Somatosensory (back to index)
Readings: Shepherd Ch 12

The somatic senses are those that are relayed to the CNS through receptors located in the body covering (integument).  Since the function of the integument of different animals is diverse we will limit our initial discussion to the somatosensory system of vertebrates particularly mammals.

pg 268-9 Covering or integument in mammals is colloquially referred to as skin.  Two types of skin can be found in mammals: glabourous (hairless) and hairy.   Both are divided into two main layers: epidermis (outer) -derived from ectodermal germ layer of embryo and the dermis -derived from the mesoderm.
    The epidermis gives rise to hair, feathers, claws and glands.   It is composed of a deep layer called the Stratum Germinativum and a superficial layer called the Stratum Corneum. Cells in the Stratum Germinativum continually undergo mitosis and migrate toward the where they form the flattened cells of the Stratum Corneum.  Cells in the Stratum Corneum contain a high concentration f a stable fibrous protein (keratin) and melanocytes which contain melanin.
    The dermis is a soft flexible structure that can also be divided into two layer structure.  The more superficial layer, papillary layer contains collagen and elastic fibers while the deeper reticular layer contains fat cells.
    Discussion will proceed by focusing on the different types of receptors cells (cutaneous receptors) found in the integument of humans. There are those found in both Glabrous and Hairy skin: Free nerve endings, Pacinian Corpuscles, Merkel's discs, those found only in Glabrous tissue: Meisnner Corpuscles and those found only in Hairy tissue: Hair follicles and Ruffini endings.
    We will discuss each of these with respect to 1) location and structure of end organ, 2) activating stimuli and size of receptive field, 3) transduction mechanism, 4) type of nerve that the signal is conveyed through and  5) the type of signal that is generated in the nerve when the receptor is activated.
Activating stimuli can be chmical, mechanical, temperature.  Receptive fields describe the area in which a particular stimulus will still be able to activate a particular sensory receptor.  Types of nerves are defined by their diameter and/or conduction velocity (sensory nerves are classified as Aalpha, A-beta, A-delta and C fibers- other classifications are used for similar fibers that are found in other modalities or systems eg. motor system).  Type of signal in response to a constant stimulus can be sustained or adapting and within the classification of adapting can be either fast or slow.

I. Receptors
A. Free nerve endings- this is the most difficult receptor type to discuss because, with respect to what stimuli activate this receptor,  it is the most non-specific of the somatosensory receptors .  However,   since it is probably the oldest evolutionarily we will begin here.

  1. Structure/Location -  They are found in both glaborous and hairy skin and  branch in the dermis or deeper layers of the epidermis. These receptors are simply unmyelinated terminals of A-delta,  and C fibers.They are also found in muscles and in the gut.  Fig 12.1 pg 269
  2. Activation - this receptor is activated by  mechanical (light touch, tickle) and noxious stimuli as well as heating and cooling.  Some endings respond to only one modality (unimodal) while others respond to 2 or 3 (polymodal). Those that respond to painful or noxious stimuli are called nociceptors.Receptive fields are variable.
  3. Transduction Mechanism - (pg 271 col 2)
    1. Nociceptors will respond to chemical mediators that result from  the inflammatory response to injury ( histamine, serotonin, bradykinin).  Activation of nerve endings from A-delta fibers by these chemicals results in the immediate sharp pain (pricking pain) while activation of the C-fibers produces a sustained sensation of pain (burning pain or itching).
    2. Temperature - cooling relative to normal body temperature is sensed by free nerve endings from a subpopulation of A- delta fibers while warming is sensed by free endings from a subpopulation of C-fibers.  Individual receptors will be activated along a broad range of temperatures and therefore individual cells will have a broad sensitivity. this enhances the ability to discriminate small changes in temperature near body temperature.
    3. Light touch - stretch receptors that may be similar to the receptors found in hair cells of the auditory system.
  4. Activation/Encoding - all free nerve endings are slowly adapting.
day2
B.  Pacinian Corpuscle FAI
  1. Structure/location -  They are found in both Glabrous and Hairy skin and are situated deep in the dermis. The end organ itself seems to be composed of a free nerve terminal  of A-deltanad A-beta fibers surrounded by an incomplete shell of cell processes and collagen fibers enclosed in an outer complete lamellae.   These receptors are also located in other structures such as connective tissue of muscles, the periosteum of bones and the mesentery of the abdomen.
  2. Activation - this receptor is activated by  mechanical stimuli (Pressure, Vibration - 250-300 Hz). Displacement of 10um can produce a maximal response. Large receptive fields
  3. Transduction Mechanism - This is thought to occur in response to activation of a 'stretch-activated channel in the sensory membrane.
  4. Activation/Encoding - rapidly adapting based on studies on receptor potential.  activation of free ending by a stimulus produced a slowly, not a fast adapting response.
C. Merkel's Discs  (Merkel's nerve complex) SA I
  1. Structure/Location -  They are found in both Glabrous and Hairy skin and are situated on the border of the dermis and epidermis.  End organ is a free nerve ending associated with other cells.
  2. Activation - this receptor is activated by  light mechanical stimuli (touch pressure ).  Receptive field is small
  3. Transduction Mechanism - Most likely mechanoreceptor/stretch receptor.
  4. Activation/Encoding - Slowly adapting. Slight difference in that Merkel's disc shows an overshoot during the initial phasic part of the indentation (thus can provide info about changes in stimulus intensity as well as steady state values - pg 274 col 2 - a bit confused here!).  Merkel's discs have small receptive fields and Ruffini endings have large receptive fields.
D. Meissner Corpuscle   FA II
  1. Structure/location - Found only in Hairy skin. The end organ  itself is found at the junction between the dermis and epidermis.
  2. Activation - this receptor is activated by  mechanical  stimuli such as light touch, and vibration (30-40 Hz). High threshold of activation. Small receptive fields.
  3. Transduction Mechanism -  Most likely mechano/stretch receptors.
  4. Activation/Encoding - rapidly adapting (less so than Pacinian Corpuscles) based on Ake Vallbo's technique called neurography.  The fact that it does not adapt as rapidly does not allow it to be sensitive to very high frequency stimulation (contrast with Pacinian Corpuscles).
Summarize using Fig 12.6-
 

