Rear string-lemnic rail system

Rear string-lemnic rail system

The conveyor system conveys high-resolution mechanoreception.

The signals, which are led into the spinal cord by large and medium-sized primary-afferent nerve fibers, spread partly via lateral branches into different neuron groups in the back and front horns and partly via the ascending main branches of the primary afferents in the inner and outer hind cord bundle and then form the " hind cord path". This runs uncrossed upwards into the spinal cord and into the medulla.

This may involve the longest nerve fibers found in the human body. Take, for example, an Ia-afferent thread that "starts" in a muscle spindle in one of the short flexor muscles of the toes. This nerve fibre will then run in the sole of the foot, up along the bone, in through an intervertebral hole where its cell body is located in the spinal ganglia, continue in the hind root, enter the spinal cord and make its way all the way up to the extended marrow where it ends in the inner posterior cord nucleus; a total distance that corresponds to the body length (approx. 2 m.).

The hindstring track conveys very detailed information (so-called high-resolution mechanoreception) about pressure, touch and elongation from the skin (exteroception) as well as information from the musculoskeletal system (proprioception) about how quickly different muscles contract, how long the muscles are and how much force they develop as well as the degree of bending and/or stretching in the joints. The mixture of these different detailed information about the effects of the skin and about the course of the musculoskeletal system is coordinated with each other up in the brain. Thanks to this, we can:

1/indicate with very high accuracy where an irritation occurs on the skin surface (1-point discrimination),
2/distinguish 2 excitation points on the skin from each other (2-point discrimination; 2-PD),
3/become aware of changes that occur in joints and muscles during different movements (kinaesthesia),
4/without looking, determine the nature of an object and sort out different objects from each other, e.g. you easily distinguish different Swedish coins from each other directly with your hand in your pocket (stereognosy) and
5/Perceive vibrations.
These five sensory impressions are sometimes summarized as the "backstring qualities".

The threads of the hind string bundles reach up to the lower part of the medulla oblongata. There they stop and the signals are switched over to new neurons. These are grouped together in two cores; the hindstring nuclei, (nucl. gracilis o nucl. cuneatus). The nuclei give rise to the clearly visible posterior tubercles (tuberculum gracile and tuberculum cuneatus) on the dorsal side of the medulla oblongata.

The axons of the hindstring nuclei cross directly across the mean line. The result is that the stimuli registered on the left half of the body (up to the chin) from o with this crossover are mediated in the hay half of the brain. The junction is called the lemnic junction and the threads after the crossing form a well-cohesive bundle of nerve fibers, the medial lemnisk. The medial limb pulls up through the medulla oblongata, pons and mesencephalon to end in the thalamus. There, the signals are switched to a new group of neurons which form the so-called "VPL nucleus". The axons of the VPL nucleus run out into the capsula interna and reach up to the bark of the post-centralis gyrus. There they end in the primary somato-sensory cortex, SSI (cortexlamina IV in Brodmann areas 3,1,2) with reasonably maintained somatotopia which is the background to the so-called sensory homunculus.

High-resolution mechanoreception from the face, mouth, nose and eye socket is mediated by n.trigeminus into the pons, rewired there, transmitted in the trigemino-thalamic pathway system over to the opposite side of the thalamus (VPM nucleus) and from there out into the capsula interna to the gyrus postcentralis, primary somato-sensory cortex, SSI.