Neural Coding of Somatosensory Systems

• Correlation; function-ending (von Frey)
• Lele & Waddell (1956) -- cornea
• Free nerve endings
• Krause end bulbs
• Mediate all senses

Impulse Initiation

• Generator potential (graded).
• Train of action potentials.

• Tonic Receptors - adapt slowly
• Phasic Receptors - adapt rapidly

Receptor Specificity

Depends on:

• Position in Body
• Anatomical structure
• Chemical Composition
  • Chemoreceptors - vision
  • Mechanoreceptors - thermal

Common Characteristics

• Each have 3 neurons - receptor to cortex
• First cell body is located outside CNS (dorsal root ganglion)
• 2nd cell body in spinal cord or brain stem
• 2nd neuron crosses near synapse
• 2nd neuron then enters lemniscus bound for thalamus
• 3rd cell body in thalamus (posterolateral ventral nucleus)
• 3rd neuron projects to cortex (postcentral gyrus)

NOTE: Thalamus nuclei are named by dividing them into anterior & posterior groups, dorsal & ventral groups, and medial vs lateral groups. The somatosensory nucleus of the thalamus might be referred to as the ventral posterior lateral or the posterior ventral lateral. Both mean the same thing.

Cortical Functions (postcentral gyrus)

1. Roughness discrimination lost following lesion of SI & SII; some relearning possible

2. Pain and temperature not reported with cortical stimulation

TOP

Epicritic Pathway

Tactile and Kinesthesis

Gracilis and Cuneatus Fasciculi

• Class A fibers -- heavily myelinated
• Has little emotion-arousing properties
• 1st cell body in dorsal root ganglion (Fibers enter spinal cord medially and branch)
• Fibers pass to dorsal fasciculus
• Ascend in spinal cord to synapse in nucleus gracilis and cuneatus
• 2nd neurons cross (decussate); (internal arcuate fiber)
• Ascend as medial lemniscus
• Terminate in postero-ventral lateral nucleus of thalamus
• 3rd from thalamus to postcentral gyrus

Intermediate pathway

Coarse touch and pressure

Ventral spinothalamic tract

• Class B fibers -- lightly myelinated
• 1st cell body in dorsal root ganglion
• Fibers enter cord medially
• Ascend in cord for several segments
• Synapse 2nd nucleus in dorsal gray column.
• 2nd neurons cross in ventral white commissure and form ventral spinothalamic tract (a few fibers do NOT cross) (newer tracts laid down ventrally)
• Pass as spinal lemniscus to posterolateral ventral nucleus of thalamus
• 3rd to postcentral gyrus of cortex

Primitive Protopathic Sensation

Pain and temperature

• Class C fibers--umyelinated
• Fibers enter dorsolateral fasciculus
• Synapse in 1-2 segments in substantia gelatinosa
• Decussate in ventral gray & white commissure
• Ascend as lateral spinothalamic tract (all fibers serve a nondiscriminative high emotion-arousing function)
• Most terminate in reticular formation
• Some to the posterolateral ventral nucleus of the thalamus
• 3rd to post-central gyrus of cortex

Terms to Know