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Your brain controls all your thoughts, actions, and emotions. It even controls your body in ways that you don't think about, such as your breathing and digestion. To compete in international sports competitions, an athlete must be in peak physical form. But ultimately, the athlete with the fittest mind is the winner.
Map of Tour de France
Established in 1903, this grueling race does exactly as its name, Tour de France, implies — it tours France. The Tour always finishes in Paris although the route changes slightly from year to year. Each day participants ride the equivalent of four marathons. And they do this for three weeks. Little wonder that the Tour de France is considered the ultimate test of courage and endurance.
The cyclists on the Tour ride in a "peloton," a close group. Like a flock of birds in flight, or a school of swimming fish, the members of the peloton save energy by staying close together and move as an integrated unit. From the air, the peloton looks like one long organism as it snakes up and down mountains and through the French countryside.

But when we zoom closer, we see that it is made up of many individual bodies...
Professional Bike Racer
The human body is made up of roughly 100 trillion cells, but it starts out as just a single cell. This cell divides many times to form new and different kinds of cells. Each kind of cell has a specific structure and function. Groups of similar cells make up tissues, such as muscle or skin. Different kinds of tissues form organs, such as the heart or brain. Related organs work together in organ systems, such as the nervous system or the skeletal system. Each organ system has a specific task. All your organ systems work together to help you learn, survive, and reproduce.
The nervous system is the body's internal communication network. It controls all the body's activities by sending messages between the brain, spinal cord, and nerves. The nervous system coordinates information from the senses, voluntary muscle movement, and involuntary processes like digestion and breathing. It's also involved in generating thoughts and emotions.
The endocrine system controls many of the body's functions, such as growth, energy use, and metabolism. The endocrine system consists of several glands and organs that secrete chemical messengers called hormones into the blood. Hormones are produced by different endocrine glands in the body, such as the pancreas near the stomach and the pituitary gland in the brain. The endocrine system helps coordinate emotions, reaction to stress, and sexual response.
Without the skeletal system, you'd just be a big, shapeless blob! This intricate system of bones supports the body, gives it shape, and helps protect the internal organs. It also helps your body move by providing a framework to which your muscles can attach. Bones store important minerals and produce new blood cells. Bones like the skull and vertebrae also protect your nervous system.
The muscular system is made up of the muscles and tendons that make your body move. Some muscles control voluntary body movements, such as walking and riding a bike. Others control involuntary processes like moving food through the digestive tract, or making the heart beat. The nervous system controls both kinds of movement.
In the cardiovascular system, the pumping heart moves blood throughout your body. The blood travels through veins and arteries, sending oxygen, nutrients, and hormones to cells. Blood also removes waste, fights infection, and helps control body temperature. Key activities of the cardiovascular system—such as heart rate and blood pressure—are controlled by the brain.
As you breathe, the respiratory system sends oxygen to your body's cells and carries off carbon dioxide waste. Air enters through your mouth and nose, and then travels through different airways into your lungs. In the lungs, oxygen enters the blood, and carbon dioxide is removed. The brain monitors the levels of these gases in the blood and regulates your breathing.
The digestive system breaks down food into molecules the body can use for energy. The system also absorbs water and removes waste. Food is first broken down in the mouth before it travels through the esophagus, stomach, and the small and large intestines, where the food and water are absorbed into the bloodstream. Finally, undigested waste exits the body through the rectum. The brain controls almost all the functions of the digestive system. In turn, the digestive system sends messages to the brain to let you know you're hungry—or when you've eaten too much!
The urinary system filters out unwanted waste from the blood and removes it from the body. This process begins when blood flows through the kidneys. In the kidneys, chemical waste and excess water are removed to form urine. The urine drains from the kidneys into the bladder, then exits the body through the urethra. The brain keeps the body's fluid levels balanced by determining how much urine is produced.
The reproductive system is responsible for producing new life. This system consists of sex organs, including the male testes and the female ovaries. The woman's uterus nourishes and protects the developing embryo from conception to delivery. The hormones of the reproductive system affect brain development and sexual behavior.
The skin, hair, and nails make up the integumentary system. The largest of all your organ systems, it protects your body from water loss, bacteria, and injury. It also helps regulate your body temperature. Skin is a sensory organ that sends messages to the brain about touch, temperature, and pain. The brain responds, adjusting the flow of blood and sweat.
The human brain
The human brain is a three-pound mass of pinkish gray tissue—and the most complex living structure known. Its intricate network of more than 100 billion nerve cells controls all voluntary and most involuntary activities. The large prefrontal cortex is where reasoning and planning takes place. Other brain structures help you see, hear, smell, taste, and touch the world around you, enable memory and learning, and keep your heart pumping and lungs working. Your life depends on the intricate coordination of all of these parts.
