Pons and Midbrain Anatomy

• The pons and midbrain make up the brainstem
• Pons is studied in three sections: general info, external anatomy, internal anatomy, and blood supply
• Midbrain is studied based on general information, external anatomy, internal anatomy, and blood supply

Learning Objectives

• Describe the boundaries/landmarks distinguishing the medulla, pons, and midbrain
• Identify external anatomical landmarks/structures of the pons and medulla on images
• Identify the three major levels of the pons and their associated nuclei and tracts
• Identify the two major levels of the midbrain and their associated nuclei and tracts
• Identify structures at the midbrain to diencephalon transition zone
• Describe the relationship between the ventricular system and brainstem, noting which part is associated with the midbrain and pons
• Describe and identify the major arteries that supply the pons and midbrain
• Describe signs/symptoms from brainstem lesion images affecting ALS, PC/ML, or voluntary motor pathways

Pons General Information

• The pons is the middle segment of the brainstem
• Rostral boundary - midbrain
• Caudal boundary - medulla
• Posterior - 4th ventricle and cerebellum
• The word "pons" translates to "bridge" in Latin
• Early anatomists saw the pons as a bridge connecting to the cerebellum
• The pons and cerebellum together are called the metencephalon
• The basilar section contains descending cortical fibers, pontine nuclei, and transverse fibers that enter the contralateral cerebellum
• Cortical fibers innervate neurons in the pontine nuclei, which project to the cerebellum as transverse fibers
• The tegmental portion contains sensory and motor nuclei
• Cranial nerves in tegmental portion include V, VI, VII, and VIII

Pons External Anatomy

• Anterior view shows the basilar pons and cranial nerve VI
• Lateral aspect features the middle cerebellar peduncle and cranial nerves V, VII, and VIII
• Posterior view (with cerebellum removed) reveals middle and superior cerebellar peduncles, and the facial colliculus
• Cerebellopontine angle is clinically significant; acoustic neuromas can grow here affecting CN VIII, and then CN VII

Internal Anatomy of the Pons

• Pons has three major sections that depict various nuclei and tracts: caudal, mid, and rostral
• Caudal Pons: Features include the:
  • Medial lemniscus starts to bend laterally towards the PC/ML pathway
  • ALS (anterolateral system)
  • ASCT (anterior spinocerebellar tract)
  • inferior cerebellar peduncle
  • spinal trigeminal nucleus and tract
  • medial longitudinal fasciculus (MLF)
  • medial and inferior vestibular nuclei
  • corticospinal tract
• Caudal Pons: New structures in this section include the:
  • Abducens nucleus contains cell bodies of CN VI, which innervate the ipsilateral lateral rectus extraocular muscle
  • Facial nucleus which cell bodies will innervate muscles of facial expressions
  • Middle cerebellar peduncle which axons will enter into the cerebellum
  • Pontine nuclei gives rise to pontocerebellar fibers that project to the cerebellum via the middle cerebellar peduncle
• Mid-level Pons: Previous structures include the:
  • MLF
  • middle cerebellar peduncle
  • medial Lemniscus as part of the posterior columns/medial lemniscus pathway
  • ALS
  • pontine nuclei
  • CST
• Mid-level Pons: New structures include the:
  • Lateral Lemniscus as part of the auditory pathway
  • Trigeminal motor nucleus - Cell bodies whose axons will innervate the ipsilateral muscles of mastication
  • Principal sensory nucleus - Cell bodies receiving tactile, vibratory sensations from the ipsilateral face
  • Mesencephalic nucleus and tract - pseudounipolar neurons relaying ipsilateral proprioceptive information from the mastication muscles
  • Transverse fibers in the pontine nuclei represent pontocerebellar fibers - axons from pontine nuclei traveling to the cerebellum
• Rostral Pons: Previous structures include the:
  • Mesencephalic nucleus and tract
  • MLF
  • Trigmenial motor nucleus
  • Principal Sensory nucleus
  • ALS
  • Medial Lemniscus
  • Lateral Lemniscus
  • Pontine nuclei
  • CST
  • Middle cerebellar peduncle
• Rostral Pons: New structures include the:
  • Superior Cerebellar Peduncle sends axons to and from the cerebellum
  • Locus Coeruleus contains cell bodies with a smokey blue appearance that contain Noradrenaline/Norepinephrine, and their axons project throughout the CNS
  • Cell bodies have low discharge rates during sleep and high discharge rates during stress

