Introduction
The brainstem is a highly organized intermediary structure that functionally connects the cerebral cortex to the spinal cord. It comprises only 2.5% of the entire brain, but serves an important function that is necessary for human survival. Its key vital functions are control of respiration, blood pressure and pulse. It is also part of the system that is responsible for consciousness. It is the passageway of sensory and motor neural fibers innervating different parts of the body. It is composed of three structures namely the midbrain, pons and medulla. Midbrain is designated as the upper brainstem. Pons and medulla make up the lower brainstem. Important sensory, motor and mixed nerves arise from the brainstem eventually exiting through the cranium to reach its target organ.
Pons
Pons comes from the Latin language, which literally means “a bridge”. It contains several neural conduction fibers that are directed longitudinally, transversely and backwards to deliver neural impulses to surrounding structures. The pons is located in between the midbrain and the medulla oblongata. Behind it is the cerebellum. It is grossly characterized by two bulbous structures, called the superior and middle cerebellar peduncle. These peduncles connect the pontine part of the brainstem to the cerebellum, which is a neural structure that is necessary for normal gait and balance.
Four important cranial nerves arise from the pons, namely, the vestibulocochlear nerve (8th cranial nerve), the facial nerve (7th cranial nerve), abducens nerve (6th cranial nerve) and the trigeminal nerve (5th cranial nerve). The vestibulocochlear nerve transmits nerves impulses to the ear for auditory function. It is also important for acquiring a sense of balance. The pons also contains a neural structure called, the “olivary complex”, which enables sound localization in space and reduction of noise. It is a special neural structure for hearing. The facial nerve sends motor impulses to the facial muscles, which makes it possible for humans to have facial expressions. It also provides taste sensation to the front two-thirds of the tongue, facilitates tearing and salivating. The abducens nerve controls the muscle of the eye that is responsible for looking sideways. The trigeminal nerve transmits sensory function to the face. It also provides motor function to masticatory muscles that enables chewing.
Medulla
The medulla oblongata is the lowest portion of the brainstem. It is located at the level of the foramen magnum, which is the rear opening of the cranium. Grossly, it looks like a subtle enlargement of the uppermost portion of the spinal cord, measuring 1cm in diameter and 2.5cm in length. It is characterized by two distinct elevations. The first prominent structure is called the medullary pyramids. These are neural fibers that provide motor function, which descends from the cerebral cortex down to the spinal cord. Interestingly, in the brainstem these neural fibers cross to the other side at the level of the medulla oblongata. Hence, the medullary pyramids are also known as the pyramidal decussation. This has important clinical implications because it explains why a stroke on the right side of the cerebral hemisphere manifests with weakness or paralysis of the left side of the body. Grossly, another prominent bulge is the inferior olivary nucleus, which connects the medulla to the cerebellum.
In contrast to the pons, cranial nerves that emerge from the medulla are the glossopharyngeal nerve (9th cranial nerve), vagus nerve (10th cranial nerve), spinal accessory nerve (11th cranial nerve) and hypoglossal nerve (12th cranial nerve). The glossopharyngeal nerve functions to provide taste sensation on the back part of the tongue, and activates the pharyngeal muscles to aid in swallowing. The vagus nerve is an autonomic nerve that has the longest course. It runs from the pharynx down to the splenic flexure of the colon. It helps activate muscles for swallowing and phonation. It enables chest and abdominal organs to function normally. The vagus nerve is also responsible for gag reflex, which can be elicited by stroking the back part of the tongue or the pharyngeal walls. The spinal accessory nerve provides additional neural fibers to the vagus nerve. It aids in turning the head and shrugging the shoulders by activating the appropriate muscles. The hypoglossal nerve facilitates swallowing by allowing voluntary tongue movement. Motor nerves that innervate vital organs originate from the medulla oblongata. An example is the cardiovascular center, which is responsible for contraction of the heart muscle, allowing us to have a stable blood pressure and pulse. The vasomotor center functions to regulate the distention of blood vessel walls. The respiratory center is responsible for control of several aspects of breathing, such as depth, rate and rhythm, also resides within the medulla. Other reflex centers, which mediate vomiting, coughing, sneezing, swallowing and having hiccups also pass through the medulla.
Summary
As part of the lower brainstem, both the pons and medulla oblongata are important neural structures with distinct anatomical features and physiologic functions. The pons is located superiorly, just above the medulla. It contains nerves that are important for hearing, balance, moving the facial muscles, chewing and looking sideways. The medulla is located inferiorly, just above the spinal cord. It contains neural structures that are vital to sustain human life. It facilitates swallowing, speaking, turning the head and shrugging the shoulders.