Sensory responses and expectancy wave
The electrophysiological analysis of voluntary movement is based on evaluating changes in cerebral cortex activity. When a person passively perceives isolated stimuli (for example, simply hears a click or sees a flash of light), sensory evoked potentials are recorded on the tracing. These are short bursts of activity reflecting simple signal processing without goal formation or movement preparation (there is no slow wave between stimuli).
A completely different picture is observed when an acoustic signal becomes a warning signal, and a subsequent visual signal requires mandatory action (e.g., pressing a button). In the interval between these two events, an expectancy wave (W. Grey Walter's phenomenon), also known as the contingent negative variation (CNV) or the "E-wave" (Expectancy wave), forms in the association areas (predominantly in the frontal cortex).
The dynamics of this process unfold as follows:
- Following the warning acoustic signal, a sharp peak appears — the auditory evoked potential.
- Immediately afterwards, the curve begins to rise smoothly, reflecting a slow increase in negative potential.
- The potential reaches its maximum strictly by the time of the trigger signal (the light flash).
- After the required action is performed (pressing the button), the potential drops sharply.
The clinical significance of the CNV lies in the fact that the amplitude of this wave is a direct indicator of the level of attention and the degree of readiness of a person for goal-directed action.
Preparation for movement: readiness potential
The next crucial stage directly preceding the motor act itself is the generation of the readiness potential (Bereitschaftspotential). It reflects the neuronal activation required to trigger muscle contraction.
Unlike local frontal expectancy phenomena, the readiness potential is recorded across the entire surface of the head before the onset of a voluntary action, although its maximum expression is observed in the motor cortex. EEG dynamics show that throughout most of the resting period, the isoelectric line remains stable. A sharp upward surge in electrical activity begins several hundred milliseconds before the initiation of movement (e.g., before a finger flexion).
This phenomenon proves that the motor cortex begins to form the command long before the actual contraction of muscle fibers occurs, preparing the necessary neuronal pools in advance.
Action initiation and the cellular level
The final stage of cortical electrophysiological activity is the motor potential. Unlike the diffuse readiness potential, the motor potential is a strictly local response. It arises in a specific area of the motor cortex (the precentral gyrus) that anatomically corresponds precisely to the active limb.
To study the electrophysiology of the motor cortex in detail, the method of single-unit recording of individual neurons using implanted microelectrodes is often used. In classical experiments, primates (monkeys) performing voluntary hand movements, such as flexion or extension against an external load, serve as the subjects.
When graphically displaying the results of such studies, two key parameters are compared:
- Mechanogram: shows the lever movement trajectory and the change in spatial position of the limb.
- Electroneurogram: records the electrical activity of the motor cortex, where individual action potentials (spikes) appear as vertical strokes.
Synchronous analysis of these indicators allows for an accurate determination of the relationship between individual motor cortex neuron discharges and the biomechanical characteristics of the developing muscle contraction.
Briefly: answers to topical questions
What is the role of the basal ganglia and cerebellum in generating the readiness potential and planning movement?
The basal ganglia and cerebellum participate in the initiation, planning, and processing stages of a voluntary movement program. Excitation from the association areas of the cortex is transmitted to the cerebellum and basal nuclei, after which impulses pass through the thalamus to the motor cortex, where the readiness potential is recorded before movement begins.
Functions of these structures:
- Basal ganglia — ensure initiation, complex patterns, and control of movement intensity.
- Cerebellum (Cerebellum) — performs movement sequence planning, timing, coordination, and error correction by comparing the plan with the actual body position.
According to what rule are alpha motor neurons recruited during muscle contraction generation?
Alpha motor neurons are recruited during muscle contraction according to Henneman's size principle. As muscle contraction force increases, motor units are recruited in strict order from smallest to largest.
Recruitment sequence:
- Small neurons (ME-I) — have a high input resistance and depolarize faster under weak synaptic input.
- Intermediate neurons (ME-IIA) — are recruited as contraction force increases.
- Large neurons (ME-IIB) — require powerful stimulation to reach the activation threshold.
This mechanism ensures smooth movements and energy conservation.
Remember the sequence from passive perception to action: Evoked (evoked potential to sound) → Expectancy (CNV in the frontal cortex) → Readiness (potential across the entire head) → Motor response (locally in the precentral gyrus).
Class and exam questions
What does the contingent negative variation (CNV) reflect?
It reflects the processes of expectancy, focusing attention, and preparing the subject for a goal-directed action in the interval between the warning and trigger signals.
What is the difference between a sensory evoked potential and an expectancy wave?
An evoked potential is a short burst of activity during passive perception of a stimulus. An expectancy wave is a slow increase in negative activity during preparation for a response action.
Where is the motor potential localized?
This is a strictly local response recorded in the precentral gyrus (the motor area of the cortex) in the area corresponding to the active limb.
What else to study on this topic
- Clinical significance of the expectancy wave amplitude (W. Grey Walter)
- Physiological meaning of the readiness potential (Bereitschaftspotential)
- Topography of EEG indicators: from the frontal cortex to the precentral gyrus
- Method of microelectrode recording of single-unit activity
- Comparison of mechanogram and electroneurogram data during voluntary movements
Master the entire topic in the "Physiology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Electrophysiology of the Motor Act" and 4,128 more subject pages
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 114 video lessons on the subject
Other topics in the "Motor Control" section
Free video lessons: PhysiologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.