Mechanisms of Hormone Action Figure 16 4 b

Mechanisms of Hormone Action Figure 16– 4 b Effects of Intracellular Hormone Binding.

Endocrine Reflexes • Endocrine Reflexes – Functional counterparts of neural reflexes – In most cases, controlled by negative feedback mechanisms • Stimulus triggers production of hormone whose effects reduce intensity of the stimulus

Endocrine Reflexes • Endocrine reflexes can be triggered by – Humoral stimuli • Changes in composition of extracellular fluid – Hormonal stimuli • Arrival or removal of specific hormone – Neural stimuli • Arrival of neurotransmitters at neuroglandular junctions

Endocrine Reflexes • Simple Endocrine Reflex – Involves only one hormone – Controls hormone secretion by the heart, pancreas, parathyroid gland, and digestive tract • Complex Endocrine Reflex – Involves • One or more intermediary steps • Two or more hormones • The hypothalamus

Endocrine Reflexes Figure 16– 5 Three Mechanisms of Hypothalamic Control over Endocrine Function.

Endocrine Reflexes • Neuroendocrine Reflexes – Pathways include both neural and endocrine components • Complex Commands – Issued by changing • Amount of hormone secreted • Pattern of hormone release: – hypothalamic and pituitary hormones released in sudden bursts – frequency changes response of target cells

The Pituitary Gland • Also called hypophysis • Lies within sella turcica – Diaphragma sellae • A dural sheet that locks pituitary in position • Isolates it from cranial cavity • Hangs inferior to hypothalamus – Connected by infundibulum

Pituitary Gland • Releases nine important peptide hormones • Hormones bind to membrane receptors – Use c. AMP as second messenger

Pituitary Gland Figure 16– 6 a The Anatomy and Orientation of the Pituitary Gland.

Pituitary Gland Figure 16– 6 b The Anatomy and Orientation of the Pituitary Gland.

Pituitary Gland • Median Eminence – Swelling near attachment of infundibulum – Where hypothalamic neurons release regulatory factors • Into interstitial fluids • Through fenestrated capillaries

Pituitary Gland • Portal Vessels – Blood vessels link two capillary networks – Entire complex is portal system • Ensures that regulatory factors reach intended target cells before entering general circulation

Pituitary Gland Figure 16– 7 The Hypophyseal Portal System and the Blood Supply to the Pituitary Gland.

Pituitary Gland • Two Classes of Hypothalamic Regulatory Hormones – Releasing hormones (RH) • Stimulate synthesis and secretion of one or more hormones at anterior lobe – Inhibiting hormones (IH) • Prevent synthesis and secretion of hormones from the anterior lobe • Rate of secretion is controlled by negative feedback

Pituitary Gland • Anterior lobe (also called adenohypophysis) – – Hormones “turn on” endocrine glands or support other organs Can be subdivided into three regions: 1. Pars distalis 2. Pars intermedia 3. Pars tuberalis

Pituitary Gland Figure 16– 8 a Feedback Control of Endocrine Secretion

Pituitary Gland Figure 16– 8 b Feedback Control of Endocrine Secretion.

Pituitary Gland Figure 16– 9 Pituitary Hormones and Their Targets.

Pituitary Gland

Pituitary Gland • Posterior lobe (also called neurohypophysis) – Contains unmyelinated axons of hypothalamic neurons – Supraoptic and paraventricular nuclei manufacture • Antidiuretic hormone (ADH) • Oxytocin (OXT)

Pituitary Gland Figure 16– 9 Pituitary Hormones and Their Targets.

Pituitary Gland