Week 5.1 Flashcards
What are the benefits of using intracellular methods to record the electrophysiology of neurons?
- precise measurements of a single axons
- insight into ionic mechanisms responsible for membrane potential
How does nerve conduction relate to diameter?
directly proportional
What are the constants for the relationship between axon diameter and conduction velocity for myelinated and unmyelinated axons?
- myelinated: 6
- unmyelinated: 1.7
How do the amplitudes of extracellular and intracellular APs vary with neuronal cell size?
- intracellular don’t vary with size
- extracellular increases with increasing cell size
In what order are neurons recruited by an artificial electrical stimulation?
largest to smallest
In what order are neurons recruited by a natural electrical stimulus?
smallest to largest
Name five properties of neurons that vary with axon diameter.
- conduction velocity
- amplitude of extracellularly recorded action potentials
- order of artificial activation (inverse)
- order of natural activation
- principles of anesthesia
What form of anesthesia affects smaller diameter axons first?
local anesthetics
Name four forms of anesthesia that affect larger axons first.
- temperature
- compression
- positive charge (anodal block)
- asphyxia/anoxia
Compound action potentials are the result of what electrophysiologic principle?
differing conduction velocities for axons of different sizes
How does a compound AP recording vary with distance form the stimulus?
the closer you are, the less separation there is between peaks
How does stimulus intensity affect the shape of an intracellular and extracellular AP?
- doesn’t affect intracellular because it’s an all or nothing event
- extracellular show increased amplitude and number of peaks
A-alpha fibers carry what information?
muscle spindle and GTO afferents
A-beta fibers carry what information?
- secondary muscle spindle
- skin touch and pressure
A-delta fibers carry what information?
- skin touch
- temperatura
- fast pain
- autonomic afferents
C fibers carry what information?
- autonomic afferents
- temperature
- slow pain
Which fiber type is most common in the dorsal root?
most C fibers, equal numbers of the others
What fiber types are found in the ventral root?
- A-alpha (LMN)
- A-gamma
- B (autonomic preganglionics)
- C (ventral root sensory axons)
What are B fibers in the dorsal root?
autonomic preganglionics
What are C fibers in the dorsal root?
ventral root sensory axons
What is the Bell-Majendie Law?
states that all sensory axons enter via the dorsal root and all motor neurons exit via the ventral root
What is the one exception to the Bell-Majendie Law?
group C fibers from pelvic viscera at L5-S3 with a DRG cell body but enter the spinal cord via the ventral root
Why does transection of the dorsal root not always eliminate pain from that dermatome?
because some C fibers enter the spinal cord through the ventral root
What three waves might you see during a motor nerve conduction study?
- M wave
- F wave
- H reflex
Which is seen in a motor nerve conduction study after maximal stimulus: M wave, F wave, or H reflex?
M wave and F wave
What is orthodromic?
means in the physiologic or normal direction
Why can most people not just raise half their forehead?
because the LMNs for the forehead receive bilateral UMN innervation in contrast to lower motor neurons for the lower face which receive unilateral innervation
How does -paresis differ from -plegia?
- paresis describes partial paralysis
- plegia describes no movement at all
What is paraplegia?
the inability to move both legs
How is muscle strength graded?
5: full strength
4: gravity + resistance
3: antigravity
2: joint movement
1: visible contraction
0: no contraction
How are muscle stretch reflexes graded?
0: no reflex
1: reflex only with reinforcement
2: normal
3: brisk
4: clonus
How can you assess muscle tone in clinic?
by passively moving the limb and judging resistance
Where could a lesion be located that would cause UMN signs in the face and leg?
it would have to be multiple lesions
Where could you find lesion that causes paraparesis?
