SOMATIC SENSORY SYSTEMS

 Sensory systems enable us to detect stimuli, and they participate in the development of our concepts of the world around us and of body image.  Each such system consists of a series of at least three neurons:

1^o SENSORY - Ganglion cell (unipolar with Receptor*)

synapse                                                                                   

2^o SENSORY - Projection cell (axon Decussates*)                                  

synapse                                   

3^o SENSORY - Thalamic cell (projects to Sensory Cortex)

Each of these neurons has a receptive field

Receptor

- sensory terminal whose membrane responds to a stimulus by producing a local, graded, depolarizing receptor potential (increased Na+&K+ conductance). 

Threshold Receptor Potential initiates axonal action potentials (spikes) whose frequency is proportional to the stimulus intensity.

Receptive Field

- area of skin which when stimulated either increases or decreases the firing rate of that cell.  A receptive field has an excitatory center and an inhibitory surround, properties resulting from a type of central processing called Lateral Inhibition.

 (enhances contrast between stimulated and unstimulated areas - it increases the signal to noise ratio)

Adaptation

- a decrease in receptor potential with continued stimulation - receptors may be slowly adapting or rapidly adapting.

    Adaptation may result from inactivation of Na+ or Ca++ membrane channels, activation of K+ channels or from the physical properties of extraneural structures that surround the sensory nerve terminal as in the Pacinian corpuscle, for example.

Axons that carry sensory information fall into four categories with the following conduction velocities:

        Type I       (or A alpha)       - large (80 - 120 m/s)

Type II      (or A beta) - medium-sized (35 - 75 m/s)             Myelinated

Type III     (or A delta) - small (5 - 30 m/s)

Type IV     (or C fibers) - Unmyelinated (0.5 - 2 m/s)

Somatosensory Modalities

-sensations derived from a single class of receptor

Discriminative Touch      mediated by type I&II (larger) fibers that

Pressure                           conduct at 35 - 120 meters/second and form

Proprioception                 the Medial Division of the dorsal root

Crude (Light) Touch        mediated by type II & III fibers

Pain                                   mediated by type III&IV (small) fibers that

Temperature                     conduct at 0.5 - 30 meters/second and form

                                           the Lateral Division of the dorsal root

Medial and Lateral division segregation

- Large fibers enter the Dorsal Funiculus, whereas small fibers enter Lissauer's Tract.

Ascending and Descending branches, carry sensory input for three functions:

(1) Reflexes - fibers make reflex connections with motor neurons

(2) Muscle coordination - fibers provide input to the cerebellum

(3) Conscious perception - fibers provide input to the cerebral cortex

Protopathic & Epicritic  Sensations

Protopathic- more primitive, crude sensory system that is normally dominated by a more refined system that mediates precise discrimination of touch stimuli, known as the Epicritic 

The tract that conveys Epicritic senses is called the Dorsal Column Medial Lemniscus, whereas the Spinothalamic Tract conveys Protopathic senses.  Sensory information mediated by each tract reaches the Somatosensory Cortex via a relay in the Thalamus

Thalamic nuclei can be categorized as follows:

Specific Sensory Nuclei - relay sensory information to Sensory areas of the Cortex (Somatosensory, Visual, Auditory or Vestibular)

Specific Motor Nuclei -relay motor information to Motor areas of the Cortex

Nonspecific (Intralaminar) Nuclei - project diffusely to the whole Cerebral Cortex

Association Nuclei -project to Association areas of the Cerebral Cortex

The interplay of thalamocortical activity produces different functional state:

-which the cerebral cortex is either syncronized (slow wave sleep) or desyncronized (awake or REM sleep) 

Dermatomes and Somatotopic Organization:

The skin area innervated by a single dorsal root is called a Dermatome.  A dermatomoverlaps one half of the adjacent dermatomes.

  Loss of one dorsal root does not cause complete loss of sensation in the related dermatome.

 Therefore, somatic sensory structures (nuclei, tracts and cortex) are said to be Somatotopically Organized.

DORSAL COLUMN MEDIAL LEMNISCAL SYSTEM (DCML)

This somatosensory system is phylogenetically new and is best developed in the mammals. 

Axons are rapidly-conducting, heavily-myelinated type I&II fibers. 

Input from each receptor projects directly to the somatosensory cortex with little divergence and no collaterals to the brainstem reticular formation

 The DCML system provides the principal sensory input from the body to somatosensory cortex. 

The neurons of this system have small receptive fields.  Therefore, each area of skin receives sensory terminals of many neurons (high innervation density).

   The DCML system convey.........

Epicritic Sensations: 

    (sensations that require high acuity or high resolution) 

(1) Discriminative touch - the ability to localize touch, tested by determining the 2 point touch 

      Threshold which is 1-2 mm on fingers, 70 mm on the back

(2) Stereognosis - recognition of objects by touch

(3) Graphesthesia - touch recognition of letters or numbers drawn on the skin

(4) Vibratory sense - mediated by Pacinian corpuscles and severely compromised by a lesion 

     of the DCML Vibratory sense is tested with a tuning fork that vibrates at 130 – 200 cps

(5) Proprioception - sense of body position & movement, mediated by special proprioceptors 

     of several types (muscle spindles, Golgi tendon organs & joint receptors)

    All of the above senses require high degrees of spatial and temporal resolution made possible by small receptive fields and larger, more rapidly-conducting axons.  

Dorsal Column axons synapse

in the medullary dorsal column nuclei:

    Nucleus Gracilis - from lower limb and lower body (below spinal level T6)

  Nucleus Cuneatus - from upper limb and upper body

Course of the Pathway through the Brainstem, Thalamus and Cerebral Cortex:

-Axons of  Nucleus Gracilis and  Nucleus Cuneatus  cross the midline in the Sensory Decussation (internal arcuate fibers) and form the Medial Lemniscus. 

The medial lemniscus has a vertical orientation in the midline of the medulla, assumes a horizontal orientation in the pontine tegmentum, and shifts laterally as it traverses the pons to lie dorsolateral to the red nucleus in the midbrain. 

Most fibers terminate in the Ventral Posterolateral (VPL) thalamic nucleus, whereas some fibers that mediate proprioception terminate in another subdivision of the ventral posterior nucleus (the Ventral Posterosuperior (VPS) nucleus). 

These thalamic nuclei project through the Posterior Limb of the Internal Capsule to the Postcentral Gyrus (Brodmann's areas 3a&b, 1 & 2).