Midbrain

Where is the midbrain?

It lies between the thalamus and the pons, and with the pons and the medulla it makes up the brain stem. (OpenStax A&P 2e, 13.2; StatPearls, Mesencephalon Midbrain)

At the back, the superior cerebellar peduncles join it to the cerebellum. On its front surface, the crus cerebri carries corticospinal fibers. (StatPearls, Mesencephalon Midbrain)

How does the midbrain develop?

In the embryo, the prosencephalon splits into the telencephalon and the diencephalon, and the rhombencephalon into the metencephalon and the myelencephalon. The mesencephalon changes far less and becomes the midbrain. (OpenStax A&P 2e, 13.1; StatPearls, Thalamus, Medulla Oblongata and Embryology, Central Nervous System)

How is the midbrain built?

The cerebral aqueduct runs through its center, carrying cerebrospinal fluid from the third ventricle to the fourth ventricle. (OpenStax A&P 2e, 13.2; StatPearls, Mesencephalon Midbrain)

The tectum, the dorsal part, lies behind the aqueduct and forms its roof; the tegmentum, the ventral part, lies in front of it. (OpenStax A&P 2e, 13.2 and 13.3; StatPearls, Mesencephalon Midbrain)

The tectum is four bumps, the colliculi. The inferior colliculi lie on the hearing pathway; the superior colliculi combine maps of visual, auditory and touch space and turn the eyes toward a sound or touch. (OpenStax A&P 2e, 13.2; StatPearls, Mesencephalon Midbrain, Thalamus and Superior Colliculus)

The tegmentum holds the upper reticular formation, the nuclei of cranial nerves that move the eyes, the periaqueductal gray, the red nucleus, the substantia nigra and the ventral tegmental area. (StatPearls, Mesencephalon Midbrain)

What does the midbrain do?

Sight, hearing and touch all feed the deeper layers of its superior colliculi, which line up these senses' maps of the space around you. (OpenStax A&P 2e, 13.2; StatPearls, Superior Colliculus)

The substantia nigra pars compacta sends dopamine to the striatum, the caudate nucleus and putamen, and switches the basal nuclei between their two pathways. Loss of its pathway to the putamen, the nigrostriatal pathway, underlies the movement problems of Parkinson’s disease. (OpenStax A&P 2e, 13.2; OpenStax Psychology 2e, 3.4; StatPearls, Substantia Nigra and Basal Ganglia)

The periaqueductal gray around the aqueduct is the main control center for descending pain modulation. (StatPearls, Mesencephalon Midbrain)

In the pupillary light reflex, the optic nerve signals the pretectal nucleus in the upper midbrain, which projects to the Edinger-Westphal nuclei on both sides; their parasympathetic fibers constrict both pupils. Dilation in dim light is the work of the sympathetic nervous system. The oculomotor nucleus of the midbrain controls the medial rectus muscle of the eye. (OpenStax A&P 2e, 16.3; StatPearls, Pupillary Light Reflexes and Pathway and Cranial Nerve 3 (Oculomotor))

What passes through the midbrain?

The corticospinal tract runs through it as the cerebral peduncles on its way down to the pons. (OpenStax A&P 2e, 14.3; StatPearls, Pyramidal Tract)

The cerebellum’s interposed nuclei project to the red nucleus, which sends signals on to the spinal cord along the rubrospinal tract. (OpenStax A&P 2e, 14.3; StatPearls, Red Nucleus)

The spinothalamic tract, which carries pain and temperature, passes through the midbrain on its way to the thalamus. (StatPearls, Mesencephalon Midbrain)