Hypothalamus | Anatomy2Medicine
hypothalamus

Hypothalamus

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:
      • the autonomic nervous system (ANS)
      • the endocrine system
      • the limbic 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:
        • is the prominence between the infundibulum and the mamillary bodies.
        • Infundibulum
          • is the stalk of the hypophysis.
          • contains the hypophyseal portal vessels.
          • contains the supraopticohypophyseal and tuberohypophyseal tracts.
        • Tuber cinereum

 

  • 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 gonadotropic hormones 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).

 

            • regulates water balance (conservation of water).

 

  • gives rise to the supraopticohypophyseal tract, which projects to the neurohy-
  • pophysis.

 

            • Destruction results in diabetes insipidus.

 

  • Supraoptic nucleus

 

            • synthesizes ADH and oxytocin.
            • projects to the neurohypophysis 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 to the tuberohypophyseal tract, which terminates in the hypophyseal portal system of the infundibulum.
            • effects, via hypothalamic-releasing factors, the release or nonrelease of adenohypophyseal hormones into the systemic circulation.

 

  • contains dopaminergic 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.

 

            • contain hemorrhagic 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  project serotonergic fibers
  • Locus ceruleus projects 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 the thalamus, 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 the mamillary body.

 

  • Mamillotegmental tract

 

        • conducts fibers from the mamillary nuclei to the dorsal and ventral tegmental nuclei.

 

  • Stria terminalis

 

        • 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
        • the release site for ADH and oxytocin

 

  • Tuberohypophyseal (tuberoinfundibular) tract

 

        • conducts fibers from the arcuate nucleus to the hypophyseal portal system of the infundibulum.
      • 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
        • influences preganglionic sympathetic neu- rons of the intermediolateral cell column and preganglionic neurons of the sacral parasympathetic nucleus.
        • interruption above 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

 

          • helps regulate and maintain body temperature.
          • Destruction causes hyperthermia.

 

  • Posterior hypothalamus

 

          • helps produce 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 hormones are peptides (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 hormone which is not a peptide