The human nervous system consists of two distinct parts: the first being the
central nervous system (CNS) which refers to the brain and spinal cord together
and the second is the peripheral nervous system (PNS) which refers to the
cervical, thoracic, lumbar, and sacral nerve trunks leading away from the
spine to the limbs.|
Messages related to function (such as movement) or dysfunction (such as pain)
travel from the brain to the spinal cord and from there to other regions in
the body and back to the brain again. The autonomic nervous system controls
involuntary functions in the body, like perspiration, blood pressure, heart
rate, or heart beat. It's divided into the sympathetic and parasympathetic
nervous systems. The sympathetic and parasympathetic nervous systems have
links to important organs and systems in the body.
The PNS also includes 12 pairs of cranial nerves located on the underside of
the brain. Most relay messages of a sensory nature. They include the olfactory
(I), optic (II), oculomotor (III), trochlear (IV), trigeminal (V), abducens (VI),
facial (VII), vestibulocochlear (VIII), glossopharyngeal (IX), vagus (X),
accessory (XI), and hypoglossal (XII) nerves. The cranial nerves are considered
components of the PNS, although on a structural level the olfactory, optic and
terminal nerves are more accurately considered part of the CNS.
Neuralgia, as in trigeminal neuralgia, is a term that refers to pain that arises
from abnormal activity of a nerve trunk or its branches. The type and severity
of pain associated with neuralgia vary widely.
Just like with the electrical grid that powers your home, neighborhood, city and
so forth, if a bolt of lightning strikes a power line during a storm it my cause
your power to be interrupted, or your neighborhoods, or possibly the entire
city, it all depends on where it hits. A problem at the beginning of any line
will cause a ripple effect all the way down the line, the severity and exact
location will determine how bad.
In the diagram below, the CNS is shown in red while the PNS is shown in blue.
If there is any damage or a disruption in the CNS, it shows up all the way down
the line, whether it's your finger, leg, or bladder. In some areas, such as
vision and balance, the problems never enter the peripheral nerves.
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What is Neurology?
Neurology, in a very general description, is a clinical specialty devoted to the
diagnosis and management of diseases that affect the nervous system. The nervous
system controls all of our sensation and behavior, and therefore the kinds of
symptoms that may result from disorders of the nervous system are almost without
limit. Symptoms of neurological disease in general may include poor memory, poor
balance, headache, blackouts, seizures, speech disorders, trouble swallowing,
numbness or loss of sensation, spasticity, tremor, difficulty controlling
urination, and a host of other symptoms.
Neurologists generally work with the primary care physician to diagnose and
manage neurological symptoms. Neurological problems of infants and young
children are often different from those that affect adults. Pediatric
neurologists manage neurological problems in patients less than 18 years of age.
Those who have neurological conditions that require surgical treatment are
referred to neurosurgeons since neurologists don't perform surgery.
The Central Nervous System
In very basic terms, the CNS consists of the brain, optic nerve and spinal cord,
and is where information in integrated. This is also the location where multiple
sclerosis (MS) attacks. The CNS is responsible for receiving and interpreting
signals from the PNS and also sends out signals back to it, either consciously
or unconsciously. All together it only weighs just over 3 pounds and yet it's
essentially you and who you are. The CNS also is the most protected part of the
nervous system since it is located inside the bones of the scull and spine. This
is the place where information is stored, received, processed, and acted upon.|
Sensory information is brought into the CNS from all parts of the body and
depending upon the brains response, sensory output is then sent to the parts of
the body that needs to react. This may sound simple, but is in fact complex.
As an example, imagine there is something on your arm. A signal is sent up to
your CNS, into the part of your brain that first receives the information that
something is on your arm. Your brain figures out what might be on your arm and
at the same time needs addition information. It has you turn your head, listen
and look to see what it actually is. Your brain also gets all of the other parts
of your body into action if it's determined that it could be harmful and has you
pull away if needed. Once all the needed information is collected and it's
determined whether there is a potential for harm, your brain then informs the
rest of your body the exact action that's needed. All of these actions together
should happen in less than a second - to "jerk" away or not.
Now if you think about all that is involved just for one simple action and
reaction, then image what it takes to walk or talk. It can even be an involved
process when someone walks and chews gum at the same time - no pun intended.
The CNS controls everything and any damage to it can never be taken lightly.
Without the CNS, you have control of nothing, and with damage to it, you can
loose control to so much. The loss of any small part of the CNS will cause a
disruption all the way down the line into the PNS.
The myelin sheath of CNS neuronal axons is composed of complex proteins and
lipids, including myelin basic protein (MBP) and myelin-oligodendrocyte
glycoprotein (MOG). Although the MOG is only a minor component of CNS myelin,
it's felt that it may be the most immunogenic autoantigen by eliciting strong
T-cell and B-cell responses. This raises concerns since T-cell responses that
open the blood brain barrier (BBB) allow antibodies to magnify the
The Peripheral Nervous System
The PNS extends out from where the CNS ends so it can maintain the organs and
limbs of the body (the nerves that go to muscles and innervate the skin and
sensory organs). This system of nerves isn't protected by bones and is
susceptible to injuries more easily.|
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The PNS includes sensory receptors, sensory neurons, and motor neurons. Sensory
receptors are activated by a stimulus (change in the internal or external
environment). The stimulus is converted to an electronic signal and transmitted
to a sensory neuron. Sensory neurons connect sensory receptors to the CNS. The
CNS processes the signal, and transmits a message back to an effector organ (an
organ that responds to a nerve impulse from the CNS) through a motor neuron.
