Characteristics of sensory pathways

•  Presence of distinct set of receptors, distributed over the body.

•  A minimum of 3 neurons between receptor and cerebral cortex

•  Obligatory relay in the thalamus

•  Feedback and feed-forward modulating mechanisms along the way especially at relay stations.

•  Collateral projections to the reticular formation in the brain stem.

•  Topological maps at the various relay stations and in the cerebral cortex.

•  Primary and secondary sensory areas.

•  Association sensory areas connect with the limbic system via sensory limbic connections




Spinal Sensations  

  • The receptors for pain consist of un-encapsulated endings of peripheral nerve fibres.

  • The first order neuron has cell bodies in the respective ganglion . Dendrites run in spinal or cranial nerves

  • Axons from the ganglion enter the dorsal horn via dorso-lateral tracts ( of Lissauer ) and terminate directly in laminae I and II .

  • Cell bodies of secondary neurons are situated in the chief nucleus (nucleus proprius) of the gray horn.

  • Their axons decussate through the anterior white commissure, then ascend as the lateral spinothalamic tract.

  • Axons of the spinothalamic tract synapse with tertiary neurons in the VPLC nucleus of the thalamus .

  • A few collaterals go to the reticular formation.

  • Tertiary neurons arising from the thalamus pass through the internal capsule and corona radiata project to the sensory cortical area in the post central gyrus.
Note: In the upper part of the medulla oblongata, the lateral and ventral spinothalamic tracts and the spinotectal tracts are closely associated at this level and throughout the reminder of the brain stem, together, and they constitute spinal lemniscus . Spinotectal tract terminates in the superior colliculus while the others go to the reticular formation .

Cranial Sensations

  • First Order neurons are found in the trigeminal , facial , glossopharyngeal and vagus nerves .

  • The cell bodies of the primary sensory neurons are in the trigeminal , geniculate , superior glossopharyngeal and vagal ganglia.

  • The central processes enter the pons through the sensory root and caudally in the trigeminal spinal tract /nucleus .

  • The second order neurons cross and ascend as trigeminothalamic tract .

  • This tract terminates in the medial part of the ventral posterior nucleus of the thalamus ( arcuate or semilunar nucleus )

  • A tertiary neurons from the arcuate nucleus traverse internal capsule, corona radiata to terminate in the sensory (somesthetic) cortex

Note: Some fibres from the trigeminal tract are believed to ascend uncrossed