II.1.1. The sympathetic nervous system
The sympathetic nervous system or orthosympathetic nervous
system is one of the three parts of the efferent autonomic nervous system. The
other two parts are the enteric nervous system and the parasympathetic nervous
system. The sympathetic nervous system is our system of action and struggle. It
prepares the body for stress situations. It is responsible for controlling a
large number of unconscious activities of the body, such as heart rate or
contraction of smooth muscles. It exerts its effects on target cells and organs
mainly via neurotransmitters called catecholamines (noradrenaline and, to a
lesser extent, adrenaline). Yet the sympathetic nervous system is not quite
superimposed on the adrenergic nervous system, its action sometimes passing
(some vessels, sweat glands) by a secretion of acetylcholine. For this, it
produces a massive discharge throughout the body and prepares it for action. A
violent and unexpected noise, a situation of fear or the last few seconds
before the start of a sports competition are all examples of the moment when
this massive discharge takes place. The effects of sympathetic stimulation are
important for the athlete.
· Increased heart rate and contraction force of the
heart,
· Dilatation of the coronary vessels and therefore
increased cardiac output,
· Muscle vasodilatation to bring more blood to the active
muscles,
· Vasoconstriction in other areas, diverting the blood mass
to the active muscles,
· Increased blood pressure, which improves muscle perfusion
and venous return,
· Increased metabolic level in response to increased
needs,
· Stimulation of mental activity that improves perception
and concentration,
· Liver release of glucose into the blood,
· Finally, functions that are not directly involved in
exercise function at a slower rate (renal function, digestion), which saves the
energy needed for movement.
These changes in the basal body function facilitate the motor
response. This highlights the importance of the autonomic nervous system to
acute stress or physical exercise (Wilmore & Costill,
1998).
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II.1.2. The parasympathetic nervous system
The parasympathetic nervous system or vagal system is our
defense system. It is one of three divisions of the autonomic or visceral
nervous system, with the orthosympathetic nervous system and the enteric
nervous system. The nerve fibers of the parasympathetic system originate in the
cranial (nerve III, VII, IX, and X) and sacral parts of the spinal cord. It
controls the involuntary activities of the organs, glands, and blood vessels
together with one of the other parts of the autonomic nervous system: the
sympathetic nervous system (orthosympathetic).
The parasympathetic influence is modulated by the release of
acetylcholine, the latter is responsible for the slowing of the heart rate
(cardio-moderator). This acetylcholine plays a major role in the digestive and
urinary functions; this secretion is more active when one is calm or at rest.
Its effects are generally opposed to those of the sympathetic system and leads
to:
· a drop in the heart rate,
· an increase in gastric, salivary and intestinal
secretions.
· a loosening of most sphincters of the gastrointestinal
tract.
II.2. Influence of autonomic nervous system on heart
rate
Although the heart has a specific functional autonomy, the
autonomic nervous system constantly adapts its frequency and contraction force
to different environmental conditions and influences. The parasympathetic
nervous system (via the vagus nerve or X) has a general effect on the heart
rate. The sympathetic nervous system generally increases cardiac activity.
Although these two systems interact continuously, the permanent parasympathetic
influence (vagal tone) is often the most intense, making the heart rate largely
dependent on vagal stimulation / inhibition.
II.3. Cardiac variability study
Blood pressure and heart rate fluctuate continuously and are
under the control of several regulatory systems: short-term regulation
represented by the central nervous system, baroreflex and choreflex systems;
medium-term regulation thanks to the hormonal systems (renin-angiotensin
system, vasopressin, natriuretic atrial factor ...), the tension-relaxation
phenomenon and the transfer of interstitial fluid to the plasma sector and vice
versa; and finally, a long-term regulation supported especially by the kidneys.
Blood pressure and heart
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rate are therefore not constant phenomena: they vary
constantly. This variability can be defined as the set of variations of these
parameters around an average reference value and can be broken down into two
time scales:
· Variability over a 24-hour period, still termed circadian
or long-term.
· Variability over a period of a few minutes (usually 5
minutes), termed short-term variability, including spontaneous and unannounced
variations (effort, emotion, positional change ...).
Because of its ability to rapidly modulate blood pressure and
heart rate levels through the baroreflex system primarily (short-term
regulation), the activity of the autonomic nervous system can be studied by
measuring the variability of these two parameters. Over the last twenty years,
heart rate variability has become a non-invasive marker of autonomic nervous
system activity (Jourdan, 2008). The study of cardiac
variability is done on two different temporal and frequency planes
(Neto et al, 2005).
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