions of joints and muscles
• most structurally and functionally complex of
  general sensory receptors

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Chemoreceptive Neurons

• Located in the
• respiratory center in medulla carotid bodies
  aortic bodies
• Tonic
• Communicate with brain stem not primary sensory
  cortex
• Receptors monitor Ph, carbon dioxide, and oxygen
  levels in arterial blood and CSF

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What are the sensory organs of smell?
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Olfactory Organs

• Located in nasal cavity on either side of nasal
  septum

Figure 171a
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• Made up of 2 layers
• olfactory epithelium
• olfactory receptors
• supporting cells
• basal (stem) cells
• lamina propria (basement membrane)
• areolar tissue
• blood vessels
• nerves
• olfactory glands

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• Olfactory Receptors
• Highly modified neurons with cilia
• 10 20 million which can distinguish thousands
  of chemical stimuli
• Olfactory Reception
• Involves detecting dissolved chemicals as they
  interact with odorant-binding proteins of
  receptors

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What are the olfactory pathways to the brain?
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• Axons leaving olfactory epithelium
• Form bundles that synapse on olfactory bulbs of
  cerebrum
• Travel along olfactory tract to synapse in
  olfactory cortex, hypothalamus, and portions of
  limbic system
• Only type of sensation that reaches the cerebral
  cortex without processing in the thalamus

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What are the sensory organs of taste?
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Taste (Gustatory) Receptors

• Clustered in taste buds associated with
  epithelial projections (papillae) on tongue
  surface
• Filiform papillae
• provide friction
• do not contain taste buds
• Fungiform papillae
• contain 5 taste buds each
• Circumvallate papillae
• contain 100 taste buds each

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• Taste buds contain
• basal (stem) cells
• gustatory cells
• extend taste hairs (microvilli) through taste
  pore
• Dissolved chemicals bind to taste receptors and
  cause formation of action potential

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Taste Buds
Figure 172
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What are the gustatory pathways to the brain?
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• Taste buds are monitored by cranial nerves that
  synapse within solitary nucleus of medulla
  oblongata, then on to thalamus and primary
  sensory cortex

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Primary Taste Sensations

• Sweet
• Salty
• Sour
• Bitter

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Additional Taste Sensations

• Umami
• Characteristic of beef/chicken broths and
  parmesan cheese
• Water
• Detected by water receptors