Medulla oblongata

Ventral surface

Along the midline of the ventral surface of the medulla is the anterior median fissure. This is a continuation of the anterior median fissure of the spinal cord. On either side of the fissure is a vertical protuberance known as the medullary pyramid, formed by the fibers of the corticospinal tract. Below the base of the pyramids, the median fissure is interrupted by the fascicles of the corticospinal tract that decussate and form the pyramidal decussation in the midline.

Each half of the ventral medullary surface shows two sulci: anterolateral sulcus and posterolateral sulcus. Between the sulci, and just lateral and posterior to each pyramid is another oval structure known as the olive. The olive is formed by the presence of the inferior olivary nucleus in the medulla. The sulci themselves serve as exit points of certain cranial nerves:

• The hypoglossal nerve (CN XII) exits the medulla through the anterolateral sulcus, just medial to the olive.
• Going from rostral to caudal, the glossopharyngeal (CN IX), vagus (CN X) and accessory nerve (CN XI) exit the medulla through the posterolateral sulcus, lateral to the olive.

Examine the landmarks of the medulla oblongata with our labelled illustrations, videos, quizzes and articles:

Dorsal surface

The dorsal surface of the medulla oblongata is marked by the dorsal median sulcus in the midline. This sulcus is continuous with its spinal counterpart.

On each side of the dorsal median sulcus, there are the two vertical prominences that run in parallel to the sulcus. The one that is located immediately lateral to the sulcus is formed by the gracile fasciculus that ascends from the spinal cord. The cranial part of the gracile fasciculus contains the gracile nucleus, a relay nucleus that synapses with the fibers of the gracile fasciculus. The gracile nucleus forms a visible enlargement on the dorsal surface of the cranial medulla, referred to as the gracile tubercle. Lateral to each gracile fasciculus is another vertical prominence formed by the cuneate fasciculus. It also features a large cranial enlargement, the cuneate tubercle, which is formed by the cuneate nucleus, a relay nucleus for the cuneate fasciculus.

Lateral to each cuneate nucleus is another prominence called the trigeminal tubercle, which is formed by the spinal nucleus of trigeminal nerve (CN V). Caudal to the trigeminal tubercle is the lateral funiculus of the medulla oblongata, which is directly continuous with the lateral funiculus of the spinal cord.

Learn the markings of the dorsal medulla with our study unit:

Nuclei

The nuclei of the medulla oblongata are involved in many important bodily functions. These are the following:

• Cranial nerve nuclei, which contain the nuclei of the lower four cranial nerves (IX-XII): inferior salivatory nucleus, spinal nucleus of trigeminal nerve, solitary nucleus, nucleus ambiguus, dorsal nucleus of vagus nerve, nucleus of accessory nerve, nucleus of hypoglossal nerve.
• Relay nuclei, which relay the stimuli from peripheral receptor areas to higher cortical centers: gracile, cuneate, arcuate, and olivary nuclei.
• Reticular nuclei, which belong to the reticular formation and control various autonomic functions: raphe nuclei, gigantocellular (magnocellular) nucleus, perihypoglossal (Roller’s) nucleus, lateral reticular nucleus and many other small clusters of neurons distributed throughout the base of the medulla.

Let’s analyze a cross-section of the medulla on the vagus nerve level to examine these structures and learn their relations. Imagine slicing the medulla oblongata transversely through the level of the vagus nerve. It would look like this from superior view:

Just like in the rest of the brainstem, the majority of the nuclei of the medulla oblongata lie within its dorsal half. All the nuclei are bilaterally distributed, meaning that there is one nucleus on either side of the midline. Going from medial to lateral, and from dorsal to ventral, the medulla oblongata contains the following nuclei:

• Raphe nuclei. The numerous raphe nuclei belong to the reticular formation of the medulla oblongata. They are the main source of the neurotransmitter serotonin and are also involved in modulation of mood, pain, wakefulness/arousal states and thermoregulation.
• Perihypoglossal (Roller’s) nucleus. This nucleus lies in close proximity to the hypoglossal nucleus and is a part of complex circuits involving eye movements, such as visual pursuit.
• Hypoglossal nucleus. This is a motor nucleus that gives rise to the hypoglossal cranial nerve and provides innervation for all of the intrinsic and extrinsic muscles of the tongue, except for the palatoglossus muscle.
• Dorsal nucleus of vagus nerve. This is a visceral motor nucleus that provides parasympathetic innervation to the smooth muscles of the digestive tract, lungs, and all the viscera of the abdomen and thorax.
• Medial vestibular nucleus (of Schwalbe). This nucleus is a part of the vestibular system and spans the caudal pons and rostral medulla. It receives inputs from the semicircular canals of the inner ear, and then further projects them through fibers that join the medial longitudinal fasciculus (MLF). Via the MLF, these inputs are carried to the nuclei of the oculomotor, trochlear and abducens nerves through which they mediate the vestibulo-ocular reflex.
• Cuneate nucleus. This nucleus receives sensory information about light touch, proprioception, and vibration from the ipsilateral upper limb, carried by the cuneate fasciculus. It gives off the fibers that decussate and join the medial lemniscus.
• Spinal trigeminal nucleus. This is a sensory nucleus that receives information about touch, vibration, pain, and temperature from the face.
• Nucleus ambiguus. This is a motor nucleus that provides fibers to the glossopharyngeal, vagus and accessory cranial nerves.
• Lateral reticular nucleus. This nucleus receives and integrates impulses from a variety of sources, which it further conveys to the ipsilateral half of the cerebellum. Through this connection, the lateral reticular nucleus helps the cerebellum with motor planning and coordination.
  
• Olivary nuclei, which are the dorsal accessory olivary nucleus, inferior olivary nucleus, and the medial accessory olivary nucleus. These nuclei connect with the cerebellum to aid motor coordination.

