The basic lower brain consists of the spinal cord, brainstem and diencephalon (the cerebellum and cortex are also present, but will be discussed in later sections). Within each of these structures are centers of neuronal cell bodies, called nuclei, that are specialized for particular functions (breathing, heart-rate regulation, sleep):
The spinal cord can be viewed as a separate entity from the brain or merely as a downward extension of the brainstem. It contains sensory and motor pathways from the body, as well as ascending and descending pathways from the brain. It has reflex pathways that react independently of the brain, as in the knee-jerk reflex.
Medulla - The medulla contains nuclei for regulating blood pressure and breathing, as well as nuclei for relaying information from the sense organs that comes in from the cranial nerves.
Pons - The pons contains nuclei that relay movement and position information from the cerebellum to the cortex. It also contains nuclei that are involved in breathing, taste and sleep.
Midbrain - The midbrain contains nuclei that link the various sections of the brain involved in motor functions (cerebellum, basal ganglia, cerebral cortex), eye movements and auditory control. One portion, called the substantia nigra, is involved in voluntary movements; when it does not function, you have the tremored movements of Parkinson's disease.
Thalamus - The thalamus relays incoming sensory pathways to appropriate areas of the cortex, determines which sensory information actually reaches consciousness and participates in motor-information exchange between the cerebellum, basal ganglia and cortex.
Hypothalamus - The hypothalamus contains nuclei that control hormonal secretions from the pituitary gland. These centers govern sexual reproduction, eating, drinking, growth, and maternal behavior such as lactation (milk-production in mammals). The hypothalamus is also involved in almost all aspects of behavior, including your biological "clock," which is linked to the daily light-dark cycle (circadian rhythms).
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