The major sensation from deep tissue, such as joint and muscle, is pain. In the absence of disease, we are not aware of sensory processes in the deep tissue. However, sensory information from muscle and joint continuously controls the activity of the motor system and is involved in the sense of movement and position.1 Pain significantly influences the motor control system and usually forces the patient to restrict movements.
Deep tissue pain is often dull and aching, and poorly localized, and is thus different from cutaneous pain which may be sharp and precisely localized.2 In particular, muscle pain is often aching and cramping and often referred to other deep tissue, such as other muscles, tendon, fascia, joint, and ligaments.3 In the normal deep tissue, acute and short-lasting pain sensations can be elicited by tissue-threatening mechanical stimuli, showing the excitation of nociceptors in deep tissue structures (see below under Nociceptors of deep tissue and peripheral sensitization). Clinically relevant pain in deep tissue is different. It usually appears as hyperalgesia or persistent pain at rest.2, 4 5 6 In the state of hyperalgesia, noxious stimuli cause stronger pain than normal, and pain is even evoked by mechanical stimuli whose intensity does not normally elicit pain, i.e. movements in the working range and gentle pressure, e.g. during palpation. Clinically relevant muscle pain often appears as a combination of ongoing muscle pain, tenderness, soreness (tenderness and stiffness), weakness, and paresthesias (sensation of pressure and tension) in the muscle.7,8
Some decades ago and again more recently, in order to gain more insight into the nature and origin of deep tissue pain, experimental invasive sensory testing was carried out in conscious humans. For example, pain in the normal joint can be elicited when noxious mechanical and chemical stimuli are directly applied to the fibrous structures, such as ligaments and fibrous capsule. No pain is elicited by stimulation of cartilage and stimulation of normal synovial tissue rarely evokes pain.5 Stimulation of fibrous structures with innocuous mechanical stimulation can evoke pressure sensations.5 In the muscle, pain can be elicited by noxious mechanical stimulation and also by high intensity thermal stimulation (48°C).9 Collectively these data show good correlation between the impact of noxious stimuli and the evoked pain sensations at least in the normal deep tissue. Accordingly, recordings from deep tissue afferents have revealed that deep tissue nociceptors reliably encode noxious stimuli.
Differences between cutaneous and deep tissue pain sensations have been pointed out. In addition to differences in pain sensation, autonomic responses to noxious stimuli can be different. In contrast to cutaneous pain, muscle pain typically elicits a drop in blood pressure, as well as sweating and nausea.10
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