Page 296 - Biology_F5

Nutrition

Nervous and hormonal control of the saliva, production of digestive enzymes,
gastric juice secretion activation of digestion process and
increased peristalsis. The parasympathetic
Task 5.12 system dilates blood vessels to increase
Read from internet sources and biology blood flow to the gastrointestinal tract
books about nervous and hormonal after the consumption of food, due to
FOR ONLINE READING ONLY
control of gastric juice secretion, and great or high metabolic demand placed on
then make short notes. the body by the gut. The control of gastric
juice secretion occurs in three phases,
Digestion is vital for the functions of all namely nervous phase, gastric phase, and
systems of a human body. It is under the intestinal phase. The mechanisms of the
nervous and hormonal control systems. nervous and hormonal control of digestion
Until 19 th century, biologists regarded phasewise is as detailed here under.
the digestive system as being entirely
under nervous control. However, in The nervous or cephalic phase: This is
1902 a British physiologist Sir William the initial stage which is initiated by sight,
M. Bayliss discovered secretin hormone thought, taste, or smell of food, which
as vital in the control of gastrointestinal later triggers a reflex in which nerve
functions. Three types of hormones impulses relayed from the brain cause
involved in digestion are secretin, gastrin gastric glands to release their secretions.
and cholecystokinin. Secretion and Generally, the nervous signals that trigger
release of these hormones is stimulated by this phase emanate from cerebral cortex
presence of certain food molecules in the appetite centres and are transmitted to
stomach and small intestine. the stomach through the vagus nerve.
Digestion process is controlled by This in turn causes the secretions of
two types of nervous system namely; histamine and increases hydrochloric acid
the somatic nervous system and the and gastrin secretion in the stomach. In
autonomic nervous system. The somatic comparison to other phases, the cephalic
nervous system enables the body to adapt phase contributes about 20% of the gastric
to stimuli, such as touch and smell from secretions while eating.
the external environment and is the system
that controls the gastrointestinal tract. This Gastric phase: This is the second phase
somatic nervous system is further divided and it is so called because it triggers
into sympathetic and parasympathetic gastric activity. It occurs once the food is
chewed and has arrived in the stomach.
nervous systems. Stimulation of The partially digested food substances,
sympathetic system brings about shunting such as proteins in the stomach stimulate
of blood from gastrointestinal tract. This the endocrine cells in the stomach walls to
reduces digestive activity and causes dry secrete gastrin from gastric glands, which
mouth and cessation of salivation. increase the secretion of the gastric juice.

The parasympathetic nervous system, on Normally, the ingested food stimulates
the contrary, is responsible for secretion of gastric activity by stretching the stomach

Form Five Student’s Book
289