- thoracic spinal cord affecting both UMN tract
- parasagittal region (ACA stroke)
If someone has generalized LMN weakness, the addition of numbness tells you what?
it is a peripheral neuropathy (afferents are being affected too)
If someone has LMN weakness and deep tendon reflexes are affected, what does this tell you?
that the lesion is localized to the anterior horn cell or peripheral nerve, not at the NMJ or in the muscle
Processes that cause generalized LMN weakness with normal sensation and deep tendon reflexes are typically located where?
the NMJ or muscle
What is motor neuron disease?
a generic term (aka ALS) used to describe a disease process that affects both UMNs and LMNs
ALS affects which cell population?
both upper and lower motor neurons
What are the primary features of ALS?
- bilateral, asymmetric weakness
- UMN and LMN signs
- normal sensation
- bulbar dysfunction (dysarthria or dysphagia)
What is dysarthria?
a motor speech disorder
How can you distinguish between a LMN problem that affects the NMJ or the muscle itself?
creatine phosphokinase typically elevated if problem is with the muscle
What signs and symptoms would point you to believe someone with generalized motor weakness had a myopathy?
- proximal weakness
- elevated CPK
- normal EMG
When there is diffuse muscle weakness, how can we localize it to the anterior horn cell, peripheral nerve, NMJ, or muscle?
- anterior horn cell: diminished reflexes, no numbness
- peripheral nerve: diminished reflexes and numbness
- NMJ: reflexes intact and CPK normal
- muscles: reflexes intact and CPK elevated
Antibodies target what in those with myasthenia gravis?
nicotinic AChRs or associated proteins
Describe the weakness that affects those with myasthenia gravis.
- fluctuating
- improves with rest
- propensity for extra-ocular muscles
What kind of inhibition occurs at the NMJ in those with myasthenia gravis?
competitive inhibition
The pathophysiology of myasthenia gravis depends on what two processes?
competitive inhibition and inflammatory changes
Myasthenia gravis typically affects those of what age?
those in their 20s and 50s
What gender differences are their in the onset of myasthenia gravis?
- early onset more common in females
- later onset more common in males
Describe the motor symptoms of of myasthenia gravis.
- ocular: diplopia, ptosis
- bulbar: dysarthria, dysphagia, dyspnea
- extremities: proximal weakness and head drop
What is ptosis?
eyelid droop
All new myasthenia gravis patients get checked for what other condition?
thymoma
Which myasthenia gravis patients are most likely to also have a thymoma?
those with onset after the age of 30
The most specific test for myasthenia gravis is what?
AChR antibody testing
The most sensitive test for myasthenia gravis is what?
single-fiber EMG
What is an endrophonium test?
- test for myasthenia gravis in which you block AChE and look to see if the patient transiently improves
- requires an objective measure like degree of ptosis
How does repetitive stimulation work as a diagnostic test for those with myasthenia gravis?
repeated stimulation depletes ACh stores and the amplitude of the extracellular AP will decrease only in those with myasthenia gravis
What are the cons of single-fiber EMG testing for myasthenia gravis?
it is painful and difficult to place the electrode
How is single-fiber EMG performed to test for myasthenia gravis?
- measure single muscle fiber action potentials from two in the same motor unit
- variability in the timing of the firing suggests myasthenia gravis
The mainstay therapy for myasthenia gravis is what?
immunosuppression (steroids)
What is pyridostigmine?
an AChE inhibitor with a long half life used in to treat myasthenia gravis before the advent of steroids
What time course can a patient with myasthenia gravis expect when they begin corticosteroids?
- transient worsening
- improvement between 2 and 6 weeks
- maximal effect at 6 months
- remission is steroid dependent
Which chemotherapy drugs are effective in the treatment of myasthenia gravis?
- azathioprine
- cyclosporine
- cyclophosphamide
Name five medical therapies for myasthenia gravis.
- steroids
- anticholinesterases (pyridostigmine)
- chemotherapy (azathioprine, cyclosporine, cyclophosphamide)
- IVIG
- plasmapheresis
What are the indications for plasmapheresis in patients with myasthenia gravis?
- myasthenic crisis
- severe exacerbation
- pre-thymectomy
What are the main pro and con of thymectomy in the treatment of myasthenia gravis?
- demonstrate significant improvement
- major surgery
