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
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•
Supplies no muscles of the arm
Supplies anterior forearm (except flexor carpi ulnaris)
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Carpal Tunnel Syndrome
–
Ulnar Nerve
Hand of benediction
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.
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After a stressful trigger, the parasympathetic nervous system is enhanced by the
Vagus nerve.
• The heart rate speeds up, then suddenly drops.
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Then the blood pressure drops.
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Unconsciousness results.
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Treatment: elevate the legs above the heart for a few minutes, and make sure the airway remains open.
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A cold, wet cloth on the forehead and back of the neck may make the person feel better as they recover.
REYNAUD'S PHENOMENON
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Autonomic nervous system is hyperactive in the ANS neurons that innervate the walls of blood vessels.
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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.
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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.