Hypothalamus

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The Hypothalamus

  • is a division of the diencephalon.
  • lies within the floor and ventral part of the walls of the third ventricle.
  • functions primarily in the maintenance of homeostasis.
  • subserves three systems:
    • theautonomic nervous system (ANS)
    • theendocrine system
    • thelimbic system.
  • Surface Anatomy of the Hypothalamus
    • is visible only from the ventral aspect of the brain.
    • lies between the optic chiasm and the interpeduncular fossa (posterior perforated substance).
    • includes the following ventral surface structures:
      • Infundibulum
        • is the stalk of the hypophysis.
        • contains the hypophyseal portal vessels.
        • contains the supraopticohypophyseal and tuberohypophyseal tracts.
      • Tuber cinereum
        • is the prominence between the infundibulum and the mamillary bodies.
        • includes the median eminence, which contains the arcuate nucleus (infundibular nucleus).
      • Mamillary bodies
        • contain the mamillary nuclei.
      • Optic chiasm
        • is the floor of the optic recess of the third ventricle.
      • Arterial circle of Willis
        • surrounds the ventral surface of the hypothalamus and provides its blood supply.
  • Hypothalamic Regions and Nuclei
    • The hypothalamus is divided into a lateral area and a medial area
    • lateral area and a medial area are separated by the fornix and the mamillothalamic tract.
    • Lateral hypothalamic area
      • is traversed by the medial forebrain bundle.
      • includes two major nuclei:
        • Lateral preoptic nucleus
        • Lateral hypothalamic nucleus
          • when stimulated, induces eating.
          • Lesions cause anorexia and starvation.
    • Medial hypothalamic area
      • includes the periventricular area that borders the third ventricle.
      • is divided into four regions, from anterior to posterior:
      • Preoptic region
        • contains the medial preoptic nucleus
          • regulates the release of gonadotropichormones from the adenohypophysis.
          • contains the sexually dimorphic nucleus, whose development is dependent on testosterone levels.
      • Supraoptic region
        • Suprachiasmatic nucleus
          • receives direct input from the retina.
          • plays a role in the control of circadian rhythms.
        • Anterior nucleus
          • plays a role in temperature regulation.
          • stimulates the parasympathetic nervous system
          • Destruction results in hyperthermia.
        • Paraventricular nucleus
          • Neurosecretory cells synthesize and release antidiuretic hormone (ADH), oxy-
          • tocin, and corticotropin-releasing hormone (CRH).
          • regulateswater balance (conservation of water).
          • gives rise to the supraopticohypophyseal tract, which projects to the neurohy-
          • pophysis.
          • Destruction results in diabetes insipidus.
        • Supraoptic nucleus
          • synthesizesADH and oxytocin.
          • projects to theneurohypophysis via the supraopticohypophyseal tract.
      • Tuberal region
        • Dorsomedial nucleus
          • when stimulated results in savage behavior.
        • Ventromedial nucleus
          • is considered a satiety center.
          • when stimulated, inhibits the urge to eat.
          • Bilateral destruction results in hyperphagia, obesity, and savage behavior.
        • Arcuate (infundibular) nucleus
          • is a periventricular nucleus.
          • contains neurons that produce hypothalamic-releasing factors
          • gives rise tothe tuberohypophyseal tract, which terminates in the hypophyseal portal systemof the infundibulum.
          • effects, via hypothalamic-releasing factors, the release or nonrelease of adenohypophyseal hormones into the systemic circulation.
          • containsdopaminergic neuron
          • dopamine is the prolactin-inhibiting factor(PIF).
      • Mamillary region
        • Mamillary nuclei
          • receive input from the hippocampal formation (specifically the subiculum) via the fornix.
          • receive input from the dorsal and ventral tegmental nuclei and the raphe nuclei via the mamillary peduncle.
          • project to the anterior nucleus of the thalamus via the mamillothalamic tract.
          • containhemorrhagic lesions in Wernicke encephalopathy.
        • Posterior nucleus
          • plays a role in thermal regulation (i.e., conservation and increased production of heat).
