Lateral Lemniscus

LATERAL LEMNISCUS, MEDIAL LEMNISCUS (lateral and medial loops), bundles of nerve fibers in the brainstem: serving to transmit impulses of deep and cutaneous sensitivity to the brain.

Lemniscus medialis, s. principal is (L. superior, L. Halleri, fasciculus bulbo-thalamicus), the internal or principal loop, originates in the lower parts of the medulla oblongata, in the nuclei of the posterior columns - nucl. funiculi gracilis (nucleus of the delicate bundle) and nucl. fun. cuneati (nucleus of the cuneate bundle). Here terminate the neurons of deep sensitivity, fibers of the delicate (Golgi) and cuneate (Burdukhovsky) bundles [Fig. I (st. 29-30)]. Fibers emerging from these nuclei, fibrae arcuatae internae (internal arcuate fibers), form a complete decussation in front of the central canal - decussatio lemniscorum (sensory, posterior or superior decussation), then take an ascending direction, arranging themselves in the so-called loop or interolivary layer, on both sides of the raphe, behind the pyramids and in front of the fasciculus longitudinalis post, and fasciculus praedorsalis (Fig. II). In the interolivary layer, fibers coming from nucl. funiculi cuneati occupy the anterior part, and fibers from nucl. funiculi gracilis - the posterior part. Gradually upward the loop layer increases all the time by continuing to receive fibers from the nuclei of the opposite side. In the distribution of fibers in the loop layer there seems to be a certain regularity: in the very posterior part of it run conductors from the conus terminalis, more forward - fibers from the lumbar enlargement (having relation to the innervation of the lower extremities), and in front of all, immediately behind the pyramidal tracts, pass conductors from the cervical region (innervating the upper extremities and neck). In the Varolian bridge L. t. is arranged as an elongated transverse layer on the boundary between the basis and the tegmentum of the bridge; in the lower parts of the bridge this layer is intersected by fibers of corporis trapezoidei; in the upper parts L. t. moves gradually more outward and in the isthmus cerebri reaches the lateral surface of the brain, forming trigonum lemnisci, a triangular field, about 8 mm long, bounded by sulcus lateralis mesencephali, brachium conjunctivum and brachium quadrigeminum inferius (Fig. Ill, IV).

