Hypothalamus/pituitary physiology

HUF2-102 Hypothalamus/Pituitary Physiology Notes

Background on the Embryonic Origin of the Pituitary Gland

  • The pituitary gland (hypophysis) originates embryonically from two distinct structures:

    • The anterior pituitary (adenohypophysis) develops from an upward evagination known as Rathke’s pouch from the primitive foregut (epithelial tissue).

    • The posterior pituitary (neurohypophysis) forms as a downward extension of the floor of the diencephalon (neurohypophysial diverticulum) (neural tissue).

Anatomical Position and Neighbouring Structures

  • The pituitary gland is located in the sella turcica, a bony cavity of the sphenoid bone.

  • Nearby anatomical structures include:

    • Optic chiasm

    • Cranial nerves III (oculomotor), IV (trochlear), VI (abducens)

Structural and Functional Relationship to the Hypothalamus

  • The functional relationship between the pituitary gland and hypothalamus is critical for hormone secretion control:

    • Anterior pituitary:

    • Regulated by the hypothalamic-hypophyseal portal system, which facilitates the delivery of hypothalamic hormones to the anterior pituitary.

    • Posterior pituitary:

    • Connected via a fiber tract down the pituitary stalk and contains axonal projections from neuroendocrine cells.

Neuroendocrine Control/Integration Significance

  • The hypothalamus integrates neural and endocrine signals to achieve regulatory control, with examples including:

    • Hypothalamus-anterior pituitary interactions

    • Hypothalamus-posterior pituitary interactions

    • Influence on the adrenal medulla

    • Regulation of endocrine pancreas functions

Hypothalamic Hormones and Anterior Pituitary Control

  • Hypothalamic hormones play essential roles in regulating anterior pituitary hormone secretion:

    • Example of hypothalamic hormones:

    • Gonadotrophin Releasing Hormone (GnRH)

    • Thyrotrophin Releasing Hormone (TRH)

    • Corticotrophin Releasing Hormone (CRH)

    • Growth Hormone Releasing Hormone (GHRH)

    • Somatostatin (GH inhibiting hormone)

  • Anterior pituitary hormones regulate target glands:

    • Thyroid Stimulating Hormone (TSH) - targets the thyroid gland

    • Adrenocorticotropic Hormone (ACTH) - targets the adrenal gland

Feedback Mechanisms in Hypothalamic-Pituitary-Target Gland Axis

  • The feedback mechanisms include:

    • Negative feedback: Predominant form of hormonal control;

    • Involved in the hypothalamic-pituitary-target gland regulation, where target gland hormones inhibit hypothalamic and pituitary secretion (e.g., cortisol inhibiting ACTH).

    • Positive feedback: Seen in specific conditions, such as the gonadotropin surge in females during the menstrual cycle.

    • Example:

      • Estrogen surge leading to positive feedback on GnRH and resulting LH/FSH surge.

Types of Anterior Pituitary Hormones

  • Glycoprotein Hormone Family:

    • Includes hormones like TSH, LH, FSH; characterized by α and β subunits, with biological activity determined by the β subunit.

  • Prolactin-Growth Hormone Family:

    • Includes Prolactin and Growth Hormone; shares structural similarities in peptide chains and functional activity.

Conditions of Hyperpituitarism and Hypopituitarism

  • Hyperpituitarism: Overproduction of functions resulting from:

    • Pituitary tumors (adenomas) which can cause compression effects (mass effect) leading to symptoms such as headaches and visual disturbances (e.g., bitemporal hemianopia, diplopia).

  • Hypopituitarism: Caused by various factors:

    • Pituitary destruction: Leads to decreased hormone production.

    • Sheehan’s syndrome: Postpartum hemorrhage causing pituitary necrosis.

    • Tumors causing compression on the pituitary stalk resulting in selective loss (e.g., raising prolactin).

Prolactin Regulation

  • Control of secretion: Regulated by:

    • Neuroendocrine reflex linked to suckling during lactation

    • Inhibition by dopamine (PIH) and stimulation by TRH, VIP.

  • Effects of prolactin:

    • Supports mammogenesis, lactogenesis, and influences reproductive functions.

Antidiuretic Hormone (ADH) Secretion Control

  • Control Mechanisms:

    • Stimulated by increases in plasma osmolality (via hypothalamic osmoreceptors)

    • Stimulated by decreases in blood volume (detected by baroreceptors)

    • Inhibition by alcohol and atrial natriuretic peptide (ANP).

  • Disorders of ADH secretion:

    • Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH): High levels of ADH lead to water retention and hyponatremia.

    • Diabetes Insipidus (DI): Results in polyuria; centrally or nephrogenically caused.

Oxytocin Actions and Control

  • Oxytocin regulates:

    • Uterine contractions during labor through a positive feedback mechanism initiated by cervical stretch.

    • Milk ejection reflex during breastfeeding triggered by suckling (neuroendocrine reflex).

  • Control Mechanism: Similar to ADH, oxytocin release can be influenced by sensory inputs from suckling and conditioned reflexes.

Clinical Assessment and Treatment Approaches

  • Diagnostic Tests for pituitary function:

    • Hormonal assays to measure anterior pituitary hormone levels in response to hypothalamic stimulation.

    • Imaging (MRI) for anatomical assessment (tumors, structural anomalies).

  • Treatment Options for disorders include:

    • Prolactinomas are treated with dopamine agonists (e.g., cabergoline, bromocriptine).

    • Diabetes Insipidus requires hormonal replacement or vasopressin analogues, depending on the type.

    • Management of hyperprolactinemia involves treating underlying causes and normalizing prolactin levels to restore physiological functions.