Chapter 6 - Brain Stem Organization
The brain stem contains the origin of motor cranial nerves and the termination for all of the sensory cranial nerves except for vision and olfaction (figure 15). Damage to the lateral brain stem often affects sensory cranial nerve nuclei such as the vestibular nuclei (where vertigo would be the primary complaint and nystagmus would be the main exam finding) and trigeminal nuclei (affecting facial sensations, particularly pain and temperature sense as well as the corneal reflex). The corneal reflex not only requires intact corneal sensation, but also intact facial nerves since this consensual reflex is mediated via bilateral connections with the facial nuclei.
The pharynx and larynx
Patients with cranial nerve damage often have problems with their voice or with swallowing. The nucleus ambiguus is the location of the majority of the motor neurons to the larynx, pharynx and soft palate. These fibers travel with the vagus and, to a lesser extent, the glossopharyngeal nerve. The gag reflex has a sensory limb that involves sensations from the back of the pharynx via the glossopharyngeal nerve to the medulla, with subsequent activation of vagal nerve fibers that should cause the pharynx and palate to contract bilaterally. This will elevate the soft palate. The larynx is innervated by the vagus nerve, via the recurrent laryngeal branch. This nerve enters the chest and ascends to the neck, passing posterior to the thyroid gland to reach the larynx.
Visceral sensations arriving at the brain stem with the vagus and glossopharyngeal nerves traverse the solitary tract to the solitary nucleus. These visceral sensory fibers include chemoreceptor inputs from the carotid and aortic body, baroreceptor inputs and inputs from pulmonary stretch receptors. These sensory fibers participate in a variety of visceral reflexes including the baroreceptor reflex as well as respiratory reflexes.
Cranial parasympathetic nuclei include the salivatory nuclei (that send fibers into the facial and glossopharyngeal nerves), the dorsal motor nuclei (that are the origin for most of the vagal parasympathetics) and the Edinger-Wesphal nucleus (that is the origin for oculomotor parasympathetic nerves to pupillary constrictors and the cillary muscle for the lens).
Eye movement is controlled by somatic efferent neurons in the oculomotor (midbrain), trochlear (midbrain) and abducens (pons) nuclei (see chapter 8).
Much of the core of the brain stem consists of "reticular formation" (RF). There are small neurons that perform an integrating function and large neurons that have axons that ramify extensively with collateral branches that distribute throughout the brain stem as well as ascending to the thalamus and descending to the spinal cord.
While there are some well-localized nuclei within the RF, most of the RF is involved in integrative functions. There are some regions that are more focused on particular functions (for example, the ventral respiratory group and some centers for cardiovascular control are portions of the RF). Additionally, there are regions in the rostral pons that play a role in patterning gait in many species (probably including humans) and also the RF is involved in modulating muscle tone. The paramedian pontine reticular formation (PPRF) has particular involvement in horizontal eye movements.
There are connections with various cranial nerve nuclei and patterned reflex responses (like blink and gag) and complex responses (like vomiting, coughing and swallowing) all have "centers" within the brain stem.
Descending projections from the medullary reticular formation (medullary reticulospinal tract) has a strong inhibitory effect over spinal cord interneurons (and, therefore, over reflexes). Despite what have been termed physiologic "centers", it is better to think of the RF as collections of neurons serving a variety of important functions, with some regional differences in the primary functions that are influenced.
One special case includes the midbrain reticular formation, which is the foundation for the "reticular activating system." This system involves diffuse projections through the intralaminar and midline thalamic nuclei to the cerebral cortex. Damage to midbrain reticular formation interferes with normal arousal and produces coma by interfering with this mechanism.
There are very specific neurotransmitter systems in the brain. Most of these systems are modulatory. For example, most of the serotonin in the brain arises from raphe nuclei and most of the norepinephrine arises from the locus ceruleus. These nuclei have terminations in virtually all areas of the central nervous system (figure 9) and are involved in such diffuse processes as sleep/wake, pain modulation and regulation of mood.