PERIPHERAL NERVOUS SYSTEM

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20 Οκτ 2013 (πριν από 3 χρόνια και 11 μήνες)

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PERIPHERAL NERVOUS SYSTEM



Cut nerves

If a small nerve is cut, it will regenerate because where are the cell bodies?
In the posterior root
ganglion (sensory) or anterior horn (motor). Since the cell body is about a meter away, axons can
regrow. Large nerves are harder to regrow, but you can still stitch the ends together at the
epineurium and perineurium, and you may g
et healing.


Pinched nerves

When a nerve gets pinched (e.g. herniated disc), it damages the nerve by interfering with its
action potential, causing weakness, pain, or paralysis.


Disruption of Blood S
upply

When a body part “falls asleep”, the region has b
ecome ischemic, impairing the action potential
of the nerves. Unlike the CNS, when blood is restored, the nerves recover. Damage to the CNS
tends to be permanent, but damage to the PNS tends to heal.


DAMAGE TO THE NERVOUS SYSTEM

If a person has a spinal

cord injury in their cervical region, they could have quadriplegia
(arms and legs paralyzed).


If a person has a spinal cord injury in their thoracic region, they could have paraplegia
(just legs are paralyzed).


SOME CLINICALLY IMPORTANT PERIPHERAL
NERVE
S
:

PUDENDAL NERVE
: this is the nerve that can be
anesthetized during childbirth

as an
alternative to an epidural (a pudendal nerve block is also called a saddle block because
the numb areas are where you would be touching a saddle).

PHRENIC NERVE
:
allows
the diaphragm to contract.

If it gets severed, the person can
no longer breathe without assistance.

Endoneurium
: surrounds one neuron

Perineurium
: surrounds a fascicle
(bundle) of neurons

Epineurium
: surrounds a bunch of
fascicles

SENSORY NERVES

These come out of the spinal cord and go to specific regions of the body. Each region of
the body is innervated by spinal nerves. For example, nerve C4 innervates region C4 of
the
DERMATOME
. It’s important to know these dermatome regions (not for this
c
lass), especially physical therapists and nurses.


If a patient has a shooting pain down the anterior shin, what nerve is pinched? L5.
Numbness in pinky and ring finger is what nerve? C8. If a workman’s comp patient
comes in saying his whole hand is nu
mb, no other symptoms, you know he’s lying
because the nerves don’t run that way. They also don’t run transversely across the body;
they are on one side or the other.


A
PLEXUS

is a network of nerves that primarily serves the limbs.

There are four major
p
lexuses: cervical, brachial, lumbar, and sacral.


1. CERVICAL
PLEXUS

comes out of the neck and are cutaneous nerves (sensory input
of the skin) of the neck and back of the head.


2.
BRACHIAL PLEXUS

This is the major group of nerves that supply the upper l
imbs. It runs through the
axilla. If a
person leans their armpits on their crutches, they can damage this plexus and lose the use of
their arms. The nerves in the brachial plexus change names as they go to different regions
in the arm.


MAJOR NERVES OF TH
E UPPER EXTREMITY






















Axillary Nerve
:

Supplies deltoid muscle

Paralysis causes weak shoulder.

Musculocutaneus Nerve



Supplies anterior muscles of the arm

Median Nerve



Supplies no muscles of the arm



Supplies anterior forearm (except flexor
carpi ulnaris)



Carpal Tunnel Syndrome



Hand of benediction

Ulnar Nerve

Supplies flexor carpi ulnaris



“Funny Bone”



Damage can cause claw hand; cannot
adduct or abduct fingers

Radial Nerve



Supplies muscles on the posterior arm and
forearm



Damage can cause wrist drop


MEDIAN
nerve travels under the transverse carpal ligament, and

gets

irritated in
CARPAL TUNNEL SYNDROME
.
It also gets cut when people try to slit their wrists.
The arteries are so small in the wrist; people rarely die from this type of
suicide attempt.
However, they live with a lot of tissue damage. They are not able to move the thumb
towards the little finger, so it is hard to pick up small objects. This is called “ape hand”.




