Feedback Mechanisms

Question 1

What is negative feedback?

Answer 1

This occurs when the feedback causes the corrective measures to be turned off. In doing so it returns the system to its normal level.

Question 2

What are the two types of feedback?

Answer 2

1) Positive feedback

2) Negative feedback

Question 3

Describe how negative feedback is involved in the control of blood temperature once it has risen.

Answer 3

1) Rise in blood temperature
2) Thermoreceptors in hypothalamus detect this
3) Impulses from thermoreceptors are sent to the heat loss centre in the hypothalamus
4) Impulses from the head detector are sent to the skin to cause vasodilation, sweating, lowering of hairs
5) This leads to a normal body temperature
6) The thermoreceptors in the hypothalamus detect this and cause corrective measures to be turned off.

Question 4

Describe how negative feedback is involved in the control of blood temperature once it has fallen.

Answer 4

1) Fall in blood temperature
2) Thermoreceptors in hypothalamus detect this
3) Impulses from thermoreceptors are sent to the heat gain centre in the hypothalamus
4) Impulses from the head detector are sent to the skin to cause vasoconstriction, reduce sweating, raising of hairs
5) This leads to a normal body temperature
6) The thermoreceptors in the hypothalamus detect this and cause corrective measures to be turned off.

Question 5

Describe how negative feedback is involved in the control of blood glucose concentration once it has fallen.

Answer 5

1) Fall in blood glucose concentration.
2) α-cells in the pancreas detect this and secrete glucagon
3) The liver synthesises glucose from glycogen and starts gluconeogenesis
4) This leads to a normal blood glucose concentration, which the α-cells detect
5) This causes the corrective measures to be turned off; glucagon is no longer produced.

Question 6

Describe how negative feedback is involved in the control of blood glucose concentration once it has risen.

Answer 6

1) Rise in blood glucose concentration
2) β-cells of the islets of Langerhans detect this and secrete insulin.
3) Glucose uptake by cells and its conversion from glycogen to fat occurs as a result of insulin secretion.
4) This leads to a normal blood glucose concentration, which β-cells detect.
5) This causes the corrective measure to be turned off; insulin is no longer produced.

Question 7

Why do we have separate negative feedback mechanisms that control departures from the norm in either direction?

Answer 7

To give a greater degree of homeostatic control.

Question 8

When does positive feedback occur?

Answer 8

Positive feedback occurs when the feedback causes the corrective measures to remain turned on, causing the system to deviate more from the original level.

Question 9

When does negative feedback occur?

Answer 9

When the feedback causes the corrective measures to be turned off, causing the system to remain at its normal level.

Question 10

Describe how positive feedback occurs in neurones.

Answer 10

1) Stimulus causes a small influx of sodium ions.
2) Influx increases permeability of neurone to sodium ions.
3) More ions enter, which increases permeability further.
4) This results in a very rapid build-up of action potential and response to the stimulus.

Question 11

What is the oestrous cycle?

Answer 11

The oestrous cycle is the regular pattern of changes that takes place in the reproductive system of female mammals.

Question 12

What is the menstrual cycle?

Answer 12

The menstrual cycle is where the lining of the uterus is shed along with blood between each cycle.

Question 13

What are the four main hormones that control the menstrual cycle?

Answer 13

1) Follicle-stimulating hormone
2) Luteinsing hormone
3) Oestrogen
4) Progesterone

Question 14

FSH…

• Site of secretion
• Target organ
• Function

Answer 14

Anterior lobe of pituitary gland
Ovary
Stimulates the growth and development of the follicle
Stimulates secretion of oestrogen
Enhances effect of LH in stimulating ovulation

Question 15

Oestrogen…

• Site of secretion
• Target organ
• Function

Answer 15

Follicles in the ovary
Endometrium in pituitary gland
Causes rebuilding of the uterus lining after menstruation.
Stimulates the pituitary gland to produce LH.
Inhibits the production of FSH .

Question 16

LH…

• Site of secretion
• Target organ
• Function

Answer 16

Pituitary gland
Ovary
Causes of ovulation to occur
Stimulates the ovary to produce progesterone from the corpus luteum.

Question 17

Progesterone

• Site of secretion
• Target organ
• Function

Answer 17

Corpus Luteum
Endometrium in pituitary gland
Maintains the lining of the uterus in readiness to receive the fertilised egg.
Inhibits the production of FSH from the pituitary gland.

Question 18

Describe the control of the menstrual cycle.

Answer 18

1) Shedding of the uterus lining.
2) Pituitary gland releases FSH into the blood. FSH stimulates follicles in the ovary to grow and mature. Each follicle contains an egg.
3) Growing follicles secrete small amount of oestrogen into the blood. This causes the uterus lining to build up again and inhibits FSH and LH (negative feedback).
4) As follicles grow, oestrogen production increases. When this reaches a critical point, the pituitary gland is stimulated to release more FSH and LH (positive feedback). There is a surge in FSH and LH production.
5) The surge in LH production causes ovulation (the release of an egg from an ovary).
6) After ovulation, LH stimulates the empty follicle to develop into the corpus luteum, which secretes progesterone.
7) Progesterone maintains the uterus lining and inhibits FSH and LH release (negative feedback).
8) If the egg is not fertilised, the corpus luteum degenerates - there is no more production of progesterone.
9) Due to the lower level of progesterone, the uterus lining breaks down (menstruation). FSH release is also no longer inhibited.