Feedback Mechanisms

Question 1

Stages of homeostatic control:

Answer 1

THE SET POINT- or desired level, at which system operates.
A RECEPTOR- which detects any deviation from the set point.
A CONTROLLER- which coordinates information from various sources.
AN EFFECTOR- which brings about the corrective measures needed to return the system to set point.
A FEEDBACK LOOP- informs the receptor of the changes to the system brought about by the effector.

Question 2

When does negative feedback occur?

Answer 2

When the feedback causes the corrective measures to be turned off. In doing so it returns the system to its original level.

Question 3

Describe how negative feedback affects temperature regulation.

Answer 3

If blood temp. increases, THEMRORECEPTORS in HYPOTHALAMUS send nerve impulses to heat loss centre (also in hypothalamus). This in turn sends impulses to the skin (effector organ).
VASOLDILATION, SWEATING and LOWERING BODY HAIRS all lead to reduction in blood temp.
If th efact that blood temp. has returned to normal is not fed back to the hypothalamus, it will continue to stimulate the skin to lose body heat.
Blood temperature will the fall below normal level and may continue causing hypothermia and death.
In practice:
Cooler blood returning to from skin passes through the hypothalamus. The thermoreceptors detect that blood temp. is at its normal set point and so they cease to send impulses to the heat loss centre. This in turn stops sending impulses to the skin so vasodilation, sweating, etc. cease and blood temp. remains at its normal level.

THE BLOOD HAVING BEEN COOLED TO NORMAL TEMP> HAS TURNED OFF EFFECTOR THAT WAS CORRECTING RISE> THEREFORE NEGATIVE FEEDBACK.

Question 4

What are there separate negative feedback mechanisms for?

Answer 4

To regulate departures from the norm in each direction.

Question 5

What happens if blood temperature falls?

Answer 5

Heat gain centre will cause vasoconstriction, raising of hair and reduced sweating. Negative feedback to turn them off.

Question 6

Describe negative feedback in control of blood glucose. (FALL)

Answer 6

If fall in conc. of glucose in the blood, the alpha cells in the islets of Langerhans in the pancreas produce the hormone glucagon. Glucagon causes the cnversion of glycogen to glucose and gluconeogenesis in the liver. As a result, the blood glucose level returns to normal. As blood circulates back to pancreas, the alpha cells detect the change and stop producing glucagon.

Question 7

Describe negative feedback in control of blood glucose. (RISE)

Answer 7

If rise in conc. of blood glucose, insulin will be produced from the beta cells in the pancreas. Insulin increases the uptake of glucose by cells and its conversion to glycogen and fat. The fall in blood glucose conc. that results will turn off insulin production once blood glucose levels return to normal .

Question 8

What does having separate negative feedback mechanisms that control departures from the norm in EITHER DIRECTION do>

Answer 8

Gives greater degree of homeostatic control.

Question 9

What is positive feedback?

Answer 9

Occurs when the feedback causes the corrective measures to remain turned on.

Question 10

What does positive feedback do?

Answer 10

Causes the system to deviate even more from the normal level.

Question 11

What is an example of positive feedback?

Answer 11

In neurones when a stimulus causes a small influx of sodium ions. This influx increases the permeability of the neurone to sodium ions so more ions enter, causing a further increase in permeability and even more rapid entry of ions. This results in a very rapid build up of an action potential that allows an equally rapid response to a stimulus.

Question 12

When does positive feedback occur more often?

Answer 12

When there is a breakdown of control systems. In certain diseases, such as typhoid fever, there is a breakdown of temperature regulation resulting in a rise in body temperature leading to hypothermia. In the same way, when the body gets too cold the temperature control system tends to break down, leading to positive feedback resulting in the body temperature dropping even lower.

Question 13

What is the oestrous cycle?

Answer 13

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

Question 14

When does the oestrous cycle begin in humans?

Answer 14

At puberty and continues until the menopause at 45-50 years old.

Question 15

Where do the hormones of the oestrous cycle circulate?

Answer 15

Blood plasma so reach all parts of body. However, only cells with appropriate receptors can respond to a particular hormone.

Question 16

What are the two hormones release from the pituitary gland (base of brain)?

Answer 16

• Follicle stimulating hormone (FSH): stimulates development of follicles in the ovary, which contain eggs, and stimulates the follicles in the ovaries to produce oestrogen.
• Luteinising hormone (LH): causes ovulation to occur, and stimulates the ovary to produce progesterone from CORPUS LUTEUM.

Question 17

What two hormones are produced by the ovaries?

Answer 17

OESTROGEN: causes the rebuilding of the uterus lining after menstruation and stimulates the pituitary gland to produce LH.

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

Question 18

Describe the process of the menstrual cycle.

Answer 18

• The menstrual cycle begins when the uterus lining is shed, along with some blood (days 1-5)
• From day 1, the pituitary gland releases FSH into the blood which stimulates follicles in the ovary to grow and mature.
• The growing follicles secrete small amounts of oestrogen into the blood. This low level of oestrogen causes the uterus lining to build up again and also inhibits the release of FSH and LH from the pituitary gland (negative feedback).
• As the follicles grow, more oestrogen is produced. The level of oestrogen increases until, at around day 10, it reaches a critical point where it stimulates the pituitary gland to release more FSH and LH (positive feedback).
• There is a surge in FSH and LH production. The surge in LH causes one of the follicles in the ovary to release its egg. This is called ovulation and occurs on day 14.
• After ovulation LH stimulates the empty follicle to develop into a structure called the corpus luteum, which secrete progesterone (and smaller amounts of oestrogen).
• Progesterone maintains the thick lining of the uterus and also inhibits release of LH and FSH by pituitary gland (negative feedback).
• If the egg is not fertilised, the corpus luteum degenerates and so no longer produces progesterone.
• With less progesterone, the lining of the uterus is no longer maintained and so breaks down (menstruation). less progesterone also means FSH release is no longer inhibited.
• FSH release therefore resumes and cycle repeats.

Question 19

How is positive feedback involved in hypothermia?

Answer 19

• Hypothermia is low body temperature (below 35C)
• It happens when heat’s lost from the body quicker than it can be produced.
• As body temperature falls the brain doesn’t work properly and shivering stops- this makes body temperature fall even more.
• Positive feedback takes body temperature further away from the normal level, and it continues to decrease unless action is taken.