Behind the principal loop lies a collection of gray matter, its nucleus, nucleus lemnisci medialis Roller's, s. griseum supralemniscatum. In the lower parts of the crus cerebri, at the level of the inferior colliculus, L. t. has the shape of an angle open inward; its continuation backward serves as L. lateralis, lying outward and forward of brachium conjunctivum; between them is placed the lateral nucleus of the tegmentum (KoHiker a), behind it it borders on substantia nigra, which separates it from the basis of the crus cerebri (Fig. V). In the upper parts of the latter L. t. is placed outward from nucleus ruber (Fig. VI). The central sensory conductors of some cranial nerves also enter the composition of L. t.: nn. trigemini, glosso-pharyngei and vagi. The entry of these fibers into L. t. begins in the medulla oblongata: from cells of substantiae gelatinosae, in which terminates the peripheral neuron of n. trigemini, and from the corresponding nuclei of the IX and X pairs axons are directed inward and enter the composition of the loop layer, differing from the fibers of the loop by a thinner caliber. Together with the loop these fibers terminate in the diencephalon, in the lower posterior part of nucl. externi. thalami optici and in its nucl. medianus. Together with L. m. in these nuclei terminates also a part of the Gowers bundle. It is expressed that a part of the fibers of L.m., bypassing the thalamus opticus, goes through the capsula interna to the cortex of the central gyri. In addition to ascending fibers, descending two systems also enter the composition of L.m. 1. The central pathway of some motor cranial nerves, going from the cerebral cortex to the motor nuclei of these nerves. This pathway joins L.m. from the pyramidal tract in the lower parts of the crus cerebri. In the Varolian bridge it is placed in the form of a round bundle at the inner end of L. t. and bears the name of the accessory (internal) bundle of the loop (Bechterev), loop from the crus (Flexig), pes lemnisci superficialis, s. medialis, or external pontine bundles (Schlesinger). 2. The second system, described under the name of pes lemnisci Meynert's, pes lemnisci profundus, s. lateralis, departs from the external parts of the basis of the crus cerebri, crosses substantia nigra and joins L.m. It was expressed that it is a descending bundle of globus pallidi. Blood supply of L.m. receives (depending on the level) from art. cerebelli poster., from branches of art. cerebri post. The principal loop is the central conductor of sensitivity (second neuron), carrying to the thalamus opticus (and according to some authors also to the cortex of the central gyri) sensations of deep sensitivity, muscle sense and partly of touch from the opposite half of the body, as well as of the face, since fibers from the nuclei V, IX and X of the cranial nerves enter the composition of L.m. The presence in L. t. of fibers from the pyramidal tract for the nuclei of the oculomotor nerves and for the nucleus of n. accessorii indicates its participation in motor function (see Pyramidal system). Damage to L. t. is accompanied by disturbance of deep sensitivity, muscle sense and touch on the opposite side of the body and face depending on the localization of the process at various levels of the brainstem. Isolated damage to L.m. is not observed due to the close relationship with surrounding formations and common blood supply, but disturbance of sensitivity enters as a symptom in many syndromes described at damage to the brainstem: Benedict's, Foville's, Raymond-Cestan's, Avellis's, Babinski-Nageotte's syndromes (see Alternating syndromes). Lemniscus lateralis, s. inferior, the lateral or external loop, laqueus, consists of two systems of fibers: the anterior part from central auditory conductors, the posterior - from fibers of the Gowers bundle. The auditory part, or L. lateralis proper, forms the third auditory neuron and is a complex formation; its composition includes 1) fibers from oliva superior, 2) a part of fibers of corporis trapezoidei (2nd auditory neuron) of its and opposite side, 3) striae acusticae, or medullares (Monakow's auditory stripes). In the lower parts of the Varolian bridge all these fibers gather around oliva superior, lying at the outer end of L. medialis (Fig. III); a little higher instead of oliva superior appears nucleus L. lateralis, which gives a new amount of fibers in L. 1. In the upper parts of the Varolian bridge and in the crus cerebri L. 1. lies perpendicular to L. t., approaches the surface of the brain and takes part in the formation of trigoni lemnisci (Fig. IV). At this level to it join sensory fibers of the Gowers bundle. In the lower parts of the crus cerebri a part of the auditory fibers of L. 1. terminates in corpus quadrigeminum poster. (Fig. V and VI), and the rest, the greater part - a little higher, in corpus geniculatum mediale. In these two formations begins already the 4th auditory neuron, going to the cortex of the temporal lobe (see Hearing). Fibers of the Gowers bundle together with L. t. go to the thalamus opticus. Blood supply of L. 1. is the same as that of the principal loop. By its function L. 1. is the 3rd auditory neuron, conducting auditory sensations mainly from the opposite side, and partly also from its own. Damage to L. 1. gives almost complete deafness to the opposite ear and a decrease in hearing on the side of damage. Isolated damage to the lateral loop is not observed, but it suffers in various processes capturing the corresponding places of the Varolian bridge and the crus cerebri.

E. Kononova.

LATERAL LEMNISCUS, MEDIAL LEMNISCUS formations do not cause any painful phenomena and turn out to be an accidental finding during operations or at autopsy. In other cases, due to partial compression of the lumen of the intestine, they cause stasis of the contents of the intestine, the result of which can be phenomena of general intoxication, persistent constipation and pains. The latter are localized mainly in the area of the cecum, have a constant character without typical attacks for appendicitis, intensify during walking, work, sometimes during lying on a certain side. They also indicate bloating and a feeling of distension in the area of the navel. On palpation the Mac Burney point is painless or slightly painful, and pressure on the right at 2--4 cm from the navel causes sharp pain (Borovsky). Len and Jordan consider that these bands can cause painful phenomena in the area of the gallbladder and even simulate a gastric and duodenal ulcer. In these cases with the removal of the bands all phenomena disappeared. The uncertainty of the clinical picture makes the diagnosis of this disease difficult, and only in rare cases before the operation it becomes possible upon exclusion of a number of diseases of the organs of the abdominal cavity. Upon discovery during the operation L. t. removal of the appendix, division of the bands and peritonization of the denuded from the peritoneum parts is recommended. In cases of diffuse spread of films in the abdominal cavity and sharp violation of the patency of the intestines, it is suggested to make an anastomosis between the ileum and the transverse colon or sigmoid colon.