Anatomy and fiber pathways
At the junction of the medulla oblongata and the spinal cord, the corticospinal tract divides into two functional branches:
- Lateral corticospinal tract: this is the crossed part of the tract (80–90% of the fibers), which descends in the lateral funiculus. The fibers terminate in the anterior horns of the spinal cord. At the level of the cervical enlargement, they control the neck, shoulder girdle, arms, and diaphragm; in the thoracic segments, the trunk; and in the lumbar and sacral regions, the pelvic girdle and legs.
- Anterior corticospinal tract: the uncrossed part running in the anterior funiculus on its own (homolateral) side. These fibers terminate predominantly in the cervical and thoracic segments. Their main task is to provide partial bilateral innervation to the axial muscles of the body (neck and trunk).
Lower motor neuron and motor unit
The structural and functional basis of muscle contraction is the motor unit. It includes the motor neuron itself, its axon, and the group of muscle fibers it controls.
The anatomical pathway of the impulse in the periphery is as follows: motor neuron cell bodies (anterior horns) $\rightarrow$ anterior roots $\rightarrow$ merging with posterior roots $\rightarrow$ spinal nerves $\rightarrow$ nerve plexuses $\rightarrow$ peripheral nerves $\rightarrow$ neuromuscular synapses.
The localization of motor neurons is strictly segmented:
- Upper cervical segments (C1–C4): neck muscles.
- Cervical enlargement (C5–Th2): upper extremities.
- Thoracic region: trunk muscles.
- Lumbar enlargement (Th12–S2): lower extremities.
Upper and lower motor neuron palsies
Damage to the motor pathways leads to muscle weakness—paresis or paralysis.
Upper motor neuron paresis (central paresis) occurs when the corticospinal tract is damaged. Due to increased excitability of motor neurons, deep reflexes are exaggerated and muscle tone increases, resulting in spasticity (hypertonia). Paresis is more pronounced in the distal parts (hands, feet) because the proximal parts and trunk are partially compensated by bilateral innervation. A characteristic symptom is Wernicke–Mann posture: the arm is pressed against the body and flexed (due to flexor tone), while the straightened leg moves in an arc during walking (due to extensor tone).
Lower motor neuron paresis (peripheral paresis) develops when any part of the lower motor neuron is damaged. The reflex arc is broken, leading to decreased (hyporeflexia) or absent (areflexia) reflexes. Tone drops (hypotonia). Due to the lack of neurotrophic influence, muscles decrease in bulk, resulting in hypotrophy (atrophy).
Clinical assessment of the motor system
The examination begins with an assessment of gait, posture, and muscle appearance (checking for hypotrophy and fasciculations). Then the physician assesses the range and strength of movements.
Important rule: if active movements are limited, passive movements must be examined. This helps to distinguish neurological deficit from joint pathology (e.g., arthrosis).
Strength is assessed by resistance: the patient holds a given position while the physician attempts to displace the limb. The degree of paresis is measured on a scoring scale:
- 5 points: normal strength.
- 4 points: slight decrease.
- 3 points: moderate decrease (full range of motion even against gravity).
- 2 points: significant decrease in strength.
Briefly: answers to topical questions
Where exactly in the cerebral cortex does the corticospinal tract originate?
The corticospinal tract originates from neurons in several areas of the cerebral cortex.
- Precentral gyrus — the primary motor cortex, where large Betz pyramidal cells, as well as small pyramidal and spindle-shaped cells, are located in layer V.
- Premotor area — the regions of the frontal lobe cortex located anterior to the precentral gyrus.
- Postcentral gyrus — the pathway is partially formed by neurons of the parietal lobe cortex in both hemispheres.
Through which part of the internal capsule does the corticospinal tract pass?
The corticospinal tract passes through the anterior two-thirds of the posterior limb (posterior crus) of the internal capsule (capsula interna). The fibers of the tract have a strict somatotopic organization and are located in the anterior part of the posterior limb in the following order from front to back:
- Fibers to the arm
- Fibers to the trunk
- Fibers to the leg
Where do the fibers of the corticonuclear pathway terminate?
The fibers of the corticonuclear (corticobulbar) pathway terminate on the motor cranial nerve nuclei in the brainstem. They provide voluntary regulation of facial muscles, pharyngeal muscles, and tongue muscles, synapsing on the nuclei of the following pairs:
- Trigeminal nerve (cranial nerve V)
- Facial nerve (cranial nerve VII)
- Glossopharyngeal nerve (cranial nerve IX)
- Vagus nerve (cranial nerve X)
- Hypoglossal nerve (cranial nerve XII)
Which motor cranial nerve nuclei receive only unilateral cortical innervation?
Only unilateral (contralateral) cortical innervation from the opposite hemisphere is received by two motor cranial nerve nuclei. These include:
- Hypoglossal nerve nucleus (CN XII) — provides innervation to the muscles of the tongue.
- Lower part of the facial nerve nucleus (CN VII) — is responsible for the innervation of the facial muscles of the lower half of the face.
The remaining motor nuclei (CN V, IX, X, and the upper part of the CN VII nucleus) have bilateral connections with the cortex of both hemispheres.
How is muscle strength assessed using the standard 6-point scale (from 0 to 5)?
Muscle strength is evaluated using a 6-point system depending on the range of motion and the ability to overcome resistance and gravity.
- 5 points — full strength movements (no paresis).
- 4 points — slight decrease in strength.
- 3 points — moderate decrease in strength, but full range of motion is achieved even against gravity.
- 2 points — significant reduction in strength, full range of motion is possible only with gravity eliminated.
- 1 point — minimal movement or only visible muscle contraction (without moving the limb).
- 0 points — complete absence of movement (plegia or paralysis).
To easily remember the distribution of cortical innervation: distal parts (hands, feet) are innervated strictly contralaterally (lateral pathway), while the body axis (neck, trunk) is innervated bilaterally (lateral + anterior pathways). Therefore, in central paresis, the trunk is barely affected.
Class and exam questions
Why do the hands and feet suffer more than trunk muscles in central paresis?
The trunk and proximal muscles have bilateral cortical innervation thanks to the anterior uncrossed pathway. Distal limb segments are innervated predominantly unilaterally (via the crossed lateral tract), making it harder to compensate for their damage.
What do fasciculations indicate during examination?
Fasciculations are involuntary contractions of muscle fiber bundles. They indicate altered excitability of motor neuron cell bodies (anterior horns) or their axons (anterior roots). However, it is important to remember that they are sometimes observed in healthy individuals as well.
Why test passive movements if the patient already has limited active movements?
This is necessary for differential diagnosis. Limitation of passive movement points to local joint pathology that mechanically hinders movement, rather than damage to the nervous system.
What else to study on this topic
- Complete 6-point muscle strength grading scale (criteria for 0 and 1 point).
- Mechanisms of muscular hypertonia (spasticity) development at the reflex arc level.
- Pathogenesis of muscle atrophy in lower motor neuron damage.
- Methodology for examining deep and superficial (cutaneous) reflexes.
- Topical diagnosis of lesions at various spinal cord segments.
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Other topics in Chapter 2. Movement Disorders
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