Brainstem Summary

Keywords

corticospinal tract, pyramid, medial lemniscus, spinothalamic tract, medial longitudinal fasciculus, reticular formation, periaqueductal gray, substantia nigra

Chapter Outline
Caudal Medulla, 97
Rostral Medulla, 98
Caudal Pons, 99
Rostral Pons, 100
Caudal Midbrain, 101
Rostral Midbrain, 102

The previous four chapters presented various aspects of the brainstem and its cranial nerves bit by bit. This chapter summarizes the major points, using as a vehicle the same series of drawings of brainstem sections used in Chapter 11 but with additional structures and brief descriptions added. A few of the structures (e.g., substantia nigra) are dealt with more fully in later chapters.

Caudal Medulla

1.

Nucleus gracilis. Second-order somatosensory neurons whose axons cross at this level to form the leg portion of the medial lemniscus.
2.

Fasciculus cuneatus. Uncrossed branches of primary afferents, carrying tactile and proprioceptive information from the arm.
3.

Nucleus cuneatus. Second-order somatosensory neurons whose axons cross at this level to form the arm portion of the medial lemniscus.
4.

Nucleus of the solitary tract, surrounding the solitary tract. This is much like a posterior column system for information from viscera and taste buds: central branches of afferents from cranial nerves VII, IX, and X travel through the tract to reach the nucleus. Only information from viscera reaches this caudal level.
5.

Dorsal motor nucleus of the vagus. Preganglionic parasympathetic neurons for thoracic and abdominal viscera.
6.

Central canal. Continuous rostrally with the fourth ventricle and caudally with the central canal of the spinal cord.
7.

Spinal trigeminal tract. Primary afferents from the ipsilateral side of the face, at this caudal level conveying information about pain and temperature.
8.

Spinal trigeminal nucleus. At this caudal level, second-order pain and temperature neurons whose axons cross the midline and join the anterolateral pathway. Trigeminal pain and temperature afferents from the ipsilateral face reach the nucleus via the spinal trigeminal tract.
9.

Hypoglossal nucleus. Lower motor neurons for ipsilateral tongue muscles.
10.

Anterolateral pathway. Mostly crossed fibers of second-order spinal neurons conveying pain and temperature information to the thalamus (spinothalamic tract), reticular formation, and midbrain.
11.

Location of nucleus ambiguus. Lower motor neurons for laryngeal and pharyngeal muscles (also contains preganglionic parasympathetic neurons for the heart).
12.

Reticular formation.
13.

Medial longitudinal fasciculus (MLF). At this level, the fibers of the medial vestibulospinal tract.
14.

Medial lemniscus, the principal ascending pathway for tactile and proprioceptive information. Originates in the contralateral posterior column nuclei and terminates in the thalamus (ventral posterolateral nucleus [VPL]).
15.

Raphe nuclei. Widely projecting serotonergic neurons that collectively blanket the CNS. Those in caudal brainstem levels like this project mainly to the spinal cord.
16.

Pyramid. Corticospinal fibers from the ipsilateral precentral gyrus and adjacent areas of cerebral cortex.

Important brainstem structures caudal to this level (i.e., between here and the spinal cord): pyramidal decussation (at the spinomedullary junction), where most fibers of the pyramids cross to form the lateral corticospinal tracts.

Rostral Medulla

1.

Cochlear nuclei. Second-order auditory neurons that project bilaterally to the superior olivary nucleus and into the lateral lemniscus.
2.

Fourth ventricle. Continuous rostrally with the aqueduct and caudally with the central canal.
3.

Inferior cerebellar peduncle. By the time it enters the cerebellum, it contains crossed olivocerebellar fibers, vestibulocerebellar fibers, most spinocerebellar fibers, and other cerebellar afferents.
4.

Vestibular nuclei. Second-order neurons that form vestibulospinal tracts, much of the MLF, and projections to the cerebellum and thalamus.
5.

Nucleus of the solitary tract, surrounding the solitary tract. This is much like a posterior column system for information from viscera and taste buds: central branches of afferents from cranial nerves VII, IX, and X travel through the tract to reach the nucleus.
6.

Dorsal motor nucleus of the vagus. Preganglionic parasympathetic neurons for thoracic and abdominal viscera.
7.

Hypoglossal nucleus. Lower motor neurons for ipsilateral tongue muscles.
8.

Medial longitudinal fasciculus (MLF). At this level, the fibers of the medial vestibulospinal tract.
9.

Spinal trigeminal tract. Primary afferents from the ipsilateral side of the face, including those conveying information about pain and temperature to caudal parts of the spinal trigeminal nucleus.
10.

Spinal trigeminal nucleus. Some neurons at intermediate levels like this are interneurons in the blink reflex arc.
11.

Anterolateral pathway. Mostly crossed fibers of second-order spinal neurons conveying pain and temperature information to the thalamus (spinothalamic tract), reticular formation, and midbrain.
12.

Location of nucleus ambiguus. Lower motor neurons for laryngeal and pharyngeal muscles (also contains preganglionic parasympathetic neurons for the heart).
13.

Reticular formation.
14.

Inferior olivary nucleus. Gives rise to climbing fibers that end in the contralateral half of the cerebellum (see Chapter 20 ).
15.

Medial lemniscus, the principal ascending pathway for tactile and proprioceptive information. Originates in the contralateral posterior column nuclei and terminates in the thalamus (VPL).
16.

Raphe nuclei. Serotonergic neurons that at this level are one source of descending pain-control fibers to the spinal cord.
17.

Hypoglossal nerve fibers, on their way to ipsilateral tongue muscles.
18.

Pyramid. Corticospinal fibers from the ipsilateral precentral gyrus and adjacent areas of cerebral cortex.