Somatosensory systems include:
- cutaneous senses (skin)
- kinesthesia: sensing body postition, called proprioception; or body movement, called kinesthesia
- organic senses (internal organs)

Cutaneous senses respond to: pressure, vibration, heating, cooling and noxious (painful) stimuli
Kinesthesia is subserved by muscle receptors for sensing changes in muscle length, stretch receptors in tendons, and joint receptors that sense the magnitude & direction of limb movement

ANATOMY OF THE SKIN
Epidermis: outer layer of dead skin cells; can be hairy or glabrous (hairless; fingertips, etc.)
Dermis: inner layer; contains the nerve endings and receptors

Receptor Types
By Morphology (5 types):
Free nerve endings & Ruffini corpuscles - respond to low frequency vibration, found closer to skin surface
Pacinian corpuscles - touch high freq vibration
Meissner's corpuscles - low freq vibration
Merkel's disks -

By Response Properties (3 main types):
1. Mechanoreceptors: respond to indentation (pressure) on the skin
2 subtypes:
1) Rapidly Adapting - RAs respond as pressure indents the skin, fire more rapidly to faster indentation, and cease responding to a constant stimulus (RAs include the morphological Pacinian & Meissner corpuscles)
2) Slowly Adapting - SAs respond to stimulus indentation but unlike RAs continue responding to constant pressure (Merkel cells & Ruffini endings and SAs)

<<< FIGURE: Summary of Relationship Between Morphology & Response >>>

2. Thermoreceptors: respond to specific temperatures and changes in temperature
these are free nerve endings, two diff. populations at diff. depths in skin (warmth receptors deeper in the skin)
warm fibers (maximum response is at 113 degrees Farenheit) - deeper in skin
cold fibers (maximum response is at 86 degrees Farenheit) - closer to surface
3. Nociceptors - respond to stimulation such as intense pressure, high heat, or burning (such as chemical) on the skin
"noci" is the same language root for our term "noxious"
these are also free nerve endings
- begin responding at 113 degrees Farenheit

When cells are injured they release prostoglandins, a category of hormones which sensitizes free nerve endings to histamine, which causes inflamation and other reactions
- Aspirin interferes with prostoglandin synthesis

The SOMATOSENSORY PATHWAYS
Somatosensory axons enter CNS via the spinal nerves and enter spinal cord via the dorsal root ganglia
Two Major Pathways:

From the thalamus, both pathways end up in the primary and then secondary somatosensory cortex

Somatosensory cortex has a columnar arrangement, such that within a column there are populations of cells that respond to a particular stimulus type applied to a particular part of the body

Skin Receptive Fields
in thalamus - on/off, center/surround RFs (e.g., on arm)
in parietal cortex - orientation and direction of movement (e.g., on hand)

Active touch and the perception of edged and smooth objects
Cutaneous Acuity and the two-point threshold
High acuity in areas like hand, fingers, feet, face, lips
Low acuity in trunk areas (like calf, thigh, stomach, back)

Penfield & Rasmussen and the Somatosensory Homonculus
from medial to lateral on the surface of somatosensory cortex - foot, leg, neck, shoulder, arm, face, tongue

PAIN PERCEPTION
"Pain is Good"
pain has both intrinsic and emotional components
top-down influences on pain perception-situations, emotions, culture

Gate Control Theory
-signals from some cutaneous fibers or from brain can "close the gate" or inhibit the passage of the pain message

Phantom limb - 70% of amputees report not just pain but temperature and a variety of other somatosensations, including kinesthetic and proprioception
- parietal lobe lesions impair body awareness (throwing your leg out of bed)
analgesia: reduction of pain perception w/o loss of consciousness
Stimulation produced analgesia (SPA) - especially effective when stimulating the periaqueductal grey (PAG) & medulla
- in humans, surgically implanted electrodes into PAG or somatosensory relay cells in thalamus, stimulate w/ small transmitter to relieve pain

Endogenous Opioids
e.g., enkephalins, beta-endorphins (runner's high)

PAG receives inputs from frontal cortex, amygdala, and hypothalamus;
hence learning and emotional state can modify pain perception without direct secretion of opiods
Naloxone blocks certain type of opioid receptor-a direct antagonist
Naloxone can block acupuncture and placebo effects; so these phenomenon are probably mediated by opioids