Medulla Oblongata

The medulla oblongata is a critical region of the vertebrate brain which plays an important role in keeping us alive.

The medulla is located in the lower part of the brainstem, which is positioned immediately superior to (above) the spinal cord and inferior to (below) the pons. It is an elongated and somewhat pyramidal structure approximately 3 cm in length in adults ^[1].

It is a part of the central nervous system and is the lowest part of the brainstem, which also includes the midbrain and pons. The brainstem, including the medulla, is often considered part of the hindbrain in developmental terms, along with the cerebellum ^[2].

The medulla oblongata has several critical functions, primarily related to maintaining essential life processes. These include:

1. Respiratory Regulation: The medulla contains the dorsal and ventral respiratory groups, which control the rhythm and rate of respiration respectively ^[4].

2. Cardiac Center: The medulla houses the cardiac center, which controls the heart's rate and force of contraction ^[5].

3. Vasomotor Center: This center in the medulla controls blood pressure by regulating the contraction and dilation of blood vessels ^[6].

4. Reflex Centers: The medulla also houses various reflex centers responsible for reactions such as coughing, vomiting, sneezing, and swallowing. These reflexes are often protective in nature ^[7].

5. Integration of Sensory Information and Motor Commands: The medulla serves as a relay station for the passing of signals between the spinal cord and the higher regions of the brain ^[1].

6. Regulation of Body Homeostasis: Through connections with the hypothalamus and pituitary gland, the medulla also plays a role in the autonomic regulation of homeostasis. This includes body temperature, water balance, and certain aspects of metabolism ^[8].

The medulla is often divided into different sections, including the anterior (ventral) and posterior (dorsal) portions, each with its distinct structures and functions.

The ventral surface of the medulla contains two prominent pyramids that contain the corticospinal tracts^[7]. These tracts consist of fibers that come from the cerebral cortex and descend to the spinal cord, playing a significant role in the voluntary control of muscles^[1].

Just lateral to the pyramids, the anterior portion of the medulla also houses the olivary nuclei^[9]. These structures are involved in motor control, and their main function is to send signals to the cerebellum, where they help coordinate movements and learn new motor tasks^[10].

The dorsal portion of the medulla contains the fourth ventricle, a CSF-filled space which is a part of the ventricular system of the brain^[1]. Lateral to the fourth ventricle, the dorsal medulla houses various important nuclei of the cranial nerves, including the vestibular, cochlear, glossopharyngeal, vagal, accessory, and hypoglossal nuclei^[12].

The dorsal medulla also contains the dorsal respiratory group (DRG), which is primarily responsible for the basic rhythm of respiration^[13].

In the dorsal part of the medulla, you can also find the nucleus of the solitary tract (also known as the nucleus tractus solitarius, or NTS), which receives sensory information from the facial, glossopharyngeal, and vagus nerves about visceral organs^[14].

The terms "open" and "closed" medulla refer to the structural appearance of the medulla oblongata in relation to the fourth ventricle, which is a cavity filled with cerebrospinal fluid (CSF) and located in the posterior part of the brainstem.

1. Open medulla (also known as superior or rostral medulla): This portion of the medulla refers to the section where the fourth ventricle is wide and open^[15]. It is superior to the area postrema, a chemoreceptor trigger zone for vomiting located at the floor of the fourth ventricle^[16]. The open medulla contains the inferior cerebellar peduncle that carries afferent fibers into the cerebellum, among other structures^[1]. Both the anterior and posterior sections of the open medulla contain various nuclei and tracts, with the posterior section being closest to the fourth ventricle^[17].

2. Closed medulla (also known as inferior or caudal medulla): This refers to the portion of the medulla where the fourth ventricle narrows down and merges with the central canal of the spinal cord^[9]. The anterior part of the closed medulla features the pyramid containing corticospinal fibers, and the olives which contain the inferior olivary nuclei^[18]. The posterior section of the closed medulla, like the posterior section of the open medulla, is closest to the fourth ventricle and contains various important nuclei and tracts^[19].

