Because the brain controls almost all of the functions of the human body, injury to the brain can affect many different parts of the body. Brain disorders, which may be inherited or caused by infections, and head injuries can affect the way the brain works and upset the daily activities of the rest of the body.
The nervous system is the body's communication center. The central nervous system (CNS) includes the brain and the spinal cord while the peripheral nervous system (PNS) is composed of nerves. Together, the CNS and PNS control every function of the body, from breathing and blinking to memorizing facts for a test. Nerves from the spinal cord branch outward to the rest of the body. Sensory nerves gather information from the environment and send that information to the spinal cord, which then speeds the message to the brain. The brain makes sense of that message and fires off a response. Motor neurons deliver instructions from the brain to the rest of the body.
The brain is composed of three main parts: the forebrain, midbrain, and hindbrain. The forebrain consists of the cerebrum, thalamus, and hypothalamus (part of the limbic system). The midbrain consists of the tectum and tegmentum. The hindbrain is composed of the cerebellum, pons, and medulla.
The cerebrum or cerebral cortex is the largest part of the human brain. The cerebrum is associated with higher brain function, such as thought and memory. The cerebral cortex is divided into four sections, including the frontal lobe, parietal lobe, occipital lobe, and temporal lobe.
The frontal lobe is associated with reasoning, planning, parts of speech, movement, emotions, and problem solving. The parietal lobe is associated with movement, orientation, recognition, and perception of stimuli. The occipital lobe is associated with visual processing. The temporal lobe is associated with perception, memory, speech, and recognition of auditory stimuli.
A deep furrow divides the cerebrum into two halves, the left and right hemispheres. The corpus callosum is a bundle of axons that connects these two hemispheres. The two hemispheres look symmetrical but are proven to function slightly differently. The right hemisphere is associated with creativity while the left hemisphere is associated with logic abilities.
Nerve cells make up the gray surface of the cerebrum. White nerve fibers underneath carry signals between the nerve cells and other parts of the brain and body.
The neocortex occupies the bulk of the cerebrum. This is a six-layered structure of the cerebral cortex that is only found in mammals. It is believed that the neocortex is a recently evolved structure and that it is associated with "higher" information processing among more evolved animals (such as humans and other primates).
The cerebellum, or "little brain," is similar to the cerebrum in that it has two hemispheres and a highly folded surface or cortex. This structure is associated with regulation and coordination of movement, posture, and balance.
The limbic system, often referred to as the "emotional brain," is found buried within the cerebrum. The lymbic system contains the thalamus, hypothalamus, amygdala, and hippocampus.
Underneath the limbic system is the brain stem. The brain stem is composed of the midbrain, pons, and medulla. This structure is responsible for basic vital functions such as breathing, heartbeat, and maintenance of blood pressure. Scientists describe the brain stem as the "simplest" part of the human brain. In some animals that appeared early on the evolutionary scale (such as reptiles), the entire brain resembles a human brain stem.
Disorders of the brain can occur as the result of infection, such as meningitis and encephalitis, or traumatic head injury, such as a concussion or contusion.
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common brain disorders
Meningitis: Meningitis is an inflammation of the meninges, which are the membranes that cover the brain and spinal cord. It is usually caused by bacteria or viruses, but can also be caused by certain medications or other organisms (such as fungi).
Many of the bacteria and viruses that cause meningitis are fairly common and may be associated with other routine illnesses. Bacteria and viruses that infect the skin, urinary system, and gastrointestinal or respiratory tract can spread via the bloodstream to the meninges through cerebrospinal fluid, the fluid that circulates in and around the spinal cord.
Many different types of bacteria can cause bacterial meningitis. In newborns, the most common causes are Group B streptococcus, Escherichia coli, andListeria monocytogenes. In older children, Streptococcus pneumoniae (pneumococcus) and Neisseria meningitidis (meningococcus) are often the causes.
In some cases of bacterial meningitis, the bacteria may spread to the meninges from a severe head trauma or a local infection, such as a serious ear infection (otitis media) or nasal sinus infection (sinusitis).
Bacterial meningitis can be caused by Haemophilus influenza type b (Hib) but, because of widespread childhood vaccinations, these cases are now becoming rarer. Vaccination against Hib is given to all infants. Because the disease is rare in children ages five and up, the vaccination is generally not given after age five.
Many viruses can lead to viral meningitis, including enteroviruses (such as coxsackievirus, poliovirus, and hepatitis A) and the herpes viruses. There are vaccinations for these viruses.
Encephalitis: Encephalitis is an inflammation (swelling) of the brain. Encephalitis typically occurs in one of three ways: through infections by herpes viruses, through tick or mosquito bites, or through viruses that cause childhood infections.
The herpes viruses are one group and include chickenpox (Varicella zoster), Epstein-Barr virus (EBV), and herpes simplex (the virus that causes cold sores and genital herpes).
