Every animal you can think of -- mammals, birds, reptiles, fish, amphibians -- all have brains. But the human brain is unique. It gives us the power to think, plan, speak, imagine... It is truly an amazing organ.
The brain performs an incredible number of tasks:
It controls body temperature, blood pressure, heart rate and breathing.
It accepts a flood of information about the world around you from your various senses (eyes, ears, nose, etc.).
It handles physical motion when walking, talking, standing or sitting.
It lets you think, dream, reason and experience emotions.
All of these tasks are coordinated, controlled and regulated by an organ that is about the size of a small head of cauliflower: your brain.
Your brain, spinal cord and peripheral nerves make up a complex, integrated information-processing and control system. The scientific study of the brain and nervous system is called neuroscience or neurobiology. Because the field of neuroscience is so vast, and the brain and nervous system so complex, this article will start at the beginning and give you an overview of this amazing organ.
In this edition of HowStuffWorks, we will examine the structures of the brain and what each one does. With this general overview of the brain, you will be able to understand concepts such as motor control, visual processing, auditory processing, sensation, learning, memory and emotions, which we will cover in detail in future articles.
Your brain is made of approximately 100-billion nerve cells, called neurons. Neurons have the amazing ability to gather and transmit electrochemical signals -- they are something like the gates and wires in a computer. Neurons share the same characteristics and have the same parts as other cells, but the electrochemical aspect lets them transmit signals over long distances (up to several feet or a few meters) and pass messages to each other.
Neurons have three basic parts:
Cell body - This main part has all of the necessary components of the cell, such as the nucleus (contains DNA), endoplasmic reticulum and ribosomes (for building proteins) and mitochondria (for making energy). If the cell body dies, the neuron dies.
Axon - This long, cable-like projection of the cell carries the electrochemical message (nerve impulse or action potential) along the length of the cell.
Depending upon the type of neuron, axons can be covered with a thin layer of myelin, like an insulated electrical wire. Myelin is made of fat, and it helps to speed transmission of a nerve impulse down a long axon. Myelinated neurons are typically found in the peripheral nerves (sensory and motor neurons), while non-myelinated neurons are found in the brain and spinal cord.
Dendrites or nerve endings - These small, branch-like projections of the cell make connections to other cells and allow the neuron to talk with other cells or perceive the environment. Dendrites can be located on one or both ends of the cell.
Neurons come in many sizes. For example, a single sensory neuron from your fingertip has an axon that extends the length of your arm, while neurons within the brain may extend only a few millimeters. Neurons have different shapes depending on what they do. Motor neurons that control muscle contractions have a cell body on one end, a long axon in the middle and dendrites on the other end; sensory neurons have dendrites on both ends, connected by a long axon with a cell body in the middle.
Some types of neurons: motoneuron (a), sensory neuron (b), cortical pyramidal cell (c)
Neurons also vary with respect to their functions:
Sensory neurons carry signals from the outer parts of your body (periphery) into the central nervous system.
Motor neurons (motoneurons) carry signals from the central nervous system to the outer parts (muscles, skin, glands) of your body.
Receptors sense the environment (chemicals, light, sound, touch) and encode this information into electrochemical messages that are transmitted by sensory neurons.
Interneurons connect various neurons within the brain and spinal cord.
The simplest type of neural pathway is a monosynaptic (single connection) reflex pathway, like the knee-jerk reflex. When the doctor taps the the right spot on your knee with a rubber hammer, receptors send a signal into the spinal cord through a sensory neuron. The sensory neuron passes the message to a motor neuron that controls your leg muscles. Nerve impulses travel down the motor neuron and stimulate the appropriate leg muscle to contract. The response is a muscular jerk that happens quickly and does not involve your brain. Humans have lots of hard-wired reflexes like this, but as tasks become more complex, the pathway "circuitry" gets more complicated and the brain gets involved.
The simplest possible creatures have incredibly simple nervous systems made up of nothing but reflex pathways. For example, flatworms and invertebrates do not have a centralized brain. They have loose associations of neurons arranged in simple reflex pathways. Flatworms have neural nets, individual neurons linked together that form a net around the entire animal.
Most invertebrates (such as the lobster) have simple "brains" that consist of localized collections of neuronal cell bodies called ganglia. Each ganglion controls sensory and motor functions in its segment through reflex pathways, and the ganglia are linked together to form a simple nervous system. As nervous systems evolved, chains of ganglia evolved into more centralized simple brains.