The olives, or olivary bodies, are oval-shaped structures located on the lateral surfaces of the medulla. Each olive contains the inferior olivary nucleus, which has connections with various areas of the brain and spinal cord and plays a role in motor coordination and learning. The level of the olives is generally considered the uppermost level of the medulla, situated above the decussation of the pyramids and medial lemniscus^[4].

The pyramids are a pair of white matter structures located on the ventral aspect of the medulla. These structures contain corticospinal tracts that carry motor signals from the brain to the spinal cord. At the level of the medulla-spinal cord junction, about 90% of these fibers decussate, or cross over to the opposite side, in an area known as the decussation of the pyramids^[1]. This decussation is what causes each side of the brain to control the opposite side of the body - this is known as contralateral control.

The medial lemniscus is a large bundle of axons that carries sensory information from the body to the thalamus in the brain. The fibers of the medial lemniscus decussate in the medulla. This decussation occurs a bit higher in the medulla, above the level of the pyramids but still lower than the level of the olives^[7].

The medulla oblongata is composed of white and gray matter:

• The white matter contains many important nerve tracts that either ascend to or descend from higher brain centers. These include the corticospinal tract, which is important for voluntary motor control, and various sensory tracts^[20].

• The gray matter in the medulla forms several groups of cells, known as nuclei, which have specific functions. Examples include the cardiac center (which controls the rate and strength of heart contractions), the respiratory centers (which control the rate and depth of breathing), and various other centers involved in the control of reflex activities^[20].

Within the medulla, there are various nuclei each associated with a function:

1. Nucleus Gracilis and Nucleus Cuneatus: These nuclei are associated with sensory information about touch, pressure, vibration, and conscious proprioception, which they receive from the body and transmit to the thalamus^[13].

2. Solitary Nucleus: This nucleus receives taste (gustatory) information and contributes to autonomic regulation by receiving visceral sensory information^[21].

3. Nucleus Ambiguus and Dorsal Motor Nucleus: These are involved with autonomic (parasympathetic) motor innervation of the heart and the muscles of the pharynx, larynx, and upper esophagus, contributing to swallowing and speech^[1].

4. Olives (Inferior Olivary Nuclei): These nuclei play a role in motor control and learning, as well as in processing of sensory information^[22].

5. Pyramids: These are bulges of white matter in anterior medulla, containing large motor tracts known as the corticospinal tracts. These tracts are involved in the voluntary control of muscles^[23].

6. Vestibular Nuclei: They play a role in balance and spatial orientation^[24].

Several cranial nerves emerge from and have nuclei in the medulla oblongata.

These include:

1. Glossopharyngeal Nerve (IX): This nerve is involved in taste, swallowing, salivation, and the sensation of the throat and middle ear. It also plays a role in the sensation from the carotid body and sinus (important for reflex control of blood pressure)^[16].

2. Vagus Nerve (X): Often known as the "wandering nerve" because of its extensive distribution, the vagus nerve is a key component of the parasympathetic nervous system, responsible for a range of functions, including control of the heart rate, gastrointestinal motility, sweating, speech, and the sensation from the outer ear^[25].

3. Accessory Nerve (XI): This nerve supplies motor input to two muscles in the neck: the sternocleidomastoid, which helps rotate the head, and the trapezius, which shrugs the shoulders^[26].

4. Hypoglossal Nerve (XII): This nerve provides motor control to the muscles of the tongue (except for one, the palatoglossus, which is innervated by the vagus nerve)^[27].

All these nerves have their nuclei (clusters of neuron cell bodies) located in the medulla. These nuclei serve as sites of synaptic contact with other neurons, and thus are critical for the processing and relaying of information associated with these cranial nerves.

The anatomical and functional complexity of the medulla oblongata reflects its significance in maintaining homeostasis and facilitating communication within the nervous system. Any damage or disease affecting the medulla oblongata can have serious or even life-threatening consequences, given its role in controlling such fundamental processes as heart rate, blood pressure, and breathing.