KEY TERMS

action potential: an electrochemical event that nerve cells use to send signals along their cellular extensions in the nervous system

axon: a nerve cell extension used to carry action potentials from one place to another in the nervous system

dendrite: a branching nerve cell extension that receives and processes the effects of action potentials from other nerve cells

dyskinesia: a neurologic disorder causing difficulty in the performance of voluntary movements

nucleus: a collection of nerve cell bodies in the brain, separable from other groups by their cellular form or by surrounding nerve cell extensions

soma: the body of a cell, where the cell’s genetic material and other vital structures are located

synapse: an area of close contact between nerve cells that is the functional junction where one cell communicates with another

tract: a collection of nerve fibers (axons) in the brain or spinal cord that all have the same place of origin and the same place of termination

STRUCTURE AND FUNCTIONS

The human brain is a complex structure composed of two major classes of individual cells: nerve cells (or neurons) and neuroglial cells (or glial cells). It has been estimated that the adult human brain has around 100 billion neurons and even more glial cells. An average adult brain weighs about 1,400 grams and has a volume of 1,200 milliliters. These values vary directly with the person’s body size; therefore, males typically have a 10 percent larger brain than females. However, there is no correlation between intelligence and brain size, as witnessed by brains as small as 750 milliliters or larger than 2,000 milliliters still show normal functioning.

Neurons process and transmit information. The usual structural features of a neuron include a cell body (or soma), anywhere from several to several hundred branching dendrites that are extensions from the soma, and a typically longer extension known as the “axon” with one or several synaptic terminals at its end.

The information processed and transmitted in the brain takes the form of very brief electrochemical events, typically shorter than two milliseconds, called “action potentials” or “nerve impulses.” These impulses often originate near the point at which the axon and soma are joined and then travel up to 130 meters per second along the axon to the synaptic terminals.

At the synaptic terminals, one neuron communicates its information to other neurons in the brain. These specialized structural points of neuron-to-neuron communication are called “synapses.” Most synapses are found on the dendrites and soma of the neuron that is to receive the nerve signal. A neuron may have as many as fifty thousand synapses on its surface. However, the average seems to be around three thousand. It is thought that as many as 300 trillion synapses may exist in the adult brain.

Neuroglia function as supporting cells. They have a variety of important duties that include acting as a supporting framework for neurons, increasing the speed of impulse conduction along axons, acting as removers of waste or cellular debris, and regulating the composition of the fluid environment around the neurons to maintain optimal working conditions in the brain. Neuroglia make up about half of the brain’s total volume.

The brain can be divided into two major components: gray matter and white matter, both named for their general appearance. The gray matter consists primarily of neural soma, dendrites, and axons that transmit information at relatively slow speeds. The white matter is made of collections of axons that have layers of specialized glial cells wrapped around them. This glial cell-synthesized insulation called a “myelin sheath,” enables much faster information transfer along these axons.

The brain has six major regions. Beginning from the top of the spinal cord and moving progressively upward, these regions are the medulla oblongata, the pons, the cerebellum, the mesencephalon (or midbrain), the diencephalon, and the cerebrum.

The initial lower portion of the medulla oblongata resembles the spinal cord. The medulla has a variety of functions besides simply relaying various categories of sensory information to higher brain centers. Several centers within the medulla are important for executing and regulating basic survival and maintenance duties. These duties are called “visceral functions” and include regulating the heart rate, breathing, digestive actions, and blood pressure.

The term pons comes from the Latin word meaning “bridge.” The pons serves as a bridge from the medulla oblongata to the cerebellum, situated on the brain stem’s backside. The pons contains tracts and nuclei that permit communication between the cerebellum and other nervous system structures. Some pontine nuclei facilitate the control of such voluntary and involuntary muscle actions as chewing, breathing, and moving the eyes; other nuclei process information related to the sense of balance.

The cerebellum is itself a small brain. While the cerebellum is not the origin of commands that initiate movements, it stores the memories of how to perform patterns of muscle contractions used to execute learned skills, such as serving a tennis ball. The two main functions of the cerebellum are to make fast and automatic adjustments to the body’s muscles that assist in maintaining balance and posture and coordinate the activities of the skeletal muscles involved in movements or sequences of movements, thereby promoting smooth and precise actions. These functions are possible because of the sensory information input to the cerebellum from position sensors in the muscles and joints, visual, touch, and balance organs, and even from the sense of hearing. There are also many communication channels to and from the cerebellum and other brain areas concerned with generating and controlling movements.

The mesencephalon, or midbrain, is located just above the pons. The midbrain contains pathways that carry sensory information to higher-brain centers and transmit motor signals from higher regions down to lower-brain and spinal cord areas involved in movements.

The nucleus known as the “substantia nigra” operates with nuclei in the cerebrum to generate the patterns and rhythms of activities like walking and running. Two important pairs of nuclei, the inferior and superior colliculi, are found on the backside of the mesencephalon. They coordinate visual and acoustic reflexes involving eye and head movements, such as eye focusing and orienting the head and body toward a sound source. Additional mesence- phalic nuclei are important for the involuntary control of muscle tone, posture maintenance, and the control of eye movements.

