Feedback Mechanisms

Negative feedback occurs when the feedback causes the corrective measures to be turned off so the system is returned to its origional level.

Temperature:

• the temperature of the blood increases, which is detected by thermoreceptors in the hypathalamus that send nerve impulses to the heat loss centre.
• the heat loss centre in the hypathalamus sends impulses to the skin, which causes vasodilation, sweating and lowering of body hairs to reduce blood temperature
• the cooled blood passes through the hypothalamus and thermoreceptors detect that it is back to normal. They stop sending impulses to the heat loss centre, which stops sending impulses to the skin, which causes vasodilation etc. to cease.

Blood Glucose:

• a cells detect a fall in blood glucose concentration in pancreas produce glucagon, which causes the conversion of glycogen to glucose and glucogeonesis in the liver
• as the blood glucose concentration rises to normal, the a cells detect this and stop producing glucagon

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

An example is 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 the 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.

Positive feedback occurs more often when there is a breakdown of control systems. In certain diseases, there is a breakdown of temperature regulation, resulting ina rise in body temperature, leading to hyperthermia. Similarly, in hypothermia, when the body gets too cold the temperature control systems break down, leading to positive feedback, resulting in the body temperature dropping lower.

The oestrous cycle is the regular pattern of changes that take place in the reproductive system of female mammals. It is controlled by a number of hormones that circulate in the blood plasma and so reach all parts of the body. However, only cells with the appropriate receptors can respond to a particular hormone.

Hormones:

• Follicle-Stimulating Hormone (FSH): stimulates the follicles to grow and mature and so start producing oestrogen
• Luteinising Hormone (LH): causes ovulation and stimulates the ovary to produce progesterone from the corpus luteum
• Oestrogen: produced from growing follicle and causes the rebuilding of the uterus lining. It also stimulates the production of LH
• Progesterone: maintains the lining of the uterus for recieving the egg and inhibits the production of FSH

• Day 1-5: uterus lining is shed
• Day 1: pituitary gland releases FSH into the blood which stimulates follicles in the ovary to grow and mature. Each follicle contains an egg
• The growing follicles secrete small amounts of oestrogen into the blood. This low level of oestrogen causes the uterus lining to build up and also inhibits the release of FSH and LH from the pituitary gland (negative feedback)
• As follicles grow, more oestrogen is produced. 
• Day 10: level of oestrogen stops increasing as it reaches a critical point, where is stimulates the pituitary gland to release more FSH and LH (positive feedback)
• Day 14: there is a surge in FSH and LH production. The surge in LH causes one of the follicles to release its egg. This is ovulation.
• After ovulation, LH stimulates the empty follicle to develop into the corpus luteum, which secretes progesterone and small amounts of oestrogen.
• Progesterone maintains the thick lining of the uterus and inhibits the release of FSH and LH (negative feedback).
• If the egg is not fertilised, the corpus luteum dies so no longer produces progesterone
• With less progesterone, the lining of the uterus is no longer maintained and so breaks down. Less progesterone means that FSH is no longer inhibited. 
• Day 28/1: FSH release resumes and the cycle repeats itself