Kinesiologia y nervios

Perspective

Structure and Biomechanics of Peripheral Nerves: Nerve Responses to Physical Stresses and Implications for Physical Therapist Practice
The structural organization of peripheral nerves enables them to function while tolerating and adapting to stresses placed upon them by postures and movements of the trunk, head, and limbs. They are exposed to combinations of tensile, shear, andcompressive stresses that result in nerve excursion, strain, and transverse contraction. The purpose of this appraisal is to review the structural and biomechanical modifications seen in peripheral nerves exposed to various levels of physical stress. We have followed the primary tenet of the Physical Stress Theory presented by Mueller and Maluf (2002), specifically, that the level of physical stressplaced upon biological tissue determines the adaptive response of the tissue. A thorough understanding of the biomechanical properties of normal and injured nerves and the stresses placed upon them in daily activities will help guide physical therapists in making diagnoses and decisions regarding interventions. [Topp KS, Boyd BS. Structure and biomechanics of peripheral nerves: nerve responses tophysical stresses and implications for physical therapist practice. Phys Ther. 2006;86:92–109.]

Key Words: Adaptation, Compression, Excursion, Force, Inflammation, Neurodynamics, Physiology, Plasticity, Sprains and strains, Stress. Kimberly S Topp, Benjamin S Boyd

92

Physical Therapy . Volume 86 . Number 1 . January 2006

Physical therapists with an understanding of the adaptive
herehas been an emergence in physical therapy of evaluation and intervention based on neurodynamics, the relationship between nerve physiology and nerve mechanics.1 To advance the clinical care of people with nerve injuries, it is essential to understand peripheral nerve structure and plasticity. The purpose of this appraisal is to review the structural and biomechanical properties of peripheralnerves and then to discuss how nerves respond to physical stresses. We will expand on the Physical Stress Theory presented by Mueller and Maluf2 and discuss the structural and biomechanical modifications seen in nerves exposed to various levels of physical stress. We hold the premise that physical therapists who understand the adaptive responses of nerves to specific physical stresses will be betterprepared to provide reasoned interventions to modify specific aspects of the stresses. These knowledgeable therapists also may educate patients in injury prevention and self-care and thus significantly improve function and quality of life. Nerve Structure The structural organization of peripheral nerves allows axons to conduct impulses that facilitate an individual’s interactions with the worldwhile directing and tolerating the myriad postures of the trunk, head, and limbs. Axons within a peripheral nerve are the lengthy extensions of cell bodies located in the dorsal root ganglia (sensory neurons), autonomic ganglia (autonomic neurons), or the ventral horn of the spinal cord or brain stem (motoneurons). Because their terminals are quite distant from the cell bodies, axons are insulatedfrom each other, bundled together, and protected by 3 connective tissue layers—the endoneurium, the perineurium, and the epineurium (Fig. 1). Axons, Schwann cells, and endoneurial components are bundled by a sheath of perineurium to form a nerve fascicle. Several fascicles are held together by epineurial tissue to form a nerve. Within the endoneurium, all axons are intimately associated with Schwanncells. As shown in Figure 1, the myelin of each myelinated axon is formed from the plasma membrane of a Schwann cell wrapped tightly multiple times around the axon. Thus, a single Schwann cell envelops a single myelinated axon, forming an internode. Along a myelinated axon, the points of

T

responses of nerves to specific physical stresses will be better prepared to provide reasoned...