2_Negative Endocrine Feedback Loops

Question 1

Describe the components and characteristics of a SIMPLE endocrine reflex.

Answer 1

A simple endocrine reflex

1. involves only one hormone (e.g. TSH production by thyroid gland; ANP production by the heart, insulin production by the pancreas);
2. controls hormone secretion by the heart, pancreas, parathyroid gland, and digestive tract.

In a simple endocrine reflex, a cell senses a change in the environment and then secretes a hormone in response. This simple pathway is like having a heater in your house that automatically turns on and off. When the temperature rises above a certain point, the space heater senses it and automatically shuts off.

Question 2

What body functions are controlled by thyroid hormone in mammals?

Answer 2

Thyroid hormone regulates:

• bioenergetics
• normal blood pressure
• heart rate
• muscle tone
• digestive functions
• reproductive functions

Question 3

Describe the components and characteristics of a COMPLEX endocrine reflex pathway.

Answer 3

COMPLEX endocrine feedback loop involves:

• one or more intermediary steps
• two or more hormones
• the hypothalamus (e.g. hypothalamic-pituitary axis or hormone production)

1. the physiological stimulus: variation in homeostasis (thyroid hormone level drops)
2. sensor: neorosensory cells in the hypothalamus
3. hormonal output:
4. effector: feeds back to influence magnitude of stimulus and returns variable to homeostasis
5. response:

Question 4

Which component of this COMPLEX endocrine pathway exerts negative feedback?

Is it the physiological response or circulating hormone that feeds back to shut down the pathway?

Question 5

In what way does exogenous (from a source outside of the organism) hormone normally affect hormone release by the HP axis?

Question 6

In the example of cortisol release from the adrenal gland (a peripheral organ):

What is the effect of cortisol on hormone release by the anterior pituitary gland?

Question 7

In the example of cortisol release from the adrenal gland (a peripheral organ):

What is the effect of cortisol on hormone release by the hypothalamus?

Question 8

Regulation of thyroid hormone secretion (a hormone cascade pathway)

1. _____________ levels drop below the normal range. Sensory neurones respond by sending nerve impulses to neurosecretory cells in the ______________.
2. Neurosecretory cells secrete thyrotropin releasing normone (TRH) into the blood which carries it to the ___________ pituitary.
3. TRH causes the ____________ pituitary to secrete _______________________ into the circulatory system. TSH circulates throughout the body via ___________.
4. TSH stimulates endocrine cells in the _______________ to secrete thyroid hormone (T3 and T4) into the circulatory system.
5. ____________ hormone levels increase in the blood and body tissues. Thyroid hormone acts on target cells throughout the body to control _________, maintain normal blood __________, _________ rate, and muscle __________, regulate ______ functions, and _______ functions.
6. As levels return to normal range, thyroid hormone blocks TRH release from the _______________ and TSH release from the _________ pituitary, forming a ___________-feedback loop that prevents overproduction of ___________.

Answer 8

1. Thyroid hormone levels drop below the normal range. Sensory neurones respond by sending nerve impulses to neurosecretory cells in the hypothalamus.
2. Neurosecretory cells secrete thyrotropin releasing normone (TRH) into the blood which carries it to the anterior pituitary.
3. TRH causes the anterior pituitary to secrete thyroid-stimulating hormone (TSH)(aka Thyrotropin) into the circulatory system. TSH circulates throughout the body via blood circulation.
4. TSH stimulates endocrine cells in the thyroid gland to secrete thyroid hormone (T3 and T4) into the circulatory system.
5. Thyroid hormone levels increase in the blood and body tissues. Thyroid hormone acts on target cells throughout the body to control bioenergetics, maintain normal blood pressure, heart rate, muscle tone, regulate digestive functions, and reproductive functions.
6. As levels return to normal range, thyroid hormone blocks TRH release from the hypothalamus and TSH release from the anterior pituitary, forming a negative-feedback loop that prevents overproduction of thyroid hormone.

Question 9

Identify the physiological stimulus, sensor, hormonal output, effector and response in an SIMPLE endocrine feedback loop.

Which of the components above exerts negative feedback (shuts down the reflex) in a simple feedback loop?

Answer 9

• physiological stimulus:
• sensor:
• hormonal output:
• effector:
• response:

_____________ exerts negative feedback (shuts down the reflex)