Medulla audio

Medulla audio/ interactive audios

Electronic School of Medicine

http://www.oluwoleogunranti.com

 

The medulla is the least sophisticated and the lowest part of the brain. It is placed at the lowest level  and joins the spinal cord at the spinomedullary junction. It therefore runs from the spinomedullary junction to the pontomedullary junction where it meets the pons at a sharp edge.

 

It has the following surfaces

1.        Dorsal surface

2.        Ventral surface

3.        Lateral surface

 

Function of the medulla

1.  The medulla is the seat of primitive reticular formation which begins to be formed in the lowest vertebrates such as fishes. This reticular formation is the most sophisticated motor coordinating unit in these animals. In higher vertebrates other centers have taken over the motor control and in  man the control resides in the highest level of the neuraxis, called the neocortex.

2.  The medulla controls the vegetative aspects of the body. Thus it controls the cardiovascular and respiratory systems and is able to provide a cardiac center which includes a cardioacceleratory center and cardio-inhibitory center. Also the center for the control of respiratory function is found in the medulla. Thus if the medulla is dead, there can be no chance that cardiorespiratory function will continue.

3.  The medulla provides the largest center for the nuclei of cranial nerves. In the fishes it provides the center for all cranial nerves but in man it only provides the center for the following cranial nerves 

  •  Hypoglossal

  •  Accessory

  •  Vagus

  •  Glossopharyngeal

  •  Part of vestibulocochlear nerve

(8-12th cranial nerves)

4.  It provides a highly functioning reticular formation

5.  It provides a center for the nuclei of second order neurons of the posterior column tracts. These tracts although primitive, are actually a sophistication over the previous sensory system of the lower vertebrates and higher invertebrates.

Gross anatomy

The medulla is rounded in shape with the 3 surfaces mentioned above.

Ventral surface

This surface is marked by  the junction of the medulla with the pons. It is provided with a median sulcus called anteromedian sulcus which is continuous with the same sulcus in the spinal cord. This sulcus separates the two bilateral pyramids which flank it at the upper medulla. Lateral to the pyramids at a sulcus, which is continuous with the anterolateral sulcus are the rootlets of the hypoglossal nerve. They occupy this sulcus which is termed the olivopyramidal sulcus. Lateral to this sulcus is the olive. This upper medullary structure forms a prominence lateral to the sulcus and the pyramid. Lateral to the olive and a bit to the lateral surface of the medulla at its upper part is the inferior cerebellar peduncle. But before this peduncle we have sulcus called the postolivary sulcus which houses the rootlets of the cranial nerves XI, X and IX. The 9th is placed superiorly while the 10th is in the middle. The 11th is placed inferiorly.  The remaining cranial nerves associated with the medulla issue from the cerebellopontine angle. This is the junction between the cerebellum, pons and medulla. The nerves are as follows

  • Vestibulocochlear nerve which is placed inferiorly

  • Facial nerve which is placed  superiorly

  • Nervus intermedius which is placed in between the two.

At the lower level of the medulla, the following are prominent-

1.        Pyramidal decussation. These are so clearly  evident that it is easy to see the decussation within the anteromedian sulcus. The decussation stops at the spinomedullary junction.

2.        The olive is only formed at its rostral level. Also there is no pyramid, because the structure that form the pyramids have begun to decussate.

 

Dorsal surface 

The dorsal surface can be easily described by dividing the medulla into two parts

The upper medulla, which is also called the open medulla

The lower medulla, which is also called the closed medulla.

The lower medulla has the following important features at the back of that structure

1.        Tuberculum gracilis. This is the swelling which forms the nucleus gracilis within the medullary substance

2.        Tuberculum cuneatus – this also forms the nucleus cuneatus

3.        Accessory cuneatus nucleus. The tuberculum gracilis is placed just lateral to the posteromedian septum which is continuous with that of the spinal cord. The tuberculum cuneatus is placed lateral and above the tuberculum gracilis. The accessory cuneatus nucleus is placed lateral to the tuberculum cuneatus. All the structures hitherto mentioned can be easily found inferior to the floor of the fourth ventricle.

