Kinesthesis
The perception of body position and movement of our limbs in space
Proprioception
Perception of balance mediated by kinesthetic and vestibular receptors
Somatosensation
A collective term for sensory signals from the skin and organs. What we commonly refer to as “touch
Touch receptors
Embedded on outer layer (epidermis) and underlying layer (dermis) of skin
* Multiple types of touch receptors
Each touch receptor can be categorized by three criteria:
- Type of stimulation to which the receptor responds
- Size of the receptive field
- Rate of adaptation (fast versus slow)
Tactile receptors
Called “mechanoreceptors” because they respond to mechanical stimulation: Pressure, vibration, or movement
Meissner corpuscles
Merkel cell disks
Pacinian corpuscles
Ruffini endings ‘
Kinesthetic receptors
Play an important role in sense of where limbs are, what kinds of movements are made
Muscle spindle
A sensory receptor located in a muscle that senses its tension
Importance of kinesthetic receptors
dependent on vision to tell limb positions
Thermoreceptors
- Sensory receptors that signal information about changes in skin temperature
- Two distinct populations of thermoreceptors: warmth fibers, cold fibers
- Body is constantly regulating internal temperature
- Thermoreceptors respond when you make contact with an object warmer or colder than your skin
Nociceptors:
- Sensory receptors that transmit information about noxious stimulation that causes damage or potential damage to skin
- Two groups of nociceptors:
A-delta fibers: Intermediate-sized, myelinated sensory nerve fibers that transmit pain and temperature signals
C fibers: Narrow-diameter, unmyelinated sensory nerve fibers that transmit pain and temperature signals
Benefit of pain perception
- Sensing dangerous objects
- Case of “Miss C” :
Born with insensitivity to pain
Did not sneeze, cough, gag, or blink reflexively
Suffered injuries such as burning herself on radiator and biting tongue while chewing food
Died at age 29 from infections that could have been prevented if she sensed pain
Touch sensations travel as far as 2 meters to get from skin and muscles of feet to brain!
- Information must pass through spinal cord
- Axons of various tactile receptors combine into single nerve trunks
- Two major pathways from spinal cord to brain:
Spinothalamic pathway: Carries most of the information about skin temperature and pain (slower)
Dorsal column–medial lemniscal (DCML) pathway: Carries signals from skin, muscles, tendons, and joints
Touch sensations are represented somatotopically in the brain:
- Primary somatosensory cortex called S1; secondary somatosensory cortex called S2
- Analogous to retinotopy found in vision
- Adjacent areas on skin connect to adjacent areas in brain § Homunculus: Maplike representation of regions of the body in the brain
- Brain contains several sensory maps of body in different areas of S1 and also in S2
Phantom limb:
Sensation perceived from a physically amputated limb of the body.
Parts of brain listening to missing limbs not fully aware of altered connections, so they attribute activity in these areas to stimulation from missing limb
Pain
- Pain sensations triggered by nociceptors
- Responses to noxious stimuli can be moderated by anticipation, religious belief, prior experience, watching others respond, and excitement
Example: Wounded soldier in battle who does not feel pain until after battle
Analgesia:
Decreasing pain sensation during conscious experience
- Soldier in above example: Experienced effect because of endogenous opiates - chemicals released in body to block release or uptake of neurotransmitters transmitting pain sensation to brain
- Externally produces substances have similar effect: Morphine, heroin, codeine
Gate control theory of pain
- Description of the system that transmits pain that incorporates modulating signals from the brain.
- Feedback circuit located in substantia gelatinosa of dorsal horn of spinal cord
- Gate neurons that block pain transmission can be activated by extreme pressure, cold, or other noxious stimulation applied to another site distant from the source of pain.
Pain sensitization:
- Nociceptors provide signal when there is impending or ongoing damage to body’s tissue: “Nociceptive” pain
- Once damage has occurred, site can become more sensitive: Hyperalgesia
- Pain as a result of damage to or dysfunction of nervous system: Neuropathic
- No single pain medication will alleviate all types of pain
Cognitive aspects of pain
- Pain is generally a subjective experience with two components: Sensation of painful stimulus and emotion that accompanies it
- Areas S1 and S2 are responsible for sensory aspects of pain
- Recently, researchers have identified areas of brain that correspond to more cognitive aspects of painful experiences:
Anterior cingulate: A region of the brain associated with the perceived unpleasantness of pain sensations
Prefrontal cortex: A region of the brain concerned with cognition and executive control. May contribute to pain sensitization
Pleasant touch
Mediated by unmyelinated peripheral C fibers known as “C tactile afferents” (CT afferents)
CT afferents not related to pain or itch
Respond best to slowly moving, lightly applied forces (e.g., petting, tickle)
Processed in orbitofrontal cortex rather than S1 or S2
Max von Frey (1852–1932):
measure tactile sensitivity
* Used horse and human hairs
* Modern researchers use nylon monofilaments of varying diameters
Hairs or monofilaments of varying diameters are pressed against the skin to see if the pressure can be sensed
* Sensitivity to mechanical pressure varies over the body
Face is most sensitive
Trunk and upper extremities (arms and fingers) most sensitive after face
Lower extremities (thigh, calf, and foot) less sensitive
What is the smallest raised element that can be felt on an otherwise smooth surface?
- People can detect a bump only 1 micrometer high at 75% accuracy!
- Dot triggers FA I receptors, which also help detect slippage of objects while being grasped
Surface with many dots a fraction of a micrometer high can be detected when moved across the skin via FA II receptors deep in skin
How finely can we resolve spatial details?
- Two-point threshold: The minimum distance at which two stimuli are just perceptible as separate
- Like sensitivity to pressure, spatial acuity varies across the body
Extremities (fingertips, face, and toes) show the highest acuity
