Transduction

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Transduction

• Stimulus is changed into electrical signal
• Different types of stimuli
• mechanical deformation
• chemical
• change in temperature
• Warmth, cold, nociceptors
• electromagnetic
• Rods and cones in the retina

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Sensory systems

• All sensory systems mediate 4 attributes of a
  stimulus no matter what type of sensation
• modality
• location
• intensity
• timing

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Receptor Potential

• Membrane potential of the receptor
• A change in the receptor potential is associated
  with opening of ion (Na) channels
• Above threshold as the receptor potential becomes
  less negative the frequency of AP into the CNS
  increases

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Labeled Line Principle

• Different modalities of sensation depend on the
  termination point in the CNS
• type of sensation felt when a nerve fiber is
  stimulated (e.g. pain, touch, sight, sound) is
  determined by termination point in CNS
• labeled line principle refers to the specificity
  of nerve fibers transmitting only one modality of
  sensation

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Adaptation

• Slow-provide continuous information
  (tonic)-relatively non adapting-respond to
  sustained stimulus
• joint capsul
• muscle spindle
• Merkels discs
• punctate receptive fields
• Ruffini end organs (corpusles)
• activated by stretching the skin

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Adaptation

• Rapid (Fast) or phasic
• react strongly when a change is taking place
• respond to vibration
• hair receptors 30-40 Hz
• Pacinian corpuscles 250 Hz
• Meissners corpuscles- 30-40 Hz
• (Hz represents optimum stimulus rate)

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Mechanoreceptors

• Information transmitted to the brain from
  mechanoreceptors in fingers allows us to
• feel the shape texture of objects
• play musical instruments
• type on computer keyboards
• palpate and perform adjustments
• perform a multitude of tasks using our hands
• Tactile information is fragmented by receptors
  must be integrated by the brain

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Tactile information

• The ability to recognize objects placed in the
  hand on the basis of touch alone is one of the
  most important complex functions of the
  somatosensory system. (Gardner Kandel)
• Tactile information obtained from palpation is
  crucial in the practice of chiropractic.

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Stereognosis

• The ability to perceive form through touch
• tests the ability of dorsal column-medial
  lemniscal system to transmit sensations from the
  hand
• also tests ability of cognitive processes in the
  brain where integration occurs

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Receptors in skin

• Most objects that we handle are larger than the
  receptive field of any receptor in the hand
• These objects stimulate a large population of
  sensory nerve fibers
• each of which scans a small portion of the object
• Deconstruction occurs at the periphery
• By analyzing which fibers have been stimulated
  the brain reconstructs the pattern

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Tactile

• No single sensory axon or class of sensory axons
  signals all relevant information
• Spatial properties are processed by populations
  of receptors that form many parallel pathways
• CNS constructs a coherent image of an object from
  fragmented information conveyed in multiple
  pathways

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Mechanoreceptors

• Rapidly adapting cutaneous
• Meissners corpuscles in glabrous (non hairy)
  skin
• signals edges
• Hair follicle receptors in hairy skin
• Pacinian corpuscles in subcutaneous tissue
• Slowly adapting cutaneous
• Merkels discs have punctate receptive fields
• senses curvature of an objects surface
• Ruffini end organs activated by stretching the
  skin
• even at some distance away from receptor

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Somatic Sensory Cortex

• Receives projections from the thalamus
• Somatotopic organization (homoculus)
• Each central neuron has a receptive field
• size varies in different areas of skin
• lateral inhibition can aid two point
  discrimination

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Somatosensory Cortex

• Two major pathways
• Dorsal column-medial lemniscal system
• Most aspects of touch, proprioception
• Anterolateral system
• Sensations of crude touch, nociception,
  temperature, tickle, itch and sexual sensations

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Somatosensory Cortex (SSC)

• Inputs to SSC are organized into columns by
  submodality
• cortical neurons defined by receptive field
  modality
• some columns activated by rapidly adapting
  Messiners, others by slowly adapting Merkels,
  still others by Paccinian corp.
• most nerve cells are responsive to only one
  modality e.g. superficial tactile, deep
  pressure, temperature, nociception

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Somatosensory cortex

• Brodman area 3, 1, 2 (dominate input)
• 3a-from muscle stretch receptors (spindles)
• 3b-from cutaneous receptors
• 2-from deep pressure receptors
• 1-rapidly adapting cutaneous receptors
• These four areas are extensively interconnected
  (serial parallel processing)
• Each of the 4 regions contains a complete map of
  the body surface

