finals

• Cutaneous senses


  Touch and pain
• Proprioception


  Body position
• Kinesthesis


  Body movement
• Cutaneous senses are crucial for daily activities, injury protection, and even sexual arousal
• Social touch


  A subset of cutaneous perception that can have positive effects
• Skin sensations are deemed as important for daily functioning and survival as sight and hearing
• The skin


  The heaviest organ in the human body, serving multiple functions like warning of dangers, preventing fluid loss, and protecting against bacteria, chemicals, and dirt
• Epidermis


  The outer layer of the skin, consisting of tough dead skin cells
• Dermis


  The layer beneath the epidermis, containing mechanoreceptors that respond to mechanical stimulation like pressure, stretching, and vibration
• Mechanoreceptors


  • Many tactile perceptions from skin stimulation stem from them
  • Located in the epidermis and dermis
• Merkel receptors


  Fire continuously as long as the stimulus is present, responsible for perceiving details, shape, and texture
• Meissner corpuscles


  Fire only upon stimulus application and removal, involved in controlling handgrip and detecting motion across the skin
• Ruffini cylinders

  Perceive stretching of the skin

  
• Pacinian corpuscles


  Detect rapid vibrations and fine texture
• Perception of texture often involves coordinated activity of different types of neurons working together
• Unlike other senses with specific locations, cutaneous receptors for touch are spread throughout the body
• Signals from the skin to the brain

  1. Travel to the spinal cord via dorsal roots

  2. Travel to the brain through the medial lemniscal pathway and the spinothalamic pathway

  
• Medial lemniscal pathway


  Carries signals for limb position and touch via large fibers
• Spinothalamic pathway


  Carries signals for temperature and pain through smaller fibers
• The case of Ian Waterman highlights the separate functions of these pathways: he lost touch and limb position sensation but retained pain and temperature perception
• Somatosensory cortex


  • Receives signals through the medial lemniscal and spinothalamic pathways
  • Shows the complexity of the cutaneous system

  
• Primary somatosensory cortex (S1)

  Receives signals from the thalamus

  
• Secondary somatosensory cortex (S2)

  Receives signals from the thalamus

  
• Signals also travel between S1 and S2, and to other areas like the insula and the anterior cingulate cortex (ACC), handling light touch and pain
• Somatosensory cortex


  • Organized into maps that correspond to locations on the body
  • Discovery of these maps began with Hughlings Jackson's observations of orderly seizures

  
• Homunculus


  The body map showing how adjacent skin areas link to adjacent brain areas, with some areas represented larger

  
• Cortical magnification factor


  Like the fovea's representation in vision, parts like fingers have a larger representation on the somatosensory cortex

  
• Recent research shows S1 divided into four areas, each with its map and functions
• Many brain areas communicate over various pathways, reflecting the diverse qualities sensed by the skin

  
• Braille


  A system of raised dots that allows blind individuals to read with their fingertips
• Braille character


  • Comprises one to six dots, representing letters, numbers, punctuation, and speech sounds
• Experienced Braille readers can read at about 100 words per minute, slower than visual reading but impressive given the tactile nature
• Tactile detail perception


  The ability to detect details on the skin, allowing Braille readers to identify dot patterns
• Tactile acuity


  The capacity to detect stimuli details on the skin
• Both receptor and cortical mechanisms play roles in tactile acuity, paralleling similarities between the cutaneous and visual systems
• Merkel receptor


  Fires in response to grooved stimuli, reflecting their pattern, indicating sensitivity to details
• Pacinian corpuscle

  Does not match grooved patterns, showing lower sensitivity to details
• High density of Merkel receptors in fingertips corresponds to their heightened sensitivity to details
• Better tactile acuity is associated with closer spacing between Merkel receptors, especially on the hands
• Tactile acuity is influenced not only by receptor spacing but also by cortical processes