Ascending and descending tracts

The white matter of the medulla oblongata is composed of two types of tracts:

• Motor tract: corticospinal (pyramidal) tract
• Sensory tracts: cuneate fascicle, gracile fascicle, medial lemniscus, spinal tract of the trigeminal nerve, spinothalamic tract, spinocerebellar tract, spinothalamic tract, inferior cerebellar peduncle

The medulla oblongata is also traversed by the medial longitudinal fascicle (MLF), which connects the nuclei of the oculomotor, trochlear and abducens nerves and makes sure that the movements of the eyes are well coordinated relative to the visual stimulus (saccadic eye movements).

Let’s continue analyzing the cross-section on the vagus nerve level to learn the location and relations of these tracts within the medulla:

• Pyramidal tract is the most ventral tract within the medulla. It is the very tract that projects into the pyramids seen on the ventral surface of the medulla. This is the main voluntary motor tract that travels from the cerebral cortex to the spinal cord, where it synapses with the bodies of the lower motor neurons that further innervate all the skeletal muscles of the body.
• Medial lemniscus passes just dorsal to the pyramidal tract. It synapses with the fibers from the cuneate and gracile nuclei which transmit the somatosensory information from the body. The medial lemniscus then passes through the thalamus and eventually ends within the primary somatosensory cortex.
• Medial longitudinal fasciculus (MLF) courses dorsal to the medial lemniscus.
• Inferior cerebellar peduncle passes lateral to the medial longitudinal fasciculus. This is a white matter mass that contains the tracts connecting the spinal cord with the brainstem and cerebellum. These tracts carry proprioceptive somatosensory fibers, as well as outputs from the cerebellum that regulate posture and balance of the body.
• Spinal tract of the trigeminal nerve courses ventral to the inferior cerebellar peduncle. This tract carries the sensory information from the face to the spinal nucleus of trigeminal nerve.
• Spinocerebellar tract lies anterior to the lateral lenticular nucleus. It carries proprioceptive information from the skeletal muscle to the cerebellum.
• Spinothalamic tract passes medial and ventral to the spinocerebellar tract. It conveys the information about pain and temperature from the contralateral side of the body to the thalamus. Upon passing through the thalamus, the fibers of this tract continue towards the primary somatosensory cortex.

Cranial nerves

Housing the nuclei of the lower four cranial nerves, the medulla oblongata controls the following bodily functions:

• Via the glossopharyngeal nerve (CN IX): swallowing, salivation, and visceral, general and special (taste) sensation in the oral cavity.
• Via the vagus nerve (CN X): parasympathetic supply to the head, thorax and abdomen, gland secretion control, peristalsis, phonation, taste, visceral and general sensation of these regions.
• Via the accessory nerve (CN XI): phonation and movements of the head and shoulders.
• Via the hypoglossal nerve (CN XII): movements of the tongue, speech and swallowing.

Respiratory center

The respiratory center is a complex group of nuclei located within the pons and medulla oblongata. It consists of three parts: the dorsal respiratory group, ventral respiratory group and pneumotaxic center. The former two are found within the dorsal and ventral medulla, respectively, while the latter lies within the rostral pons.

The dorsal respiratory group is in charge of inspiration or inhaling the air, and it plays the most fundamental role in the breathing process. The majority of its neurons are found within the nucleus of the solitary tract, which receives information from the peripheral chemoreceptors about the blood oxygen saturation. Upon receiving this information, the dorsal respiratory group stimulates the phrenic nerve to contract the diaphragm, as well as the thoracic spinal nerves to contract the intercostal muscles. The end result is inspiration.

The ventral respiratory group consists of the rostral part of the nucleus ambiguus, and a small satellite nucleus called the nucleus retroambiguus which lies caudally to the former. These neurons are inactive during normal, non-forced breathing. However, when in need of increased pulmonary ventilation, the dorsal respiratory group stimulates the ventral group, which in turn stimulates the accessory respiratory muscles.

Let’s just briefly explain the pontine pneumotaxic center to round up the story about breathing. This center lies within the parabrachial nucleus of the rostral pons and it is connected with the dorsal respiratory group of neurons. The main function of the pneumotaxic center is to “turn off” the inspiratory signal from the dorsal respiratory group, thus dictating the respiratory cycle and the length of inspiration. Depending on the physiological condition, the pneumotaxic center allows 0.5 to 5 seconds long inspiration.

Vasomotor center

The control system of the arterial blood pressure lies within the vasomotor center of the medulla oblongata. Its function is to collect baroreceptor signals from the aortic body about the blood pressure, and to initiate an autonomic response according to those information in the following way:

• To stimulate the parasympathetic nervous system through the vagus nerve and influence the heart rate.
• To initiate the vasoconstricting sympathetic nervous system response through the spinal cord, all in order to control the diameter of all the arteries, arterioles, veins and venules of the body.

The vasomotor center is composed of the reticular nuclei of the medulla, which are divided into three groups:

• Vasoconstrictor area, which lies in the anterolateral part of the rostral medulla and connects with the spinal cord neurons that later contribute to the peripheral sympathetic nerves which cause systemic vasoconstriction. The vasoconstriction is most potent in the kidneys, skin, spleen and intestines, and much less potent in the brain and skeletal muscles.
• Vasodilator area, which lies in the anterolateral part of the caudal medulla. They connect with the vasoconstrictor area and inhibit its function when in need, thus causing systemic vasodilation. Moreover, this area stimulates the vagus nerve which directly influences the heart rate by decreasing it.
• Sensory area, that lies within the nucleus of the solitary tract and receives baroreceptor stimuli from the aortic body via the glossopharyngeal nerve. The signals from this nucleus are then sent to both the vasoconstrictor and vasodilator area in order to control their activity.