Major Divisions of the Brain
Diencephalon - thalamus, hypothalamus
Brains evolved from ganglia of invertebrates. Regardless of the animal, brains have the following parts:
Brainstem - The brainstem consists of the medulla (an enlarged portion of the upper spinal cord), pons and midbrain (lower animals have only a medulla). The brainstem controls the reflexes and automatic functions (heart rate, blood pressure), limb movements and visceral functions (digestion, urination).
Cerebellum - The cerebellum integrates information from the vestibular system that indicates position and movement and uses this information to coordinate limb movements.
Hypothalamus and pituitary gland - These control visceral functions, body temperature and behavioral responses such as feeding, drinking, sexual response, aggression and pleasure.
Cerebrum (also called the cerebral cortex or just the cortex) - The cerebrum consists of the cortex, large fiber tracts (corpus callosum) and some deeper structures (basal ganglia, amygdala, hippocampus). It integrates information from all of the sense organs, initiates motor functions, controls emotions and holds memory and thought processes (emotional expression and thinking are more prevalent in higher mammals).
As you proceed from fish toward humans, you can see that the cortex gets bigger, takes up a larger portion of the total brain and becomes folded. The enlarged cortex takes on additional higher-order functions, such as information processing, speech, thought and memory. In addition, the part of the brain called the thalamus evolved to help relay information from the brainstem and spinal cord to the cerebral cortex.
The spinal cord can be viewed as a separate entity from the brain or merely as a downward extension of the brainstem. It contains sensory and motor pathways from the body, as well as ascending and descending pathways from the brain. It has reflex pathways that react independently of the brain, as in the knee-jerk reflex.
Lower animals (fish, amphibians, reptiles, birds) do not do much "thinking," but instead concern themselves with the everyday business of gathering food, eating, drinking, sleeping, reproducing and defending themselves. Therefore, their brains reflect the major centers that control these functions. We perform these functions as well, and so have a "reptilian" brain built into us.
Underside of the brain, showing the brainstem and cranial nerves
The basic lower brain consists of the spinal cord, brainstem and diencephalon (the cerebellum and cortex are also present, but will be discussed in later sections). Within each of these structures are centers of neuronal cell bodies, called nuclei, that are specialized for particular functions (breathing, heart-rate regulation, sleep):
Medulla - The medulla contains nuclei for regulating blood pressure and breathing, as well as nuclei for relaying information from the sense organs that comes in from the cranial nerves.
Pons - The pons contains nuclei that relay movement and position information from the cerebellum to the cortex. It also contains nuclei that are involved in breathing, taste and sleep.
Midbrain - The midbrain contains nuclei that link the various sections of the brain involved in motor functions (cerebellum, basal ganglia, cerebral cortex), eye movements and auditory control. One portion, called the substantia nigra, is involved in voluntary movements; when it does not function, you have the tremored movements of Parkinson's disease.
Thalamus - The thalamus relays incoming sensory pathways to appropriate areas of the cortex, determines which sensory information actually reaches consciousness and participates in motor-information exchange between the cerebellum, basal ganglia and cortex.
Hypothalamus - The hypothalamus contains nuclei that control hormonal secretions from the pituitary gland. These centers govern sexual reproduction, eating, drinking, growth, and maternal behavior such as lactation (milk-production in mammals). The hypothalamus is also involved in almost all aspects of behavior, including your biological "clock," which is linked to the daily light-dark cycle (circadian rhythms).
Internal view of the lower brain
The cerebellum is folded into many lobes and lies above and behind the pons. It receives sensory input from the spinal cord, motor input from the cortex and basal ganglia and position information from the vestibular system. The cerebellum then integrates this information and influences outgoing motor pathways from the brain to coordinate movements. To demonstrate this, reach out and touch a point in front of you, such as the computer monitor -- your hand makes one smooth motion. If your cerebellum were damaged, that same motion would be very jerky as your cortex initiated a series of small muscle contractions to home in on the target point. The cerebellum may also be involved in language (fine muscle contractions of the lips and larynx), as well as other cognitive functions.
The cerebrum contains gray matter (neurons with no myelin) and white matter (myelinated neurons that enter and leave the cortex).
The cerebrum is the largest part of the human brain. The cortex contains all of the centers that receive and interpret sensory information, initiate movement, analyze information, reason and experience emotions. The centers for these tasks are located in different parts of the cortex. Before we discuss what each part does, let's look at the parts of the cerebrum.
Major Parts of the Cerebral Cortex
The cortex dominates the exterior surface of the brain. The surface area of the brain is about 233 to 465 square inches (1,500 to 2,000 cm2), which is about the size of one to two pages of a newspaper. To fit this surface area within the skull, the cortex is folded, forming folds (gyri) and grooves (sulci). Several large sulci divide the cortex into various lobes: the frontal lobe, parietal lobe, occipital lobe and temporal lobe. Each lobe has a different function.
