Notes

The human skin is the largest organ of the human body.

Somatosensory System Functions

• **Exteroceptive Functions
  • Mechanoreception - pressure or touch (tactile sensitivity)
  • Thermoreception - temperature (thermal sensitivity)
  • Nociception - noxious (damaging or potentially damaging) stimuli (noxious sensitivity). Noxious stimulus refers to the capacity of a stimulus of injuring our skin, regardless if it is painful or not.
• Proprioceptive Functions (Kinaesthesis) - information about position and movement of limbs and body in space.
• Interoceptive Functions - information from internal organs.

Mechanoreception

• Form Perception (pressure is part of form perception)
  • E.g., identification of objects by touch alone; Braille reading; measured experimentally in terms of "two-point threshold" (minimal distance between the stimuli need to feel that there are 2 pencils pressing on your skin).
  • Note (Huge) variation in sensitivity across skin surface.
• Texture Perception
  • Our ability to analyse surfaces by touching them which is closely associated with vibration perception. Texture perception is very important for rodents.
• Vibration Perception
  • Flutter (frequencies < 40 Hz)
  • Vibration (frequencies 40 -- 400 Hz)

Skin and Skin Receptors

There are many different types of receptors in the skin (number is still debated), the skin is a huge organ (large surface) filled with receptors.

• Size of skin as receptor surface
• Glabrous (hairless) and Hairy skin.
• Distinctions between:
  • Encapsulated and Unencapsulated receptors
  • Superficial (cutaneous) and Deep receptors. (The location in the skin give them a different type of sensitivity.

At the base of the hair follicle, we have some nerves wrapped around the hair. Just below the epidermis, we have Merkel's disk, free nerve ending and Meissner's corpuscle. Deeper layers of the skin have the Pacinian corpuscle and Ruffini's endings.

• Superficial Encapsulated Receptors:
  • Merkel Receptor/Disk
  • Meissner Corpuscle
• Deep Encapsulated Receptors:
  • Ruffini Ending
  • Pacinian Corpuscle
• Unencapsulated Receptors - free nerve endings.

Because of their location in the skin and the nature of their specializations, different encapsulated receptor types have different forms of cutaneous sensitivity. This was first discovered not by looking at receptors themselves but by recording from single Cutaneous Afferent Fibers (can be done in humans and in animals, e.g., Swedish researcher sticking electrodes in his arms). All of the mechanoreceptor afferent axons are medium diameter (6 -- 12 μm), well-myelinated fibers with conduction velocities of 35-75 m/s. They are called Aβ Fibers.

Cutaneous Afferents Some of these receptors quickly adapt, while others don't. Fibers are classified as either:

• Rapidly Adapting (RAs)
• Slowly Adapting (SAs)
• Also in terms of the size of their Receptive Field (The size of the area on the skin from which they can be activated) (it is also related to the position of the receptor, indeed superficial receptors tend to have smaller RF than deeper ones).

A stimulus probe movement consists of applying a pressure through a probe on the skin and subsequent release.

In the picture above, we can notice that the top row adapts very fast to the pressure stimulus and signal it. Ruffini's endings on the contrary never completely adapts to the new stimulus and doesn't stop firing.

Schematic Illustration of Determination of Receptive Field

As we can see from the picture, Meissner's corpuscles tend to have small RF. On the contrary, Pacinian corpuscles are relatively deep and have large receptive fields. RAs respond only at the beginning and end of sustained displacements (i.e., to transients) but respond well to higher frequency vibrations. Two types:

• RA I: Meissner Corpuscles (10 - 200 Hz) (small RF)
• RA II: Pacinian Corpuscles (70 - 1000 Hz) (large RF)

SAs respond throughout sustained displacements of the skin, and are thus suited to coding the duration and magnitude of mechanical stimuli. Two types:

• SA I: Merkel Receptors/Disks (small RF)
• SA II: Ruffini Endings (large RF)

Encapsulation has a very important role to play. The sensitivity characteristics of a particular afferent fiber reflects the specialized nature of the encapsulated ending (e.g., If you dissect away the onion-like layers of a PC, the fiber from that PC becomes sensitive to sustained displacement) and its position on the skin. By removing the capsule and leaving only the axon, they realized that the receptor potential was relatively different, to the point of changing a fast-adapting receptor to a slowly adapting one. Papers show that these properties are due to the mechanical properties of this structure.

