Ch. 15 pt. 1&2

• Ian Waterman


  • Worked as an apprentice in a butcher shop
  • Had a small cut in his finger that likely developed into an infection
  • Gradually lost control over his limbs and ended up lying in bed without conscious control over any part of his body from his neck down
  • His muscles still worked and his brain was receiving signals from his body conveying sensations such as pain and differences in temperature
  • But the brain seemed to have lost the notion of where the different parts that it was supposed to move were located
• Nerve Fibers


  • Can be either a sensory fiber or motor fiber
  • Motor fibers send signals to our muscle fibers telling them to contract
  • Sensory fibers start either in the skin or in the muscle and come in different sizes
  • The largest ones convey information concerning touch, muscle sensitivity or sense of movement
  • The smallest ones convey information concerning muscle fatigue, temperature and certain forms of pain
• Parts of the Somatosensory System

  • Cutaneous Senses: perception of touch and pain from stimulation of the skin
  • Proprioception: ability to sense position of the body and limbs
  • Kinesthesis: ability to sense movement of body and limbs
• Skin: protects the organism by keeping damaging agents from penetrating the body

  • Heaviest organ in the body
  • Epidermis is the outer layer of the skin, which is made up of dead skin cells
  • Dermis is below the epidermis and contains mechanoreceptors that respond to stimuli such as pressure, stretching, and vibration
• Mechanoreceptors located close to surface of the skin

  • Merkel Receptor: fires continuously while stimulus is present, responsible for sensing fine details
  • Meissner Corpuscle: fires only when a stimulus is first applied and when it is removed, responsible for controlling hand-grip
• Mechanoreceptors located deeper in the skin

  • Ruffini Cylinder: fires continuously to stimulation, associated with perceiving stretching of the skin
  • Pacinian Corpuscle: fires only when a stimulus is first applied and when it is removed, associated with sensing rapid vibrations and fine texture
• Medial Lemniscal Pathway: consists of large fibers that carry proprioceptive and touch information
• Spinothalamic Pathway: consists of smaller fibers that carry temperature and pain information
• Somatosensory Cortex


  • Signals travel from the thalamus to the somatosensory receiving area (S1) and the secondary receiving area (S2) in the parietal lobe
  • Body map (homunculus) on the cortex in S1 and S2 shows more cortical space allocated to parts of the body that are responsible for detail
  • Plasticity in neural functioning leads to multiple homunculi and changes in how cortical cells are allocated to body parts
• Two-Point Threshold: minimum separation needed between two points to perceive them as two units
• Grating Acuity: placing a grooved stimulus on the skin and asking the participant to indicate the orientation of the grating
• Raised Pattern Identification: using such patterns to determine the smallest size that can be identified
• There is a high density of Merkel receptors in the fingertips
• Merkel receptors are densely packed on the fingertips, similar to cones in the fovea
• Body areas with high acuity have larger areas of cortical tissue devoted to them, paralleling the "magnification factor" seen in the visual cortex for the cones in the fovea
• Areas with higher acuity also have smaller receptive fields on the skin
• Pacinian Corpuscles (PCs)


  Nerve fibers associated with PCs respond best to high rates of vibration
• The structure of the PC is responsible for the response to vibration: fibers without the PC only respond to continuous pressure
• Spatial Cues: determined by the size, shape, and distribution of surface elements, such as bumps and grooves

  • feeling textures without moving fingers
• Temporal Cues: determined by the rate of vibration as skin is moved across finely textured surfaces

  • when texture is fine, must move fingers across the surface
• Duplex theory of texture perception: two receptors may be responsible for this process, using spatial and temporal cues

  1. Slow Adapting - coarse texture (just touching)
  2. Rapid Adapting - fine texture
• Past research showed support for the role of spatial cues, while recent research by Hollins and Reisner shows support for the role of temporal cues
• Active Touch: touch in which a person actively explores an object, usually with fingers and hands
• Passive Touch: occurs when touch stimuli are applied to the skin, as when two points are pushed onto the skin to determine the two point threshold

  • touch is being done TO you (ex. "something is touching my arm")
• Haptic Perception: perception in which three-dimensional objects are explored with the fingers and hand

  • involving the sensory system, motor system, and cognitive system
• Psychophysical research has shown that people can accurately identify most common objects within 1 or 2 seconds using active touch, using a number of distinctive movements called exploratory procedures (EPs)
• Cortical Neurons: specialized, with some having center-surround receptive fields similar to the visual system

  • others responding to more specialized stimulation of the skin
  • Some neurons in the monkey's somatosensory cortex respond when the monkey grasps a specific object
• Cortical responding is affected by attention, as shown by studies recording neuron responses in S1 and S2 to raised letters scanned across a monkey's finger under different attentional conditions
• Pain: a multimodal phenomenon containing a sensory component and an affective or emotional component
• Types of Pain


  • Inflammatory Pain: caused by damage to tissue or inflammation
  • Neuropathic Pain: caused by lesions or damage to the nervous system
  • Nociceptive Pain: caused by activation of receptors in the skin called nociceptors, which respond to tissue damage or potential damage
• Gate Control Model of Pain

  Pain signals enter the spinal cord and are then transmitted to the brain, with additional pathways that can open or close a "gate" in the spinal cord to modulate the strength of the signal
• Nociceptors activate a circuit of excitatory synapses, sending excitatory signals to transmission cells, while mechanoreceptors carry information about non-painful tactile stimulation
• Central control pathways carry signals down from the cortex, related to cognitive functions like expectation, attention, and distraction
• Placebo Effect: decrease in pain from a substance that has no pharmacological effect, due to positive expectation
• Attention is one way to decrease pain by distracting a person from the source of the pain
• Emotions can also affect pain, as shown by studies on the effects of viewing positive/negative pictures or listening to pleasant/unpleasant music while experiencing a pain stimulus
• • Vision --> lateral geniculate nucleus
  • Hearing --> medial geniculate nucleus
  • Touch --> ventrolateral geniculate nucleus
• Primary Somatosensory Cortex (S1): concerned with processing information coming from bodily senses

  • sense of touch is directly processed
• Secondary Somatosensory Cortex (S2): stores the information from sense of touch from S1
• How sensitive different parts of the body are relates to how much acuity each has