PERIPHERAL NERVOUS SYSTEM

Nervous System Classification-1• Somatic Nervous System

– Motor nerves to skeletal muscle (somatic motor neurons)• Upper and lower motor neurons

– Reflexes• Sensory, interneurons, lower motor neurons

– Sensory nerves (somatosensory neurons)• Autonomic Nervous System

– Motor nerves to smooth and cardiac muscle (visceral motor neurons)• Sympathetic• Parasympathetic

Nervous System Classification-2• Efferent Nervous System

• Somatic Nervous System• Upper and lower motor neurons to skeletal muscle (somatic motor

neurons)• Visceral (Autonomic) Nervous System• Motor nerves to smooth and cardiac muscle (visceral motor neurons)

• Sympathetic• Parasympathetic

• Afferent Nervous System• Sensory nerves (somatosensory neurons)

• Reflexes

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.

Somatosensory Neurons• The somatosensory system is made up of a number of different receptors,

including thermoreceptors (action potential is triggered by heat or cold), photoreceptors (triggered by light), mechanoreceptors (triggered by movement) and chemoreceptors (triggered by chemicals).

• Each somatosensory neuron specializes in conducting one type of sensation, such as proprioception, mechanoreception (touch), thermoception (temperature), and nociception (pain).

• The sensory receptors cover the skin and epithelia, skeletal muscles, bones and joints, internal organs, and the cardiovascular system.

Somatic Senses• Pain• Haptic perception

– Deep Pressure– Light Touch

• Vibration• Thermoception (Hot/Cold)• Two-point discrimination• Proprioception• Special senses

Sense of Touch• Touch includes mechanoreception (deep pressure, light touch, vibration and

proprioception), pain (nociception) and heat (thermoception).

Sense of Pain• A nociceptor is a receptor of a sensory neuron (nerve cell) that responds to

potentially damaging stimuli by sending signals to the spinal cord and brain. • This process, called nociception, usually causes the perception of pain.• They travel in the dorsal column pathway in the spinal cord, ascend to the

thalamus in the brain, and go to the cerebral cortex.• 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.

• One pain disorder is called Complex regional pain syndrome (CRPS)• Formerly called Reflex Sympathetic Dystrophy (RSD)• Chronic progressive disease characterized by severe pain, swelling, and changes

in the skin. • It often affects an arm or a leg and may spread to another part of the body and is

associated with dysregulation of the autonomic nervous system resulting in multiple functional loss, impairment, and disability.

• Symptoms include burning and shooting pains and edema that occur more often during times of stress.

• Treatment is often unsatisfactory. Early multimodal therapy can cause dramatic improvement or remission of the syndrome in some patients.

Treatment for Pain• Somatosensory Rehabilitation of Pain is a method whose aim is to treat conditions

of a reduced sense of touch or sensation (hypoesthesia) in order to decrease neuropathic pain.

• Neuropathic pain, with a prevalence of 6.9% of the general population, represents an important public health problem.

• Examples of pain syndromes include:• Carpal Tunnel Syndrome (CTS) concerns 2.7% of the general population.• Complex Regional Pain Syndrome (CRPS) concerns 26/100,000.• Patients with chronic pain might use transcutaneous electrical nerve stimulation

and other physical therapy modalities, which bring substantial pain relief to large numbers of people.

These are routinely tested by doctors in a physical exam, especially for people with diabetes and lupus.

Haptic perception (Sense of Touch)– Deep Pressure– Light Touch

• While touch (also called tactile perception) is considered one of the five traditional senses, the impression of touch is formed from several modalities including pressure, skin stretch, vibration and temperature.

• There are separate neurons for each of those sensations, and each group of neurons must be evaluated during a physical exam.

• In medicine, the colloquial term "touch" is usually replaced with "somatic senses" to better reflect the variety of mechanisms involved.

• Haptic perception is the process of recognizing objects through touch. It involves a combination of somatosensory perception of patterns on the skin surface (e.g., edges, curvature, and texture) and proprioception of hand position and conformation (what position do my finger go into while tracing the surface of the object?).

