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PERIPHERAL NERVOUS SYSTEM
Endoneurium: surrounds one neuron
Perineurium: surrounds a fascicle
(bundle) of neurons
Epineurium: surrounds a bunch of
fascicles
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 get 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 Supply
When a body part “falls asleep”, the region has become 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 NERVES:
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.
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
class), 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 numb, 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
plexuses: 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 limbs. 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 THE 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
KLUMPKE’S PARALYSIS (brachial plexus damaged during birth)
Acquired Brachial Plexus injuries
Crutch paralysis (total upper extremity paralysis)
Claw Hand / 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.
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”
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 high heels.
• An injury to the fibular nerve may result in “foot drop”, where the foot cannot be
dorsiflexed.
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 that 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
effects, so not used anymore.
Do people really have multiple personalities, 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 smarter 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 females.
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 less sleep when you get older?
Actually, the sleep 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 processing 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 coordinate 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 learned. 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 tickle 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 touch, 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 affectionately. "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 obviously 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 sensation.
AUTONOMIC NERVOUS SYSTEM (ANS)
We don’t have voluntary control over these nerves. They are involved digestion, blood
flow, urination, defecation, 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 (there 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
two 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 these ganglia (those of the sympathetic division of the ANS) are
lined up along the vertebral column, 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 with 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 sympathetic 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 of the POST-GANGLIONIC NEURONS
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 punched 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: ↓heart 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 neurons 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 parasympathetic 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 remains 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 spasms 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 not 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.