The periaqueductal grey area is an area of neuron cell bodies that surrounds the cerebral aqueduct. It is continuous with the grey substance of the third ventricle.

The tectum comprises the ‘roof’ or dorsal portion of the midbrain and contains the corpora quadrigemina, which includes the superior and inferior colliculi and all the substances that lie dorsal to the cerebral aqueduct in the midbrain area of the neuraxis. The superior colliculi are involved with visual reflexes, and project to the lateral geniculate bodies of the thalamus. The inferior colliculi are involved with reflexes associated with sound, and project to the medial geniculate bodies of the thalamus.

The tegmentum consists of the bulk of the matter of the brainstem and comprises the area ventral to the cerebral aqueduct and fourth ventricle. It contains the bulk of the brainstem nuclei and the reticular formation of the midbrain, pons, and medulla.

The substantia nigra is a broad layer of pigmented neurons that separates the basis from the tegmentum. It extends from the upper surface of the pons to the hypothalamus. It projects to and receives projections from the neostriatum, thalamus, subthalamic nucleus, superior colliculi, and the reticular system of the brainstem. The neuron of this region utilise dopamine as a neurotransmitter.

The oculomotor nuclei are the motor nuclei of the superior rectus, inferior rectus, and medial rectus muscles of the eye.

These bilateral structures are ovoid groups of nuclei composed of two different cell types, the magnocellular and parvocellular groups of neurons. The magnocellular neurons are large, multipolar cells located in the caudal area of the red nuclear mass. These neurons receive bilateral projections both from sensorimotor cortical areas via the corticorubral tracts and from collaterals via the corticospinal tracts. The cortical projections and their target neurons in the red nucleus are somatotopically organised. Axon projections from the magnocellular neurons form the rubrospinal tracts, which cross in the brainstem and project in a somatotopically organised fashion, mainly to the interneurons of the intermediate grey areas of the spinal cord. Some rubrospinal fibres terminate directly on ventral horn motor neurons as well. Some axons that form the rubrospinal tracts terminate on neurons in the pontomedullary reticular formation and the motor nuclei of various cranial nerves, forming the rubroreticular system and the rubrobulbar tracts, respectively (Brown 1974). Reciprocal, bilateral projections to the superior colliculi are also present and form the rubrotectal tracts (Fig. 13.6). The rubrospinal and corticospinal tracts form the lateral motor system of the spinal cord. The medial motor system is composed of the reticulospinal and vestibulospinal tracts.

Figure 13.6 Afferent and efferent projections of the red nucleus.

The parvocellular neurons of the red nucleus are small pyramidal- and spherical-shaped neurons, mostly located in the rostral areas of the red nuclear mass. These neurons receive projections from the dentate nucleus of the contralateral cerebellum, and from the ipsilateral globus pallidus pars externa, substantia nigra, and subthalamic nuclei. These neurons project to the ipsilateral thalamus (Fig. 13.6).

This structure is a highly myelinated axon tract that descends from the interstitial nucleus of Cajal in the lateral wall of the third ventricle through the midbrain, pons, and medulla to the spinal cord where it becomes continuous with the anterior intersegmental fasciculus. The MLF acts as a major communications conduit between all of the cranial nerve nuclei and all related structures including the reticular formations of the mesencephalon, pons, and medulla. The medial vestibulospinal tract axons project with the MLF to the spinal cord (Brodal et al. 1962). Other axons from the vestibular nuclei ascend in the MLF to more rostral structures including the extraocular cranial nuclei. The MLF is the first fibre tract in the developing embryo to become myelinated and probably acts as a major pathway providing stimulus to developing neurons in the early stages of embryonic development.

The medial lemniscus is a bundle of axons that forms a triangular structure medial to the spinothalamic tract. The bundles are formed from axons of the contralateral dorsal column nuclei, the nucleus gracilis and cuneatus, which have decussated and formed the internal arcuate fibres which become continuous with the medial lemnisci. The fibres are joined by axons of the trigeminal sensory nucleus to project to the ipsilateral thalamus. Axons from the dorsal column nuclei terminate on neurons in the ventral posterior lateral nucleus of the thalamus, whereas axons from the trigeminal sensory nucleus terminate on neurons in the ventral posterior medial nucleus of the thalamus (Guyton & Hall 1996).

