Peripheral Nervous System part 1

Question 1

Two main components of peripheral nerves

Answer 1

Axons

Myelin sheath compose of Schwann cells

Question 2

carried out by the motor unit

Answer 2

Somatic motor function

Question 3

Somatic motor function is carried out by the motor unit, which consists of

Answer 3

1) lower motor neuron located in the anterior horn of the spinal cord or in the brainstem
2) an axon that travels to a target muscle as part of a nerve
3) the neuromuscular junctions
4) multiple innervated myofibers

Question 4

Somatic sensory function depends on

Answer 4

1) the distal nerve endings
2) an axon that travels as part of a peripheral nerve to the dorsal root ganglia
3) a proximal axon segment that synapses on neurons in the spinal cord or the brainstem

Question 5

outnumber somatic fibers in the peripheral nervous system, but signs and symptoms related to their involvement are generally not prominent features of peripheral neuropathies, with a few important exceptions

Answer 5

Autonomic nerve fibers

Question 6

are each conveyed by axons that can be distinguished based on their diameter

Answer 6

Specific sensation like

Pain
Temperature
Touch

Question 7

Thin unmyelinated f ibers mediate autonomic functions as well as pain and temperature sensation and have the

Answer 7

slowest conduction speeds

Question 8

Large diameter axons with thick myelin sheaths transmit light touch and motor signals and have

Answer 8

fast conduction speeds

Question 9

In the case of myelinated axons, individual Schwann cells make exactly one myelin sheath that wraps around a single axon to create a myelinated segment called an

Answer 9

internode

Question 10

Internodes are separated by unmyelinated gaps referred to as ________, which are uniformly spaced along the length of the axon

Answer 10

nodes of Ranvier

Question 11

The axons are bundled together by three major connective tissue components

Answer 11

epineurium, which encloses the entire nerve;
perineurium, groups subsets of axons into fascicles;
endoneurium, which surrounds individual nerve fibers

Question 12

The morphologic hallmarks of axonal neuropathies can be produced experimentally by cutting a peripheral nerve, which results in a prototypical pattern of injury described as

Answer 12

Wallerian degeneration

Question 13

Atonal neuropathies

Within a day of injury, the distal axons begin to fragment and the associated myelin sheaths unravel (Fig. 27-3) and disintegrate into spherical structures

Answer 13

Myelin ovoids

Question 14

Atonal neuropathies

The repair process is successful only if the two transected ends remain closely approximated. A failure of the outgrowing axons to find their distal target can produce a “

Answer 14

pseudotumor” termed traumatic neuroma—

Question 15

Axonal neuropathies

nonneoplastic haphazard whorled proliferation of axonal processes and associated Schwann cells that results in a painful nodule

Answer 15

Traumatic neuroma

Question 16

Axonal neuropathies

Consequently, the electrophysiologic hallmark of axonal neuropathies is a

Answer 16

reduction in signal strength owing to the dropout of axons from affected peripheral nerves

Question 17

Axonal neuropathies

Nice to know

Answer 17

The changes observed following experimental nerve transsections only partially resemble those seen in various axonal neuropathies. One key difference is that in these disease states (unlike nerve transection) damage occurs over an extended period of time. As a result, degenerating and regenerating axons co-exist in a single biopsy. With time, damage tends to outpace repair, resulting in progressive loss of axons

Question 18

Axons are the primary target of the damage in this large group of peripheral neuropathies

Answer 18

Axonal neuropathies

Question 19

In these disorders, Schwann cells with their myelin sheaths are the primary targets of damage whereas axons are relatively preserved

Answer 19

Demyelinating neuropathies

Question 20

results in degeneration of the distal axon and its associated myelin sheath, with atrophy of denervated myofibers

Answer 20

Acute Axonal injury

Question 21

produces random segmental degeneration of individual myelin internodes, while sparing the axons

Answer 21

Acute demyelination disease

Question 22

Regeneration of axons after injury allows reinnervation of myofibers. The regenerated axon is myelinated by proliferating Schwann cells, but the new internodes are 

Answer 22

shorter and the myelin sheaths are thinner than the original ones

Question 23

Demyelinating disease

The electrophysiologic hallmark of these disorders is

Answer 23

slowed nerve conduction velocity, reflective of the loss of myelin

Question 24

Demyelinating disease

Individual myelin sheaths degenerate in a seemingly random pattern, re sulting in discontinuous damage of myelin segments. In response to this damage, Schwann cells or Schwann cell precursors proliferate and initiate repair through the formation of new myelin sheaths,

