chapter 48

• Mechanoreceptive


  stimulated by mechanical displacement.

  
  

  Tactile: touch, pressure, vibration, tickle, itch

  
  

  Proprioceptive: static position, rate of change
• Thermoreceptive.

  AND

  Nociceptive


  Thermoreceptive.

  detect heat and cold.

  
  

  Nociceptive.

  detect pain and any factor that damages tissue.
• Pathways for the Transmission of Sensory Information

  Almost all sensory information enters spinal cord through dorsal roots of spinal nerves.

  
  

  Anterolateral system

  
  

  Dorsal column-medial lemniscal system
• Meissner corpuscles

  Location and Function

  Small Field Tactile Receptors

  
  

  Location: non-hairy skin close to surface (fingertips, lips, eyelids, nipples

  and external genitalia).

  
  

  Function: motion detection, grip control
• Meissner Corpuscles

  Stimuli, Adaptation, Receptive, nerve fibers


  Stimuli: skin motion, low frequency vibration

  
  

  Adaptation: rapid adaptation

  
  

  receptive field: 22 mm2

  
  

  type Aβ nerve fibers
• Merkel Discs

  Location and Function


  Small field

  
  

  Location: tip of epidermal ridges

  
  

  Function: form and texture perception
• Merkel Discs

  Stimuli, Adaptation, receptive field, nerve fibers


  Stimuli: edges, points, corners, curvature

  
  

  slow adaptation

  
  

  receptive field: 9 mm2

  
  

  type Aβ nerve fibers
• Pacinian corpuscle.

  Location and Function , stimuli

  Large field

  
  

  dermis and deeper tissues

  
  

  perception of distant events through transmitted vibrations;
• Pacinian Corpuscle

  Stimulus, Adaptation, Receptive field , fibers

  very rapid adaptation

  
  

  entire finger or hand

  
  

  type Aβ nerve fibers
• Ruffini corpuscle

  Location

  Function

  Stimuli

  dermis

  
  

  tangential force; hand shape; motion detection

  
  

  skin stretch
• Ruffini Corpuscle

  
  

  Adaptation

  Receptive Field

  Fibers

  slow adaptation

  
  

  receptive field: 60 mm2

  
  

  type Aβ nerve fibers
• Free nerve endings (Myelinated).


  surface of body and elsewhere

  
  

  pain, temperature

  
  

  slow adaptation

  
  

  type Aδ nerve fibers
• Free nerve endings (unmyelinated)


  surface of body and elsewhere

  
  

  pain, temperature, itch

  
  

  slow adaptation

  
  

  type C
• Dermatome


  area of skin supplied by sensory neurons that arise from a spinal nerve ganglion.
• Dorsal Column-medial lemniscal System structure and path

  large myelinated nerve fibers

  
  

  Three neurons to sensory cortex / decussates in medulla oblongata
• Dorsal column system

  Spatial and modalities

  High degree of spatial orientation maintained throughout the tract .

  
  

  a high degree of spatial fidelity

  
  

  Transmits touch, vibration, position, fine pressure.
• The Anterolateral System structure and path


  smaller myelinated and unmyelinated fibers

  slow transmission

  
  

  three neurons to sensory cortex / decussates in spinal cord
• Anterolateral System Spatial and modalities


  low degree of spatial orientation.

  
  

  broad spectrum of modalities.

  
  

  pain, thermal sensations, crude touch and pressure, tickle and itch, sexual sensations.
• clinical significance of dermatomes

  Localizing cord lesion

  
  

  Viruses such as varicella zoster hibernate in ganglia causing rash in associated dermatome

  
  

  Referred pain
• The Somatosensory Cortex


  Located in the postcentral gyrus.

  
  

  Highly organized distinct spatial orientation.

  
  

  Contralateral transmission
• Homunculus and somatosensory cortex


  Unequal representation of the body.

  
  

  lips have greatest area of representation followed by face and thumb.

  
  

  trunk and lower body have least area of representation.
• Somatosensory Association Function

  Function is to decipher complex sensory associations.

  
  

  Loss of these areas

  inability to recognize complex objects

  neglect of contralateral world and even refusal to acknowledge ownership of contralateral body.
• Somatosensory Area I


  primary somatosensory area

  
  

  Brodmann's areas: 1, 2, 3
• Somatosensory association area

  Location

  Brodmann's areas: 5,7

  
  

  in partriel
• Somatosensory Association Area Input

  Receives input from somatosensory cortex, ventrobasal nuclei of thalamus, visual and auditory cortex
• Layers I and II of Cortex


  receive diffuse input from lower brain centers
• Layer IV of the cortex

  Incoming signals enter layer IV and spread both up and down
• Layers II and III of the cortex


  neurons send axons to closely related portion of the cortex presumably for communicating between similar areas.
• Layers of V and VI of the cortex

  send axons to more distant parts of the nervous system, layer V to the brainstem and spinal cord, layer VI to the thalamus.
• Cortical Layer I and II morphology


  I molecular layer

  II external granular layer
• columns of the cortex


  Within the layers the neurons are arranged in columns.

  
  

  Each column serves a specific modality

  .

  Different columns interspersed among each other.

  
  

  interaction of the columns occurs at different cortical levels. the beginning of the analysis of the meaning of the signals.
• Cortical Layer III and IV morphology

  III layer of small pyramidal cells

  
  

  IV internal granular layer
• Cortical Layer V and VI morphology

  V large pyramidal cell layer

  
  

  VI layer of fusiform or polymorphic cells
• Lateral inhibition


  the capacity of an excited neuron to reduce activity of neighboring neurons;

  
  

  it improves degree of contrast
• Two point discrimination


  measures the minimum distance at which two stimuli are resolved as distinct;

  
  

  reflects how finely innervated an area of skin is.

  
  

  density of mechanoreceptors varies
• Lateral Inhibition at dorsal column system

  dorsal column nuclei, ventrobasal nuclei of thalamus, cortex
• somatic senses


  nervous mechanisms that collect sensory information
• special senses

  vision, hearing, smell, taste, and equilibrium.
• mechanoreceptive, thermoreceptic, nociceptive

  somatic senses can be divided into
• factors which damages tissue activates this sensation

  pain