axons of most upper motor neurons cross, or "decussate," to the opposite side of the brain in the motor decussation, which is located in the lowermost segment of the brain stem (the medulla oblongata)
spinal cord, second lumbar (L2) cord level, and lower motor neuron whose cell body is shown with four dendritic trunks—
Figure 1.5. Upper and LI lower motor neurons. The cell body of an upper motor neuron is shown above in the left cerebral cortex, and its target—the cell body of a motor neuron whose axon is destined to g1
innervate the right quadriceps femoris muscle —is on the right side of the spinal cord.
cerebral cortex, motor region for thigh muscles half-brain, left side, medial view frontal lobe example of a lower motor neuron with an extensive dendritic tree axon collateral
\axon of upper motor neuron
36 ANATOMi CH HATHA it».A
of the lower motor neurons, especially on the side opposite to the site of the injury. Our will can no longer be expressed actively and smoothly. The ultimate result of this, at least in severe cases in which a vascular mishap occurs at a site where the axons of other motor systems are interrupted along with those of the upper motor neurons, is not flaccid paralysis but spastic paralysis, in which the muscles are rigid and not easily controlled. A semblance of motor function remains because other parts of the nervous system, parts that have been spared injury, also send axon terminals to the lower motor neurons and affect motor function. The problem is that these supplemental sources of input cannot be controlled accurately, and some of them facilitate the lower motor neurons to such an extent that skeletal muscles are driven into strong and uncontrolled states of contraction. Although most of the time the condition does not result in total dysfunction, severe spastic paralysis is only mildly less devastating than flaccid paralysis; some active voluntary movements are possible, but they are poorly coordinated, especially those that make use oi the distal muscles of the extremities (fig. 1.6).
spinal cord injuries
If the entire spinal cord is severed or severely damaged at some specific level, there are two main problems. First, sensory information that comes into the spinal cord from below the level of the injury cannot get to the cerebral cortex and thereby to conscious awareness. The patient is not aware of touch, pressure, pain, or temperature from the affected region of the body. Second, motor commands from the brain cannot get to the lower motor neurons that are located below the injury. Spinal cord injuries at different, levels illustrate these conditions: a spinal cord transection in the thoracic region would result in paraplegia—paralysis and loss of sensation in the lower extremities; and a spinal cord transection in the lower part of the neck would cause quadriplegia—paralysis and loss of sensation from the neck down, including all four extremities (fig. 2.12). Injuries such as these are usually the result of either automobile or sports accidents.
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