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Exercise Science and Sport Studies
Sara Campbell

1 LECTURE 5: BIOMECHANICS OF PERIPHERAL NERVES AND SPINAL NERVE ROOTS ■ the spinal column ends at L2 and then turns into the cauda equina ■ peripheral nerves have both a motor and sensory component I. Anatomy and Physiology of Peripheral Nerves: the peripheral nerve fibers structure and fx ■ have both a sensory and motor function ■ transmit impulses and are the connection to the cell body and the end organs ■ nodes of ranvier, axons, myelin sheaths, schwann cells ○ myelin sheath and nodes of ranvier help propagate impulse (compromised in MS) ■ peripheral nerve is highly vascularized because of the need of high oxygen ○ unlike tendons and ligaments ○ need to be vascular because they need a lot of nutrients to fx properly ■ with tearing, compression, or swelling of the nerve oxygen supply is compromised which can cause less sensation or motor fx ■ figure 5.4: layers of CT surrounding the nerve ○ epineurium- outer layer ○ perineurium- middle layer ○ endoneurium- inner layer; surrounds the nerve axon (sheath) II. Anatomy and Physiology of Spinal Nerve Roots: the spinal nerve fibers structure and fx ■ sensory or dorsal root- afferent ■ motor or ventral root- efferent ■ highly vascular system ■ figure 5.2: schematic representation of a typical spinal nerve as it emerges from its dorsal and ventral nerve roots ○ dorsal nerve root- goes to the sensory nerve ○ ventral nerve root- goes to the motor nerve ○ afferent nerve- (sensory nerve) receive impulse from sensory receptors and send impulse to NS ○ efferent nerve- (motor neurons) receive impulse from other neurons and send impulse to effectors (muscles, glands) to initiate a response III. Biomechanical Behavior of Peripheral Nerves: stretching or tensile injuries of peripheral nerves ■ nerves are very strong (need to be during movement) ■ median and radial nerves can take up to 220N and 150N ■ initially under a 20% load, the nerve has a direct relationship between elongation (strain) and stress, at 30% elongation the perineural sheaths rupture; after this point there is incomplete recovery of the nerve ■ surgery to repair a severed nerve may cause problems as well; tension is required to bring the two points together to suture them and this causes a few problems ○ stretches and angulates local feeding vessels (figure 5.8) ○ reduces the fascicular cross sectional area ○ impairs the intraneural nutritive capillary flow ○ can cause a reduction in nerve action potential ( leading to impaired sensory or motor response) ■ figure 5.8: schematic representation of a peripheral nerve and blood supply during stages of stretching ○ stage I: the blood vessels are normally coiled to allow for the physiological movements of the nerve ○ stage II: under gradually increasing elongation these vessels become stretched and the blood flow to them is impaired ○ stage III: the cross sectional area of the nerve is reduced during stretching and the intraneural blood flow 2 is further impaired ○ complete cessation of all blood flow to the nerve occurs at 15% elongation IV. compression injuries of peripheral nerves ■ can cause numbness, pain, and muscle weakness ■ paresthesias- abnormal sensation, numbness, tingling ■ what is worse to have? muscle weakness ○ can cause a drop foot if a person can't DF ankle or flex toe ○ gluteus medius- major problem with walking if compromised ■ what will come back? depends on how much the nerves regenerate ○ sometimes numbness is permanent- ex with ACL reconstruction surgery ■ leaning on elbow can cause paresthesia ■ saturday night palsy- compression of nerves in the axillary region, including the radial nerve, causes numbness and tingling in the arm ■ crutch use- if lean on crutches can cause numbness and tingling in the arms because of compression of nerves in the axillary region, when resting crutches keep them close to the ribs and abduct the arm V. critical pressure levels ■ 30 mmHg of local compression up to 4-6 hours can cause functional changes within the nerve secondary to decreased blood flow; CTS ■ low pressure compression (30-80 mmHg) at intermittent intervals may cause swelling and subsequent scarring ■ axonal transport is effected distally from the compression site (resulting in a problem in sensory/ motor fx) ■ direct versus indirect pressure on the nerve ○ direct pressure creates a more severe result than indirect pressure VI. mechanical aspects of nerve compression ■ larger diameter nerve fibers undergo a great form of deformation than thinner diameter nerve fibers ○ larger deals more with motor responses so muscle weakness becomes a problem ■ the clinical aspect of this is that the large diameter nerves carry motor responses while the thinner nerve fibers carry pain signals ■ no known research to indicate if there is a relationship btwn type of compression, direct or lateral, and functional consequences VII. duration of pressure versus pressure level ■ higher pressures produce more damage than lower pressures but is still time dependent ■ time is an important factor mostly due to ischemia (lack of blood supply) ■ nerve compression at 30 mmHg for 2-4 hours is reversible; beyond that there may be irreversible compression ■ S/P T4-T6 LAMNI patient VIII. biomechanical behavior or spinal nerve roots ■ nerve roots in the thecal sac lack epineurium and perineurium but still have elastic and tensile properties ■ the tensile strength of ventral nerve roots is between 2-22 N and the dorsal nerve roots from the thecal sac is 5-33 N ○ dorsal is stronger because supplies muscles ■ also there is a difference between the tensile ultimate load and the foraminal segment of the nerve versus the intrathecal portion; five times as great at the foraminal segment ○ foramen is stronger because it has to withstand the sliding of the nerve; especially if a nerve is
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