Lemniscale system

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The medial lemniscus formed by the convergence of fasciculus gracilis and fasciculus cuneate after the second neuron connected were and the contralateral crossed page.

As a lemniscal system or loop system ( Latin lemniscus , loop ') of general somatic afferents ( ASA ), the ascending long conduction pathways (1.) of the posterior cord and (2.) of the lemniscus medialis (middle loop) and (3.) their projections are sequentially identified combined to form the cerebral cortex .

This functional system is of particular importance for sensitive perception in mammals . It derives information from various mechanoreceptors of the body wall on the type and location differed in such a way that stimuli differentiated by sensory qualities with high spatial resolution haptic finely perceived to be:

for touch, pressure, vibration (especially mechanoreceptors of the skin ; but not nociceptors and thermoreceptors )
  • Deep sensitivity : fine perception of posture, tension and movement of one's own body
With a sense of strength, position, and movement (especially proprioceptors in the muscle spindles , Golgi tendon organs and joint capsules )

There are also other functional systems, both generally somatosensitive with similar performance of pathways of primary afferent neurons in the head area with the lemniscus trigeminalis , as well as special somatosensory ( SSA ), for example for the auditory sense (auditory system) with the lemniscus lateralis , or for the sense of balance ( vestibular system).

The pathways for the ( protopathic ) perception of pain and temperature - and also for the coarse perception of pressure, touch or tickling - are functionally general somatosensory systems with different performance (see anterolateral system ).

anatomy

Cross section through the cervical medulla - in the posterior cord (blue, above in the picture, 3) the fasciculi gracilis (3a) and cuneatus (3b)

The nerve cell bodies ( perikaryen ) of the primary afferent (1st) neurons are located in the spinal ganglia . Their ascending axons run in the posterior cord ( funiculus posterior , in animals: funiculus dorsalis ) of the spinal cord . From the middle of the thoracic spine, each posterior cord is divided by a glial septum into:

  • Fasciculus gracilis (GOLL bundle, after Friedrich Goll ), which lies towards the middle (medial) and contains nerve fibers from the lower segments that supply the lower (in animals: rear) trunk sections and (rear) leg.
  • Fasciculus cuneatus (BURDACH bundle, after Friedrich Burdach ), which lies on the side of the GOLL bundle and contains nerve fibers from the upper segments that supply the upper (front) trunk and arm (forelimbs).
The lemniskalen system are neurons chains of three afferent neurons basis - the second are in the marrow brain , the third in the midbrain and project to the parietal lobe ( parietal lobe ) of the end-brain

The two rail systems of primary sensory neurons of the dorsal columns end on each side in separate core regions , the gracilis nucleus and the nucleus cuneatus , both in the medulla oblongata caudal to the fourth ventricle ( Fossa rhomboidea located). The switch to secondary sensitive neurons takes place in these back- strand cores.

The transmission of the impulses is divergent on the part of an upstream primary afferent neuron and convergent with respect to a downstream secondary afferent neuron. Here, each secondary mostly only receives primary afferents of the same sensory modality or quality; in this way, impulses triggered only by receptor cells of a similar type converge on the neuron. The receptive fields of neighboring neurons roughly correspond to the spatial neighborhoods according to their location in body regions ( somatotopia ).

Axons originate from these second (2nd) neurons, which still cross in the medulla oblongata as Fibrae arcuatae internae (“inner curved fibers”) on the other side ( Decussatio lemniscorum medialium ) and as Lemniscus medialis (medial loop) on the opposite side pull to the thalamus , more precisely to specific thalamic nuclei in the lateral nucleus ventralis posterior .

In the thalamus, primarily in the mutual nucleus ventralis posterolateralis (VPL), the switchover to the third (3rd) neuron of the lemniscal path takes place. These tertiary sensitive neurons project via the corona radiata to the cerebral cortex and initially reach cortical neurons in areas known as the primary somatosensory cortex (SI) and located in the anterior parietal lobe (parietal lobe) (on the postcentral gyrus ).

This is also a matter of somatotopic projections , so the information passed on is processed with reference to the spatial proximity of the body surface, but regions are represented in different sizes according to the receptor density ( homunculus ). Because of the crossed fibers of the 2nd afferent neurons in the medullary brain , one half of the cerebral cortex receives the processed information from sensitive units from the opposite side of the body (see also: contralaterality of the forebrain ).

Clinical Aspects

Unilateral damage to the posterior cord leads to sensory disorders on the same side of the body (ipsilateral), as the junction only takes place further towards the brain.

Disturbances occur here

  • fine perception (epicritical sensitivity),
  • the sense of vibration ( pallesthesia ),
  • two-point discrimination (the ability to perceive spatially closely spaced stimuli as separate),
  • the stereognosis (ability to recognize the shape, texture and consistency of an object by touch) and
  • the sense of position and movement (proprioceptive perception, kinaesthesia ).

The disturbed sense of position and movement also leads to disturbances in the course of movement (spinal ataxia ). If the cerebellum is intact, these can be partially compensated for by visual perception .

literature

  • Martin Trepel: Neuroanatomy. Structure and function. 3rd, revised edition. Urban & Fischer, Munich et al. 2004, ISBN 3-437-41297-3 .
  • Franz-Viktor Salomon: nervous system, systema nervosum . In: Salomon / Geyer / Gille (Hrsg.): Anatomy for veterinary medicine . Enke Stuttgart, 2004, pp. 464-577. ISBN 3-8304-1007-7