Peripheral Nervous System Flashcards
What forms the peripheral nervous system?
- axons of sensory, motor, and autonomic neurons
- specialized sensory endings
- entire postganglionic autonomic neruorns
What do peripheral nerves consist of?
parallel bundles of axons surrounded by three connective tissue sheaths: endoneurium, perineurium, and epineurium
What connective tissue separates individual axons?
endoneurium
What connective tissue surrounds fascicles?
perineurium
What connective tissue encloses the entire nerve trunk?
epineurium
What do peripheral nerves supply?
both the viscera (autonomic) and somatic structures
Are somatic peripheral nerves usually sensory, motor, or mixed?
mixed
What do the cutaneuous branches of somatic peripheral nerves supply?
the skin and subcutaneous tissues
What do the muscular branches of somatic peripheral nerves supply?
the muscles, tendons, and joints
What are peripheral axons classified according to?
speed of conduction and diameter
What do the junctions of anterior rami form?
nerve plexuses
What are the 4 nerve plexuses?
- Cervical
- Brachial
- Lumbar
- Sacral
The cervical plexus arises from the anterior rami of C_ to C_
C1 - C4
What does the cervical plexus supply?
- sensory information from the posterior scalp to the clavicle
- motor innervation of the anterior neck muscles and diaphragm
The brachial plexus arises from the anterior rami of C_ to T_
C5 - T1
What does the brachial plexus supply?
The entire motor and sensory innervation of the upper limb
The lumbar plexus arises from the anterior rami of L_ to L_
L1 - L4
What does the lumbar plexus supply?
The skin and muscles of the anterior and medial thigh
The sacral plexus arises from the anterior rami of L_ to S_
L5 - S4
What does the sacral plexus supply?
the posterior thigh and most of the leg and foot
Why is movement essential for nerve health?
It optimizes the health of nerves by promoting blood flow throughout the nerves and the flow of axoplasm through the axons
What does movement cause the axoplasm to do?
thin and flow more easily, facilitating axoplasmic transportation
Where do motor axons synapse with muscle fibers?
at neuromuscular junctions
Is summation of action potentials is required to depolarize the postsynaptic membrane at the neuromuscular junction?
No
What are the 3 signs of peripheral nerve damage?
- Sensory
- Autonomic
- Motor
What are the 4 types of abnormal sensations?
- Hyperalgesia
- Dysesthesia
- Paresthesia
- Allodynia
Following peripheral nerve damage, what do autonomic changes depend on?
the pattern of axonal dysfunction
If a single nerve fiber is damaged when do signs occur?
Only when the nerve is completely severed
If many peripheral nerves are damaged, what problems may arise?
- difficulty regulating blood pressure
- heart rate
- sweating
- bowel and bladder functions
- impotence
Motor signs of peripheral nerve damage include what?
paresis or paralysis
Following peripheral nerve damage what occurs?
- rapidly progressing muscle atrophy
- muscle fibers begin to develop a generalized sensitivity to ACh along the muscle membrane
What occurs when muscle fibers begin to develop a generalized sensitivity to ACh along the muscle membrane?
Fibrillation, which is spontaneous contraction of individual muscle fibers
How are fibrillations observed?
Only through needle EMG, not on the skin
What types of tropic changes occur when a peripheral nerve is denervated?
- muscles atrophy
- skin becomes shiny
- nails become brittle
- subcutaneous tissues thicken
Due to blood supply changes, loss of sensation, and lack of movement following peripheral nerve damage what is a common occurrence?
- ulceration of cutaneous and subcutaneous tissues
- poor healing of wounds and infections
- neurogenic joint damage
Describe Mononeuropathy
It is a form of peripheral neuropathy that involves a single nerve and is considered a focal dysfunction
Describe Multiple mononeuropathy
It is a form of peripheral neuropathy that involves several nerves and is considered a multifocal dysfunction
Describe Polyneuropathy
It is a form of peripheral neuropathy that involves many nerves and is a generalized disorder that typically has a distal and symmetrical presentation
What types of traumas may injure peripheral nerves?
- repetitive stimuli
- prolonged compression
- wounds
What are the 3 classifications of traumatic peripheral nerve injury?
- Traumatic myelinopathy
- Traumatic axonopathy
- Severance
What does traumatic myelinopathy refer to?
The loss of myelin limited to the site of injury
Peripheral myelopathies interfere with what?
the function of large-diameter axons
What causes traumatic myelinopathy?
