In cells, the nucleus is traditionally considered to the be the center of the cell: a meeting center where information is stored, processed, and distributed. Brainstem nuclei serve a similar purpose, as they are the central networks through which nerve cells and nerves originate and perform their functions. Brainstem nuclei are thus responsible for many of the feelings and functions that humans experience every day, but do not necessary consciously think about. Each nucleus may be classified as either sensory or motor.
The brainstem is the most primitive portion of the brain. It is located between the main brain and the spinal cord and consists of the midbrain, the pons, and the medulla. A wide network of nerves rest inside these structures, and each of these nerves begins at one or more brainstem nuclei.
In general, nerves may serve either a sensory or a motor function. As such, they either contribute to a human’s five senses or they aid in movements. Therefore, brainstem nuclei are referred to as either sensory or motor nuclei depending on the nature of the nerves that derive from them. If a nerve has sensory and motor capabilities, it will be attached to both sensory nuclei and motor nuclei.
While sensory nuclei are typically found on the sides of the brainstem, motor nuclei are affixed in the middle — or medial — portion of the brainstem. Most of these nuclei are connected to only one nerve, but on occasion multiple nerves may arise from a solitary nucleus.
There are 12 cranial nerves that originate from the brainstem nuclei. These nerves perform various individual functions, including moving the eye, moving the face, and aiding in smelling or tasting. The oculomotor nerve, for example, is a motor nerve responsible for keeping certain eye muscles mobile. The nucleus for this nerve is known as the oculomotor nucleus. Other nuclei that contain corresponding nerves include the following: the abducens nuclei, the trochlear nuclei, the vestibular nuclei, and the hypoglossal nuclei.
While most brainstem nuclei are named after the nerves that they create, a few exceptions do exist. The Edinger Westphal nucleus, for example, is located around the oculomotor nuclei and boasts the nerves responsible for pupil constriction. Further, a nerve that carries both sensory and motor capacities relating to the face and mouth — the trigeminal nerve — has two nuclei origins: the mesencephalic nucleus and the motor nucleus. Further, the versatile vagus nerve has three brainstem nuclei: the nucleus ambiguus, the secretomotor parasympathetic nucleus, and the solitary nucleus. In addition, some nerves, such as the solitary nucleus, have connections to more than one cranial nerve and therefore do not share a name with any particular nerve.