Brain Basics PDF

CHAPTER 1 Brain Basics 1 Brain Basics T he brain is literally t...

CHAPTER 1 Brain Basics 1 Brain Basics T he brain is literally the “nerve The frontal lobes are at the front of center” of your body — it the brain, immediately above the eyes. contains billions of neurons Parts of these lobes coordinate volun- that transmit information from the tary movements and speech, memory body and the outside world, and and emotion, higher cognitive skills then programs our responses — con- like planning and problem-solving, and scious and unconscious movements, many aspects of personality. thoughts, emotions, and memories. The parietal lobes are located at What’s more, your brain can do all the top of the brain, immediately be- these things simultaneously: You can hind the frontal lobes. They integrate throw a ball while talking to a friend, sensory signals from the skin, process plan dinner while you’re shopping, or taste, and process some types of daydream about a balloon ride as you visual information. drive to work. Your brain can pull off The back of the brain houses the these feats of multitasking because it occipital lobes. They process visual is split into many distinct regions spe- information and are responsible for cialized for specific tasks and abilities. recognizing colors and shapes and Henry Vandyke Carter integrating them into complex Major Brain Landmarks visual understanding. The largest part of the human The temporal lobes lie on the brain is the cerebrum. It is sides of the brain, at and below the divided into two large, separate level of the eyes. They carry out Pictured are the brain’s four principal lobes. The frontal lobe, responsible for attention, planning, and decision-making, is labeled blue. The temporal lobe, associated with language, hemispheres, one on the left side, the some visual processing and interpret memory, and emotion, is labeled in green. The parietal lobe, which integrates information from other on the right. The hemispheres auditory information. The hippo- the senses, is labeled in yellow. And the occipital lobe, responsible for vision, is labeled in pink. are connected by bundles of nerve campus consists of curved structures fibers that carry information from one lying beneath the cerebral cortex; it ments like blinking and focusing, and over half the brain’s neurons. Like the side of your brain to the other. The is a region of the temporal lobes that trigger reflexes to sounds. An example cerebrum, the cerebellum is deeply largest of these bundles forms a bridge encodes new memories. Another deep is the startled jump when you are sur- folded, divided into two hemispheres, between the cerebral hemispheres and structure within each temporal lobe, prised by a loud noise. Other regions and carries out a variety of functions. is called the corpus callosum. the amygdala, integrates memory of the midbrain inhibit unwanted For example, it coordinates voluntary The surface of the cerebrum is and emotion. body movements and help coordinate movements and helps the brain learn a deeply folded layer of nerve tissue The hippocampus and amygdala sensory input and motor output to new motor skills. It also has roles in called the cerebral cortex. Its deep are part of the limbic system, a group manage the fine motor control that spatial and temporal perception. A folds increase the area of the cerebral of structures deep within the brain enables you to write with a pen or play patient with cerebellar damage might cortex, creating space in this surface that help regulate our emotion and a musical instrument. have a jerky, arrhythmic gait or might layer for more neurons, which increase motivation. Other parts of the limbic Some of these regions — along be unable to accurately touch his the brain’s processing power. Just as system include the thalamus, which with parts of the forebrain — form a finger to his nose. explorers use landmarks like rivers integrates sensory information and collection of structures called the basal Below the cerebellum is the pons, and mountain ranges to describe and relays it to other parts of the brain, and ganglia, which helps regulate complex which influences breathing and pos- map continents, neuroscientists use the hypothalamus, which sends hor- body movements. ture. Another part of the hindbrain, the deepest divisions of the cerebrum monal signals to the rest of the body The hindbrain plays roles in glu- the medulla, carries nerve pathways to identify regions of each hemisphere through the pituitary gland. These cose regulation and sleep and includes connecting the brain to the spinal cord as separate lobes — distinct regions structures, together with the cerebral several regions that help control move- and contains neural networks that help that have characteristic functions. This cortex, make up the forebrain. ment. The cerebellum, tucked under- control basic functions like swallow- “brain map” will serve as a useful trail The midbrain sits beneath the neath the occipital