9.4 Pituitary Gland Functions

Introduction to the HPA

  • Definition and Importance

    • The hypothalamus is a region of the brain that regulates internal environment and homeostasis.

    • The hypothalamic-pituitary axis (HPA) is viewed as the "command center" of the endocrine system.

    • The HPA is vital in:

    • Regulating numerous bodily functions through hormone release.

    • Facilitating a complex interplay between the hypothalamus and the pituitary gland.

    • Coordinating messages between the endocrine system and the nervous system.

    • Many stimuli from the nervous system must pass through the HPA to trigger hormonal responses.

Anatomy of the Hypothalamus and Pituitary Glands

  • Location and Structure

    • The hypothalamus is situated in the diencephalon of the brain, specifically anterior and inferior to the thalamus.

    • The relationship between the hypothalamus and the posterior pituitary illustrates overlap between nervous and endocrine systems.

  • Functions

    • The hypothalamus has both neural and endocrine functions, producing and secreting various hormones.

    • The pituitary gland (hypophysis) is a small organ suspended from the hypothalamus by the infundibulum (or pituitary stalk).

    • Anatomically, the pituitary gland resides in the sella turcica of the sphenoid bone of the skull.

    • Composed of two main lobes:

    • Posterior Pituitary Lobe (Neurohypophysis): Composed of neural tissue.

    • Anterior Pituitary Lobe (Adenohypophysis): Composed of glandular tissue derived from the embryonic digestive tract.

    • The intermediate zone (pars intermedia) is part of the anterior pituitary lobe.

  • Hormone Overview

    • A summary of the hormones secreted by the pituitary gland is presented in Table 9.1 (specific details to be provided in that table).

Hypothalamic Controls

  • Complexity

    • The hypothalamus functions as a complex endocrine structure regulating many body processes.

    • Specialized neurons in the hypothalamus, known as neurosecretory cells, respond to neurotransmitters and produce hormones.

  • Regulatory Functions

    • The hypothalamus releases two types of hormones:

    • Regulatory Hormones: Control the release of hormones from the anterior pituitary gland.

    • Directly produced Hormones: Such as ADH (antidiuretic hormone, also known as vasopressin) and oxytocin, which are released by the posterior pituitary.

  • Mechanism of Action

    • Both hormones pass through axons terminating in the posterior pituitary, acting as an extension of the hypothalamus.

    • The hypothalamus has autonomic centers that control the endocrine cells of the adrenal medulla via the autonomic nervous system.

  • Hormone Functions

    • Physiological details regarding ADH and oxytocin include:

    • ADH: Released in response to high ion concentrations (e.g., hypernatremia) or low blood pressure (e.g., hypovolemia).

      • Targets the kidneys to decrease urine formation by retaining water, thereby increasing blood volume and blood pressure (hence also known as vasopressin).

      • Regulated via a negative feedback loop.

    • Oxytocin: Released during childbirth through a positive feedback mechanism, prevalent during milk let-down in nursing.

Posterior Pituitary Gland

  • Structure and Functionality

    • The posterior pituitary (neurohypophysis) is an extension of hypothalamic neurons.

    • Neuron cell bodies are located in the hypothalamus and their axons, descending as the hypothalamic-hypophyseal tract, form the posterior pituitary.

    • Hormones stored and secreted include oxytocin and ADH, produced in the hypothalamus, not in the posterior pituitary itself.

  • Storage and Release Mechanism

    • Hormones travel down axons into storage at posterior pituitary terminals.

    • Upon receiving signals from hypothalamic neurons, hormones are released into the bloodstream from these terminals.

Anterior Pituitary Gland

  • Regulation of Secretion

    • The secretion of hormones from the anterior pituitary (adenohypophysis) is regulated by two classes of hormones from the hypothalamus:

    • Releasing Hormones (-RH): Stimulate hormone secretion from anterior pituitary.

    • Inhibiting Hormones (-IH): Inhibit hormone secretion from anterior pituitary.

    • Overview of releasing hormones:

    • Thyrotropin-Releasing Hormone (TRH)

    • Corticotropin-Releasing Hormone (CRH)

    • Gonadotropin-Releasing Hormone (GnRH)

    • Growth Hormone-Releasing Hormone (GH-RH)

    • Prolactin-Releasing Factor (PRF)

    • Overview of inhibiting hormones:

    • Growth Hormone-Inhibiting Hormone (GH-IH)

    • Prolactin-Inhibiting Hormone (PIH)

  • Hypophyseal Portal System

    • Hypothalamic hormones enter the anterior pituitary through a network of capillaries (hypophyseal portal system), ensuring high hormone concentration by bypassing systemic circulation.

    • Hormones produced by the anterior pituitary then enter a secondary capillary plexus before draining into systemic circulation.

  • Secretion of Anterior Pituitary Hormones

    • The anterior pituitary secretes seven key hormones that stimulate various organs and glands. Summary includes:

    • Thyroid-Stimulating Hormone (TSH)

    • Adrenocorticotropic Hormone (ACTH)

    • Follicle-Stimulating Hormone (FSH)

    • Luteinizing Hormone (LH)

    • Growth Hormone (GH)

    • Prolactin (PRL)

    • Melanocyte-Stimulating Hormone (MSH)

    • Tropic Hormones: TSH, ACTH, FSH, and LH are termed tropic hormones as they stimulate other endocrine glands.

Pars Intermedia

  • Function of Pars Intermedia

    • The cells secrete Melanocyte-Stimulating Hormone (MSH).

    • MSH is crucial in melanin production in response to UV light.

    • In humans, MSH production levels remain constant across different skin tones, but pregnancy can increase levels leading to darker skin pigmentation, especially in areolas and labia minora.

Summary of Hypothalamic Control

  • Control Mechanisms: The hypothalamus controls numerous functions via:

    • Direct Nervous System Control

    • Indirect Hormonal Control: Via regulatory hormone release into the hypophyseal portal system to the anterior pituitary.

    • Direct Hormone Release: Such as ADH and oxytocin.

  • Target Organs:

    • Directly affects organs such as the adrenal glands, thyroid gland, liver, mammary glands, and reproductive organs, leading to physiological responses as outlined in Figure 9.13.