A-alpha fibers:

• Mechanical stimulation
• Wide-diameter, myelinated sensory nerve fibers that transmit signals from proprioceptive receptors
  • in muscles and tendons.

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 signal

Tactile receptors:

• Embedded in outer layer (epidermis) and underlying layer (dermis) of skin

3 Touch Criterias

1. Type of stimulation to which the receptor responds
2. Size of the receptive field
3. Rate of adaptation (fast versus slow)

Meissner corpuscles

fast adaptation, small receptive field
(FA I)

Merkel cell neurite complexes

slow adaptation, small
receptive field (SA I)

Pacinian corpuscles

• fast adaptation, large receptive field (FA II)

Ruffini endings

slow adaptation, large receptive field (SA II)

Kinesthetic receptors:

• Mechanoreceptors in muscles,
  tendons, and joints.
• Play an important role in sense of where limbs are, what kinds of movements are made

Muscle spindle:

• located in a muscle that senses its tension.
• Receptors in tendons
  • signal tension in muscles attached to tendons
• Receptors in joints
  • react when joint is bent to an extreme angle

Thermoreceptors

• signal information about changes
  in skin temperature
• Two distinct populations of thermoreceptors:
  • warmth fibers, cold fibers
• Body is constantly regulating internal temperature
  • 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

nociceptors

A-delta fibers

Intermediate-sized, myelinated sensory
nerve fibers that transmit pain and temperature signals

nociceptors

C fibers:

Narrow-diameter, unmyelinated sensory nerve fibers that transmit pain and temperature signals.

nociceptors

• quick sharp pain (A-
  delta fibers)
  • followed by throbbing sensation (C fibers)
  • Difference in speeds is due to myelination.

Detection and Pain

Nociceptive neurons detect the thermal and chemical
stimulation that produces pain

ThermoTRP channel

Thermally sensitive transient receptor potential ion channel found in sensory neurons

Classic categories of discriminative touch:

1. tactile,
2. thermal
3. pain itch

Newly uncovered fifth component of touch

• pleasant touch
  • Mediated by unmyelinated peripheral C fibers (“C tactile afferents” )(CT afferents)
  • CT afferents not related to pain or itch

Pleasant touch

• Respond best to slowly moving, lightly applied forces (e.g., petting)
• Processed in orbitofrontal cortex rather than S1 or S2

Information and Spinal Cord

Axons of various tactile receptors combine into single
nerve trunks

Labeled lines:

• Each fiber type from the skin codes a particular touch sensation.
• becomes interconnected in spinal cord,
  • allows complex patterns to emerge.

Inputs to spinal cord organized somatotopically

• adjacent areas of the skin project to adjacent areas in the spinal cord
• Primary somatosensory cortex called S1; secondary somatosensory cortex called S2
• Analogous to retinotopic mapping found in vision

Two major pathways from spinal cord to brain

• Spinothalamic pathway
• Dorsal column-medial lemniscal (DCML) pathway:

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.

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.

Body image

The impression of our bodies in space

Body image and weight

• distorted towards top- heaviness.
• Expanded shoulders and upper arms
  • People rate upper half of body to be larger than lower half.
• Consistent with somatotopic mapping in cortex and
  sensory homunculus

Pain

• 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.

Benefit of pain perception

Sensing dangerous objects

Pain is generally a subjective experience with two
components:

• sensation of the painful stimulus and the emotional response to it
• Areas S1 and S2 are responsible for sensory aspects of pain

Cognitive 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.

Gate control theory

• transmits pain that incorporates modulating signals from the brain
• Bottom-up pain signals from nociceptors can be blocked via a circuit located in the spinal cord.
• Neurons in the dorsal horn actively inhibit pain transmission

Analgesia Example

• EX: Soldier experienced analgesic effect
  • because of endogenous opiates—chemicals released in body to block release or uptake of neurotransmitters transmitting pain sensation to brain

Analgesia

• Endogenous opiates may be responsible for certain
  placebo effects.
• Externally produced substances have similar effect:
  • morphine, heroin, codeine

Pain sensitization

Hyperalgesia

• A heightened response to a normally painful stimulus.
• Nociceptors signal ongoing damage to body’s tissue— “nociceptive” pain.
• result of damage to or dysfunction of nervous
  system—neuropathic.
• No single pain medication will alleviate all types of pain

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
  • they attribute activity in these areas to stimulation from missing limb.
    • Body image is inaccurate.

Amputees feeling the amputated hand WHEN
face or remaining limbs are stimulated

• Face area is located next to hand and arm area on
  sensory homunculus.
• Neural crosstalk leads to sensory crosstalk.

Neural plasticity:

the ability of neural circuits to undergo changes in function or organization as a result of previous activity

Max von Frey (1852–1932)

• developed an elegant way to measure tactile sensitivity.
  • Used horse and human hairs
• Modern researchers use nylon monofilaments of
  varying diameters

Diameters of Monofilaments/Hair

The smaller the diameter = the less force applied to the skin before it buckles

Sensitivity to mechanical pressure varies over the body

• Face is most sensitive.
• Trunk and upper extremities (arms and fingers) next most sensitive.
• Lower extremities (thigh, calf, and foot) less sensitive

Body and Acuity

• spatial acuity varies across
  the body.
• Extremities (fingertips, face, and toes) show the highest acuity