The diencephalon above the midbrain contains the two important brain structures known as the “thalamus” and “hypothalamus.” The thalamus is the final relay for all sensory signals (except the sense of smell) before arriving at the cerebral cortex (the cerebrum’s outer covering of gray matter). The hypothalamus is important for regulating drives and emotions. It serves as a master link between the nervous and endocrine systems.

The thalamus is a collection of different nuclei. Some cooperate with nuclei in the cerebrum to process memories and generate emotional states. Other nuclei have complex involvement in the interactions of the cerebellum, cerebral nuclei, and motor areas of the cerebral cortex.

The relatively small hypothalamus plays many crucial roles that help to maintain stability in the body’s internal environment. It regulates food and liquid intake, blood pressure, heart rate, breathing, body temperature, and digestion. Other significant duties encompass the management of sexual activity, rage, fear, and pleasure.

The final major brain region is the cerebrum, the largest of the six regions and the seat of higher intellectual capabilities. Sensory information reaching the cerebrum also enters a person’s conscious awareness. Voluntary actions originate in cerebral neural activities.

The cerebrum is divided into two cerebral hemispheres, each covered by the gray matter known as the “cerebral cortex.” Below the cortex is the white matter, which consists of massive bundles of axons carrying signals between various cortical areas, down from the cortex to lower areas and up into the cortex from lower areas. Embedded in the white matter are also several cerebral nuclei.

The cerebral cortex has the primary sensory areas for each of the senses and other areas whose major duties deal with the origin and planning of motor activities. The association areas of the cortex integrate and process sensory signals, often initiating appropriate motor responses. Cortical integrative centers receive information from different association areas. The integrative centers perform complex information analyses (such as predicting the consequences of various possible responses) and direct elaborate motor activities (such as writing).

The cerebral nuclei, also called the “basal nuclei” or “basal ganglia,” form components of brain systems that have complex duties such as regulating emotions, controlling muscle tone, coordinating learned movement patterns, and processing memories.

The electrochemical signal that constitutes an action potential in a neuron sent along the neuron’s axon to the synaptic contacts formed with other neurons in the brain is the basic unit of activity in neural tissue. Although the electrical voltage generated by a single action potential is very small and difficult to measure, the tremendous number of neurons active at any moment results in voltages large enough to be measured at the scalp with appropriate instruments called “electroencephalographs.” The recorded signals are known as an electroencephalogram (EEG).

Although interpreting an EEG can be compared to standing outside a football stadium filled with screaming fans and trying to discern what is happening on the playing field by listening to the crowd noises, it still provides clinically useful information. It is used regularly in clinics around the world each day. The typical EEG signal appears as a series of wavy patterns whose size, length, shape, and location of best recording on the head provide valuable indications concerning the conditions of brain regions beneath the recording electrodes placed on the scalp.

DISORDERS AND DISEASES

Brain development is a complex, multistep process that continues long after birth. Brain developmental disorders occur when some part of the brain fails to form normally, resulting in conditions that typically manifest during childhood and affect the normal growth and development of the brain.

Brain developmental disorders affect a child’s cognition, language, social interaction, motor skills, and behavior. Autism spectrum disorder (ASD), for example, is a neurodevelopmental disorder characterized by difficulties in social interaction, communication challenges, repetitive behaviors, and restricted interests. ASD varies widely and may cause severe or mild behavioral issues. Attention-deficit/hyperactivity disorder (ADHD) is characterized by persistent inattention, hyperactivity, and impulsivity that significantly interfere with a child’s functioning and development. ADHD affects attention span, impulse control, and self-regulation.

Other brain developmental disorders affect brain regions dedicated to learning and memory. Intellectual disability (ID) significantly limits intellectual functioning and adaptive behaviors. It is typically diagnosed before age 18 and is characterized by deficits in reasoning, problem-solving, and adaptive skills. Specific learning disorders include difficulties in acquiring and using specific academic skills, such as reading (dyslexia), writing (dysgraphia), or mathematics (dyscalculia). Children with learning disorders may have normal intelligence but struggle with specific areas of learning. Developmental language disorder (DLD) presents persistent language development and communication difficulties. Children with DLD may have trouble understanding and expressing language, impacting their communication ability.

Other development brain disorders significantly affect physical abilities. Cerebral palsy is a cluster of movement disorders caused by damage or abnormalities in the developing brain that affect muscle coordination, balance, and motor control, leading to difficulties with movement and posture.

Still, other brain developmental disorders affect physical and cognitive abilities. Down syndrome is a genetic disorder caused by an extra copy of chromosome 21. Down syndrome is associated with intellectual disability, characteristic facial features, and various medical conditions. It adversely affects cognitive, language, and motor development. Fetal alcohol spectrum disorders (FASDs) are an aggregate of conditions caused by prenatal exposure to alcohol. FASDs cause varying cognitive, behavioral, and physical impairments, including intellectual disabilities, learning difficulties, and growth deficiencies. Fragile X syndrome is a genetic disorder that is the leading cause of inherited intellectual disability and is associated with cognitive impairments, learning difficulties, social challenges, and various physical features.

