midbrain is the shortest oart of brainstem located between forebrain and hindbrain. It connects pons and cerebellum with the forebrain and most of it lies in the posterior cranial fossa.
Midbrain has a small cavity inside known as aqueductus of silvius that connects 3 and 4 ventricles
Main prominent features on the anterior durface of midbrain:
delimited superior by the optic chiasm and inferiorly by the deep transverse groove
on the lateral sides there are the 2 cerebral peduncles.
inbetween in cerebral peduncles is the interpeduncular foosa that contains from inf to sup the posteior perforated substance, the mammillary bodies and the infundibulum.
The cerebral peduncles are crossed by the optic tract, basal vein, post and sup cerebellar arteries. On its medial side the oculomotor nerve merges snd on its lateral side the trochlear nerve merges.
Anterior surface of midbrain
Posterior surface of midbrain is featured by:
Superior colliculi: involved in visual processing and coordination of eye mov
Inferior colliculi: involved in auditory processing, localisation of sound and reflexive response to auditory stimuli. The colliculus together are called corpora quadrigemia.
The colliculi are crossed by the cruciform sulcus which has 2 limbs an vertical and an horizontsl limbs.
At the cranial end of cruciform sulcus pineal gland is situated and at its caudal end the frenellum velli and superior medullary velum are located.
At the frenulum veli the trocheal nerve crosses in the superior medularry vellum and travel to the ant surface of midbrain
Dorsal surface of midbrain
Midbrain is subdivided into 2 main parts:
the tectum: which is the posterior area that is made up of the superior and inferior colliculi
And each half of midbrain anterior to the aqueducts of silvius is the cerebral peduncles, which are them subdivided into crus cerebri (ant), substancia nigra (middle), and tegmentum (post)
Main parts of midbrain
The crus cerebri of cerebral peduncle represents thick cylindrical ropes of whith matter. It has 3 part:
medial2/3: containing corticospinal and corticonuclear tracts that travel towards spinal cord
lat 1/6: contains temporopontine, parietopontine and occiptopontine fibers
middle 1/6: frontopontine tract
Separating crus cerebri from tegmentun is the substancia nigra which is a collection mass of pigmented cells that produces neuromelanin.
It has 2 parts:
pars compacts: made up of neurons that produce dopamine. It sends dopaminergic projections to striatum forming nigrostriatal pathway crutial in voluntary mov.
pars reticulata: GABA producing neurons that sends projections to thalamus and modulationg motor function. The 2 pars work together in basal ganglia circuit to ensure smooth and coordinated movements.
Tegmentum of midbrain is located ventral to cerebral aqueduct and dorsal to substancia nigra.
At level of inferior colliculi it contains:
rubrospinal tract
tectospinal tract
medial leminiscus
trigeminal leminiscus
spinal leminiscus
lateral leminiscus
reticular formation ( ventral tegmental nucleus, medial longitudinal fasciculus, and interstitial nucleus of cajal).
The central gray matte r of midbrain aka periaqueductal gray is a mass of neuronal cell bodies that encloses the cerebral aqueduct of silvius. It function in modulating pain by realising endogenous opioids.
Cross section of midbrain at level of inferior colliculi shows the periaqueduct of gray and inside it we Can find:
trochlear n nuclei: anterior to cerebral aqueduct. Fibers emerge from it, pass laterally and posteior around the grsy matter decussates inside the infeior medullary velum and emerges just bellow inferior colliculi. Motor innervation to superior oblique muscle of eye.
Laterally to cerebral aqueduct is the Mesencephalic nucleus of trigeminal nerve: made of unipolar sensory neurons that receives propioceptive info from muscle of face
The inferior colliculi contains the inferior colliculi nucleus whcih are relay station along the auditory pathway, relaying majority of fibers of lateral leminiscus. The pathway then continues with fibers through the inferior brachium then to medial geniculate body.
internal structure of midbrain at level of inferior colliculi
The crus cerebri internal structure remains the same at both levels.
The central grey matter at level of superior colliculi contains:
mesencephalic nucleus in same site as inf colliculi
Oculomotor nucleus complex which is divided into:
Group of nuclei that sends fiber to innervate inferior rectus, middle rectus, inferior oblique, levator palpabre superior, and superior rectus.
