Pons

• Pons is 2.5cm long structure part of brain stem between midbrain and medulla oblongata. It lies in posterior cranial fossa on the clivus, anterior to cerebellum.
• Pons act as a relay station from medulla, cerebrum and cerebellum.

  Ventral pons contains pontine nucleus that relay motor signals from cortex to cerebellum which are essential for coordinating voluntary movements. It also contains the pneumotaxic and apneustic centers, which are involved in relagulation of breathing pattern.
• Most prominent features of anterior/ventral pons are:

  • Basilar groove which lodges the basilar artery (fusion of the vertebral arteries)
  • lateral to basilar groove there are transversely situated markers formed by the transversely running fibers of corticopontocerebellar tracts.
  • On the anterolateral aspect there the motor and sensory roots of trigeminal nerve which is the site that marks the transition between pons and middle cerebellar peduncle, which connects it to the cerebellum.
• The superior border of pons is marked by the presence of posterior cerebral artery which is being sandwiched by the 3 (above) and 4 (bellow) cranial nerves.

  The lower of pons on the anterior surface is the bulbopontineal sulcus just above the pontomedullary junction. it marks the transion between pons and medulla.

  At the ponto medullary juntion 3 cranial nerves merges: abducens (most medially), facial and vestibolochloear nerves (most laterally). Next to the facial nerve there is nervus intermedius which carries sensory and parasympa fibers of facial nerve.
• Dorsal surface of pons

  • Diamond shaped depression
  • Hidden by the cerebellum
  • Forms part of the floor of 4th ventricle
• Lower limit of pons on posterior surface


  Striae medullaris (nerve fibers that run transversely along floor of 4th ventricle)
• On each side of median sulcus

  • Vertical prominence called median eminence
• Inferior end of median eminence

  • Elevation called facial colliculus
  • Formed by the motor fibers of facial nerve looping around abducens nucleus
• Lateral to median eminence

  • Sulcus limitans
  • Reaches a bluish spot called locus coeruleus (noreadrenergic neurons)
• Lateral to sulcus limitans

  • Vestibular area
  • Acoustic tubercle of the rhomboid fossa
• Cross section of pons shows two distinct regions: basal pons and tegmentum.

  • Basal pons contains common structures at all levels of pons: number of descengi tracts of myelinated fibers (white matter); many nuclei (gray matter) and transverse pontine fibers that continue laterally to become middle cerebellar peduncle
  • tegmentum pontis: contains different structures at different levels of pons in cross-section. It forms floor of 4ventricle and contains cranial nerves nuclei + their associated fiber tract
• Basilar part of pons forms the prominent bulbous ventral segment of pons. It contains:

  • longitudinal corticospinal/corticonuclear/corticopontine tracts
  • transverse corticopontocerebellar tracts
  • pontine nucleus scattered throughout these fibers
• The longitudinal fibers are mainly part of corticospinal tract but some fiber are from tracts that start in cerebral cortex and terminate in pons

  • corticospinal
  • corticonuclear
  • corticopontine
  • frontopontine
• The transverse fibers are coming from pontine nucleus decussates in the middle crossing opposite site forming middle cerebellar peduncle to reach cerebellum. These are corticopontocerebellar fibers.
• Nuclei in basilar part of pons are mainly pontine nucleus and these are formed by two types of neurons: excitatory glutaminergic neurons and inhibitory gabanergic neurons. It receives input from cerebral cortex and relays them to cerebellum via the middle cerebellar peduncles a pathway Essential in coordination of motor moviments.
• Tegmentum of pons lies dorsal to basilar part and anterior to 4ventricle. Its continuous with tegmentum of medulla and midbrain.

  • It contains seversl gray matter nuclei and their associated tract + the pneumotaxic and apneustic centers
• Trapezoid body


  • Bundle of auditory fibers
  • Cochlear n enters brainstem and synapse at the cochlear nuclei
  • some Fibers cross the midline and synapse in superior olivary complex. Other cross over and move upwards as dorsal and intermediate acoustic striae
  • Both of these fiber come together to continue upwards as lateral leminiscus and synapse in inferior colliculi
• Medial leminiscus


  • Anteromedial to trapezoid body on each side
  • Sensory pathway that carries info about fine touch, vibration and propioception
• Spinal leminiscus


  • Anteromedial to trapezoid body on each side
  • Sensory pathway that carries info about pain, temperature and crude touch
• On lateral sides of tegmentum there is the ventral and dorsal pairs of cochlear nuclei and btw them the inferior cerebellar peduncle.

  Anterior to them is the vestibular nuclei and thr trigeminal spinal nuclei and their associated spinal tract
• Blood supply of pons in mainly by the pontine branches of basilar artery and terminal branches of vertebral artery.
• Trigeminal nuclei in pons 3 main terminal nuclei and one motor nuclei.

