2. Brainstem

**Posterior fossa:**

* Protected space surrounded by calvarium, contains:
	+ Brainstem anteriorly, cerebellum posteriorly
	+ Cerebral aqueduct and fourth ventricle
	+ CSF cisterns containing CNs, vertebrobasilar arterial system and veins
* CSF cisterns suspend & cushion brainstem and cerebellum

**Brainstem**

* Anatomic divisions
	+ Midbrain (mesencephalon): Upper brainstem, connects pons and cerebellum with forebrain
	+ Pons: Mid portion of brainstem, relays information from brain to cerebellum
	+ Medulla: Caudal (inferior) brainstem, relays information from spinal cord to brain
* Functional divisions
	+ Ventral part: Contains large descending white matter tracts: Midbrain cerebral peduncles, pontine bulb, medullary pyramids
	+ Dorsal part: Tegmentum, common to midbrain, pons and medulla; contains CN nuclei and reticular formation

Anatomy Relationships

* Posterior fossa boundaries
	+ Tentorium cerebelli superiorly
	+ Bony clivus anteriorly
	+ Temporal bones and calvarium laterally
	+ Foramen magnum and calvarium inferiorly

Midbrain

* Anterior: paired cerebral peduncles (crus cerebri) containing corticospinal, corticobulbar and corticopontine tracts, separated by interpenduclar fossa.
* Dorsal tegmentum: anterior to to cerebral aqueduct
	+ White matter tracts: Medial longitudinal fasciculus, medial lemniscus, lateral lemniscus, spinothalamic tract, central tegmental tract
	+ Gray matter: Substantia nigra and red nucleus (upper midbrain area)
	+ Upper midbrain: Contains CN3 nucleus with nerve emerging from between the cerebral peduncles anteriorly (at superior colliculus level)
	+ Lower midbrain: Contains CN4 nucleus with nerve emerging from the dorsal surface of the midbrain and wind around the lateral aspect (at inferior colliculus level)
* Tectum (quadrigeminal plate): posterior to cerebral aqueduct
	+ four rounded prominence - the superior (connected to lateral geniculate) and inferior colliculli (connected to auditory system)
	+ Periaqueductal gray matter

Pons: bulbous mid portion of brainstem located between midbrain (superiorly) & medulla oblongata (inferiorly)

* Ventral: longitudinal fibers primarily from corticospinal, corticobulbar & corticopontine tracts. It is continuous with cerebral peduncles superiorly and medullary pyramids inferiorly.
* Dorsal tegmentum: White matter tracts & CN nuclei. Continuation of midbrain tegmentum superiorly and medullary tegmentum inferiorly
	+ White matter tracts: medial longitudinal fasiculus, medial lemniscus, lateral lemniscus, trapezoid body, spinothalamic tract, central tegmental tract
	+ **Upper pons:** Contains main nuclei of CN5
		- CN5 nuclei located throughout brainstem and upper cord. Carries bulk of motor, main sensory and mesencephalic nuclei located in pons
	+ **Lower pons:** Contains nuclei of CN6, 7 & 8.
		- CN6 nucleus is located in pontine tegmentum near midline, anterior to fourth ventricle
			* Axons of facial nerve (CN7) loop around abducens nucleus creating a bulge in floor of fourth ventricle, the facial colliculus
		- CN7 nucleus has three main nuclei within pons: Motor, superior salivatory, solitary tract. Located in ventrolateral aspect of tegmentum of lower pons
		- CN8 nuclei has cochlear and vestibular nuclei. The vestibular nuclei is beneath lateral recess along floor of fourth ventricle (rhomboid fossa) while the dorsal and ventral cochlear nuclei on lateral surface of inferior cerebellar peduncle (restiform body)

Medulla: caudal part of brainstem composed of gray matter formations, CN nuclei (CN9-12) & white matter tracts. Divided into two parts.

