Axonal Pathways Traversing the Brain Stem 1 Descending Pathways

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The direct motor pathway from the cerebral cortex to the spinal cord comprises myelinated axons of moto-neurons in the cerebral cortex. These axons form bilateral thick bundles which traverse midbrain, pons, and medulla. As shown in the different cross sections in Fig. 2, the pathway is often loosely referred to as the pyramidal tract. In the midbrain the anterior region through which the pyramidal tract passes is called the crus cerebri or the basis pedunculi. In the pons the fibers are divided into smaller bundles by bundles of axons which sweep around the front of the brain stem. In the medulla, the corticospinal fibers are again situated medially and anteriorly, and the paired pathways are referred to as the pyramids.

Figure 2 (continued)

There are other well-recognized descending pathways whose names provide helpful clues to their neuroanatomy. The rubrospinal tract consists of axons of neuronal cell bodies located in the red nuclei in the midbrain. The vestibulospinal tract consists of axons of neuronal cell bodies located in the vestibular nuclei in the dorsolateral part of the rostral medulla ob-longata. Other descending pathways include the so-called "reticulospinal" tracts. These axons descend in less well-defined pathways. The hypothalamus and certain regions of the pons and medulla contain neurons (presympathetic neurons) whose axons descend to the spinal cord via the dorsolateral funiculi and innervate sympathetic preganglionic neurons located in the intermediolateral columns of the thoracic and upper lumbar spinal cord.

Other neurons in the medulla and pons function as parasympathetic premotor neurons with axons descending to the sacral portion of the spinal cord to innervate the parasympathetic preganglionic neurons that regulate genital and bowel-bladder function.

2. Ascending Pathways

Cell bodies in the dorsal root ganglia of the spinal cord whose peripheral processes detect light touch,

Figure 2 (continued)

vibration, and joint position sense have a centrally directed process that passes into the ipsilateral dorsal column of the spinal cord, ascends to the dorsal region of the medulla oblongata, and synapses on a second-order sensory neuron in the gracile and cuneate nuclei (Figs. 2a and 2b). The axon of the second-order neuron passes ventrally and medially before crossing to the other side and ascending through the medulla, pons, and midbrain in a distinctive pathway known as the medial lemnisus (Figs. 2a-2g). Lemniscal axons synapse in the thalamus.

In contrast, cell bodies in the dorsal root ganglia of the spinal cord whose peripheral processes detect pain and temperature sensation have a centrally directed axon which synapses in the dorsal horn near the level of entry. The axon of the second-order neuron passes ventrally and medially, crossing the midline just ventral to the central canal. The axon then ascends as the spinothalamic tract, through the spinal cord, medulla oblongata, pons, and midbrain, before synap-sing in the thalamus.

Other bundles of axons ascend as components of the spinocerebellar pathways, traversing the lateral part of the medulla oblongata and entering the cerebellum via the inferior or superior cerebellar peduncles. In the medulla oblongata, an additional ascending pathway to the cerebellum consists of axons of inferior olivary neurons (Figs. 2b and 2c). These olivocerebellar axons pass medially from the olive, crossing the midline and entering the laterally positioned inferior cerebellar peduncle en route to the cerebellum.

Table I

Classification of Brain Stem Neurons

Somatic motoneurons

Peripheral striated muscle innervated

Cranial nerve

Brain stem nucleus

Extraocular muscles

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