Biomechanics of Peripheral Nerves and Spinal Nerve Roots PDF

Biomechanics of peripheral nerves and spinal nerve roots 1. Anatomy and physiology of peripheral nerves PERIPHERAL NERVOUS SYSTEM: - Function: to allow the communication between internal and external media with the central nervous system - It is made up by: - Peripheral...

Biomechanics of peripheral nerves and spinal nerve roots 1. Anatomy and physiology of peripheral nerves PERIPHERAL NERVOUS SYSTEM: - Function: to allow the communication between internal and external media with the central nervous system - It is made up by: - Peripheral nerves: to drive nerve impulses. There are afferent or sensitive nerves (to CNS) and efferent or motor (from CNS) - Ganglia: craniospinal (afferents) and vegetative (efferents) - Nerve endings: sensitive or receptor; and motor or effector PERIPHERAL NERVE: - Group of nerve fascicles that are held together by vascularized connective tissue - There are three protective layers: - Endoneurium: thin layer of lax connective tissue that surrounds each nerve fiber - Perineurium: connective tissue surrounding each nerve fascicle - Epineurium: dense connective tissue surrounding the entire nerve TYPES OF NEURON IN PERIPHERAL NERVOUS SYSTEM: - Structural, trophic and functional unit of the nervous system that conducts electrochemical impulses to control, modulate and integrate the functions of the tissues - Types of neuron: - Sensitive neuron: located in afferent nerves. It sends information from receptors to the CNS - Somatic motor neuron: located in efferent nerves. It sends information from CNS to the skeletal muscles - Autonomous motor neuron: located in efferent nerves. It sends information from CNS to smooth muscle, cardiac muscle, and glands; through and autonomic ganglia AUTONOMOUS PERIPHERAL NERVOUS SYSTEM: - Sympathetic autonomous SNS - Location: thoraco-lumbar - Function: to prepare the body to stressful situations or physical activity - Answer: “fight or run away” - Parasympathetic autonomous PNS - Location: craniosacral - Function: to act on the basal activities of the organism - It prevails during the rest time SIMPLE THEORY OF NERVOUS SYSTEM PHYSIOLOGY: - Sensory reports: - Conscious (senses) - Unconscious - Integration - Central nervous system - Motor responses: - Voluntary - Involuntary NEURAL DRIVE: - Thanks to the action potential, that is rapid change in the membrane potential that propagates along the entire length of the nerve - All-or-none principle: once the stimulus reaches threshold, the action potential is always the same, even if the intensity of the stimulus changes - 5 stages: - Latency period: the resting membrane potential remains stable - Slow depolarization: decrease in potential difference across the membrane - Rapid depolarization: membrane potential quickly becomes more positive - Rapid repolarization: membrane potential tends to return to its resting value - Hyperpolarization: the cell has a more negative membrane potential than that of rest IONIC CHANGES IN ACTION POTENTIAL: - Sodium channel: - Rest state: the conformation prevents the passage of sodium ions → M gate closed and H gate open - Activated state.on depolarization the M gate opens → sodium enters - Inactivated state: 5 milliseconds after the activated one but algo on depolarization → H gate closed - Potassium channel: - Rest state: the conformations prevents the release of potassium ions → N gate closed - Activated state: on depolarization the N gate takes 5 milliseconds to open → potassium comes our - When membrane potential returns to the rest value, the N gate closes slowly, thus allowing hyperpolarization  2. Anatomy and physiology of spinal nerve roots SPINAL NERVE ROOTS - Functions: to provide functional and structural neural continuity between peripheral nerves and spinal cord - Histological characteristics: lack of connective sheaths, epineurium and perineurium, typical of the peripheral nerves - More sensitivity: more susceptible to irritation and damage due to the comprehensive and tensile forces - Relative to nerve, individual roots are about 50% smaller in cross-sectional and 70% smaller in mass/unit length. ANATOMY OF SPINAL NERVE ROOTS - Location: junctional foramina or intervertebral foramina - There are two roots: - Posterior root: aferent → sensitive - Anterior root: efferent → motor - Two roots unite from the trunk of the spinal nerve - The trunk is divided into: - Anterior or ventral ramus - Posterior or dorsal ramus NERVE PLEXUSES The axons of the anterior ramus are usually configured in the form of nerve plexuses - Cervical plexus: - Roots C1-C4 (neck, thorax and head) - Brachial plexus: - Roots C5-T1 (upper limb) - Lumbar plexus: - Roots L1-L4 (abdomen, genitals and lower limb) - Sacral plexus: - Roots L4-S4 (perineal and gluteal region, and lower limb) - Coccygeal plexus: - Roots S4-S5 (coccygeus region) PHYSIOLOGY OF SPINAL NERVE ROOTS - 4 stages: - The sensory information arrives through the posterior root (spinal ganglia) to the posterior horn of the cord - Synapse with an alpha motor-neuron - Afferent fibers exit through the anterior horn of the cord - The impulse travels through the anterior root towards the muscle for generating the motor response - This occurs in spinal reflex arcs that do not require higher processing - If the afferent signal is more complex, it must go up to the brain and the response must go the other way  