Structure and anatomy of the corticomuscular pathway
The motor pathway is anatomically and functionally divided into two main parts: the central and peripheral neurons. The pathway originates in the cells of the frontal and parietal lobes and terminates in the skeletal muscles of the trunk, limbs, and head.
- The central neuron (pyramidal or corticospinal tract) originates in the pyramidal neurons of the precentral gyrus, the premotor area of the frontal lobe, and partially in the postcentral gyrus.
- The corticobulbar tract is responsible for the voluntary regulation of facial expressions, pharyngeal, and lingual muscles, terminating on the motor nuclei of cranial nerve pairs V, VII, IX, X, and XII.
- In the white matter, the axons form the corona radiata, pass through the posterior limb of the internal capsule, and descend along the base of the brainstem.
Somatotopic organization and the motor homunculus
In the cortex of the precentral gyrus, there is a strict somatotopic representation of the body, known as the motor homunculus. Projections are distributed from top to bottom:
- Upper section (including the paracentral lobule): innervation of the lower limb and trunk (from toes to hip).
- Middle section: innervation of the upper limb, where the hand and its fingers occupy a disproportionately large area of the cortex.
- Lower section: innervation of the muscles of the face, neck, mouth, tongue, and larynx.
This arrangement determines the topography of lesions in brain pathologies.
Topography of the pathway in the brainstem and spinal cord
Descending fibers pass through all brainstem structures at the level of the cranial nerve nuclei:
- In the cerebral peduncles, fibers pass at the level of pairs III and IV.
- In the pons — at the level of pairs V, VI, and VII.
- In the medulla oblongata, visible thickenings called pyramids are formed. At the border with the spinal cord, the pyramidal decussation occurs.
- After the decussation, the lateral and anterior corticospinal tracts are formed, which terminate on the motor neurons of the anterior horns of the spinal cord.
From the spinal cord, axons exit via the anterior roots into the peripheral nerves and reach the muscle fibers through the neuromuscular synapse.
Clinical manifestations and classification of paresis
The main symptom of damage to the corticomuscular pathway is muscle weakness:
- Paresis — a partial decrease in strength with the preservation of minimal voluntary movements.
- Paralysis (plegia) — the complete absence of voluntary contractions.
Topographic lesion variants:
- Monoparesis: one limb is affected.
- Hemiparesis: damage to one half of the body on one side.
- Paraparesis: symmetrical damage to both upper or lower limbs.
- Triparesis: involvement of three limbs.
- Tetraparesis: damage to all four limbs.
Differential diagnosis excludes movement limitations due to pain, bone and ligament pathology, or mental disorders.
Briefly: answers to topical questions
What structures form the peripheral motor neuron?
The peripheral motor neuron (the peripheral division of the motor system) is formed by nerve cell bodies and their processes extending to the effector muscles. Its anatomical composition includes:
- Motor nuclei of cranial nerves — located in the brainstem.
- Motor neurons of the anterior horns of the spinal cord — cell bodies.
- Anterior roots — formed by the axons of motor neurons.
- Spinal nerves — formed by the union with the posterior roots.
- Nerve plexuses — cervical and lumbosacral.
- Peripheral nerves — contain motor fibers.
- Neuromuscular synapses — transmit the impulse to striated muscle.
How are the fibers of the motor pathways topographically arranged in the internal capsule?
The fibers of the motor pathways pass through the genu and the anterior two-thirds of the posterior limb of the internal capsule (capsula interna), being arranged as compactly as possible. Somatotopic arrangement of fibers from front to back:
- Genu — fibers passing to the face (corticonuclear tract).
- Posterior limb (anterior part) — fibers passing to the arm, then to the trunk, then to the leg (corticospinal tract).
The anterior limb contains frontopontine and frontothalamic fibers, while the posterior part of the posterior limb contains somatosensory, visual, and auditory pathways.
What is included in the anatomical composition of the segmental-reflex apparatus of the spinal cord?
The segmental-reflex apparatus of the spinal cord includes:
- Spinal cord segments: cervical — 8, thoracic — 12, lumbar — 5, sacral — 5, coccygeal — 1–3.
- Motor neurons of the anterior horns of the gray matter, on which the branches of afferent fibers terminate.
- Posterior roots, through which the central processes of sensory neurons enter the spinal cord.
- Anterior roots, through which the axons of motor neurons exit.
What clinical symptoms differentiate central paralysis from peripheral paralysis?
| Sign | Central (spastic) paralysis | Peripheral (flaccid) paralysis |
|---|---|---|
| Muscle tone | Spastic hypertonia, usually uneven | Hypotonia; the muscle is flabby, flaccid |
| Reflexes | Hyperreflexia, increased response amplitude, and expansion of reflexogenic zones | Hyporeflexia or areflexia |
| Pathological manifestations | Pathological reflexes, clonus, synkinesias | Muscular atrophy in denervation; muscle "degeneration" phenomenon |
The somatotopic arrangement in the precentral gyrus is easy to memorize from top to bottom using an "upside-down" principle: from the paracentral lobule with the legs through the trunk and arms to the face and tongue at the very bottom near the lateral sulcus.
Class and exam questions
What structures form the upper motor neuron of the cortico-muscular pathway?
The upper motor neuron (pyramidal tract) originates from the precentral gyrus, the premotor area of the frontal lobe, and partially the postcentral gyrus. Its axons pass through the corona radiata, the posterior limb of the internal capsule, and the brainstem to the spinal cord.
What is the difference between paresis and paralysis?
Paresis is a decrease in muscle strength while maintaining the ability to make at least minimal voluntary movements. Paralysis or plegia is characterized by the complete inability to perform voluntary muscle contractions.
Which cranial nerves have bilateral cortical innervation?
The motor nuclei of cranial nerves V, VII (upper part), IX, and X have connections with the cortex of both hemispheres. The exception is the nucleus of the XII nerve and the lower part of the nucleus of the VII nerve, which receive only contralateral innervation.
What else to study on this topic
- Detailed analysis of brainstem pathways
- Features of the lateral and anterior corticospinal tracts
- Clinical presentation and symptoms of internal capsule lesions
- Functional significance of the motor homunculus
- Structure and functions of the neuromuscular synapse
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