Hypothalamus and Neuroendocrine Control

Introduction

Location – The hypothalamus is a small bilaterally symmetrical diencephalic structure, located below the thalamus posterior to the optic chiasma; it includes the infundibulum as well as the mammillary bodies.

Role – The hypothalamus serves as coordinator of input from the internal and external environments to assure organized endocrine, autonomic, behavioral, and somatic responses to specific stimuli.

The critical role of the hypothalamus is adaptation of the organism and species for maintenance of homeostasis in response to changes in both the internal and external environments. This can be accomplished by (1) regulation of visceral functions and/or (2) changes in behavior. The hypothalamus is appropriately organized and interconnected with autonomic centers (in the brainstem) and motivational centers (in the limbic system and neocortex) to integrate these functions into well-defined and specific behavioral patterns which will accomplish the necessary adaptive response.

Design

Divided into 3 regions (medial to lateral):

(1) Periventricular – tends to have endocrine functions

(2) Medial – where most of the integration occurs, receives input from the limbic system

(3) Lateral – a "relay"; large fiber tracts go through the hypothalamus here

Divided into 3 regions anterior to posterior

(1) Anterior – includes three nuclei

Paraventricular – 3 divisions:

(1) parvocellular involved in endocrine function
(2) magnocellular project to pituitary along with the Supraoptic nucleus
(3) autonomic involved in visceral regulation

Supraoptic – projects axons to the neural lobe of the pituitary along with the magnocellular division (above)

Suprachaismatic – gets direct input from retina, is the "biological clock", regulates diurnal and circadian rhythms.

(2) Middle – includes cells of the arcuate nucleus which project to the median eminence and release hormones that influence the anterior pituitary. Also contains cell groups involved in feeding.

(3) Posterior – includes the mammillary bodies. Involved in thermoregulation.

Connections

Ins and Outs

IN: receptors (baro, chemo, temp, etc) and cortical pathways (what’s the overall state of the animal?)
OUT: to Autonomic system, Endocrine system, Cortex (limbic and neocortex), and Somatic (via spinal cord)

Limbic system – connected via the stria terminalis (projects to anterior hypothalamus), a ventral tract (projects to lateral hypothalamus), and the fornix (projects to the mammillary bodies)
Brainstem – the medial forebrain bundle contains afferents that arise from the Nucleus Tractus Solitarius (NTS) and efferents to both sympathetic and parasympathetic cell groups in the spinal cord. The dorsal longitudinal fasiculus orginates in the midbrain central gray and projects diffusely to the hypothalamus.
Pituitary – in communication via the infundibulum

Structure-Function

THIRST – ß blood volume and/or Ý plasma Osm trigger thirst.

ß blood volume is sensed by the baroreceptors of the R atrium Þ NTS Þ hypothalamus

Ý plasma Osm is sensed directly by cells of the Anterior hypothalamus

OUT

magnocellular cells and the supraoptic nucleus produce and secrete more ADH
Direct projections to the neocortex trigger behavioral changes (urge to drink)

HUNGER – either ß blood metabolites/sugar etc, or by circadian rhythm (telling you it’s time to eat)

ventromedial (satiety center); lateral and dorsomedial (hunger center)

Suprachiasmatic nucleus influenced by light, regulates rhythms

The hypothalamus itself can sense carbohydrates, fat, and amino acids in the blood

A lesion of ventromedial nucleus of the hypothalamus = cats that eat continuously whereas lesions of the lateral hypothalamic zone abolishes eating and drinking behavior Þ death of cat

The lateral hypothalamus makes you "lateral"; The medial hypothalamus makes you "medial"

OUT

Hunger causes the gut to be stimulated via the Dorsal motor nucleus of X, to get the gut ready for food
Projections to the amygdala (which controls visceral and emotional functions) as well as cortex (behavior)

RAGE – hypothalamus is not involved in initiating rage, but in coordinating the response

"Sham rage" can be created in cats by either stimulating the dorsalmedial nucleus or destroying the ventromedial nucleus
IN

complex, from frontal cortex

the dorsal longitudinal fasiculus leaves the hypothalamus, travels the length of the brainstem giving off branches to all the descending reticular nuclei (which supply the autonomic nervous system), all the parasympathetic nuclei of III, VII, IX, and X, the respiratory and cardiac centers, and the motor nuclei of the cranial nerves.

TEMPERATURE REGULATION

A "SET POINT" is accomplished by the combined efforts of the septal area and the anterior hypothalamus. The septal area will respond to interleukins during infection by raising the set point and allowing a fever.
anterior hypothalamus wants to lower temperature (vasodilation, pseudomotor)
posterior hypothalamus wants to raise temperature (inhibits ant. Hypothalamus, initiates shivering)
OUT

aside from projecting to the autonomic system, the hypothalamus projects to the cortex to influence behavior.

CIRCADIAN RHYTHM – seasonal changes in metabolism, etc are mainly modified by the anterior hypothalamus’s influence on the endocrine system via the pituitary – suprachiasmatic