Brain and Nervous System

18 May

Brain and Nervous System

Your brain is the most complex and least understood organ in your body. It inter- prets information gathered through the senses, initiates all body movement, stores information for later use, and controls thought and behavior. The cerebel- lum helps maintain posture and balance and the coordination of movements. The brain stem controls vital functions such as breathing. The uppermost part of the brain stem, known as the midbrain, controls some reflex actions and is involved in voluntary eye movements.

Your forebrain—the cerebrum and the structures inside it—manages abstract reasoning, learning, communication, sensation, emotion, and all other higher functions. The cerebrum is coated with a thin layer of gray tissue, called the cere- bral cortex, where information processing occurs. Voluntary responses, such as movement and speech, are initiated inside the cerebrum, influenced by structures that control emotional state and modify perceptions. The hypothalamus is an important center of emotion and controls body temperature and sleep. The thal- amus serves as a clearinghouse for information moving between the cerebrum and the brain stem and spinal cord. The hippocampus indexes memories. The basal ganglia (clusters of nerve cells adjacent to the thalamus) integrate move- ments.

Although your nervous system has many types of cells, the neuron serves as the main functional unit and is often referred to by the terms nerve cell or brain cell. Neurons have a central cell body, threadlike extensions called dendrites that spread out like short branches of a tree to receive messages from other nerve cells, and axons, which transmit messages between nerve cells. Other cells wrap around the axon, providing insulation and helping signals travel faster. At the end of each axon is a synapse, where signals pass from one nerve cell to another.

Chemicals called neurotransmitters enable brain cells to communicate with each other. Some neurotransmitters, such as acetylcholine or norepinephrine, make neurons react quickly. A neurotransmitter called gamma-aminobutyric acid (GABA) makes cells less excitable. Other neurotransmitters are serotonin, dopamine, and glutamate. Some medications used to treat neurological disorders adjust the level of specific neurotransmitters to restore normal function.

Your spinal cord, a complex bundle of nerves, carries messages between your brain and your body. Your brain and spinal cord together make up the central nervous system. Nerves that branch off from your spinal cord form the periph- eral nervous system. Eight sets of nerves branch off from the cervical spine (neck), 12 sets from the thoracic spine (chest), five sets from the lumbar spine (lower back), and six sets from the sacrum (base of the backbone and tailbone). These nerves transmit instructions to all parts of the body and carry sensory information to the brain.

The Nervous System

The central  nervous  system is made up of the brain and spinal cord.The nerves that emanate from the spinal cord to the rest of the body make up the peripheral nervous system.

With spinal injuries, the exact location of nerve damage determines the func- tion lost. For example, nerves in the arms or legs may be injured by compression. This occurs in carpal tunnel syndrome (see page 311), which affects peripheral nerves that pass through the wrist. Some disorders such as diabetes can affect many nerves at the same time, resulting in a condition called polyneuropathy.

Common Neurological Symptoms

The following are common symptoms of neurological disorders. Familiarity with these symptoms will help you to recognize the onset of a disorder or a new characteristic of an existing disorder.

•  Seizure. A seizure is excessive electrical activity in the brain that can result in temporary loss of consciousness, memory, or motor control. The most com- mon type of seizure, called a generalized seizure, begins with a loss of con- sciousness and motor control followed by violent, repetitive jerking of the limbs. During a partial seizure, a person usually remains conscious, although he or she may have hallucinations involving smell or vision, experience repet- itive involuntary movements, or exhibit unusual behavior.

•  Tics. Tics are involuntary and repetitive movements or actions that can be sim- ple or complex. Simple tics are sudden, brief movements such as eye blinking, shoulder shrugging, facial grimacing, head jerking, yelping, and sniffing. Complex tics are coordinated patterns of movement involving several muscle groups such as jumping, smelling objects, touching the nose, touching other people, biting the lips, banging the head, and shouting obscenities or repeating the words of others.

•  Aphasia. Aphasia refers to a disturbance or loss of language skills (compre- hension, expression, or both) caused by injury to tissues in the language cen- ters (called Broca’s area and Wernicke’s area) of the brain. Injury to Broca’s area causes problems with language expression. Damage to Wernicke’s area affects language comprehension. Global aphasia describes a loss of both lan- guage comprehension and expression caused by widespread damage to that side of the brain. Nominal aphasia refers to difficulty naming objects or think- ing of a particular word. This may be caused by injury to a specific language center of the brain or by widespread brain dysfunction.

•  Apraxia. Apraxia refers to the inability to perform certain tasks, such as tying one’s shoes or dressing, because of loss of the ability to recall the sequence of steps that are necessary to perform such tasks. The person understands the task and sometimes can perform individual components of the task but cannot complete the entire task.