lobe at the very ing, heart rate, and breathing. Together, guide as you explore the brain in the thalamus. It includes distinct groups back of the brain, is the second-largest the midbrain, pons, and medulla chapters ahead. of neurons that coordinate eye move- part of the brain in volume, containing make up the brainstem. society for neuroscience | Brain Facts 11  1 Brain Basics Brain Basics 1 Brain Evolution expanded even further, packing new representation of the outside world. sensory input or concentrates on a works provide feedback that helps NEURAL CIRCUITS It’s hard to believe that our neurons into layers and folds gen- The image is even further refined as task. Theta waves and delta waves are integrate sensory and motor signals. Each region of your brain complex human brain evolved erating more complex tissues with signals are sent down two parallel typical of sleep. Theta waves are slower For example, the brain’s basal ganglia analyzes only a specialized from a simple tube. The earliest increased processing power. processing streams. In one stream, than alpha waves, ranging from 4 to are part of a feedback loop that takes subset of all the information that is vertebrates probably had brains much neurons in the temporal lobe recognize 7 Hz, while delta waves, which occur information from cortical areas that received, but all regions use the same like the one in the modern lancelet NEURAL NETWORKS and identify objects; in the other,neu- during deep sleep, are very slow, with elicit movement and produces signals basic mechanism to process informa- Amphioxus — little more than a wide Information moves from one rons in the parietal lobe detect the frequencies less than 3.5 Hz. Alpha that feed back to the cortex to excite tion. When signals arrive at a brain spot in the hollow nerve cord running region of your brain to spatial location of those objects. And and delta waves are typically of higher or inhibit specific movements. Loops region, they engage local neural down its back. But while the lancelet’s another via chains of neurons that that’s only the visual input from the amplitude (stronger) than beta or theta that connect the brainstem and the circuits — interconnected neurons brain looks simple, it still contains can transmit signals over long dis- film! New technologies that allow us to waves but, when measured with elec- cerebellum also influence the timing that turn entering signals into output specialized regions where neurons tances. When the nerve fibers of look with increasing detail at the brain patterns that can be sent to other process specific kinds of information, region-spanning neurons form distinct regions being activated as we perform parts of the brain. like the presence of light or the chemi- bundles, these are called nerve tracts. different functions are giving us in- The cerebral cortex is packed cals drifting through the water. In its Examples of major nerve tracts include creasing insight into the fine regions of with neural circuits. Neurons are Neurons are organized into a early development, the human brain the corpus callosum (the thick bundle the brain used for specific tasks. organized into a stack of distinct began as a simple tube, and even today of neurons connecting your left and layers that span the thickness of the it is divided into the same kinds of right cerebral hemispheres) and the Network Activity cortex like shelves in a bookcase. regions as the brains of our ancestors. smaller anterior commissure that Creates Brain Waves Circuits are arranged in columns, as In early vertebrates, the “brain” transmits signals between the left The visual cortex also sends sig- stack of distinct layers that span each neuron forms connections with end of the nerve cord developed three distinct bulges as neurons were and right temporal lobes. A group of nerve tracts connect- nals back to the thalamus to become integrated with other sensory infor- the thickness of the cortex like cells in the layers above and below. The neurons in a column form a added, improving processing in sen- ing a series of regions in the brain is mation; this is an example of a “thal- shelves in a bookcase. single chain, and signals that enter sory and motor reflex regions. These called a neural network. Neural net- amocortical loop,” a two-way circuit the circuit travel down that chain bulges became the forebrain, the works route signals through the brain that connects the thalamus with parts from one neuron to the next. Each midbrain, and the hindbrain. In the along a linear pathway, analyzing and of the cortex and back. As neuronal time the signal is fed forward, it is forebrain, the region able to detect organizing different types of informa- signals loop through the thalamus transformed in some way, building chemicals expanded to form the ol- tion within fractions of a second. and cortex, they produce rhythmic, trodes on your scalp, all these signals and strength of motor signals; some outputs that encode complex infor- factory