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

Medulla oblongata

The pyramids are alongside the anterior median fissure. Just above the spinomedullary junction, the fissure is invaded by the decussation of the pyramids, where fibers of the two pyramids intersect while crossing the midline. Lateral to the pyramid is the olive, and behind the olive is the inferior cerebellar peduncle. Attached between pyramid and olive is the hypoglossal nerve (XII). Attached between olive and inferior cerebellar peduncle are the glossopharyngeal (IX), vagus (X), and cranial accessory (XIc) nerves. The spinal accessory nerve (XIs) arises from the spinal cord and runs up through the foramen magnum to join the cranial accessory.

Dorsal view (Figure 3.2B)

The roof or tectum of the midbrain is composed of four colliculi. The superior colliculi belong to the visual system, and the inferior colliculi belong to the auditory system. The trochlear nerve (IV) emerges below the inferior colliculus on each side.

The diamond-shaped fourth ventricle lies behind the pons and upper medulla oblongata, under cover of the cerebellum. The upper half of the diamond is bounded by the superior cerebellar peduncles, which are attached to the midbrain. The lower half is bounded by the inferior cerebellar peduncles, which are attached to the medulla oblongata. The middle cerebellar peduncles enter from the pons and overlap the other two.

Near the midline in the midregion of the ventricle is the facial colliculus, which is created by the facial nerve curving around the nucleus of the abducens nerve. The vestibular area, and the vagal and hypoglossal trigones overlie the corresponding cranial nerve nuclei. The obex is the inferior apex of the ventricle.

Below the fourth ventricle, the medulla oblongata shows a pair of gracile tubercles flanked by a pair of cuneate tubercles.

Sectional views

In the midbrain, the central canal of the embryonic neural tube is represented by the aqueduct. Behind the pons and upper medulla oblongata (Figure 3.3), it is represented by the fourth ventricle, which is tent-shaped in this view. The central canal resumes at midmedullary level; it is continuous with the central canal of the spinal cord, although movement of cerebrospinal fluid into the cord canal is negligible.

Figure 3.3 (A) Sagittal section of the brainstem. Cerebrospinal fluid descends along the aqueduct into the fourth ventricle and emerges into the subarachnoid space via three apertures including the median aperture containing the arrow. (B) Transverse section of midbrain at the level indicated in (A). The substantia nigra separates the tegmentum from the two crura cerebri. The interpeduncular fossa is so called because the entire midbrain is said to be made up of a pair of cerebral peduncles. PAG, periaqueductal gray matter.

The intermediate region of the brainstem is called the tegmentum, which in the midbrain contains the paired red nucleus. Ventral to the tegmentum in the pons is the basilar region. Ventral to the tegmentum in the medulla oblongata are the pyramids.

The tegmentum of the entire brainstem is permeated by an important network of neurons, the reticular formation. The tegmentum also contains ascending sensory pathways carrying general sensory information from the trunk and limbs. Illustrated in Figures 3.4–3.6 are the posterior column–medial lemniscal (PCML) pathways, which inform the brain about the position of the limbs in space. At spinal cord level, the label PCML is used here because these pathways occupy the posterior columns of white matter in the cord. In the brainstem, the label PCML is used because they continue upward as the medial lemnisci.

Figure 3.4 Transverse sections of midbrain. (A) At level of superior colliculi. (B) At level of inferior colliculi. In this and following diagrams, the corticospinal tract (CST) and posterior column–medial lemniscal (PCML) pathway connected to the left cerebral hemisphere are highlighted. PAG, periaqueductal gray matter.

Figure 3.5 Transverse sections of pons. (A) Upper pons. (B) Lower pons. SCP, MCP, ICP, superior, middle, inferior cerebellar peduncles. CST, corticospinal tract; PCML, posterior column–medial lemniscal pathway.

