Neuroanatomy L6: Mechanisms that underlie movement changes with pain Flashcards
Are there limitations to using SEMG to assess changes in SMU discharge during an acute pain experiment? Explain your answer
EXAM QUESTION
- Yes!
- Other muscles which help to maintain the task (being picked up- potnetial cross talk)
- SEMG picking up from a large surface area of muscle. This is a problem because it doesn’t pick up redistribution (or changes in force)
- (HINT EXAM: why can’t it pick up redistribution)= summation of AP from multiple motor units (some that decrease or being recruited but might be the same EMG- not picking up changes
There are many motor adaptations that are possible during acute pain. Describe the changes in SMU discharge that are observed during acute pain and if these changes are associated with a decrease in load within the painful part.
EXAM QUESTION
- Decraese in discharge rate (certain motor neurons- on average- due to pain)
- Some discharge faster
- Newly recruited (specific with pain)
- No longer recruited (only in control
- If simple adaptation (eg. nervous system can use other side- bilateral) = decrease stress with pain
- Not always see decrease in painful part (eg. simple knee extension)
- Can see (in RF) because other muscles can take over and compensate
By 2020: 1/7 Australians expected to be living with chronic pain. In addition, 62% chronic pain burden related to _______ / ____ injury
musculoskeletal problems; acute
Movement changes with acute and chronic pain well described. Give an example of how gross movements change with pain in leg.
Gross movement changes, like limping when you have injured your leg.
How does movement changes due to pain have a short and long term effect?
Short term: benefit (avoid pain / protect painful part)
Long term: Contribute to long term persistence or reoccurrence of pain e.g altered activation patterns linked to more rapid progression KOA.
short term adaptations that have long term consequences
What are the 2 electromyography (EMG) types?
- Surface electrode
- Fine wire
In which type of ECG is the signal messier? Why?
The signal is messier in the recording from the surface electrodes - this is due to picking up electrical activity from a larger area of the muscle (and possibly from other muscles) underneath the electrodes.
What effect does recruitment of motor units have on the discharge rate?
Increases discharge rate
Every time a motor unit discharges, a little bit of force is produced. As more force is produced, the discharge rate of the motor units _______ (increases/decreases) (shown in the space between action potentials on the graph), and there is also recruitment of _____ motor units.
increases; new
What is Lund’s pain adaptation theory?
- Group III and IV afferent fibres stimulated by nociceptive input
- Inhibition of agonists (decrease activity) and excitation of antagonists (increase activity)
- Decreased amplitude and velocity of movements to reduced further risk of injury
In animal studies, when the cortex is removed, there is no need to maintain the task. However, what happens when you need to maintain the task during pain?
We don’t see the same changes occur.
Human studies, intact cortex - not all literature supports the pain adaptation theory. In some studies there is an ____ effect, or no difference at all. There is a lot of ______ in surface EMG recordings of people in pain.
opposite; variability
What is a motor unit?
Motor unit = motor neurons in the spinal cord, the axon, and all of the muscle fibres it innervates.
Motor unit discharge rate is _____ (high/lower) during force matched contractions with pain. What happens to the motor units when biceps is injected with salty water to induce pain while still produced the same amount of force?
lower
eg. biceps contract at 10Hz and motor units recruited once salty water is injected and contraction occurs again at 10Hz = decreased motot units
How is force maintained if motor unit discharge rate is lower during pain? List 3 ways that force is not maintained by?
Thus it doesn’t fully support that there is a generalised inhibition of the agonist muscles (Lund’s theory of pain adaptation).
Additional changes in motor unit recruitment
- Adjacent body segments / posture
- Synergist muscle activity
- Muscle fibre membrane properties
4.
Is force maintained by adjacent body segments / posture to compensate for the decrease in motor unit discharge rate?
NO
In order to examine force produced by a specific muscle, we need to ensue that POSTURE is well maintained in order to reduce compensation techniques to produce the force measured.
Is forced maintained by synergist muscle activity to compensate for the decrease in motor unit discharge rate?
NO
same discharge rate in synergist muscle group
There is good evidence that it is not just a synergist muscle taking over from an injured muscle.
Is force maintained by muscle fibre membrane properties to compensate for the decrease in motor unit discharge rate?
NO
The changes in membrane properties don’t relate exactly to the changes in force with reduced single motor discharge rate.
Is force maintained by additional changes in motor unit recruitment to compensate for the decrease in motor unit discharge rate?
YES
maintaining force (recruiting more and new motor units) with decreased discharge rate
ADD
ADD
There is recruitment of ____ motor units during pain. Why?
This recruitment of new motor units was to help maintain the force in the pain condition.
There is a(n) _______ (increase/decrease) in discharge rate during pain?
decrease
The adaptation the occurred during the acute pain was maintained in_______ of motor units after the pain ceased.
50%
How do the changes in motor unit recruitment during pain influence surface EMG measures?
Both increased and decreased SMU activity with pain
(SEMG = summation of all SMU activity)
Pain adaptations may be overlooked using SEMG
What are the changes in EMG with surface electrodes during pain?