 At the upper medulla, the 4th ventricle comes into view. It has the following features

  • Floor which is diamond shaped and therefore called rhomboid fossa. A roof which is covered by tela choroidea and the inferior medullary velum at the back of the medulla. The rhomboid fossa (and indeed the whole of the 4th ventricle) extends to the back of the pons. Its lower limit or inferior edge forms the calamus scriptorius. At the lateral edge of the calamus scriptorius is the obex. Lateral and slightly superior to the obex is the hypoglossal trigone. Above this trigone is the vagal trigone. At the junction between the pons and the medulla, we have the striae medullares. At this junction, the two lateral recesses of the 4th ventricle are found. They are the foramina of Luschka. Surrounding the junction between the pons and medulla is the vestibular area which is found within the tuberculum acousticum. But above the vestibular area and directly behind the pons is the facial colliculus. This colliculus is made up of internal structures as follows

  • Abducent nucleus

  • Internal genu of the facial nerve

Lateral surface 

The lateral surface of the medulla has only the inferior cerebellar peduncle. This peduncle begins in the lower medulla and appears on the lateral aspect of the upper medulla to course into the cerebellum. It will carry fibers from the inferior olive to reach the cerebellar nuclei.

 Medial surface

Certain structures are visible at the medial surface and they include the 4th ventricle and pyramidal tract.

 

INTERNAL ORGANIZATION OF MEDULLA

The medulla can be described again from the point of view of two parts i.e. open (upper) medulla and closed (lower) medulla

Lower medulla

The configuration of the spinal cord changes gradually as the spinomedullary junction is reached.

1.        The formation of the spinal tract and nucleus of the trigeminal (V) nerve at the level of C4 up to higher cervical levels changes the configuration of the substantia gelatinosa so that it becomes bulbous. This configuration is maintained to higher levels of he medulla.

2.        2. The formation of the pyramidal tract and their decussation also changes the configuration of the ventral horn of the grey matter. It is cut off at its bottom by the decussating fibers of the lower medulla. So that the medullary grey matter now looks like  a butterfly together with the bulbous spinal tact of V, rather than the usual H-shape of the spinal grey matter.

3.        The formation of the corticospinal tract in the lower medulla leads to infringement on the grey matter of the ventral horn so that a reticular pattern is now formed. This reticular pattern is the beginning of the reticular formation.

4.        The posterior column tract now begins to terminate on their various nuclear which are the nucleus gracilis nucleus cuneatus and the accessory cuneate nucleus. The second order neurons of the pathway now begin to issue from their nuclei. They will project as internal arcuate fibers which sweep across the dorsal to the ventral aspect of the lower medulla in order to reach the contralateral medial lemniscus for these fibers decussate ventrally and form the medal lemniscus of the opposite side.

5.        The formation of the medial lemniscus  is a feature of the all levels of the medulla beginning from the lower medulla.

6.        Spinal lemniscus is the continuation of the lateral funiculus which begins from spinal cord levels. They include the spinothalamic tract and the spinocerebellar tract.

7.        Central canal is still maintained at this level

 

Upper medulla

At the upper medulla, the following structures are seen internally

  • Inferior olive

  • Pyramid

  • Inferior cerebellar peduncle

  • 4th ventricle and the cranial nuclei

  • Medullary reticular formation

  •  

Inferior Olive

The olive, which is seen externally at the open or upper medulla, is called internally the inferior olive or the inferior olivary nuclear complex. It has the following nuclei groups

  • Principal inferior olivary nucleus

  • Accesory dorsal olivary nucleus

  • Accessory medial olivary nucleus.

The olive projects 2/3 of the fibers which enter the inferior cerebellar peduncle in order to reach the cerebellum. Hence its main function is to provide a center for the interconnection between the medulla and the cerebellum. Being inferior in the neuraxis it is therefore quite primitive.

The olive has afferent projections from various parts of the brain and spinal cord

  • Spino-olivary tract

  • Cortico-olivary tract (cortex)

  • Fibers form midbrain – periaqueductal gray and the red nucleus

The fibers coming from the red nucleus are called rubro-olivary tract.

The white matter of the olive is called the amiculum olivae

Pyramid

The pyramid contains corticospinal tracts which are three in number

  • Lateral corticospinal tract, which begins decussation at the lower medulla and ends its decussation at the spinomedullary junction.

  • Anterolateral corticospinal tract

  • Anterior corticospinal tract.

The above 2 tracts are uncrossed.

Inferior cerebellar peduncle

This begins at the upper medulla and courses lateral to the 4th ventricle and anterior to it. It is also lateral and dorsal to the olive. It enters the cerebellum and sweeps across the vestibular and cochlear nuclei at the pontomedullary junction.