Figure 3.6 Transverse sections of medulla oblongata. (A) Level of inferior olivary nucleus (ION). (B) Level of sensory decussation. (C) Level of motor decussation. CST, Corticospinal tract; ICP, inferior cerebellar peduncle; PCML, posterior column–medial lemniscal pathways.

The most important motor pathways from a clinical standpoint are the corticospinal tracts (CSTs), the pathways for execution of voluntary movements. The CSTs are placed ventrally, occupying the crura of the midbrain, the basilar pons, and the pyramids of the medulla oblongata.

Note that, in the medulla oblongata, the PCML and CST decussate: one of each pair intersects with the other to gain the contralateral (opposite) side of the neuraxis (brainstem–spinal cord). The four most important decussations are illustrated in Box 3.1.

In the following account of seven horizontal sections of the brainstem, the positions of the cranial nerve nuclei are not included.

Midbrain (Figure 3.4)

The main landmarks have already been identified. The medial lemniscal component of PCML occupies the lateral part of the tegmentum (upper section), on its way to a sensory nucleus of the thalamus immediately above this level. The CST has arisen in the cerebral cortex, and it is descending in the midregion of the cerebral crus on the same side.

Figure 3.7 Horizontal section of fixed cadaver, taken at level of the midbrain. The cerebellum is seen through the tentorial notch.

(From Liu et al. 2003, with permission of Shantung Press of Science and Technology.)

The decussation of the superior cerebellar peduncles straddles the midline at the level of the inferior colliculi (lower section).

Pons (Figure 3.5)

In the upper section, the cavity of the fourth ventricle is bordered laterally by the superior cerebellar peduncles, which are ascending (arrows) to decussate in the lower midbrain. In the floor of the ventricle is the central gray matter. The medial lemniscus occupies the ventral part of the tegmentum on each side. The basilar region contains millions of transverse fibers, some of which separate the CST into individual fascicles. The transverse fibers enter the cerebellum via the middle cerebellar peduncles and appear to form a bridge (hence, pons) connecting the cerebellar hemispheres. But the individual transverse fibers arise on one side of the pons and cross to enter the contralateral cerebellar hemisphere. The transverse fibers belong to the giant corticopontocerebellar pathway, which travels from the cerebral cortex of one side to the contralateral cerebellar hemisphere, as depicted in Box 3.1.

The lower section contains the inferior cerebellar peduncle, about to plunge into the cerebellum. The CST bundles have reunited prior to entering the medulla oblongata.

Medulla oblongata (Figure 3.6)

Follow the CST from above down. It descends through sections A and B as the pyramid. In C, it intersects with its opposite number in the motor decussation, prior to entering the contralateral side of the spinal cord.

Follow the PCML pathway from below upward. In section C, it takes the form of the gracile and cuneate fasciculi, known in the spinal cord as the posterior columns of white matter. In section B, the posterior columns terminate in the gracile and cuneate nuclei. From these nuclei, fresh sets of fibers swing around the central gray matter and intersect with their opposite numbers in the sensory decussation. Having crossed the midline, the fibers turn upward. In section A, they form the medial lemniscal component of PCML.

On the left side of the medulla is shown the posterior spinocerebellar tract. Its function (non-conscious) is to inform the cerebellum of the state of activity of the ipsilateral (same side) skeletal muscles in the trunk and limbs.

The upper third of the medulla shows the wrinkled inferior olivary nucleus, which creates the olive of gross anatomy.

Sections of brainstem in situ are in Figures 3.7–3.11.

Figure 3.8 Enlargement from Figure 3.7.

Figure 3.9 Horizontal section taken at the level of upper pons. The fourth ventricle is slot-like at this level; on each side is a superior cerebellar peduncle traveling upward and medially from dentate nucleus toward the contralateral thalamus.

(From Liu et al. 2003, with permission of Shantung Press of Science and Technology.)

Figure 3.10 Horizontal section taken through the middle of the pons. (A) In axial brain scans, the pons would be in the position shown, i.e. in the roof of the fourth ventricle. (B) In standard anatomic descriptions including histologic sections (cf. Ch. 17), the pons occupies the floor of the fourth ventricle, as shown here. Note the massive size of the middle cerebellar peduncles.