Focal compression of a peripheral nerve (such as nerve entrapment or repetitive mechanical stimuli)
What syndrome is a common form of traumatic myelinopathy?
carpal tunnel syndrome
What does traumatic axonopathy disrupt? What does this cause?
axons, which causes wallerian degeneration distal to the lesion
What size of axons are affected by axonopathies?
All sizes of axons, which means reflexes, somatosensation, and motor functions are all significantly reduced or absent.
Following traumatic myelinopathy are axons able to regenerate? Why or why not?
Yes, because myelin and connective tissues remain intact.
When does traumatic severance occur?
when nerves are physically divided by excessive stretching or a laceration
What does traumatic severance result in?
The immediate loss of sensation and/or muscle paralysis in the area supplied
Following traumatic severance can the peripheral nerves regenerate?
Yes, if the proximal and distal nerve stumps are apposed and scarring does not interfere
What does multiple mononeuropathy involve? What does this produce?
Two or more nerves in different parts of the body, which produces a random, asymmetrical presentation of signs
When multiple mononeuropathy typically occur?
When diabetes or vasculitis (inflammation of blood vessels) cause ischemia of the nerves
What are the hallmark signs of polyneuropathy?
symmetrical involvement of sensory, motor, and autonomic fibers, often progressing from distal to proximal
Where do symptoms of polyneuropathy typically occur?
In the feet and then appear in the hands. These are the areas of the body supplied by the longest axons
Are polyneuropathies the result of trauma or ischemia?
No
Polyneuropathies can be of what 3 origins?
- toxic
- metabolic
- autoimmune
What are the most common causes of polyneuropathies?
- diabetes
- nutritional deficiencies secondary to alcoholism
- autoimmune diseases
Polyneuropathy in what syndrome is characterized by more severe effects on the motor system than the sensory system?
Guillain-Barré
What is the major difference between Guillain-Barré polyneuropathy and most neuropathies?
Paresis is worse proximally
What is the most commonly inherited form of polyneuropathy?
Charcot-Marie-Tooth disease
What does Charcot-Marie-Tooth disease typically cause?
Paresis of muscles distal to the knee with resulting foot drop, step-page gait, frequent tripping, and muscle atrophy
What is the typical onset of Charcot-Marie-Tooth disease?
adolescence or in young adults
What is Myasthenia gravis?
an autoimmune disease that damages ACh receptors at the neuromuscular junction, repeated use of a muscle leads to increasing weakness.
What are myopathies?
disorders intrinsic to muscles
What is an example of a myopathy?
Muscular dystrophy
Describe muscular dystrophy
Random muscle fibers degenerate, leaving fewer muscle fibers than normal which results in less force being produced than by a healthy motor unit
Are sensation and autonomic function affected in muscular dystrophy? Why or why not?
No, because the nervous system is not affected by myopathy
Are coordination, muscle tone, and reflexes affected in muscular dystrophy?
Not until muscle atrophy becomes so severe that muscle activity cannot be elicited.
Dysfunction of peripheral nerves and the muscles they innervate can be evaluated by what?
electrodiagnostic studies
How can electrodiagnostic studies be diagnostic?
By recording electrical activity from nerves and muscles by nerve conduction and EMG studies reveal the pathologic location
Nerve conduction studies can be used to differentiate among what 4 things?
- Primarily myelinopathy and axonopathy processes
- Upper motor neuron and lower motor neuron paresis
- Mononeuropathy and polyneuropathy
- Local conduction block and Wallerian degeneration
What can differentiate between nerve and muscle disorders?
Electromyography (EMG)
How can myelinopathies be diagnosed via motor nerve conduction studies?
Indicated by long distal latency times, slow conduction velocity, and normal amplitude
How can axonopathies be diagnosed via motor nerve conduction studies?
Indicated by severely prolonged distal latency and a marked decrease in amplitude. Conduction velocity is normal.
What are the 3 clinical signs of peripheral neuropathy revealed in patients 50 years of age and older?
- Absence of ankle jerk reflex despite facilitation
- Impaired vibration sense of the great toe
- Impaired position sense of the great toe
- presence of two or three signs correlate highly with electrodiagnostic evidence of peripheral neuropathy
What increases as the severity of neuropathy increases?
reports of a pins and needles sensation
What should individuals with peripheral neuropathy that affects sensation do daily?
inspect the involved areas to monitor for wounds and for a reddening of the skin that persists more than a few minutes
What has endurance exercise following peripheral nerve damage been shown to do?
enhance sensory and motor recovery
What has resistance training following peripheral nerve damage been shown to do?
delay functional recovery
What devices are frequently used to stabilize weight-bearing joints following peripheral nerve injury?
Orthoses
