Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
Study Guidelines
- 1.
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 ( Fig. 17.1A,B ). From the basal plate the general somatic efferent (GSE) column supplies the striated muscles of the trunk and limbs and the general visceral efferent (GVE) column supplies preganglionic neurons of the autonomic system. From the alar plate, neurons in the general visceral afferent (GVA) column receive 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.
-
GSE column . Supplies the striated musculature of the orbit (via the oculomotor, trochlear, and abducens nerves) and tongue (via the hypoglossal nerve).
-
GVE column . Gives rise to the cranial parasympathetic system introduced in Chapter 13 . The target ganglia are the ciliary, pterygopalatine, submandibular, and otic ganglia in the head and neck and the vagal ganglia in the neck, thorax, and abdomen.
-
GVA column . Receives afferents from the visceral territory of the glossopharyngeal and vagus nerves.
-
GSA column . Receives afferents from the skin and mucous membranes, mainly over the trigeminal nerve from the skin and mucous membranes of the oronasofacial region, and the dura mater.
Three additional cell columns ( Fig. 17.1C,D ) serve branchial arch tissues and the inner ear, as follows.
-
Special visceral (branchial) efferent (SVE) column . Supplying efferents to the 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 ).
-
Special visceral afferent (SVA) column . Receives afferent fibres from the taste buds that develop from endodermal lining of branchial arches.
-
Special somatic afferent (SSA) column . Receives afferents from the vestibular (balance) and cochlear (hearing) organs of the inner ear.
Fig. 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) from the trunk and limbs to the contralateral cerebral hemisphere. It was also explained that corticospinal fibres of the pyramidal tract arising from motor areas of the cerebral cortex synapse on the 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 true for the brainstem . The descending motor fibres terminating in the brainstem are referred to as corticobulbar . As shown in Fig. 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 the motor nucleus of the 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 extraocular motor ocular nuclei. Why? Because these nuclei do not receive a direct corticobulbar supply. Instead their predominantly contralateral supply originates from the brainstem 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 ( Fig. 17.4 ).
The general layout of the reticular formation ( Fig. 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.
Fig. 17.6 depicts the main components of the medial longitudinal fasciculus (MLF) . This fibre bundle extends the entire length of the brainstem, originating from different structures and serving different functions at different levels of the brainstem. 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
This presentation departs from the traditional method used in this textbook which is to describe photographs or diagrams at successive brain 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.
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 caudal levels rostrally to the thalamus (commencing with Fig. 17.10 ) and efferent pathways can be followed caudally from rostral levels (commencing with (see Fig. 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 identified at each level. This is because occlusion of a small artery supplying arterial blood to the brainstem or a tumour developing in the brainstem will most likely affect the function of more than one neuronal system that is either present at the lesion site or is projecting axons through the affected organ. At each level, miniature replicas of the diagrams in Fig. 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 would be to initially browse through the sections and then recheck the location of individual items during later reading.
Overview of Three Pathways in the Brainstem
Fig. 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 that subsequently give rise to axons contributing to the central tegmental tract (CTT), 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.
The corticospinal tract , discussed in Chapter 16 , is shown in Fig. 17.9 . Also included are corticobulbar projections to the facial and hypoglossal nuclei.
C1 Segment of the Spinal Cord ( Fig. 17.10 )
Blue
The gracile and cuneate fasciculi constitute the dorsal column of the spinal cord. Their axons are ipsilateral central processes of dorsal root ganglion cells whose peripheral processes receive information from the large tactile nerve endings in the skin, including Meissner and Pacinian corpuscles, and from neuromuscular spindles and Golgi tendon organs. The fasciculi terminate ipsilaterally in the gracile and cuneate nuclei (see Fig. 17.12 ).
Unlike the dorsal column, the anterolateral tract contains crossed axons. As indicated in Fig. 15.10 , the second-order neurons traverse the ventral white commissure at all segmental levels before ascending to the thalamus.
The dorsolateral tract of Lissauer contains fine first-order sensory fibres that divide and span several cord segments prior to synapsing in the dorsal grey horn.
The spinal (descending) tract of the trigeminal nerve contains nociceptive and thermoceptive first-order neurons about to synapse in the dorsal grey horn of segments C2 and C3.
Red
The large red area on the left side of the cord represents the (crossed) lateral corticospinal tract . The ventral corticospinal tract has not yet crossed.
Anterior motor neurons projecting from the ventral grey horn occupy the ventral root of spinal nerve C1 and the uppermost root of the spinal accessory nerve.
The lateral vestibulospinal tract (uncrossed) descends in the ventral funiculus to activate antigravity muscles ipsilaterally. The medial vestibulospinal tract (partly crossed) descends in the caudal MLF to activate head-righting reflexes.
Lateral to the ventral grey horn is the autonomic projection from the hypothalamus. Its functions include activation of sacral parasympathetic neurons causing contraction of the bladder and rectum.
Green
The dorsal spinocerebellar tract (from the posterior thoracic/Clarke nucleus) conveys high-speed unconscious proprioception from the ipsilateral trunk and limbs, notably from muscle stretch receptors.
The pontine reticulospinal tract is descending ipsilaterally to supply motor neurons innervating antigravity muscles. The medullary reticulospinal tract supplies flexor motor neurons.
Spinomedullary Junction ( Fig. 17.11 )
Blue
The gracile and cuneate fasciculi continue to occupy the dorsal white column, with the spinal tract and nucleus of the trigeminal nerve alongside. The position of the spinal lemniscus is also unchanged.
Red
The dominant feature in this diagram is the decussation of the pyramids . Observe the right pyramid: 80% of its fibres cross the midline by decussating with its opposite numbers, to form the left lateral corticospinal tract ; 10% form the ipsilateral ventral corticospinal tract which will cross lower down; and 10% remain ipsilateral among the fibres of the right lateral corticospinal tract.
Within the lateral tegmentum is the lateral vestibulospinal tract . The red spots in the medial longitudinal fasciculi represent the medial vestibulospinal tract , which descends bilaterally within them.
Green
The dorsal spinocerebellar tract is nearing its point of departure into the inferior cerebellar peduncle. The paramedian and lateral reticular formation occupy the tegmentum.
Middle of the Medulla Oblongata ( Fig. 17.12 )
Blue
The left dorsal column of the spinal cord ascends to the mid-medulla before turning ventrally. The gracile fasciculus synapses in the gracile nucleus and the cuneate fasciculus in the cuneate nucleus . Second-order neurons give rise to internal arcuate fibres , which cross over in the sensory decussation and then ascend (to the ventroposterolateral (VPL) nucleus of the thalamus) as the medial lemniscus . The anterolateral system (ALS) contains the neospinothalamic, spinoreticular, and spinotectal tracts. The vestibulospinal tract is descending from the vestibular nucleus to the spinal cord.
Red
The pyramid contains the corticospinal tract prior to the pyramidal decussation; the hypoglossal nerve emerges at its lateral edge. Lateral to the XII nucleus is the dorsal nucleus of vagus nerve . The ‘cranial’ accessory nerve has emerged from the nucleus ambiguus; it will be incorporated into the vagus below the jugular foramen. The dorsal longitudinal fasciculus (DLF) contains autonomic fibres descending from the hypothalamus to the spinal cord.
Green
The projections from inferior and accessory olivary nuclei to the contralateral cerebellar cortex are shown. The paramedian and lateral reticular formation and the inferior cerebellar peduncle are seen again now.
You're Reading a Preview
Become a Clinical Tree membership for Full access and enjoy Unlimited articles
If you are a member. Log in here