Medulla oblongata

The medulla oblongata (Latin: medulla oblongata) is the most inferior part of the brainstem. The medulla oblongata lies in the posterior cranial fossa towards the clivus. The medulla oblongata contains many essential tracts passing through, nuclei for vital functions of the body (the heart rate, blood pressure, and breathing), and the cranial nerve nuclei. It continues with the pons above and the spinal below, directly above the first cervical spinal nerve's pair. 

External anatomy of the medulla oblongata

The medulla oblongata is 2.5-3.5 cm long, 1.5 cm wide, with 2 -2.5 cm wide in its widest part. The medulla has a cone shape with a truncated apex, while its base is facing the pons. The medulla oblongata has two surfaces: the ventral (anterior) and dorsal (posterior). 

Ventral surface

The ventral surface faces the occipital bone's basilar part and the dens of axis, separated from the surface by the meninges and the atlantooccipital and atlantoaxial ligaments. The ventral surface has five sulci lying towards the clivus: the anterior median fissure, left and right anterolateral sulci, left and right posterolateral sulci

In the middle of the ventral surface is the anterior median fissure, a continuation of the anterior median fissure of the spinal cord. Superiorly, the anterior median fissure ends at the pontine border. Two more sulci are present on both sides of the anterior median fissure: the anterolateral and posterolateral sulcus. Between the anterior median fissure and anterolateral sulcus lies the pyramid, while between the anterolateral sulcus and posterolateral sulcus - the olive

The olive is separated from the pyramid not only by the sulcus but also by the hypoglossal nerve. Both pyramids are white matter structures consisting of the corticospinal tract. Below the pyramids, in the anterior median fissure, a pyramidal decussation is formed. The pyramidal decussation is set up by the motor fibers of the corticospinal tract disrupting the anterior median fissure. 

The olive contains the superior and inferior olivary nuclei. The anterolateral and posterolateral sulci are the exit points for some cranial nerves: the hypoglossal nerve (CN XII) leaves the medulla oblongata through the anterolateral sulcus, while the glossopharyngeal (CN IX), vagus (CN X), and accessory nerve (CN XI) leave through the posterolateral sulcus

Dorsal surface 

The upper part of the dorsal surface is more expansive and forms the rhomboid fossa, while the lower part is narrower. The dorsal surface is facing the fourth ventricle. The dorsal surface has a midline groove called the posterior median sulcus. The posterior median sulcus is a continuation of the posterior sulcus of the spinal cord. The posterior median sulcus ends approximately in the middle of the medulla oblongata, where the central canal widens into the fourth ventricle. 

On both sides of the posterior median sulcus are the right and left posterior intermediate sulci. The gracile fascicle lies between the posterior median sulcus and the posterior intermediate sulcus. The gracile fascicle ends as the gracile tubercle that contains the gracile nucleus

Another fascicle, the cuneate fascicle that ends as the cuneate tubercle containing the cuneate nucleus, is found between the posterior intermediate and posterolateral sulci. Both the gracile and cuneate fascicles contain sensory fibers. Lateral to the cuneate nucleus is the spinal nucleus of the trigeminal nerve. The nucleus forms the trigeminal tubercle. The dorsal surface's upper margins are thick and form the inferior cerebellar peduncles

Internal anatomy of the medulla oblongata

At the cross-section, the medulla oblongata has three parts: basis, tegmentum, and tectum. The basis is the most ventral part of the medulla oblongata and contains the pyramidal decussation. The tegmentum is the middle part and has the inferior olivary nucleus and the nuclei of the cranial nerves V, IX-XII. The dorsal part is the tectum housing the inferior medullary velum, the posteroinferior part of the fourth ventricle. 

As any other part of the brain, the medulla oblongata consists of grey and white matter. The grey matter is formed by the nuclei, while the white matter - by the ascending and descending tracts. The cell bodies of the neurons shape the nucleus. The tract is a structure of the axon bundles connecting the nuclei with different parts of the CNS. 

Nuclei of the medulla oblongata

The medulla oblongata contains three groups of nuclei: the cranial nerve, relay, and reticular nuclei. 

