Anatomical basis and skeletotopy
The spinal cord is located within the vertebral canal. Its structural unit is the segment—an area from which a single pair of spinal nerves arises. The following divisions are distinguished:
- Cervical (8 segments, C_I–C_{VIII})
- Thoracic (12 segments, Th_I–Th_{XII})
- Lumbar (5 segments, L_I–L_V)
- Sacral (5 segments, S_I–S_V)
- Coccygeal (1–3 segments, Co_I–Co_{III})
It is important to account for skeletotopy: the spinal cord is shorter than the vertebral column. Consequently, a mismatch exists between the spinal segment level and the correspondingly numbered vertebra. In the cervical region, the difference is one vertebra, in the upper thoracic region two, and in the lower thoracic region three. The roots of the lower segments must descend toward their respective intervertebral foramina, forming the cauda equina (cauda equina).
To innervate the limbs, there are two enlargements: the cervical enlargement (C_{IV}–Th_I, innervation of the upper limbs) and the lumbar enlargement (L_{II}–S_{III}, innervation of the lower limbs).
Internal structure: gray and white matter
The spinal cord is divided into two symmetrical halves by the anterior median fissure and the posterior median sulcus. The central canal runs through the center.
Gray matter (clusters of neurons) resembles a butterfly on cross-section and forms the horns:
- Anterior horns: contain motor neurons.
- Posterior horns: contain sensory neurons.
- Lateral horns: autonomic centers. Not present throughout; sympathetic neurons lie at the C_{VIII}–L_I level, while parasympathetic neurons lie in the S_{II}–S_{IV} segments.
White matter surrounds the gray matter and is divided into funiculi (funiculi): anterior, lateral, and posterior. The posterior funiculi contain the gracile and cuneate fasciculi, which mediate proprioceptive, tactile, and vibratory sensation.
Roots and nerve plexuses
Two roots emerge from each segment:
- Posterior (sensory): formed by the processes of the spinal ganglion (ganglion spinale), where the first sensory neuron resides.
- Anterior (motor): consists of motor neuron axons.
By uniting, they form a spinal nerve. Upon exiting the vertebral canal, nerve branches form plexuses: cervical (C_{II}–C_{IV}), brachial (C_V–Th_I), lumbar (Th_{XII}–L_{IV}), and sacral (S_I–S_{III}). An exception is the thoracic segments, which form intercostal nerves.
Topical lesion syndromes
Damage to specific structures yields a clear clinical picture that allows localization of the pathology level:
- Anterior horn lesion: results in peripheral paresis of muscles with characteristic fasciculations (involuntary fine muscle twitches). A lesion in the cervical enlargement disrupts upper limb function, while one in the lumbar enlargement disrupts the lower limbs.
- Posterior horn lesion: produces a segmental-dissociated type of sensory impairment. Pain and temperature sensation are lost on the side of the lesion, whereas tactile and joint-muscle sensation are preserved (their pathways ascend via the posterior funiculi).
- Lateral horn lesion: autonomic (trophic and vasomotor) disturbances appear. A lesion at the C_{VIII}–Th_{XII} level impairs sympathetic innervation of the eye, leading to Horner's syndrome.
- Lateral funiculus lesion: central paresis (of the leg or hemiparesis) occurs on the side of the lesion, while contralateral loss of pain and temperature sensation occurs in a tract (conduction) pattern. The sensory border lies 2–3 dermatomes below the lesion.
- Posterior funiculus lesion: joint-muscle sense is lost, and vibratory and tactile sensation are diminished.
Briefly: answers to topical questions
Which ascending and descending tracts pass through the lateral funiculi of the spinal cord?
Motor and sensory pathways run through the lateral funiculi of the spinal cord.
- Lateral corticospinal tract — located centrally.
- Rubrospinal tract — located centrally.
- Anterior spinothalamic tract — located peripherally.
- Lateral spinothalamic tract (Tractus spinothalamicus lateralis) — located peripherally, conducts pain and temperature sensation.
- Anterior spinocerebellar tract — located peripherally.