•  Amnesia. Amnesia refers to loss of the ability to store information in memory or to recall information stored in memory. Amnesia can be classified broadly as immediate (lasts for a few seconds), intermediate (lasts for a few days), or

long term (lasts indefinitely). Sudden amnesia can be a sign of a serious neu- rological disorder.

•  Delirium and dementia. Delirium refers to a temporary condition in which a person is disoriented and also may be irritable, fearful, delusional, or con- fused. Dementia refers to a permanent and progressive decline in intellectual function, insight, judgment, memory, and personality. In dementia, aphasia and apraxia (see above) may occur, along with depression, anxiety, paranoia, or an inability to recognize familiar faces, locations, or objects (agnosia).

•  Tremor. Tremors are involuntary, rhythmic, back-and-forth movements caused by alternating contraction and relaxation of muscles. The most common type is a slight rapid tremor in the hands when the arms are stretched out in front of the body. Anxiety, fatigue, stress, and certain drugs can also cause tremors. Tremors that occur during intentional movements may result from injury to the cerebellum, the area of the brain that controls balance and coordination. Tremors that occur when a person is at rest may be due to Parkinson’s disease.

Stroke

A stroke (also called a cerebrovascular accident) occurs when brain tissue is deprived of its blood supply. A stroke is the equivalent of a heart attack, but in the brain. Strokes can result from a blockage of blood flow to the brain (called an ischemic stroke) or from a ruptured blood vessel that bleeds into the brain (called a hemorrhagic stroke). Damage to the brain from a stroke begins within seconds of the interruption in blood flow.

About 80 percent of all strokes are ischemic strokes that result from a block- age in a blood vessel in the neck or the brain. Ischemic strokes can cause severe disability. There are three ways that blood flow can be blocked in an artery lead- ing to the brain: when a clot forms inside the blood vessel (called cerebral throm- bosis), when a clot travels from another part of the body and becomes lodged in a blood vessel (called cerebral embolism), or when an artery becomes severely narrowed (called stenosis).

Hemorrhagic strokes can result from rupture of an aneurysm (a weak, thin blood vessel wall); from changes in small arteries resulting from high blood pressure or diabetes; or, less commonly, from an arteriovenous malformation (see “Other Neurological Disorders,” page 341). When the bleeding occurs within the brain itself, the stroke is classified as an intracerebral hemorrhage. If the bleeding occurs within the membranes between the brain and the skull, it is called a subarachnoid hemorrhage.

Sometimes blood flow to the brain is diminished or blocked for just a few minutes. This temporary cutoff of blood is called a transient ischemic attack (TIA). The symptoms of a TIA clear up quickly. Just as angina indicates an increased risk for heart attack, a TIA is a warning sign for a future stroke. Famil- iarize yourself with the following warning signs of stroke.

Warning Signs of a Stroke

Symptoms of a stroke often appear suddenly, without warning. If you have one or more of the following symptoms, call your doctor, 911, or your local emergency number immediately. The sooner you get treatment  for a stroke, the more likely you are to recover. These symptoms may be accompanied by drowsiness, nausea, or vomiting.

•   sudden numbness or weakness in your face, an arm or a leg, or on one side of your body

•   sudden confusion or difficulty speaking or understanding speech

•   sudden difficulty with vision (such as dimness or double vision) in one or both eyes

•   sudden difficulty walking, dizziness, or loss of balance or coordination

•   sudden severe headache with no obvious cause

A stroke can occur in anyone at any age. However, the following factors can increase your risk of having a stroke:

•   High blood pressure is the leading risk factor for stroke. If your blood pressure is high (more than 140/90), work with your doctor to lower it (see page 217). High blood pressure in blood vessels in the brain can cause them to rupture. It also can cause blood vessels to narrow by thickening their walls. Obstructions such as blood clots and tiny pieces of fatty plaque may lodge in blood vessels that supply blood to the brain.

•   Smoking has been linked to the buildup of fatty deposits in the walls of the carotid arteries, the main arteries in the neck that supply blood to the brain. Blockage of these arteries is a leading cause of stroke in white Americans. In addition, smoking raises blood pressure, reduces the amount of oxygen in the blood, and makes blood thicker and more likely to clot. (For information on quitting smoking, see page 107.)

•   Heart disease can produce blood clots that may break loose and block blood vessels in or leading to the brain. Preventing or treating heart disease can reduce the risk of devel- oping blood clots.

•   Prior TIAs or strokes indicate that a problem exists in the blood vessels supplying the brain. Contact your doctor as soon as possible if you think you have had a TIA. Care- fully following your doctor’s advice will reduce your risk for stroke.

