L3 - Hypothalamus - Structure and Function

what is the hypothalamus?

• a key brain region that maintains homeostasis by linking the nervous system to the endocrine system

• the hypothalamus keeps the body’s internal environment stable and helps the brain control basic survival functions

• the hypothalamus is connected to various brain structures, and also directly connected to the body, as it is the site of brain-body and body-brain communication

how is the hypothalamus able to maintain homeostasis?

• many of the hypothalamic neurons make and transmit/secrete specialised neurohormones that centrally regulate other hormones of the body to maintain homeostasis

• many of the cells of the hypothalamus have receptors that respond to body hormones or metabolites, they can sense what is going on in the body based on the levels of these hormones/metabolites

what are some of the main functions of the hypothalamus?

• regulate body temperature

• electrolyte balance

• blood pressure

• regulate sleep/wake cycle

• circadian rhythm

• coordinate stress and emotional responses

• control hormone release

what is the hypothalamus referred to as?

central regulator of physical homeostasis in the body

what is allostasis?

• the hypothalamus adapts to re-set optimal set points which are re-set as the cells of the hypothalamus have a life-long plasticity important for survival

where is the hypothalamus located?

• located at the ventral most part of the brain and forms the ventral forebrain

what is the hypothalamus composed of?

• it is composed of multiple nuclei which are clusters of neurons organised into distinct regions

• the nuclei are grouped into types based on their anatomical regions

what are the types of nuclei in the hypothalamus?

• paraventricular nuclei - contains paraventricular nucleus (PVN)

• tuberal nuclei - includes arcuate nucleus (ARCN) and ventromedial nucleus (VMN)

• mammillary nuclei - contains mammillary bodies, mammillary and supramammilary nuclei

how are the hypothalamic nuclei arranged?

• they are arranged around glial-rich regions called the median eminence and the pituitary stalk/posterior pituitary

what are glial-rich regions?

• regions that form key brain-body interfaces and allow close interaction with the pituitary gland

what do the hypothalamic nuclei occupy?

• distinct subdomains each containing specialised neuronal populations that produce specific neurohormones or neuropeptides, enabling regulation of core homeostasis processes like energy balance, sleep and stress

what is physical homeostasis mediated by?

neurons in particular nuclei that act via neurotransmitters and neurohormones

how is energy balance regulated?

• primarily regulated within the tuberal hypothalamus, particularly the arcuate nucleus (ARCN)

• the ARCN contains two sets of neurons called POMC and NPY neurons

• POMC neurons are activated when energy stores are high (full) and they reduce food intake

• NPY neurons are activated when energy stores are low (staving) and promote food intake

• these two neuron sets act antagonistically to regulate energy balance, food intake, digestion and metabolic rate

how is sleep regulated?

• sleep and wakefulness are controlled by neurons in the lateral hypothalamus and zona incerta

• these regions contain antagonistic neuronal populations, with some neurons promoting sleep and some promoting wakefulness

how is stress regulation mediated?

• mediated by neurons in the paraventricular nucleus and ventromedial hypothalamus

• these neurons produce key factors such as cotricotropin releasing hormone and steroidal factor 1 (SF-1), which activate the hypothalamic-pituitary-adrenal (HPA) axis and coordinate physiological stress responses

how is the hypothalamic circuitry organised?

• it is organised into interconnected neurons of nuclei and neurons

• the neurons are linked into interlinked circuits

• such organisation allows to see how one behaviour has an impact on another

• it allows different behaviours to influence one another, explaining how changes in one physiological or behavioural state can directly impact another

how do interfaces allow direct communication from brain to body?

• some neurons (neurons 1-4) have their cell bodies in the paraventricular nucleus

• these neurons project their axons to glial-rich domains such as the median eminence

• they secrete regulatory neurohormone/neuropeptide at the median eminence and release them into the portal capillary network which forms the interface between the brain and the body

• the hormones are transported to the anterior pituitary where they trigger the endocrine cells to release hormones into the circulation

• other neurons (5-6) are specialised paraventricular nucleus neurons involved in indirect neuroendocrine control

→ this organisation demonstrates how the hypothalamus converts neural signals into endocrine responses to regulate body physiology