Muscle spindle reflex

Nerves differ in their sensitivity to local anaesthetics. When lidocaine (lignocaine) is applied to a mixed peripheral nerve the onset of the block is in the order, vasodilatation (B fibres), loss of pain and temperature (C and A6 fibres), muscle spindle reflex (Ay fibres), motor and pressure (A(3 fibres) and large motor and proprioception (Aa fibres). This phenomenon is called differential block. There are other minor variations in this ranking order among the local anaesthetics. The basis of differential block is thought to be the result of variability in the sensitivity of different nerves to the same agent. 

Muscle spindles are intrafusal mechanoreceptors that are widely distributed within skeletal muscle fibers in the belly of the muscle. They exist in parallel with the much larger extrafusal skeletal muscle fibers, and the cotmective tissue around the muscle spindles is continuous with the connective tissue around the other muscle fibers. The muscle spindles mediate a response to a load placed on the muscle this is known as the load reflex. Muscle spindles have a dampening function, as well. They prevent some types of oscillation and jerkiness in body movement. In fact, tremors noted, especially during times of extreme anxiety or stimulation, represent a failure to dampen this response smoothly. In addition to serving at a subconscious level, the muscle spindle reflex is invoked in voluntary motor activity. 

Servo-assist function of the muscle spindle reflex. If the extrafusal fibers contract less than the intrafusal fibers, then the muscle spindles will maintain a stretch reflex to further excite the extrafusal fibers. This technique is used in active resistive myofascial therapy. 

Botulinum toxin induces weakness of striated muscles by inhibiting transmission of motor neurons at the neuromuscular junction. This has led to its use in conditions with muscular overactivity, such as dystonia. Transmission is also inhibited at y neurons in muscle spindles, which may alter reflex overactivity. 

A simple example of the muscle spirrdle reflex is the patellar tendon reflex. The knee, when flexed to 90 degrees, places the quadriceps muscle into a relatively stretched position The sudden strike of a hammer against the tendon resrrlts in a dynamic stretch of the spindle and firing of the la (and possibly II) fiber the alpha motor nemon is then stimulated and in tiun induces quadriceps contraction arrd knee extension (Fig. 6-4B). 

Slow stretch of the length of the receptor portion of the muscle spindle produces the static stretch reflex. The rtumber of impulses transmitted fi-om the primary arrd secondary endings increase in proportion to the amount of stretch. 

FIG. 6-4 (A) Neutral position. (B) Stretch of extrafusal and intrafusal alpha motor neuron stimulated to reflexively initiate extrafusal muscle contraction. (C) Gamma motor neuron innervation of muscle spindle. Contraction of ends stretches spindle, activating lb fibers, lb sensory fiber activates alpha motor neuron to cause extrafusal muscle contraction. (D) Compression of whole muscle or contraction of extrafusal muscle results in shortening of muscle spindle and deactivation of sensory fiber firing. 

Both the muscle spindle and the Golgi tendon organ reflexes serve the same basic purpose to... 

Stretch reflex. The stretch of a muscle excites the muscle spindle mechanism, resulting in reflex contraction of that muscle. This is to be avoided when applying passive myofascial techniques to contracted or con-tractured muscles. It can be avoided by applying slow, even force and releasing that force slowly and evenly. However, the stretch reflex is to be used during active myofascial... 

The deep tendon reflexes (DTRs) are automatic responses of muscle spindles if the tendons were stretched suddenly [1], Such reflex can be obtained at different parts of the body such as the biceps, triceps, knee or ankle. DTRs are often showed as immediate muscle contraction once tap is applied on the tendon [1],... 

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