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Somatosensory Cortex

• Detailed features of a stimulus are communicated
  to the brain
• in early stages of cortical processing the
  dynamic properties of central neurons and
  receptors are similar (eg rapidly adapting
  cutaneous receptors connected to rapidly adapting
  2nd and 3rd order neurons)
• in the later stages of cortical processing the
  central nerve cells have complex feature
  detecting properties and integrate various
  sensory inputs

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Somatosensory Cortex

• 3 different types of neurons in BM area 1,2 have
  complex feature detection capabilities
• Motion sensitive neurons
• respond well to movement in all directions but
  not selectively to movement in any one direction
• Direction-sensitive neurons
• respond much better to movement in one direction
  than in another
• Orientation-sensitive neurons
• respond best to movement along a specific axis

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Other Somatosensory Cortical Areas

• Posterior parietal cortex (BM 5 7)
• BM 5 integrates tactile information from
  mechanoreceptors in skin with proprioceptive
  inputs from underlying muscles joints
• BM 7 receives visual, tactile, proprioceptive
  inputs
• intergrates stereognostic and visual information
• Projects to motor areas of frontal lobe
• sensory initiation guidance of movement

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Secondary SSC (S-II)

• Secondary somatic sensory cortex (S-II)
• located in superior bank of the lateral fissure
• projections from S-1 are required for function of
  S-II
• projects to the insular cortex, which innervates
  regions of temporal lobe believed to be important
  in tactile memory

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Sensory innervation of Spinal joints

• Tremendous amount of innervation with cervical
  joints the most heavily innervated
• Four types of sensory receptors
• Type I, II, III, IV

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Type I mechanoreceptors

• Outer layers of joint capsul
• fire at a degree proportional to joint movement
  or traction
• low threshold
• dynamic-fire with movement
• slow adapting
• tonic effects on lower motor neuron pools

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Type II Mechanoreceptors

• Deeper layers of joint capsul
• low threshold
• rapidly adapting
• completely inactive in imobilized joints
• functions in joint movement monitering
• phasic effects on lower motor neuron pools

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Type III Mechanoreceptors

• Recently found in spinal joints
• very high threshold
• slow adaptation
• joint version of Golgi tendon organ

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Type IV receptors

• Nociceptors
• very high threshold
• completely inactive in physiologic normal joint
• activation with joint narrowing, increased capsul
  pressure, chemical irratation

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Pain Analgesia

• Noxious Insults to Body stimulate Nociceptors
• Nociceptors are activated by
• Mechanical Stimuli
• Thermal Stimuli
• Chemical Stimuli

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Sensations of Pain

• Pricking
• Burning
• Aching
• Stinging
• Soreness

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Pain vs. Nociception

• Nociception-reception of signals in CNS evoked by
  stimulation of specialized sensory receptors
  (nociceptors) that provide information about
  tissue damage
• Pain-perception of adversive or unpleasant
  sensation that originates from a specific region
  of the body

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Perception of Pain

• All perception involves an abstraction and
  elaboration of sensory inputs
• highly subjective nature of pain is one the
  factors that makes it difficult to define and
  treat clinically

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Pain

• Conspicuous sensory experience that warns of
  danger
• Chronic pain is a massive economic problem- in US
  more than 2 million people are incapacitated by
  pain at any give time
• Drives most chiropractic practices

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Nociceptors

• Least differentiated of all sensory receptors
• Can be sensitized by tissue damage
• hyperalgesia
• repeated heating
• axon reflex may cause spread of hyperalgesia in
  periphery
• sensitization of central nociceptor neurons as a
  result of sustained activation

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Sensitization of Nociceptors

• Potassium from damaged cells-activation
• Serotonin from platelets- activation
• Bradykinin from plasma kininogen-activate
• Histamine from mast cells-activation
• Prostaglandins leukotriens from arachidonic
  acid-damaged cells-sensitize
• Substance P from the 1o afferent-sensitize

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Nociceptive pathways

• Fast
• A delta fibers
• glutamate
• neospinothalamic
• mechanical, thermal
• good localization
• sharp, pricking
• terminate in VB complex of thalamus

• Slow
• C fibers
• substance P
• paleospinothalamic
• polymodal/chemical
• poor localization
• dull, burning, aching
• terminate RF
• tectal area of mesen.
• Periaqueductal gray

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Nociceptive pathways

• Spinothalamic-major
• neo- fast (A delta)
• paleo- slow (C fibers)
• Spinoreticular
• Spinomesencephalic
• Spinocervical (mostly tactile)
• Dorsal columns- (mostly tactile)

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Cardinal signs of inflammation

• Rubor-redness
• Calor-heat
• Tumor-swelling
• Dolar-pain

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Pain Control Mechanisms

• Peripheral
• Gating theory
• involves inhibitory interneruon in cord impacting
  nocicep. projection neurons
• inhibited by C fibers
• stimulated by A alpha beta fibers
• TENS

