He Myothetic reflex Is a reflex mechanism of the organism, designed to avoid injuries. This reflex occurs when the neural receptors located in the middle of the muscles are activated upon detecting a sharp stretch inside the muscle.
When a muscle lengthens, so-called neuromuscular spindles-which are receptors that detect changes in muscle length-are activated. This increases the Activity of motor neurons , Which causes the fiber to contract and can withstand stretching.
The receptors inside the muscle send a signal to the spinal cord to contract the muscle and another signal to relax the antagonist muscle. Thus, the muscle is always maintained at the same length, protecting the tissues.
Mechanism of the myotatic reflex
Reflexes are automatic and subconscious mechanisms that respond to changes within or outside the body. Its function is to maintain the homeostasis of the body - that is to say the autonomic reflexes that include the blood pressure, frequency of heartbeats and respiration.
In the case of the myotatic reflex, it corresponds to the contraction of the muscle that occurs in response to its stretching. It is not controlled by the brain , But corresponds to a monosynaptic response that is transmitted to the spinal cord.
It is a simple reflex and a programmed response of the organism in case the muscle is passively stretched, such as a change in the surface of the ground when walking.
It can be activated externally, for example by placing a load on the muscle or striking the knee with a reflex hammer; Or activated internally, when motor neurons are stimulated from the inside. This is observed when a person is cold and begins to tremble, stimulated by the Neurons Muscles to warm up.
When the muscle is stretched, an impulse is sent through the sensory neurons to the segment of the spinal cord corresponding. This nerve does Synapses With a second nerve within the same spinal cord, without needing to reach the brain.
Then, the nerve impulse is simply transmitted through the spinal cord to the muscle, causing the muscular response in seconds.
Anatomy of the myotatic reflex
The circuit inside the spinal cord is responsible for a number of sensory and motor reflex actions, such as the case of the myotatic reflex. In this case, the reflex reflex simplicity is triggered by the direct response of the motor neurons that innervate the muscle that has been stretched.
Muscles have the so-called neuromuscular spindles, which in turn are located inside the intrafusal muscle fibers ("fusus"is a Latin term for"spindle"), which have tenderness to muscle movements.
Motor neurons or motor neurons are responsible for the activation of these intrafusal muscle fibers. They are only innervated in these fibers, therefore they are also known as fusimotor neurons.
These motor neurons also known as gamma motoneurons are responsible for regulating the sensitivity of the myotatic reflex by contracting or relaxing intrafusal fibers within the neuromuscular spindle.
There are two types of intrafusal fiber nuclear fiber bags and nuclear fiber fiber. Its difference lies in its sensitivity differences. Nuclear fiber bags respond to small stretches, while the nuclear fibers are activated in sustained stretches, responding according to the degree of stretching.
The function of these motoneurons is not to support the force of the muscle-work contraction charged to the extrafustal fibers but to modify the sensitivity of the neuromuscular spindles for the stretching.
With the release of acetylcholine - which corresponds to a neurotransmitter related to the transmission of nerve impulses - the motor neurons are activated, contracting the intrafusal muscle fibers and at the same time elongating the non-contractile central portions. This builds strength and resists stretching, while the antagonistic muscles relax.
It is worth noting that 31% of all motor nerve fibers in the muscle are fibers belonging to the neuromuscular spindles. Any signal transmitted to the motoneurons stimulates gamma motor neurons simultaneously, making the intrafusal and extrafusal fibers of the muscle fibers contract at the same time, generating movement.
Usefulness of the myotatic reflex
Myothetic or stretching reflex helps maintain body balance. When the person crouches or leans, the skeletal muscles tighten and contract, and that helps maintain the structure and prevent the body from falling.
Even when standing, it is this reflex that keeps the skeletal muscles in a complex order of contraction and relaxation, making the necessary adjustments to maintain an upright posture of the body.
The myotatic reflex is significantly connected to the spine, the receptors of the muscles are directly related to the spinal nerves attached to the vertebrae and spinal cord.
Interestingly, if a stretch is sustained in a muscle for an extended period of time, and the muscle is held in the stretching position, the neuromuscular spindle becomes accustomed to the new length, reducing the myotatic reflex. Myotatic reflex receptors can be trained to increase muscle length and increase flexibility, as occurs in professional athletes and dancers.
Myothetic reflex in clinical practice
When a person suffers an attack or a major injury to the spinal cord, spastic paralysis often occurs, where the muscles increase their tone abnormally, with rigidity and lack of elongation. Thus, myotatic reflex in the flexor muscles of the arms and extensor muscles of the legs has a hypersensitivity
The result of this are abnormal postures, stiffness and contractures. Hypertonia - which is too high in muscle tone - is also a result of hypersensitivity of motor neurons. This can be seen in Cardiovascular accidents , Paralysis and Parkinson's disease .
Also the myotatic reflex is used as a diagnostic method in the case of injuries in muscles or nervous system. When the response to the stimulus is zero, the result is always abnormal. The response is expected to be always fast, indicating a normal reflex. On the other hand, a rhythmic and repetitive muscle contraction, known as clonus, is also an abnormal response.
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