bulbs, and with the evolution Have you ever wondered what oscillating, electrical patterns that can are in the microvolt range: 20–200 μV of these loops incorporate tracts mation — so you can recognize of image-producing eyes, light-sens- happens in your brain when you watch be detected with an electroencephalo- for alpha and delta waves, and 5-10 from the cerebral cortex that en- your grandmother’s face in a crowd ing regions expanded and began a movie? Your brain turns a panoply graph (EEG). These signals are com- μV for beta and theta waves. able environmental and emotional or plan where to run to catch a processing more complex visual sig- of moving shapes into recognizable monly called brain waves. There are context to influence your body’s thrown ball. nals. The cerebellum appeared as the characters and scenery. The process four distinct types, each recognized Neural Networks Organize movements. Networks that loop the Neuroscientists think each hindbrain and expanded the regions begins with photoreceptors, cells in by its characteristic shape on an EEG and Integrate Information hippocampus into sensory cortex column in the cortex is dedicated that control escape movements and the retina that trigger electrical signals display or printout. Your brain and spinal cord con- pathways help your brain analyze to one very specific processing task. orient the body in space. Both these in response to specific wavelengths Your awake brain typically produc- tain many distinct neural networks. whether environmental signals are But a column’s final output can be functions are far more important to of light. Once those signals reach the es alpha waves and beta waves. Alpha These include spinal tracts — chains familiar or are part of a new situa- influenced by the activity of nearby an actively swimming fish than to a optic nerve, they travel through the waves originate mainly in the parietal of neurons that pass signals through tion. Related networks linking the circuits. Every neuron in a circuit sedentary lancelet buried in the sand. optic tract to the thalamus, where and occipital lobes when your brain the brainstem and the spinal cord. hippocampus to the thalamus and has other connections to neurons in Regions that could rapidly process neurons respond to the shape, color, or is relaxed and eyes are closed, and are Signals either travel upward from hypothalamus allow your memory neighboring columns. Since every visual and auditory information and movement of objects on the screen and characterized by frequencies between sensory receptors in skin and mus- to influence conscious behavior as neuron behaves like a microprocessor, trigger appropriate escape, feeding, or pass their signals to the primary visual 8 and 13 Hz. (The Hertz is a mea- cles to the thalamus and parts of the well as unconscious physiological summing all the signals it receives mating behaviors also expanded in ver- cortex in the occipital lobe, at the back sure of frequency; 1 Hz = 1 cycle per cortex that interpret touch and pres- responses. Reflex loops are circuits before sending one of its own, the tebrates. Over time, those new types of the brain. Neurons in the primary second.) Beta waves are somewhat sure; or they travel downward from eliciting action well before thoughts; strength of signals from neighbor- of neurons made the forebrain balloon visual cortex, in turn, detect the edges faster, with frequencies ranging from brain regions that induce movement, these actions are controlled locally by ing circuits can dynamically shift out, forming the cerebral hemispheres. of objects within the field of vision 14 to 30 Hz. Beta waves are typically passing through the medulla and information going in and out of the a neuron’s response. This dynamic In early mammals, cortical tissues in and integrate the signals from each produced by the frontal and parietal spinal cord before projecting to the spinal cord or subcortical regions of organization may help the brain react the cerebrum and the cerebellum eye, creating a three-dimensional regions of your brain when it processes body’s muscles. Other neural net- the brain, and never reach the cortex. flexibly to different situations. 12 Brain Facts | society for neuroscience society for neuroscience | Brain Facts 13  1 Brain Basics Brain Basics 1 Excitatory and and other primates, that ratio is closer (hyperpolarized) by opening ion signals can’t directly impact the next Inhibitory Neurons to 1:1. However, the ratio of glia to channels in the dendrites. If the sum neuron; rather, chemical signals called Individual neurons are either neuron from region to region varies of all the signals at the dendrites rises neurotransmitters cross the synapse. This excitatory or inhibitory. The majority considerably. The central nervous sys- to match the membrane’s threshold process is called neurotransmission. of neurons in your brain — about tem contains four main types of glial voltage, a series of voltage-sensitive ion When an action potential