18.5: Major Somatic Sensory Pathways
Sensory impulses related to touch, pressure, vibration, and proprioception from various body parts, such as the limbs, trunk, neck, and posterior head, travel to the cerebral cortex through the posterior column-medial lemniscus pathway. The pathway’s name derives from the two white-matter tracts that convey the impulses: the spinal cord's posterior column and the brainstem's medial lemniscus. First-order sensory neurons extend their axons into the spinal cord, forming the posterior columns consisting of two tracts, the gracile fasciculus and the cuneate fasciculus. Sensations from the lower part of the body travel through the gracile fasciculus tract, while sensations from the upper parts travel through the cuneate fasciculus tract. The axons synapse with second-order neurons located in the gracile or cuneate nucleus of the medulla, respectively, which then cross to the opposite side of the medulla and enter the medial lemniscus tract. The axons of the second-order neurons synapse with third-order neurons in the thalamus, which then project their axons to the primary somatosensory cortex.
Sensory impulses for pain, temperature, itch, and tickle from body parts such as the limbs, trunk, neck, and posterior head travel up to the cerebral cortex through the anterolateral or the spinothalamic pathway. The first-order neurons of this pathway connect sensory receptors of the body parts with the spinal cord. These first-order neurons have their cell bodies located in the posterior root ganglion, and their axon terminals synapse with second-order neurons located in the posterior gray horn of the spinal cord. The axons of the second-order neurons cross to the opposite side of the spinal cord and ascend to the brainstem as the spinothalamic tract. These axons end in the ventral posterior nucleus of the thalamus, where they synapse with third-order neurons. The axons of these third-order neurons project to the primary somatosensory area on the same side of the cerebral cortex as the thalamus.
The anterior and posterior spinocerebellar tract are the primary pathways through which proprioceptive impulses reach the cerebellum. These pathways transmit sensory information related to posture, balance, and coordination of skilled movements. The neurons of this pathway either do not cross over or cross over twice, so they reach the cerebellum on the same side as that of the impulse. Although these sensory impulses are not consciously perceived, they regulate movements and maintain balance.
