Osteotendinous reflexes: what they are, how they work, and associated pathologies

In neuroscience, it is known as a reflex to the nervous activity developed in the spine (and the brainstem) consisting of an involuntary response to a sensory stimulus, either internal or external. Generally, we associate reflexes with rapid and uncontrollable jerking movements, but another example of this activity is also the activation of a gland and the secretion of a given compound into the stream blood.

In any case, at a general level all reflexes are involuntary, unplanned, sequential and practically instantaneous. The initiation of a reflex is achieved thanks to the neural pathways and reflex arcs, that is, the nerve pathway that runs through the vertebral arch and controls a given reflex act. It should be noted, at this point, that there are 2 types of reflex arches: autonomic (affecting internal organs) and somatic (affecting muscles).

With all this information, we are able to paint a general picture that allows us to understand what reflections are and what they are for. Anyway, this time we are going to talk specifically about

tendon reflexes, muscle contractions in response to stretching within a muscle.

• Related article: "Reflex arc: characteristics, types and functions"

What are tendon reflexes?

In humans, when a muscle is struck vigorously, it immediately contracts due to a reflex arc composed of 2 neurons, which also involve the segment of the spinal trunk that innervates the analyzed muscle structure. These are the tendon reflexes themselves. For this special type of reflex to occur, the following physiological elements must be present:

• Receptor: in this case we are dealing with muscle receptors (spindles), which will capture the sudden “stretching” of the unit after the external stimulus.
• Afferent nerve fiber: this is made up of the axon of the sensory neuron. It is found in the spinal ganglia and innervates the neuromuscular spindle (sensory receptors within the belly of the muscle).
• Integrating center: it is located in the spinal cord and the synapse between the afferent and efferent neurons is produced there.
• Efferent nerve fiber: it is the axon of the motor neuron. It carries the signals of the motor nerves from the anterior horn of the spinal cord to the muscles.
• Muscular unit: it is the one that carries out the contraction response itself and is innervated by the efferent fiber. In other words, it is about the structure that responds to external stimuli.

The osteotendinous reflexes that are usually explored, depending on the area stimulated, are the bicipital, tricipital, style-radial, ulnar pronator, patellar and achilles.. The type of reflex and the response shown always reveal something about the state of the elements of the nervous system involved in its appearance.

When you want to assess the state of the reflex arches, the professional applies a slight force to an area of ​​the body, which translates into a slight elongation of the muscle fiber. This act activates the neuromuscular spindle, formed by a set of sensory receptors inside the muscle that detect changes in its total length.

These receptors send an afferent impulse to the spinal cord, where a direct synapse occurs with the motor neuron. The latter emits the efferent signal back to the muscle, allowing it to contract. As you can see, this is a very simple circuit: it has to be like that, because thanks to the proximity of the structures involved, the tendon reflexes occur so quickly.

The Medical Importance of Tendon Reflexes in Medicine

At this point, it should be noted that there are several conditions that can be suspected by the patient's tendon reflexes. On the one hand, hyperreflexia refers to a pathological situation in which the individual suffers hyperactive or repeated reflexes over time (clones).

Apart from muscle spasms, autonomic hyperreflexia causes changes in heart rate, excessive sweating, high blood pressure, and changes in skin color. The most common cause of this clinical entity is an injury to the spinal cord, although it can also occur from certain syndromes, drug side effects, or after head trauma serious.

On the other hand, hyporeflexia and areflexia are events in which the muscle does not produce any response to the application of force. It is a situation that reflects a failure or interruption in the reflex arc, either in the efferent nerve fiber or in the afferent nerve fiber. or, on the other hand, it shows conditions in the patient such as hypothyroidism, blood electrolyte disturbances or myopathies.

• You may be interested in: "Parts of the Nervous System: anatomical structures and functions"

The Osteotendinous Reflex Scale

Tendon reflexes are quantified in the clinic when a nervous or neuromuscular pathology is suspected in the patient. To carry out this type of test, the muscular structure to be analyzed must be in a neutral position, but before Therefore, the professional must locate the tendon associated with the musculature (for this the patient must flex the muscle).