•   Diabetes (see page 365) increases fatty deposits inside blood vessels throughout the body, including the brain. If blood glucose levels are high at the time of a stroke, the damage to the brain can be severe. Keeping diabetes under control will help reduce the risk of stroke.

If you have one or more of the above disorders or conditions, work closely with your doctor to reduce your risk for stroke. If you smoke, the sooner you quit, the sooner your risk for stroke will begin to decline. Most people at risk for stroke are advised by their doctors to take anticoagulants (blood thinners) such as warfarin, or platelet inhibitors (drugs that discourage formation of blood clots) such as aspirin, clopidogrel, or ticlopidine.

If the arteries that supply blood to your brain—the carotid arteries—are nar- rowed, your doctor may recommend a carotid endarterectomy (a surgical proce- dure to remove fatty deposits from the walls of the carotid arteries). Before surgery, your carotid arteries will be examined for blockages using a procedure called cerebral angiography (see “Diagnostic Procedures,” page 342). In people who have significant narrowing of the carotid arteries that has caused a stroke or TIAs and who are at low risk for complications, carotid endarterectomy can sig- nificantly reduce the risk of stroke.

The severity of a stroke may range from mild to disabling to life-threatening, depending on the type of stroke and its location in the brain, the amount of tissue affected, and the amount of time that passes before treatment begins. Because each side of the brain controls the opposite side of the body, a stroke in the left side of the brain affects the right side of the body, and a stroke in the right side of the brain affects the left side of the body. Strokes in areas of the brain that carry out specific functions (such as speech, sight, or memory) will interfere with or possibly eliminate that ability. Strokes in vital brain structures can result in coma or even death. In a hemorrhagic stroke, bleeding from a ruptured artery can irri- tate other blood vessels in the brain, causing them to go into spasm and damage adjacent brain tissue.

The amount of damage caused by a stroke is directly related to promptness of treatment. Ideally, treatment should begin during the stroke. If injected within 3 hours of the onset of symptoms, a thrombolytic (clot-dissolving) drug such as tissue plasminogen activator (tPA) can greatly reduce long-term damage from an ischemic stroke. When contacting emergency medical personnel, be sure to tell them that you (or a family member or friend) have symptoms of a stroke.

For hemorrhagic strokes, doctors bring blood pressure under control, if nec- essary. In some cases blood may be drained to reduce pressure on surviving brain tissue and prevent further damage. The person may need to undergo sur- gery to repair a bleeding artery or to remove blood clots.

When blockages or other types of damage occur in small blood vessels in the brain, they may gradually destroy brain tissue in many areas. Over time, symp- toms of dementia may develop, including confusion, loss of short-term memory, getting lost in familiar places, bladder or bowel incontinence, emotional distur- bances, difficulty following instructions, or difficulty handling money. Symp- toms usually begin after age 60.

Recovery after a Stroke

Loss of function after a stroke may be temporary or limited; some people recover most of the functions they have lost and continue to live independent lives. However, some people who have strokes need assistance with daily tasks (such as bathing or dressing). Others may become permanently dependent on family members, friends, or healthcare providers for all their daily care. Many people who do not immediately or fully recover from a stroke work with professional therapists who help them adapt to their limitations and function as independently as possible.

A physical therapist can help the person recover physical strength and mobility and prevent immobility by providing treatments such as exercise, massage, and manipulation. A physical therapist also can help the person learn to use equipment such as a walker or a cane properly. An occupational therapist can help the person regain muscle control and coordination and learn to compensate for his or her limitations. A speech therapist can work with the person to recover as much of his or her speech as possible and can teach the person (and his or her family) other methods of effective communication. A speech ther- apist also can help the person deal with breathing and swallowing problems.

After a stroke, many people experience feelings of depression (see page 345). They may feel frustrated or isolated, especially if they have not been able to return to their usual routine or if they are having problems communicating with others. Symptoms of depres- sion include sleeplessness, indifference, and withdrawal. For most of these people, the depression is temporary. It may be helpful for the person to join a support group to share experiences and information with others who are in a similar situation. Talking with a psy- chiatrist or another mental health professional may help the person cope with and over- come his or her depression. To treat prolonged depression, a doctor may prescribe antidepressant medications such as tricyclic antidepressants, serotonin reuptake inhibi- tors, monoamine oxidase inhibitors, or bupropion.

Some people who have had a stroke may experience inconsistent and unpredictable mood changes. For example, they may laugh or cry inappropriately or may become irrita- ble without apparent cause. In such cases it is important for family members and friends to understand that the person cannot control this behavior and that he or she will benefit from their patience and ongoing support.

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