15 Brainstem Summary
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.
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).
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 (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.
Rostral Medulla
1. Cochlear nuclei. Second-order auditory neurons that project bilaterally to the superior olivary nucleus and into the lateral lemniscus.
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.
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).
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).
Caudal Pons
1. Dentate nucleus. The deep nucleus connected to the lateral part of the cerebellar hemisphere and the source of most of the fibers in the superior cerebellar peduncle (see Chapter 20).
2. Interposed nucleus. The deep cerebellar nucleus connected to the medial part of the cerebellar hemisphere and the source of most of the remaining fibers in the superior cerebellar peduncle (see Chapter 20).
3. Fastigial nucleus. The deep cerebellar nucleus connected to the cerebellar vermis (see Chapter 20).
4. Superior cerebellar peduncle, just forming in the cerebellum. The principal output pathway from the cerebellum.
6. Vestibular nuclei. Second-order neurons that form vestibulospinal tracts, much of the MLF, and projections to the cerebellum and thalamus.
7. Abducens nucleus. Lower motor neurons for the ipsilateral lateral rectus, as well as the interneurons that project through the contralateral MLF to medial rectus motor neurons.
8. Internal genu of the facial nerve. Facial nerve fibers, most of them on their way to ipsilateral muscles of facial expression, that hook around the abducens nucleus before leaving the brainstem.
9. Medial longitudinal fasciculus (MLF). At this level, fibers from vestibular nuclei and abducens interneurons, active in coordinating eye movements.
10. 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.
11. Spinal trigeminal nucleus. Some primary afferents of the spinal trigeminal tract, particularly those carrying tactile information, end at this relatively rostral level.
14. Raphe nuclei. Widely projecting serotonergic neurons that collectively blanket the CNS. Those in intermediate brainstem levels like this are one source of descending pain-control fibers to the spinal cord, and also project to other brainstem levels and the cerebellum.
15. Middle cerebellar peduncle. Fibers from contralateral pontine nuclei that end as mossy fibers (see Chapter 20) in all areas of cerebellar cortex.
17. Anterolateral pathway. Mostly crossed fibers of second-order spinal neurons conveying pain and temperature information to the thalamus (spinothalamic tract), reticular formation, and midbrain. At this level, it also includes a contribution from the spinal trigeminal nucleus.
18. Superior olivary nucleus. First site of convergence of fibers representing the two ears and the source of many of the fibers of the lateral lemniscus.
19. Medial lemniscus, the principal ascending pathway for tactile and proprioceptive information. Originates in the contralateral posterior column nuclei and terminates in the thalamus (VPL).
21. Pontine nuclei and pontocerebellar fibers. Efferents from the cerebral cortex descend through the cerebral peduncle to reach ipsilateral pontine nuclei, whose axons cross in the basal pons and form the contralateral middle cerebellar peduncle.
Important brainstem structures between Figures 15-4 and 15-3, near the pontomedullary junction: cochlear nuclei (just beginning in Fig. 15-3), the second-order neurons of the auditory pathway; inferior cerebellar peduncle enters the cerebellum; attachment points of the abducens, facial, and vestibulocochlear nerves.
Rostral Pons
1. Lateral lemniscus. Ascending auditory fibers from the cochlear and superior olivary nuclei, representing both ears.
4. Periventricular gray. Continuous with the periaqueductal gray, the site of origin of the descending pain control pathway that relays in raphe nuclei of the rostral medulla and caudal pons (among other connections).
6. Anterolateral pathway. Mostly crossed fibers of second-order spinal neurons conveying pain and temperature information to the thalamus (spinothalamic tract), reticular formation, and midbrain. At this level, it also includes a contribution from the spinal trigeminal nucleus.
7. Medial lemniscus, the principal ascending pathway for tactile and proprioceptive information. Originates in the contralateral posterior column nuclei and terminates in the thalamus (VPL). At this level, it also includes a contribution from the trigeminal main sensory nucleus on its way to VPM.
9. Medial longitudinal fasciculus (MLF). At this level, fibers from vestibular nuclei and abducens interneurons, active in coordinating eye movements.
10. Raphe nuclei. Widely projecting serotonergic neurons that collectively blanket the CNS. Those in rostral brainstem levels like this project mainly to the cerebrum and cerebellum.
