Stages of Motor Reaction Formation
Before a muscle contracts, covert processes take place in the higher divisions of the central nervous system. Any movement begins with afferent synthesis, followed by decision-making and the formation of a general behavioral goal.
The process of involving brain structures occurs strictly in a cascade and is divided into three key stages:
- Initiation and Planning (Stage A). Associative areas of the cerebral cortex are activated. They transmit the initial excitation to the subcortical level — to the cerebellum and basal nuclei.
- Program Processing (Stage B). Neurons of the cerebellum and basal nuclei process the received movement "draft". From them, impulses are directed to the thalamic nuclei, which serve as a relay station, and from there the corrected signal returns back to the motor cortex.
- Execution (Stage C). Motor areas of the cortex are activated. Descending excitation is transmitted to the motor neurons of the brainstem and spinal cord for the direct execution of movement.
Three neural circuits of motor control
To make a movement smooth, precise, and proportional, the brain uses three interacting loops.
Coordination Loop (Cortex — Cerebellum) Provides planning, temporal coordination, and real-time error correction.
- Via the corticoponto-cerebellar pathway (through the pontine nuclei — pons), data arrays from the motor, premotor, and somatosensory cortex enter the cerebellum (cerebellum). This is a kind of copy of the motor command.
- The cerebellum compares the planned action with actual proprioceptive information from the spinal cord, calculating the error.
- Via the thalamocortical feedback loop (from the dentate nucleus of the cerebellum through the ventrolateral thalamic nuclei), the corrective signal returns to the motor cortex.
Regulatory Loop (Cortex — Basal Ganglia) Responsible for movement initiation and muscle tone regulation. Includes the striopallidal system (caudate nucleus, putamen, globus pallidus). The basal ganglia exert an inhibitory or disinhibitory effect on the motor cortex via the thalamus, thereby "permitting" or "prohibiting" the initiation of programs. They also have descending pathways to the brainstem (red nucleus, reticular formation) to control background tone and automated actions.
Executive Loop (Direct Pathway) This is the pyramidal system. Excitation is transmitted from the primary motor cortex (precentral gyrus) directly via the corticospinal tract to the alpha motor neurons of the anterior horns of the spinal cord. This pathway is what provides fine, discrete movements of the fingers and hands.
Interaction of posture and local movement
Under natural conditions, an animal or a human does not perform movements in isolation — any action requires a preliminary postural adjustment. This is well illustrated by a classical experimental model in a dog.
A dog was trained to bend its hind limb in response to a sound (with pain reinforcement). Normally, upon hearing the sound, the animal first shifts its weight to three limbs (postural movement), creating support, and only then lifts the fourth (local goal-directed movement).
Experimental dissociation demonstrated their different nature:
- If pain reinforcement is canceled or the motor cortex is removed, the local movement (limb lifting) completely disappears.
- However, the postural movement (weight shift) is preserved.
This proves that posture maintenance is formed with the participation of subcortical structures. Postural excitation spreads significantly faster than local excitation and is more inert. In an integrated behavioral act, these two mechanisms are seamlessly combined thanks to a unified dynamic efferent synthesis.
Briefly: answers to topical questions
Which descending brainstem tracts transmit signals from the basal ganglia to control background tone?
Signals from the basal ganglia to control background tone are transmitted via descending pathways from brainstem nuclei.
Participating tracts:
- Rubrospinal tract (tractus rubrospinalis) — originates from the red nucleus, transmits effector impulses from the striopallidal system, and participates in the regulation of muscle tone.
- Reticulospinal tract (tractus reticulospinalis) — originates from the reticular formation, participates in regulating the activity level of the spinal cord, posture, and background muscle tone.
Which types of spinal motor neurons participate in the execution of voluntary movement?
Alpha and gamma motor neurons of the spinal cord jointly participate in the execution of voluntary movement through a co-activation mechanism.
Types of neurons:
- Alpha motor neurons — receive direct impulses from the pyramidal system, innervate skeletal muscle, and provide precise, discrete voluntary movements.
- Gamma motor neurons — innervate intrafusal muscle fibers; their activation allows monitoring muscle length during contraction and provides an indirect reflex activation of alpha motor neurons to maintain tone.
Which afferent pathways deliver proprioceptive information from the spinal cord to the cerebellum?
Proprioceptive information from the spinal cord to the cerebellar cortex is delivered by the anterior and posterior spinocerebellar tracts.
Afferent tracts:
- Posterior spinocerebellar tract (tractus spinocerebellaris posterior) — Flechsig's tract, conducts impulses from muscle and tendon receptors, passing through the inferior cerebellar peduncle to the cortex of the cerebellar vermis.
- Anterior spinocerebellar tract (tractus spinocerebellaris anterior) — Gowers' tract, ascends in the lateral funiculi and reaches the cerebellar vermis through the superior cerebellar peduncle.
To memorize the three loops, use the acronym COR: Coordination (cerebellum: smoothness and error calculation), Executive (pyramidal tract: direct command to muscles), Regulatory (basal ganglia: tone and go-ahead for initiation).
Class and exam questions
Can a full-fledged behavioral act be artificially induced by stimulating the cortex?
No. As studies by J. Delgado showed, electrical stimulation of the cortex triggers coordinated and natural movements, but obtaining a sequential chain of actions (full-fledged behavior) in this way is impossible.
Why is the thalamus needed in the regulatory and coordination loops?
The thalamus acts as an obligatory relay station. The basal ganglia and cerebellum do not send commands to the cortex directly, but route them through thalamic nuclei, which integrate the signals and send them to the motor areas.
What will happen to movement if the striopallidal system is damaged?
The regulatory loop will be disrupted. This will lead to problems with the initiation of voluntary movements, changes in background muscle tone, and the loss of control over automated actions due to impaired control over the thalamus and brainstem.
What else to study on this topic
- Mechanisms of afferent synthesis in movement preparation
- The role of giant Betz pyramidal cells in the direct executive loop
- The corticoponto-cerebellar pathway and the mechanism of efference copy
- J. Delgado's experiments on electrical stimulation of brain structures
- Descending influences of the basal ganglia on the brainstem reticular formation
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 "Formation of Voluntary Movements" and 4,128 more pages of the subject
- 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.