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 CHAPTER 2 Senses & Perception 2 Senses & Y ou can think of your sense or- but the lens can thicken or flatten to on the other hand, pick up fine detail for reading and driving. In the United gans as the brain’s windows on bring near or far objects into better and color, allowing you to engage in States and other developed countries, the external world. The world focus on the retina. Much like a cam- activities that require a great deal of death or degeneration of photoreceptors itself has no actual images, sounds, era capturing images on film, visual visual acuity. The human eye contains in the macula, called macular degener- Perception tastes, and smells. Instead, you are sur- rounded by different types of energy and molecules that must be translated into perceptions or sensations. For this input is mapped directly onto the retina as a two-dimensional reversed image. Objects to the right proj- ect images onto the left side of the three types of cones, each sensitive to a different range of colors (red, green, or blue). Because their sensitivities overlap, differing combinations of the ation, is a leading cause of blindness in people older than 55. Neurons in each of the three layers of the retina typically receive inputs extraordinary transformation to work, retina and vice versa; objects above three cones’ activity convey informa- from many cells in the preceding layer, your sense organs turn stimuli such are imaged at the lower part and vice tion about every color, enabling you but the total number of inputs varies as light waves or food molecules into versa. After processing by specialized to see the familiar color spectrum. In widely across the retina. For example, electrical signals through the process of cells in several layers of the retina, that way, your eyes resemble computer in the macular region where visual transduction. These electrical messages signals travel via the optic nerves to monitors that mix red, green, and blue acuity is highest, each ganglion cell are then carried through a network of other parts of your brain and undergo levels to generate millions of colors. receives input (via one or more inter- cells and fibers to specialized areas of further integration and interpretation. Because the center of the retina neurons) from just one or very few your brain where they are processed contains many more cones than other cones, allowing you to resolve very fine and integrated into a seamless percep- The Three-Layered Retina retinal areas, vision is sharper here than details. Near the margins of the retina, tion of your surroundings. The retina is home to three in the periphery. In the very center of however, each ganglion cell receives types of neurons — photo- the retina is the fovea, a small pitted signals from several photoreceptor VISION receptors, interneurons, and ganglion area where cones are most densely cells. This convergence of inputs Vision is one of your most compli- cells — which are organized into packed. The fovea contains only red explains why your peripheral vision cated senses, involving many processes several layers. These cells communicate and green cones and can resolve very is less detailed. The portion of visual that work simultaneously enabling you extensively with each other before fine details. The area immediately space providing input to a single gan- to see what is happening around you. sending information along to the brain. around the fovea, the macula, is critical glion cell is called its receptive field. It is no surprise, then, that the visual Counterintuitively, the light-sensitive system involves about 30 percent of photoreceptors — rods and cones humans’ cerebral cortex — more than — are located in the most peripheral any other sense does. Vision has been layer of the retina. This means that after studied intensively, and we now know entering through the cornea and lens, more about it than any other sensory light travels through the ganglion cells system. Knowledge of how light ener- and interneurons before it reaches the gy is converted into electrical signals photoreceptors. Ganglion cells and comes primarily from studies of fruit interneurons do not respond directly to flies (Drosophila) and mice. Higher- light, but they process and relay level visual processing has mostly information from the photoreceptors; Ward, et al. The Journal of Neuroscience, 2014. been studied in monkeys and cats. the axons of ganglion cells exit the In many ways, seeing with your retina together, forming the optic nerve. eyes is similar to taking pictures with There are approximately 125 an old-fashioned camera. Light passes million photoreceptors in each human through the cornea and enters the eye eye, and they turn light into electrical through the pupil. The iris regulates signals. The process of converting one how much light enters by changing form of energy into another occurs in the size of the pupil. The lens then most sensory systems and is known as bends the light so that it focuses on transduction. Rods, which make up Here, in the back of the eye, is one of the first stops visual information makes on its way to the inner surface of your eyeball, on about 95 percent of photoreceptors the brain. In this image of a mouse retina, axons of nerve cells are labeled in yellow. They extend through a small opening in the back of the eye — labeled in black — through the a sheet of cells called the retina. The in humans, are extremely