arrives 80 percent of them — are excitatory, cells: astrocytes, microglia, ependymal channels opens automatically, triggering at the axon terminal, the voltage sending signals that push their neigh- cells, and oligodendrocytes. Astrocytes an electrical impulse called an action change triggers ion channels in the bors toward firing. In many parts of form a network inside the brain that potential, which moves down the axon membrane to open, which lets calci- the cerebral cortex, the most common regulates ion concentrations around towards the next neuron in the circuit. um ions flow into the cell. When the type of excitatory neuron is the pyra- neurons, provides them with nutrients, calcium ions bind to packages of neu- midal cell, named for its cone-shaped and helps regulate the formation of SYNAPSES AND rotransmitter molecules called synaptic cell body. Each pyramidal cell has two new connections between neurons. NEUROTRANSMISSION vesicles, the vesicles fuse with the cell sets of branched dendrites — one set Microglia are the main “immune cells” Signals are passed from one membrane at the axon terminal and at the apex and another set of shorter of the brain. They function mainly neuron to the next at junctions empty their contents into the synap- dendrites at the base — that collect as phagocytes — helping protect the called synapses. In most circuits, a tic cleft. Afterwards, pieces of axon signals from neurons in every layer brain from infections and cellular synapse includes the end of an axon, the terminal membrane cycle back into the of the cortex. A multi-branched axon damage — but can also regulate the dendrite of an adjacent neuron, and a soma as new vesicles, which are refilled Mariana Ruiz Villarreal sends a single electrical signal to mul- formation of new neuronal connec- space between the two called the with neurotransmitter molecules. tiple destinations. The 20 percent of tions. Ependymal cells make the cere- synaptic cleft. Amazingly, this separa- Many substances act as nero- your brain’s neurons that are inhibitory brospinal fluid that cushions the brain tion between neurons was only verified transmitters, including amino acids, send signals that suppress the activity This is the neuron, the building block of the nervous system. Neurons come in many shapes inside the skull, and oligodendrocytes (by electron microscopy) in the 1950s. gases, small organic chemicals, and of neighboring neurons and regulate and sizes, but most have some basic features. The cell body contains structures such as the improve neuron function by wrapping The cleft is wide enough that electrical short peptides. Neurons can synthesize nucleus. Dendrites, the arms extending from the cell body, receive signals from other neurons at the activity of a circuit. axons in a fatty sheath called myelin. junctions called synapses. The neuron sends signals via the axon, a long cable that ends with Every neural circuit contains both the axon terminals. The axon terminals release chemical messengers called neurotransmitters. excitatory and inhibitory neurons. Ion Channels and Neurons that pass signals forward simultaneously send inhibitory signals the neuron’s nucleus and most of its Action Potentials through a circuit and eventually send to adjacent columns, reducing their cytoplasm, along with molecular Ions are electrically charged atoms outputs to other parts of the brain activity. In feedback inhibition, machinery for building and transport- that can only cross a neuron’s cell tend to be excitatory, while inhibitory however, neurons send signals to their ing proteins critical to the cell’s membrane through tunnel-like pro- neurons are typically local and often downstream excitatory neighbors and function. Dendrites are branched teins called ion channels. These loop their responses back to earlier to interneurons that reach back and projections that extend from the cell tunnel-like proteins act like gates, al- segments of a circuit. The interplay inhibit preceding layers of the same body and collect incoming signals lowing some ions to enter or leave the between these signals in a circuit seems circuit. Both are examples of recurrent from other neurons. The neuron’s cell, but keeping others out. Ions that to be important in learning, tuning neural networks, in which neurons electrical signals travel down its axon enter or leave the cell change the volt- and smoothing the signals sent to the inside interconnected circuits send — another extension from the cell age difference across the membrane. body and other parts of the brain. feedback signals to one another. body that may branch before ending in This change in voltage influences the Ferreira, et al. The Journal of Neuroscience, 2015. Seizure disorders like epilepsy could be axon terminals, where the signal is neuron’s likelihood of generating an caused by imbalances in the activity of NEURONS AND GLIA passed across a synapse to other cells. electrical signal. excitatory and inhibitory neurons. The functional