Lab 4 - External and Internal Anatomy of the Spinal Cord

Brainstem

Next we will follow the Posterior Column-Medial Lemniscal pathway through the brainstem and identify some important landmarks along the way. Follow the following text as you navigate through the different levels of the brainstem. (Not all of the items in bold will be identified in this slide.)

Lower Medulla

This is a section through the lower, "closed" medulla near its junction with the spinal cord. Note that the section passes through the pyramidal (motor) decussation and that the spinal trigeminal nucleus replaces the posterior horn of the spinal cord. Within the gracile fasciculus, the gracile nucleus appears as a light band along the posterior median septum. The cuneate nucleus, which is barely visible at this level of the medulla, forms a small knob at the base of thecuneate fasciculus. These nuclei are the sites of termination of the 1° posterior column-medial lemniscal afferents.

Medulla at the Obex

This is a section through the medulla at the level of the obex, the point at which the central canal opens into the fourth ventricle, and includes the sensory decussation. Other structures characteristic of the "open" or rostral medulla are the pyramids and the inferior olivary nuclei. Notice that the gracile nucleus is much smaller than the cuneate nucleus. The axons of these two nuclei, 2° somatosensory afferents, sweep through the interior of the medulla as the internal arcuate fibers, form the sensory decussation and collect in the contralateral medial lemniscus.

Upper (Open) Medulla

This section is at the level of the "open" medulla where the hypoglossal nuclei and the dorsal vagal motor nuclei form the hypoglossal and vagal trigones, respectively, in the floor of the fourth ventricle. The pyramids and the inferior olivary nuclei continue to be located in the inferior half of the medulla. The pyramids form the medulla base while the bulk of the medulla - extending from the floor of the fourth ventricle down to the pyramids form the medulla tegmentum. The medial lemniscus forms a vertical band of fibers along the midline, posterior to the pyramids. Recall that the medial lemniscus consists of crossed 2° somatosensory afferent fibers of the nuclei gracilis and cuneatus.

Caudal Pons

This is a section passing through the pons. Recall that the pons is characterized by the middle cerebellar peduncles. The pontine nuclei and their decussating axons now surround the fibers that formed the pyramids of the medulla. The medulla base is replaced by the pons base - which consists of the pontine nuclei, their axons and the corticofugal (also called pyramidal tract) fibers. The medulla tegmentum is replaced by the pons tegmentum - which extends from the floor of the fourth ventricle down to the pons base. The inferior olivary nuclei are absent and the abducens nuclei and facial nerve genu form the facial colliculi in the floor of the fourth ventricle. Note that within the pons tegmentum, the fibers in the medial lemniscus are moving rostrally from a medial, vertical position to a more lateral, horizontal position. As the medial lemniscus ascends in the pons it gradually changes shape, flattening into a band along the anterior surface of the pons tegmentum. Recall that the medial lemniscus consists of crossed 2° somatosensory afferent fibers of the nuclei gracilis and cuneatus.

Mid Pons

This section passes through the mid-pons at the level of entry of the trigeminal nerve root into the pons tegmentum. The decussated axons of the pontine nuclei continue to collect on the lateral surface of the pons to form the middle cerebellar peduncles. The fibers of the medial lemniscus continue to shift laterally to form a flat band of longitudinally coursing, crossed, 2° somatosensory afferent fibers near the anterior border of the pons tegmentum. Recall that the neurons of the posterior column-medial lemniscal pathway process information used for discriminative touch and proprioception from the body.

Rostral Pons

This section passes though the rostral pons at the level of the isthmus between the fourth ventricle and cerebral aqueduct. The medial lemniscus has moved laterally and lines the floor of the pons tegmentum. Recall that the medial lemniscus consists of crossed 2° somatosensory afferent fibers and carry information used for discriminative touch and proprioception from the body.

Caudal Midbrain

This is a section through the midbrain at the level of the inferior colliculus and the decussating fibers of the superior cerebellar peduncle. Notice that the corticofugal fibers (called the occipitotemperopontine, pyramidal and frontopontine tracts by DeArmond) are again located on the anterior surface of the brain stem and now form the crus cerebri, a.k.a. as the cerebral peduncles in the gross brain stem. The midbrain base, which replaces the pons base, is formed by the crus cerebri, and the midbrain tegmentum, which replaces the pons tegmentum, extends from the midbrain base up to the floor of the cerebral aqueduct. The roof of the cerebral aqueduct, i.e., the inferior colliculi, forms the midbrain tectum
In the midbrain, the crossed 2° somatosensory afferent fibers carrying discriminative touch and proprioception information from the body continue to ascend the neuroaxis in the medial lemniscus. At this level of the midbrain, the fibers of the medial lemniscus have been displaced laterally by the decussating fibers of the superior cerebellar peduncles and are beginning to move posteriorly along the lateral edge of the midbrain tegmentum.