3. Biomechanical behaviour of peripheral nerves MECHANICAL PROPERTIES OF PERIPHERAL NERVES - Sliding properties thanks to the elasticity of connective layers → to allow the nerve trunks to adapt to the different positions of the segments of the limbs - Wavy architecture → adapt their length depending on the stretching forces - Progressive stretch → gradual rupture of the most voluminous fascicles and the most fragile, prior to the rupture of the nerve trunk. It may compromise the vascular supply due to an increase in intrafascicular pressure - Nerve compression is worse tolerated by nerves rich in fascicles and whose epineurium thickness is small TENSILE STRESS - It produces excursion → displacement of the nerve relative to the surrounding nerve bed - If it is elongated → convergence: nerve glides toward the moving joint - If the tension is relieved → divergence: nerve glides away from the moving joint (realigned) - Relationship stress-strain: - “Toe region” → nerve is markedly elongated relative to the applied load - “Linear region” → straightening of wavy connective tissue and constant elongation - “Ultimate load” → transition between the recoverable and permanent strain - “Plastic region” → nerve reaches its ultimate elongation and undergoes mechanical failure COMPRESSIVE STRESS - It causes displacement f internal contents of the nerve in transverse and longitudinal directions - Extra-neural compression causes an immediate displacement of endoneurial fluid to the edges - The damage to axons and myelin is greatest at the edges of the compression zone, where the shear forces are highest - At the edges: - Myelin retraction - Widening of nodes - Paranodal demylinizaton 4. Biomechanical behavior of spinal nerve roots MECHANICAL PROPERTIES OF SPINAL NERVE ROOTS: DIFFERENCES WITH PERIPHERAL NERVES - The proportional limit force is frequently the maximum force tolerated by the roots → point of failure - The force sustained by the roots at the proportional limit is positively correlated with the cross-sectional area - In relation to nerve, roots present much smaller values of strength, stiffness, density and proportional limit force - Relatively little applied force → obvious damage in nerve roots  5. Nerve injury and pain NERVE INJURY CLASSIFICATION Seddon and Sunderland established a classification based on the severity and the possibility of repair - Neuropraxia (Grade 1): - Does not involve loss of nerve continuity - Functional loss - Transient - Axonotmesis (Grade 2, 3 and 4): - Complete interruption of the nerve axon and myelin - Mesenchymal structures are preserved - Complete denervation but greater prospect of recovery - Neurotmesis (Grade 5): - Disconnection of a nerve - Functional and structural loss - Recovery is nor possible without surgical intervention NEURAL DEGENERATION AFTER INJURY - In neuropraxia grade 1 → pathological changes are mild or absent - In axonotmesis grade 2 → Wallerian degeneration: - Physical fragmentations of both axons and myelin - Neurotubules and neurofilaments become disarrayed and axonal contour becomes irregular - In axonotmesis grade 3: - Retractation of the ends of the nerve fibers - Intrafascicular scar tissue - In axonotmesis grade 4 and neurotmesis grade 5: - Endoneurial tubes are disrupted, and axons and Schwann cells are no longer confined - The nerves ends become a swollen mass. NEURAL REGENERATION - Sequence of regeneration: 1. Neuronal cell body 2. Segment between the cell body and the injury site 3. Injury site 4. Segment between the injury site and the end organ 5. End organ - Possible sequels after regeneration: - Impairment in functional motor recovery, specially in terms of proprioception - Failures in sensory recovery: decrease or increase of sensitivity and pain NEUROPATHIES - Injury of the peripheral nervous system in which the second neurons of the pyramidal pathway are affected - Symptoms: - Hypotonia - Abnormalities in the electromyogram - Hypoactive or absent tendon reflexes - Loss of strength, more evident in small muscle groups - Ethiopatolgy: - Metabolic abnormalities: diabetes, hypotension, hypothyroidism - Infections: HIV, leprosy - Autoimmune diseases: Guillain-Barré syndrome - Toxics: heavy metals drugs, alcohol - Paraneoplastic syndromes - Hereditary: Charcot Marie - Tooth - Drugs: antiepilepsy CLASSIFICATION OF NEUROPATHIES - Topographic classification: - Polyneuropathies: affect many nerves symmetrically distally - Mononeuropathies: affect one nerve progressively - Multiple mononeuropathies: affect different nerve trunks asymmetrically - Radiculopathies: affect nerve roots - Classification by evolution time: - Acutes: less than a week - Subacutes: some weeks or months - Chronic: several months or years - Clinical classification: - Sensitive - Motor - Mixed or sensitive-motor - Classification depending on electroneuromyography: - Demyelinating - Axonal - Mixed TRAINING IN NERVE INJURY NERVE INTERRUPTION - Work a lot on proprioception - If there is sensitive disorder, working with different textures and adequate the environment to the person - If there is motor disorder, set very simple goals so that the person does not get frustrated NEUROPATHY - Look for the cause (cancer, diabetes…) and focus efforts on adapting the training to improve that cause - Be careful with the sensitivity disorders - To include neuropedagogy of pain and strategies of dual tasks in cases of great pain.