unit of neural In some neurons, axons are only a In mammals, the voltage difference Within circuits, neurons can be circuits and networks is the fraction of a centimeter long; in others, across the membrane of a resting neu- organized in a number of different neuron, a specialized cell that can they may extend more than a meter. ron is around -70 millivolts (mV), more input architectures, each affecting how transmit electrical signals to other Neurons are associated with sup- negative inside the cell than on its outer a circuit manages information. In a nerve cells, muscles, or glands. Neu- port cells called glia. Neuroscientists surface. That membrane potential is feed-forward inhibitory circuit, inhib- rons come in a broad range of shapes have long believed that glia outnumber affected by signals arriving from other itory interneurons connect neighbor- and sizes, but all of them have a cell neurons by 10:1 (or more). However, neurons in its circuit, which can make Dendrites — the arms extending from a neuron’s cell body — receive information from other neurons at sites called synapses. Each dendrite can have thousands of synapses, which ing neural circuits in such a way that body, dendrites, and an axon. The cell recent investigations suggest that in the membrane potential less negative together form complex circuits that govern brain function. The synapses on this mouse excitatory signals in one column body, also called the soma, contains some regions of the brains of humans (depolarized) or more negative neuron are labeled in yellow and red. 14 Brain Facts | society for neuroscience society for neuroscience | Brain Facts 15  1 Brain Basics Brain Basics 1 small non-peptides like dopamine or channels, altering the voltage across time. Once they detach, the ion RECEPTORS AND NEURONS, GENES, and are associated with open, unfolded acetylcholine inside the axon termi- the postsynaptic membrane. Local channels return to their resting state MOLECULAR SIGNALING AND GENE EXPRESSION chromatin, while unexpressed genes nal. But an axon terminal doesn’t glial cells (astrocytes) mop up any ex- and stop altering the charge across Neurons have receptors for many By this point, it should be clear are typically in tightly packed regions. contain the molecular machinery for cess neurotransmitters at the synapse. their membrane. The neurotrans- molecules that can change the way that neurons inside the brain can differ Chemical changes that tighten or building proteins, so peptide-based This process prevents them from mitters are either broken down or they function. These molecules include in appearance and function. They can spread out chromatin complexes can, neurotransmitters are built in the continuously activating receptors. reabsorbed by the axon terminal in a hormones, which send the brain produce different types of neurotrans- respectively, shut down or activate the ribosome-rich space of the cell body. There are two broad types of process called reuptake. specific cues about the condition and mitters, determining whether their genes on that segment of DNA. These Vesicles containing neurotransmit- receptors on the postsynaptic mem- The excitatory and inhibitory activity of distant tissues in the body; signals have excitatory or inhibitory changes are reversible, giving neurons ter “cargos” bud off from the wall brane. In an ionotropic receptor, a neurons described above can be neuromodulators such as the endo- effects in their circuits. They can have flexibility to alter the genes they identified by the specific neurotrans- cannabinoids, cannabis-like chemicals different assortments of neurotrans- express in response to hormonal cues mitters that they make. Excitatory that seem to suppress neurotransmit- mitter receptors, determining the cells’ and environmental changes. neurons make neurotransmitters that ter release; and prostaglandins, small sensitivity to the effects of specific The genes that affect neuron open ion channels that depolarize lipids that change the brain’s response neurotransmitters. And, in their cell structure and function can also differ Many different molecules act the dendrite’s membrane; inhibitory (increasing pain sensitivity) to pain membranes, neurons possess different between individuals. Gene variants neurons make neurotransmitters that and inflammation. combinations of receptors capable of or alleles reflect differences in the hyperpolarize it. The brain’s most Individual neurons have receptors detecting neuromodulators that influ- nucleotide sequences that make up common excitatory neurotransmitter for different subsets of hormones and ence neuronal behavior — for exam- a gene. While different alleles code as neurotransmitters, and each one is glutamate; the brain’s most com- neuromodulators. In each case, these ple, hormones such as vasopressin, for forms of the same protein, the fits into specific receptors like a mon inhibitory neurotransmitter is gamma-aminobutyric acid (GABA). molecules are signals that trigger a