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Midbrain, Hindbrain, Spinal Cord


Study Guidelines

  • 1.

    Be able to recognise and label the locations of the ascending and descending pathways in the horizontal sections of the brainstem and spinal cord.

  • 2.

    Be able to describe or trace the four decussations that occur as part of a simple motor action. ( Box 3.1 deserves special attention because it indicates why certain pathways cross the midline and others do not. The brainstem crossings are formally addressed in Chapter 15 , Chapter 16 .)

    Box 3.1
    Four Decussations

    Fig. 3.7, (A) The stage is set. The subject’s right hand is about to click a mouse while the eyes are directed elsewhere. The coronal section identifies key structures. (B) Afferents. The left parietal lobe constructs a map of the right hand in relation to the mouse, based on the information sent to the left somatic sensory cortex (postcentral gyrus) from the skin and deep tissues. The information is relayed by three successive sets of neurons from the skin and by another set of three from the deep tissues. The first set in each case is composed of first-order or primary afferent neurons. These neurons are called pseudo-unipolar , because each axon emerges from a single point (or pole) of the cell body and divides in a T-shaped manner to provide continuity of impulse conduction from tissue to central nervous system. The primary afferent neurons terminate by forming contacts known as synapses on the multipolar (more or less star-shaped) cells of the second-order (secondary) set. The axons of the second-order neurons project across the midline before turning up to terminate on third-order (tertiary) multipolar neurons projecting to the postcentral gyrus. Primary afferents activated by contacts with the skin of the hand (S1) terminate in the posterior horn of the grey matter of the spinal cord. Second-order cutaneous afferents (S2) cross the midline in the anterior white commissure and ascend to the thalamus within the spinothalamic tract (STT), to be relayed by third-order neurons to the hand area of the sensory cortex.

  • 3.

    Identify the major ‘constituents’ of the midbrain, pons, and medulla (prominent structures) and the location of the dorsal column–medial lemniscal pathways and corticospinal tracts and their decussations, as well as the superior cerebellar peduncles.

  • 4.

    List the spinal cord segments and describe the anatomic reason for the prominent enlargements.

  • 5.

    Describe the relationships of the three cerebellar peduncles to the fourth ventricle as seen in cross-sections.

The midbrain connects the diencephalon to the hindbrain. As explained in Chapter 1 , the hindbrain is made up of the pons, medulla oblongata, and cerebellum. The medulla oblongata joins the spinal cord at the spinomedullary junction within the foramen magnum of the skull.

In this chapter, the cerebellum (part of the hindbrain) is considered after the spinal cord for the sake of continuity of motor and sensory pathway descriptions.

Brainstem

Ventral View ( Figs. 3.1 and 3.2A )

Midbrain

The ventral surface of the midbrain shows two cerebral peduncles bordering the interpeduncular fossa . The optic tracts wind around the midbrain at its junction with the diencephalon. Lateral to the midbrain is the uncus of the temporal lobe. The oculomotor nerve (CN III) exits the brainstem on the medial surface of the cerebral peduncle, whereas the trochlear nerve (CN IV) exits the brainstem from the lateral surface of the cerebral peduncle.

Fig. 3.1, Ventral view of the brainstem in situ.

Fig. 3.2, (A) Anterior and (B) posterior view of the brainstem.

Pons

The bulk of the pons is composed of transverse fibres (the pontocerebellar tract ) that raise numerous surface ridges. On each side, the pons is marked off from the middle cerebellar peduncle by the attachment of the trigeminal nerve (V). The middle cerebellar peduncle enters into the hemisphere of the cerebellum.

At the lower border of the pons are the attachments of the abducens (VI), facial (VII), and vestibulocochlear (VIII) nerves ( Table 3.1 ).

Medulla Oblongata

The pyramids are on either side of the anterior median fissure. Just above the spinomedullary junction, this fissure is traversed by the decussation of the corticospinal tracts , where the majority of the corticospinal fibres from the two pyramids decussate in the midline. Lateral to the pyramids are the olives while posterior to the olives lies the inferior cerebellar peduncle. Attached between the pyramid and the olive in the preolivary sulcus are the rootlets of the hypoglossal nerve (CN XII). Attached between the olive and inferior cerebellar peduncle in the postolivary sulcus are the glossopharyngeal (CN IX), vagus (CN X), and cranial accessory (CN XIc) nerves . The spinal accessory nerve (CN XIs) arises from the upper five spinal segments of the spinal cord and enters the foramen magnum to join the cranial accessory nerve.

Table 3.1
The Cranial Nerves.
Number Name
I Olfactory, enters the olfactory bulb from the nose
II Optic
III Oculomotor
IV Trochlear
V Trigeminal
VI Abducens
VII Facial
VIII Vestibulocochlear
IX Glossopharyngeal
X Vagus
XI Accessory
XII Hypoglossal

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