Lemniscus (anatomy)

A lemniscus is a ribbon-like bundle of secondary sensory fibres in the brainstem, and the name comes from the Greek word for a ribbon or band.

The principal lemnisci are the medial, lateral, trigeminal and spinal lemnisci, each carrying a different sensory modality toward the thalamus or another relay.^[1]

Because the bundles run through the brainstem, their damage produces characteristic crossed sensory syndromes that are important in clinical neurology.

Structure

• The medial lemniscus is formed by axons of the gracile and cuneate nuclei, which cross in the medulla as internal arcuate fibres and ascend in the ventral brainstem.
• The medial lemniscus runs through the medulla, pons and midbrain and terminates in the ventral posterolateral nucleus of the thalamus.
• The lateral lemniscus carries auditory fibres from the cochlear nuclei and superior olivary complex to the inferior colliculus.
• The trigeminal lemniscus carries facial touch and pain fibres from the principal and spinal trigeminal nuclei to the ventral posteromedial nucleus.
• The spinal lemniscus is the brainstem continuation of the spinothalamic tract, and it carries pain and temperature fibres from the spinal cord.
• The bundles are not encapsulated nerves but compact fascicles, and their position is used as a landmark in brainstem cross-sections.
• The medial lemniscus lies near the midline in the medulla and shifts laterally in the pons, which is why brainstem lesions affect it differently at different levels.

Function

• The medial lemniscus carries fine touch, vibration and conscious proprioception from the body to the thalamus and then to the somatosensory cortex.
• The lateral lemniscus carries auditory information and contributes to sound localisation and temporal processing.
• The trigeminal lemniscus carries tactile and nociceptive information from the face and oral cavity.
• The spinal lemniscus carries pain and temperature information from the contralateral body and is part of the anterolateral system.
• All of these pathways are crossed, so a lesion above the crossing produces sensory loss on the opposite side of the body or face.
• Lemniscal fibres are somatotopically organised, and the body is represented in a medial-to-lateral order that is preserved into the thalamus.

Vasculature and innervation

• The brainstem is supplied by perforating branches of the vertebral artery, basilar artery and cerebellar arteries, and the lemnisci depend on these small vessels.
• The medial lemniscus in the medulla is supplied by branches of the anterior spinal artery and the vertebral artery.
• The lateral lemniscus in the pons and midbrain is supplied by branches of the basilar artery and the superior cerebellar artery.
• The lemnisci have no nerve supply of their own, because they are bundles of axons rather than contractile or glandular tissue.
• Small perforating artery disease can therefore produce a purely sensory stroke that involves the lemniscus without motor signs.

Development

• All lemniscal systems arise from the neural tube, and their nuclei develop from the alar plate of the hindbrain and midbrain.
• The medial lemniscus depends on the dorsal column nuclei, which migrate and differentiate in the caudal medulla during the first trimester.
• The lateral lemniscus develops with the cochlear nuclei and superior olivary complex, and its fibres grow toward the inferior colliculus.
• Myelination of the lemnisci continues after birth, and it is one reason why some sensory functions mature gradually in infancy.
• Developmental disorders of the dorsal column nuclei can produce congenital sensory ataxia, although such cases are rare.

Clinical significance

• A lesion of the medial lemniscus in the medulla causes loss of fine touch and proprioception on the contralateral body, often with preserved pain and temperature sensation.
• Dejerine syndrome, also called medial medullary syndrome, combines contralateral medial lemniscus signs with ipsilateral tongue weakness.
• Wallenberg syndrome, or lateral medullary infarction, damages the spinal and trigeminal lemnisci and produces crossed pain and temperature loss.
• Brainstem haemorrhage can compress the medial lemniscus and produce severe sensory loss with a good motor recovery, a dissociation that puzzles clinicians.
• Multiple sclerosis can demyelinate lemniscal fibres and produce tingling, numbness and sensory ataxia.
• Lesions of the lateral lemniscus rarely cause deafness because auditory fibres are bilateral, but they can impair sound localisation.
• The trigeminal lemniscus is involved in lateral medullary and pontine lesions, which is why facial sensory loss is a key localising sign.

Comparative anatomy

• The medial lemniscus is highly developed in mammals, and its size correlates with the importance of fine touch and proprioception in the species.
• Birds have a large nucleus of the lateral lemniscus that processes auditory information, and their auditory brainstem differs from the mammalian plan.
• Reptiles and amphibians have less distinct lemniscal bundles, and their sensory pathways are more diffuse.
• The spinal lemniscus is present in all amniotes, but its relative size varies with the importance of pain signalling in the species.

History

• The term lemniscus was used by nineteenth-century neuroanatomists for ribbon-like fibre bundles, and it was applied to both sensory and auditory pathways.
• Dejerine described the medial medullary syndrome in the early twentieth century, linking the medial lemniscus to crossed sensory loss.
• Wallenberg described lateral medullary infarction in 1895, and his name remains attached to the syndrome that involves the spinal and trigeminal lemnisci.
• Terminologia Anatomica lists lemniscus medialis, lateralis, trigeminalis and spinalis as preferred terms.

Research and controversies

• Diffusion tensor imaging can trace the lemnisci in vivo, but the tracts are small and partial volume effects remain a limitation.
• Whether the spinal lemniscus deserves a separate name or is simply the brainstem part of the spinothalamic tract is a matter of nomenclature rather than anatomy.
• The functional role of the trigeminal lemniscus in facial pain is still debated, because parallel pathways carry similar information.
• Auditory brainstem implants target the cochlear nuclei and lateral lemniscus, and their outcomes depend on the preservation of these pathways.
• In the pons the medial lemniscus lies in the ventral tegmentum, where it is separated from the basilar pons by the medial longitudinal fasciculus and other tracts.
• The trigeminal lemniscus runs close to the medial lemniscus, so pontine lesions can produce combined body and face sensory loss.
• High-resolution magnetic resonance imaging can detect lemniscal infarction as a small restricted diffusion lesion in the brainstem.