Having found the structure, a rapid and sudden force is applied to the relaxed tendon area, which should be translated into a rapid and involuntary muscle contraction, or what is the same, the osteotendinous reflex that concerns us here. This can be valued in the following categories:

0 = there is no response from the muscle and it is always considered a pathological situation. 1 (+) = a mild but obvious muscle response. There are traces of response or a complete one can be promoted with repetitions of the stimulus. It can be normal or be indicative of a pathology of a neuromuscular nature. 2 (+) = a fast muscle contraction response. Get into normalcy. 3 (+) = a very energetic contraction response. It can be normal or indicate a pathology on the other side of the spectrum. 4 (+) = the application of force always causes repeated (clonic) reflexes. It is an abnormal situation in all cases and indicates a clear maladjustment at the nervous level.

Whether an osteotendinous reflex from 1 to 3 is normal or abnormal depends on its previous state, that is, what results the patient obtained in the past with respect to the same tests. A more accurate diagnosis can be reached based on other tests that assess muscle tone, contraction force, and other possible pathological evidence..

It should also be noted that the result of these analyzes is subjective, since it depends on the perception of the healthcare professional and the examinations they have carried out in the past. It is not so important that a doctor classifies one reflex as 2 and another as 2+, but rather to date the difference in response of the tendon reflexes in different parts of the body of the same patient. The absence (or decrease) of a reflex in one part of the arm and its normality in the analogous limb indicates that there is a problem, for example.

A multitude of contraptions can be used to cause slight elongation of the muscle fiber to be tested, but specialized small hammers are always recommended for testing. These They come in 3 types according to their shape: triangular (Taylor), T-shaped (Tromner) and circular (Queen square). All are effective in causing reflections, but it is recommended to avoid using the Taylor model in those patients with hyperreflexia, as it is the least effective in promoting reflexes osteotendinous.

On the other hand, although it sounds strange, sometimes the use of the fingers is also used (very useful in patients with hyperreflexia) and, even, the edge of a Smartphone can be used. It is much more important to find the point where the pressure should be applied than the material with which it is made.

Resume

The world of osteotendinous reflexes is very complex, since a series of concepts of neuromuscular physiology must be clear that only those specialized in the field can acquire. If we want you to have a clear idea, this is the following: the reflex arc of reflections osteotendinous is composed of 2 neurons, an afferent and an efferent, which communicate in the integrating center. The response to the pressure stimulus is very fast and can be quantified numerically.

The fact that the patient has hypo or hyperreflexia is always indicative of a pathology, either in the neurons of the circuit or in the internal spinal cord itself. Detecting these abnormalities is essential to put in place accurate diagnostic mechanisms and start treatment as soon as possible. For this reason, tendon reflexes are extremely important in medical practice at the neuromuscular level.

Bibliographic references:

• Dick, J. P. R. (2003). The deep tendon and the abdominal reflexes. Journal of Neurology, Neurosurgery & Psychiatry, 74 (2), 150-153.
• Lemoyne, R., Dabiri, F., & Jafari, R. (2008). Quantified deep tendon reflex device, second generation. Journal of Mechanics in Medicine and Biology, 8 (01), 75-85.
• Péréon, Y., Tich, S. N. T., Fournier, E., Genet, R., & Guihéneuc, P. (2004). Electrophysiological recording of deep tendon reflexes: normative data in children and in adults. Neurophysiologie Clinique / Clinical Neurophysiology, 34 (3-4), 131-139.
• Rodriguez-Beato, F. Y., & De Jesus, O. (2020). Physiology, Deep Tendon Reflexes. StatPearls [Internet].
• Walker, H. K. (1990). Deep tendon reflexes. Clinical Methods: The History, Physical, and Laboratory Examinations. 3rd edition.