Midbrain anatomy: tectum, tegmentum and cerebral peduncles

What it is

The midbrain is the smallest part of the brainstem and links the pons with the diencephalon.

Location

It lies between the pons and the thalamus and hypothalamus, passing through the tentorial notch.

Function

It is involved in visual and auditory reflexes, eye movements, and motor control, and contains nuclei for cranial nerves III and IV.

Body system

Nervous · single, midline structure

FMA ID

FMA61993 (Foundational Model of Anatomy)

Key facts

• Divided into tectum, tegmentum and cerebral peduncles.
• Contains the substantia nigra and red nucleus.
• The cerebral aqueduct runs through it, surrounded by periaqueductal grey matter involved in pain control.

Clinical relevance

Loss of dopaminergic neurons of the substantia nigra underlies Parkinson disease.

The midbrain is the most superior (highest) of the three regions of the brainstem. It links the forebrain with the pons and cerebellum, and it passes through the gap in the tentorium cerebelli, the fold of dura that separates the cerebrum from the cerebellum. (TeachMeAnatomy: The Midbrain.)

The cerebral aqueduct, a narrow channel about 1 to 2 mm across, runs through the midbrain and connects the third ventricle above with the fourth ventricle below. Everything behind the aqueduct is the tectum; everything in front of it is the tegmentum and cerebral peduncles. (Radiopaedia: Cerebral aqueduct.)

The midbrain develops from the mesencephalon, the middle of the three primary brain vesicles. Unlike the forebrain and hindbrain vesicles, the mesencephalon does not divide further, and the midbrain stays a small part of the adult brain. (OpenStax Anatomy and Physiology 2e, 13.1: The Embryologic Perspective.)

From the front, the midbrain shows two large cerebral peduncles that converge as they run down to the pons. The hollow between them is the interpeduncular fossa. Its floor, the posterior perforated substance, is pierced by many small blood vessels. The oculomotor nerve (cranial nerve III) emerges between the peduncles, and the optic tract curves around the upper border of the midbrain. (TeachMeAnatomy: The Midbrain.)

From behind, the roof of the midbrain carries four rounded swellings, the colliculi (corpora quadrigemina): a superior pair and an inferior pair, separated by a cross-shaped groove and lying just below the pineal gland. On each side, the brachium of the superior colliculus links it with the retina, and the brachium of the inferior colliculus carries auditory fibres to the medial geniculate body of the thalamus. (TeachMeAnatomy: The Midbrain.)

The trochlear nerve (cranial nerve IV) leaves the brainstem just below the inferior colliculus. It is the only cranial nerve to emerge from the back of the brainstem, and it is the thinnest cranial nerve. Its fibres cross to the opposite side before they exit, then the nerve winds forward around the midbrain to reach the superior oblique muscle of the eye. (Radiopaedia: Trochlear nerve.)

In cross-section the midbrain has three parts. The tectum (Latin for roof) lies behind the aqueduct and is made of the tectal (quadrigeminal) plate with the superior and inferior colliculi; it has no counterpart elsewhere in the brainstem. The tegmentum is the central core, continuous with the tegmentum of the pons and medulla. The cerebral peduncles at the front are made of large tracts running to and from the cerebrum. (Radiopaedia: Midbrain.)

TeachMeAnatomy describes the same layout slightly differently: each cerebral peduncle is divided by the pigmented substantia nigra into a front part, the crus cerebri, and a back part, the tegmentum. The crus is separate on each side, while the tegmentum is continuous across the midline. (TeachMeAnatomy: The Midbrain.)

The crus cerebri carries descending fibres from the cortex. From medial to lateral these are frontopontine fibres, corticospinal and corticobulbar fibres from the primary motor cortex, and temporopontine fibres. (TeachMeAnatomy: The Midbrain.)

The periaqueductal grey matter is a column of grey matter around the cerebral aqueduct. It takes part in the descending control of pain signals and in circuits for emotion. The mesencephalic nucleus of the trigeminal nerve lies at its side, and the oculomotor and Edinger-Westphal nuclei lie in front of it. (Radiopaedia: Periaqueductal grey matter.)

At the level of the inferior colliculus the trochlear nucleus lies in the central grey, with the medial longitudinal fasciculus in front of it. The superior cerebellar peduncles cross (decussate) in the middle of the tegmentum, with the reticular formation at the sides. Four lemnisci (medial, spinal, trigeminal and lateral) run between the central grey and the substantia nigra. (TeachMeAnatomy: The Midbrain.)

At the level of the superior colliculus the large red nuclei take the central position, the oculomotor nucleus replaces the trochlear nucleus, and the lateral lemniscus has ended. Fibres of the oculomotor nerve run forward through the tegmentum from the nucleus to the interpeduncular fossa. (TeachMeAnatomy: The Midbrain.)