Viruses like the West Nile virus, which is transmitted through a mosquito bite, and bacteria that cause Lyme disease and Rocky Mountain spotted fever, which are transmitted through tick bites, can also cause encephalitis.
Viruses that cause measles, mumps, and German measles (rubella) may cause meningitis. These forms of meningitis are rare today because of widespread immunizations
Some cases of encephalitis are mild, and symptoms only last for a short time. However, it is possible to develop severe cases of encephalitis that may even be life-threatening.
Being exposed to a virus or bacterium that can cause encephalitis does not mean that the individual will automatically develop encephalitis. In fact, it is rare for infections to develop into encephalitis.
One form of encephalitis, called ethylmalonic encephalopathy, is a rare inherited disorder that affects several body systems. About 30 individuals with this condition have been identified worldwide, mostly in Mediterranean and Arab populations. Although ethylmalonic encephalopathy appears to be very rare, researchers suggest that some cases have been misdiagnosed as other neurologic disorders.
Traumatic brain injury: Traumatic brain injury (TBI), also called acquired brain injury or head injury, occurs when a sudden trauma causes damage to the brain.
TBI may result when the head suddenly and violently hits an object or when an object pierces the skull and enters brain tissue. Symptoms of a TBI can be mild, moderate, or severe, depending on the extent of the damage to the brain. A person with a mild TBI may remain conscious or may lose consciousness for a few seconds or minutes. Other symptoms of mild TBI include headache, confusion, lightheadedness, dizziness, blurred vision or tired eyes, ringing in the ears, bad taste in the mouth, fatigue or lethargy, a change in sleep patterns, behavioral or mood changes, and trouble with memory, concentration, attention, or thinking. An individual with a moderate or severe TBI may display these symptoms, but may also have a headache that gets worse or does not improve, repeated vomiting or nausea, convulsions or seizures, an inability to awaken from sleep, dilation of one or both pupils of the eyes, slurred speech, weakness or numbness in the extremities, loss of coordination, and increased confusion, restlessness, or agitation.
TBI is a major public health problem, especially among males aged 15-24 and among elderly men and women 75 years and older. Children aged five and younger are also at high risk for TBI. Males account for two thirds of childhood and adolescent head trauma cases.
Each year in the United States, approximately one million people are treated for head injuries in hospital emergency rooms and approximately 270,000 people experience a moderate or severe TBI. Approximately 50,000 people die from head injury annually, and approximately 80,000 of these survivors live with significant disabilities as a result of the injury.
Half of all TBIs are due to transportation accidents involving automobiles, motorcycles, bicycles, and pedestrians. These accidents are the major cause of TBI in people under age 75. For those aged 75 and older, falls cause the majority of TBIs. Approximately 20% of TBIs are due to violence, such as firearm assaults and child abuse, and about 3% are due to sports injuries. Half of TBI incidents involve alcohol use.
Traumatic brain injury is a frequent cause of major long-term disability in individuals surviving head injuries sustained in war zones.
Outcome for patients with head injury depends heavily on the cause. In the US, for example, patients with TBIs from falls have an 89% survival rate, while only 9% of patients with firearm-related TBIs survive.
TBI is classified as either closed or open (penetrating). A closed head injury means the individual received a hard blow to the head from striking an object. An open, or penetrating, head injury means the individual was hit with an object that broke the skull and entered the brain. This usually happens when moving at high speed, such as going through the windshield during a car accident. It can also happen from a gunshot to the head. There are several types of traumatic brain injuries, including concussions and contusions, which are bruises (hematomas) on the brain. A subdural hematoma is a collection of blood on the surface of the brain. Cerebral contusions are bruises on the brain, usually caused by a direct, strong blow to the head. Cerebral lacerations are tears in brain tissue, which often accompany visible head wounds and skull fractures.
Cerebral contusions and lacerations are usually more serious than concussions. Contusions may be caused by the sudden acceleration of the brain that follows a jolt, which may be delivered by a forceful blow to the head, or by the sudden deceleration that occurs when a moving head strikes an immovable object. The brain can be damaged at the point of impact and on the opposite side by striking the inside of the skull.
Contusions and lacerations may cause only minimal physical damage to the brain and few symptoms. However, if swelling or bleeding is severe, these injuries can lead to a severe headache, dizziness, and vomiting. Depending on which area of the brain is damaged, the ability to think, control emotions, move, feel, speak, see, hear, and remember may be impaired. The person may become irritable, restless, or agitated. One side of the body may become weak or feel numb. Confusion may develop. A more severe injury causes swelling within the brain, damaging brain tissue further. Severe brain damage is often accompanied by other injuries, especially scalp injuries, skull fractures, and injuries of the chest and spine.