Your brain is divided into two halves, or hemispheres. A deep groove runs down its center, separating the two hemispheres. Each hemisphere is almost a mirror image of the other but each has a slightly different pattern of bumps and grooves. And each hemisphere controls the opposite side of the body and is associated with different functions. The left hemisphere is specialized for speech, writing, language, and calculation. The right hemisphere is specialized for spatial abilities, face recognition, and some musical abilities.
The corpus callosum is a thick bundle of nerve fibers that connects the two hemispheres of the brain. It allows the two hemispheres to communicate, which is important for coordinating left- and right-brain functions.
The cerebrum is the largest part of the brain and controls all conscious thoughts, experiences, and actions. It is divided into right and left hemispheres, which are joined by the corpus callosum. Its outer folded layer is called the cerebral cortex.
The cerebral cortex is the folded gray tissue that covers the surface of each cerebral hemisphere. It is responsible for language, music, calculations, imagining, thinking and planning. It controls our ability to move our arms, legs, head, eyes, tongue—any body part we can move deliberately. It determines our intelligence, emotions, and personality. It also processes sensory information for vision, hearing, and speech. Almost everything we do consciously depends on the cortex.
The frontal lobes are located behind the forehead. This area of the brain is associated with higher-level thinking, such as problem solving, reasoning, and some aspects of speech. It also contains the motor cortex, which controls voluntary movement.
The temporal lobes, above the ears, are involved in hearing,identifying objects, understanding language, and storing memories. They also play a role in emotions.
The parietal lobes on the top of the head process senses like touch, pain, temperature, pressure, and spatial awareness. They are also associated with voluntary movement, attention, language, and some mathematical abilities.
The occipital lobes at the back of the brain interpret visual information like color, light, shape, and movement. The left and right occipital lobes interpret messages from the opposite halves of each eye. For example, the left occipital lobe receives visual signals from the left half of the retinas in both the right and left eyes, and these halves, in turn, see objects in the opposite (right) halves of the visual field. The two lobes are connected, so the information is combined to produce a single image.
The prefrontal cortex in the forward part of the frontal lobe helps control the highest levels of thinking, such as planning, reasoning, and imagination. It is also involved in conscious functions such as empathy, self-perception, and the ability to interact appropriately with others. This part of the brain is especially well-developed in humans.
Inside The human brain
Delve beneath the cerebrum, and you'll find other essential structures. These parts of the brain, including the limbic system, the brain stem, and the cerebellum, are mostly involved in unconscious actions. Some control body functions such as heart rate and breathing, while others regulate your body temperature and sleep cycle. Still others maintain your balance or relay sensory information between parts of the brain.
The brain stem is the “stalk” of the brain below the cerebrum that connects to the spinal cord. It controls processes basic for survival, such as heart rate, breathing, digestion, and sleep. It is the main route of communication between the rest of the brain, the spinal cord, and the nerves that run throughout the body.It also has its own set of nerves that send and receive signals to the face, mouth, tongue, eye muscles, ears, and balance-sensing vestibular organs.
The limbic system is a ring-shaped group of structures involved in emotions, instincts, and memory formation. Together with the brain stem, it manages essential survival functions such as temperature, blood pressure, heart rate, and blood sugar.
The thalamus serves as a two-way relay station for messages traveling into and out of the brain. It filters and directs most sensory information from outside the body to appropriate parts of the brain. This walnut-sized structure also plays a role in storing and retrieving memories.
The hypothalamus is a small structure below the thalamus that regulates body conditions such as body temperature, thirst, hunger, and blood pressure. It also controls the release of hormones from the pituitary gland, linking the nervous system to the endocrine system.
The amygdala plays a key role in emotions and forming emotional memories. This almond-shaped structure integrates your senses and links them with your emotions. It also affects basic behaviors such as feeding, sexual arousal, and the “fight-or-flight” reaction to stress.
The hippocampus is a seahorse-shaped brain structure involved in memory, learning, and emotion. It forms new memories and organizes them with related memories and emotions.
The pea-sized pituitary gland is one of the key structures in the endocrine system. It releases the hormones that regulate growth, sexual development, and the activity of other endocrine organs in the body. Found in the center of the skull, it works closely with the nearby hypothalamus.
The pineal gland regulates the body’s internal clock. It monitors the amount of incoming light and produces the hormone melatonin, which affects your sleep-wake cycle. This small, cone-shaped gland is part of the endocrine system.