Answer 24

but these again tend to be shorter and thinner than the original ones

Question 25

result from destruction of neurons, leading to secondary degeneration of axonal processes

Answer 25

Neuronopathies

Question 26

Neuropathies are examples of insults that may lead to neuronopathies

Answer 26

Infections like herpes zoster and toxins like platinum compounds

Question 27

Because the damage is at the level of the neuronal cell body, peripheral nerve dysfunction caused by neuronopathies is equally likely to affect proximal and distal parts of the body unlike _________________which preferentially affect the distal extremities

Answer 27

peripheral axonopathies

Question 28

Neuropathies affect a single nerve and result in deficits in a restricted distribution dictated by normal anatomy

Answer 28

Mononeuropathies

Question 29

Neuropathies are common causes of mononeuropathy

Answer 29

Trauma, entrapment, and infections

Question 30

Neuropathies are characterized by involvement of multiple nerves usually in symmetric fashion

Answer 30

Polyneuropathies

Question 31

Poly neuropathies In most cases axons are affected in a length dependent fashion leading to deficits that start in the feet and ascend with disease progression. The hands often start to show involvement by the time deficits extend to the level of the knee, resulting in a characteristic “_____________” distribution of sensory deficits

Answer 31

stocking and glove

Question 32

Neurophaties describes a disease process that damages several nerves in a haphazard fashion

Answer 32

Mononeuritis multiplex

Question 33

Mononeuritis multiplex An affected patient might have a right wrist drop from involvement of the right radial nerve and a left foot drop from peroneal nerve damage. ___________ is a common cause of this pattern of injury

Answer 33

Vasculitis

Question 34

Neuropathies affect nerve roots as well as peripheral nerves, leading to diffuse symmetric symptoms in proximal and distal parts of the body

Answer 34

Polyradiculoneuropathies

Question 35

is a demyelinating peripheral neuropathy that may lead to life-threatening respiratory paralysis

Answer 35

Guillain-Barré syndrome

Question 36

The disease is characterized clinically by weakness beginning in the distal limbs that rapidly advances to affect proximal muscle function (“ascending paralysis

Answer 36

GBS

Question 37

GBS Histologic features are inflammation and demyelination of spinal nerve roots and peripheral nerves

Answer 37

radiculoneuropathy

Question 38

In most cases, Guillain-Barré syndrome is thought to be an acute-onset

Answer 38

immune-mediated demyelinating neuropathy

Question 39

GBS Approximately two thirds of cases are preceded by an ________________from which the affected individual has recovered by the time the neuropathy becomes symptomatic

Answer 39

acute, influenza-like illness

Question 40

GBS Infections with ______________, have significant epidemiologic associations with Guillain-Barré syndrome

