Complex Multilevel Reflex Pathways

What is involved in the multilevel reflex pathway?

• Hypothalamus-Pituitary Axis

• Hypothalamus + Pituitary Glands

What is the posterior pituitary gland made of?

Neural / nerve tissue

Does the posterior pituitary create hormones?

No it is simply a storage and release site for the hypothalamus

What hormones are created by the hypothalamus and released by the posterior pituitary?

• Oxytocin

• Vasopressin

Posterior Pituitary Characteristics

• Made of neural tissue

• Cell bodies of neurons start in the hypothalamus and synthesize hormones

• Hormones travel down the axon to the posterior pituitary and leave to enter the blood

• Posterior pituitary produces two neurohormones

What is a real-life example of oxytocin in use?

• During labor, oxytocin is released to cause uterine contractions

• Also released during nursing, mating, or social bonding

• Sometimes called “love hormone” it promotes the tendency towards bonding

What is a real-life example of vasopressin (ADH - Antidiuretic Hormone) in us?

• Released when blood is too concentrated (aka not enough water)

• Acts on the kidneys to reabsorb water and concentrate urine

• Released during sleep so as not to wake up to urinate

• Extremely short half-life, about 45 seconds in the blood before being broken down

Anterior Pituitary Characteristics

• Made of real glandular tissue

• Controlled partially by hormones secreted by the hypothalamus

• The hypothalamic portal system carries hormones from the hypothalamus to the anterior pituitary

• Produces its own hormones that are released into the blood (trophic hormones)

Trophic Hormones

Hormones that will influence other endocrine tissues/glands

What are the six anterior pituitary glands?

• Prolactin

• Thyroid Stimulating Hormones (TSH)

• Adrenocorticotropic Hormone (ACTH)

• Growth Hormone (GH)

• Follicle Stimulating Hormone (FSH)

• Luteinizing Hormone (LH)

Prolactin

• Lactation and Reward

• Prolactin controls milk production

• Peptide Hormone

• Released in response to eating, mating, estrogen, ovulation, or nursing

What inhibits prolactin?

Instead of being stimulated by the hypothalamus, it is inhibited by dopamine

Thyroid Stimulating Hormone (TSH)

• Thyroid-stimulating hormone (TSH) travels to the thyroid gland to tell it to produce T3 and T4

• TSH is controlled by thyrotropin-releasing hormone (TRH) from the hypothalamus

What is the TSH pathway?

• Hypothalamus releases TRH

• The anterior pituitary releases TSH

• The thyroid releases T3 and T4

• This is called the HPT axis (Hypothalamic-Pituitary-Thyroid Gland)

Adrenocorticotropic Hormone (ACTH)

• A peptide hormone that increases cortisol production from the adrenal cortex (Outer layer of the adrenal gland)

• ACTH is controlled by Corticotropin-releasing hormone (CRH) from the hypothalamus

What is the pathway of the Adrenocorticotropic Hormone (ACTH)?

• Hypothalamus releases CRH

• The anterior pituitary releases ACTH

• The adrenal cortex releases cortisol

Growth Hormone (GH)

• A peptide hormone that stimulates bone/tissue growth, metabolic effects and many other effects

• Controlled by growth hormone-releasing hormone (GHRH) from the hypothalamus

• Doesn’t have one target; Has many effects throughout the body

• Excess GH causes acromegaly, aka enlargement; Too little causes growth failure

Follicle Stimulating Hormone (FSH)

• FSH travels to the gonads and stimulates gamete production

• Sperm in males, egg maturation in females

• Both FSH and LH are gonadotropins

• Both are controlled by Gonadotropin-releasing hormone (GnRH) from the hypothalamus

Gonadotropins

Hormones that act on gonads

What are examples of Gonadotropins?

• Follicle Stimulating Hormone (FSH)

• Luteinizing Hormone (LH)

Luteinizing Hormone (LH)

• LH targets the gonads and adrenal glands, triggers ovulation, and stimulates estrogen and progesterone production from the ovaries

• LH in males tells the testes to produce testosterone

How does FSH and LH work together?

• FSH matures the follicle (The eggs)

• LH triggers its release and turns the remaining follicle structure into a progesterone-producing body

• Together, they run the entire reproductive cycle

Dopamine (Prolactin Inhibitory hormone)

Inhibit prolactin production

Thyrotropin Releasing Hormone (TRH)

Stimulates TSH

Corticotrophin Releasing Hormone

Stimulates ACTH

Growth Hormone Releasing Hormone

Stimulates GH

Gonadotrophin Releasing Hormone

Stimulates FSH & LH

What is the Hypothalamic Pituitary Adrenal Axis negative feedback loop?

• Step 1: Stress Detected: Hypothalamus detects stress signals (Nervous system, sensory input, or circulating stress signals in blood)

• Step 2: CRH released: Hypothalamus releases CRH into the hypophyseal portal blood (Private blood circuit connecting hypothalamus and anterior pituitary)

• Step 3: Anterior Pituitary responds: Anterior pituitary cells receive CRH, are stimulated, and release ACTH into the general bloodstream

• Step 4: ACTH travels to the adrenal gland: ACTH circulates through the body and reaches the adrenal cortex (aka outer layer of the adrenal gland sitting on top of each kidney)

• Step 5: Cortisol released: Adrenal cortex responds to ACTH by producing and releasing cortisol and other steroid hormones into the blood

• Step 6: Cortisol effects: Cortisol produces 7+ widespread effects

What are the effects or cortisol?

• Raise blood glucose

• Suppresses nonurgent functions (Digestion, inflammation, immune activity)

• Increase blood pressure

• Sharpens focus

• And more, all designed to help cope with the stressor

What is the long loop negative feedback?

• When cortisol suppresses the hypothalamus + anterior pituitary

• Cortisol to Hypothalamus: Cortisol suppresses CRH production. The hypothalamus gets message of enough cortisol, stop production

• Cortisol to Anterior Pituitary: Cortisol suppresses ACTH production. Pituitary gets the same message, stops releasing ACTH

• Long loop since the feedback signal travels the full distance from the adrenal gland all the way back to the brain

Short Loop Negative Feedback

• Short loop does not wait for cortisol

• ACTH feeds back to the hypothalamus and suppresses CRH production

• The pituitary sends the hypothalamus an early warning to ease off the signal