PONS

The pons is situated between the medulla oblongata and the mesencephalon. It is composed of a dorsal portion or tegmentum[1] and a ventral portion. The tegmentum constitutes the floor of the fourth ventricle. It is formed by the following nuclei and tracts:

 

  • Motor nucleus of the trigeminal nerve[2]

  • Pontine sensory nucleus of the trigeminal nerve[3]

  • Locus coeruleus[4]

  • Pontine reticular nuclei

  • Rubrospinal tract[5]

  • Medial lemniscus[6]

  • Lateral lemniscus[7]

  • Tectospinal tract[8]

  • Medial longitudinal fasciculus[9]

  • Dorsal longitudinal fasciculus[10]

 

The ventral portion is formed by pontine nuclei (they receive the corticopontine tract) and longitudinal and transverse fibers. The longitudinal fibers form the pyramidal tract[11] and the corticopontine tract. The transverse fibers are located on the ventral surface of the pons and enter the cerebellum as the middle cerebellar peduncle, connecting the cerebral cortex with the cerebellum.

 

[1] From the Latin tegmentum, “cover”.

[2] From the Latin tres, “three”; geminus, “twins”.

[3] The pontine sensory nucleus of the trigeminal nerve receives temperature and tactile sensations. Its neurons cross the midline to meet at the medial lemniscus and ascend toward the ventrocaudal nucleus of the thalamus.

[4] From the Latin ceruleus, “dark blue”. It is formed by adrenergic fibers located on the medial border of the rostral cerebellar peduncle. In the dog and cat two pontine micturition centers exist: the commonly known pontine micturition center (PMC) responsible for urine emptying and a second, which functions as the urine storage facilitator center. The first is located in the locus ceruleus (or coeruleus), and the second in the nucleus subceruleus (subcoeruleus). Micturition is controlled by coordination of both centers. From Nishizawa, O, and Sugaya, K. in “Cat and dog: higher center of micturition”. Neurol. Urodyn, 1994; 13 (2): 169-179.

[5] The rubrospinal tract was first described by Von Monakow.  It is formed by fibers that originate in the red nucleus. The decussate in the ventral portion of the mesencephalic tegmental decussation  and descend throughout the lateral funiculus of the spinal cord, terminating in the ventral horn.

[6] The medial lemniscus was first described by Reil. It is formed by fibers originating from the gracile and cuneate nuclei. After leaving these nuclei, these fibers go ventrally and medially, forming the arcuate fibers that will cross the midline at the level of the medial lemniscal decussation. After decussating, they ascend toward the thalamus. The medial lemniscus transmits proprioceptive information.

[7] The lateral lemniscus is formed by auditory fibers that, from the trapezoid body, ascend to the medial geniculate body of the thalamus. Meynert differentiated the medial and lateral lemnisci but it was Forel who named them.

[8] The tectospinal tract is formed from the fibers which, from the tectum mesencephali, decussate in the dorsal portion of the mesencephalic tegmental decussation and descend ventrally to the medial longitudinal fasciculus until they reach the spinal cord. There, they can be found in the ventral and the lateral funiculi (medial to the rubrospinal tract). They synapse on the interneurons of nuclei from the brain stem and the ventral horn.

[9] The medial longitudinal fasciculus is situated in the dorsal part of the brain stem, bordering the raphe. It is associated with vestibulo-ocular reflexes. It descends toward the spinal cord where it is located in the ventral funiculus.

[10] The dorsal longitudinal fasciculus is formed by myelinated and unmyelinated, descending or ascending fibers. It sits in the central gray matter of the myelencephalon, in the floor of the fourth ventricle and in the dorsal gray commissure of the spinal cord. It connects the hypothalamus with the spinal cord.

[11] The pyramidal tract corresponds with a longitudinal eminence that can be seen on the ventral surface of the medulla oblongata. It is formed by the corticonuclear tract and the corticospinal tract. The first is made up of fibers that reach the motor nuclei of the brain stem from the motor cortex. The second connects the motor cortex with the spinal cord.