• People can rapidly and accurately identify three-dimensional objects by touch.They do so through the use of exploratory procedures, such as moving the fingers over the outer surface of the object or holding the entire object in the hand.

• Haptic perception is active exploration, whether with a body part or a tool, such as feeling the contours of something by using a stick to touch it.

• Haptic perception allows the creation of "virtual", illusory haptic shapes with different perceived qualities.

• Loss of the sense of touch is a catastrophic deficit that can impair walking and other skilled actions such as holding objects or using tools.

Disorder of Sense of Touch• Allochiria (from the Greek meaning "other hand") is a neurological disorder in

which the patient responds to stimuli presented to one side of their body as if the stimuli had been presented at the opposite side.

• It is associated with spatial transpositions, usually symmetrical, of stimuli from one side of the body (or of the space) to the opposite one. Thus a touch to the left arm will be reported as a touch to the right arm

• If the auditory or visual senses are affected, sounds (a person's voice for instance) will be reported as being heard on the opposite side to that on which they occur and objects presented visually will be reported as having been presented on the opposite side.

• Commonly from damage to the right parietal lobe.Allochiria can also occur to other senses.

• Motor Allochiria• Reflex Allochiria• Audio Allochiria• Visual Allochiria• Gustatory Allochiria

Sense of Vibration• Pallesthesia refers to the sensation of mechanical vibration on or near the body.• Vibration sense may be lost as a result of a number of lesions to the nervous

system, often in conjunction with other deficits.• The word "pallesthesia" is derived from the Greek pallein, meaning "to shake."• Pallesthesia may be tested for using a vibrating mechanical tuning fork, placed on

a bony prominence.

Thermoception (Sense of Hot/Cold)• Thermoception is the sense of heat and the absence of heat (cold) by the skin. • There are specialized receptors for cold (declining temperature) and for heat. • Cold receptors help an animal’s sense of smell, telling wind direction. Heat

receptors help animals that can use radiation to detect prey, such as snakes.• The information travels up the spinothalamic tract in the spinal cord and ascends

to the thalamus.• The thermoceptors in the skin are quite different from the homeostatic

thermoceptors in the brain (hypothalamus), which provide feedback on internal body temperature.

Two-point discrimination• Two-point discrimination is the ability to discern that two nearby objects touching

the skin are truly two distinct points, not one. It is often tested with two sharp points during a neurological examination and is assumed to reflect how finely innervated an area of skin is and diagnosing tactile agnosia (don’t know what is touching me).

• The therapist randomly alternates between touching the patient with one point or with two points on the area being tested (e.g. finger, arm, leg, toe).

• The patient is asked to report whether one or two points was felt. The smallest distance between two points that still results in the perception of two distinct stimuli is recorded as the patient's two-point threshold.

• Performance on the two extremities can be compared for discrepancies.

PROPRIOCEPTION• There are sensors within the muscles and joints that measure the amount of force,

movement, and position. • Proprioception neurons travel up the spinocerebellar tract. The brain can then

interpret whether you are off balance, then send a command to the muscles to contract and straighten yourself up so you don’t fall.

• Note that this sense of balance is NOT the same as the sense of balance from equilibrium in the ears. Proprioception neurons are located within the muscles and joints.

• During a physical exam, a doctor will test the patient’s proprioception ability by telling them to close their eyes and place their finger on their nose. This may indicate a lesion in the cerebellum. Who else may ask you to do this test? Alcohol disrupts the cerebellum.

Muscle spindles• Muscle spindles are sensory receptors within the belly of a muscle that primarily

detect changes in the length of this muscle. The brain can then determine the position of body parts.

• Muscle spindle responses also play an important role in regulating the contraction of muscles, by activating motor neurons via the stretch reflex to resist muscle stretch.

• Proprioceptors are sensory receptors that report on internal events in your muscles and joints.