Cranial nerves

The cranial nerve's nuclei are from the cranial nerve V, IX-XII. The nuclei are:

  • The spinal nucleus of the trigeminal nerve (CN V) is a sensory nucleus gathering information about touch, vibration, pain, and temperature from the ipsilateral side of the face. The spinal nucleus of the trigeminal nerve has three subnuclei: the oral subnucleus, caudal subnucleus, and interpolar subnucleus. The oral subnucleus is responsible for the discriminative tactile sensation from the orofacial region. The caudal subnucleus is linked to nociception and thermal sensations from the head. The interpolar nucleus is related to the tactile sense and dental pain.
  • The nucleus ambiguus is the common motor nucleus of the glossopharyngeal nerve (CN IX), vagus nerve (CN X), and the accessory nerve (CN XI) and is a motor nucleus lying deep in the reticular formation. The nucleus contains neuronal bodies that innervate the muscles of the soft palate, pharynx, larynx, and upper esophagus, taking part in speech and swallowing. The nucleus ambiguus holds preganglionic parasympathetic neurons, which innervate postganglionic parasympathetic neurons in the heart.
  • The solitary nucleus is the common sensory nucleus of the facial nerve (CN VII), the glossopharyngeal nerve (CN IX), and the vagus nerve (CN X). The solitary nucleus receives general visceral information from mechanoreceptors and chemoreceptors within the heart, lungs, airways, gastrointestinal tract, pharynx, and liver via the glossopharyngeal and vagus nerves, while from the nasal cavity, soft palate, and sinus cavities - via the facial nerve. The information about taste arrives through all three nerves. The solitary nucleus projects to the sensory thalamus and so to the cerebral cortex.
  • The inferior salivatory nucleus of the glossopharyngeal nerve (CN IX) is a parasympathetic nucleus. The inferior salivatory nucleus provides parasympathetic fibers to the glossopharyngeal nerve and innervates the parotid gland through the otic ganglion.
  • The dorsal nucleus of the vagus nerve (CN X) is a parasympathetic nucleus. The fibers originating from the nucleus supply the heart, lungs, bronchi, esophagus, stomach, small intestine, and large intestine until the right two-thirds of the transverse colon.
  • The nucleus of the hypoglossal nerve (CN XII) is a motor nuclei. The nucleus gives rise to the fibers that innervate all of the tongue's intrinsic and extrinsic muscles, except for the palatoglossus muscle.

Relay nuclei

The relay nuclei pass on the stimuli from the peripheral receptor areas to higher cortical centers. The relay nuclei are:

  • The cuneate nucleus, one of the dorsal column nuclei, is a sensory nucleus located in the cuneate tubercle. Via the cuneate fasciculus, the cuneate nucleus receives information about slight touch, proprioception, and vibration from the ipsilateral upper limb. The fibers originating from the nucleus join the medial lemniscus, a part of the dorsal column-medial lemniscus pathway going to the thalamus.
  • The gracile nucleus, the other dorsal column nucleus, is a sensory nucleus within the gracile tubercle. Via the gracile tubercle, the gracile nucleus receives information about slight touch, proprioception, and vibration from the ipsilateral lower limb. The fibers from the nucleus join the medial lemniscus.
  • The arcuate nucleus receives information from the corticospinal tract (motor pathway) and passes it to the cerebellum via the anterior external arcuate fibers and the inferior cerebellar peduncle.
  • The olivary nuclei are three nuclei: dorsal accessory olivary nucleus, medial accessory olivary nucleus. These nuclei are subdivisions of the bigger nucleus - the inferior olivary nucleus. The olivary nuclei coordinate impulses from the spinal cord to the cerebellum regulating motor coordination and learning.

Reticular nuclei

The reticular nuclei are a part of the reticular formation. The nuclei control different autonomic functions. The reticular nuclei are:

  • The raphe nuclei are considered to be the main source of serotonin. The raphe nuclei take part in the modulation of mood, pain, arousal, and thermoregulation.
  • The gigantocellular nucleus gives fibers that innervate the hypoglossal nucleus.
  • The perihypoglossal nucleus receives information from the cerebral cortex, vestibular nuclei, accessory oculomotor nuclei and transfers it to the cerebellum, thalamus, and cranial nuclei taking part in eye movements.
  • The lateral reticular nucleus receives information from different sources and passes on to the ipsilateral hemisphere of the cerebellum.

