The anterolateral pathways are composed of three tracts: the spinothalamic, spinoreticular, and spinomesencephalic tract. Separate aspects of pain are conveyed through the spinothalamic tract, so we will follow the course of those neurons first. The first-order neuronal cell body of axons forming the spinothalamic tract is located in a spinal ganglion (eFig. 9.52). Axons then enter the spinal cord through the posterior root to reach the posterior horn. From here, axons have two courses: some synapse immediately on second-order neurons in the posterior horn gray matter (lamina I and V), and others have axonal collaterals that ascend or descend one to two spinal cord segments in the posterolateral tract of Lissauer before synapsing with the second-order neurons in the gray matter (eFig. 9.51). Axons of the second-order neurons then cross obliquely over two to three spinal cord segments within the anterior commissure of the spinal cord to join the anterolateral tract on the contralateral side (eFig. 9.52). These second-order axons ascend through the CNS to reach the third-order neuronal cell bodies in the ventral posterior lateral nucleus of the thalamus (eFig. 9.52). Axons from the third-order neurons then project through the posterior limb of the internal capsule to reach the primary somatosensory cortex (eFig. 9.52).
The spinoreticular and spinomesencephalic tracts have a similar beginning as the spinothalamic tract. The principal difference is the target structure of the second-order axons. Rather than project to the thalamus as the spinothalamic tract does, the spinoreticular tract projects to the reticular formation in the brainstem to convey the emotional and arousal aspects of pain (eFig. 9.52), and the spinomesencephalic tract projects to the periaqueductal gray matter and superior colliculi in the midbrain for the central modulation of pain (eFig. 9.52).
First-order neuronal cells bodies of the posterior column–medial lemniscal pathway are located in a spinal ganglion (eFig. 9.53). Axons then enter the spinal cord through the posterior root to reach either the gracile fasciculus (gracile means “thin”), which carries information from the lower limb and trunk, or the cuneate fasciculus (cuneate means “wedge-shaped”), which carries information from the upper limb and neck. These first-order axons then ascend ipsilaterally to the caudal medulla and synapse with the second-order neuronal cell bodies within the nucleus gracilis and nucleus cuneatus (eFig. 9.53). Axons of these second-order neurons then cross over as the internal arcuate fibers to form the medial lemniscus in the contralateral medulla (eFig. 9.53). These second-order axons ascend through the brainstem to reach the third-order neuronal cell bodies in the ventral posterior lateral nucleus of the thalamus (eFig. 9.53). Axons from the third-order neurons then project through the posterior limb of the internal capsule to reach the primary somatosensory cortex (eFig. 9.53).
Descending tracts through the spinal cord are involved in voluntary movements; postural movements; and coordination of head, neck, and eye movements. These pathways originate from the cerebral cortex and brainstem and are influenced by sensory input and feedback circuitry from the cerebellum and basal ganglia. Structures that influence regulation of motor planning and voluntary control will be discussed in subsequent sections. In this section we will review the tracts of the medial and lateral motor systems. The tracts in each of these systems are composed of an upper motor neuron with cell bodies located in the cerebral cortex or brainstem and a lower motor neuron with cell bodies located in the spinal cord gray matter. We will begin by exploring the tracts of the lateral motor system first.