Summary Classification Table

Hairy Skin

• Merkel disks, Pacinian corpuscles and free nerve endings as in glabrous skin. While Ruffini's endings and Meissner's corpuscles are less common in hairy skin.
• Innervation of hair follicles (by free nerve endings or more specialized endings).
• A recent discovery -- hairy skin has "soft touch" receptors that project to cortical regions associated with emotion and sexual arousal.

Nociception

• Nociceptors respond to noxious mechanical and thermal stimuli (i.e., stimuli that produce or threaten to produce tissue damage) and also to chemicals released by damaged tissue (e.g., histamine, etc.).
• Note distinction between noxiousness as a quality of the stimulus and pain as perceptual and emotional response.
• Two types of nociceptor:
  • High Threshold Mechanoreceptors
  • Polymodal Nociceptors
• Receptors are free nerve endings.
• Afferent axons are of different types (which have very different conduction velocities):
  • High Threshold Mechanoreceptors: Aδ Fibers.
  • Polymodal Nociceptors: C Fibers.

Axon types in nerves from skin and from muscles (Note different terminologies associated with different origins).

Different fiber types give rise to different pain sensations:

• "First" or "Cutaneous Pricking" pain - mediated by Aδ Fibers.
• "Second" or "Burning" pain - mediated by C Fibers.

Thermoreception

• Thermoreceptors: "Warm" fibers and "Cold" fibers.
• They respond to increase or decrease in temperature, respectively, from steady state and to maintained temperature.
• At intermediate skin temperatures (approx. 30-35 degrees) there is ongoing discharge in both warm and cold fibers, and a change in skin temperature reciprocally modifies the discharge in the two fiber types. At a lower adapting temperature (e.g., 26 degrees) only cold fibers are active, and an increase in temperature first decreases the discharge in cold fibers, and then - with larger increments - begin to engage the previously silent warm fibers.
• Large receptive fields, so temperature sensations are not well localized.
• Note relativity of thermal sensations (i.e., change in temperature is what is felt as warm or cold) and similar relativity of neuronal responses. (Three buckets of water experiment)
• Receptors are free/bare nerve endings.
• Afferent axons:
  • Warm Fibers: C Fibers
  • Cold Fibers: Aδ Fibers

Proprioception (The Kinaesthetic Sense)

• Provides information about the position and movement of the limbs and body in space (e.g., limb movement without vision; passive movement).
• Proprioceptive information supplements "efference copy" (It refers to perform precise, well-coordinated movements) information from motor system.
• Information is provided by three classes of receptors:
  • Cutaneous Mechanoreceptors
  • Joint Receptors
  • Muscle Spindles (Muscle fibers that are measuring the state of tension of the muscle) and Golgi Tendon Organs

Cutaneous Mechanoreceptors and Joint Receptors

• Cutaneous Mechanoreceptors
  • Information about skin stretch from Ruffini/SAII afferents.
  • Of different importance in different regions (important around hands, mouth and feet).
• Joint Receptors
  • Ruffini-type endings and Pacinian corpuscles in joint capsule.
  • But not essential, because anesthesia or removal of joint capsules does not result in loss of limb position sense (they seem to be supplementary).

Muscle Spindles and Golgi Tendon Organs

• Golgi tendon organs (Ib fibers) respond to tendon stretch.
• Muscle spindles (type Ia and II axons) are coiled around intrafusal muscle fibers; respond to muscle stretch; type I axons constitute the sensory component of the knee-jerk reflex.

Dorsal Root Ganglia

All somatic fibers are the axons of dorsal root ganglion cells (or trigeminal ganglion cells in the case of the head and neck). DRG cells have no dendrite, but a single bifurcating axon - sends one process to the periphery and another to the CNS.

Dermatomes

The axons of individual dorsal root innervate a restricted region of skin that is the same in everyone -- dermatome. A dermatomal map is a stereotyped pattern of dermatomes that allows for diagnosis of location of injury or infection of dorsal roots on basis of skin distribution of sensitivity (e.g., in shingles).