What does TMS stand for?
Transcranial Magnetic Stimulation
What does TMS result in?
- Indirect trans-synaptic activation of corticospinal neurons
- Direct Activation of corticospinal neurons
How is TMS measured?
- TMS sends electrical signal through loops to excitable tissue (brain)
- Short but strong electrical signal sent, creating magnetic field which induces ionic currents to excite tissue underneath
When is TMS administered?
- Various time periods of preparing movement
- different neural excitabilities
What does the TMS induce?
- MEP (motor evoked potential)
What does MEP stand for?
- Motor evoked potential
What is the TMS evoked MEP’s amplitude used to measure?
- neural excitability
Describe and Define the motor evoked potential and motor silent period
- MEP: result of activating corticospinal neurons in the primary motor cortex
- Silent period: Refractory period + activation of inhibitory interneurons of motor cortex
Describe the H-Reflex
- Same curcuit as stretch relfex
- Electrical stimulate nerve bundle
- ## High intensity: activate motor neurons in muscle directly creating M-wave (before H-wave)
Describe the basic “monosynaptic” reflex circuit
- electric signal at nerve bundle
- Recruit large sensory afferent
- Sensory afferent synapse recruit motor neuron in spinal cord
- Generates muscle response (H-wave)
Distinguish between mechanisms for stretch reflex and H-reflex
- Stretch reflex when you physically stretch the nerve bundle and the muscle reacts
- H-reflex is when the nerve bundle is electrically stimulated and generates muscle activation
Explain the concept of reflex excitability
- The H-wave amplitude directly reflects the reflex excitability
Describe the time course of corticospinal excitability as measured with TMS prior to and after movement initiation
- Premovement - increased corticospinal excitability
- Postmovement - increase in corticospinal excitability
- Postmovmenet - Decrease in corticospinal excitability
Explain the premovment -increase corticospinal excitability measured with TMS
- Occurred 80ms before emg onset
- Higher activation as it gets to response
- Reaction Time with subthreshold
Describe teh Postmovement increase in corticospinal excitability seen with TMS measurement
- 0-100ms after the emg burst
- M1 excitability still remained baseline, but decreasing
- 100-160ms, second increase in excitability, may be related to subthrshold, second agonist burst
Describe teh Postmovement decrease in corticospinal excitability that is measured with TMS
- Corticospinal excitability is reduced from 550-1000ms after EMG offset
Describe and explain the “dual nature” of corticospinal activation during the temporal preparation for an upcoming action
- Both inhibitory and excitatory are activated during corticospinal action of the temporal preparation stage
- Exticitatory for the movement preparing processes
- Inhibitory to forbid body from initiating the motor program
Explain teh mechanism for concurrent neural activation and suppresion
- Shown by reduction in MEP amplitude (inhibition)
- Shown by shortening silent period (activation)
Describe and discuss the modulation of corticospinal excitability during the inhibition of a prepared action
- MEP of corticospinal activity will gradually increase exponentially until the “go” signal cannot be withhled at around 150ms prior to target
Describe how the H-reflex can be used as a probe for motor preparation
H-reflex can be stimulated during different times of motor preparation in a reaction time task
- Foreperiod interval
- Reaction time interval (prior to movment onset - 25-75ms)
Describe and explain the modulation of spinal excitability during foreperiod and reaction time intervals
During Short, Constant Foreperiod Intervals
- Depression of H-reflex magnitude (supressed response)
- To “tune” motor-neuron pool for upcoming interval
During reaction time intervals
- Facilitation of H-reflex magnitude (facilitated response)
