2 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 Chapters 15 and 16.

3 Get to grips with the nomenclature used for parts of the midbrain.

4 Become able to divide the 31 spinal nerves into five groups.

5 Relate the three cerebellar peduncles to the fourth ventricle as seen in cross-sections.

Box 3.1 Four decussations

Figure Box 3.1.1 (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.

Figure Box 3.1.1 (B) Afferents. The left parietal lobe constructs a map of the right hand in relation to the mouse, based on 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 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 gray 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.

The most significant deep tissue sensory organs are neuromuscular spindles (muscle spindles) contained within skeletal muscles. The primary afferents supplying the muscle spindles of the intrinsic muscles of the hand belong to large unipolar neurons whose axons (labeled M1) ascend ipsilaterally (on the same side of the spinal cord) within the posterior funiculus, as already seen in Figure 3.5. They synapse in the nucleus cuneatus in the medulla oblongata. The multipolar second-order neurons send their axons across the midline in the sensory decussation (seen in Figure 3.6).The axons ascend (M2) through pons and midbrain before synapsing on third-order neurons (M3) projecting from thalamus to sensory cortex.

PCML, posterior column–medial lemniscal pathway.

Figure Box 3.1.1 (C) Cerebellar control. Before the brain sends an instruction to click the mouse, it requires information on the current state of contraction of the muscles. This information is constantly being sent from the muscles to the cerebellar hemisphere on the same side. As indicated in the diagram, M1 neurons are dual-purpose sensory neurons. At their point of entry to the posterior funiculus, they give off a branch, here labeled C1, to a spinocerebellar neuron that projects (C2) to the ipsilateral cerebellum. From here, a cerebellothalamic neuron (C3) is shown projecting across the midbrain to the contralateral thalamus, where a further neuron (C4) relays information to the hand area of the motor cortex in the precentral gyrus.

(D) Motor output. Multipolar neurons in the left motor cortex now fire impulses along the upper motor neurons that constitute the corticospinal tract (CST), which crosses to the opposite side in the motor decussation, as already noted in Figure 3.6. The CST synapses on lower motor neurons projecting from the anterior horn of the spinal gray matter to activate flexor muscles of the index finger and local stabilizing muscles.

Note that a copy of the outgoing message is sent to the right cerebellar hemisphere by way of transverse fibers of the pons (TFP) originating in multipolar neurons located on the left side of the pons.

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 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 (Figures 3.1 and 3.2A)

Midbrain

The ventral surface of the midbrain shows two massive 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 (III) emerges from the medial surface of the peduncle. The trochlear nerve (IV) passes between the peduncle and the uncus.

Figure 3.1 Ventral view of the brainstem in situ.

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

Pons

The bulk of the pons is composed of transverse fibers 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 plunges 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 (see Table 3.1).

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