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Autonomic Nervous System
The nervous system comprises the brain and various types of nerves, including afferent nerves (from the Latin, ad = towards; ferro = I carry), which carry sensory impulses from all parts of the body to the brain and efferent nerves (ex = from; ferro = I carry) through which "messages" are conducted from the brain to the muscles and all of the organs of the body. The somatic part of the nervous system has sensory components which convey sensations from the eyes, the nose and other sensory organs to the brain (mainly the cerebral cortex) where most of the impulses reach our awareness, and motor components transmitting impulses to the skeletal muscles in the limbs and trunk permitting voluntary control of movements. The autonomic nervous system conveys sensory impulses from the blood vessels, the heart and all of the organs in the chest, abdomen and pelvis through nerves to other parts of the brain (mainly the brain stem which consists of the medulla, pons and hypothalamus). These impulses often do not reach our consciousness, but elicit largely automatic or reflex responses through the efferent autonomic nerves, thereby eliciting appropriate reactions of the heart, the vascular system, and all the organs of the body to variations in environmental temperature, posture, food intake, stressful experiences and other changes to which all individuals are exposed.
There are two major components of the autonomic nervous system, the sympathetic and the parasympathetic systems. The afferent nerves sub serving both systems convey impulses from sensory organs, muscles, the circulatory system and all the organs of the body to the controlling centers in the medulla, pons and hypothalamus. From these centers efferent impulses are conveyed to all parts of the body by the parasympathetic and sympathetic nerves. The impulses of the parasympathetic system reach the organs of the body through the cranial nerves # 3, 7, 9, & 10, and some sacral nerves to the eyes, the gastrointestinal system, and other organs. The sympathetic nerves reach their end-organs through more devious pathways down the spinal cord to clusters of sympathetic nerve bodies (ganglia) alongside the spine where the messages are relayed to other nerve bodies (or neurons) that travel to a large extent with the blood vessels to all parts of the body. Through these nervous pathways, the autonomic nerves convey stimuli resulting in largely unconscious, reflex, bodily adjustments such as in the size of the pupil, the digestive functions of the stomach and intestines, the rate and depth of respiration and dilatation or constriction of the blood vessels.
The Parasympathetic Nervous System
When
a stimulus arises in an organ, such as a bright light shone into the eyes, the
message is conducted through sensory fibers to the midbrain to give rise to an
appropriate stimulus that travels through the parasympathetic fibers of the
oculomotor (third cranial) nerves to the pupils, resulting in automatic
contraction of the pupillary muscles to constrict the aperture and so reduce the
amount of light reaching the sensory cells in the retina of the eyes. Similarly,
the stimuli associated with the entry of food into the stomach are conveyed by
afferent fibers of the vagus nerve to the command station or nucleus of the
vagus in the brain whence messages are automatically conveyed through efferent
fibers of the vagus back to the stomach. These stimulate the secretion of
gastric juices and peristaltic contractions of the stomach to mix the food with
the secreted digestive juices and gradually to convey the gastric contents into
the intestines where a similar process is initiated through essentially the same
parasympathetic nerve pathways. Fortunately, emptying of the rectum and of the
urinary bladder is not entirely automatic but is subject to parasympathetic
impulses that are voluntarily controlled. Thus, filling of the urinary bladder
with urine stimulates stretch-sensitive receptors in the wall of the bladder
whence the message is conveyed to the midbrain where the stimulus to bladder
contraction and opening of the sphincters is voluntarily initiated to allow the
discharge of the contained urine.
Similarly the very complex requirements of giving birth to a
baby are initiated by stimuli to dilatation of the cervix, and involuntary
contractions of the uterine musculature with delivery of the fetus assisted by
voluntary contraction of the abdominal muscles.
The Sympathetic Nervous System
The
sympathetic nervous system is even more automatic and only exceptionally
susceptible to any voluntary control. When the environmental temperature is
raised on a hot summers day, the increased temperature initiates several
automatic responses. Thermal receptors convey stimuli to sympathetic control
centers of the brain from which inhibitory messages travel along the sympathetic
nerves to the blood vessels of the skin resulting in dilatation of the cutaneous
blood vessels, thereby greatly increasing the flow of blood to the surface of
the body from where heat is lost by radiation from the surface of the body.