Answer 40

``` Campylobacter jejuni, cytomegalovirus, Epstein-Barr virus, and Mycoplasma pneumoniae, or prior vaccination ```

Question 41

GBS No infectious agent has been demonstrated in affected nerves, and an __________ is favored as the underlying cause

Answer 41

immunologic reaction T cell or lymphocytes

Question 42

GBS The dominant histopathologic finding is ______________ manifested as perivenular and endoneurial infiltration by lymphocytes, macrophages, and a few plasma cells

Answer 42

inflammation of peripheral nerves,

Question 43

GBS  affecting peripheral nerves is the most prominent lesion, but damage to axons is also seen, particularly when the disease is severe

Answer 43

Segmental demyelination

Question 44

GBS Clinical features

Answer 44

ascending paralysis and areflexia Deep tendon reflexes disappear early in the process loss of pain sensation, is often present but is usually not a prominent feature Nerve conduction velocities are slowed Cerebrospinal f luid (CSF) protein levels are elevated due to inflammation

Question 45

GBS _________________and _______________appear to be beneficial, apparently because these remove pathogenic antibodies and suppress immune function, respectively

Answer 45

Plasmapheresis and intravenous immunoglobulin

Question 46

This is the most common chronic acquired inflammatory peripheral neuropathy, characterized by symmetrical mixed sensorimotor polyneuropathy that persists for 2 months or more

Answer 46

Poly(radiculo)neuropathy

Question 47

PRN By definition, signs and symptoms must be present for at least 2 months but often the disease evolves over years, usually with relapses and remissions. While typically there is a symmetric, mixed sensorimotor polyneuropathy, some patients may present with predominantly sensory or motor impairment

Answer 47

Nice to know

Question 48

PRN Clinical remissions can often be achieved with immunosuppressive therapies, such as

Answer 48

glucocorticoids, intravenous immunoglobulin, plasmapheresis, and biologic agents directed against T cells or B cells

Question 49

PRN The _____________distinguish chronic inflammatory demyelinating polyradiculoneuropathy from Guillain-Barré syndrome

Answer 49

time course and the response to steroids

Question 50

PRN T cells as well as humoral factors are implicated in the inflammatory process. Molecules expressed at the ___________________ appear to be the target of the immune response

Answer 50

Schwann cell-axon junction and in noncompact areas of myelin

Question 51

PRN Complement-fixing __________ can be found on the myelin sheath, and the deposition of these opsonins leads to recruitment of macrophages that strip myelin from axons

Answer 51

IgG and IgM

Question 52

PRN show evidence of recurrent demyelination and remyelination associated with proliferation of Schwann cells

Answer 52

Sural nerve biopsies

Question 53

PRN When excessive, this proliferation leads to the formation of so-called ___________—structures in which multiple layers of Schwann cells wrap around an axon

Answer 53

onion-bulbs

Question 54

Systemic autoimmune diseases like _________________) can be associated with peripheral neuropathies that often take the form of distal sensory or sensorimotor polyneuropathies. These neuropathies are distinct from vasculitic peripheral neuropathies, which can arise as secondary manifestations of these same diseases

Answer 54

rheumatoid arthritis, Sjögren syndrome, or systemic lupus erythematosus (SLE

Question 55

is a noninfectious inflammation of blood vessels that can involve and damage peripheral nerves

Answer 55

Vasculitis

Question 56

About one third of patients with vasculitis have peripheral nerve involvement, and __________ may be the presenting feature

Answer 56

neuropathy

Question 57

Vasculitis often presents as

Answer 57

mononeuritis multiplex

Question 58

Peripheral nerves involved by vasculitis typically show

Answer 58

patchy axonal degeneration and loss

Question 59

Infectious neuropathies Many infectious processes affect peripheral nerves. Among these, ______________ cause relatively specific pathologic changes in nerves that are the

Answer 59

leprosy, diphtheria, and varicella-zoster

Question 60

In lepromatous leprosy, Schwann cells are invaded by ____________, which proliferate and eventually infect other cells

Answer 60

Mycobacterium leprae

Question 61

There is evidence of segmental demyelination and remyelination and loss of both myelinated and unmyelinated axons

Answer 61

Leprosy

Question 62

As the infection advances, endoneurial fibrosis and multilayered thickening of the perineurial sheaths occur

Answer 62

Leprosy

Question 63

Lepromatous Leprosy Affected individuals develop a symmetric polyneuropathy that is most severe in the _________________ because lower temperatures favor mycobacterial growth

Answer 63

relatively cool distal extremities and in the face

Question 64

Lepromatous Leprosy The infection prominently involves _________, and the resulting loss of sensation contributes to injury, since the patient is rendered unaware of injurious stimuli and damaged tissues. Thus, large traumatic ulcers may develop

Answer 64

pain fibers,

Question 65

The inflammation injures cutaneous nerves in the vicinity; axons, Schwann cells, and myelin are lost, and there is fibrosis of the perineurium and endoneurium

Answer 65

Tuberculoid leprosy

Question 66

In tuberculoid leprosy, affected individuals have much more

Answer 66

localized nerve involvement

Question 67

causes various neurologic manifestations in the second and third stage of the disease. These include polyradiculoneuropathy and unilateral or bilateral facial nerve palsies

Answer 67

Lyme disease

Question 68

