4. Complex Feedback Loops

Complex Feedback Loops: Nervous and Endocrine Systems

Integration of Nervous and Endocrine Systems

• Endocrine responses are often controlled by the brain.
• The hypothalamus initiates nervous system responses and integrates signals from the central nervous system.
• This integration allows for a comprehensive response considering various factors (e.g., body temperature, stress).
• Examples of processes controlled in this way include reproduction, stress, body metabolism, and growth.
• Thyroid hormones and body metabolism require relatively immediate responses but still need integrated information.

Hypothalamus and Pituitary Gland

• The hypothalamus is a brain structure containing neurons grouped by function.
• The hypothalamus connects to the posterior and anterior pituitary glands.
• Posterior Pituitary:
  • Neurons in the hypothalamus have axons extending directly to the posterior pituitary.
  • Axon terminals release hormones in the posterior pituitary.
• Anterior Pituitary:
  • Neurons in the hypothalamus have axons ending in the hypothalamus.
  • These neurons release substances that travel via blood vessels to the anterior pituitary.

Hypothalamic-Anterior Pituitary Axis

• Neurons in the hypothalamus produce hormones (releasing hormones).

• Examples:

  • TRH (Thyrotropin-Releasing Hormone)
  • CRH (Corticotropin-Releasing Hormone)
  • GnRH (Gonadotropin-Releasing Hormone)

• Releasing hormones stimulate the anterior pituitary to release trophic hormones.

• The name of the releasing hormone indicates the target hormone to be released from the anterior pituitary.

  • Example: Thyrotropin-releasing hormone stimulates the release of thyroid-stimulating hormone.

• The sequence of hormones released forms a feedback loop.

• Specific cells in the anterior pituitary (e.g., thyrotrophs, gonadotrophs) produce specific hormones.

  • Thyrotrophs: Thyroid Stimulating Hormone $\rightarrow$ Thyroid Hormone
  • Gonadotrophs: Luteinizing Hormone and Follicle Stimulating Hormone $\rightarrow$ Estrogen, Testosterone, Progesterone

Posterior Pituitary Hormones

• Hormones are made in the hypothalamus and released from the posterior pituitary.
• Hormones then directly enter the bloodstream to exert their effects.
• Two key hormones:
  • Oxytocin
  • ADH (Antidiuretic Hormone or Vasopressin)

Anterior Pituitary Hormones and Portal System

• Cells in the hypothalamus produce and release hormones into a portal system.
• Hypothalamic Portal Vein:
  • Direct connection from one set of capillaries to another (unusual arrangement).
  • Allows hormones from the hypothalamus to directly target cells in the anterior pituitary.
  • Minimizes the amount of hypothalamic hormone needed to produce a response.
• Anterior pituitary hormones are released into the bloodstream and travel to target glands.
• This multi-hormone sequence amplifies the signal and enhances control.
• Hormones from the anterior pituitary:
  • Thyroid Stimulating Hormone (TSH)
  • Adrenocorticotropic Hormone (ACTH)
  • Follicle-Stimulating Hormone (FSH)
  • Luteinizing Hormone (LH)
  • Growth Hormone (GH)
  • Prolactin