Pituitary Gland Dr. Shaikh Mujeeb Ahmed. Lecture Objectives Explain the hypothalamus as the major...

Pituitary Gland

Dr. Shaikh Mujeeb Ahmed

Lecture Objectives

• Explain the hypothalamus as the major integrative site for the neuroendocrine system.

• Contrast the anterior and posterior pituitary lobes with respect to cell types, vascular supply, development, and innervations.

• Describe the general characteristics of hypothalamic releasing and inhibiting (hypophysiotropic) hormones and describe their route of transport from the hypothalamus to the anterior pituitary.

• Identify appropriate hypothalamic hormones that control the secretion of each of the anterior pituitary hormones.

• Diagram short-loop and long-loop negative feedback control of anterior pituitary hormone secretion.

• List the target tissues for oxytocin and describe its effects on each.• Name the stimuli for oxytocin release during parturition or lactation.• Describe the stimuli and mechanisms that control vasopressin secretion.

The pituitary is two fused glands

Pituitary Gland

• Hypophysis• Small gland (0.5 – 1 gm)located in bony cavity

just below hypothalamus– Thin stalk connects pituitary gland to

hypothalamus• Consists of two anatomically and functionally

distinct lobes– Posterior pituitary (neurohypophysis)

• Composed of nervous tissue

– Anterior pituitary (adenohypophysis)• Consists of glandular epithelial tissue

Anatomy of the Pituitary Gland

Anterior/Posterior pituitary gland secretions

• Anterior Pituitary: 6 hormones• Growth hormone (GH)

• Thyroid stimulating hormone (TSH)

• Adrenocorticotropic hormone (ACTH)

• Follicle stimulating hormone (FSH)

• Luteinizing hormone (LH)

• Prolactin (PL)

• Posterior Pituitary: 2 hormones• Vasopressin (ADH)

• oxytocin

Terminology

• Tropic hormone:

• A hormone that controls the secretion of another hormone is known as a trophic (tropic) hormone.

Pituitary Gland

• Release of hormones from both anterior and posterior pituitary is controlled by hypothalamus

• Posterior pituitary– Along with hypothalamus forms

neuroendocrine system– Does not actually produce any hormones– Stores and releases two small peptide

hormones• Vasopressin (ADH)

– Conserves water during urine formation• Oxytocin

– Stimulates uterine contraction during childbirth and milk ejection during breast-feeding

Relationship of the Hypothalamus and Posterior Pituitary

1. The hormone, vasopressin and oxytocin are synthesized in supraoptic and paraventricular nuclei in the hypothalamus.

2. The hormone travels down the axon to be stored in the neuronal terminals within the posterior pituitary.

3. On excitation of the neuron, the stored hormone is released from these terminals into the systemic blood for distribution throughout the body.

Neurohormones: secreted into the Blood by Neurons

Figure 7-12: Synthesis, storage, and release of posterior pituitary hormones

Oxytocinvasopressin

ProlactinGrowth hormoneTSHACTHFSHLH

Anterior Pituitary Hormones

secretion of anterior pituitary hormones: a typical endocrine regulatory mechanism

important releasing/inhibiting hormones produced in hypothalamus:- Corticotropin-releasing hormone (CRH)

- Thyrotropin-releasing hormone (TRH)

- Gonadotropin-releasing hormone (GnRH)

- Growth-hormone-releasing hormone (GHRH)

- Prolactin-releasing hormone (PRH)

- Prolactin-inhibitory hormone (PIH)

Regulation of pituitary hormones secretion by the hypothalamus

Hypothalamic Releasing and Inhibiting Hormones

• Secretion of each anterior pituitary hormone is stimulated or inhibited by one or more hypothalamic hypophysiotropic hormones

Hypothalamic-hypophysealPortal system

• Hypothalamus

• Pituitary stimulation–from hypothalamic trophic hormones

• Endocrine gland stimulation–from pituitary trophic hormones

Endocrine Control: Three Levels of Integration

Endocrine Control: Three Levels of Integration

Figure 7-13: Hormones of the hypothalamic-anterior pituitary pathway

Negative Feedback Loops

• Regulate amount of hormones in blood

• Hormone levels stay within range needed for appropriate responses

• Eliminates waste of hormones

Negative Feedback Controls: Long & Short Loop Reflexes

Negative feedback loops in the hypothalamicanterior

pituitary pathway

Short long

Negative Feedback Controls: Long & Short Loop Reflexes

Control pathway for cortisol secretion

Vascular Link Between the Hypothalamus and Anterior Pituitary

• Antidiuretic hormone (ADH, vasppressin)

- chemistry: 9 amino acid peptide, produced primarily by supraoptic nucleus and small amount by paraventricular nucleus

- actions:

1) water retention by the kidney urine volume and ECF

2) vasoconstriction (in large amounts)

- regulation/stimuli:

1) blood (or ECF) osmolality/osmoreceptors 2) blood volume

3) others: alcohol, nicotine, barbiturates, etc.

- abnormality: Diabetes Insipidus

Posterior pituitary hormones

Vasopressin Secretion

Diabetes Insipidus

• Diabetes insipidus is a disorder of the posterior lobe of the pituitary gland.

• ADH is deficient. • Causes could be head trauma, brain tumor, or

irradiation of the pituitary gland, infections of the CNS (meningitis, encephalitis, tuberculosis) or tumors (eg, metastatic disease, lymphoma of the breast or lung). Another cause is failure of the renal tubules to respond to ADH; this may be related to hypokalemia,hypercalcemia, and a variety of medications (eg, lithium,Declomycin).

Clinical Manifestations

• Enormous daily output of very dilute, water-like urine.

• Intense thirst: The patient drinks 2 to 20 liters of fluid daily.

• Fluid intake cannot be limited; otherwise the patient will experience crave for fluid and would develop hypernatremia and severe dehydration.

Assessment and Diagnostic Findings

• Fluid deprivation test for 8 to 12 hours. • The patient is weighed frequently during the test. • Perform plasma and urine osmolality studies before

and after the test. • Low specific gravity and weight loss, rising serum

osmolality, and elevated serum sodium levels. • Terminate the test if tachycardia, excessive weight

loss, or hypotension develops.• Plasma levels of ADH (vasopressin) and plasma and

urine osmolality.

Aims to:

• Replace ADH

• Ensure adequate fluid replacement

• Identify and correct the underlying cause

Medical Management

Oxytocin

- chemistry: 9 amino acid peptide, produced primarily by paraventricular nucleus and small amount by supraoptic nucleus

- actions: 1) regulating breast milk release

2) contraction of pregnant uterus

- regulation/stimuli:

suckling by a nursing infant

crying sounds from a baby

fear and stress inhibit release

References

Human physiology, Lauralee Sherwood, seventh edition.

Text book physiology by Guyton &Hall,11th edition.

Physiology by Berne and Levy, sixth edition.

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