■ Convergence Projection
Two alternative connections in which afferences are switched over to the efferent neuron exist in the bone marrow:
• An afferent nociceptive impulse from the skin and muscle or an internal organ is switched to one inter-neuron in the bone marrow that is responsible for both afferences, before this neuron is again switched to the efference, to respond to the stimulus.
• Skin, muscle, and visceral afferences have a shared end path, before the stimulus is conducted to the efference.
Afferent information is not only conducted to the efference to respond to the stimulus, but also through the spinothalamic tract into the central nervous system (CNS). As the afferent stimulus reaches it, the CNS is unable to distinguish whether the nociceptive impulse comes from the skin/muscle or an internal organ (Fig. 14.1). Because our body, or rather the CNS,
Fig. 14.1 Formative paths of transmitted pain.
Posterior root i i Spinothalamic tract
Sympathetic trunk ramus communicans
■ Viscera has learned in the course of life that nociceptive, that is, harmful irritations generally hit the body from the outside, they are interpreted as coming from the skin or muscle: a pain stimulus that has been passed on for conscious perception through the spinothalamic tract is recognized as pain referred from the segmental^ associated skin.
Afferent impulse activity from a trigger point is treated by the CNS like a nociceptive afference from an internal organ: The perception of pain takes place in the skin, that is, in the segmentally associated reference zone.
■ Convergence Facilitation
Many afferent nerves carry out a background activity. We can say that they generate a type of base noise, an impulse activity that does not stem from external (or internal) stimuli, but has to be explained neurophysio-logically as a lowered stimulus threshold due to changes in the ion channels. Consequently, action potentials are more likely to be triggered. This can be considered as a protective mechanism towards nociceptive stimuli that can therefore be recognized and responded to more quickly.
If such background activity in an area of the skin is heightened (convergence-facilitated) by a series of afferent nociceptive stimuli from an internal organ or trigger point and conducted to a neuron of the spinothalamic tract into the CNS (see Convergence Projection above), the pain is then perceived as very strong in that area of the skin.
■ Axonal Ramifications
The dendrites of afferent nerves can branch out into diverse directions so that different areas of the body are captured sensitively by this nerve. This can result in the misinterpretation of an afferent stimulus by the CNS: the individual regions of the body can no longer be differentiated from the axon mound—as a consequence, the pain is perceived as stemming from the entire area that is innervated by the neuron.
■ Sympathetic Nerves
It is possible that these nerves help in sustaining the referred pain by releasing substances that further sensitize the nociceptive afferences of the pain area and lower their stimulus threshold. It is also conceivable that sympathetic innervation causes the blood supply of the afferences from the pain area to be reduced.
■ Metabolic Derailment
The trigger point zone is an area of the muscle that is characterized by metabolic derailment. We find here a combination of increased demand for energy with simultaneous lack of oxygen and energy. This situation most likely results from the reduced blood circulation in the area. A vicious cycle develops that ends with the formation of trigger points in the muscle zone with lowered energy supply. Already existing trigger points can likewise be sustained by metabolic derailment.
■ Muscle Stretching Affects Muscle Metabolism
When contracted sarcomeres (see below) are extended to their greatest length by stretching, this has immediate results for the muscle. First, this reduces the consumption of adenosine triphosphate (ATP) and normalizes metabolism. Second, it decreases muscle tension.
If a metabolic derailment has released substances into the muscle (such as prostaglandin) that can set into motion certain pathomechanisms relevant to trigger points, their concentration declines when the metabolism is returned to normal. It is also suspected that the irritability of the afferent nociceptic nerve fibers is normalized by a balanced metabolism.
■ The Hypertonic Palpable Muscle Spindle
The hypertonic palpable muscle spindle is a rope-like muscle segment of 1-4 mm thickness surrounding the trigger point, which is noted during palpation because of its greater tightness compared with the surrounding muscle. This spindle stands out due to its hyperesthetic property, to the point of clear painful-ness. It is easiest to palpate this hypertonic muscle spindle when its muscle fibers are stretched just to the point where the fibers that are not integrated into the spindle remain relax.
Stretching or strong contraction of the spindle, or pressure on the trigger point within the muscle spindle can cause localized pain and also, after a certain period of latency, referred pain.
Muscle fibers in normal muscles contain sarcomeres that are all the same length. They arrange themselves lengthwise to allow for maximum muscle strength. To achieve this, the actin and myosin filaments must overlap to a certain degree. If they overlap too much or too little, the strength of the muscle is reduced.
Muscle fibers of hypertonic muscle spindles differ histologically: the length of the sarcomeres varies within the spindle. Thus, the sarcomeres around the trigger point are shortened without showing any electromyographic activity—they are contracted. Compensating for this, we find lengthened sarcomeres at the end of the muscle spindle near the musculotendinous junction.
This special property explains why a muscle with a hypertonic palpable muscle spindle exhibits reduced stretchability (contracted sarcomeres) as well as reduced strength (shortened and lengthened sarcomeres-sarcomeres outside the ideal length) (Figs. 14.2-14.4).
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