DAMAGE TO BRACHIAL PLEXUS



Congenital (brachial plexus
damaged during birth)



Klumpke’s paralysis



Acquired Brachial Plexus injuries



Crutch paralysis (total upper extremity paralysis)



Claw Hand



Carpal Tunnel Syndrome, Ape hand, Hand of benediction



Wrist Drop (Waiter’s Hand)



3.
LUMBAR PLEXUS

FEMORAL NERVE

is

the
main nerve to the anterior thigh.


4. SACRAL

PLEXUS

are spinal nerves from L4
-
S5

Some of the fibers from the lumbar plexus mix with the sacral plexus, so these are often
referred to together as the lumbosacral plexus.




























SCIATIC NERVE

is the largest branch of the sacral plexus and the largest nerve in the
body; it
leaves the pelvis through the sciatic notch.
A short, thick muscle (Piriformis
muscle) covers the sciatic notch, and when it contracts, it can pinch the sciatic nerve,
causing a type of sciatica (sciatic nerve irritation) known as Piriformis syndrome.

This can be alleviated by stretching exercises.
However, sciatica can also be caused if
there is a herniated lumbar disc, in which case stretching exercises make it worse.


The sciatic nerve branches out into the
TIBIAL

and
FIBULAR

nerves

to supply the
thigh, leg, and foot.


TIBIAL NERVE



Sometimes a small branch of the tibial nerve in the foot gets pinched between the
metatarsal heads, and the irritation causes nerve swelling and pain.



It is called a neuroma (“nerve tumor”) and manifests as pain in the ball of the
foot, made worse with hi
gh heels.



An injury to the fibular nerve may result in “foot drop”, where the foot cannot be
dorsiflexed.






LOWER EXTREMITY NERVES


Obturator Nerve


Supplies adductor muscles

Sciatic Nerve


Supplies back of thigh, leg and foot

Femoral Nerve



Supplies anterior Thigh

Tibial Nerve



Supplies posterior leg and foot

Common Fibular Nerve


Superficial branch




Supplies lateral side of leg


Deep branch



Supplies anterior leg



Injury causes “Foot Drop”


AUTONOMIC NERVOUS SYSTEM

(ANS)


We don’t have voluntary control over these nerves. They are involved digestion, blood
flow, urination, defecatio
n, glandular secretion.
Therefore, the ANS supplies the glands,
smooth muscle, and cardiac muscle, but NOT the skeletal muscle. For this reason, the
ANS is also called the general visceral motor system. All of the neurons of the ANS are
motor neurons
(th
ere are no sensory neurons in the ANS). The ANS differs from the CNS
reflex arc because the ANS has
two

motor neurons in the periphery (the cell body of one
is in the spinal cord and the cell body of the other is in the periphery), whereas the CNS
has one
motor neuron, and its cell body is within the spinal cord, not in the periphery.
The ANS neuron comes from the spinal cord and synapses on the cell body of another
neuron, which then synapses on the target (gland, blood vessel, etc). The area where the
t
wo neurons come together is the
AUTONOMIC GANGLIA
. The first neuron is the
PRE
-
GANGLIONIC NEURON.

The second neuron is the
POST
-
GANGLIONIC
NEURON
. Some of t
hese ganglia
(those of the sympathetic division of the ANS)
are
lined up along the vertebral colum
n,
as

a structure called the sympathetic trunk ganglia.



The ANS motor unit is characterized by having more than one motor neuron, the axons
may be myelinated or unmyelinated, conduction is slow, and the axons are thin.


The ANS has two divisions:
sympathetic and parasympathetic.


