Pain Impulse Transmission and Perception
疼痛冲动的传播和感知
Two small-diameter sensory nerves, called the A-delta and C fibers, respectively, respond to stimulation by generating nerve impulses that produce pain sensations. The A-delta fibers are small, myelinated fibers that respond quickly to acute pain. The C fibers are unmyelinated and are slow conducting. Pain impulses from the skin, subcutaneous tissues, muscles, and deep visceral structures are conducted to the dorsal horn of the spinal cord on these fibers. In the spinal cord, these nerves form synapses with spinal cord nerves that then send impulses to the brain.
In addition, large-diameter sensory nerves enter the dorsal horn of the spinal cord. These so-called A fibers do not transmit pain impulses; instead, they transmit sensations associated with touch and temperature. The A fibers, which are larger and conduct impulses more rapidly than do the smaller fibers, can actually block the ability of the smaller fibers to transmit their signals to the secondary neurons in the spinal cord. The dorsal horn, therefore, can be both excitatory and inhibitory with regard to pain impulses that are transmitted from the periphery.
The impulses reaching the dorsal horn are transmitted upward toward the brain by a number of specific ascending nerve pathways. These pathways run from the spinal cord into the thalamus, where they form synapses with various nerve cells that transmit the information to the cerebral cortex, along the spinothalamic tracts. According to the gate control theory the transmission of these impulses can be modulated or adjusted all along these tracts. All along the spinal cord the interneurons can act as “gates” by blocking the ascending transmission of pain impulses. It is thought that the gates can be closed by stimulation of the larger A fibers and by descending impulses coming down the spinal cord from higher levels in such areas as the cerebral cortex, the limbic system, and the reticular activating system.
The inhibitory influence of the higher brain centers on the transmission of pain impulses helps to explain much of the mystery associated with pain. Several factors, including learned experiences, cultural expectations, individual tolerance, and the placebo effect, can activate the descending inhibitory nerves coming from the upper CNS. These other factors need to be considered and incorporated into pain management strategies, which usually involve the use of drugs. For example, the placebo effect, stress reduction, acupuncture, and back rubs (which stimulate the A fibers) all can play important roles in the effective management of pain.
1. In planning pain reduction interventions, which pain theory provides information most useful to nurses?
A. Specificity theory
B. Pattern theory
C. Gate-control theory
D. Central-control theory
2. A 12-year-old student fall off the stairs, grabs his wrist, and cries, “Oh, my wrist! Help! The pain is so sharp, I think I broke it.” Based on this data, the pain the student is experiencing is caused by impulses traveling from receptors to the spinal cord along which type of nerve fibers?
A. Type A-delta fibers
B. Autonomic nerve fibers
C. Type C fibers
D. Somatic efferent fibers
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答案 Answers
1. D. Central-control theory Rationale: No one theory explains all the factors underlying the pain experience, but the central-control theory discusses brain opiates with analgesic properties and how their release can be affected by actions initiated by the client and caregivers. The gate-control, specificity, and patter theories do not address pain control to the depth included in the central-control theory. 2. A. Type A-delta fibers Rationale: Type A-delta fibers conduct impulses at a very rapid rate and are responsible for transmitting acute sharp pain signals from the peripheral nerves to the spinal cord. Only type A-delta fibers transmit sharp, piercing pain. Somatic efferent fibers affect the voluntary movement of skeletal muscles and joints. Type C fibers transmit sensory input at a much slower rate and produce a slow, chronic type of pain. The autonomic system regulates involuntary vital functions and organ control such as breathing.
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