Pain and temperature sensitivity of the trunk
Transmission of impulses from nociceptors (from Lat. nocere — to harm) located in the skin, muscles, joints, and internal organs follows a three-neuron scheme:
- 1st neuron: Pseudounipolar cells of the spinal ganglion. Their peripheral process collects information from tissues, and the central process as part of the dorsal root goes to the posterior horns of the spinal cord.
- 2nd neuron: Cells of the posterior horns of the spinal cord. Their axons decussate—cross to the opposite side through the anterior gray commissure 1–2 segments above the level of entry. This is how the spinothalamic pathway is formed in the lateral funiculus.
- 3rd neuron: Ventrolateral nucleus of the thalamus. From here, axons pass through the posterior limb of the internal capsule to the postcentral gyrus of the parietal lobe cortex.
Sensitivity of the head and face
Innervation of the face is provided by three branches of the trigeminal nerve (n. trigeminus): ophthalmic, maxillary, and mandibular.
- Tactile and deep sensitivity: First-order neurons lie in the trigeminal ganglion (gasserian ganglion). Second-order neurons lie in the principal sensory nucleus of the pons. Their axons decussate, join the medial lemniscus, and proceed to the thalamus.
- Pain and temperature sensitivity: Proximal branches of neurons from the trigeminal ganglion descend to the long spinal trigeminal nucleus (nucleus tractus spinalis nervi trigemini). After decussation in the brainstem, they join the spinothalamic tract.
The end point for all pathways is the postcentral gyrus of the opposite hemisphere.
Anatomy of the dorsal root and ganglion
Afferent fibers in the dorsal root are strictly structured:
- Medial part: thick myelinated fibers from muscle spindles (proprioception).
- Central part: fibers of touch, pressure, and discriminative sensitivity.
- Lateral part: thin unmyelinated fibers (pain and temperature).
The spinal ganglion itself looks like a thickening of the root and consists of pseudounipolar neurons. Their single process divides in a T-shape into a peripheral process (giving off branches to numerous receptors) and a central process (proceeding to the CNS). An important morphological feature: there are no synapses inside the ganglion itself.
Dermatomes and topical diagnostics
A dermatome is an area of skin innervated by a single segment of the spinal cord. On the trunk, they appear as circular bands; on the limbs, they are distributed longitudinally (for example, C6 innervates the thumb, and S1 innervates the little toe).
Adjacent dermatomes overlap each other, but unevenly. Zones of tactile sensitivity overlap very widely, whereas pain and temperature overlap significantly less.
Clinical significance: in lesions of a single nerve root (radicular syndrome), the deficit in touch is almost imperceptible, whereas the loss of pain sensitivity is pronounced. It is the testing of pain that is used for precise topical diagnostics. Unlike roots, the innervation zones of peripheral nerves overlap weakly, so their damage is much easier to recognize and is often accompanied by motor disorders.
Nerve plexuses and polysegmental innervation
Peripheral nerves are formed within plexuses (plexus), where a complex redistribution of fibers takes place. Sensory fibers of a single peripheral nerve diverge into several dorsal roots. Conversely, fibers of a single spinal segment pass through different nerves to reassemble at the periphery and innervate their dermatome.
Motor innervation of muscles (myotomes) is also polysegmental in nature. Due to this mechanism, when only a single root is damaged, the muscle is not completely paralyzed, as it continues to receive signals from adjacent healthy segments.
Briefly: answers to topical questions
Which pathways form the medial lemniscus (lemniscus medialis)?
The medial lemniscus (lemniscus medialis) is formed by decussated axons of second-order neurons of all conscious ascending sensory pathways of the brainstem, with the exception of auditory pathways. It includes:
- Bulbothalamic tract — axons from the gracile and cuneate nuclei transmitting proprioceptive and tactile sensitivity.
- Spinothalamic pathways — decussated axons of the anterior and lateral spinothalamic tracts.
- Central tract from the pontine trigeminal nucleus — tactile sensitivity fibers from the pharynx, ear, and meninges.
Which tracts transmit proprioceptive sensitivity from the trunk and limbs?
Proprioceptive sensitivity from the trunk and limbs is transmitted via ascending tracts of the posterior and lateral funiculi of the spinal cord. Conscious and unconscious proprioception pathways are distinguished:
- Gracile fasciculus (fasciculus gracilis, Goll's tract) — transmits conscious proprioceptive sensitivity from the lower limbs.
- Cuneate fasciculus (fasciculus cuneatus, Burdach's tract) — transmits conscious proprioceptive sensitivity from the upper limbs.
- Posterior spinocerebellar tract (Flechsig's tract) — transmits unconscious proprioception to the cerebellum.
- Anterior spinocerebellar tract (Gowers' tract) — also transmits unconscious proprioceptive sensitivity.
What symptoms occur in lesions of the thalamus?
Symptoms in thalamic lesions depend on the localization of the lesion and include sensory, motor, and cognitive disorders. The following symptoms occur:
- Sensory disturbances: severe pain syndrome ("thalamic pain", anesthesia dolorosa), paroxysms of pain and dysesthesia, transient thalamic neglect.
- Motor disorders: "thalamic astasia" (weakness without decreased muscle strength), contralateral ataxia, intention tremor or resting tremor, choreoathetoid hyperkinesia, dysmetria, adiadochokinesia, formation of the "thalamic hand".
- Cognitive and mental disorders: impairment of consciousness and attention, acute confusion, transient amnesia, mood disorders, delirium with confabulations.
Also, if the lesion extends, vertical gaze palsy may develop.
To quickly recall key trunk dermatomes, use topographical landmarks: Th4 runs strictly at the nipple level, Th10 at the umbilicus level, and Th12 descends to the inguinal crease.
Class and exam questions
Where does the decussation of the spinothalamic tract occur?
The axons of the second-order neurons decussate to the contralateral side via the anterior gray commissure of the spinal cord 1–2 segments above the level of their entry.
Why is tactile sensitivity barely affected when a single root is damaged?
Dermatomes for tactile sensitivity overlap very widely. The zone of the affected root is compensated by signals traveling along adjacent healthy segments.
Why is the cauda equina formed?
During development, the spinal cord grows slower than the vertebral column and ends at the level of the upper lumbar vertebrae. The lower nerve roots have to descend far down the vertebral canal to their exit foramina, forming a cluster around the filum terminale.
What else to study on this topic
- Skeletotopy: mismatch between spinal cord segment levels and vertebral bodies
- Mechanisms of central and peripheral sensitization in pain pathophysiology
- Antinociceptive system and gate control theory of pain
- Detailed diagram of limb innervation (myotomes and dermatomes)
- Differential diagnosis of nerve root and peripheral nerve lesions
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