Dilatation of the blood vessels in this way tends to lower the blood pressure
and to promote oozing or transudation of the fluid from the capillaries which
may result in swelling of the dependent limbs. Thus, fine adjustments in
sympathetic control of vascular contraction and "tone" are required to prevent
excessive vascular dilatation and undue reduction in blood pressure. Otherwise,
this might result in severe gravitational pooling of blood in the lower limbs
thereby reducing blood flow to the brain and causing fainting spells, to which
individuals with impaired sympathetic nervous functions are very susceptible.
The sympathetic nervous system responds to environmental heat in another
important way. The rise in body temperature is sensed by the hypothalamic center
from which stimuli emanate via sympathetic nerves to the sweat glands, resulting
in appropriate sweating. This serves to cool the body by the loss of heat
resulting from evaporation of the sweat, aided by a cool breeze. The only really
voluntary input that we have to facilitate cooling in a warm environment is to
get into a pool, a cold shower, or an air-conditioned room! We cannot
voluntarily influence the dilatation of our blood vessels or the adequacy of our
sweating in response to heat in other ways.
Control of the rate and strength of cardiac contractions is also under the
predominant control of the sympathetic nervous system. Thus, a fall in blood
pressure resulting from traumatic injury causing blood loss is sensed by
pressure-sensitive parts of the arteries called baroreceptors. Evidence of
reduced arterial distension is sensed by these baroreceptors and conveyed by the
parasympathetic (mainly the glossopharyngeal) nerves to the cardiovascular
control center in the medulla, called the nucleus tractus solitarii. From these
nuclei sympathetic stimuli conveyed by the cardiac nerves cause acceleration of
the heart rate, probably complemented by simultaneous reduction in the
parasympathetic stimuli via the vagus nerves which slow the heart rate. Although
pain, anxiety, fear and injuries or blood loss would involuntarily increase the
sympathetic stimulation to cardiac acceleration, most of us are unable to
influence either this effect or the consequences of blood loss per se on cardiac
acceleration.
Transmission of Autonomic Stimuli
Like other nerves, those of the autonomic nervous system
convey their messages to the appropriate end organs (blood vessels, viscera,
etc.) by releasing transmitter substances to which the receptors of the target
cells are responsive. The most important of these transmitters in the autonomic
nervous system are acetylcholine and norepinephrine. In the parasympathetic
system, acetylcholine is responsible for most of these transmissions between
the afferent and efferent nerves of the system and between the efferent nerve
endings and the cells or organs that they subserve. Acetylcholine also serves to
transmit nerve-to-nerve messages in the afferent nerves and the brain centers of
the sympathetic nervous system. However, the final transmission of messages from
the sympathetic nerves to the end-organs or cells that they innervate is
conveyed by the release of norepinephrine (noradrenalin) with at least one
important exception, namely the sympathetically conveyed stimulus to the sweat
glands which is transmitted by acetylcholine. A stimulus to contraction of the
blood vessels is required in order to maintain the blood pressure when we arise
from bed in the morning, so as to prevent fainting from excessive pooling of
blood in the lower body. This stimulus is conveyed by norepinephrine release
within the walls of the blood vessels from the nerve endings of the sympathetic
nerves that innervate each blood vessel.
The Autonomic Nervous System
Structure Sympathetic Stimulation Parasympathetic Stimulation Iris (eye muscle) Pupil Dilation Pupil Constriction Salivary Glands Saliva production reduced Saliva production increased Oral/Nasal Mucosa Mucus production reduced Mucus production increased Heart Heart rate and force increased Heart rate and force decreased Lung Bronchial muscle relaxed Bronchial muscle contracted Stomach Peristalsis reduced Gastric juice secreted; motility increased Small Intestine Motility reduced Digestion increased Large Intestine Motility reduced Secretions and motility increased Liver Increased conversion of
glycogen to glucoseKidney Decreased urine secretion Increased urine secretion Adrenal medulla Norepinephrine and
epinephrine secretedBladder Wall relaxed
Sphincter closedWall contracted
Sphincter relaxed
*It should be noted that the autonomic nervous system is
always working. It is NOT only active during "fight or flight" or
"rest and digest" situations. Rather, the autonomic nervous system acts to
maintain normal internal functions and works with the somatic nervous system.
