<P> The midbrain is divided into three parts: tectum, tegmentum, and the ventral tegmentum . The tectum (Latin: roof), which forms the ceiling . The tectum comprises the paired structure of the superior and inferior colliculi and is the dorsal covering of the cerebral aqueduct . The inferior colliculus, is the principal midbrain nucleus of the auditory pathway and receives input from several peripheral brainstem nuclei, as well as inputs from the auditory cortex . Its inferior brachium (arm - like process) reaches to the medial geniculate nucleus of the diencephalon . Superior to the inferior colliculus, the superior colliculus marks the rostral midbrain . It is involved in the special sense of vision and sends its superior brachium to the lateral geniculate body of the diencephalon . The tegmentum which forms the floor of the midbrain, and is ventral to the cerebral aqueduct . Several nuclei, tracts, and the reticular formation are contained here . The ventral tegmentum is composed of paired cerebral peduncles . These transmit axons of upper motor neurons . </P> <P> The midbrain consists of: </P> <Ul> <Li> Periaqueductal gray: The area of gray matter around the cerebral aqueduct, which contains various neurons involved in the pain desensitization pathway . Neurons synapse here and, when stimulated, cause activation of neurons in the nucleus raphe magnus, which then project down into the posterior grey column of the spinal cord and prevent pain sensation transmission . </Li> <Li> Oculomotor nerve nucleus: This is the third cranial nerve nucleus . </Li> <Li> Trochlear nerve nucleus: This is the fourth cranial nerve . </Li> <Li> Red nucleus: This is a motor nucleus that sends a descending tract to the lower motor neurons . </Li> <Li> Substantia nigra pars compacta: This is a concentration of neurons in the ventral portion of the midbrain that uses dopamine as its neurotransmitter and is involved in both motor function and emotion . Its dysfunction is implicated in Parkinson's disease . </Li> <Li> Reticular formation: This is a large area in the midbrain that is involved in various important functions of the midbrain . In particular, it contains lower motor neurons, is involved in the pain desensitization pathway, is involved in the arousal and consciousness systems, and contains the locus coeruleus, which is involved in intensive alertness modulation and in autonomic reflexes . </Li> <Li> Central tegmental tract: Directly anterior to the floor of the fourth ventricle, this is a pathway by which many tracts project up to the cortex and down to the spinal cord . </Li> <Li> Ventral tegmental area: A dopaminergic nucleus located close to the midline on the floor of the midbrain . </Li> <Li> Rostromedial tegmental nucleus: A GABAergic nucleus located adjacent to the ventral tegmental area . </Li> </Ul> <Li> Periaqueductal gray: The area of gray matter around the cerebral aqueduct, which contains various neurons involved in the pain desensitization pathway . Neurons synapse here and, when stimulated, cause activation of neurons in the nucleus raphe magnus, which then project down into the posterior grey column of the spinal cord and prevent pain sensation transmission . </Li>

What is the function of the brainstem and cerebellum