Medicinal and aromatic plants – industrial profiles

Stress: biochemical and physiological implications
Changes in the body which occur outside of the brain, as a result of stress, are not under 
conscious control but are mediated by the sympathetic branch of the autonomic nervous system.
The activity of most organs of the body is controlled by the autonomic nervous system and as 
a general rule the sympathetic system may be considered to be activated in times of flight or
fight which will include stress. Stress-related changes in the body will also be mediated by hor-
mones, such as those released from the adrenal gland. Stimulation of the sympathetic system,
and adrenaline released from the adrenal gland, will increase heart rate and stroke volume and by
dilating and contracting different blood vessels will cause blood to be distributed to those organs
such as skeletal muscle, heart and lungs which are involved in exercise.

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Maria Lis-Balchin and Stephen Hart
Smooth muscle will also be either contracted or relaxed such that the body is prepared for
exercise, thus bronchial muscle relaxes and sphincters of the gastro-intestinal system contract. If
one considers the fight response in animals, smooth muscle contracts to give dilated pupils and
make hair stand on end. In both man and other animals, stimulation of the sympathetic system
will cause metabolic changes which favour activity, such as an increase in blood glucose. The
nerves of the sympathetic system which innervate smooth, cardiac and vascular smooth muscle
all release noradrenaline as their neurotransmitter and the differential response, either contrac-
tion or relaxation, is brought about by the presence of different adrenoceptors on the innervated
tissue.
In general, 

-adrenoceptors mediate contraction and 

-adrenoceptors relaxation, but of
course there are exceptions to this rule. Further differentiation and control of the system is
obtained by the presence of sub-types of 

- and 

-adrenoceptors. Occupation of a receptor by an
appropriate agonist results in a change in cell activity (such as contraction or relaxation) which
is mediated via a secondary messenger within the cell. Alpha-2 adrenoceptors mediate their
actions via a fall in cyclic AMP (cAMP), while beta-adrenoceptor activation is associated with a
rise in cAMP. Alpha-adrenoceptors are linked to the phosphoinositide pathway. In general, con-
traction is associated with an increase in the concentration of calcium ions within the muscle
fibre while relaxation involves either a removal of calcium, the blocking of calcium channels or
the opening of potassium channels.
Many tissue of the body receive a dual innervation from the two branches of the autonomic
nervous system (sympathetic associated with activity and parasympathetic with feeding and the
restoration of energy). In the gastro-intestinal tract we have this dual innervation plus an addi-
tional plexus of nerves in the wall of the intestine, often called the enteric nervous system, which
involves several other neutotransmitters.
It is on account of this rich innervation of the intestine that we have studied the action of
essential oils on the smooth muscle of the guinea-pig ileum 
in vitro
.