Page 254 - Biology_F5

Coordination and Irritability

regulated by a feedback mechanism. Hormone (TRH) by the hypothalamus and
Feedback mechanisms are self regulatory Thyroid Stimulating Hormone (TSH) by
mechanisms in which when there is a anterior pituitary gland. Therefore, in this

disturbance or deviation in a system, case, the product of a series of reactions
series of events occur to either remove the controls its own production by turning
disturbance (negative feedback) or make off the pathway, when it reaches a certain
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the system to deviate further (positive level.
feedback). Usually, it is a negative Negative feedback is also observed in
feedback mechanism, which regulates production and regulation of insulin in the
the release of hormones. In rare cases body. A rise of sugar level in the blood is
positive feedback mechanisms may occur. detected by β cells of islets of Langerhans
An example of the negative feedback of the pancreas, which release the insulin
mechanism is the release of thyroxine hormone. The hormone will then convert
hormone (Figure 4.36). glucose into glycogen, which is stored

Small effect in the liver and muscles. This causes the
Hypothalamus level of sugar in the blood to decrease.
The lower level of glucose then causes
TRH
Thyroxine the b cells to reduce production of insulin.
Main effect
Anterior pituitary Inturn, this triggers the release of glucagon
from α-cells of islets of Langerhans of
TSH the pancreas. This hormone converts
glycogen into glucose.
Thyroid Thyroxine

Thyroxine The interaction between hormonal and
nervous systems
Blood/tissue
Coordination process is achieved when
Increased BMR nervous and endocrine systems act
Key: together. Although the nervous system
Inhibition and the endocrine system are two different
Stimulation systems, both release chemical substances
Figure 4.36: Factors regulating thyroxine as a means of communication between
secretion and homeostatic control of the cells. The principal role of both systems
metabolic rate
is to coordinate and control various
Thyroxine helps in regulating the physiological activities in organisms. The
metabolic rate, growth and development major centres for linking the two systems
of an organism. The control of the release are the pituitary gland (control centre for
of thyroxine is determined by the level endocrine glands) and the hypothalamus
of thyroxine with four iodine atoms (the control centre for the nervous system).
(T4) circulating in the blood. If there is The hypothalamus collects information
high concentration of T4 in the blood, it from the brain and blood vessels passing
inhibits further production by inhibiting through it to the pituitary gland. The
the production of Thyrotropin Releasing pituitary gland directly or indirectly

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