The cerebellum is the second largest part of the brain. It controls posture and balance. Replace this with, "It also helps with the timing and coordination of our movements, making them smooth and precise. Recent research also suggests a role in higher cognitive processes.
Web of neurons
All of the parts of the brain function and communicate through a network of brain cells called neurons. Scientists estimate that the brain contains some 100 billion neurons, more than the number of stars in the night sky. Because one neuron can connect with over 10,000 others, the human brain contains trillions of connections! Messages race through and between these tiny cells, driving all thoughts and behaviors, conscious and unconscious. Neurons are made up of a cell body, branching dendrites that receive signals from other neurons, and a long fiber—an axon—that sends these signals to the next neuron.
A neuron is a nerve cell, the basic unit of the central nervous system. Unlike other cells in the body, neurons are specialized to transmit electrical messages, or nerve impulses, from one part of the brain to another or from the brain to different parts of your body. Information travels through a neuron from the dendrite to the cell body, or soma, through the axon, and—using neurotransmitters—across a gap, or synapse, to the next neuron.
An axon is a neuron’s long, tube-like “arm;” it sends information on to the next neuron. Messages travel from the cell body along the axon as electrical signals, but are passed to the target cell, across a gap, as chemicals called neurotransmitters. Like wires, axons are long and thin. These fibers can reach up to three or more feet in length.
A dendrite is an extension from the neuron that receives messages from other neurons. Named after the Greek word meaning “tree,” these branch-like projections relay messages to the cell body before they’re passed down the axons and on to the other neurons.
The myelin sheath is an insulating fatty layer that surrounds many axons. It helps speed the transmission of electrical signals down the axon, allowing much faster communication. Myelin sheaths are especially important in the axons of the peripheral nerves. These nerves can be up to several feet in length as they extend down to our fingers and toes!
Neuroplasticity refers to the brain’s natural ability to change or adapt. These changes occur in the complex network of neurons that make up your brain. Many experiences, thoughts, or memories create new or stronger connections among neurons. Even in the adult brain, some new neurons are born and migrate out into the cortex, looking for new roles. At the same time, neural connections and neurons that aren’t used or are ineffective wither away and die.
When scientists study brain activity, they’re monitoring the communication—or nerve impulses—among neurons. Neurons in the brain communicate with each other through a series of chain reactions. Messages pass along neurons as electrical impulses, but are passed along to other neurons in the form of chemicals called neurotransmitters. These chemicals flow across a tiny gap called a synapse, after which they bind with the receptors of a target neuron. The neurotransmitters thus set off an electrical impulse that travels down the dendrites of the next neuron.
Neurotransmitters are the chemical messengers that travel between two neurons. When nerve impulses reach the end of an axon, these chemicals are released into the synapse. If the neurotransmitters bind to the target neuron’s receptors, they can send, or else inhibit, the messages from traveling on to the next neuron.
A receptor (or receptor site) is a microscopic slot on the surface of a receiving neuron where neurotransmitters can attach. In order for this to happen, the shape of the neurotransmitter must fit into the receptor molecule just as a key fits into a lock. If enough neurotransmitters bind to their matched receptors, an electrical impulse can be started in the receiving neuron. However, some neurotransmitters will inhibit electrical impulses, leading to less or no electrical impulses going down the dendrites of a receiving neuron.
A synaptic vesicle is a microscopic pouch within a neuron’s axon terminal that holds and sometimes releases neurotransmitters into a synapse. Electrical impulses traveling from the cell body trigger the release of neurotransmitters from the synaptic vesicle.
The action potential is the electrical impulse that travels through a neuron, along the axon, and triggers the release of neurotransmitters. An action potential can be activated when many neurotransmitters bind to a neuron’s receptor sites. However, some neurotransmitters actually inhibit the action potential.
The synapse includes the space between two neurons where they communicate with each other, i.e., the axon terminal and the receptors on dendrites. Since neurons never touch, the synapse has a miniscule gap—the synaptic gap or synaptic cleft—where messages flow from the sending neuron’s axon in the form of chemical neurotransmitters. Most neurons receive messages along their dendrites, but synapses can also occur between two axons, or an axon and a cell body.
Synaptic pruning is the process of “cutting back” the connections between neurons that aren’t used. Individual neurons form thousands more connections than are needed. Over time, those that are used become stronger, and those that aren’t, disappear. Synaptic sprouting is the process of increasing the connections among neurons that are being used. These changes depend on your actions and experiences and are referred to as “synaptic plasticity.”