The Midbrain

The superior and posterior part of the midbrain is called the tectum. There

are two enlargements on both sides of the tectum known as the colliculi.

The superior colliculus controls visual reflexes such as tracking the flight of

a ball, while the inferior colliculus controls auditory reflexes such as turning

toward the sound of a buzzing insect. Above and between the colliculi lies

the pineal gland, which contains melatonin, a hormone that greatly influences

the sleep-wake cycle. Melatonin levels are high when it is dark and low

when it is light. High levels of melatonin induce sleepiness, which is one reason

that people sleep better when it is darker. Another structure near the

colliculi is the periaqueductal gray (PAG) area of the ventricular system.

Stimulation of the PAG helps to block the sensation of pain.

Beneath the tectum is the tegmentum, which includes some structures

involved in movement. Red nucleus activity is high during twisting movements,

especially of the hands and fingers. The substantia nigra smooths out

movements and is influential in maintaining good posture. The characteristic

limb trembling and posture difficulties of Parkinson’s disease are attributable

to neuronal damage in the substantia nigra.