This is one of the three peduncles that interconnect the cerebellum  with the brainstem. It carries fibers the posterior spinocerebellar tract (1/3) and the cuneocerellar tract. But two third of its fibers are projections from the inferior olive in the tract called the olivocerebellar tract.

Medullary reticular formation

The medullary reticular formation is the highest motor coordinating center in lower vertebrates like fishes. It projects fibers to the spinal cord as reticulospinal tract and fibers reaching from various parts of the brain keep it under control. Hence we have the corticoreticular fibers. Also fibers project from medullary reticular formation to higher cortical centers. These fibers represent the so called ascending reticular activating system (ARAS) or reticular formation for short. This formation is responsible for arousal and hence keeps the individual at its very conscious self. It is acted upon by various fibers coming from the neuraxis. It is blocked by certain anesthetic agents such as ketamine.

The medullary reticular formation is divided into two parts

  • Lateral group

  • Medial group  The medial group is further divide into

  1. Nucleus reticularis gigantocellularis (large cells)

  2. And a nucleus reticularis parvocellularis  (small cells)

 

Cranial nerve nuclei in the 4th ventricle

Hypoglossal nerve

The nucleus of this nerve lies in the hypoglossal trigone at the extreme inferior edge of the floor of the 4th ventricle.  It lies above the obex and lateral to the area postrema. But below the vagal trigone. It contains nucleus of this nerve, which carries GSE fibers to the intrinsic and extrinsic muscles of the tongue excepting the palatoglossus. It is a new addition to the cranial nerves of vertebrates for fishes and lower vertebrates lack the hypoglossal nerve.

It may also carry GSA fibers to the meninges of the posterior cranial fossa, but this is doubtful. When damaged, it will cause an ipsilateral hemiparesis of tongue. In the condition known as inferior  alternating hemiplegia there is a contralateral hemiplegia and an ipsilateral hemiparesis of tongue presumably due to the closeness of the pyramids to the hypoglossal nucleus so that damage to one may also affect the other. Because the fibers of the hypoglossal nerve are not crossed, we then obtain an ipsilateral hemiparesis of tongue.

Accessory nerve

The accessory nerve is the 11th cranial nerve. It has two components

  • Spinal accessory

  • Cranial accessory

The spinal accessory begins at the level of C2 and its fibers are from the ventral horn of these cervical levels. The fibers enter the cranial cavity via the foramen magnum and joins its counterpart, the cranial accessory as it emerges from the nucleus ambiguus, which is associated with the vagal trigone. The two meet at the cranial cavity and then separate at the jugular foramen. The cranial accessory joins the vagus and is distributed to the recurrent laryngeal nerve to supply the intrinsic muscles of the larynx. The spinal accessory moves inferiorly to enter the posterior triangle of the neck and disappears behind the trapezius and supplies that muscle and the sternocleidomastoid.

Vagus nerve

This nerve is the 10th cranial nerve. It is very extensive and it is provided for by a special trigone in the floor of the 4th ventricle. This trigone called the vagal trigone is placed lateral and superior to the hypoglossal trigone. It contains mainly the dorsal nucleus of the vagus, which is the main nucleus associated with the vagus nerve only. Other nuclei groups associated with the vagus are as follows

1. Nucleus of the tractus solitarius

2. Inferior salivatory nucleus

3. Nucleus ambiguus

4. Nucleus of the spinal tract of V

 

Dorsal nucleus of vagus

This nucleus is only associated with the vagus nerve. It is the specialized nucleus for myomotor function and therefore supplies all areas of the vagus distribution with myomotor fibers for smooth muscles in foodways and airways. The vagus extends its supply to the gastrointestinal tract after the laryngeal and pharyngeal supply to reach the splenic flexure. Parasympathetic supply below or inferior to the splenic flexure is via the sacral outflow and not the vagus. Hence the vagus is the most extensive in supply and distribution of all cranial nerves.

It also supplies the heart muscles with cardioinhibitory vagal fibers and hence when damaged may cause problem with the heart. It supplies all the smooth muscles of the larynx, and trachea together with the gastrointestinal tract up to the splenic flexure.

It is placed most medial in the area associated with the vagus at the floor of the fourth ventricle

Nucleus of the tractus solitarius.