          • Lesions result in poikilothermia, the inability to thermoregulate.
  • Major Hypothalamic Connections
    • Afferent connections to the hypothalamus
      • Septal area and nuclei and orbitofrontal cortex
      • Hippocampal formation
      • Amygdaloid complex
      • Primary olfactory cortex (area 34)
      • Mediodorsal nucleus of the thalamus
      • Brainstem nuclei
        • Tegmental nuclei (dorsal and ventral)
        • Raphe nuclei  projectserotonergic fibers
        • Locus ceruleusprojects noradrenergic fibers
    • Efferent connections from the hypothalamus
      • Septal area and nuclei
      • Anterior nucleus of the thalamus
      • Mediodorsal nucleus of the thalamus
      • Amygdaloid complex
      • Brainstem nuclei and spinal cord
      • Adenohypophysis
        • via the tuberohypophyseal tract and hypophyseal portal system
      • Neurohypophysis
        • via the supraopticohypophyseal tract
  • Major Fiber Systems
    • Fornix
      • projects from the hippocampal formation to the mamillary nucleus, anterior nucleus of thethalamus, and septal area.
      • is the largest projection to the hypothalamus.
      • Bilateral transection results in an acute amnestic syndrome.
    • Medial forebrain bundle
      • traverses the entire lateral hypothalamic area.
      • interconnects the septal area and nuclei, the hypothalamus, and the midbrain tegmentum.
    • Mamillothalamic tract
      • projects from the mamillary nuclei to the anterior nucleus of the thalamus.
    • Mamillary peduncle
      • conducts fibers from the dorsal and ventral tegmental nuclei and the raphe nuclei to themamillary body.
    • Mamillotegmental tract
      • conducts fibers from the mamillary nuclei to the dorsal and ventral tegmental nuclei.
    • Striaterminalis
      • is the most prominent pathway from the amygdaloid complex.
      • interconnects the septal area, the hypothalamus, and the amygdaloid complex.
    • Ventral amygdalofugal pathway
      • interconnects the amygdaloid complex and the hypothalamus.
    • Supraopticohypophyseal tract
      • conducts fibers from the supraoptic and paraventricular nuclei to the neurohypophysis
      • therelease site for ADH and oxytocin
    • Tuberohypophyseal (tuberoinfundibular) tract
      • conducts fibers from the arcuate nucleus to the hypophyseal portal system of theinfundibulum.
    • Dorsal longitudinal fasciculus
      • extends from the hypothalamus to the caudal medulla.
      • projects to the parasympathetic nuclei of the brainstem.
    • Hypothalamospinal tract
      • contains direct descending autonomic fibers
      • influencespreganglionic sympathetic neu- rons of the intermediolateral cell column and preganglionic neurons of the sacral parasympathetic nucleus.
      • interruptionabove T1 results in Horner syndrome.
  • Functional Considerations
    • Autonomic function
      • Anterior hypothalamus
        • has an excitatory effect on the parasympathetic nervous system.
      • Posterior hypothalamus
        • has an excitatory effect on the sympathetic nervous system.
    • Temperature regulation
      • Anterior hypothalamus
        • helpsregulate and maintain body temperature.
        • Destruction causes hyperthermia.
      • Posterior hypothalamus
        • helpsproduce and conserve heat.
        • Destruction causes the inability to thermoregulate.
    • Water balance regulation
      • ADH controls water excretion by the kidneys.
    • Food intake regulation
      • two hypothalamic nuclei play roles in the control of appetite:
        • Ventromedial nucleus
        • Lateral hypothalamic nucleus
          • is called the hunger or feeding center.
          • Destruction causes starvation and emaciation.
    • Hypothalamic-releasing and release-inhibiting factors
      • are produced in the arcuate nucleus of the median eminence.
      • are transported via the tuberohypophyseal tract to the hypophyseal portal system.
      • effect the release or nonrelease of adenohypophyseal hormones.
      • adenohypophyseal hormonesarepeptides (hypophysiotropins), which include:
      • Thyrotropin-releasing hormone (TRH)
      • Gonadotropin-releasing hormone (GnRH)
      • Somatostatin (growth hormone–inhibiting hormone)
      • Growth hormone–releasing hormone (GHRH)
      • CRH
      • PIF and prolactin-releasing factor (PRF) (PIF is dopamine.)
        • Dopamine is the only adenohypophyseal hormonewhich is not a peptide


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