Study guidelines
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This chapter largely deals with the identification of structures in transverse sections of the brainstem. A separate study guide is provided for the sections.
- 2.
Four brainstem decussations should recall those described in Box 3.1.
- 3.
Note that in magnetic resonance images, brainstem orientation is the reverse of the anatomic convention.
General arrangement of cranial nerve nuclei
In the thoracic region of the developing spinal cord, four distinct cell columns can be identified in the grey matter on each side ( Figure 17.1A, B ). In the basal plate the general somatic efferent (GSE) column supplies the striated muscles of the trunk and limbs. The general visceral efferent (GVE) column contains preganglionic neurons of the autonomic system. In the alar plate the general visceral afferent (GVA) column receives afferents from thoracic and abdominal organs. A general somatic afferent (GSA) column receives afferents from the body wall and the limbs.
In the brainstem these four cell columns can be identified; but they are fragmented, and not all contribute to each cranial nerve. Their connections are as follows.
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GSE column . Supplies the striated musculature of the orbit (via the oculomotor, trochlear, and abducens nerves) and tongue (via the hypoglossal nerve).
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GVE column . Gives rise to the cranial parasympathetic system introduced in Chapter 13 . The target ganglia are the ciliary, pterygopalatine, otic, and submandibular ganglia in the head and neck and the vagal ganglia in the neck, thorax, and abdomen.
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GVA column . Receives from the visceral territory of the glossopharyngeal and vagus nerves.
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GSA column . Receives from skin and mucous membranes, mainly in trigeminal nerve territory whose most important components are the skin and mucous membranes of the oronasofacial region, and the dura mater.
Three additional cell columns ( Figure 17.1C, D ) serve branchial arch tissues and the inner ear, as follows.
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Special visceral (branchial) efferent (SVE) column . To branchial arch musculature of the face, jaws, palate, larynx, and pharynx (via facial, trigeminal, glossopharyngeal, vagus, and cranial accessory nerves). These striated muscles have visceral functions in relation to food and air intake (hence, visceral ).
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Special visceral afferent (SVA) column . Receives from taste buds located in the endoderm lining the branchial arches.
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Special somatic afferent (SSA) column . Receives from vestibular (balance) and cochlear (hearing) organs in the inner ear.
Figure 17.2 shows the position of the various nuclei in a dorsal view of the brainstem.
In this chapter, details of the internal anatomy of the brainstem accompany nine representative transverse sections and their captions. Connections (direct or indirect) with the right cerebral hemisphere have been highlighted in accordance with information to be provided.
Background information
As stated earlier, exteroceptive and conscious proprioceptive information is transferred (by anterolateral and dorsal column–medial lemniscal pathways, respectively) from the left trunk and limbs to the right cerebral hemisphere. It was also explained that corticospinal fibres of the pyramidal tract arising from motor areas of the cerebral cortex supply contralateral ventral horn cells and give a small ipsilateral supply of similar nature, and that those arising from the parietal lobe project to the contralateral dorsal grey horn.
The same arrangement holds good for the brainstem . The descending motor fibres terminating in the brainstem are corticobulbar . As shown in Figure 17.3 , the motor nuclei receiving bilateral corticobulbar input are the motor nuclei of cranial nerve V, the motor nuclei of cranial nerve VII for the upper part of the face, and the nucleus ambiguus (cranial nerves IX and X). Note that the motor nucleus receiving totally crossed corticobulbar input is the motor nucleus of cranial nerve VII for the lower face, whereas the corticobulbar input to motor nucleus of hypoglossal nerve is more crossed than uncrossed. The corticobulbar input is entirely contralateral to the somatic sensory nuclei.
Absent from this figure are the three pairs of motor ocular nuclei. Why? Because these nuclei do not receive a direct corticobulbar supply. Instead their predominantly contralateral supply synapses on adjacent cell groups known as gaze centres that have the function of synchronising conjugate (conjoint parallel) movements of the eyes.
For a basic understanding of neural relationships in the brainstem, it is also essential to appreciate hemisphere linkages to the inferior olivary nucleus and to the cerebellum ( Figure 17.4 ).
The general layout of the reticular formation ( Figure 17.5 ) is borrowed from a figure in Chapter 24 devoted to this topic. It may be consulted when reading under this heading in successive descriptions.
Figure 17.6 depicts the main components of the medial longitudinal fasciculus (MLF) . This fibre bundle extends the entire length of the brainstem, changing its fibre composition at different levels. This figure, too, may be consulted during study of the brainstem sections to be described, following inspection of the C1 segment of the spinal cord.
Study guide
The presentation departs from the traditional method, which is to describe photographs or diagrams at successive levels in ascending order without highlights. In the present approach:
- 1.
The various nuclei and pathways are highlighted and labelled on the side having primary affiliation with the right cerebral hemisphere.
- 2.
The nuclei and pathways are colour coded by systems, for example red for motor, blue for sensory, and green for connections of the cerebellum and reticular formation.
- 3.
Highlighting together with colour coding makes it possible to study individual systems in vertical, ‘multiple window’ mode. The descriptive text related to the brainstem sections enables a logical sequence of study whereby afferent pathways can be followed from below upwards to the thalamic level (commencing with Figure 17.10 ) and efferent pathways can be followed from above downwards (commencing with Figure 17.19 ). It must be emphasised that, following study in the vertical mode, a horizontal approach must be undertaken, with the location of the various systems to be identified at each level. This is because occlusion of a small artery of supply to the brainstem may affect function in a patch that may include several distinct nuclei or pathways.
Figure 17.10
C1 segment of the spinal cord.
Figure 17.19
Midbrain–thalamic junction.
At each level, miniature replicas of the diagrams in Figure 17.7 are inserted to assist left–right orientation.
Special note: Readers unfamiliar with the internal anatomy of the brainstem may be disconcerted by the amount of new information contained in the series of sections to be described. It may be reassuring to know that all the information will come up again in later chapters. Therefore a sensible approach could be to undertake an initial browse through the sections and to recheck the location of individual items during later reading.
Overview of three pathways in the brainstem
Figure 17.8 shows the dorsal column–medial lemniscal and anterolateral pathways already described in Chapter 15 . Recall that the latter comprises the neospinothalamic tract serving pain and temperature and the reticulospinal tract serving dull aching pain. This pathway terminates in the reticular nuclei of the brainstem forming the central tegmental tract , which terminates in the intralaminar nuclei of the thalamus. The third component of the anterolateral system is the spinotectal tract that terminates in the midbrain (at the level of the superior colliculus) and is responsible for the coordination of head and eye movements.
