Spinal Cord Injury Progress, Promise, and Priorities (2005) / Chapter Skim
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2 Progression of Spinal Cord Injury
2 Progression of Spinal Cord Injury
Pages 30-63
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From page 30... ...
The choreography of tightly interwoven responses that lead to dysfunction is known as injury pathophysiology. The final outcome of serious spinal cord injury is shattering: loss of reflexes, loss of sensation, and paralysis (i.e., the loss of control over muscles and movement of the body)
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From page 31... ...
The outer edge of the spinal cord is the white matter (Figure 2-1) , which contains the branching portions of nerve cells known as axons.
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. The type of spinal cord injury, as well as its level and severity, dictates its functional impact and prognosis.
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From page 33... ...
Similarly, the motor level is determined by manual testing and grading of the strengths, on a scale of 0 to 5, of 10 muscle groups that control different motor functions, including limb, bowel, and bladder functions. Although the sensory and motor levels may differ somewhat, they both come under an umbrella "neurological level," which is defined as the most
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From page 34... ...
TABLE 2-2 Types of Spinal Cord Injuries Type of Percentage of Spinal Cord Injury Total Injuries Description Contusion 25 to 40 Bruising, but not severing, of the spinal cord Laceration 25 Severing or tearing of the spinal cord and introduction of connective tissue into the spinal cord, typically from gunshot or knife wounds Solid cord injury 17 Axon injury and demyelination SOURCES: Bunge et al., 1993, 1997; Harper et al., 1996; Hulsebosch, 2002. caudal segment of the spinal cord with both normal sensory and motor functions.
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From page 35... ...
The ASIA Impairment Scale provides clinicians with a standard way of grading the functional severity of a spinal cord injury (Table 2-3)
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. THREE PHASES OF SPINAL CORD INJURIES A spinal cord injury immediately injures or kills cells, but it also causes delayed damage and death to cells that survive the original trauma.
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PROGRESSION OF SPINAL CORD INJURY 37 TABLE 2-5 Cell Types Involved in Spinal Cord Injuries Cell Type Function and Description Neuron · Carries information within the brain to the rest of the body by conducting electrical signals from neuron to neuron · Several functional types: motor neuron, sensory neuron, autonomic neuron, and interneuron Astrocyte · A type of glial cell found in the CNS · Sequesters potassium ions during neural activity · Removes excess neurotransmitters (e.g., glutamate and -aminobutyric acid) · Reacts to injury with hypertrophy and cell division, an increase in protein filaments, and formation of a glial scar Oligodendrocyte · A type of glial cell found in the CNS · Forms myelin that insulates the neurons' axons to expedite transmission of electrical signals · One oligodendrocyte myelinates multiple axons · Produces molecules, including Nogo-A, that inhibit neurite outgrowth Schwann cell · Found in the peripheral nervous system · Forms myelin that insulates the neurons' axons to expedite transmission of electrical signals · One Schwann cell myelinates only one axon · Promotes neurite outgrowth · Migrates into the spinal cord after injury Schwann cell Axon Continued
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From page 38... ...
38 SPINAL CORD INJURY: PROGRESS, PROMISE, AND PRIORITIES TABLE 2-5 Continued Cell Type Function and Description Endothelial cell · Forms the lining of blood vessels · Upon injury, up-regulates cell adhesion molecules on the endothelial cell membrane, which helps to recruit inflammatory cells to the site · When injured, releases cytokines and chemokines, which contribute to inflammation Neutrophil · Removes microbial intruders and tissue debris · Emits substances that activate other inflammatory cells and glial cells and that injure neurons Monocyte · Migrates to area of injury and differentiates into macrophages · Releases inflammatory cytokines and free radicals · Emits growth factors · Removes tissue debris Microglia · Found in the CNS · Has actions similar to those of macrophages Tlymphocyte · Emits inflammatory cytokines · Kills cells (cytotoxic killing) · Neuroprotection in part by possible secretion of growth factors SOURCES: Reprinted with permission, from Lentz, 1971.
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From page 39... ...
. Neuron death, by whatever mechanism, contributes to the losses of sensory, motor, and autonomic functions that occur after spinal cord injury.
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From page 40... ...
, and apoptotic cell death. Formation of Free Radicals Free-radical formation, usually from oxygen atoms, gives rise to a series of pathological reactions inside cells, including the breakdown of lipids in the cell membrane, a process known as lipid peroxidation.
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From page 41... ...
