midbrain ninja nerd

• Midbrain


  A part of the brainstem located between the cerebrum and the pons
• Analyzing the midbrain


  1. Take cross-sections at the level of the inferior colliculus

  2. Take cross-sections at the level of the superior colliculus

  3. Discuss the internal structures and their functions
• Midbrain anatomy


  • Ventral portion
  • Dorsal portion with inferior and superior colliculi
• Crus cerebri


  The ventral portion of the midbrain where descending motor pathways are located
• Subdivisions of the crus cerebri

  • Corticospinal fibers
  • Corticonuclear fibers
  • Frontal pontine fibers
  • Other cortico-pontine fibers
• Location of motor pathways in the crus cerebri

  • Medial portion - head and upper body
  • Middle portion - trunk
  • Lateral portion - lower extremities
• Tegmentum


  The region between the crus cerebri and the inferior/superior colliculi
• Substantia nigra


  A structure in the tegmentum, with a pars compacta (dopaminergic) and a pars reticularis (GABAergic)
• Ascending sensory pathways in the tegmentum

  • Medial lemniscus
  • Trigeminal lemniscus
  • Spinal lemniscus
  • Lateral lemniscus
• Locus coeruleus


  A nucleus in the tegmentum that produces norepinephrine and is involved in the descending pain pathway
• Dorsal raphe nucleus


  A nucleus in the tegmentum that is also involved in the descending pain pathway
• Mesencephalic nucleus of the trigeminal nerve


  A nucleus in the tegmentum that receives proprioceptive information from the trigeminal nerve
• Cerebral aqueduct


  A fluid-filled tube connecting the third and fourth ventricles, surrounded by the periaqueductal gray matter
• Medial longitudinal fasciculus


  A pathway connecting the vestibular nuclei to cranial nerve nuclei III, IV, and VI to coordinate eye movements
• Tectospinal fibers


  Fibers carrying information from the tectum (superior and inferior colliculi) to the spinal cord to control eye and neck movements
• Rubrospinal tract


  A descending pathway from the red nucleus to the spinal cord, controlling flexor muscles of the distal extremities
• Decussation of the superior cerebellar peduncles

  A crossing of fibers in the ventral tegmentum
• Moving head towards a direction

  1. Inferior colliculus responsible

  2. Tectospinal fibers control
• Red fibers


  • Descending fibers via the rubrospinal pathway
  • From red nucleus to lower motor neurons
  • Controlling motor movement of flexor muscles and distal extremities
• Rubrospinal tract


  Fibers from red nucleus going down to control motor movement
• Decussation of the superior cerebellar peduncles

  • Lattice-like structure in the ventral portion of the tegmentum
  • Fibers ascending from the cerebellum crossing over to the contralateral red nucleus or thalamus
• Cerebral peduncles


  Combination of the tegmentum and the crust cerebri
• Trochlear nerve


  • Cranial nerve 4
  • Motor nerve carrying sensations to the superior oblique extraocular muscle
  • Causes inferior movement and intorsion of the eye
• Tectum


  • Posterior portion of the midbrain
  • Consists of the superior colliculus and inferior colliculus
• Crust cerebri


  • Anterior portion of the midbrain
  • Contains corticospinal, corticonuclear, and corticopontine fibers
• Substantia nigra


  • Pars compacta releases dopamine
  • Pars reticularis releases GABA
• Medial, trigeminal, and spinal lemnisci

  • Structures present at the superior colliculus level
  • Lateral lemniscus ends at the inferior colliculus level
• Red nucleus


  • Located in the tegmentum at the superior colliculus level
  • Crosses ventrally forming the ventral tegmental decussation
• Tectospinal fibers


  Originate from the superior colliculus, cross dorsally forming the dorsal tegmental decussation, then descend as the tectospinal tract
• Oculomotor nerve


  • Cranial nerve 3
  • Carries general somatic efferent fibers to control eye movements and pupil constriction
• Edinger-Westphal nucleus


  • Associated with the oculomotor nerve
  • Carries parasympathetic (general visceral efferent) fibers to control pupil constriction and lens accommodation
• Pretectal nucleus


  Receives visual input and coordinates the pupillary light reflex via the Edinger-Westphal nucleus
• Superior colliculus


  • Part of the tectum
  • Receives visual, auditory, and sensory information to coordinate tracking of moving objects
• Lateral lemniscus


  Carries auditory information from the cochlear nuclei to the inferior colliculus
• Auditory pathway


  1. Vestibulocochlear nerve synapses on cochlear nuclei

  2. Fibers cross to other side

  3. Ascend via lateral lemniscus

  4. Synapse on inferior colliculus

  5. Inferior colliculus sends to medial geniculate nucleus

  6. Medial geniculate nucleus relays to primary auditory cortex
• Brachium of inferior colliculus


  Connection between inferior colliculus and medial geniculate nucleus
• Inferior colliculus


  Can send information to superior colliculus
• Superior colliculus


  Can activate tactospinal pathway and cranial nerve nuclei to move head, neck and eyes towards auditory stimulus
• Descending motor pathways


  1. Upper motor neurons in cortex

  2. Fibers descend through internal capsule and midbrain

  3. Some synapse on pontine nuclei (cortico-pontine fibers)

  4. Some continue to medulla and decussate at pyramids (corticospinal fibers)

  5. Some synapse on brainstem nuclei (cortico-nuclear/cortico-bulbar fibers)

  6. Cortico-pontine fibers connect to cerebellum via middle cerebellar peduncles
• Corticospinal fibers


  Descend to spinal cord