• They report on muscle stretch and joint position. • They generate electrical impulses that will travel up neurons to the CNS.• A muscle spindle is one type of proprioceptor.

Proprioception Disorders• Not enough vitamin B1 (memory problems)• Too much vitamin B6 (balance problems)

– Damage to proprioceptors can occur from consuming excess vitamin B6 (pyridoxine).

– Patients cannot tell where their body parts are unless they look at them. – They have difficulty with all motor tasks including walking, eating,

dressing, etc.– They must use their vision to watch each body part to make it move in the

right direction.• Proprioception is often tested by having the patient close their eyes and saying if

their fingers are up or down.• Proprioceptors send information to the cerebellum. That’s how you know your

legs are crossed before you stand up. • Somatic senses and proprioception are NOT considered special senses.

Special senses (discussed in another lecture)• The special senses are those that have specialized organs devoted to them:• vision (the eye)• hearing and balance (the ear)• smell (the nose)• taste (the tongue)

Somatosensory Information Pathway• Somatic sense information goes into the spinal cord, travels up a tract and into the

primary somatosensory area in the parietal lobe of the cerebral cortex.• The mapping of the body surfaces in the brain is called a homunculus and plays a

fundamental role in the creation of body image. • This brain-surface ("cortical") map is not immutable, however. Dramatic shifts

can occur in response to stroke or injury.

SENSORY CUTANEOUS NERVES• These come out of the spinal cord and go to specific regions of skin on the body. • For example, nerve C4 innervates the skin region C4 of the DERMATOME

MAP. • It’s important to know these dermatome map 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.

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).

Disruption of Blood Supply• When a body part “falls asleep”, the region has become ischemic (lack of blood

flow), impairing the action potential of the nerves. Unlike the CNS, when blood is restored to the PNS, the nerves recover. Damage to the CNS tends to be permanent, but damage to the PNS tends to heal.

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.

Tens unit• Transcutaneous electrical nerve stimulation• TENS is usually applied at high frequency with an intensity below motor

contraction. It just blocks pain impulses.• This is a different machine than a muscle stimulator, but they look the same.• TENS is available without a Rx.

Corydalis powder for painPut ½ teaspoon into one cup of hot fluid (tea, etc) and drink it. Alleviates chronic pain in about 20 minutes!

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.

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.

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 PLEXUSThis 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 muscleParalysis 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 benedictionUlnar Nerve

Supplies flexor carpi ulnaris• “Funny Bone”• Damage can cause claw hand; cannot

adduct or abduct fingersRadial 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 PLEXUSFEMORAL NERVE is the main nerve to the anterior thigh.

4. SACRAL PLEXUS are spinal nerves from L4-S5Some 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 high heels.• An injury to the fibular nerve may result in “foot drop”, where the foot cannot be

dorsiflexed.

Nervous System Classification• Somatic Nervous System

– Motor nerves to skeletal muscle (somatic motor neurons)• Upper and lower motor neurons

– Reflexes• Sensory, interneurons, lower motor neurons

– Sensory nerves (somatosensory neurons)• Autonomic Nervous System

– Motor nerves to smooth and cardiac muscle (visceral motor neurons)• Sympathetic• Parasympathetic

LOWER EXTREMITY NERVES

Obturator NerveSupplies adductor muscles

Sciatic NerveSupplies back of thigh, leg and foot

Femoral Nerve Supplies anterior Thigh

Tibial Nerve Supplies posterior leg and foot

Common Fibular NerveSuperficial branch

Supplies lateral side of legDeep 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, 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 DIVISIONThis 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 DIVISIONThe 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. It causes the diaphragm to go into spasm.

PARASYMPATHETIC DIVISIONUnlike 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.

• “Rest and Digest”• Involved in vegetative activities, such as digestion, urination, defecation• Has postganglionic cell bodies in terminal ganglia, located either near or within

target organs• Has both preganglionic and postganglionic neurons that secrete acetylcholine• Has preganglionic cell bodies located in the cervical (Vagus nerve) and sacral

areas.

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.