• Central
• Direct electrical to brain -gt analgesia
• Nociceptive control pathways descend to cord
• Endogenous opiods

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Endogenous opioids

• Periaquedutal gray
• enkephalin projections to Raphe
• Raphe N.
• serotonin projections to the cord
• Inhibitory interneurons in cord
• release enkephalin which can cause presynatic
  inhibition of incoming C fibers and A delta fibers

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Pain control

• Endogenous opioid peptides and receptors are
  located at key points in the pain modulatory
  system

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Surgery to Alleviate Pain

• Over the years surgical intervention to treat
  pain has been tried at every level of the nervous
  system from the primary afferent fiber to the
  cortex
• Procedures not very sucessful
• Pain can return with new sensations often unlike
  anything the patients have felt before
• spontaneous aching, shooting pain, numbness,
  cold, heaviness, burning, etc.

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Surgery to alleviate pain (cont)

• Central pain syndromes often cause more distress
  than the pain the operation was intended to
  relieve
• Many instances of chronic pain result from
  spontaneous lesions to central sites in
  nociceptive pathways
• cases of intractable pain resulting from vascular
  damage to CNS (Dejerine Roussy)

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Headache

• Referred pain to surface of head
• Intracranial origins
• meningitis
• inflammation of meninges
• migraine
• vasocontraction/vasodilatation
• irritation of meninges
• e.g. abuse of alcohol
• constipation

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Headache (cont.)

• Extracranial origins
• Muscle spasm
• connective tissue bridges between muscle dura
  in upper cervical spine
• Irritation of nasal passages and/or sinuses
• eye disorders
• cervical joint dysfunction
• spill over of signals from cervical joints (C2)
  to nucleus of CN V
• traction of dura
• mandibular branch of CN V has a recurrent or
  meningeal branch which innervates part of dura

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Relationship of Cervical Spine to HA

• CN V sensory innervation of most of head face.
  (Three divisions).
• CN V nucleus of termination extends all the way
  down to level of C2.
• Some of cervical joint afferents synapse directly
  in CN V nuclei.
• C2 afferents synapse both in dorsal horn (DRG)
  CN V nuclei.
• Overlap between CN V C2 can cause headache
  associated w/ cervical dysfunction

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Muscle Receptors

• Muscle contain 2 types of sensory receptors
• muscle spindles respond to stretch
• located within belly of muscle in parallel with
  extrafusal fibers (spindles are intrafusal
  fibers)
• innervated by 2 types of myelinated afferent
  fibers
• group Ia (large diameter)
• group II (small diameter)
• innervated by gamma motor neurons that regulate
  the sensitivity of the spindle
• golgi tendon organs respond to tension
• located at junction of muscle tendon
• innervated by group Ib afferent fibers

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Muscle Spindles

• Nuclear chain
• Nuclear bag
• dynamic
• static
• A typical mammalian muscle spindle contains one
  of each type of bag fiber a variable number of
  chain fibers (? 5)

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Muscle Spindles

• sensory endings
• primary-usually 1/spindle include all branches
  of Ia afferent axon
• innervate all three types
• much more sensitive to rate of change of length
  than secondary endings
• secondary-usually 1/spindle from group II
  afferent
• innervate only on chain and static bag
• information about static length of muscle

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Golgi tendon organ (GTO)

• Sensitive to changes in tension
• each tendon organ is innervated by single group
  Ib axon that branches intertwines among braided
  collagen fascicles.
• Stretching tendon organ straightens collagen
  bundles which compresses elongates nerve
  endings causing them to fire
• firing rate very sensitive to changes in tension
• greater response associated with contraction vs.
  stretch (collagen stiffer than muscle fiber)

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CNS control of spindle sensitivity

• Gamma motor innervation to the spindle causes
  contraction of the ends of the spindle
• This allows the spindle to shorten function
  while the muscle is contracting
• Spindle operate over wide range of muscle length
• This is due to simultaneously activating both
  alpha gamma motor neurons during muscle
  contraction. (alpha-gamma coactivation)
• In slow voluntary movements Ia afferents often
  increase rate of discharge as muscle is shortening

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CNS control of spindle sensitivity

• In movement the Ia afferents discharge rate is
  very sensitive to variartions in the rate of
  change of muscle length
• This information can be used by the nervous
  system to compensate for irregularities in the
  trajectory of a movement to detect fatigue of
  local groups of muscle fibers

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Summary

• Spindles in conjunction with GTOs provide the
  CNS with continuous information about the
  mechanical state of a muscle
• For virtually all higher order perceptual
  processes, the brain must correlate sensory input
  with motor output to accurately assess the bodies
  interaction with its environment