This nucleus is placed lateral to the dorsal nucleus of vagus. It contains cell bodies of second order neurons of the taste pathway. They will carry SVE fibers to reach the gustatory nucleus of the cortex (temporal lobe) area 43. They have the cell bodies of their first order neurons placed in the ganglion nodosum, which is the inferior vagal ganglion. This ganglion is associated with GSA and SVA fibers while the superior vagal ganglion is associated with the GSA fibers

Nucleus ambiguus

This is a special medullary nucleus, which is associated only with structures derived from the pharyngeal arches. Hence it carries  SVE fibers. It is placed between the nucleus of tractus solitarius and the dorsal nucleus of vagus around the vagal trigone. It supplies all the muscles of the larynx through superior laryngeal nerve and also the recurrent laryngeal nerve. The superior laryngeal nerve supplies the cricothyroid and the cricopharyngeus while the recurrent laryngeal nerve, which was initially carried in the cranial accessory but is later distributed by the vagus, supplies all the intrinsic muscles of the larynx excepting the above two.

Nucleus of spinal tract of V. 

This nucleus is the most lateral of all nuclei associated with the vagus. It carries general sensation of the somatic system from the area of the auricle and the external auditory meatus. These are very small fibers whose cell bodies are placed in the superior vagal ganglion. Their central processes then run to the medulla and synapse with their second order neurons at the nucleus of the spinal tract of V. The tract of the second order neurons form the trigeminothalamic tract. They will project into the thalamus and reach the ventral posteromedian nucleus of the thalamus (VPM) where the 3rd order neurons issue to reach the sensory cortex.

Inferior salivatory nucleus 

This nucleus is the caudal extension of the pontine superior salivatory nucleus. As its name implies, it is secretomotor and therefore it carries GVE (general somatic efferent) fibers to the glands in the foodways and airways supplied by the vagus and also to the glands of the soft palate and epiglottis. It is also the areas (soft palate and epiglottis) that SVA fibers associated with the vagus supplies for they have scattered taste buds on their surfaces.

Functional components of the vagus

1.                      GSA fibers associated with auricular sensation

2.                      GVA fibers for general visceral sensation in areas supplied by the vagus. Their cell bodies lie in the inferior vagal ganglion called  ganglion nodosum

3.                      SVA fibers. These are taste fibers from the soft palate and epiglottis

4.                      SVE fibers for the muscles of larynx derived from the pharyngeal arches

5.                      GVE secretomotor to all areas supplied by the vagus which includes intestinal and gastric glands to provide digestive and other juices.

Glossopharyngeal nerve

This nerve does not have a special trigone but its nuclei are associated with the vagal trigone as follows

Nucleus ambiguus, which provides SVE fibers to the muscle derived from the 3rd pharyngeal arch and supplied by the IX. This is the stylopharyngeus

Inferior salivatory nucleus. The fibers associated with the IX supply the glands of the posterior 1/3 of the tongue and the oropharynx, which is the main area of supply by the IX

Nucleus of the tractus solitarius- caries SVA taste fibers to the area of IX – oropharynx and the posterior 1/3 of tongue

Nucleus of the spinal tract of V. This carries GSA fibers, which run in the Jacobsons’ nerve to supply the bony aspect of auditory tube, inner aspect of tympanic membrane and the tympanic cavity.

Functional components of IX

1.                      GSA fibers which run in the Jacobsons’ nerve

2.                      GVA fibers for the oropharynx and posterior 1/3 of tongue

3.                      GVE fibers for glands in the oropharynx and posterior 1/3 of tongue

4.                      SVE fibers to stylopharyngeus

5.                      SVA fibers – taste fibers to the posterior 1/3 of tongue and the oropharynx

The IX escapes from the medulla at the upper aspect of the postolivary sulcus above the rootlets of escape of X which in turn is above the rootlets of escape of the XI.

It then enters the jugular foramen also accompanied by the X and XI. Together they transverse this foramen in order to escape into the neck region. At the neck region, the IX seeks for its muscle of supply, after given off branches to the oropharynx and the posterior 1/3 of the tongue. It enters the stylopharyngeus in order to supply that muscle.

 

Vestibulocochlear nerve (VIII)

This nerve essentially is two- the cochlear and the vestibular nerves. The nerve is the only cranial nerve (or indeed any nerve at all) that is SSA. It is therefore sensory to the inner ear apparatus such as the semicircular canals the vestibule and the cochlear. 

