Dorsal column lesion (conscious proprioception)
The dorsal columns convey deep sensitivity impulses that reach consciousness (sense of body position, movement, and support). Fibers from the lower limbs run medially (fasciculus gracilis), while those from the upper limbs join at the cervical level and lie laterally (fasciculus cuneatus).
Damage to the columns themselves, the medulla oblongata nuclei, or the medial lemniscus leads to a characteristic symptom complex:
- Sensory (statolocomotor) ataxia: instability during standing and walking due to the loss of the sense of support. A specific sign is a sharp deterioration of balance in the Romberg test with closed eyes.
- Astereognosis: inability to recognize an object by touch with closed eyes.
- Agraphesthesia: inability to recognize numbers or letters "drawn" by the doctor on the patient's skin.
- Loss of joint-position and vibration sense (the patient does not feel the tuning fork vibration on the bone).
Lateral spinothalamic tract lesion
This tract is responsible for conducting pain and temperature stimuli from free nerve endings. Second-neuron fibers make an oblique decussation in the anterior white commissure of the spinal cord and ascend in the lateral funiculus of the contralateral side.
A lesion in the ventral part of the spinal cord causes a dissociated sensory loss:
- Pain and temperature sensitivity are lost.
- The side of the lesion is contralateral (opposite to the focus).
- The level of sensory disorders is determined 1–2 segments below the actual anatomical focus (due to the oblique course of fibers during decussation).
- Tactile sensitivity is completely preserved because it is conducted via other intact tracts.
Anterior spinothalamic tract and touch
The anterior tract conducts touch and pressure impulses from cutaneous receptors. Its main anatomical feature is that the central processes of first-order neurons can ascend on their own (ipsilateral) side by 2–15 segments before synapsing on second-order neurons. Only then do the axons of the second-order neurons cross to the opposite side.
Due to such extensive pathway duplication, a lesion of the anterior tract at the thoracic or lumbar level is often asymptomatic or presents with minimal tactile disturbances (impulses bypass the lesion along the ipsilateral side). Only when the lesion is localized in the cervical region is mild hypesthesia possible in the contralateral leg.
Pathway topography in spinal cord cross-section
The white matter of the spinal cord is strictly structured by function:
- Dorsal columns: contain exclusively ascending pathways of conscious proprioception (fasciculi gracilis and cuneatus).
- Lateral funiculi: mixed. They include afferent tracts (posterior and anterior spinocerebellar, lateral spinothalamic) and descending motor pathways (lateral corticospinal, rubrospinal).
- Anterior funiculi: contain the ascending anterior spinothalamic tract and a number of descending pathways (anterior pyramidal, vestibulospinal).
Cortical projection and sensorimotor integration
The final destination of most sensory pathways is the postcentral gyrus of the parietal lobe (primary somatosensory cortex). The projection of body parts here is inverted ("sensory homunculus"):
- The lower part represents internal organs, face, lips, and tongue.
- The middle part contains an extensive zone for the hand and arm.
- The upper part and the medial surface of the hemisphere project the trunk, legs, and genitals.
Sensory cortical fields partially overlap with the motor areas of the precentral gyrus. This functional combination is called the sensorimotor cortex. This integration allows the CNS to instantly analyze incoming afferent signals and transform them into precise motor commands via feedback regulatory loops.
Briefly: answers to topical questions
Which types of sensitivity are impaired on the side of the lesion in Brown-Séquard syndrome?
On the side of the lesion in Brown-Séquard syndrome, deep sensitivity is impaired, and at the level of the lesion itself, all types are lost. Below the level of the lesion, the following are lost:
- Proprioceptive sensitivity — joint-muscle sense.
- Vibratory sense.
- Tactile sensitivity.
- Discriminative sensitivity.
At the level of the lesion itself, there is a complete loss of all types of sensitivity.
Where are the cell bodies of the first, second, and third neurons of the lateral spinothalamic tract located?
The cell bodies of the lateral spinothalamic tract neurons are sequentially located in the dorsal root ganglia, posterior horns of the spinal cord, and the thalamus.
- First-order neurons — pseudounipolar cells whose cell bodies lie in the dorsal root ganglia.
- Second-order neurons — funicular neurons located in the posterior horns of the spinal cord.
- Third-order neurons — located in the ventral posterolateral nucleus of the thalamus.
How does sensory ataxia differ from cerebellar ataxia?
Sensory (sensitive) ataxia differs from cerebellar ataxia by the following features:
- Dependence on vision: with open eyes, coordination is significantly compensated by visual control, while in the dark or with closed eyes, ataxia increases sharply.
- Coordination tests: finger-nose and heel-knee tests are performed well under visual control, but are impaired with closed eyes.
- Associated signs: sensitive ataxia lacks intention tremor, dysarthria, and nystagmus.
- Gait: wide base of support and a loud slapping step sound are possible.
Which fibers form the medial lemniscus in the medulla oblongata?
The medial lemniscus (lemniscus medialis) in the medulla oblongata is formed by axons of second-order sensory pathway neurons conducting proprioceptive and tactile impulses. These fibers originate from neuronal cell bodies located in the following structures:
- Nucleus gracilis (nucleus gracilis) — Goll's nucleus.
- Nucleus cuneatus (nucleus cuneatus) — Burdach's nucleus.
The axons of these nuclei decussate, forming the bulbothalamic tract, which runs as part of the medial lemniscus to the thalamus.
The somatotopy rule of the lateral spinothalamic tract (the law of eccentric location): the lower the body segment (sacrum, lumbar), the more laterally (outward) its fibers lie in the funiculus. Fibers from the neck and arms lie most medially.
Class and exam questions
Why does ataxia worsen in the dark when the posterior columns are damaged?
The patient loses proprioceptive sensitivity and ceases to feel the support under their feet. During the day, this deficit is partially compensated by vision, but with closed eyes or in the dark, visual control disappears, leading to a sharp loss of balance.
What is a dissociated sensory loss?
This is a specific syndrome in which only pain and temperature sensitivity are lost (due to damage to the lateral spinothalamic tract), while tactile sensitivity and joint-muscle sense are completely preserved.
Why does a lesion of the anterior spinothalamic tract often present with minimal symptoms?
Fibers of the first-order neurons of this tract ascend on their own side by 2–15 segments before decussating. Due to this, most impulses simply bypass the local lesion along the ipsilateral side.
What else to study on this topic
- Anatomy and functions of the anterior spinocerebellar tract (unconscious proprioception).
- Course of sensory pathways through the internal capsule and corona radiata.
- Intrinsic (internal) integrative apparatus of the spinal cord.
- The role of the thalamus in the primary perception of pain and temperature.
- Ascending pathways to the brainstem: spinoreticular and spinotectal tracts.
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