Endocrine System and Hypothalamus

Endocrine Glands

• Endocrine glands are ductless glands that secrete hormones directly into the bloodstream.
• Hormones reach their target sites via the bloodstream and exert their actions there.
• The endocrine system consists of:
  • Hypothalamus
  • Pituitary gland
  • Thymus gland
  • Thyroid gland
  • Pancreas
  • Adrenal gland
  • Reproductive system

Hypothalamic-Pituitary Unit

• The hypothalamus acts as a bridge between the nervous system and the endocrine system.
• The posterior pituitary contains axons of neurons from the hypothalamus.
• Hormones synthesized in the hypothalamus are transported to the posterior pituitary for secretion.
• The hypothalamus synthesizes neurohormones, transporting them to the anterior pituitary through a specialized portal vascular system.
• These neurohormones regulate anterior pituitary hormone secretion.

Hypothalamopituitary Connection

• A vascular connection exists between the hypothalamus and the anterior pituitary known as the hypothalamo-hypophysial portal circulation.
• A nervous connection between the hypothalamus and the posterior pituitary is called the hypothalamo-hypophysial tract.

Control of Endocrine System

• A negative feedback loop exists between hormone levels in the blood and the related gland to maintain hormonal stability.
• This involves:
  • Short feedback loops: Feedback between the anterior pituitary and hypothalamus.
  • Long feedback loops: Feedback between the target gland and the hypothalamus.

Neurosecretory Cells of Hypothalamus

• The hypothalamus's neurosecretory cells are controlled by:
  • Signals from various brain areas in response to external, internal stimuli, emotional and physical stress via neurotransmitters.
  • Short feedback loops from anterior pituitary hormones.
  • Long feedback loops from endocrine glands.
• The hypothalamus releases:
  • Releasing hormones
  • Inhibiting hormones

Anterior Pituitary Hormones

• The anterior pituitary is regulated by hypothalamic releasing/inhibiting hormones.
• It also experiences long feedback loops from hormones produced by various endocrine glands.
• Anterior pituitary secretes:
  • Thyroid stimulating hormone (TSH)
  • Adrenocorticotropic hormone (ACTH)
  • Gonadotropic hormones (FSH & LH)
  • Growth hormone (GH)
  • Prolactin hormone

Pituitary Gland

• The pituitary gland is often referred to as the "master gland" of the endocrine system.
• Anterior pituitary hormone secretion is controlled by:
  • Hypophysiotropic releasing or inhibiting hormones.
  • Long feedback loops from hormones of endocrine glands.
• The pituitary gland secretes:
  • Thyroid stimulating hormone (TSH)
  • Adrenocorticotropic hormone (ACTH)
  • Gonadotropic hormones
  • Growth hormone (GH)
  • Prolactin hormone
  • Oxytocin hormone
  • Antidiuretic hormone (ADH)

Anterior Pituitary Hormones and Function

• Thyroid stimulating hormone (TSH): Stimulates growth and secretion of the thyroid gland.
• Adrenocorticotropic hormone (ACTH): Stimulates growth and secretion of the adrenal gland.
• Growth Hormone (GH): Stimulates growth of bones and tissues, leading to increased weight and height.
• Prolactin: Responsible for milk synthesis.
• Gonadotropic hormones (FSH & LH): In females - stimulate maturation of ova and synthesis of female sex hormones. In males - stimulate spermatogenesis and production of male sex hormones.

Functions of Growth Hormone

• GH influences somatic growth by:
  • Growth of long bones: Stimulates proliferation of epiphyseal cartilage, leading to linear bone growth.
  • Growth of soft tissues: Includes muscles and organs.
• GH has metabolic effects:
  • Fat metabolism: Promotes lipolysis.
  • Carbohydrate metabolism: Possesses an anti-insulin effect.
• GH mechanisms of action:
  • Direct action on cells: Diabetogenic action, lipolysis, increased protein synthesis, production of IGF-I by the liver.
  • Indirect action: Through intermediaries like somatomedins (IGF-I).