Lecture 13: Principles of Neurostimulation Flashcards
List the underlying objectives of patterned electrical stimulation applied to motor or sensory pathways in the CNS or PNS
alleviate chronic pain (e.g., gate control theory)
replace lost sensory modalities (hearing, vision, touch)
reanimate paralyzed muscles that remain innervated,
augment voluntary use of partially paralyzed limbs,
restore voluntary use of completely paralyzed limbs,
augment sensory-motor integration (reflex pathways)
modulate activity levels in central pathways (neuromodulation)
assist voluntary control of prosthetic limbs in amputees
What are the underlying objectives to delivering patterned electrical stimulation to the CNS or PNS
alleviate chronic pain (e.g., gate control theory)
replace lost sensory modalities (hearing, vision, touch)
reanimate paralyzed muscles that remain innervated,
augment voluntary use of partially paralyzed limbs,
restore voluntary use of completely paralyzed limbs,
augment sensory-motor integration (reflex pathways)
modulate activity levels in central pathways (neuromodulation)
assist voluntary control of prosthetic limbs in amputees
What are 2 physiological Mechanisms that Control Muscle Force Production
- Orderly Recruitment of Motoneurons/Motor Units (small, oxidative, type 1 (slow) are first to be recruited; size principle)
- Motor Unit Firing Rate Modulation (slow to higher frequency of firing to develop force)
Why do small diameter motor neurons fire first?
If the same current enters both neurons, the generated voltage is higher in smaller motor neurons as it has higher resistance. Thus, will reaches action potential firing threshold
What happens when you increase current in a motoneuron that is already above its threshold for recruitment?
firing frequency increases (rate modulation)
Describe type 1 motor units
recruited first! ^^ - low action potential threshold due to high resistance
oxidative
slow axonal conduction velocity (small axon)
slow muscle contraction time
smaller soma size
higher soma input resistance
larger EPSP size
Describe type 2 motor units
glycolytic
fast axonal conduction velocity (large axon)
fast muscle contraction time
Type FR motor units
fast twitch, fatigue resistance
aka MHC2a
FF
fast twitch and fast fatiguing
MHC2B
What determines the different types of motor units?
Myosin heavy chain isoforms
What are the 4 motor unit types
slow twitch
fast twitch, fatigue resistance
fast twitch, fatigue intermediate
fast twitch, fatigable
how can we externally induce an action potential?
* describe cathodal and anodal nerve stimulation
briefly depolarize membrane by adding negative charges outside the target axons using cathodes
Anions (-) flow from the cathode,
into the tissue, and back to the
anode. Negative charges will accumulate extracellularly, depolarizing the axon membrane
Anodal Nerve Stimulation
Cations (+) flow from the anode, into
the tissue, and back to the cathode.
Positive charges will accumulate extracellularly; the axon
membrane is hyperpolarized
What is the nerve fibre’s response to extracellular stimulation?
What axons are more easily depolarized with electrical stimulation?
electrical stimulation of axons usually causes bidirectional generation of action potentials
large diameter axons are more easily depolarized with electrical stimulation than small axons (Ohm’s Law)
What happens when you bring anode close to nerve fibre?
inverted transmembrane potential observed
transient hyperpolarization can result in anodal break excitation (membrane depolaration and action potential generation not right next to anode but some distance away and in both directions)
In paralyzed muscle, what do all motor units convert to?
How does electrical stimulation therapy help with this?
fast type, fast fatiguing (MHC-2B);
electrical stimulation rebuilds muscle force capacity, and converts it back to slower types. increasing its resistance to fatigue.
List the 4 Controllable Electrical Stimulation Parameters
- Stimulation Pulse Amplitude (in V or better, in mA); better to measure in mA (current); voltage depends on resistance
- Stimulation Pulse Duration (in μS or mS)
- Stimulation Pulse Polarity (-/+)
- Inter-pulse Interval (Stimulation Frequency). If stimulation frequency is too high, you may catch a neuron during an absolute refractory period thus it won’t be excitable.
Define: rheobase value/threshold current
the minimal electric current required to excite a tissue (as nerve or muscle) given an indefinitely long time during which the current is applied
What are the factors that affect the rheobase value for a motorneuron
- Electrode position on skin
- Skin resistance
- Subcutaneous fat layer
- Edema
- Denervation (will atrophy –> smaller muscle fibre also means more difficult to stimulate; no neuron to stimulate. less easy to stimulate by direct stimulation) /reinnervation (lower threshold for stimulation?)
Why are shorter excitation pulses safer?
- less electrode corrosion (less rust) and
- less tissue damage.
Explain the difference between physiological vs electrical recruitment
- When the motoneurons in a pool are physiologically recruited, SMALLER neurons tend to be recruited FIRST.
- When external electrical stimulation is applied to a nerve, the LARGEST axons
tend to be recruited FIRST. - added current is easier to enter the large axons with lower resistance.
group 1As will be recruited first.
What is the strength duration curve?
relates the intensity of a threshold stimulus to its duration. As the duration of a test stimulus increases, the strength of the current required to activate a single fiber action potential decreases
current strength that lies above the curve and along the curve causes stimulation. anything below the curve is subthreshold and won’t cause stimulation
if you make the pulse width longer, you can get away with using a smaller current
chronaxie
the minimum time required for excitation of a neuron by a constant electric current of twice the threshold voltage
Charge balanced biphasic stimulation. Why is this needed?
negative and positive phases of charge.
“Zero Net Charge” is required to minimize electrode corrosion and tissue damage.
you want the second pulse to fall in the refractory period so it does nothing
Describe the charge-duration curve
Demonstrates how charge injected into tissue increases with pulse width (amplitude x pulse width = charge). To minimize charge injection, shorter pulses are recommended.
Also minimzes, electrode corrosion and tissue damage