The muscle spindle is one of the two types of sensory receptors in skeletal muscles. One of its functions is to give the cerebral cortex - therefore, the individual - proprioceptive information; that is, the ability to recognize the place in space of its anatomical segments.
The importance of knowing this anatomical structure lies in the fact that studies have been carried out that indicate its possible involvement in the sensory and motor development of humans, as well as in the expression of various pathological signs inherent in clinical syndromes, such as motor neuron syndrome. lower or higher.
Features
In summary, the functions of the muscle spindle can be summarized in two major actions:
- Transmission of proprioceptive information from the body segments to the cortex.
- Generate an environment of functional relaxation before stretching, in such a way that it is responsible for the prevention of injuries due to muscle overstretching.
Physiology
Intrafusal fibers exist in relation to two types of nerve fibers: with afferent fibers, which collect the stretching information from the muscle and send it to the spinal cord; and with efferent nerve fibers, which send motor information from the spinal cord to the spindle.
Sensitive information travels through two types of fibers. The first are called dynamic or type 1 and transmit to the spinal cord information related to changes in size and speed of the muscle belly. They do this by recording variations in the action potential.
Second are the static or type 2 calls, which only respond to changes in muscle length.
The motor innervation of the neuromuscular spindle is given by fibers known as gamma motor neurons, which are found in the anterior horns of the spinal cord.
Skeletal muscles physiologically have the function of contracting and, successively, returning to their resting position (stretching).
However, there must be a functional limit to this stretch; said limit is monitored by the central nervous system through the muscle spindles as described below:
- For the movement of muscle groups, the generation of an action potential is necessary, which will generate the necessary conformational changes that will ultimately result in muscle contraction. This will be followed by a period of relaxation or stretching of those contracted muscle fibers.
- This stretching results in the activation of the proximal ends of the intrafusal fibers, the opening of ion channels and, consequently, the generation of an action potential by depolarization and the conduction of information related to the stretching of the muscle fibers.
- Finally, the intrafusal fibers receive impulses that are conducted through gamma motor fibers (cells that maintain tension and the sensory capacity of the muscle spindle) and propagate them towards extrafusal fibers, resulting in the generation of force and resistance to stretching, causing a purely functional relaxation.
Constitution and location
The muscle spindle is located within the muscle fibers of skeletal muscles. Skeletal muscles are all those muscle groups that are in direct relationship with bone tissues and that respond to the will.
That is, the mobilization of skeletal muscles is linked to the individual's desire, with certain exceptions such as pathological states or in the case of tendon reflexes.
With regard to the constitution of the spindle, an elongated cylindrical structure stands out, the central part of which is thicker in relation to the surrounding tissue.
Inside it there may be more than two muscle fibers with functional and specialized characteristics such as stretch mechanoreceptors (mechanical stretch receptors). Since these transformed fibers are in the central part of the spindle, they are called intrafusal fibers.
Two components are histologically described within intrafusal fibers: a component that varies from 2 to 4 fibers, also known as nuclear sac bag fibers; and another component that goes from 4 to 12 fibers, whose core is arranged in straight chains and, therefore, are called nuclear chain fibers.
On the other hand, the term extrafusal fibers corresponds to all those skeletal muscle fibers that are not part of the neuromuscular spindle, and this term is coined for the sole purpose of differentiating them from intrafusal fibers.
Pathologies
Certain clinical entities are described after trauma to the central nervous system or clinical pictures secondary to diseases.
One of these cases is cerebrovascular disease, in which there is an alteration in the sensitivity of the neuromuscular spindles and, consequently, the stretch reflexes will be altered, expressing themselves in the form of pathological postures, spastic paralysis of limbs or muscle groups.
According to studies that follow the natural history of chronic tension headaches as well as migraine headaches, hypotheses have been obtained according to which the neuromuscular spindle has a leading pathophysiology of these clinical entities.
Physiopathologically, the condition is attributed to a progressive, sustained and chronic sympathetic stimulation of the neuromuscular spindles, which leads to excessive tension in the latter, to acute painful episodes and to symptoms in the context of a tension headache.
References
- Moreno F. Histological description of the neuromuscular spindle. Salutem Scientia Spiritus 2015; 1 (1): 48-52
- Arthur Prochazka and Sergiy Yakovenko. "Locomotor control: from spring-like reactions of muscles to neural prediction". Recovered from: ualberta.ca
- Prochazka A. Proprioceptive feedback and movement regulation. In: Exercise: Regulation and Integration of Multiple Systems, edited by Rowell L, and Sheperd JT. New York: American Physiological Society, 1996, p. 89-127.
- Functioning of the muscle spindle. Recovered from: accessmedicina.mhmedical.com
- Muscle spindle dysfunction. Recovered from: encolombia.com