Tendon Reflexes

TENDON REFLEXES, the contraction of a muscle in response to the irritation of a tendon. Tendon reflexes are not elicited from all the muscles of the body: those from the extensors of the leg and thigh (the knee and Achilles reflexes) are the most constant, and their absence in the normal state occurs only in exceptional cases. Tendon reflexes on the upper extremities with the biceps and triceps are somewhat less constant. In the region of the face there is the mandibular or masseter reflex. In addition to these reflexes, which have the greatest clinical significance, a number of other tendon reflexes were described (by Bekhterev) that differ considerably in their constancy and great variability of manifestation. These include the dorsal foot reflex, or Mendel-Bekhterev reflex, the scapulo-humeral reflex, or reflex from the infraspinatus muscle, the reflex from the flexors of the fingers of the hand, and others. Tendon reflexes are the simplest type of reflex reactions, carried out through the spinal cord, and, compared with other reflexes, are distinguished by the shortness of their latent period. This is explained by the shortness of the reflex path traversed by the excitation, as well as by the direct transmission of excitation from the sensory neuron to the motor neuron without the mediation of intermediate (insertion) neurons. The mechanism of the origin of tendon reflexes is explained in different ways. Some (Hoffmann) consider that the essential factor is the rapid and jerky stretching of the muscle upon a blow to its tendon, serving as a specific irritant for special receptors located in the muscle itself. The afferent arc consists of the sensory fibers that originate from these receptors, first in the composition of the motor peripheral nerves, and then through the posterior roots of the spinal cord to the motor nuclei of the corresponding muscles in the anterior horns of the spinal cord, from which originate the motor fibers that travel in the composition of the anterior roots and peripheral nerves to these same muscles. Thus, tendon reflexes can be regarded as a proprioceptive reflex of a muscle upon itself ("Eigen-reflex"). According to another opinion (Forster), tendon reflexes are caused directly by the irritation of receptors located in the tendons themselves; the sensory fibers that originate from these receptors, entering the spinal cord, give off collaterals to the motor nuclei not only of the corresponding muscles but also of other muscular groups (agonists, antagonists, and synergists) of the same and the opposite side. The state of tendon reflexes depends both on the state of the peripheral organ (bones, joints, muscles) and on the state of the peripheral and central nervous system. Modifications of tendon reflexes can be expressed in a lowering of the reflexes to the point of their complete extinction, an increase in them, and perversion. Violations of the integrity of the peripheral organ in the form, for example, of ruptures of tendons, muscular atrophy of various origins, and others, as well as a violation of the conduction of the reflex arc upon the pathological process affecting any of its divisions (neuritis, radiculitis, tabes dorsalis, foci in the gray matter of the spinal cord), lead to a lowering and extinction of tendon reflexes from the corresponding muscles. These same factors can be the cause of perversion of tendon reflexes, expressed in the contraction of other muscular groups in the absence of contraction of the muscle to which the irritated tendon belongs. In the initial stages of disease, there is often an increase in tendon reflexes, which depends on the irritation of the pathological process of the sensory or motor neurons of the reflex arc. Modifications of tendon reflexes upon lesions of the central nervous system that localize above the level of the closure of the reflex arc depend on the nature and place of the lesion. A complete transection of the spinal cord, especially if it occurs suddenly, as for example in hemorrhages into the spinal cord, is often accompanied by the complete extinction of tendon reflexes whose centers are located below the level of the lesion. This extinction of tendon reflexes depends mainly on the onset of the nervous system current (diaschisis). Lowering and extinction of tendon reflexes is observed with increased intracranial pressure (new formations of the brain, hydrocephalus, etc.), apparently due to the compression of the posterior roots of the spinal cord by the cerebrospinal fluid. Lesions of the pyramidal system lead, as a rule, to a greater or lesser increase in tendon reflexes depending on the degree of the lesion. An increase in tendon reflexes can be expressed in an acceleration and strengthening of the contraction of the muscle, in its multiple contraction in response to a single irritation up to the transition into clonus, and in the irradiation of excitation both longitudinally and transversely through the spinal cord. Lesions of various divisions of the extrapyramidal system, in contrast to the pyramidal system, do not directly influence the state of tendon reflexes to a significant extent, but can indirectly modify them, for example, due to the change in tone of the corresponding muscles caused by them. In addition to organic lesions of the central nervous system, modifications of tendon reflexes that can reach a considerable degree are observed with a general increase in the excitability of the nervous system of functional origin. Exhaustion after intense physical exertion, for example after intense sports training or prolonged heavy physical work, after an epileptic seizure, leads to a lowering of tendon reflexes. The same is observed under chloroform anesthesia, with hyperthermia, and also with various endogenous and exogenous intoxications, for example with diabetes, nephritis, and hypothyroidism. With other intoxications, for example with strychnine poisoning, with uremia, and tetanus, the opposite, an increase in tendon reflexes, is observed. The mechanism of the origin of changes in tendon reflexes under the influence of the above-mentioned factors on the nervous system is different depending on which particular division of the central or peripheral nervous system the agent acts upon.

G. Polakop