Imbalances of the CNS

Imbalances of the CNS

Brain dysfunctions are common and varied and may be caused by many different factors. This section focuses on brain trauma, cerebrovascular accidents, degenerative brain disorders, spinal cord trauma, poliomyelitis, and amyotrophic lateral sclerosis.

Brain Trauma

One of the leading causes of accidental death in the United States is head injury that results in brain trauma. In a car accident, for example, the brain is damaged by local injury at the site of the blow (the coup injury) and the reverse effect as the brain con-tacts the opposite end of the skull (the contrecoup injury). An alteration in brain function after a blow to the head, which is usually temporary, is called a concussion . Symptoms of a concussion include dizziness and loss of consciousness. Concussions, regardless of their severity, can damage the brain. If serious, the brain is bruised, resulting in permanent neurological damage. This is known as acontusion, which can range from the patient remaining con-scious to varying lengths of loss of consciousness. If the brain stem is contused, a coma develops that may last for hours or from which the patient will never awaken.

Brain trauma may also involve subdural hemorrhage or subarachnoid hemorrhage. These occur when blood vessels in one of these brain areas are ruptured. The patient is often lucid at first after the trauma, but then develops neurolog-ical deterioration due to the hemorrhage. Accumu-lating blood compresses brain tissue and increases intracranial pressure. When the brain stem becomes forced inferiorly through the foramen magnum, the patient’s blood pressure, respiration, and heart rate become uncontrolled. Treatment of intracranial hemorrhage is via surgery to remove the localized hematoma (blood mass) and repair vessel ruptures. Traumatic head injury may also cause swelling of the brain, called cerebral edema, which can aggravate a brain injury or even be fatal itself.

Cerebrovascular Accident (Stroke)

The most common nervous disorder is a cerebrovas-cular accident, which is also called a stroke or a brain attack. Strokes are the third leading cause of death in North America and occur when the brain’s blood cir-culation is blocked, causing the death of brain tissue (FIGURE 12 -18). Deprivation of blood to a body tis-sue, called ischemia, impairs delivery of oxygen and nutrients. Strokes are usually caused by blood clots that block cerebral arteries. The second most common cause is rupture of blood vessels in the brain, often due to hypertension (FIGURE 12-19). Aneurysm is the third most common cause of stroke (FIGURE 12- 20). Usually, the survivor of a cerebrovascular accident is paralyzed on one side of the body (hemiplegia).

The most critical component of a cerebrovascular accident is its long-term effect that often results in the death of brain neurons. Neurons that are completely deprived of oxygen disintegrate relatively quickly, releasing an overabundance of glutamate (an excit-atory neurotransmitter).

In strokes, glutamate acts as an excitotoxin, which overexcites the surrounding cells until they die. High levels of calcium ions damage mitochondria of brain cells and initiate specific protein synthesis to cause cell death via free radicals and inflammatory agents. Stroke treatments include plasminogen activator to dissolve blood clots and robotic surgery. New advancements include the use of stem cells to replace damaged brain neurons. Temporary episodes of reversible cerebral ischemia are known as transient ischemic attacks. They last between five and 50 minutes, causing tempo-rary paralysis, numbness, or speech impairment.

Degeneration of the Brain

The three primary degenerative brain disorders include Alzheimer’s disease, Parkinson’s disease, and Huntington’s­ disease. Alzheimer’s disease is a ­progressive disease that eventually results in demen-tia (mental deterioration). Up to 50% of people over age 85 die because of some Alzheimer-related factor. Symptoms include disorientation, short attention span, and loss of language abilities. The brain is over-come with senile plaques that form between neurons and neurofibrillary tangles inside the neurons. The brain begins to shrink as its cells die (FIGURE 12 -21). This primarily occurs in the basal forebrain and ­hippocampus. Glutamate excitotoxicity is also involved in the progression of Alzheimer’s disease.

