MIDBRAIN DrParvathy S Nair

• Anatomy

• Parts of midbrain

• Tracts and Nuclei

• Imaging

• Vascular anatomy

• Lesions

• The midbrain is the most rostral part of the brainstem and sits above the pons and is adjoined rostrally to the thalamus.

• Midbrain/Mesencephalon• From the Greek mesos - middle, and

enkephalos – brain• “bridge portion”

• 1 of 3 primary brain vesicles– Prosencephalon– Mesencephalon– Rhombencephalon

MIDBRAIN

CEREBRAL PEDUNCLE

CRUS CERBRI

TEGMENTUM

TECTUM

SUPERIOR COLLICUL

US

INFERIOR COLLICUL

US

• The tectum (“roof”) is the dorsal part of the mesencephalon (midbrain).

• Consists of the inferior and the superior colliculi.– The superior colliculus is involved in preliminary visual

processing and control of eye movements. – The inferior colliculus is involved in auditory processing. It

receives input from various brain stem nuclei and projects to the medial geniculate nucleus of the thalamus, which relays auditory information to the primary auditory cortex.

– Both colliculi also have descending projections to the paramedian pontine reticular formation and spinal cord.

– Collectively the colliculi are referred to as the corpora quadrigemina.

• The tegmentum (from Latin for "covering”) is a general area within the brainstem. It is located between the ventricular system and distinctive basal or ventral structures at each level.

• The midbrain tegmentum is the part of the midbrain extending from the substantia nigra to the cerebral aqueduct in a horizontal section

• The nuclei of CN III and IV are located in the tegmentum portion of the midbrain.

Cerebral Peduncle

• The cerebral peduncle, by most classifications, is everything in the mesencephalon except the tectum.

• The region includes the midbrain tegmentum, crus cerebri and pretectum.

• The large ventral bundle of efferent fibers is referred to as the crus cerebri or the pes pedunculi.

Red nucleus• The red nucleus and substantia nigra are

subcortical centers of the extrapyramidal motor system.

• With a developed corticospinal tract, in humans red nucleus does not have a crucial role

• The majority of red nucleus axons do not project to the spinal cord, but instead (via its parvocellular part) relay from the motor cortex to the cerebellum through the inferior olivary complex.

Substantia nigra• Pigmented layer- melanin granules• Dorsal to peduncles and ventral to the red

nucleus . • Composed zona compacta and zona reticulata• The GABAergic neurons in the pars reticulata

convey the final processed signals of the basal ganglia to the thalamus and superior colliculus.

• The dopaminergic neurons of pars compacta mainly deals with motor control

Tracts

• Main tracts– Crus cerebri– Dentatorubrothalamic (red nucleus)– Medial longitudinal fasciculus– Spinothalamic tract– Medial leminiscus

TRACTS

SENSORY• TRIGEMINAL• SPINAL• MEDIAL• LATERAL• DECUSSATION OF SUPERIOR

CERBELLAR PEDUNCLE

MOTOR• CORTICOSPINAL• CORTICONUCLEAR• TEMPOROPONTINE• FRONTOPONTINE• MED. LONG. FASCICULUS

• Cruscerebri (“crus”-structure resembling leg)

– 1. corticopontine fibers (frontopontineprojection)

medial 5th

• fibers from the precentral gyrus (motor strip) to the

nuclei of cranial nerves V (trigeminal), VII (facial) and XII

(hypoglossal).

– 2. corticospinal fibers (middle three 5ths)

– 3. temporopontine fibers (lateral 5ths)

• they originate in the temporal lobe and end in the nuclei

pontis.

MLF

• Medial longitudinal fasciculus: Connects the cranial

nerve nuclei III ,IV and VI together, and integrates

movements directed by the gaze centers (frontal eye

field) and information about head movement (from

vestibulocochlear nerve).

• It is an integral component of saccadic eye

movements as well as vestibulo-ocular and

optokinetic reflexes

• Spinothalamic tract – pain, temperature,touch,pressure

• Medial lemniscus – sensory input from the gracile and cuneate nuclei

to the thalamus

Nuclei

1. Nucleus of Oculomotor nerve.

2. Nucleus of Edinger- Westphal: supplies preganglionic parasympathetic fibers to the eye, constricting the pupil, accommodating the lens , and convergence of the eyes

3. Nucleus CN IV(trochlear)

Axial oblique 0.8-mm-thick SSFP MR image shows the nerve (small arrows) where it emerges from the interpeduncular cistern (large arrow), which lies medial to the cerebellar peduncle (p)

Coronal 0.8-mm-thick SSFP MR image shows the oculomotor nerve (white arrow) in cross section between the posterior cerebral artery (white arrowhead) and the superior cerebellar artery (black arrowhead), which are distal branches of the basilar artery (black arrow).

Trochlear nerves (arrows) where they emerge from the dorsal midbrain to cross the ambient cisterns. The characteristic course of the trochlear nerves allows their differentiation from the nearby superior cerebellar artery (arrowheads).

Blood Supply

Quadrigeminal (from PCA) :inf. Colliculi

Superior cerebelar :tectum

Posterior choroidal: cerebral peduncles,

lateral sup colliculi, thamalus, choroid

plexus 3rd ventricle

Anterior choroidal: from MCA or IC, to

cerebral peduncles, or supeiror midbrain

Posterior Cerebral arteries

• Claude Syndrome– Ipsi CN III, contra

tremor, ataxia and incoordination

• Rubral tremor (Holme’s tremor)– A coarse, slow (4Hz) tremor, especially present in

the upper extremities, that is found at rest, postural and intention.

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