series of chemical reactions inside the estradiol, or cortisol. All cells in your body, including variants can produce structural dif- ferences that affect their function. An key fits a lock. Glutamate is an amino acid used cell. The process starts when one of neurons, contain the same DNA allele might code for a version of an as a neurotransmitter by approximate- these molecules binds to its specific housing the same genes. Differences enzyme that is less effective than the ly half the excitatory synapses in the receptor. If the receptor is on the among your neurons result from dif- usual version, and specific alleles of brain. It can bind to several types of surface of the cell, the bound molecule ferences in which genes direct cellular some genes can even cause neurolog- ionotropic receptors; the most im- changes the receptor’s shape across the activities, a process called gene expres- ical diseases. For example, Tay-Sachs of the Golgi apparatus — the cell’s neurotransmitter binds directly to portant of these are AMPA receptors cell membrane and starts a chain of in- sion. Each cell (or cell type) builds disease, a fatal degenerative neurolog- protein-packaging organelle — then part of an ion channel. The chan- and NMDA receptors. When acti- tracellular reactions. This signal trans- proteins from a slightly different ical condition, is caused by muta- bind to proteins called kinesins that nel is normally closed; the receptor vated, the action of AMPA receptors duction pathway ultimately modifies subset of genes in its genetic code, the tions in a gene that codes for part work their way down the axon along protein changes its shape when the is fast and brief; NMDA receptors neuronal function, either by shifting same way different children will build of a fat-metabolizing enzyme called microtubules, filamentous parts of the neurotransmitter attaches, widen- activate more slowly, particularly in the cell’s ion balance or by changing different structures from the same beta-hexosaminidase A. Because the cellular skeleton. ing the tunnel in the center of the response to waves of multiple action the activity of specific enzymes. starting set of Lego blocks. variant enzyme is poor at breaking After neurotransmitters are ion channel so that ions can move potentials. Interactions between these If a molecule can diffuse through The mechanisms causing neu- down specific fats, these build up in released from an axon terminal, through. Metabotropic receptors are receptors appear to be important in the cell membrane — as occurs with rons to express some genes and not neurons and become toxic. There they drift across the synaptic cleft more complex. The receptor and the learning and memory. steroid hormones like estradiol or cor- others are currently an area of intense are many cases where small changes until they reach the outer surface ion channel are different proteins lo- GABA is the brain’s most im- tisol — its receptor might be a protein research. Many of these mechanisms in genetic sequence affect how our of the dendrite, a region that looks cated at a distance from one another, portant inhibitory neurotransmitter. inside the neuron’s soma. When the depend on chemical changes to brain can function, and in the next thick or dense in highly magnified but they are linked by a cascade of It binds to two groups of receptors; hormone binds to its receptor, the com- chromatin, the complex of protein 10 years — with our capacity to se- images. This region, the postsyn- biochemical steps that are triggered one group is ionotropic, the other plex can transform into a transcription and DNA that compactly packages quence a person’s entire genome now aptic density, has a high concentra- when a neurotransmitter binds to the metabotropic. Ionotropic GABA factor that is capable of entering the cell the long DNA molecule inside the possible — we will be able to move tion of neurotransmitter receptors. receptor. This response is less rapid receptors have ion channels that let nucleus, binding to specific genes and nucleus. Genes that a cell is using to much closer to understanding the Many different molecules act as and activates a series of events inside negatively charged chloride ions enter changing their activity. build proteins need to be accessible genetic basis of brain disorders. neurotransmitters, and each one fits the postsynaptic cell. The result may the cell. Metabotropic GABA recep- into specific receptors like a key fits be opening an ion channel some dis- tors open ion channels that release a lock. Receptors are linked to ion tance away or activating other intra- potassium ions. In both instances, ion channels in such a way that, when cellular molecules. movement pushes membrane poten- neurotransmitter molecules dock on Neurotransmitter molecules only tial downward and inhibits a neuron their receptors, they open those bind to their receptors for a short from firing. 16 Brain Facts | society for neuroscience society for neuroscience | Brain Facts 17