212 Cutenous Senses, Chemical Sense

• Ian experienced severe difficulties due to nerve destruction in muscles, tendons, and joints, leading to the loss of sensing body position
• Somatosensory system


  Includes cutaneous senses (touch and pain), proprioception (body position), and kinesthesis (body movement)

  
• Cutaneous senses are crucial for daily activities, injury protection, and even sexual arousal
• Social touch, a subset of cutaneous perception, can have positive effects
• Skin sensations are deemed as important for daily functioning and survival as sight and hearing
• Skin


  The heaviest organ in the human body, serves multiple functions including warning of potential dangers, preventing body fluids from escaping, 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 travel to the brain

  1. Signals from the entire body are sent to the spinal cord via bundles of fibers called dorsal roots

  2. Signals then travel to the brain through two main pathways: the medial lemniscal pathway and the spinothalamic pathway

  3. Medial lemniscal pathway carries signals for limb position and touch via large fibers

  4. 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
• Fibers from both pathways cross over to the opposite side of the body during their journey to the brain and synapse in the thalamus
• Primary somatosensory cortex (S1)

  One of the areas that receive signals from the thalamus
• Secondary somatosensory cortex (S2)


  Another area that 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
• Hughlings Jackson noted orderly seizures, hinting at brain maps; later, Penfield mapped them via surgery
• 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
• Although great strides have been made in computer vision, computers still can't perceive as well as humans
• Problem of computers


  • Lacking the extensive knowledge base that humans naturally accumulate from birth
  • When confronted with unfamiliar objects, the computer may only identify them based on superficial similarities
• Early computer scientists believed they could quickly achieve human-like perception in computers within a decade due to the seeming simplicity of human perception
• Perception


  The experiences that result from stimulation of the senses
• Sensation


  Commonly associated with basic processes occurring at the onset of sensory systems, like when light enters the eye, sound waves reach the ear, or food touches the tongue. It involves elementary experiences
• Perception


  Linked to intricate processes that encompass higher-order mechanisms, including interpretation and memory, often occurring in the brain. It involves the higher brain functions involved in interpreting events and objects

  
• Perceptual Process


  Starts with a stimulus in the environment and ends with the conscious experiences of perceiving, recognizing, and acting with respect to the object
• Neural processing


  Involves not just understanding the workings of neurons but also their interactions and operations within different brain areas

  
• The perceptual process is considered simplified because its steps don't always unfold in a linear sequence
• Perception and recognition may occur simultaneously or even in reverse order, challenging the conventional one-follows-the-other notion
• Distal stimulus


  The actual object or event present in the external world that gives rise to the proximal stimulus
• Proximal Stimulus


  The raw sensory data that the perceptual system processes to create a meaningful perception of the external world

  
• Receptor processes


  Include transduction and the shaping of perception by the properties of the receptors
• Neural Processing


  Involves interactions between the electrical signals traveling in networks of neurons
• Behavioral response


  Includes perception, recognition and action which are generated