- Posterior spinocerebellar tract (Tractus spinocerebellaris posterior) — located peripherally, conducts impulses from muscle and tendon receptors to the cerebellum.
What is Chipault's rule for determining the skeletotopy of spinal cord segments?
Chipault's rule defines the correspondence between spinal cord segments and vertebrae.
- Upper 4 cervical segments — correspond to their own vertebrae.
- Lower 4 cervical and upper 4 thoracic segments — lie one vertebra higher.
- Middle 4 thoracic segments — lie two vertebrae higher.
- Lower 4 thoracic segments — lie three vertebrae higher.
- Lumbar segments — lie at the level of the 12th thoracic vertebra.
- Sacral and coccygeal segments — project at the level of the 1st lumbar vertebra.
What symptoms are characteristic of Brown-Séquard syndrome?
Brown-Séquard syndrome (hemisection of the spinal cord) is characterized by symptoms developing on both the ipsilateral and contralateral sides.
- At the level of the lesion (ipsilateral) — flaccid paresis and loss of all modalities of sensation.
- Below the level of the lesion (ipsilateral) — spastic paresis, loss of proprioceptive, vibratory, tactile, and discriminative sensation. Pain and temperature sensation are preserved.
- Below the level of the lesion (contralateral) — analgesia and thermoanesthesia (loss of superficial sensory modalities: pain and temperature).
How does central paresis differ from peripheral paresis?
Central and peripheral paresis differ in their clinical manifestations.
| Sign | Central paresis | Peripheral paresis |
|---|---|---|
| Muscle tone | Spastic increase | Flaccid paresis |
| Deep reflexes | Exaggeration, hyperreflexia | Not specified in the provided general form for peripheral paresis in the sources |
| Pathological reflexes | Presence of pathological reflexes | Absence of pathological reflexes is not indicated in the sources |
| Muscle atrophy | Mild, late, due to disuse | Early-onset and pronounced in lower motor neuron lesions |
| Additional | Defense reflexes and pathological synkinesias | Fasciculations are frequently observed in anterior horn lesions |
What symptoms occur in complete transverse spinal cord injury at the level of the thoracic segments?
Complete transverse spinal cord injury at the thoracic level leads to severe impairments below the level of the lesion.
- Motor impairments: bilateral central paralysis of the legs; complete flaccid paralysis may occur in the acute period due to spinal shock.
- Sensory impairments: complete anesthesia, i.e., loss of all types of sensation strictly below the level of the lesion.
- Pelvic disorders: loss of voluntary control over urination and defecation, sexual dysfunction.
- When the upper thoracic region is affected: movement and sensation in the arms are preserved; respiratory disorders and paralytic ileus may develop.
For quick memorization of root functions, use a simple rule: "Posterior means sensory, anterior means motor." The posterior root always carries sensory information from the spinal ganglion, while the anterior root transmits motor commands to muscles.
Class and exam questions
Why is the level of sensory loss located lower than the lesion itself in lateral funiculus involvement?
This is due to the anatomy of the spinothalamic tract. Its fibers cross to the opposite side not strictly horizontally, but ascending obliquely upward, which is why the boundary of sensory disturbances is detected 2–3 dermatomes below the actual lesion level.
What is the cauda equina and how is it formed?
The cauda equina is a bundle of nerve roots (lumbar, sacral, and coccygeal) that descends in the vertebral canal below the termination of the spinal cord itself (below the 1st–2nd lumbar vertebrae) toward their respective intervertebral foramina.
What additional methods are used to diagnose spinal cord lesions?
In addition to standard MRI and CT, electromyography (EMG), short-latency somatosensory evoked potentials, and motor responses from transcranial magnetic stimulation are used.
What else to study on this topic
- Formation and innervation zones of the cervical and brachial plexuses
- Anatomy of the lumbosacral plexus and its main nerves
- White matter pathways: ascending and descending tracts
- Clinical evaluation of autonomic and pelvic disorders
- Brown-Séquard syndrome and other levels of complete transverse spinal cord lesions
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