SYMPATHETIC DIVISION

This is involved in
↑heart rate and blood pressure, ↑metabolic activity (increased blood
glucose), decreased peristalsis (decreased food digestion) dilation of bronchioles, control
of blood flow to the

skin, and sweating. E.g. when running, ↑heart rate = sympathetic.
When hot


sweat = sympathetic. The term “Fight or Flight” is inaccurate; it refers to
the ↑ heart rate, etc, but the sympathetic division is also active when relaxing on a nice
beach wi
th a cool drink on a hot day, because whenever you’re sweating, that’s the
sympathetic division.


ANATOMY OF THE SYMPATHETIC DIVISION

The neurons exit the spinal cord at the thorax and lumbar regions. The axons of most
pre
-
ganglionic neurons in the sympat
hetic division are fairly short, and they synapse
quickly on a ganglion. All these ganglia together are the
SYMPATHETIC TRUNK
(CHAIN) GANGLIA
. There are about 22
-
24 on each side. There are also nerves that
connect the ganglia to each other. The axons o
f the
POST
-
GANGLIONIC N
EURONS

are very long, and go to the target organs. Some pre
-
ganglionic neurons bypass the chain
ganglia and go directly to the abdomen. They create a group of ganglia in the abdomen
called the
SOLAR PLEXUS
(“sun”). When you get pun
ched in the abdomen, you are
punched in the solar plexus, and get the wind knocked out of you.



PARASYMPATHETIC DIVISION

Unlike the sympathetic division, the
axons of the
preganglionic
neurons in

the
parasympathetic division are long, and
axons of
the postganglionic
neurons

are short.
The nerve cell bodies (peripheral ganglia) of the parasympathetic division are closer
to the organs being innervated than in the sympathetic division. In fact, the cell
bodies are either next to or inside of the target

organs. Therefore, they have short
post
-
ganglionic fibers.


The function of this division is often antagonistic (opposite) of the sympathetic, but
actually, they work together. The parasympathetic division inhibits cardiac contraction,
so there is: ↓hear
t rate, constricts bronchioles, activates digestive system, and causes
salivation, urination, and defecation. When you are lounging on the beach, the heart rate
decreases (parasympathetic), but the sweat increases (sympathetic).


The parasympathetic ne
urons

come out of either the brain or the sacral region of the
spinal cord. The majority of the parasympathetic outflow from the head is by the
vagus
nerve.


Vasovagal Syncope (Fainting)



The most common type of fainting.



After a stressful trigger, the pa
rasympathetic nervous system is enhanced by the
Vagus nerve.



The heart rate speeds up, then suddenly drops.



Then the blood pressure drops.



Unconsciousness results.



Treatment: elevate the legs above the heart for a few minutes, and make sure the
airway re
mains open.



A cold, wet cloth on the forehead and back of the neck may make the person feel
better as they recover.


REYNAUD'S PHENOMENON



Autonomic nervous system is hyperactive in the ANS neurons that innervate the
walls of blood vessels.



It causes spas
ms of peripheral blood vessels, cuts off some blood supply, and
causes the fingers and toes to be white, or in severe cases, blue.



Emotional stress and being cold tend to trigger the discoloration.







VISCERAL (“organ”) SENSES

A

visceral

nerve
innervates involuntary effectors

(smooth muscle in organs)
.

A
somatic
nerve innervates

voluntary effectors (skeletal muscle).


Internal organs also have sensory nerves that tell you when you have eaten enough or
your bladder is full. Not all organs have
sensory nerves, for instance, you can’t feel when
you have high blood pressure. You can also have visceral reflexes, which trigger the
parasympathetic system to contract the bladder when full, etc. Reflexes are hard to
localize.


Pain in an organ may n
ot be where the organ is. Heart pain usually manifests in the left
side of chest, the left shoulder, arm, but not the heart. This is
REFERRED PAIN
. Pain
in the lungs usually shows up as neck pain. These areas of referred pain are important to
know, but

not for this class.



FUN NERVOUS SYSTEM DISCUSSION QUESTIONS

Does the kind of amnesia you see

in the movies really exist?

Memory loss is usually only temporary and involves a losing memory of only a
short period of the person’s life.

Is there really a

medication t
hat acts like a truth serum?

