Download - 08 Midbrain
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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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