Tracts of the medulla oblongata

The white matter in the way of the ascending and descending tracts goes through the medulla oblongata. Most of the following tracts only pass the medulla oblongata, but some of them originate from the medulla oblongata. The ascending tracts are:

  • The gracile fascicle passes tactile, vibratory, and proprioceptive information. This information is received from the lower limbs and lower parts of the body (Th5-Co1).
  • The cuneate fascicle receives tactile, vibratory, and proprioceptive information from the upper limbs and upper body (C1-Th5). The cuneate fascicle together with the gracile fascicle contains nerve fibers from the spinal nerve's sensory ganglia. The fibers travel to their corresponding nuclei. The axons of the nuclei form the internal arcuate fibers that cross each other and form the decussation of the medial lemniscus. The fibers beyond the decussation are called the medial lemniscus and via the thalamus send information about proprioception and a sense of touch to the primary somatosensory cortex.
  • The spinothalamic fibers come from the lateral and anterior spinothalamic tracts. The fibers pass the pain, temperature, and touch sensation to the thalamus. The anterior spinothalamic tract connects the nucleus proprius in the spinal cord with the thalamus. The anterior spinothalamic tract carries information about light touch and pressure sensation. The lateral spinothalamic tract connects the substantia gelatinosa in the spinal cord with the thalamus. The lateral spinothalamic tract transmits information about pain and temperature.
  • The anterior spinocerebellar tract carries the skeletal muscle's unconscious proprioceptive information from the lower limbs to the cerebellum.
  • The posterior spinocerebellar tract carries the proprioceptive, touch, and pressure information from the lower limbs and trunk to the cerebellum.
  • The olivocerebellar tract sends balance information from the olivary nucleus to the cerebellum.
  • The spinal tract of the trigeminal nerve carries the sensory information from the face to the trigeminal nerve's spinal nucleus.

The descending tracts are:

  • The pyramidal tract consists of the corticospinal fibers and corticonuclear fibers. The pyramidal tract is the common voluntary movement path. The corticospinal fibers regulate voluntary movements through the spinal cord's nerves. They are located closer to the ventral surface, in the pyramids, where approximately 80% of the fibers cross, forming the pyramidal decussation. The fibers that cross form the lateral corticospinal tract, while the fibers that do not cross - the anterior corticospinal tract. The corticonuclear fibers create the voluntary movement path regulating movements through the cranial nerves. Within the medulla oblongata, they synapse with cranial nerves IX, X, XI, XIII motor nuclei.
  • The rubrospinal tract regulates automatic movements and takes part in forming the movement coordination path. The tract connects the ruber nucleus with the motor nuclei of the spinal cord.
  • The tectospinal tract is a reflector vision and hearing path. The tract transmits impulses that cause a protective reaction to unexpected vision or hearing organ irritation. The tectospinal tract connects the superior colliculus with the motor nuclei of the spinal cord.
  • The reticulospinal tract regulates the arousal of the brain and the activity of the reflexes. The tract connects the reticular formation cells within the brainstem with the spinal reticular formation and motor nuclei in the spinal cord.
  • The medial longitudinal fascicle coordinates the eyeball movement with the head and neck movements. The fascicle connects the motor nuclei of the cranial nerve III, IV, VI, and XI with the motor nuclei in the spinal cord's cervical segment. The medial longitudinal fascicle takes part in the automatism of the reflective movement together with the vestibulospinal tract and olivospinal tract.
  • The vestibulospinal tract is a reflector balance path that connects the vestibular nuclei with the motor nuclei in the spinal cord.
  • The olivospinal tract is a reflector balance path connecting the olivary nucleus with the motor nuclei in the spinal cord.

Cross-section of the medulla oblongata

The internal anatomy of the medulla oblongata differs within different cross-sections: at the decussation of the pyramids, decussation of the lemnisci, level of the olives

The decussation of the pyramids

The pyramidal decussation is the major decussation point of the descending motor fibers. The pyramidal decussation is the lower part of the medulla oblongata and crosses the posterior, lateral, and anterior funicles continuing from the spinal cord. The corticospinal fibers separate the grey matter from the anterior funicle. 

