The brainstem and reticular formation

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13 The brainstem and reticular formation

image Clinical cases for thought

Anatomy of the brainstem

The brainstem is composed of the following anatomical areas (Figs 13.1, 13.2, and 13.3):

We will approach the description of the brainstem by outlining the structures observed at various levels in cross-sectional dissections.

Mesencephalon

The structures of the mesencephalon can be observed in a cross-section of midbrain at the superior colliculus and the inferior colliculus (Figs 13.4 and 13.5).

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Figure 13.4 A cross-sectional view of the mesencephalon at the superior colliculus.

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

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Figure 13.5 A cross-sectional view of the mesencephalon at the inferior colliculus.

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

Red nucleus

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.

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).

Pons

The structures of the pons can be observed through a cross-section at the level of the trigeminal nerves (Fig. 13.7), just superior to the cerebral peduncles.

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Figure 13.7 A cross-sectional view of the pons at the level of the trigeminal nerves.

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

Medulla

Structures of the medulla can be viewed by cross-sections at:

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

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

The reticular formation (RF)

The reticular formation receives little attention in traditional neurology textbooks. It is an area that spans all levels of the brainstem, from the thalamus to the spinal cord, and is responsible for integrating information from the brain and periphery and linking sensory, motor, and autonomic nuclei of the brainstem. The reticular formation therefore mediates complex reflexes and functions such as eye movements, posture, feeding, breathing, homeostasis, arousal, sleep, control of vasomotor tone and cardiac output, and pain. The reticular formation is composed of continuous groups of neurons interconnected via polysynaptic pathways that can be both crossed and uncrossed in nature. The RF receives projections from virtually all sensory modalities and projects to all areas of the neuraxis including direct projections to the cortex (Webster 1978).

Afferent projections to the RF include:

Efferent projections from the RF include: