As shown in Figures 1, 2, 3, neurokinin-like immunoreactive (NK-ir) structures are widely distributed throughout the human brainstem. In general, the distribution of the immunoreactive structures (fibers and cell bodies), as well as the density of such structures, were quite similar in the four brainstems studied. In the three brainstem regions (medulla oblongata, pons and mesencephalon), the highest density of immunoreactive structures was generally observed in their dorsal parts. In addition, the clusters of cell bodies containing NK were almost always observed in the dorsal part of the human brainstem. Finally, in general, the NK-ir cell bodies observed in the human brainstem were large (showing the longest diameter between 20–55 μm).
Distribution of NK-ir structures in the human medulla oblongata
NK-ir cell bodies
A low density of immunoreactive cell bodies containing NK was observed caudally in the medullary central gray (Figs. 1A; 4A), whereas more rostrally (Fig. 1B) such clusters of NK-ir perikarya showed a moderate density. At the latter level (Fig. 1B), a moderate density was also observed in the dorsal motor nucleus of the vagus and a low density below the nucleus of the solitary tract. More rostrally (Fig. 1C) three populations of immunoreactive cell bodies were observed: the first located along the midline (high density); the second in the dorsal motor nucleus of the vagus (high density)(Fig. 5A,5B); and the third in the reticular formation (nucleus reticularis gigantocellularis included)(moderate density), above the dorsal accessory olivary nucleus and the central tegmental tract (Fig. 6A,6B). In addition, a moderate density of immunoreactive cell bodies containing NK was observed in the raphe obscurus (not shown in Figures).
NK-ir fibers
A high density of immunoreactive fibers containing NK was observed in the following regions: in the caudaldorsal part of the cuneate nucleus (Fig. 1A), the medullary central gray (Figs. 1A,1B; 4A), the dorsal motor nucleus of the vagus (Figs. 1A,1B; 5A,5B), the nucleus of the solitary tract (Figs. 1B,1C; 5A), the raphe obscurus, the spinal trigeminal nucleus (gelatinosa part) (Figs. 1A,1B,1C; 7B), the inferior salivatory nucleus, the anterolateral system (Fig. 1A,1B,1C), the nucleus of the accessory nerve (Fig. 1A), the lateral part of the reticular formation (Figs. 1B,1C; 7A), the nucleus ambiguus (Fig. 1B,1C), the lateral reticular nucleus (Fig. 1B,1C), and the dorsal accessory olivary nucleus (Figs. 1C; 6B). A moderate density was found in the gracile nucleus (Fig. 1A,1B), the caudal-ventral part of the cuneate nucleus (Fig. 1A), as well as in the whole nucleus (Fig. 1B), the caudal end of the motor hypoglossal nucleus (Fig. 1A), the rubrospinal tract (Fig. 1A,1B,1C), the medial C1 cells (Fig. 1A), the spinal trigeminal tract (Fig. 1B,1C), the medial part of the reticular formation (nucleus reticularis gigantocellularis included) (Fig. 1B,1C), the motor hypoglossal nucleus (Fig. 1B,1C), the medial accessory olivary nucleus (Fig. 1B), the medial vestibular nucleus (Fig. 1C) and in the spinal (inferior) vestibular nucleus (Fig. 1C).
A low density of immunoreactive fibers containing NK was found in the fasciculus gracilis (Fig. 1A), the fasciculus cuneatus (Fig. 1A,1B), the spinal trigeminal tract (Fig. 1A), the reticulospinal tract (Fig. 1A,1B,1C), the dorsal spinocerebellar tract (Fig. 1A), the vestibulospinal tract (Fig. 1A,1B), the spinal trigeminal nucleus (magnocellular part) (Figs. 1A; 7B), the pyramidal tract (Fig. 1A,1B,1C), the lateral cuneate nucleus (Fig. 1B,1C), the restiform body (Fig. 1B,1C), the medial lemniscus (Fig. 1B,1C), the principal part of the inferior olivary nucleus (Fig. 1B,1C), and in the midline (Fig. 1C), whereas single immunoreactive fibers were observed in the ventral spinocerebellar tract (Fig. 1A,1B,1C), the medial longitudinal fascicle (Fig. 1A,1B,1C), the glossopharyngeal nerve, the tectospinal tract (Fig. 1A,1B,1C), the central tegmental tract(Fig. 1B,1C), the dorsal part of the medial longitudinal fascicle(Fig. 1B,1C), the medial accessory olivary nucleus (Fig. 1C), the solitary tract (Figs. 1C; 5A), and in the glossopharyngeal fibers that join the spinal trigeminal tract.
Distribution of NK-ir structures in the human pons
NK-ir cell bodies
We observed a moderate density of immunoreactive cell bodies in the caudal (Fig. 2A) and in the rostral (close to the central tegmental tract)(Fig. 2C) reticular formation, between the superior cerebellar peduncle and the anterolateral system, as well as in the tegmental central gray (Fig. 2C). In the nucleus prepositus hypoglossi (Fig. 2A), a low density of immunoreactive cell bodies was found.
