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Divisions

Midbrain is formally divided into dorsal and

ventral parts at the level of cerebral

aqueduct

The dorsal portion is known as tectum

which largely consists of inferior and

superior colliculi

The ventral portion is known as tegmentum

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Divisions

Tegmentum is bounded ventrally by themassive fibre system of the crus cerebri

The term cerebral peduncle issometimes used as a synonym for cruscerebri

Or the cerebral peduncle refers to thewhole midbrain on either side excludingthe tectum

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Caudal Part

In the caudal part of the midbrain the inferiorcolliculus constitutes part of the ascendingacoustic projection

Ascending auditory fibres run in the laterallemniscus which terminates in the inferiorcolliculus

Efferent fibres from the colliculus terminate inthe medial geniculate nucleus of the thalamus

This nucleus projects to the auditory cortex ofthe temporal lobe

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Rostral Part

The superior colliculus of the rostralarea of the midbrain is part of the visualsystem

Its main afferents are corticotectal fibresoriginating from the visual cortex of theoccipital lobe and from the frontal eye

field of the frontal lobe

These inputs are concerned with

controlling movements of the eyes

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Eye Movements

These movements of eyes are thoseoccurring when a moving object isfollowed

Or when the direction of the gaze isaltered (saccadic eye movement)

Corticotectal fibres from the visualcortex are involved in theaccommodation reflex

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Pretectal Nucleus

A small number of visual fibres runningin the optic tract terminate just rostral tothe superior colliculus in the pretectal

nucleus

This nucleus has connections with

parasympathetic neurons controlling thesmooth muscle of the eye and is part ofthe circuit mediating the pupillary lightreflex

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Cerebral Aqueduct

Ventral to the colliculi the cerebral

aqueduct runs the length of the midbrain

Surrounding the aqueduct is a pear

shaped arrangement of grey matter

called periaqueductal grey

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Nuclei

In the ventral part of the periaqueductalgrey at the level of the inferior and superiorcolliculi lie the trochlear and oculomotornuclei respectively

These innervate the extraocular musclescontrolling the eye movements

Close to the nuclei runs the mediallongitudinal fasciculus which links them tothe abducens nucleus in the pons and is

important in the control of gaze

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Superior Cerebellar

Peduncle

At the level of the inferior colliculus the

central portion of the tegmentum is

dominated by the superior cerebellar

peduncles

These fibres originate in the cerebellum

coursing ventromedially as they run intothe midbrain

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Red Nucleus

Beneath the inferior colliculus the superiorcerebellar peduncles decussate in themidline

Rostral to the decussation at the level ofthe superior colliculus the portion of thetegmentum is occupied by red nucleus

Some of the fibres of the superiorcerebellar peduncles terminate in the rednucleus

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Red Nucleus

The red nucleus is involved in motor control

Its other major source of afferents is the motorcortex of the frontal lobe

Efferent fibres from the red nucleus cross inthe ventral tegmental decussation anddescend to the spinal cord in the rubrospinal

tract

The red nucleus also projects to the inferiorolivary nucleus of the medulla via the centraltegmental tract

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Substantia Nigra

The most ventral part of the midbraintegmentum is occupied by the substantianigra

A subdivision of this nucleus known as parscompacta

It consists of pigmented melanin containingneurones that synthesise dopamine astheir transmitter

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Substantia Nigra

These neurones project to the caudate

nucleus and putamen of the basal ganglia

in the forebrain

Degeneration of the pars compacta of the

substantia nigra is associated with

Parkinsons disease

Other non pigmented subdivision of the

substantia nigra is called the pars reticulata

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Substantia Nigra

Pars reticulata is considered to be a

functional homologue of the medial

segment of the globus pallidus which isalso part of the basal ganglia

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Crus Cerebri

Ventral to the substantia nigra lies themassive crus cerebri

This consists entirely of descending corticalefferent fibres that have left the cerebralhemisphere by traversing the internalcapsule

Approximately the middle 50% of the crusconsists of corticobulbar and corticospinalfibres

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Fibres

The corticobulbar fibres end

predominantly in or near the motor

cranial nerve nuclei of the brain stem

The corticospinal fibres traverse the

pons to enter the medullary pyramid and

thence the corticospinal tract

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Middle Cerebellar Peduncle

On either side of the corticobulbar andcorticospinal fibres the crus cerebricontains corticopontine fibres that originate

from widespread regions of the cerebralcortex and terminate in the pontine nucleiof the ventral pons

From the pontine nuclei connections areestablished with the cerebellum via themiddle cerebellar peduncle which areinvolved in the coordination of movement

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