This semilunar structure, also referred to as the basis, is located anterior to the substantia nigra and is composed of the corticospinal, corticonuclear, and corticopontine fibre tracts. The corticospinal fibres terminate on the ventral horn neurons of the contralateral spinal cord. The corticonuclear fibres terminate mainly on contralateral cranial nerve nuclei throughout the brainstem. The corticopontine fibres are composed of projections from the frontal and temporal cortical areas and terminate on the interneurons of the nuclei pontis. These nuclei then project mostly to the contralateral cerebellum.

This structure, which is also referred to as the brachium conjunctivum, proceeds from the upper white substance of the cerebellar hemisphere to the tegmentum where it completely decussates at the level of the inferior colliculus. It is composed of:

These tracts form bilateral structures that ascend in the spinal cord in the ventral lateral fasciculi. They terminate in the vermis and intermediate zones of the ipsilateral cerebellum. These tracts relay information to the cerebellum about what information or commands have arrived at the ventral horn cells. These pathways are part of the efference copy mechanism of the cerebellar motor system.

This tract carries fibres from the contralateral dorsal cochlear nucleus to the inferior colliculus.

These bilateral structures, which are also referred to as the brachium ponti, are the largest of the cerebellar peduncles. They carry fibres from the pontine nuclei to the contralateral neocerebellum. They are a component of the corticopontocerebellar pathways.

The reticular formation of the pons is continuous with the reticular formations of the medulla and the mesencephalon. This area receives input from virtually all areas of the neuraxis and projects widely throughout the neuraxis. (See below for more detail.)

These structures were discussed earlier; see above.

This cavity is bounded by the pons and medulla ventrally and by the cerebellum dorsally. It is continuous with the cerebral aqueduct above and the central canal of the medulla below and has a capacity for cerebrospinal fluid (CSF) of about 20 ml. The floor of the fourth ventricle, which is also referred to as the rhomboid fossa, is formed by the dorsal surfaces of the pons and medulla. The fourth ventricle acts as a component of the CSF system (Chusid 1982).

Structures found at a cross-section at the inferior olive (fig. 13.8)

Choroid plexus of 4th

This structure is also referred to as the tela choroidea of the fourth ventricle and is composed of a layer of pia matter that has become highly vascularised. The choroid plexus produces CSF.

Figure 13.8 A cross-sectional view of the medulla at the level of the inferior olive.

(from Standring 2008 Gray’s Anatomy 40th edn. Edinburgh, Elsevier, with permission)

Inferior olivary nucleus

These bilateral groups of nuclei are located within the olive of the medulla. They receive projections from the cortex, from other brainstem nuclei, from the ipsilateral parvocellular red nucleus, and from the ipsilateral spinal cord. These neurons project axons, referred to as climbing fibres, via the inferior cerebellar peduncles to all areas of the cerebellar cortex. These structures are part of a group of projections and nuclei referred to as the inferior olivary nuclear complex, which forms a complicated loop from the cerebellar cortex to the dentate nucleus to the contralateral red nucleus to the inferior olivary nucleus and back to the contralateral cerebellar cortex (Fig. 13.9)

Figure 13.9 The functional loop of the inferior olivary complex.

Structures found in a cross-section at the lemniscal decussation (fig. 13.10)

Fasciculus gracilis

These bilateral tracts run dorsally in the medulla medial to the cuneate fasciculi. They are formed by the axons of the dorsal root ganglion cells that detect proprioception and touch in the lower limbs and trunk, usually below the T6 level. The axons enter the spinal cord via the dorsal root and ascend ipsilaterally in the fasciculus gracilis to the gracile nucleus.

Figure 13.10 A cross-sectional view of the medulla at the level of the lemniscal decussation.

(from Standring 2008 Gray’s Anatomy 40th edn. Edinburgh, Elsevier, with permission)

Structures found at a cross-section through pyramidal decussation (fig. 13.11)

Fasciculus gracilis and cuneatus

This structure was discussed earlier; see above.

Figure 13.11 A cross-sectional view of the medulla through pyramidal decussation.

(from Standring 2008 Gray’s Anatomy 40th edn. Edinburgh, Elsevier, with permission)