Pilomotor Fibers

Pilomotor fibers, REFLEX. Pilomotor fibers innervate in the skin the smooth muscles-arrector pili (see), the contraction of which causes the raising of skin hairs and the formation of goosebumps (see). P. v. belong to the sympathetic system, they originate in nuclei located in the lateral horns of the spinal cord from C8 to L1 (nucleus sympathicus lateralis superior); there also originate vasoconstrictor and sweat fibers; at present the exact localization in the nucleus for each of the aforementioned types of fibers is not yet sufficiently clarified. The cells of the nuclei are round or oval, multipolar, with short dendrites; their size is from 12 to 60 μ, their number for each side of the spinal cord is about 88,500 (Thomas). Through the anterior roots, pilomotor fibers leave the spinal cord and enter the composition of the mixed nerve, from which through rami communicantes albi they reach the nodes of the sympathetic trunk, where they end, forming preganglionic fibers (fibrae preganglionares); their place of termination serves not only the nearby nodes, but also more distant ones above and below. In the nodes, postganglionic fibers (fibrae postganglionares) originate, which through rami communicantes grisei return to the mixed spinal nerve, giving rise to the peripheral nerve; in the composition of this latter, pilomotor fibers reach the skin, in the thickness of which they divide dichotomically several times, then enter the m. arrector pili and end in the muscle fibers with thickenings. Not all peripheral nerves contain pilomotor fibers, but only certain ones: for the upper limb, pilomotor fibers go in the nn. musculo-cutaneus, brachialis internus, radialis; for the trunk - in nn. intercostales; for the lower limb - in nn. femoralis, saphenus internus, cutaneus femoris lateralis et posterior; for the face - in the n. trigeminus (into the cranial cavity the fibers penetrate through the plexus caroticus internus). The zones of innervation of the pilomotor centers of the spinal cord (from C8 to L1) do not coincide with the zones of the sensory nerves of the skin due to the fact that preganglionic fibers go not only to the corresponding sympathetic node, but also to those above and below, and consequently from there they pass into various peripheral nerves. The pilomotor centers innervating the upper limbs are located in the D8-D11 segments of the spinal cord, partly also in D12; the higher the segments, the higher parts of the limb they innervate. For the lower limbs, similar centers are contained in the segments from D12 to L5; in the first three thoracic segments - centers for the innervation of the head, neck and upper parts of the trunk; in D12-L1 - for the lower parts of the trunk. Experimental research and some clinical data indicate the existence of pilomotor centers in higher parts of the brain - in the cortex and in the corpus striatum, but so far it has not been possible to establish the exact localization of these centers, nor the course of the fibers connecting these centers with the lower ones. It is only known that these fibers together with the vasoconstrictor and sweat pathways go in the brain stem through formatio reticularis and when passing into the spinal cord form a decussation at the level of the pyramidal decussation. Pilomotor reflex. Under the influence of irritation, pilomotor muscles contract and cause the formation of goosebumps. The irritation can affect the muscle directly (local reaction) or occur with the participation of the pilomotor centers of the central reflex arc; in the latter case, one speaks of the pilomotor reflex (syn.: pilomotor, pilomuscular). In the first case, the appearance of goosebumps is limited only to the place of irritation, in the latter - it captures a more extensive territory. Irritants causing the pilomotor reflex can be of physical and psychological nature. The main irritant is cold in all its manifestations - exposure of the skin at low temperature, application of pieces of ice, wet linen, moistening with ether. Light touch, tickling, electric current also cause the pilomotor reflex. In addition to skin irritation, the pilomotor reflex appears with sharp cries, unpleasant sounds, dissonances, during sneezing, yawning, coughing, when taking cold or hot drinks, with strong urges to urinate, to defecate, etc. Many emotional experiences - fear, anger, joy, pleasure - are accompanied by the appearance of the pilomotor reflex. The mere approach of already experienced irritants, such as cold linen, a piece of ice, etc., or even only the representation of both emotional experiences and some physical irritants, can cause the appearance of the pilomotor reflex. An intense pilomotor reflex is accompanied by a sensation similar to shivering. When examining the pilomotor reflex, certain conditions are necessary: good uniform light, warm room, absolute silence and turning off vision. The reflex appears most readily on the neck, shoulders, buttocks, etc. The hairs rise slowly and gradually (from 7 to 10 sec.) and even more slowly return to their original position (14-20 sec.); the curve of hair elevation resembles the curve of smooth muscle contraction. The hairs on the head, beard, armpits, on the eyelashes and eyebrows remain motionless. On the sole, on the palm, on the dorsal surface of the hand there is no pilomotor apparatus. With unilateral irritation, the pilomotor reflex is unilateral and only slightly extends to the other side; it can be partial and involve only part of the trunk. The intensity of the pilomotor reflex, its appearance and spread are not the same in all individuals; it appears faster and is more intense in anemic and exhausted individuals, in alcoholics, in persons with a labile nervous system. Pathology of the pilomotor reflex. Various diseases of the nervous system and skin affect the pilomotor reflex. In diseases of the brain (hemorrhages, softening), the pilomotor reflex changes depending on the time of examination. Examinations performed soon after the disease indicate the disappearance of the reflex, which appears after about three months; its appearance also depends on the state of sensitivity - in anesthesias the pilomotor reflex is absent. Pathological processes irritating the brain cause an increase in tone in the mm. arrectores pilorum, and the pilomotor reflex intensifies on the affected side. In some diseases of the spinal cord, such as tabes, poliomyelitis, amyotrophic lateral sclerosis, the pilomotor reflex remains normal; in syringomyelia, its changes are very diverse depending on the localization of the focus and its size. In transverse diseases of the spinal cord, the pilomotor reflex differs above and below the site of the lesion. Above, the reflex remains normal, due to the preservation of connection with the higher centers - encephalic pilomotor reflex. Below the site of the lesion, the pilomotor reflex first disappears, while the local reaction greatly intensifies; then in the lower segment spinal automatism develops, in which the pilomotor system also participates; the spinal pilomotor reflex appears simultaneously with the protective reflexes (see); at first it spreads in separate islands, and then - depending on the irritants; it is caused by the same irritants as the protective reflexes. In diseases of the peripheral nerves containing pilomotor fibers, the pilomotor reflex disappears only in the area innervated by the diseased nerve. Disease of the sympathetic system causes the disappearance of the pilomotor reflex in the corresponding areas. The pilomotor reflex changes in skin diseases when the pilomotor muscles are involved in the process. Hypertrophy of the mm. arrectores pili is observed in prurigo, urticaria, eczema; they atrophy together with the skin, for example in scleroderma; they disappear in leprosy, syphilis, skin tbc. In Basedow's disease, the reflex is greatly weakened, and sometimes disappears completely.