Not really; it’s just a relaxing drug that makes you less inhibited.

What is on the rags that villains use to
make their victims pass out?

Chloroform and ether used to be used for anesthesia. Too many toxic side
e
ffects, so not used anymore.

Do people really have multiple p
ersonalities, like in Sybil?

Yes, it’s caused by severe emotional stress, such as abuse as a child.

Can you get scared to death?
It can increase the likelihood of a heart attack,
but only if the

person was going to have one in the next few weeks anyway.

Is it true that left
-
handed people are smarte
r than right

handed people?
Not
really. Right handed people use their left brain and lefties use their right brain
more, but it’s not cut and dry. The

right brain is responsible for visual and spatial
skills, and the left brain controls language and speech. Lefties tend to be better in
math, and people who are good at math tend to be good at music, too. Males are
twice as likely to be left handed than f
emales.

Can

aluminum cause Alzheimer's?
If so, the Tin Man would have needed a
brain instead of a heart! Actually, it does play a small role in Alzheimer’s.

Why are older people such bad drivers?
People over 65 are now 12% of the
population. They tend to

have decreased ability to see, especially at night, and
our reflexes slow as we age, yet we don’t realize it and we think we can react as
quickly as we used to. Watch out; your day is coming!

Why do you need les
s sleep when you get older?

Actually, the s
leep needs remain the same, but older people tend to have more
difficulty falling asleep and are awakened by night cramps, etc. their bodies
compensate by giving them more REM time (dreams).



What's the longest a person has gone without sleep?

The record
is 11 days.


How does the human brain measure up to a computer?

The
human brain

could hold more data than any hard drive currently available

simulating
basic actions in robots, like talking, laughing, and walking, requires an amazing amount
of memory and p
rocessing power. But when it comes to multitasking, computers have
near limitless potential, whereas people have a hard time patting our stomachs and
rubbing our heads at the same time.


If one person yawns, why does everyone else in the room want to yawn
?

55% of people who witness someone yawn will yawn within five minutes. If a visually
impaired person hears a tape of someone yawning, he or she is likely to yawn as well. It’s
a holdover from a period in evolutionary history when
yawning served to coordin
ate

the
social behavior of a group of animals. A
recent study

postulates that contagious yawning
could be part of the "neural network involved in empathy."


What makes people ticklish?

Being ticklish

is our defense against creepy crawlies like spiders and
bugs, a

physiological response alerting us to a specific type of threat. That is why vulnerable
parts of our bodies
--

feet, chest, and armpits, are among the most
ticklish. Although
being ticklish is neurological, scientists contend that it is also learne
d. One
theory

sees
ticklishness as a personality
-
based response to

perceived attack. Antsy folks may laugh
uncontrollably at the lightest touch, or even without being touched at all, while folks
made of sterner stuff won't budge during more aggressive tick
le attacks. If you close your
eyes and try to remain calm while you are tickled, you can decrease panic, reduce
giggles, and dull sensation. And, no matter how hard you try, it is nearly impossible to
tickle yourself. Tickling satisfies our human need to t
ouch, and can be a
form of
communication between friends, family, and lovers, playing a key role in

the evolution of
social and sexual behavior. Chimps tickle each other during play, parents tickling little
kids, and lovers tickling each other affectionate
ly. "If you think the social component is
not important, try tickling a stranger.


Why do we only use 10% of our brains? Does anyone use more?

That is a myth; we use all of our brain. Some people have more efficient synapses than
others. There is evidence
that if we don't exercise mental skills and capacities they tend to
atrophy, just as muscle does.


Why does scratching an itch make it stop?

The itch reflex likely shares some of the same neural mechanisms and pathways as the
pain reflex, but they're obvio
usly not the same thing.

Scratching provides a "counter irritation" that distracts the brain from the original itch.

Others believe
scratching

releases pain
-
reducing endorphins. The pain neurons

become
temporarily overwhelmed, which masks the itching sens
ation.