SENSORY SYSTEMS

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• SENSORY SYSTEMS
• 1. General Principles of Sensory Transduction
• Adequate stimulus
• a specific stimulus that activates sensory
  receptor(s)
• e.g. Pacinian corpuscle
• Change in membrane conductance
• Generator potential
• Action potential

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• Stimulus intensity and frequency of action
  potentials
• frequency code of stimulus intensity
• stimulus intensity ? generator potential ?
  frequency of action potential
• This allows the receptor to have a extreme range
  of response, from very weak to very intense
• population code
• More sensory receptors are activated as stimulus
  gets greater

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• B. Modality of sensation
• Each type of sensation (pain, touch, sound, etc.)
  is called modality of sensation
• The quality of a stimulus is encoded by the
  pathway of transmission
• Different modality of sensation is transmitted
  by a specific nerve fiber and determined by where
  the nerve fiber terminates in the brain
• Labeled-line code principle
• The specificity of nerve fibers for transmitting
  only one modality of sensation

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• B. Mechanisms of adaptation
• can occur at the brain and the receptor
• sensory adaptation at receptor site

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• 1. Somatic Sensory System
• A. Types of somatic sensory receptors
• tactile receptors (mechanoreceptors)
• stimuli touch, pressure, vibration
• e.g. Pacinian corpuscle
• hair receptor
• Ruffini ending

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• thermal receptors
• stimuli heat and cold
• nociceptors (pain receptors)
• stimuli painful touch, cut, extreme temperatures
• proprioceptive receptors
• detect position and movement
• static proprioceptors
• tonic discharge
• dynamic proprioceptors/kinesthesia
• phasic discharge

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• joint capsules
• muscle spindle receptors
• general principles for somatic sensory receptors
  mediated transmission
• specificity
• cross-over of most pathways
• topographical organization
• B. The dorsal root ganglion
• a cluster of cell bodies
• different size of nerve fibers

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• large fibers D 13-22 ?m, glutamate as
  neurotransmitters
• myelinated, fast action potential conduction
  70-120m/sec
• small fibers D 1-5 ?m, substance P as
  neurotransmitters
• unmyelinated, slow conduction of action
  potential 2-15m/sec
• two major pathways dorsal column and
  anterolateral
• C. The spinal cord in somatic sensory function
• anatomy of the spinal cord

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• useful anatomic terms rostro-caudal axis
  dorsal/ventral medial/lateral
  anterior/posterior
• transverse section gray matter and white matter
• segments of the spinal cord
• 31 segments 8 cervical (C1-C8)
• 12 thoracic (T1-T12)
• 5 Lumbar (L1-L5)
• 5 sacral (S1-S5)
• 1 sacro-coccygeal
• dermatomes
• lesions of the spinal cord

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• D. The thalamus in somatic sensory function
• One of the brain areas that receives most types
  of sensory information
• VPL receives somatic sensory information
• E. The somatic sensory cortex
• somatotopic organization
• F. Receptive field of neurons
• The area monitored by a single receptor cell is
  called the receptive field for that cell
• The receptive fields differ in different parts of
  body.
• small in more sensitive areas like fingertips,
  tongue (1 mm in diameter)
• large in less sensitive areas such as in the back
  (7 cm)

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• Lateral inhibition
• Two-points threshold discrimination test
• Pain sensation
• Pathway anterolateral pathway
• Neurotransmitter substance P
• Modification of pain sensation
• Brain stem
• Other sensory fibers (touch, pressure)
• Higher brain center
• The gating theory
• Referred pain

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Referred pain. Nociceptors from several
locations converging on a single ascending tract
in the spinal cord may cause referred pain.