develop several patterns of peripheral neuropathy that are poorly understood, but all appear to be related in some way to immune dysregulation

Answer 68

AIDS

Question 69

Early stage HIV infection can be associated with mononeuritis multiplex and demyelinating disorders that may resemble

Answer 69

Guillain-Barré syndrome or chronic inflammatory demyelinating polyradiculoneuropathy

Question 70

More commonly, later stages of HIV infection are associated with a ______________ that is often painful

Answer 70

distal sensory neuropathy

Question 71

most commonly found in the developing world and is a continuing medical problem because of incomplete immunization or waning immunity in adults

Answer 71

Diphtheria

Question 72

It produces an acute peripheral neuropathy associated with prominent bulbar and respiratory muscle dysfunction, which can lead to death or long-term disability

Answer 72

Diphtheria

Question 73

is one of the most common viral infections of the peripheral nervous system

Answer 73

Varicella-zoster

Question 74

Most common is the involvement of thoracic or trigeminal nerve dermatomes

Answer 74

VZV

Question 75

is the most common cause of peripheral neuropathy

Answer 75

Diabetes

Question 76

Several distinct clinicopathologic patterns of diabetes-related peripheral neuropathy are recognized (described later), but the most common by far is an

Answer 76

ascending distal symmetric sensorimotor polyneuropathy

Question 77

The mechanism of diabetic neuropathy is complex and not completely resolved; both _________________ are believed to contribute to the damage of neurons and Schwann cells

Answer 77

Metabolic and secondary vascular changes

Question 78

Diabetes Pathogenesis Nice to know

Answer 78

Hyperglycemia causes the nonenzymatic glycosylation of proteins, lipids, and nucleic acids. The resulting advanced glycosylation end products (AGEs) may interfere with normal protein function and activate inflammatory signaling through the receptor for AGE. Excess glucose within cells is reduced to sorbitol, a process that depletes NADPH and increases intracellular osmolality

Question 79

Diabetes In individuals with a distal symmetric sensorimotor neuropathy, the predominant pathologic finding is an 

Answer 79

axonal neuropathy

Question 80

Diabetes Distal symmetric diabetic polyneuropathy typically presents with sensory symptoms, like

Answer 80

numbness, loss of pain sensation, difficulty with balance, and paresthesias or dysesthesias

Question 81

Diabetes are so called “positive” symptoms—painful sensations that result from abnormal discharges of damaged nerves

Answer 81

Paresthesias or dysesthesias

Question 82

Diabetes leads to considerable morbidity, in particular an increased susceptibility to foot and ankle fractures and chronic skin ulcers, which may eventually lead to amputations

Answer 82

Neuropathy

Question 83

DM Another manifestation is ________________system; this affects 20% to 40% of individuals with diabetes mellitus, nearly always in association with a distal sensorimotor neuropathy

Answer 83

dysfunction of the autonomic nervous

Question 84

DM Diabetic autonomic neuropathy has protean manifestations, including

Answer 84

postural hypotension, incomplete emptying of the bladder (resulting in recurrent infections), and sexual dysfunction

Question 85

DM Some affected individuals, especially older adults with a long history of diabetes, develop a peripheral neuropathy that manifests with asymmetric presentations, including

Answer 85

mononeuropathy, cranial neuropathy and radiculoplexus neuropathy

Question 86

DM The latter is a devastatingly painful acute disorder that presents in the distribution of the

Answer 86

brachial or lumbosacral nerve plexus

Question 87

Typically this is a distal, symmetric neuropathy that may be asymptomatic or may be associated with muscle cramps, distal dysesthesias, and diminished deep tendon reflexes

Answer 87

Uremic neuropathy

Question 88

Uremic neuropathy In these patients ___________is the primary event; occasionally there is secondary demyelination. Regeneration and recovery are common after dialysis

Answer 88

axonal degeneration

Question 89

Thyroid dysfunction Hypothyroidism can lead to compression mononeuropathies such as carpal tunnel syndrome or cause a distal symmetric predominantly sensory polyneuropathy

Answer 89

Both mono and poly neuropathies

Question 90

Thyroid dysfunction In rare cases, hyperthyroidism is associated with a neuropathy resembling

Answer 90

GuillainBarré syndrome

Question 91

deficiency classically results in subacute combined degeneration with damage to long tracts in the spinal cord (Chapter 28), and also peripheral nerves

Answer 91

Vitamin B12 (cyanocobalamin)

Question 92

Deficiencies of _______ have all been associated with peripheral neuropathy

Answer 92

``` vitamin B1 (thiamine), vitamin B6 (pyridoxine), folate, vitamin E, copper, and zinc ```

Question 93

Toxic neuropathies Important causes of toxic peripheral nerve damage include alcohol (independent of associated nutritional deficiencies), heavy metals (lead, mercury, arsenic, and thallium), and organic solvents but the most notorious are

Answer 93

chemotherapeutic agents

Question 94

Toxic neuropathies , microtubule inhibitors that interfere with axonal transport

Answer 94

vinca alkaloids and taxanes

Question 95

Toxic neuropathies , which may cause a neuronopathy

Answer 95

cisplatin

Question 96

is a common cause of mononeuropathy and may be a presenting symptom of cancer

Answer 96

Direct infiltration or compression of peripheral nerves by tumor

Question 97

General Types of Peripheral Nerve Injury

Answer 97

* Axonal neuropathies * Demyelinating neuropathies * Neuronopathies