Brain diseases or conditions are acquired later in life but may also adversely affect the brain. Alzheimer’s disease, for example, is a progressive neurodegenerative disease characterized by memory loss, cognitive decline, and behavioral changes. It is the most common cause of dementia. Parkinson’s disease is a chronic and progressive movement disorder caused by the degeneration of dopamine-producing cells in the brain that leads to tremors, rigidity, bradykinesia (slowed movement), and postural instability. Multiple sclerosis is an autoimmune disease that affects the central nervous system, including the brain and spinal cord. It causes inflammation, demyelination, and damage to nerve fibers, resulting in various neurological symptoms such as fatigue, impaired coordination, and sensory disturbances.

Epilepsy is a brain-based disorder characterized by recurring seizures. Seizures occur due to abnormal electrical activity in the brain, leading to temporary behavior, sensation, or consciousness disruptions.

Brain injuries include traumatic brain injury that results from a blow, jolt, or penetrating object to the head. Traumatic brain injuries can lead to a range of physical, cognitive, and behavioral impairments depending on the severity and location of the injury. Another brain injury is a stroke. Strokes are caused by interruptions of blood supply to the brain, leading to brain cell damage and loss of function. Strokes can result from blood vessel blockage (ischemic stroke) or a burst blood vessel (hemorrhagic stroke).

Genetic diseases that affect the brain include Huntington’s disease. This genetic disorder is characterized by the progressive degeneration of brain cells, leading to movement abnormalities, cognitive decline, and psychiatric symptoms. Amyotrophic lateral sclerosis (ALS) is a progressive neurodegen- erative disease affecting nerve cells that control voluntary muscles. It results in muscle weakness and paralysis, ultimately affecting speech, swallowing, and breathing.

Infectious brain diseases include meningitis, characterized by inflammation of the protective membranes (meninges) surrounding the brain and spinal cord. Viral, bacterial, or fungal infections cause meningitis. Meningitis can lead to fever, headache, neck stiffness, and neurological complications.

Brain tumors are abnormal growths of brain cells that can be benign (noncancerous) or malignant (cancerous). Tumors can cause various neurological symptoms depending on their location and size.

PERSPECTIVE AND PROSPECTS

Given the complexity of the human brain, understanding its structure and function is the ultimate challenge to medical science. Physicians and neuroscientists must understand how the brain functions to treat brain disorders. An appreciation of this can be gleaned by studying the history of some approaches used to treat brain disorders.

For example, in the Middle Ages, it was common to treat people with epilepsy by cutting open the patient’s scalp and pouring salt into the wound (all of which was performed without anesthesia since anesthetics were not yet known). Physicians performed this gruesome surgical treatment because they thought that poisoning the spirits that possessed the patient would force the offending spirits to leave.

As modern science discovered the cellular basis of life, physicians gradually replaced such measures with treatments directed toward the biochemical imbalances, infections, or interruptions of blood flow that cause many brain disorders. The development of nonsurgical techniques permitting the visualization of the brain regions that are active or inactive during various tasks or illnesses advanced the understanding of brain function and improved diagnosis, the planning of effective treatments, and the tracking of either the improvement or the deterioration of patients.

Late in the 1970s, the disease known as acquired immunodeficiency syndrome (AIDS) attracted the attention of the world’s scientists. AIDS is caused by the human immunodeficiency virus (HIV). Many AIDS patients experience neurological problems, including movement, memory loss, and cognitive disturbances. Although HIV cannot infect neurons or the neuroglia that make the myelin sheath (oligodendrocytes), these cells suffer extensive cell death. Half of the neurons may die in some cerebral cortical areas. Paradoxically, the two neuroglial that HIV can infect, microglia and astrocytes, produce HIV particles that move into the bloodstream but show little cell death. The HIV-infected astrocytes shed several HIV-specific proteins (e.g., Tat, Nef, and Rev). These viral proteins cause cell damage and promote inflammation, resulting in the death of nearby neurons and oligodendrocytes. HIV, therefore, transforms astrocytes, cells that normally support and protect neurons, into poisoning centers that damage nearby neurons and oligodendrocytes.

SIGNIFICANCE

Occupational therapy is closely related to the brain as it focuses on helping individuals with brain-related conditions or injuries improve their functional abilities, regain independence, and engage in meaningful activities. The brain is responsible for controlling various cognitive, sensory, motor, and psychosocial functions, and when the brain is affected by injury or disease, it can impact a person’s daily life. Occupational therapy interventions target these areas to promote optimal functioning. Here are some key aspects of occupational therapy related to the brain:

Overall, occupational therapy is vital in addressing the functional limitations, cognitive impairments, motor control difficulties, and psychosocial challenges associated with brain-related conditions or injuries. By providing individualized interventions, occupational therapists aim to enhance the overall well-being and quality of life for individuals affected by brain conditions.

Further Reading