12. Pontine nuclei and pontocerebellar fibers. Efferents from the cerebral cortex descend through the cerebral peduncle to reach ipsilateral pontine nuclei, whose axons cross in the basal pons and form the contralateral middle cerebellar peduncle.
13. Corticospinal and corticobulbar fibers, from the ipsilateral precentral gyrus and adjacent cortical areas.
Important brainstem structures between Figures 15-5 and 15-4: trigeminal main sensory nucleus (midpons), the second-order neurons for large-diameter trigeminal afferents; trigeminal motor nucleus (midpons), the lower motor neurons for muscles of mastication.
Caudal Midbrain
1. Inferior colliculus. A stop in the auditory pathway where the lateral lemniscus ends and the brachium of the inferior colliculus begins, in turn carrying auditory information to the medial geniculate nucleus of the thalamus.
2. Periaqueductal gray. Site of origin of the descending pain control pathway that relays in raphe nuclei of the rostral medulla and caudal pons (among other connections).
3. Cerebral aqueduct, continuous rostrally with the third ventricle and caudally with the fourth ventricle.
4. Lateral lemniscus ending in the inferior colliculus, the next stop in the central auditory pathway.
6. Medial longitudinal fasciculus (MLF). At this level, fibers from vestibular nuclei and abducens interneurons, active in coordinating eye movements.
7. Raphe nuclei. Widely projecting serotonergic neurons that collectively blanket the CNS. Those in rostral brainstem levels like this project mainly to the cerebrum.
8. Anterolateral pathway. Mostly crossed fibers of second-order spinal neurons conveying pain and temperature information to the thalamus (spinothalamic tract), reticular formation, and midbrain. At this level, it also includes a contribution from the spinal trigeminal nucleus.
9. Medial lemniscus, the principal ascending pathway for tactile and proprioceptive information. Originates in the contralateral posterior column nuclei and terminates in the thalamus (VPL). At this level, it also includes a contribution from the trigeminal main sensory nucleus on its way to VPM.
11. Decussation of the superior cerebellar peduncles. Cerebellar output fibers crossing on their way to the red nucleus and thalamus (VL).
13. Corticospinal and corticobulbar fibers from the ipsilateral precentral gyrus and adjacent cortical areas.
14. Last few pontine nuclei. Source of pontocerebellar fibers that cross the midline and form the middle cerebellar peduncle.
Important brainstem structures between Figures 15-6 and 15-5: Trochlear nerves decussate and exit from the dorsal surface of the brainstem (pons-midbrain junction).
Rostral Midbrain
1. Superior colliculus. Involved in visual attention and eye movements. Inputs from the retina and visual cortex reach it through the brachium of the superior colliculus.
2. Periaqueductal gray. Site of origin of the descending pain control pathway that relays in raphe nuclei of the rostral medulla and caudal pons (among other connections).
3. Cerebral aqueduct, continuous rostrally with the third ventricle and caudally with the fourth ventricle.
4. Brachium of the inferior colliculus. Ascending auditory fibers on their way from the inferior colliculus to the medial geniculate nucleus of the thalamus.
5. Anterolateral pathway. Mostly crossed fibers of second-order spinal neurons conveying pain and temperature information to the thalamus (spinothalamic tract), reticular formation, and midbrain. At this level, it also includes a contribution from the spinal trigeminal nucleus.
6. Medial lemniscus, the principal ascending pathway for tactile and proprioceptive information. Originates in the contralateral posterior column nuclei and terminates in the thalamus (VPL). At this level, it also includes a contribution from the trigeminal main sensory nucleus on its way to VPM.
8. Last of the medial longitudinal fasciculus (MLF), conveying information from vestibular nuclei and abducens interneurons to the oculomotor nucleus.
9a. Edinger-Westphal nucleus; preganglionic parasympathetic component of the oculomotor nucleus, supplying the pupillary sphincter and the ciliary muscle.
10. Substantia nigra (reticular part). Inputs from the caudate nucleus and putamen, outputs to the thalamus and other sites (see Chapter 19).
11. Substantia nigra (compact part). Dopaminergic neurons whose axons terminate in the caudate nucleus and putamen.
12. Red nucleus. Interconnected with cerebellar circuitry, and the source of the small rubrospinal tract (see Chapter 20).
13. Ventral tegmental area. Dopaminergic neurons whose axons terminate in limbic and frontal cortical sites.