These fibers cross the pyramidal decussation to join the contralateral lateral funicle. It affects the location of the central grey matter and central canal by pushing them dorsally. At the decussation of the pyramids, the gracile and cuneate nuclei are found in the medulla oblongata's dorsal part. Both nuclei are located in front of their respective fascicles, as well as bilaterally to the posterior median sulcus. The cuneate fascicle and gracile fascicle continue from the dorsal column of the spinal cord.

Posterolaterally, in the medulla oblongata, the spinal nucleus of the trigeminal nerve and its tract is found. The spinal tract is more posterior than the spinal nucleus. In this level, the medial longitudinal fascicle is lateral to the lateral corticospinal tract. The medial accessory olivary nucleus is also lateral to the lateral corticospinal tract. 

The decussation of the medial lemniscus

In the cross-section at the decussation of the lemnisci level, the gracile and cuneate fasciculi continue going up within the dorsal part. At this level, the gracile and cuneate nuclei form the gracile and cuneate tubercles below the lower margin of the fourth ventricle. The cuneate nucleus is lateral and superior to the gracile nucleus. The gracile and cuneate nuclei give off the internal arcuate fibers that form the medial lemniscus pathway. 

The internal arcuate fibers go anteriorly, lateral to the central canal, hypoglossal nucleus, and the medial longitudinal fascicle. Then the internal arcuate fibers travel medial and decussate, forming the medial lemniscus that ascends to the thalamus. The decussation is dorsal to the pyramids and ventral to the central grey matter. The decussation of the medial lemniscus is known as sensory decussation. The pyramidal tract is found ventrally to the decussation, while the medial longitudinal fasciculus is dorsal. 

Ventrally to the gracile nucleus is the dorsal motor nucleus of the vagus nerve, the solitary tract, and the solitary nucleus. Also, the hypoglossal nucleus is seen at this level, being close to the midline. Laterally to the medial lemniscus and behind the inferior olivary nucleus is the nucleus ambiguus, while the inferior olivary nucleus itself is found dorsal to the pyramid and lateral to the medial lemniscus. 

At the level of the olives

In the cross-section at the level of the olives, additional structures to previously mentioned are seen. This level is the superior region and is wider. Here, the central canal has moved more posteriorly and widened into the fourth ventricle. The medial longitudinal fascicle, tectospinal tract, and medial lemniscus are situated medially. In front of the hypoglossal and dorsal vagal nuclei and lateral to the medial longitudinal fascicle is the reticular formation

At this level, also the medial vestibular nucleus and solitary nucleus are seen located lateral to the dorsal vagal nucleus, while the inferior vestibular nucleus and posterior cochlear nucleus are lateral to the medial vestibular nucleus. The anterior cochlear nucleus is above the inferior cerebellar peduncle. At this level, the anterior spinocerebellar tract is lateral, superficial, and above the CN X fibers leaving the medulla oblongata. The arcuate nucleus is found in the pyramid.

The rhomboid fossa

The rhomboid fossa is formed by the dorsal surfaces of the medulla oblongata and pons. The fossa has a rhomboid shape with four margins and four angles. The upper margins are made by the superior cerebellar peduncles, while the lower margins – the inferior cerebellar peduncles. The upper and lower angles are narrow, but the lateral angles are wide. The surface of the fossa is not completely smooth. 

Vertically, in the midline is the median sulcus that divides the fossa into two equal halves. Transversely from the lateral angle to the median sulcus travel the striae medullares of the fourth ventricle. They divide the rhomboid fossa in the upper and lower part while also creating the border between the dorsal surfaces of the medulla oblongata and pons. Lateral to the median sulcus is the sulcus limitans

Between the median sulcus and sulcus limitans, a small prominence is seen – the medial eminence. Above the striae medullares is a small prominence called the facial colliculus. The neurons near the sulcus limitans in the upper part of the rhomboid fossa contain a pigment that changes the brain colour. Because of that, this place is known as the locus coeruleus

Below the striae medullares, the medial eminence gets narrower and triangular-shaped. This place is called the triangle of the hypoglossal nerve. Another triangle, the triangle of the vagus nerve, is located laterally from the previous triangle. The surface of the rhomboid fossa is slightly elevated in both lateral angles and is called the vestibular area.