(From Liu et al. 2003, with permission of Shantung Press of Science and Technology.)

Figure 3.11 Coronal section of brainstem and cerebellum at the level shown at top. Note that the section passes through the tegmentum of the midbrain. The spinal and trigeminal lemnisci are entering the posterior–posterolateral nuclei of thalamus. The periaqueductal gray matter is sectioned longitudinally; the aqueduct itself is seen below the third ventricle.

Spinal Cord

General features

The spinal cord occupies the upper two-thirds of the vertebral canal. Thirty-one pairs of spinal nerves are attached to it by means of anterior and posterior nerve roots (Figure 3.12A). The cord shows cervical and lumbar enlargements that accommodate nerve cells supplying the upper and lower limbs.

Figure 3.12 Spinal cord. On the left is an anterior view of the cord with nerve attachments enumerated. On the right are (A) cervical enlargement level, (B) thoracic level, and (C) lumbar enlargement level, showing the arrangement of the largest motor and sensory pathways in the white matter, namely the corticospinal tract (CST) and the posterior column–medial lemniscal pathway (PCML) comprising the gracile fasciculi.

Internal anatomy

In transverse sections, the cord shows butterfly-shaped gray matter surrounded by three columns or funiculi of white matter (Figure 3.12B): an anterior funiculus in the interval between the anterior median fissure and the emerging anterior nerve roots; a lateral funiculus between the anterior and posterior nerve roots; and a posterior funiculus between the posterior roots and the posterior median septum.

The gray matter consists of central gray matter surrounding a minute central canal, and anterior and posterior gray horns on each side. At the levels of attachment of the 12 thoracic and upper two or three lumbar nerve roots, a lateral gray horn is present as well. Posterior nerve roots enter the posterior gray horn, and anterior nerve roots emerge from the anterior gray horn.

Axons pass from one side of the spinal cord to the other in the anterior white and gray commissures deep to the anterior median fissure.

The CST descends the cord within the lateral funiculus. Its principal targets are neurons in the anterior gray horn concerned with activation of skeletal muscles. Special note: In Chapter 16, it will be seen that a small, anterior CST separates from the main bundle and descends within the anterior funiculus. Accordingly, the proper name of the bundle depicted here is the lateral CST.

In the cord, the PCML pathway is represented by the gracile and cuneate fasciculi. The fasciculi are composed of the central processes of peripheral sensory neurons supplying muscles, joints, and skin. Processes entering from the lower part of the body form the gracile (’slender’) fasciculus; those from the upper part form the cuneate (‘wedge-shaped’) fasciculus (Figures 3.13 and 3.14).

Figure 3.13 Midline sagittal section of fixed cadaver, displaying spinal cord and cauda equina in situ. It should be borne in mind that the cauda equina contains not only the motor and sensory nerve roots of the lumbosacral plexus supplying the lower limbs but also the autonomic motor nerves supplying smooth muscle of hindgut (sigmoid colon and rectum), bladder, uterus, and erectile tissues.

(From Liu et al. 2003, with permission of Shantung Press of Science and Technology.)

Figure 3.14 Remarkable photograph of coronal section of fixed cadaver, confirming the high level of commencement of the cauda equina as viewed from in front. In clinical context, this photograph is a reminder of the hazard to somatic (notably sciatic) and parasympathetic (notably to bladder and rectum) nerves incurred by crush fractures of lumbar vertebrae.

(From Liu et al. 2003, with permission of Shantung Press of Science and Technology.)

Cerebellum

The cerebellum is made up of two hemispheres connected by the vermis in the midline (Figure 3.15). The vermis is distinct only on the undersurface, where it occupies the floor of a deep groove, the vallecula. The hemispheres show numerous deep fissures, with folia between. About 80% of the cortex (surface gray matter) is hidden from view on the surfaces of the folia.