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

Touch  The skin senses are connected to the somatosensory cortex located in the brain’s parietal lobes.  Pathway: Sensory receptors -> the spinal column - > brainstem -> cross to opposite side of brain -> thalamus -> somatosensory cortex  The skin’s 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.

Touch  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.  (Example: Rubber Hand Illusion)  Top-Down vs. Bottom-Up Processing Top-Down vs. Bottom-Up Processing  Rubber Hand Illusion: ayer_embedded Rubber Hand Illusion: ayer_embedded

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. What happens when someone loses their kinesthetic sense? --Man Who Lost His BodyMan Who Lost His Body

The Vestibular System  The Vestibular system responds to gravity and keeps you informed of your body’s location in space.  It provides your sense of balance or equilibrium.

The Vestibular System  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.  Dizziness

Pain  Pain is your body’s 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.  What would happen if you can’t feel pain?  The Girl Who Can’t Feel Pain The Girl Who Can’t Feel Pain

Pain  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.

The Gate-Control Theory of 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 nerve 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 nerve 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

The Gate-Control Theory  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 Control Theory  Gate-Control Theory of Pain: Gate-Control Theory of Pain  Slow/Small Fibers: in spinal cord OPEN GATE FEEL PAIN  Fast/Large Fibers: can block pain messages from reaching brain  Animation

The Gate-Control Theory  The gate also receives input from 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.

Figure 4.53 Pathways for pain signals

Phantom Limb Pain  The workings of the gate can also be influenced to some extent by a variety of cognitive variables such as attention, suggestion, and imagination among others.  Article Article  Phantom Limb sensations Phantom Limb sensations

Natural Pain Relievers  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 spicy food, and when people believe they are receiving a painkiller.

Congenital Insensitivity to Pain (Hereditary and Sensory Autonomic Neuropathies)  Read about real people with this disorder:  /story?id= /story?id=