NK-ir fibers
In the lateral part (Fig. 2A,2B) and in the medial-most area (Fig. 2C) of the reticular formation, the nucleus of the solitary tract (Fig. 2A), the superior salivatory nucleus (Fig. 2A), and in the tegmental central gray (Fig. 2B,2C) a high density of NK-ir fibers was visualized. A moderate density was observed in the following regions: the rubrospinal tract (Fig. 2A,2B), the anterolateral system (Fig. 2A,2B,2C), the medial part of the reticular formation (Fig. 2A,2B), the medial parabrachial nucleus (close to the superior cerebellar peduncle)(Fig. 2B), the spinal trigeminal nucleus (Fig. 2A), the superior vestibular nucleus, the nucleus prepositus hypoglossi (Fig. 2A), and the nuclei of the lateral lemniscus (Fig. 2B,2C), whereas a low density of immunoreactive fibers containing NK was found in the spinal trigeminal tract(Fig. 2A), the restiform body (Fig. 2A), the medial lemniscus (Fig. 2A,2B,2C), the central tegmental tract (Fig. 2A,2B,2C), the lateral and medial vestibular nuclei (Fig. 2A), the motor facial nucleus (Fig. 2A), the superior olive (Fig. 2A,2B), the corticospinal fibers (Fig. 2A,2B,2C), the pontine nuclei (Fig. 2A,2B,2C), the abducens nucleus (Fig. 2A), the principal trigeminal nucleus (Fig. 2B), the caudal end of the chief sensory nucleus (Fig. 2B), the superior cerebellar peduncle (Fig. 2A,2B,2C) and in the middle cerebellar peduncle (Fig. 2C), Finally, single immunoreactive fibers were visualized in the ventral spinocerebellar tract (Fig. 2A,2B), the medial longitudinal fascicle (Fig. 2A,2B,2C), the tectospinal tract (Fig. 2A,2B,2C), the dorsal part of the medial longitudinal fascicle (Fig. 2A,2B), the solitary tract(Fig. 2A), the internal genu of the facial nerve (Fig. 2A), the facial nerve (Fig. 2A), the trapezoid body (Fig. 2A), the trapezoid nucleus(Fig. 2A), the pontocerebellar fibers (Fig. 2A,2B,2C), the ventral trigeminothalamic tract(Fig. 2A,2B,2C), the juxtarestiform body (Fig. 2A), and the trigeminal nerve (Fig. 2C).
Distribution of NK-ir structures in the human mesencephalon
NK-ir cell bodies
In the mesencephalon two populations of cell bodies containing NK were observed. The first (high density) was located above the periaqueductal gray and the central nucleus of the inferior colliculus and extended into the latter nucleus (Figs. 3A; 8A,8B; 9A,9B). The second was observed in the periaqueductal gray (high density) (Figs. 3B; 10A) and in the reticular formation (low density) (Fig. 3B). In the caudal and rostral regions of the periaqueductal gray a low density of NK-ir perikarya was found in the ventromedial part of the nucleus (Fig. 3A,3C).
NK-ir fibers
A high density of NK-ir fibers was found above the periaqueductal gray and the central nucleus of the inferior colliculus (Fig. 3A), the reticular formation (Fig. 3A,3B,3C), the periaqueductal gray (Figs. 3A,3B,3C; 10A), the lateral zone of the inferior colliculus (Fig. 3A), the lateral lemniscus(Fig. 3A), the dorsal trigeminothalamic tract (Fig. 3A,3B,3C), the superior colliculus (layers 2–6)(Figs. 3B,3C; 10B), the substantia nigra(Fig. 3B,3C), and in the pallidonigral, nigrostriatal and corticonigral fibers (Fig. 3B,3C); a moderate density in the spinotectal tract (Fig. 3A,3B,3C), the spinothalamic tract (Fig. 3A,3B,3C), the ventral trigeminothalamic tract(Fig. 3A,3B,3C), the superior cerebellar peduncle (Fig. 3A), the dorsal part of the pontine nuclei (Fig. 3B), the interpeduncular nucleus(Fig. 3B,3C), and the rubrospinal tract (Fig. 3C).
In the central nucleus of the inferior colliculus (Figs. 3A; 8B; 9A,9B), the medial lemniscus (Fig. 3A,3B,3C), the central tegmental tract (Fig. 3A,3B,3C), the corticospinal fibers (Fig. 3A), the pontine nuclei (Fig. 3A), the crus cerebri(Fig. 3A,3B,3C), the red nucleus (Fig. 3C), the superior colliculus (layer l)(Figs. 3B; 10B), the superior cerebellar peduncle (Fig. 3C) a low density of NK-ir fibers was found, whereas single fibers were observed in the medial longitudinal fascicle (Fig. 3A,3B,3C), the tectospinal tract (Fig. 3A,3B), the pontocerebellar fibers (Fig. 3A), the trochlear nerve (Fig. 3B), the oculomotor nucleus(Fig. 3C), the dorsal and ventral tegmental decussations (Fig. 3C), and in the oculomotor nerve(Fig. 3C).