Appunti neuroanatomia - De Caro

Spinothalamic pathways

The spinothalamic tracts consist of 2nd order neurons that convey pain, temperature, and pressure information to the somatosensory region of the thalamus. The lateral spinothalamic tract is sited in the lateral funiculus, lying medial to the ventral spinocerebellar tract. The ventral spinothalamic tract lies in the anterior funiculus medial to the point of exit of the ventral nerve roots and dorsal to the vestibulospinal tract point on reaching the lower brain stem. On reaching the lower brain stem, spinothalamic tract axons separate. Axons in the ventral tract join the medial lemniscus, while axons in the lateral tract continue as the spinal lemniscus. Somatotopy is maintained throughout the medulla oblongata and pons. In the midbrain, fibres in the spinal lemniscus conveying pain and temperature sensation from lower limb extend dorsally, whereas those from the trunk and upper limb are more ventrally placed. Both lemnisci ascend to end in the thalamus.

Spinocerebellar tracts

There are two principal spinocerebellar tracts: dorsal and ventral. They occupy the periphery of the lateral aspect of the spinal white matter and carry proprioceptive and cutaneous information to the cerebellum for the coordination of movement. The dorsal spinocerebellar tract lies lateral to the lateral corticospinal tract. It begins at about the level of the second or third lumbar segment and enlarges as it descends. Axons of the tract originate ipsilaterally from the larger neurons of Clarke’s column. In the medulla, the dorsal spinocerebellar tract passes through the inferior cerebellar peduncle to terminate ipsilaterally in the rostral and caudal part of the vermis. The ventral spinocerebellar tract lies immediately ventral to the dorsal spinocerebellar tract. The tract begins in the upper lumbar region and ascends through the medulla oblongata to reach the upper pontine level; it then descends in the dorsal part of the superior cerebellar peduncle and terminates, mainly contralaterally, in the anterior cerebellar vermis.

Dorsal columns

The dorsal funiculus on each side of the cord consists of two large ascending tracts: the fasciculus gracilis and cuneatus, separated by a posterointermediate septum. They are also known as dorsal columns. The dorsal columns contain fibres carrying proprioception and exteroceptive information. The fasciculus gracilis is present throughout the length of the spinal cord and contains the long ascending fibers from the sacral, lumbar, and lower six thoracic spinal nerves. The fasciculus cuneatus is situated laterally in the upper thoracic and cervical segments of the spinal cord and is separated from the fasciculus gracilis by a septum. The fasciculus cuneatus contains the long ascending fibers from the upper six thoracic and all the cervical spinal nerves. The fibers of the fasciculus gracilis and fasciculus cuneatus ascend ipsilaterally and terminate by synapsing on the second-order neurons in the nuclei gracilis and cuneatus of the medulla oblongata. The axons of the second-order neurons, called the internal arcuate fibers, cross the median plane, decussating with the corresponding fibers of the opposite side in the sensory decussation. The fibers then ascend as a single compact bundle, the medial lemniscus, through the medulla oblongata, the pons, and the midbrain. The fibers terminate by synapsing on the third-order neurons in the ventral posterolateral nucleus of the thalamus. The axons of the third-order neuron leave and pass through the posterior limb of the internal capsule and corona radiata to reach the somesthetic area in the postcentral gyrus of the cerebral cortex. The contralateral half of the body is represented inverted, with the hand and mouth situated inferior. Many fibers in the fasciculus cuneatus from the cervical and upper thoracic segments, having terminated on the second-order neuron of the nucleus cuneatus, are relayed and travel as the axons of the second-order neurons to enter the cerebellum through the inferior cerebellar peduncle of the same side. The pathway is referred to as the cuneocerebellar tract, and the fibers are known as the posterior external arcuate fibers. The function of these fibers is to convey information of muscle joint sense to the cerebellum.

Pyramidal tract

The pyramidal system is the chief organizer and executor of voluntary movements. The pyramidal tracts derive their name from the medullary pyramids of the medulla oblongata, which they pass through. Functionally, these tracts can be subdivided into two: corticospinal tracts – supplies the musculature of the body; and corticobulbar tracts – supplies the musculature of the head and neck. The origin of the corticospinal tract is in the primary motor cortex, premotor cortex, and supplementary motor area. It descends through the internal capsule. After the internal capsule, the neurons pass through the crus cerebri of the midbrain, the pons, and into the medulla. A crossover occurs at the caudal end of the medulla, also called decussation of the pyramids. At the level of the pyramids, approximately 85% to 90% of the corticospinal fibers cross over (decussate), providing contralateral motor control of muscles. The remaining 15% of fibers forms the ventral (direct) corticospinal tract. In the spinal cord, most axons of the corticospinal tract terminate in the intermediate gray matter and the ventral horn. The corticobulbar tracts arise from the lateral aspect of the primary motor cortex.