Cochlear nerve

The cochlear nerve has the cell bodies of its neurons in the spiral ganglia arranged along the cochlea apparatus of the osseous labyrinth, spiral laminae. The peripheral processes receive impulses from hair cells which lie in the organ of Corti. The central processes of the neurons then issue from the spiral ganglia. They are actually bipolar neurons rather than pseudounipolar. The central processes gather together to form the cochlear nerve which approaches the internal acoustic meatus accompanied by the facial nerve, the nervus intermedius and the labyrinthine artery. Its fibers sweep across the superior edge of the inferior cerebellar peduncle at rostral level of the medulla. The fibers then terminate on the dorsal and cochlear nuclei of the floor of the 4th ventricle at the so called tuberculum acousticum found in the vestibular area.

Second order neurons now issue from these nuclear groups to projects upwards, sweeping across the medial lemniscus and also the superior olive to reach the trapezoid body all of which are found in the pontine tegmentum. The fibers pass through the lateral lemniscus to reach the midbrain and terminate at the inferior colliculus of the midbrain tectum.

3rd order neurons now issue from the inferior colliculus to project upwards. Their destination will be the medial geniculate body of the metathalamus. From the medial geniculate body the geniculotemporal tract arise and terminate on the temporal auditory cortex. This is at the transverse temporal gyrus of the temporal lobe.

It is possible for fibers to reach the medial geniculate body directly from the dorsal or ventral cochlear nuclei of the medulla or for fibers to run from the inferior colliculus to terminate in the  temporal cortex. Whatever be the case, the fibers which reach the superior colliculus or the medial geniculate body are responsible for auditory reflexes. Those which reach the cortex are responsible for voluntary mechanisms for which man is well recognized.

Along the pathway of the auditory system we have several nuclei which occur

1.                      Nucleus of the superior olive

2.                      Nucleus of the trapezoid body

3.                      Nucleus of the lateral lemniscus

4.                      Also we have the brachium of the inferior colliculus, which is the structure that projects fibers from the inferior colliculus to the medial geniculate body.

 

Vestibular nerve

This nerve has its cell bodies placed in the vestibular ganglia. It is responsible for kinetic and static equilibrium. Its peripheral processes reach the receptors which are hair cells placed in the vestibular apparatus (maculae of utricle and saccule) and semicircular ducts (ampullae) of the inner ear. The peripheral processes from 5 separate nerves (anterior canal, posterior canal, lateral canal, utricular and saccular nerves) before they are guided into the vestibular ganglion. The central processes issue from the bipolar neurons of the vestibular ganglia and move centrally as the vestibular nerve, accompanied by the cochlear nerve and the facial nerve into the internal acoustic meatus. It then enters the lower part of the pons and the upper part of the medulla, also sweeping across the inferior cerebellar peduncle to terminate in four characteristic nuclei as follows

  • Medial vestibular nucleus

  • Lateral vestibular nucleus

  • Inferior vestibular nucleus

  • Superior vestibular nuclei

These nuclei have afferent fibers from all parts of the brain including the cerebral cortex. Fibers also reach from the spinal cord and higher cortical centers via the medial longitudinal fasciculus.

Efferent projections now issue from these nuclei and project into the spinal cord via the vestibulospinal tract and other parts of the neuraxis. Some will project into the inferior cerebellar peduncle to reach the cerebellum.

 

 

Professional Neuroanatomy Theory

 

 

 

 

 
 

 

 

 

 


History taking  
Learn clinical examination  
Long cases  
Short cases   
Clinicopathological cases  
Female examination 
Electronic clinical demonstrations  
Electronic clinical conferences  
Electronic clinicopathological conferences  
Integrated Organ examination 
Heart sounds  
Lung sounds
Osteology encyclopedia

Videopage

Main Subject Course Links

Anatomy Anesthesia Biochemistry Chemical pathology Community Health
Dermatology ENT Gynecology Hematology Imaging
Medicine Medical microbiology Obstetrics Ophthalmology Pathology
Pediatrics Pharmacology Physiology Psychiatry    Surgery/Orthopedics
eLab eOSCE eProcedures eInvestigations eSchool/Videopage
eOrgans eLocator Anatomy Museum eDissector eFractures
All diseases eClerking eTreatment eDoctor ePatient

 

 

 

 

© Electronic School of Medicine
Creator: Oluwole Ogunranti