No change during pain (while using surface electrodes)
What are 2 changes in EMG (discharge rate and recruitment of motor units) with surface electrodes during pain?
- Single motor unit decreased discharge rate - shown by the fine wire electrodes.
- Increase in recruitment of motor units to maintain the force, despite loss of discharge rate.
What does it mean that the surface electrode EMG shows no change, while the fine wire showed changes?
Surface EMG wasn’t sufficient to show the changes found with fine wire EMG.
Not a simple generalised inhibition of agonist muscles during pain. Why?
There are research implications
SEMG not sensitive to altered motor unit recruitment during pain
Care when interpreting EMG in pain studies
What drives altered motoneurone discharge during pain? List 2.
- Input to motoneurone pool from nociceptive stimulation
- Changes to descending drive proposed but not tested e.g. pain inhibitory system, motor cortex (Changing the way the brain is controlling movement)
Does motoneurone discharge change during anticipation of pain when there is no nociceptive afferent stimulation?
(Note: same central mechanism- but not peripheral nociceptive input)
Yes….. Higher centers likely involved with altered discharge during pain (not simply spinal)
The same changes in motor unit recruitment were shown in the anticipation and real pain conditions.
Is there a change in motor response during anticipated pain?
Gait, reaction time, postural responses
When we anticipate pain (but there is no actual pain present) there is a change in the motor response and the way we move.
______ (increase/decrease) of discharge rate of single motor units (in anticipation and pain)
decrease
Some motor units (above line of equality)- discharge ____ (faster/slower) with pain and anticipation
faster
Some motor units (grey area)- discharge during control but not in ____ and ______.
control; pain; anticipation
______ recruited during pain (not during control)
Neurally
Why is motor unit discharge altered during pain? Is there a purpose or is it an epiphenomenon?
Something that is happening in the body, but not driven to make a change for any particular purpose.
What is hypothesis 1 relating to larger units: protective & faster force production? What does it mean?
greater nociceptive input to smaller neurons
If smaller motor units are inhibited during pain (ie. a threatening situation) maybe this allows us to recruit the larger motor units to produce more force quicker (ie. as an evolutionary trait).
What is hypothesis 2 relating to alter load within painful part: protective? What does it mean?
changes in biomechanics with pain –> protect injured part
Is recruitment of new motor units during pain associated with a change in force direction?
Yes
What are elastography? How does it work?
Able to measure stiffness within tissue
Mechanical pertubation >> propagation of mechanical waves along the length of the tissue.
Velocity of the waves direclty related to tissue stiffness
During single joint task, stress _____ (is/is not) systematically altered within the painful part *absolute change*. Thus, is stress reduced in the painful part?
is not
Everybody did change a little bit - there was some increases and some decreases.
Not clear- to support hypothesis
Are the motor adaptions to acute pain task dependent? (clear alternative, during a bilateral task)
Hypothesis was that if someone did a task using two legs, and pain was in one leg, that the subject would compensate by using the unaffected leg more.
Are there motor adaptions to acute pain task dependent? (clear alternative, during a bilateral task), specifically cycling at 130w and 90rpm (multi-joint task)?
Pedal power and EMG decreased in the painful limb
There are lots of different ways to adapt in this situation by changing movement at the different joints during multi-joint tasks
Are there motor adaptions to acute pain task dependent? (tasks with differing degrees of freedom)- knee extension
Slight decrease in stress of VM
Are the motor adaptions to acute pain task dependent? (tasks with differing degrees of freedom)- single leg squat?
Didn’t see a significant decrease in stress in VM
Are the motor adaptions to acute pain task dependent? (tasks with differing degrees of freedom)- bilateral leg squat
In the bilateral squat task there was a significant decrease in stress in the VM. Shows between leg compensation.
Are the motor adaptions to acute pain task dependent? (neural control of muscles)
No change in stiffness in VM
Decrease in stiffness in RF
Compare adaption when pain is induced into VM or RF.
The VM and VL have high common drive - the same information goes to both muscles to produce the same motor task.
But the rectus femoris does not have the same amount of common drive.
RF unable to compensate for vastii
Vasti able to compensate for RF
Volume of RF is only ___% of Vasti. What does that mean?
16
This might mean that it doesn’t have the same ability to compensate for the vasti muscles, because of the smaller size (reduced ability to produce force).
What happens to stiffness with pain in the VM?
No change in stiffness - because the rectus femoris can’t compensate for the vasti muscles.
What happens to stiffness with pain in the RF?
Decrease in stiffness - ability of the vastus muscles to compensate. pain in RF –> had some decrease in shear modulus
In contrast to a generalised inhibition during pain: Redistribution of activity within and between muscles during _____ pain and ___ of pain. This does not necessarily _____ after pain has ceased. Redistribution not observed using SEMG, and changes in SEMG do not necessarily reflect changes in ___. This is associated with a “______” change in force and stress (we do change, but not all in the same way). Systematic ____ (increase/decrease) in stress/force when a clear option to compensate is available. The pain adaptation needs to be considered with direct reference to constraints related to the ______.
acute; anticipation; resolve; force; absolute; decrease; task