The spinal and cranial nerves branch out of the spinal cord along its entire
length. Any damage to a specific area prior to these nerves would cause a
disruption from that point outward.
The PNS is divided into two separate systems being the Autonomic Nervous System
(ANS) and the Somatic Nervous System (SNS).
The Autonomic Nervous System
The ANS is the part of the PNS that acts as a control system, maintaining
homeostasis in the body. These nerves are more involved in regulating vital
internal functions and these activities are generally performed without
conscious control. The ANS affects heart rate, digestion, respiration rate,
salivation, perspiration, diameter of the pupils, micturition (urination), and
sexual arousal. Whereas most of its actions are involuntary, some, such as
breathing, work in tandem with the conscious mind.
The ANS can be divided by subsystems into the Parasympathetic Nervous System
(PSNS), Sympathetic Nervous System (SNS), and Enteric Nervous System (ENS). It
can also be divided functionally, into its sensory and motor systems.
Parasympathetic Nervous System (PSNS)
|The PSNS is a branch of the ANS. It promotes a
"rest and digest" response, promotes calming of the
nerves return to regular function, and enhances digestion.
||The PSNS is a branch of the ANS. It promotes a "rest and
digest" response, promotes calming of the nerves return to
regular function, and enhances digestion.
||Dilates blood vessels leading to the gastro-intestinal (GI) tract,
increasing blood flow. This is important following the consumption of
food, due to the greater metabolic demands placed on the body by the gut.
||Can also constrict the bronchiolar diameter when the need
for oxygen has diminished.
||Dedicated cardiac branches of the Vagus and thoracic spinal
accessory nerves impart parasympathetic control of the Heart or Myocardium.
||During accommodation, it causes constriction of the pupil and lens.
||Stimulates salivary gland secretion, and accelerates peristalsis, so,
in keeping with the rest and digest functions, appropriate PNS activity
mediates digestion of food and indirectly, the absorption of nutrients.
||It's also involved in erection of genitals, via the pelvic
splanchnic nerves 2 to 4.
Sympathetic Nervous System (SNS)
|The SNS is a branch of the ANS. Promotes a "fight or flight" response,
corresponds with arousal and energy generation, and inhibits digestion.
||Diverts blood flow away from the GI tract and skin via vasoconstriction.
||Blood flow to skeletal muscles, the lung is not only maintained, but
enhanced (by as much as 1200%, in the case of skeletal muscles).
||Dilates bronchioles of the lung, which allows for greater alveolar
||Increases heart rate and the contractility of cardiac cells (myocytes),
thereby providing a mechanism for the enhanced blood flow to skeletal muscles.
||Dilates pupils and relaxes the lens, allowing more light to enter the eye.
||Provides vasodilation for the coronary vessels of the heart.
Enteric Nervous System (ENS)
The ENS is a branch of the ANS that directly controls the
The Somatic Nervous System
The SNS is the part of the PNS associated with the voluntary control of body
movements through the action of skeletal muscles, and with reception of external
stimuli, which helps keep the body in touch with its surroundings such as touch,
hearing, and sight.
The system includes all the neurons connected with skeletal muscles, skin and
sense organs. The SNS consists of efferent nerves responsible for sending brain
signals for muscle contraction. This part of the nervous system is responsible
for gathering information around you, sending it up to the CNS for processing,
and then awaits instructions on what to do and then does it.
The SNS, or voluntary nervous system, enables humans to react consciously to
environmental changes. It includes 31 pairs of spinal nerves and 12 pairs of
cranial nerves. This system controls movements of skeletal (voluntary) muscles.
Thirty-one pairs of spinal nerves emerge from various segments of the spinal
cord. Each spinal nerve has a dorsal root and a ventral root. The dorsal root
contains afferent (sensory) fibers that transmit information to the spinal cord
from the sensory receptors. The ventral root contains efferent (motor) fibers
that carry messages from the spinal cord to the effectors. Cell bodies of the
efferent fibers reside in the spinal cord gray matter. These roots become nerves
that innervate or transmit nerve impulses to muscles and organs throughout the
Twelve pairs of cranial nerves (as in the image to the right) transmit from
special sensory receptors information on the senses of balance, smell, sight,
taste, and hearing. Cranial nerves also carry information from general sensory
receptors in the body, mostly from the head region. This information is
processed in the CNS and the resulting orders travel back through the cranial
nerves to the skeletal muscles that control movements in the face and throat,
such as for smiling and swallowing. In addition, some cranial nerves contain
somatic and autonomic motor fibers.