E. Ruffini endings SA II

  1. Structure/Location - Found in hairy skin.  End organ is situated in the reticular layer of the dermis.
  2. Activation - this receptor is activated by  light mechanical stimuli (touch pressure).  Large receptive fields.
  3. Transduction Mechanism - mechano/stretch receptors.
  4. Activation/Encoding - Slowly adapting.
F. Hair follicle endings
  1. Structure/Location - Found only in hairy skin.  Free nerve endings are wrapped around hair follicle in the papillary layer of the dermis.
  2. Activation - this receptor is activated by  light mechanical stimuli (30-40 Hz vibration).
  3. Transduction Mechanism - Most likely mechano/strech receptors.
  4. Activation/Encoding - fast adapting
  5. * Takes the functional place of Meisnner Corpuscles found in Glabrous skin.

 
G. Special muscle receptors: these are included here because they contribute to both the reflex pathway and the ascending somatosensory pathways. day 3
II.  Pathways
    note: You should be able to keep track of where information enters i.e level of cord, where it travels within fiber pathways and where it crosses (decussates) and synapses.

All sensory receptors from the body have cell bodies in Dorsal root ganglian (DRG).  DRG fibers enter the cord through the Dorsal Entry Zone (Tract of Lissauer).

  1. reflex pathway - (Ch 19) - spinal cord - muscles
  2. Ascending pathway

  3. a) Spinothalmic pathway aka anteriolateral system. (mostly conveys pain information).  Spinal cord (superficial layers I-II)- contralateral white matter of cord (lateral spinothalamic tract)- thalamus ()- sensory cortex.  Conveys  mostly pain and temperature information but also contain some tactile and joint information.  Give off collaterals in brainstem that activate reticular formation and contribute to arousal and consciousness.
      paleospinal cerebellar - synapses in reticular nuclei.
      neospinal cerebellar - synapses in thalamus-neocortex.
    b) Lemniscal Pathway (mostly conveys information about touch).  Spinal cord - ipsilateral white matter to Nucleus Cuneatus and Gracile (dorsal columns)- contralateral white matter (medial lemniscus) - thalamus (ventral posterior lateral -VPL- nucleus of the thalamus) - somatosensory cortex. Conveys mostly complex information about touch and pressure. Give off collaterals in brainstem that activate reticular formation and contribute to arousal and consciousness.

    c) Spinocerebellar (mostly conveys proprioceptive information. Spinal cord - ipsilateral and contralateral white matter to cerebellum.
     

day 4
Review of idea that topography or somatotopy is maintained through most levels of the nervous system.
    1. Spinal Cord -
    2. Thalamus - somatosensory information from the body terminates in a nucleus called the ventral posteriolateral nucleus of the thalamus (VPL).  Information from the face terminates in an adjacent region referred to VPM in subprimates these regions are combined and called the ventrobasal complex (VBC).
    3. Somatosensory cortex.
 

*Facial information comes through cranial nerves and travels together  with the sensations from the body in various nuclei. Touch information  arrives in the VPM of the thalamus and travels to the cortex with touch information from the body that has synapsed in the VPL.  Pain/Temperature takes a similar path.

III. Perception
Figure 12.6 Receptive Fields vs Adaptation.
(pg 276 col 1 )

Feeling of diffuse pain in the abdomen.  Mediated by free nerve endings in the gut.  Activation of these nerves does not produce a topographical representation of the gut at various levels of the anterolateral pathway.

Hot and cold