* **Ventral medulla:**
	+ Corticospinal tracts (pyramidal tracts) make up bulk separated by ventral median fissure.
	+ Medullary olives
* **Dorsal tegmentum:** White matter tracts & CN nuclei
	+ White matter tracts: Medial longitudinal fasiculus, medial lemniscus, spinothalamic tract, central tegmental tract, spinocerebellar tract
	+ CN nuclei: CN9, 10 & 11 (bulbar portion) in upper & mid medulla; CN12 nuclei in mid medulla
		- CN9 nuclei consists of nucleus ambiguus, solitary tract nucleus, inferior salivatory nucleus. CN9 exits medulla in postolivary sulcus above CN10
		- CN10 nuclei consists of nucleus ambiguus, solitary tract nucleus, dorsal vagal nucleus. CN10 exits medulla in postolivary sulcus between CN9 and 11
		- CN11 bulbar nuclei exits medulla in postolivary sulcus below CN10
		- CN12 nuclei in mid medulla, produces the bulge in fourth ventricle and exits anterior medulla in pre-olivary sulcus

* **Medulla external features**
	+ Inferior cerebellar peduncle (restiform body)
		- Arise from superior aspect of dorsal medulla; peduncles diverge and incline to enter cerebellar hemispheres
		- Nuclei of CN8 located along dorsal surface
	+ Gracile and cuneate tubercles
		- Form lower aspect of dorsal medulla
		- Produced by paired nuclei gracilis (medial) and cuneatus (lateral)
		- Dorsal median sulcus separates gracile tubercles

* Reticular formation
	+ Occupies central tegmentum, afferent and efferent connections
	+ Important in consciousness, motor function, respiration and cardiovascular control

Relations of the brainstem:

* Anterior
	+ Clivus superiorly and the basiocciput inferiorly
* Posterior
	+ Quadrigeminal cistern superiorly and 4th ventricle, cerebellum and cisterna magna inferiorly

* Superior
	+ Cerebral hemispheres

* Inferior
	+ Spinal cord
* Lateral
	+ Petrous temporal bone and tentorium

Blood supply:

**Arterial supply**

Midbrain

* Central branches of the superior cerebellar artery (branches of the basilar artery)

Pons

* Supplied by the pontine branches of the basilar artery

Medulla

* The upper anterior part is supplied by the vertebral and basilar arteries
* The posterior part is supplied by the posterior inferior cerebellar arteries

**Venous drainage**

Midbrain

* Posterior mesencephalic veins curve around each side of the midbrain and drain to the great cerebral vein

Pons

* Blood drains via the anterior pontomesencephalic vein (to the basal vein of Rosenthal and then to the great cerebral vein) and the basilar plexus and inferior petrosal sinuses

Medulla

* Anteriorly venous drainage is to the inferior petrosal sinus
* Posteriorly venous drainage is to the occipital sinus
* Inferior medullary veins communicate with spinal veins

Cerebellum: largest part of hindbrain, integrates coordinations & fine-tuning of movement & regulation of muscle tone

* Cortical gray matter, central white matter & four paired deep gray nuclei

* Three surfaces: Superior (tentorial), inferior (suboccipital), anterior (petrosal)
* Two hemispheres and a midline vermis
	+ Divided into lobes & lobules by transverse fissures
	+ Major fissures: Primary (tentorial), horizontal (petrosal), prebiventral/prepyramidal (suboccipital) cerebellar fissures
* Three paired peduncles
	+ Superior cerebellar peduncle (brachium conjunctivum) connects to cerebrum via midbrain
	+ Middle cerebellar peduncle (brachium pontis) connects to pons
	+ Inferior cerebellar peduncle (restiform body) connects to medulla

Vertebrobasilar system

* Midbrain: Perforating branches from basilar, superior cerebellar & posterior cerebral arteries
* Pons: Superior cerebellar artery, perforating branches of basilar artery
* Medulla: Anterior spinal artery, vertebral artery penetrating branches, posterior inferior cerebellar artery
* Cerebellum: Superior cerebellar, posterior inferior cerebellar & anterior inferior cerebellar arteries