Hormones in Animals (2)

EndocrinologyD R Davies

School of Biological Sciences

Purves Life: the Science of Biology Chapters 41 (Animal Hormones) and 15 (Cell Signalling)

Lecture 14 Learning Outcomes

At the end of this lecture you should be able to:• Describe in general terms how endocrine control

mechanisms regulate homeostasis• Explain the endocrine functions of the hypothalamus

and pituitary gland• Discuss at least one complex endocrine control

mechanism involving the interplay of several different hormones

Feedback Control Systems • Thus a feedback control system will consist of an

effector (gland) which produces and maintains a variable signal (hormone concentration).

• This is measured by a sensor (receptor) which monitors the changes in the signal and sends a further signal to the pituitary gland (comparator) which compares the feedback signal to a reference (called the set point)

• produces an error signal in the form of a trophic hormone which regulates the effector and restores the variable to its proper value,

Control Systems

• Feedback loop• Effector (gland)• Variable Hormone

concn

• Sensor (hormone receptor)

• Comparator (pituitary)

• Error signal

The hypothalamus and pituitary

• The hypothalamus is a region of the brain controlling many aspects of physiological activities such as hunger, thirst, sleep and body temperature and consisting of a distinctive cells which produce hormones regulating these activities.

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Pituitary Gland

The hypothalamus can regulate physiological activity via three routes:

•Via the autonomic nervous system e.g. via the release of adrenaline by the adrenal medulla

•Neurones in the hypothalamus produce releasing hormones into the blood portal system causing the release of hormones by specific cells in the anterior pituitary gland (adenohypophysis) . These tropic hormones are then released into the general circulation where they affect a whole range of physiological activities in various target cells

•Via the posterior pituitary gland (neurohypohysis) where the release of two closely related peptide hormones (oxytocin and vasopressin) at nerve endings. These peptides have totally different functions

Hormonal Regulatory Mechanisms

• Positive Feedback e.g. increase in oestradiol triggers ovulation

• Negative Feedback e.g circulating thyroid hormone inhibits release of TRH and TSH

Negative Feedback Loops

Much of endocrine physiology is concerned with the maintenance of homeostasis. Control systems are of different degrees of complexity but usually consist of a feedback loop with a signal from the output used to regulate the input. Here the feedback signal produces an opposite response to a change in the variable.

Hormonal Regulatory Mechanisms

• Pulsatile Secretion Hypothalamic hormones are released in bursts typically 60 - 180 min apart

• Permissive Effects e.g hormones may regulate the action of other hormones, oestrogens increase the number of progesterone receptors

• Synergistic Effects

Hormonal Regulatory Mechanisms

• Up-Regulation- hormones increase the number of their own receptors thus increasing the sensitivity of the target cell to the hormone

• Down-Regulation- hormones decrease the number of their own receptors thus reducing the sensitivity of the target cell to the hormone

• The components of the signal transduction mechanism may also become sensitized or desensitized to the signal

Regulation of reproductive function

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

• Androgens, such as testosterone, trigger thedevelopment of male characteristics

– In male elephant seals, androgens account forbodies weighing 2 tons or more, a thick hide,and aggressive behavior

Figure 26.12

Hormonal Regulation

of Ovulation

Hormonal Changes in Pregnancy

Hormonal regulation of cyclic ovulation• Pregnancy is triggered by an increase in

oestradiol (oestrogen) arising from the developing ovarian follicle

• This is monitored by the hypothalamus and pituitary and results in an increased frequency of GnRH secretion followed by a surge in LH and FSH

• LH surge causes the rupture of the ovarian follicle and the release of the oocyte

• Oestradiol causes development of the endometrium lining the uterus and oestrogen and progesterone together maintain the uterus as a preparation for pregnancy

Hormonal Regulation of Pregnancy

• If pregnancy is maintained the coprpus luteum is maintained by the production of chorionic gonadotrophin (hCG)

• the levels of oestradiol and progesterone increase

• These steroid hormones maintain the development of the endometrium in a suitable state for implantation.

• Eventually the placenta takes over the production of the two steroids

• The levels of oestrogens and progesterone continue to increase through pregnancy until just before full term

Role of the adrenal gland in the response to stress

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

• How the adrenal glands control our responsesto stress

Figure 26.10

Adrenalgland

Mineralocorticoids

Adrenalmedulla

SHORT-TERM STRESS RESPONSE

1. Glycogen broken down to glucose;increased blood glucose

2. Increased blood pressure

3. Increased breathing rate4. Increased metabolic rate

5. Change in blood-flow patterns, leadingto increased alertness and decreaseddigestive and kidney activity

STRESS

Glucocorticoids

ACTH

ACTH

Adrenalmedulla

Adrenalcortex

Kidney

Nervesignals

Spinal cord(crosssection)

Nervecell

Nervecell

Epinephrine andnorepinephrine

LONG-TERM STRESS RESPONSE

1. Retention ofsodium ionsand water bykidneys

2. Increasedblood volumeand bloodpressure

1. Proteins and fatsbroken down andconverted toglucose, leading toincreased bloodglucose

2. Immune systemmay besuppressed

Hypothalamus

Releasing hormone

Anterior pituitary

Blood vessel

Adrenal cortex