Somatic Senses

1
Somatic Senses

• The Skin Senses of Touch, Temperature, and Pain.
  
• Also Includes Kinesthesia and the Vestibular
  System.

2
Touch

• The skin senses are connected to the
  somatosensory cortex located in the brains
  parietal lobes. (Pathway Sensory receptors -gt
  the spinal column -gt brainstem -gt cross to
  opposite side of brain -gt thalamus -gt
  somatosensory cortex)
• The skins sensitivity to stimulation varies
  tremendously over the body, depending in part on
  the number of receptors in each area. For
  example, we are 10 times more accurate in sensing
  stimulation on our fingertips than stimulation on
  our backs.
• In general, our sensitivity is greatest where we
  need it most on our faces, tongues, and hands.
• Touch is not only a bottom-up property of your
  senses, but also a top-down product of your brain
  and your expectations.
• Self-produced tickle vs. unexpected tickle from
  another source
• The rubber-hand illusion (pg. 253-254 in your
  textbook)

3
Figure 4.52 Receptive field for touch. A
receptive field for touch is an area on the skin
surface that, when stimulated, affects the firing
of a cell that responds to pressure on the skin.
Shown here is a centersurround receptive field
for a cell in the thalamus of a monkey.
Fg. 4-52, p. 158
4
Temperature

• Some skin receptors are sensitive to warmth and
  cold. Many receptors that respond to temperature
  also respond to touch.
• There is no simple relationship between what we
  feel at a given spot and the type of specialized
  nerve ending found there. Only pressure has
  identifiable skin receptors. Other skin
  sensations are variations of the basic four
  (pressure, warmth, cold, and pain)
• For example
• Touching adjacent cold and pressure spots
  triggers a feeling of wetness, which you can
  experience by touching dry, cold metal.
• Stimulating nearby cold and warn spots produces
  the sensation of hot (try this yourself with a
  demonstration you can do at home! I put the
  description on your school fusion page. The
  document is called Warm Plus Cold Equals Hot)

5
Pain

• Pain is your bodys way of telling you something
  has gone wrong. It is an adaptive mechanism that
  makes you respond to conditions that threaten
  damage to your body.
• There is no one type of stimulus that triggers
  pain (as light triggers vision). Instead, there
  are a lot of different nociceptors sensory
  receptors that detect hurtful temperatures,
  pressure, or chemicals.

6
The Gate-Control Theory of Pain

• Melzack and Wall developed this theory, which
  explains why analgesic drugs (pain relievers like
  aspirin), competing stimuli (like acupuncture)
  and even the mere expectation of treatment
  effects (like placebos) can sometimes block pain.
• With this theory, pain depends on the relative
  amount of traffic in two different sensory
  pathways which carry information from the sense
  organs to the brain.
• Slow/Small fibers
• No myelin sheaths, so messages delivered more
  slowly. Very intense stimuli (like that caused
  by a tissue injury) send strong signals on these
  slow fibers.
• Slow/small fibers open the gate you feel pain
• Fast/Large fibers
• Deliver most sensory information to the brain.
  Covered by fatty myelin sheaths so delivery is
  faster.
• Fast/large fibers close the gate block pain
  signals

7
The Gate-Control Theory (cont.)

• Fast/Large fibers can block pain messages in the
  slow/small fibers. They can close a kind of
  spinal gate, preventing the slow fibers
  messages from reaching the brain.
• Consequently, the level of pain you experience
  from a wound results from the combination of
  information coming through these two pathways.
  When you hit your finger with a hammer, you
  automatically try to close the gate by
  vigorously shaking your hand to generate
  fast-fiber signals that block the pain.
• The gate also receives input from the inhibitory
  system located in the brainstem. When activated,
  this brainstem mechanism also has the effect of
  shutting the gate and blocking further
  transmission of pain impulses. This might be
  responsible for some of the incidents in which
  people suffer serious injury but apparently
  experience little or no pain.
• Massage, electric stimulation, acupuncture, etc.,
  stimulates gate-closing activity in the large
  neural fibers. The workings of the gate can also
  be influenced to some extent by a variety of
  cognitive variables such as attention,
  suggestion, and imagination.

8
Figure 4.53 Pathways for pain signals
9
Natural Analgesics

• Serotonin and endorphins are naturally occurring
  substances that block synapses in fibers carrying
  pain signals. The body releases endorphins in
  painful situations when experiencing labor pains
  during childbirth, when eating very hot, spicy
  food, and when people believe they are receiving
  a painkiller.

10
Congenital Insensitivity to Pain (Hereditary
Sensory and Autonomic Neuropathies)

• Read articles about real people with this
  disorder
• http//www.helproberto.com/
• http//abcnews.go.com/GMA/OnCall/story?id1386322
• House episode, Insensitivity http//www.amazon.c
  om/Insensitive/dp/B000WEAYSM
• Greys Anatomy episode, Sometimes a Fantasy
• http//www.cucirca.com/2007/01/18/greys-anatomy-s
  eason-3-episode-3-sometimes-a-fantasy/

House performs surgery on an awake patient.
11
Kinesthesia

• Kinesthesis involves knowing the position of the
  various parts of the body. Kinesthetic receptors
  lie in the joints, indicating how much they are
  bending, or in the muscles, registering tautness
  or extension.

The kinesthetic sense tells us where our body
parts are, so that we can coordinate actions like
making copies of our buttocks.
12
The Vestibular System

• The Vestibular system responds to gravity and
  keeps you informed of your bodys location in
  space. It provides your sense of balance or
  equilibrium. The semicircular canals make up the
  largest part of the vestibular system these are
  fluid filled canals that contain hair cells
  similar to those in the basilar membrane. When
  your head moves, the fluid moves, moving the hair
  cells, and initiating neural signals that travel
  to the brain.