The oculomotor nucleus lies at the level of the superior colliculus, just above the trochlear nucleus. Its fibres join those of the Edinger-Westphal nucleus, pass through the red nucleus and emerge from the medial side of the cerebral peduncle. (Radiopaedia: Oculomotor nucleus.)

The superior colliculi receive input from the retina and drive reflex movements of the eyes, head, body and limbs in response to light. The inferior colliculi process sound and drive reflex movements in response to sounds; they project to the medial geniculate nucleus of the thalamus, which relays hearing to the auditory cortex. (Radiopaedia: Superior colliculus.)

OpenStax adds that the superior colliculus combines visual, auditory and touch information to turn the eyes towards a sound or touch. Throughout the brainstem, including the midbrain, the tegmentum contains cranial nerve nuclei and regions that regulate cardiovascular and respiratory function. (OpenStax Anatomy and Physiology 2e, 13.2: The Central Nervous System.)

The oculomotor nerve supplies four of the six muscles that move the eye (superior, medial and inferior rectus and inferior oblique) and the levator palpebrae superioris, which lifts the upper eyelid. Its parasympathetic fibres constrict the pupil and adjust the lens for near vision. (Radiopaedia: Oculomotor nerve.)

The Edinger-Westphal nucleus is the small parasympathetic nucleus of the oculomotor nerve. Its fibres synapse in the ciliary ganglion, and the postganglionic fibres reach the constrictor pupillae and ciliary body through the short ciliary nerves. It is needed for the pupillary light and accommodation reflexes. (Radiopaedia: Edinger-Westphal nucleus.)

The red nucleus receives input from the dentate nucleus of the cerebellum and from the cortex, and sends fibres to the olive and, through the rubrospinal tract, to the spinal cord. It helps coordinate muscle tone, body position and gait. (Radiopaedia: Red nucleus.)

The substantia nigra lies in the front of the midbrain, where the tegmentum meets the cerebral peduncle. It has a compact part, dark because it contains melanin, and a reticular part. (Radiopaedia: Substantia nigra.)

The pars compacta releases dopamine onto the striatum and makes movement more likely; the pars reticulata is one of the output nuclei of the basal nuclei. In Parkinson disease the dopamine neurons of the pars compacta die, so less dopamine reaches the striatum and movement becomes slow and difficult. Treatment with L-DOPA, a dopamine precursor, is common, but there is no cure. (OpenStax Anatomy and Physiology 2e, 13.2: The Central Nervous System.)

On high-resolution MRI, a healthy cluster of dopamine cells called nigrosome-1 gives a swallow tail appearance. Loss of this appearance is reported to be a reliable sign of Parkinson disease. (Radiopaedia: Substantia nigra.)

The midbrain is supplied by the vertebrobasilar (posterior) circulation, through small penetrating branches of the basilar artery, the superior cerebellar artery and the posterior cerebral artery. (Radiopaedia: Midbrain.)

TeachMeAnatomy lists the same vessels in more detail: the posterior cerebral artery and its peduncular branch, the superior cerebellar artery, the posterior choroidal artery and the interpeduncular branches of the basilar artery. (TeachMeAnatomy: The Midbrain.)

In 4 to 12% of people a single artery, the artery of Percheron, arises from one posterior cerebral artery and supplies the paramedian thalamus on both sides and, most often, the upper midbrain. A blockage causes infarcts in both thalami and the midbrain; patients may be drowsy or comatose and have vertical gaze palsy and memory problems. (Radiopaedia: Artery of Percheron.)

Weber syndrome is a midbrain stroke, usually from blocked branches of the posterior cerebral artery, that damages the cerebral peduncle and the fibres of the oculomotor nerve on one side. It causes an oculomotor palsy on the same side (drooping eyelid and double vision) with weakness of the opposite side of the body. (Radiopaedia: Weber syndrome.)

Benedikt and Claude syndromes also come from blocked perforating branches of the posterior cerebral artery. They involve the red nucleus and cause an oculomotor nerve palsy with tremor, ataxia and involuntary movements on the opposite side of the body. (Radiopaedia: Red nucleus.)

Parinaud syndrome (dorsal midbrain syndrome) comes from pressure on or damage to the back of the midbrain, most often by a tumour of the pineal region. Its classic triad is difficulty looking up, pupils that react better to near objects than to light, and convergence-retraction nystagmus. (Radiopaedia: Parinaud syndrome.)

Because the cerebral aqueduct is so narrow, a blockage there causes obstructive hydrocephalus, with enlargement of the lateral and third ventricles. (Radiopaedia: Cerebral aqueduct.)

The nearest named structures in the 3D model, measured from the centre of each part:

• Cerebral aqueduct · nervous
• Peduncle of midbrain · nervous
• Superior colliculus · nervous
• Posterior commissure · nervous
• Inferior colliculus · nervous
• Postcommunicating part of posterior cerebral artery · cardiovascular
• Brachium of inferior colliculus · nervous
• Third ventricle · nervous
• Mammillary body · nervous
• Optic tract · nervous