Mouse-over the part labels of the brain to see where those parts are located.
When viewed from above, a large groove (interhemispheric fissure) separates the brain into left and right halves. The halves talk to each other through a tract of white-matter fibers called the corpus callosum. Also, the right and left temporal lobes communicate through another tract of fibers near the rear of the brain called the anterior commissure.
If you look at a cutaway view of the brain, you see that the cortical area above the corpus callosum is divided by a groove. This groove is called the cingulate sulcus. The area between that groove and the corpus callosum is called the cingulate gyrus, also referred to as the limbic system or limbic lobe. Deep within the cerebrum lies the basal ganglia, amygdala and hippocampus.
This ends our tour of the major structures of the cortex. Now, let's see what they do.
The brain is "hard-wired" with connections, much like a building or airplane is hard-wired with electrical wiring. In the case of the brain, the connections are made by neurons that connect the sensory inputs and motor outputs with centers in the various lobes of the cortex. There are also connections between these cortical centers and other parts of the brain.
Several areas of the cerebrum have specialized functions:
Parietal lobe - The parietal lobe receives and processes all somatosensory input from the body (touch, pain).
Fibers from the spinal cord are distributed by the thalamus to various parts of the parietal lobe.
The connections form a "map" of the body's surface on the parietal lobe. This map is called a homunculus.
The homunculus looks rather strange because the representation of each area is related to the number of sensory neuronal connections, not the physical size of the area. (See What Does Your "Homunculus" Look Like? Mapping Your Brain for details on how to determine your own homunculus.)
Homonculus, a sensory map of your body. The homunculus looks rather strange because the representation of each area is related to the number of sensory neuronal connections, not the physical size of the area.
The rear of the parietal lobe (next to the temporal lobe) has a section called Wernicke's area, which is important for understanding the sensory (auditory and visual) information associated with language. Damage to this area of the brain produces what is called "sensory aphasia," in which patients cannot understand language but can still produce sounds.
Frontal lobe - The frontal lobe is involved in motor skills (including speech) and cognitive functions.
The motor center of the brain (pre-central gyrus) is located in the rear of the frontal lobe, just in front of the parietal lobe. It receives connections from the somatosensory portion in the parietal lobe and processes and initiates motor functions. Like the homunculus in the parietal lobe, the pre-central gyrus has a motor map of the brain (for details, see A Science Odyssey: You Try It - Probe the Brain Activity).
An area on the left side of the frontal lobe, called Broca's area, processes language by controlling the muscles that make sounds (mouth, lips and larynx). Damage to this area results in "motor aphasia," in which patients can understand language but cannot produce meaningful or appropriate sounds.
Remaining areas of the frontal lobe perform associative processes (thought, learning, memory).
Diagram highlighting the functional areas of the brain
Occipital lobe - The occipital lobe receives and processes visual information directly from the eyes and relates this information to the parietal lobe (Wernicke's area) and motor cortex (frontal lobe). One of the things it must do is interpret the upside-down images of the world that are projected onto the retina by the lens of the eye.
Temporal lobe - The temporal lobe processes auditory information from the ears and relates it to Wernicke's area of the parietal lobe and the motor cortex of the frontal lobe.
Insula - The insula influences automatic functions of the brainstem. For example, when you hold your breath, impulses from your insula suppress the medulla's breathing centers. The insula also processes taste information.
Hippocampus - The hippocampus is located within the temporal lobe and is important for short-term memory.
Amygdala - The amygdala is located within the temporal lobe and controls social and sexual behavior and other emotions.
Basal ganglia - The basal ganglia works with the cerebellum to coordinate fine motions, such as fingertip movements.
Limbic system - The limbic system is important in emotional behavior and controlling movements of visceral muscles (muscles of the digestive tract and body cavities).
Water on the Brain
Your brain and spinal cord are covered by a series of tough membranes called meninges, which protect these organs from rubbing against the bones of the skull and spine. For further protection, the brain and spinal cord float in a sea of cerebrospinal fluid within the skull and spine. This cushioning fluid is produced by the choroid plexus tissue, which is located within the brain, and flows through a series of cavities (ventricles) out of the brain and down along the spinal cord. The cerebrospinal fluid is kept separate from the blood supply by the blood-brain barrier.
Ventricle system of the brain
As you can see, your brain is a complex, highly organized organ that governs everything you do. Now that you are familiar with the anatomy of the brain, look for future articles to address its specific functions.