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

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

Rubber Hand Illusion: http://www.youtube.com/watch?v=sxwn1w7MJvk&feature=player_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 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

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: 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 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: http://abcnews.go.com/GMA/

OnCall/story?id=1386322