Reticular Formation

• Reticular formation spans the brainstem
• Network of nuclei and neurons (reticular & raphe nuclei without distinct boundaries)
• Name from "reticulum," Latin for "little net"
• Functions as a major integration and relay center for crucial neural pathways essential for survival and coordinating various functions
• Influences control of heart and respiratory rate, pain modulation, habituation, coordination of somatic motor movements, sleep-wake cycles, circadian rhythm, consciousness, and arousal
• Ventral lateral reticular areas aid in controlling heart rate and respiration

Cranial Nerve Nuclei in Pons

• Nuclei of CN V (partial), VI, VII & VIII (partial) can be found here
• Superior and inferior salivatory nuclei (of IX) are too small to identify in the lab

Blood Supply to the Pons

• Supplied by branches of the basilar artery
• Basilar artery stroke: due to blockage of the basilar artery/branches, can result in "locked-in" syndrome with total body and face paralysis, some eye movements and pupillary reflex may be spared
• Large strokes damaging nuclei of the ascending reticular activating system (ARAS) bilaterally can lead to coma or death

Midbrain General Information

• Rostral Boundary: Diencephalon
• Caudal Boundary: Pons
• Cerebral aqueduct runs through it
• Cranial nerve nuclei III & IV are located here
• Midbrain has a basis pedunculi (anteriorly), a tegmentum (middle), and a tectum (posteriorly)
• Basis pedunculi contains the substantia nigra and cerebral peduncles (also called the crus cerebri)
• Tectum contains the superior and inferior colliculi, also known as the quadrigeminal plate

Cranial Nerve Nuclei in Midbrain

• Nuclei of CN III and IV can be found here
• Parts of the nuclei for CN V can also be found in the midbrain, but are too small to identify in lab.

Internal Anatomy of the Midbrain

• Midbrain has two major transverse sections: caudal, and rostral
• Caudal Midbrain previous structures
  • Lateral lemniscus (these axons synapse on the inferior colliculus, part of auditory pathway)
    • MLF
    • ALS
    • Medial Lemniscus
• Caudal Midbrain new structures
  • Inferior colliculus - part of auditory pathway
  • Cerebral aqueduct - part of ventricular system
  • Periaqueductal gray - enkephalins present to block pain transmission
  • Trochlear nucleus (CN IV) - cell bodies innervate superior oblique muscle
  • Decussation of superior cerebellar peduncles cross here, ascend to cortex or synapse in red nucleus
  • Substantia nigra - part of basal nuclei (ganglia), contains dopamine, death of these cells results in Parkinson's disease
  • Cerebral peduncles contains axons from many sources; middle 3/5 contains corticospinal tracts which are axons from upper motor neurons descending to the spinal cord
• Rostral Midbrain previous structures:
  • Cerebral Aqueduct
  • Periaqueductal Gray
  • ALS
  • Medial Lemniscus
  • Substantia Nigra
  • Cerebral Peduncles
• Rostral Midbrain new structures:
  • Superior colliculus - visual system
  • Oculomotor nucleus (CN III) - projects axons to innervate 4 extraocular muscles
  • Edinger-Westphal nucleus - parasympathetic nucleus to CN III; contains preganglionic parasympathetic cell bodies whose axons will synapse on the ciliary ganglion and cause pupillary constriction
• Axons of oculomotor nuclei exit between the two cerebral peduncles (in the interpeduncular fossa), and are important when discussing lesions
  • Red nucleus projects as rubrospinal tract, influencing lower motor neurons in spinal cord

Blood Supply to the Midbrain

• Branches of the superior cerebellar artery and posterior cerebral artery supply the midbrain