Brainstem

Amadi O. Ihunwo, PhDSchool of Anatomical Sciences

Lecture Outline

ConstituentsBasic general internal features of brainstem External and Internal features of ◦ Midbrain◦ Pons◦ Medulla

Constituents of Brainstem – External Features

Basic internal features of brainstemin a cross section

Ascending/descending tractsCranial nerve nucleiIntrinsic nucleiNeuromodulatory nucleiTegmental/Reticular nuclei

Ascending and Descending fibre pathways

These fibres carry information in the form of ascending sensory, or descending motor pathways. E.g.

Ascending trigeminal sensory pathway and the Descending pyramidal motor pathway.

Location of the cranial nerve nuclei in the brainstemFunctions: motor, sensory (general & special), parasympathetic

Intrinsic NucleiForm connections and undertake functions within regions of the brainstem. E.g. Inferior olive (IO)

Correlation of impulses from cerebellum, spinal cord, cerebral cortex, and corpus striatum and channels into the cerebellum for the coordination of learned patterns of movement.

IO

Cross section of medulla

Neuromodulatory Nuclei

Modulate neuronal activity over large parts of the entire brain. Function: ◦ Control over levels of

consciousness, perception of pain, regulation of CVS and Respiratory system.

Connect with cranial nerve nuclei, cerebellum, brain stem and spinal motor mechanisms and so influence movement, posture and muscle tone.

Doya K (2002) in Neural Networks

RETICULAR FORMATION (RF)

• Ill-defined collections of neurones and fibres with diffuse connections traversing the whole brainstem.

• Continuous below with the reticular intermediate spinal grey laminae.

• Modulate motor, pain & autonomic functions associated with respiratory and cardiovascular systems

Midbrain – External Features

Swellings – Mammary bodies & ColliculiTuber cinereum & infundibulumCrus cerebriCranial Nerves III & IVInterpeduncular (Anterior Perforated space)Superior cerebellar peduncle

Interpeduncular Fossa of Midbrain

BoundariesAnterior: optic chiasma Anterolateral: optic tractPostero-lateral: crus cerebriBehind: antero-superior surface of pons

Contents◦ posterior perforated substance◦ mammillary bodies◦ tuber cinereum◦ infundibulum◦ arterial circle of Willis

Midbrain: Internal features at levels of superior colliculus

Component parts & contentsTectum/ColliculiTegmentumSubstantia Nigra and Red nucleus (Intrinsic)Crus cerebri with nerve fibresCranial Nerve III (Oculomotor)Reticular formation

Midbrain: Internal features at level of inferior colliculus

Component parts & contentsTectum/ColliculiTegmentumSubstantia Nigra (Intrinsic)Crus cerebri with nerve fibresSuperior cerebellar peduncles & decussation (central)Cranial Nerve IV (Trochlear)Reticular formation

Note absence of red nucleus!!!

Pons – External Features

SwellingsPontomedullary junctionCranial Nerves Middle cerebellar peduncleFloor of 4th ventricle (Pontine part)

PONS - COMPONENTS

2 main parts: Ventral & Dorsal (Tegmentum)A) Dorsal sensory/motor tegmental portionB) Ventral pontine nuclei with pontocerebellar fibres

• Cranial Nerve Nuclei –V, VI, VII, VIII

• Tracts E.g. Medial (ML), Spinal (SL) & Lateral (LL) lemnisci

• Reticular Formation

Medulla – External Features

1) Sulcus (ventral median, ventrolateral, dorsal median & dsorsolateral)2) Swellings: pyramid & olive (ventral)2) Cranial Nerves 4) Inferior cerebellar peduncle5) Floor of 4th ventricle (Medullary part)

Medulla Oblongata: Components

1) Composed mostly of continuations of fibre tracts of spinal cord2) Decussations of Internal arcuate fibres to form medial lemniscus3) Decussation of Corticospinal tract4) Cranial nerve nuclei (VIII – XII)5) Gracile, Cuneate & Olivary Nuclei6) Nucleus of spinal tract of trigeminal N

MRI OF BRAINSTEM

A- MedullaB- PonsC- Midbrain

QuestionsList the external features of the midbrain (pons or medulla)What are the five basic internal features in a cross-section of the brainstem?With a well labelled diagram, describe the boundaries and contents of the interpeduncular fossaDraw a well labelled cross-section of the midbrain at the level of the superior colliculus. List any three internal differences with cross-section at level of inferior colliculus.