W6 - Hypothalamus, Pituitary and Thyroid Gland

Endocrine/ Neuroendocrine System

• function

• composition

• key players 

• regulation of growth, metabolism, and reproduction

• glands and secretory cells

• hormones/neurohormones

Neuroendocrine Stimulus in the CNS

neural control/aspect in the secretion of hormones which is the hypothalamus; neurohormone release through efferent neurons

Types of Neurohormones

• catecholamines = class of hormones and neurotransmitters that take part in stress response, produced by adrenal medulla, brain, nervous tissue

• hypothalamic neurohormones

  • stored and secreted by the post. pituitary

  • controls hormone release by the ant. pituitary

Releasing vs. Effector Hormone

• releasing - acts on endocrine cells to secrete hormones (eg. GHRH and GHIH produced by the hypothalamus)

• effector - acts on target producing a physiological response (eg. GH released by the ant.pituitary gland)

Hormonal Secretion Regulation

regulated through feedback

• negative = responds to maintain homeostasis

• positive (less common) = amplifies a stimulus  

What links the NS and the Endocrine System

the hypothalamus which is connected to the pituitary glands via the infundibulum 

Endocrine Functions of the Hypothalamus

1. stimulates the release of effector hormones from the ant. pituitary that control other endocrine organs

2. produces ADH and oxytocin which is stored and released in the post. pituitary 

3. contains sympathetic neurons that innervate the adrenal gland which secretes epinephrine and norepinephrine 

Anterior vs. Posterior Pituitary

• anterior/ adenohypophysis = true endocrine gland, produce and secretes hormones

• posterior/ neurohypophysis = extension of neural tissue and secretes neurohormones produced in the hypothalamus

Hypothalamic-hypophyseal Portal System

major interface between the brain and the endocrine system which is via a portal system

1. trophic neurohormones are released into the capillaries of the portal system

2. portal veins carry them directly to the ant.pituitary

3. endocrine cells in the ant.pituitary release their peptide hormones into the second set of capillaries for bodily distribution 

Blood System of the Posterior Pituitary

• arterial blood enter via the inferior hypophyseal artery → branch of the ICA

• venous/plexus drained via efferent hypophyseal veins → which is drained into the cavernous sinus 

Posterior Pituitary Gland stores and releases

oxytocin and vaspressin, important to note that the hypothalamus produces these hormones

Vasopressin

an anti-diuretic hormone (ADH) produced when the hypothalamic osmoreceptors detect changes in blood solute concentrations

ADH Effects

• increases water reabsorption by the kidneys (inhibits urine output)

• constriction of blood vessels (BP)

• decreases water loss by sweat glands

Oxytocin Functions 

• childbirth = levels rise in response to positive feedback, stimulated by the stretching of the cervix

• breast milk = stimulated  by breast feeding, sensory neurons innervating the nipples project info to hypothalamus 

Anterior Pituitary Hormones

• a major endocrine gland

• two sections

  • pars intermedia = MSH → skin

  • pars distalis = hormones to mammary glands, musculoskeletal system, thyroid gland, adrenal cortex, sex organs

Growth Hormones (GH)

• function

• regulation

• feedback control type

• functions in growth by releasing the insulin-like growth factor (IGF) in the liver

• GHRH and GHIH which is released by the hypothalamus 

• negative feedback

Adrenocorticotropic Hormone (ACTH)

• function

• regulation

• feedback control type

• stimulates synthesis and secretion of cortisol by the adrenal cortex

• release is stimulated by the corticotropin releasing hormone (CRH) from hypothalamus (no IH)

• controlled by negative feedback

Follicle Stimulating Hormone (FSH) and Luteinizing Hormone (LH)

• function

• regulation

• feedback control type

• development of gametes and sex hormones (acts on ovaries/testes)

• stimulated by gonadotropin releasing hormone (GnRH) from hypothalamus

• controlled by positive and negative feedbacks

Prolactin (PRL)

• function

• regulation

• initiates and maintains mammary gland development and milk production 

• regulated by PRH and PIH from the hypothalamus

Melanocyte Stimulating Hormone (MSH)

• function

• regulation

• exact role is unknown, but enhances skin pigmentation

• release is stimulated by CRH and MIF from the hypothalamus

Thyroid Stimulating Hormone (TSH)

• function

• regulation

• feedback control type

• TSH stimulates the synthesis and secretion of thyroid hormones from the thyroid gland

• stimulated by the TRH from hypothalamus

• negative feedback

Thyroid Follicle Contents

• follicular cells = synthesize thyroid hormones

• colloid = viscous fluid that stores inactive thyroid hormones

• parafollicular cells = lies between thyroid follicles, secrete calcitonin

Types of Thyroid Hormones

• thyroxine (T4) = 90% production, inactive hormone which gets converted in T3 in the circulation

• triiodothyronine (T3) = active form

Key Functions of Thyroid Hormone

• increases basal metabolic rate and body temp → increased production of ATP

• fetal development and growth in children (nervous and skeletal)

• increases HR and contractility

Transportation of Thyroid Hormones

T3 and T4 are lipid soluble and in blood they bound to thyroxine-binding globulin (TBG)

Effect of TSH on Follicular Cells

TSH binds to the receptors on follicular cells and

• increases activity of Na+/I- transporter

• increases thyroglobulin synthesis

• release of stored thyroid hormones

• increases number and size of follicular cells 

Calcium Balance Regulation

• calcitonin builds bone

• parathyroid resorbs bone