All the oculomoto nucleus have somatic efferent components and supply the extraocular muscles
The central gray matter at level of superior colliculi contains the edinger-westphal nucleus which is an accessory oculomotor nucleus that send parasympathetic fibers via oculomor nerve to innervate sphincter pupillae causing pupil constriction. It is responsible for pupillary light reflex and accommodation of eye.
The midbrain is supplied by the following arteries:
Basilar artery via its posterior cerebral and superior cerebellar arteries. Basilar artery also supplies the midbrain via direct branches.
Branches of posterior communication and anterior choroidal arteries.
Red nucleus
Belongs to extrapyramidal syste.
lus. Fibers cross immediately on ventral portion of tegmentum forming ventral tegmental decussation. Fibers then become rubrospinal tract seen at level of inferior colliculus.
2. Projects efferents to inferior olivary nucleus and spinal cord (rubrospinal tract) fibers that decussates and forms the ventral tegmental decussation
Dorsal tegmental decussation
Formed by tectospinal tract fibers
Afferent fibers from the superior colliculi
Dorsal to red nucleus is the dorsal tegmental decussation
red nucleus:
receives input from cerebellum (cerebellorubral fibers) about body position and movement + cerebral cortex (corticorubral fibers) about planned movements. Output to spinal cord (rubrospinal tract) which influences motor contro + inferior olive which is involved in motor planning and coordination.
Efferent fibers from red nucleus:
rubrospinal tract
rubrobulbar tract: fibers to motor nucleus of trigeminal and facial nerve
rubroreticular fibers
rubroolivary fibers
Superior colliculi
A visual reflex center involved in coordination of head and eye movements and primary integrating center for visual responses. Sends axons that decussate at ventral tegmental decussation and descend as tectospinal tract.
Tectobulbar tract (fibers to cranial nerve of oculomotor, abducens and trocheal nerve)
Tectospinal tract and tectobulbar tract
Involved in movement of eye and neck towards source of sound and towards cutaneous stimulation
Other efferents sent by superior colliculi
Lateral geniculate body via superior brachium
The afferent light reflex howevers end in pretectsl nucleus situated laterally to superior colliculi. It receives input from retina and visual cortex and projects fibers to edinger-westphal nucleus on both sides causing pupil to contract in response to light stimuli.
Mid brain at level of inferior colliculi
List all structures seen on midbrain at level of superior colliculus
medial, trigeminal, spinal leminiscus
mesencephalic nucleus of trigeminal
periaqueduct grau + cerebral aqueduct
medial longitudinal fasciculus
red nucleus
tegmental decussation
oculomotor n + edinger-westphal nucleus
pretectal nucleus
substancia nigra
crus cerebri
structures related to midbrain
Crus cerebri: longitudinal column present in front of midbrain. House depression called interpeduncular fossa.
cerebral aqueduct: CSF filled cavity
Tectum: posterior to cerebral aqueduct. Contains superior + inferior colliculus.
Tegmentum: Present btw cerebral aqueduct and substancia nigra.
Substancia nigra: separates tegmentum from crus cerebri.
Ventral tegmental decussation
rubrospinal tract: originate in red nucleus, its fibers decussate in ventral tegmental area before descending into spinal cord.
Rubrospinal tract plays role in control of motor function
Dorsal tegmental decussation
Also known as decussation of superior cerebellar peduncle.
Superior cerebellar peduncle are major output pathway of cerebellum. They carry info from deep cerebellar nuclei to red nuclei and thalamus. Fibers of superior cerebellar peduncles decussate at level of dorsal tegmental decussation
Inferior colliculus: part of corpora quadrigemia, which includes 4 rounded elevations on dorsal surface of midbrain.
Main auditory relay station. Integrates info from auditory sources and processes it for futher transmission to higher auditory centers.
Afferents: choclear nucleus via lateral leminiscus.; superior olivary complex.
Efferents: Medial geniculate body of thalamus which then projects to auditory cortex; superior colliculus; spinal cord via tectospinal tract.