Parkinson’s disease results from degeneration of dopamine-releasing neurons in the substantia nigra, and usually begins when a person is in his or her 50s or 60s. Symptoms include “pill- rolling” hand movements, tremor of the hands at rest, a stony facial expression, slowness in movements, a shuffling gait when walking, and a forward-bending posture when walking. Although of unknown cause, Parkinson’s disease may occur due to abnormalities in mito-chondrial proteins and their degradation pathways. Treatments involve the medications elodea (L-dopa) and darnel, deep brain stimulation, gene therapy, and stem cell implantation.

Huntington’s disease usually develops during middle age and is a fatal, hereditary disorder. Hun-tingtin proteins mutate and accumulate, causing death of brain tissue. The basal nuclei and cerebral cortex degenerate. Symptoms include nearly continuous jerky movements (chorea), which are involuntary, and even-tual severe mental deterioration. Huntington’s disease is usually fatal within 15 years of symptom onset. This dis-order’s symptoms oppose Parkinson’s disease because there is overstimulation of the motor drive instead of inhibition. Treatments include medications to block the effects of dopamine and stem cell implantation.

Spinal Cord Trauma

The spinal cord is able to stretch extensively, yet direct pressure on it may cause serious loss of function. Localized damage to either the cord itself or the spinal roots causes either paralysis or paresthesias­. ­Flaccid paralysis of the skeletal muscles occurs because of severe ventral root or ventral horn cell damage. Because nerve impulses do not reach the skeletal muscles, they become unable to move invol-untarily or voluntarily and atrophy because of lack of stimulation. Spastic paralysis occurs if just the upper motor neurons of the primary motor cortex are dam-aged. This is because the spinal motor neurons remain intact, with spinal reflex activity continuing to irregu-larly stimulate the muscles. The muscles stay healthy for a longer time but lose voluntary control. They may become permanently shortened as a result.

If the spinal cord is cut in half (transected), total motor and sensory loss occurs in body regions below the site of the damage. For example, transection between the T ₁ and L₁ levels affects both lower limbs (paraplegia). If transection occurs in the cervical region, all four limbs are affected (quadriplegia). Paralysis of one side of the body (hemiplegia) is usu-ally caused by brain injury and not spinal cord injury.

Spinal shock is a collection of symptoms caused by spinal cord transection. There is a transient period of functional loss after injury and immediate depression of all reflex activity that is caudal to the lesion site. All muscles below the injury site become paralyzed and insensitive, blood pressure is reduced, and reflexes of the bowel and bladder stop. Within a few hours after injury, neural function usually returns. If this does not occur within 48 hours, paralysis is usually permanent.

Poliomyelitis

Poliomyelitis is defined as inflammation of the gray matter of the spinal cord caused by the poliovi-rus, which usually enters the body via water that is contaminated with feces, destroying the ventral horn motor neurons. Symptoms of poliomyelitis begin with headache, fever, muscle weakness and pain, and loss of specific somatic reflexes. Paralysis eventually develops, with affected muscles experiencing atrophy. Poliomy-elitis may be fatal due to cardiac arrest or paralysis of the respiratory muscles. The polio vaccines have nearly eradicated the disease worldwide.

The poliomyelitis epidemic of the 1940s and 1950s claimed many victims, and many survivors are today experiencing postpolio syndrome signified by sharp, burning muscular pain, extreme lethargy, and pro-gressive muscle weakness and atrophy. Of unknown cause, postpolio syndrome is believed to be related to continual loss of neurons, which occurs throughout normal aging. A polio survivor may not have sufficient reserve neurons to compensate for this loss over time.

Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis, commonly referred to as Lou Gehrig’s disease, is a neuromuscular condition that progressively destroys the ventral horn motor neurons and pyramidal tract fibers, causing loss of the ability to swallow, speak, and breathe. This condition is usually fatal within five years of onset. It is caused by a combination of genetic and environmental factors. Mutations are inherited in 10% of patients, with spon-taneous mutations probably occurring in the remain-der. Recent advancements have localized the mutation to genes involved in RNA processing. Excitotoxic cell death is probably involved because of the presence of excess amounts of extracellular glutamate. The only treatment that has prolonged the lives of amyotrophic lateral sclerosis patients is the drug called riluzole, which interferes with glutamate signaling.