OVERVIEW OF BRAINSTEM ANATOMY
The three levels of the brainstem from superior to inferior are the midbrain, pons, and medulla. The midbrain is just inferior to the bilateral thalami, and the medulla transitions inferiorly into the cervical spinal cord. Most simply, the brainstem can be thought of as a “spinal cord for the head and neck”: Just as the spinal cord has sensory information coming in and motor information going out for the extremities and torso, the brainstem has sensory information coming in and motor information going out for the head and neck. In addition to somatic sensory information, however, the brainstem also receives vestibular, auditory, taste, and visceral sensory information. Motor functions of the brainstem include control of ocular, pupillary, facial, laryngeal, pharyngeal, and visceral musculature (Figs. 9–1 and 9–2).
Schematic of brainstem anatomy. A: Posterior view of the brainstem with cerebellum removed revealing the locations of the cranial nerve nuclei. B–E: Axial sections demonstrating locations of cranial nerve nuclei and exiting cranial nerves in the midbrain (B), upper pons (C), midpons (D), and medulla (E). Reproduced with permission from Waxman S: Clinical Neuroanatomy, 27th ed. New York, NY: McGraw Hill; 2013.
Schematic for locations of cranial nerve nuclei. Motor nuclei for CNs innervating skeletal muscle are medial (CNs 3, 4, 6, and 12). Motor nuclei for CNs innervating branchial muscle are more lateral (CN 5 [motor for muscles of mastication], CN 7, CN 9, CN 10). Somatic sensory and special sensory cranial nerve nuclei are most lateral (CNs 5 and 8). LT: Light touch. P/T: Pain/Temperature.
Understanding the brainstem requires a general framework for what is there and where it is. As far as what is in the brainstem, there are five general categories of structures:
The descending motor pathways for the extremities and torso (corticospinal tracts; see Ch. 4)
The ascending somatosensory pathways from the extremities and torso (dorsal columns and spinothalamic tracts; see Ch. 4)
The cranial nerve nuclei and associated structures. The cranial nerve nuclei are the collections of cell bodies whose axons give rise to the cranial nerves. Before the cranial nerves exit the brainstem, their fibers are referred to as fascicles.
Connections with the cerebellum (the cerebellar peduncles; see Ch. 8)
The reticular activating system and ascending neurotransmitter-specific projection pathways: substantia nigra (dopamine), locus coeruleus (norepinephrine), median raphe nuclei (serotonin), pedunculopontine nuclei (acetylcholine)
As points of orientation for where structures are in the brainstem, the following principles apply at all three levels of the brainstem (see Fig. 9–1):