The nuclei of the cranial nerves V-XI are located in the white matter of the rhomboid fossa. In descending way, they are:

  • The nuclei of the trigeminal nerve (CN V):
    • The motor nucleus of the trigeminal nerve located in the locus coeruleus;
    • The sensory nucleus of the trigeminal nerve, which is responsible for sense of touch and found lateral to the motor nucleus;
    • The mesencephalic nucleus has a proprioceptive sensation. It goes as a line upward and ends in the midbrain;
    • The spinal nucleus has a pain and temperature sensation. It goes down through all medulla oblongata and ends within the upper cervical segments of the spinal cord.
  • The nucleus of the abducens nerve (CN VI) is a motor nucleus located within the area of the facial colliculus.
  • The nuclei of the facial nerve (CN VII):
    • The nucleus of the facial nerve is a motor nucleus located lateral and deeper to the facial colliculus;
    • The superior salivatory nucleus is the nucleus of the intermedia nerve (a part of the facial nerve). This nucleus is parasympathetic above the striae medullares;
    • The solitary tract nucleus is the nucleus of the intermedia nerve. It is a sensory nucleus below the striae medullares and common with the glossopharyngeal and vagus nerve.
  • The nuclei of the vestibulocochlear nerve (CN VIII). The vestibulocochlear nerve consists of two parts: the vestibular and cochlear nerve:
    • The vestibular nuclei are sensory nuclei within the vestibular area, more within the medulla oblongata;
    • The cochlear nuclei are sensory nuclei located within the vestibular area, more within the pons.
  • The nuclei of the glossopharyngeal nerve (CN IX):
    • The nucleus ambiguus is a motor nucleus, common with the vagus nerve and accessory nerve and found lateral to the triangle of the hypoglossal nerve;
    • The solitary tract nucleus is a sensory nucleus, common with the vagus and intermedia nerves;
    • The inferior salivatory nucleus is a parasympathetic nucleus, below the striae medullares of the fourth ventricle.
  • The nuclei of the vagus nerve (CN X):
    • The nucleus ambiguus is a motor nucleus common with the glossopharyngeal and accessory nerve;
    • The solitary tract nucleus is a sensory nucleus common with the glossopharyngeal and intermedia nerves;
    • The dorsal nucleus of the vagus nerve is a parasympathetic nucleus within the triangle of the vagus nerve.
  • The nuclei of the accessory nerve (CN XI):
    • The nucleus ambiguus is a motor nucleus common with the glossopharyngeal and vagus nerve;
    • The spinal nucleus of the accessory nerve is a motor nucleus located in the upper six cervical segments of the spinal cord.
  • The nucleus of the hypoglossal nerve (CN XII) is a motor nucleus within the triangle of the hypoglossal nerve.

The way the nuclei are distributed within the rhomboid fossa has a specific regulatory. The motor nuclei are located the closest to the midline, while a bit more lateral is the parasympathetic nuclei, with the sensory nuclei being located the furthest from the midline.

The axons of the rhomboid fossa’s sensory neurons take part in forming the ascending tracts transmitting the sensory impulses to the higher brain parts. The motor tracts, regulating voluntary movements through the cranial nerves and originating from the cerebral cortex, end in the motor nuclei. 

Vasculature of the medulla oblongata

The medulla oblongata is supplied by:

The anterior spinal artery supplies the beginning at the central canal, also known as the ventral border of the fourth ventricle, and continues to surround the pyramids supplying them. The posterior spinal artery supplies the posterior part of the medulla oblongata below the olives. Other parts of the medulla oblongata are supplied by the vertebral and inferior cerebellar arteries. 

The veins draining the medulla oblongata drain into the occipital sinus dorsally and the basilar plexus of veins, and the inferior petrosal sinus ventrally. The medulla oblongata veins connect with the spinal veins.