They are recruited there from the circulatory system, especially by vascular endothelial cells, which up-regulate and express adhesion molecules on their cell membranes to help guide neutrophils to the site of injury. Once the neutrophils have entered the spinal tissue, they remove microbial intruders and tissue debris.
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What triggers apoptosis after spinal cord injury? An answer to this question would immediately open up new targets for treatments that could prevent apoptosis from occurring.
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From page 43... ...
. Therefore, besides calcium influx, there are likely other triggers of apoptosis in spinal cord injury.
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From page 44... ...
NG2 (a proteoglycan) Meningeal cell NG2 Semaphorins Activated microglia Free radicals Nitric oxide Arachidonic acid derivatives SOURCE: Fawcett and Asher, 1999.
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From page 45... ...
SPONTANEOUS HEALING Often overlooked amid the litany of pathological changes that occur after a spinal cord injury is the natural ability of the spinal cord to heal itself. In fact, most individuals with spinal cord injuries, especially those with incomplete injuries, show some degree of functional recovery, and some show substantial degrees of recovery (Tator et al., 1998)
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From page 46... ...
Mature oligodendrocytes cannot divide or migrate. Yet, imma TABLE 2-7 Mechanisms of Spontaneous Recovery in the Spinal Cord · Remyelination by Schwann cells entering the spinal cord after injury · Remyelination by oligodendrocyte precursors · Recovery of conduction in demyelinated axons · Strengthening of existing synapses · Regrowth and sprouting of intact axons to form new circuits · Release of growth factors and guidance molecules · Shift of function to alternate circuits
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From page 47... ...
Research is needed to determine if the same or similar mechanisms are involved in axon guidance following injury in the adult CNS. BIOLOGICAL BASES OF FUNCTIONAL LOSSES No daily activity can be taken for granted for someone with a spinal cord injury.
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From page 48... ...
Thus, an injury or disruption to the motor pathways leading to and from the brain could cause a patient to lose motor function. Differences in Degree of Cortical Control on Motor Function The circuitry between the primary motor cortex and the motor neurons of the ventral horn of the spinal cord is very complex.
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From page 49... ...
Signals commanding the initiation of a movement are generated in the primary motor cortex of the brain. These signals are modulated before they reach the muscle.
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From page 50... ...
This sensory information bypasses ascending information to the brain and is conveyed directly to lower motor neurons, resulting in involuntary or reflex movements. Role of the Central Pattern Generator in Humans Experiments performed by Shik, Severin, and Orlovsky in the 1960s provided evidence of a central pattern generator (CPG)
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From page 51... ...
in the spinal cord, which controls rhythmic motor activity, including walking. site of lesion Stimulating electrode Shik and colleagues experimented with a cat whose brain stem was severed but that was still able to walk on a treadmill when a specific region of the brain stem was stimulated.
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Pain and Its Causation Pain is a common and debilitating outcome of spinal cord injuries. Most studies find that 60 to 80 percent of individuals report chronic pain after a spinal cord injury.
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. The new classification system organizes spinal cord injury pain under two broad categories -- nociceptive and neuropathic -- along with five subclassifications (each of which has further clinical subtypes and possible pathologies; see Table 2-8)
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From page 54... ...
. Much of what is known about the pathophysiology of the pain that occurs after a spinal cord injury comes from studies with a host of animal models of different types of injuries.
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From page 55... ...
In individuals with complete spinal cord injuries above the level of the sacral cord, disruption of the pathway from the spinal cord to the brain can lead to bladder problems related to the lack of coordination between the detrusor and the sphincter muscles (see below)
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From page 56... ...
The impact of a spinal cord injury on voluntary control of the bowel is known as neurogenic bowel. Neurogenic bowel comes in two types-reflexic and areflexic -- depending on the location of the injury.
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A large proportion of men and women with spinal cord injuries report reduced sexual desire (Alexander et al., 1993; Sipski and Alexander, 1993) and reduced fertility (Elliot, 2002)
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1993. Sexual activities, desire, and satisfaction in males pre- and post-spinal cord injury.
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From page 59... ...
1995. Mechanisms underlying the recovery of lower urinary tract function following spinal cord injury.
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2003a. Primary cortical motor neurons undergo apoptosis after axotomizing spinal cord injury.
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2001. Plasticity of motor systems after incomplete spinal cord injury.
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2000. Pain following spinal cord injury: Animal models and mechanistic studies.
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From page 63... ...
2002. Spinal Cord Injury Levels & Classification.
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