  • Main sensory nucleus: receives info related to dircriminative touch, propioception and vibration from face
  • spinal trigeminal nuclei: extends from midpons to upper cervical cord, receives sensory info related to pain, T and sensation from face.
  • Mesencephalic nuclei: in midbrain. Receives proprioceptive info from muscle of mastication and temporomandibular joint.

  Motor nucleus of trigeminal n: in midpons. Contains motor neurons that innervates muscles of mastication.
• The spinal trigeminal nuclei extend from mid- pons into the upper cervicsl spinal cord.

  it has. 3 part: pars oralis in pons, pars interpolaris in medulla and pars caudalis in cervical spinal cord.

  Together with the main sensory nuclei they receive fibers coming from DRG.

  They give fibers that decussates in the midline and form the trigeminal lemniscus which later join the lateral leminuscus.
• Cross section of pons at level of facial colliculi

  
• Cross section of pons at level of floor of 4ventricle

  
• Dorsal surface of pons

  
• Ventral surface of pons

  
• facial colliculi

  • elevation on dorsal aspect of pons formed by the looping of fibers from the facial nerve nuclei around abducens nuclei, these later pierces spinal leminiscus and exist as the facial nerve.

  
• Trigeminal nuclei in pons 3 main terminal nuclei and one motor nuclei.

  • Main sensory nucleus: receives info related to dircriminative touch, propioception and vibration from face
  • spinal trigeminal nuclei: extends from midpons to upper cervical cord, receives sensory info related to pain, T and sensation from face.
  • Mesencephalic nuclei: in midbrain. Receives proprioceptive info from muscle of mastication and temporomandibular joint.

  Motor nucleus of trigeminal n: in midpons. Contains motor neurons that innervates muscles of mastication.

  
• List all structures seen in pons at level of trigeminal nucelus

  At basilar pons: pontine nuclei

  At tegmental pons

  1. Trapezoid body: crossing of acoustic striae ( auditory fibers from cochlear)
  2. Medial lemniscus (sensory info from dorsal column to thalamus)
  3. Trigeminal lemniscus
  4. Spinal lemniscus
  5. Lateral lemnicus
  6. Ventral spinocerebellar tract
  7. Rubrospinal tract
  8. Tectospinal tract
  9. Medial longitudinal fasciculus
  10. Main sensory nuclei of trigeminal
  11. Principal pontine nucleus of trigeinal
  12. Superior medullary velum
  13. superior cerebellar peduncles

  
• List all structures seen at pons at level of facial colliculus

  Basilar pons: pontine nuclei

  Tegmentum:

  • Trapezoid body
  • Medial lemniscus
  • Spinal lemniscus
  • Ventral spinocerebellar tract
  • Spinal nucleus of trigeminal
  • Motor nucleus of facial
  • Superior salivatory nucleus of facial
  • Nucleus of tractus solitarius of facial
  • Facial nerve fibers
  • Abducens nucleus
  • Vestibular nuclear complex
  • Dorsal + ventral cochlaer nuclei
  • Supeior olivary nuclei
  • Rubrospinal tract
  • Tectospinal tract
  • Medial longitudinal fasciculus
  • Facial colliculus

  
• Medial lemniscus

  • Carries sensory information (touch, vibration, propioception from body excluding face to the thalamus)
• Lateral lemniscus

  • Carries hearing information from the cochlear nucleu to inferior colliculus in the midbrain
  • The fibers forming it cross in the trapezoid body

  
• Spinal lemniscus

  • Transmits pain, temperature, crude touch sensations from body via the ventral and lateral spinothalamic tracts in the spinal cord to the thalamus

  
• Trigeminal lemniscus

  • Carries sensory information from the face to the thalamus (Pain, Temperature, Crude touch,Pressure)
  • These sensations are picked up by CN V (trigeminal) and this tract originates when fibers divide from the spinal nucleus of CN V at a lower level in the pons

  
• Ventral spinocerebellar tract

  • Ascending fibers carrying proprioceptive information from muscles, tendons and ligaments to the cerebellum
  • Forms from lateral white column fibers at the levels of L2 to Coccis
  • This is used by the cerebellum as feedback on motor movement
  • Aids movement co-ordination

  
• Rubrospinal tract


  Descending motor pathway to flexor muscles

  
• Tectospinal tract


  Descending motor information from inferior and superior colliculi to muscles of the head, neck and eyes

  • Allows us to respond quickly to visual and auditory stimuli

  
• Medial longitudinal fasciculus

  • Ascending tract that connects the Nuclei of Cranial Nerves III, IV and VI
  • Co-ordinates eye movements

  
• Motor nucleus of trigeminal

  Motor supply to structures of 1st pharyngeal arch (muscles of mastication)

  1. temporalis
  2. Masseter
  3. Medial pterygoid
  4. Lateral Pterygoid
  5. Suprahyoid muscles