Topics and Speakers Peter W. Carmel, MD, DMSc
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Synopsis
For millennia, the tiny hypothalamus has been considered one of the most critical structures of the brain. Dr Peter W. Carmel reviews current understanding of the functional anatomy of the hypothalamus and the related structure, the pituitary. The hypothalamus controls homeostasis by regulating all the critical states and drives necessary for life: body temperature, drives for food and water, sleep and wakefulness, defense and stress mechanisms, circulatory volume and blood pressure, circadian rhythms, and both sexual and affective behavior.
Dr Carmel begins the lecture with a review of the anatomy and structure of the hypothalamus. Surprisingly small and remarkably dense, this phylogenetically ancient structure has links to the oldest portions of the brainstem as well as the cortex and the limbic system. Carmel details the various divisions of the hypothalamus, specifically the cell groups, or nuclei, and the paraventricular organization, which is based largely on the fiber bundles. Both the afferent and efferent connections in the hypothalamus display bilateral representation. This duplication provides the hypothalamus protection against damage. The afferent connections, which send information into the brain, have multiple short-chain pathways. These incoming connections receive strong input from the brainstem.
Because of its profound combination of sympathetic and parasympathetic effects, the hypothalamus has been dubbed the head ganglion of the autonomic nervous system. Unlike a simple monosynaptic reflex, which executes its effect rapidly, the hypothalamus exerts its effects through multiple short-chain pathways that build the given response slowly over time. Frequent feedback loops allow for set-point mechanisms and oscillators.
Neuroendocrine control in the hypothalamus is divided between two regions, the large cells and the small cells. The large, or magnocellular, neurons release hormones such as oxytocin, antidiuretic hormone, and vasopressin. The small, or parvocellular, neurosecretory neurons produce the nine hypothalamic releasing factors.
The hypothalamus coordinates drive-related activities through three sets of connections: to the limbic, pituitary, and brainstem. By linking to the limbic system, the hypothalamus regulates emotional drives. With its outputs to the pituitary, it regulates and coordinates hormonal drives, and with connections to visceral and somatic nuclei of the brainstem and spinal cord, it controls autonomic drives.
Since the hypothalamus regulates homeostasis, damage to this structure can disrupt thermal regulation and the state of alertness, and can cause disorders of endocrine, caloric, and osmolar functions, as well as affective disorders, disorders of autonomic balance, and memory and learning disorders.
Dr Carmel concludes the lecture with an examination of pituitary function, examining how the pituicytes have the ability to secrete hormones into the bloodstream. In addition to describing normal pituitary function, he details several pituitary disorders, including pituitary tumors, acromegaly, hypogonadism, cranial pharyngioma, and Cushing's syndrome.
