Oestrous cycle and +/- feedback

Question 1

What is negative feedback?

Answer 1

A stimulus triggers a response that opposes the stimulus and brings the measure back to its normal value.
These mechanisms are homeostatic and keep an internal environment constant.

Question 2

What is positive feedback?

Answer 2

A stimulus leads to a response which often increases the stimulus and doesn’t have a fixed normal value
This ISNT homeostatic

Question 3

What is the oestrous cycle?

Answer 3

Cycle in female mammals which describes when they are most sexually fertile and menstruate if not pregnant

Question 4

Which are the two hormones that are produced by the pituitary gland (brain)?

Answer 4

FSH and LH

Question 5

What is the role of FSH (follicle stimulating hormone)?

Answer 5

Stimulates the development of follicles in the ovary which contain eggs and stimulates the follicles in the ovaries to produce oestrogen.

Question 6

What is the role of LH (Luteinising hormone)?

Answer 6

Causes ovulation to occur and stimulates the ovary to produce progesterone from the corpus luteum.

Question 7

What are the two hormones that are produced by the ovaries?

Answer 7

Oestrogen and Progesterone

Question 8

What is the role of oestrogen?

Answer 8

Causes the rebuilding of the uterus lining after menstruation and stimulates the pituitary gland to produce LH.

Question 9

What is the role of progesterone?

Answer 9

Maintains the lining of the uterus in readiness to receive the fertilised egg and inhibits the production of FSH from the pituitary gland.

Question 10

What happens between days 1-5?

Answer 10

• The menstrual cycle begins when the uterus lining begins to shed along with some blood.
• From day 1 the pituitary gland releases FSH into the blood which stimulates follicles in the ovary to grow and mature, each follicle contains an egg.

Question 11

What happens around days 6-10?

Answer 11

• The growing follicles secrete small amounts of oestrogen into the blood and this low level of oestrogen causes the uterus lining to build up and inhibits the release of FSH and LH from the pituitary gland. (negative feedback)
• As the follicles grow more oestrogen is produced and the level increases until around day 10. This then stimulates more FSH and LH to be produced (positive feedback)

Question 12

What happens around days 11-14?

Answer 12

There is a surge in FSH and LH and the surge in LH causes a follicle in the ovary to release an egg. This is ovulation and happens on day 14.

Question 13

What happens after ovulation?

Answer 13

• LH stimulates the empty follicle to develop into a corpus luteum which secretes progesterone
• Progesterone then maintains a thick lining of the uterus and inhibits the release of FSH and LH (negative feedback)

Question 14

What happens if the egg is not fertilised?

Answer 14

The corpus luteum degenerates and no longer produces progesterone, this means the lining of the uterus is no longer maintained and breaks down (menstruation)
Less progesterone means that FSH release is no longer inhibited. This means FSH release resumes and the cycle repeats itself.