<P> The ventral tegmental area (VTA) 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>

Which structure of the brain stem is primarily related to alertness