Hypothalamus

1. Extensively interconnected with the limbic system; receives blood supply from branches of the PComm
2. Amygdala has many nuclei that are reciprocally connected to the hypothalamus
   1. amygdala gives rise to two efferent projections: stria terminalis (innervates nucleus accumbens and hypothalamus) and ventral amydalofugal pathway (provides input to the hypothalamus, dorsal medial nucleus of the thalamus and rostral cingulate gyrus)
      1. stria terminalis: parallels the course of the lateral ventricle and the caudate nucleus; separates the caudate and the thalamus. Stria terminalis is consistently located medial to the caudate nucleus in the roof of the temporal horn of the lateral ventricle and floor of the body of the lateral ventricle; stria terminalis courses within a groove formed at the junction of the thalamus and the caudate nucleus (terminal vein runs with the stria terminalis)
   2. lesions and stimulation of the amygdala produce autonomic responses, emotional behaviors and feeding similar to stimulation and lesioning of the lateral or medial regions of the hypothalamus
3. posterior extent of the hypothalamus is delimited by the mammillary bodies; the anterior extent is delimited by the optic chiasm, preoptic area, and lamina terminalis – See Figure 4
4. hypothalamus is divided in a lateral to medial direction into 3 regions:
   1. lateral
      1. has long fibers that project to the spinal cord and cortex along with extensive short-fiber ascending and descending pathways; medial forebrain bundle is the major tract that runs through the lateral region, it contains many aminergic neurons originating in the brain stem that project to the cortex
      2. includes the lateral preoptic nucleus and the lateral nucleus
         1. lateral nucleus when stimulated induces eating and lesions cause anorexia and starvation
   2. medial
      1. separated from the lateral region of the hypothalamus by the fornix; contains most of the nuclei of the hypothalamus; has 4 regions
         1. preoptic region
   3. medial preoptic nucleus is known as the sexually dimorphic nucleus because it regulates the release of gonadotropic hormones and is dependent upon testosterone levels
      1. supraoptic region
         1. suprachiasmatic nucleus with control of circadian rhythms via innervation by the retina
         2. anterior nucleus - anterior nucleus plays a role in temperature regulation and destruction results in hyperthermia
         3. paraventricular nucleus releasing ADH, oxytocin, and CRH (both paraventricular and supraoptic nuclei produce vasopressin and oxytocin)
         4. paraventricular nucleus sends descending axons to the brainstem and all levels of the spinal cord
         5. supraoptic nuclei is composed of mostly large cells
      2. tuberal region
         1. dorsomedial nucleus – when stimulated results in savage behavior
         2. ventromedial nucleus – satiety center, when stimulated, inhibits the urge to eat
         3. arcuate produces dopamine, endorphins, and prolactin; efferent projection goes to the median eminence
      3. mamillary region
         1. mamillary nuclei which projects to the anterior nucleus of the thalamus
         2. posterior nucleus – plays a role in thermal regulation with lesions resulting in poikilothermia
            1. dorsal longitudinal fasciculus extends from the posterior hypothalamic nucleus to the caudal medulla and projects to autonomic centers of the brainstem with both ascending and descending fibers
   4. periventriuclar
      1. consists of those parts of the hypothalamus immediately bordering the third ventricle
5. each nucleus of the hypothalamus subserves a variety of functions but can be classified into three groups: those that project to the posterior pituitary, those that project to the median eminence (which goes to the anterior pituitary), and those associated with the autonomic nervous system
   1. most fibers systems of the hypothalamus are bi-directional including the structures caudal to the hypothalamus which are the medial forebrain bundle, the mammillotegmental tract, and the dorsal longitudinal fasciculus; rostral structures are interconnected to the hypothalamus by means of the mamillothalamic tract (which projects to the anterior nucleus of the thalamus), fornix, and stria terminalis
   2. each neuroendocrine neuron synthesizes neurohormones or precursor peptides in the cell body that are transported down the axon to the terminal; each neuron type contains more than one peptide
   3. the large neuroendocrine neurons (magnocellular) are located in the paraventricular and supraoptic nuclei: these release oxytocin and vasopressin into the general circulation via the posterior pituitary
   4. the small neuroendocrine (parvocellular) neurons are located in several hypothalamic regions: the medial basal region, the arcuate and tuberal nuclei, the periventricular region, and the preoptic (and paraventricular nuclei according to some texts)
      1. these neurons release their hormones into the portal plexus in the median eminence
         1. median eminence lacks neuronal cell bodies and is derived from the anterior pituitary
      2. they include thyrotropin releasing hormone, gonadotropin hormone releasing hormone, corticotropin releasing hormone, growth hormone releasing hormone, somatostatin
6. hypothalamus receives one way afferent connections directly from the retina; these fibers terminate in the suprachiasmatic nucleus which is involved in circadian rhythms
   1. the surpachiasmatic nucleus sends descending sympathetic tracts to the intermediolateral cell columns and superior cervical ganglia and then ascends to innervate noradrenergic terminals on pinealocytes
7. stimulation of the lateral and posterior hypothalamus causes profound sympathetic nervous system activation; lesions of the posterior and lateral hypothalamus result in placidity
   1. stimulation causes autonomic and somatic responses characteristic of anger: increased blood pressure, raising of the body hair, pupillary constriction (fight or flight responses)
   2. lesions of the posterior part of the hypothalamus produce hypothermia or poikilothermia, emotional lethargy and sleepiness (parasympathetic overload)
   3. lesions of the posterior hypothalamus can also cause neurogenic pulmonary edema and systemic hypertension (this part of the hypothalamus is tonically inhibited by the lateral regions of the hypothalamus)
8. stimulation of the anterior and medial regions causes parasympathetic stimulation
   1. bilateral lesions in the anterior parts of the hypothalamus (specifically in the temperature sensitive neurons in the preoptic area) result in hyperthermia with disruption of the heat dissipating mechanisms of the body
9. lesions of the medial hypothalamus causes highly excitable and easily triggered aggressive responses (sympathetic overload)
10. satiety center is in the ventromedial nucleus of the hypothalamus while the appetite center is located in the ventrolateral nucleus
    1. lesions in the medial hypothalamus result in overeating and obesity
    2. lesions in the lateral hypothalamus cause failure to eat and thrive