Mesencephalon

Tectum

1.) “roof” of midbrain (posterior)

2.) Posterior to cerebral aqueduct

3.) Composed of four collicular bodies (Super and inferior collicular pairs)

Cerebral Aqueduct

1.) Channels for CSF exchange between third and fourth ventricle

2.) surrounded by periaqueductal gray

Periaqueductal Gray

Important Nuclei for CN III and IV found in or near this gray area. Endorphins and Enkephalins (natural pain killers) are present in this area

Cerebral Peduncles

1.) Ventral to cerebral aqueduct

2.) Includes: Tegmentum (ventral to aqueduct), Substantia Nigra (towards middle, and Crus Cerebri (ventral and lateral aspect )

Cranial Nerve III (Oculomotor Nerve)

1.) Apparent Origin is into interpeduncular fossa (into cistern)

2.) Six nuclei are located in periaqueductal gray area

3.) Oculomotor Nuclei (Cells bodies of LMN fond here): Superior, medial and inferior rectus and inferior oblique; Levator palpebrae superioris

4.) Accessory Oculomotor (Edinger-Westphal’s): Preganglionic parasympathetics (ciliary muscle and sphincter pupillae) (CN 3,7,9,10 all carry this)

Cranial Nerve IV (Trochlear Nerve)

1.) innervates the superior oblique muscle (turns eye down and out)

2.) CN with fewest number of fibers

3.) Apparent origin is on posterior side of brainstem

4.) Fibers cross at A.O.

Tegmentum

1.) just ventral to cerebral aqueduct

2.) CN III and IV located here in periaqueductal gray

3.) Reticular formation of midbrain is here

4.) Medial lemniscus

5.) Red Nucleus

Red Nucleus (tegmentum)

1.) may or may not be considered part of reticular formation

2.) Superior part extends into subthalamic region of diencephalon

3.) Divided into inferior large cell area (Magnocellular) and highly vascularized superior area (Parvocellular)

4.) Most rubrospinal fibers originate in magnocellular area, smaller in humans than other animals

Red Nucleus input fibers from

1.) Central Cerebral Nuclei (Primarily interposed, none from fastigial)

2.) Cerebral Cortex

Red Nucleus Function

Contralateral motor responses necessary for postural control and muscle tone control primarily in flexor musculature

Substantia Nigra

1.) located between tegmentum and Crus

2.) Large and extends into subthalamus of diencephalon

3.) “Black Substance” found only in mammals, most developed in humans

4.) Afferent and Efferent exchanged between caudate and Putamen

5.) Secretes Dopamine from Tyrosine and melanin

Damage Substantia Nigra

Parkinson’s

1.) Substantia nigra produces Dopamine and is part of the extrapyramidal system (coordinate skilled voluntary movements and maintain muscle tone, etc)

2.) Shaking of digits, hands, tongue while at rest 95% (resting tremor)

3.) Odd Postures, pain, shuffling gait, loss of speech in later stages

4.) Treated with L-Dopa

Crus Cerebri (descending Pyramidal Fibers)

1.) Ventrolateral Aspect of cerebral peduncle

2.) Composed of descending pyramidal tracts of fibers from the cerebral cortex

Crus Cerebri carries descending tracts from cerebral cortex

1.) Corticospinal fibers (to cord)

2.) Corticopontine (to pons)

3.) Corticobulbar fibers (to medulla oblongata)

4.) Corticomesencephalic Fibers: Terminate on Nuclei for CN III and IV to initiate voluntary eye movements

Superior Colliculi deal with

Visual reflexes

Inferior colliculi relays

auditory information to thalamus

Tectospinal tract arises from the

superior colliculus. Move our heads away from objects moving rapidly through our field of vision

Tectospinal tract influences the neurons in the upper cervical through

CN XI. It, in turn, contracts traps and SCM to prevent head injury (reflex pathway)