sensitive, al- optic nerve to higher vision centers. The axons must penetrate another layer of cells known rigid cornea does the initial focusing, lowing you to see in dim light. Cones, as astrocytes, labeled in blue, that provide nutritional support to the retina. society for neuroscience | Brain Facts 19  2 Senses & Perception Senses & Perception 2 How Is Visual shout out “Stop!” without thinking. they start to favor one eye and often tects, such as a sound’s pitch, loudness, Information Processed? Ongoing research now questions this lose vision in the other. Vision can be duration, and location. Your auditory Every time you open your strict division of labor and suggests that restored in such cases, but only if the system analyzes complex sounds, break- eyes, you distinguish shapes, crosstalk between streams may actually child is treated at a young age; beyond ing them into separate components or colors, contrasts and the speed and create a conscious experience. Clearly, the age of 8 or so, the blindness frequencies, as a result, you can follow direction of movements. You can easily in recognizing an image the brain becomes permanent. Until a few particular voices in a conversation or distinguish your coffee mug from the extracts information at several stages, decades ago, ophthalmologists waited instruments as you listen to music. peanut butter jar in front of you. You compares it with past experiences, and until children were 4 years old before can also tell that the tree outside the passes it to higher levels for processing. operating to align the eyes, prescribing Can You Hear Me Now? window stands still and the squirrel is exercises or using an eye patch. Now Whether it’s the dreaded alarm in Miquel Perelló Nieto. scurrying up the tree (not vice versa). Eyes Come in Pairs strabismus is corrected well before age 4, the morning, the ringtone on your cell But how is a simple two-dimensional Seeing with two eyes, called binoc- when normal vision can still be restored. phone, or your favorite jogging music, retinal image processed to create such ular vision, allows you to perceive depth Loss of function or death of hearing involves a series of steps that complex imagery? or three dimensions, because each eye photoreceptors appears to lie at the convert sound waves in the air into Vision begins with light. The light bouncing off an object passes through the optical lens and Visual processing begins with sees an object from a slightly different heart of various disorders that cause electrical signals that are carried to the hits the retina at the back of the eye. Receptors in the retina transform light into electrical comparing the amounts of light hit- signals that carry information to the vision processing centers in the brain. angle. This only works if the eyes’ blindness. Unfortunately, many are brain by nerve cells. Sound in the form ting small, adjacent areas on the retina. visual fields overlap and if both eyes are difficult to treat. Extensive genetic of air pressure waves reaches the pin- The receptive fields of ganglion cells information from the left side of the of cells, new processing streams pass equally active and properly aligned. A studies and the use of model organisms nae of your ears, where the waves are “tile” the retina, providing a complete retinas of both eyes continue together the information along to other parts person with crossed eyes, a condition have identified a variety of genetic funneled into each ear canal to reach two-dimensional representation (or on the left side of the brain; informa- of the visual cortex. As visual infor- called strabismus, misses out on much defects that cause people to go blind, the eardrum (tympanic membrane). map) of the visual scene. The recep- tion from the right side of both retinas mation from the primary visual cortex depth perception. Information from making it possible to design gene or The eardrum vibrates in response to tive field of a ganglion cell is activated proceeds on the right side of the brain. is combined in other areas, receptive the perspective of each eye is preserved stem cell therapies that can recover these changes in air pressure, send- when light hits a tiny region on the Visual information is then relayed fields become increasingly complex and all the way to the primary visual cortex photoreceptors. Researchers are work- ing these vibrations to three tiny, retina that corresponds to the center of through the lateral geniculate nucleus, selective. Some neurons at higher levels where it is processed further. Two eyes ing on potential treatments for genetic sound-amplifying bones in the middle its field; it is inhibited when light hits a region of the thalamus, and then to of processing, for example, respond also allow a much larger visual field blindness, and gene therapies have ear: the malleus (hammer), incus (an- the donut-shaped area surrounding the primary visual cortex at the rear only to specific objects and faces. to be mapped onto the primary visual already enabled some patients with loss vil), and stapes (stirrup). The last bone the center. If light strikes the entire of the brain. Studies in monkeys suggest that vi- cortex. Because some of the nerve fibers of central vision (macular degenera- in the chain (the stapes) acts like a tiny receptive field — the donut and its sual signals are fed into several parallel exiting each eye cross over at the optic tion) or other forms of blindness to see piston, pushing on the oval window, hole — the ganglion cell responds only Visual Cortex: but interacting processing streams. Two chiasm, signals from the left visual field better. Work is also underway to send a membrane that separates the air- weakly. This center-surround antago- Layers, Angles, and Streams of these are the dorsal stream, which end up on the right side of the brain electrical signals directly to the brain filled middle ear from the fluid-filled, nism is the first way our visual system The primary visual cortex, a thin heads up toward the parietal lobe, and and vice versa, no matter which eye via ganglion cells rather than attempt- snail-shell-shaped cochlea of the inner maximizes the perception of contrast, sheet of neural tissue no larger than a the ventral stream, which heads down the information comes from. A similar ing to restore lost photoreceptors, an ear. The oval window converts the which is key to object detection. half-dollar, is located in the occipital to the temporal lobe. Traditionally, arrangement applies to movement and approach very similar to the use of mechanical vibrations of the stapes Neural activity in the axons of lobe at the back of your brain. Like these streams were believed to carry touch. Each half of the cerebrum is cochlear implants to treat deafness. into pressure waves in the fluid of the ganglion cells is transmitted via the the retina, this region consists of many out separate processing of unconscious responsible for processing information cochlea, where they are transduced optic nerves, which exit the back of layers with densely packed cells. The vision, which guides behavior and from the opposite side of the body. HEARING into electrical signals by specialized each eye and travel toward the back of middle layer, which receives messages conscious visual experiences. If you see Hearing is one of your most receptor cells (hair cells). the brain. Because there are no photo- from the thalamus, has receptive fields a dog running out into the street, the Treating Visual Disorders important senses, alerting you to an ap- receptors at this site, the exit point of similar to those in the retina and can ventral or “What” stream would inte- Many research studies using proaching car and telling you where it’s From Pressure the optic nerve results in a small “blind preserve the retina’s visual map. Cells grate information about the dog’s shape animals have provided insights coming from long before it comes into Wave to Electrical Signal spot” in each eye, which our brains above and below the middle layer have and color with memories and experi- into treatment of diseases that affect sight. Hearing is also central to social An elastic membrane, called fortuitously “fill in” using information more complex receptive fields, and ences that let you recognize the dog as eyesight. Research with cats and interactions. It allows you to communi- the basilar membrane, runs from the other eye. On their way to they register stimuli shaped like bars or your neighbor’s. The dorsal or “Where” monkeys has helped us find better cate with others by processing and inter- along the inside of the cochlea like a the brain, signals travel along nerve edges or with particular orientations. stream would combine various spatial therapies for strabismus. Children with preting complex messages in the form of winding ramp, spiraling from the fibers from both eyes which first For example, specific cells can respond relationships, motion, and timing to strabismus initially have good vision in speech sounds. Like the visual system, outer coil, near the oval window, to converge at a crossover junction called to edges at a certain angle or moving in create an action plan, but without a each eye but, because they cannot fuse your hearing (auditory) system picks up the innermost coil. The basilar mem- the optic chiasm. Those fibers carrying a particular direction. From these layers need for conscious thought. You might the images coming from both eyes, several qualities of the signals it de- brane is “tuned” along its length to 20 Brain Facts | society for neuroscience society for neuroscience | Brain Facts 21  2 Senses & Perception Senses & Perception 2 different frequencies (pitches). When where different auditory neurons Treating Hearing Loss ating a distinct activity pattern. This fluid inside the cochlea ripples, the respond to different frequencies. Some Loss of hair cells is responsible for “signature” pattern of activity is then Braubach, et al. The Journal of Neuroscience, 2013. membrane moves, vibrating to higher- cortical neurons, however, respond to the majority of cases of hearing loss. transmitted to the olfactory bulb and Ma, et al. The Journal of Neuroscience, 2009. pitched sounds (like the screech of sound qualities such as intensity, Unfortunately, once they die, hair on to the primary olfactory cortex audio feedback) near the oval window duration, or a change in frequency. cells don’t regrow. Current research is located on the anterior surface of the and to lower-pitched sounds (like a Other neurons are selective for complex therefore focusing on how inner ear temporal lobe. Olfactory information bass drum) in the center. sounds, while still others specialize in structures like hair cells develop and then passes to nearby brain areas, Rows of small sensory hair cells various combinations of tones. At function, exploring new avenues for where odor and taste information are located on top of the vibrating higher levels, beyond the primary treatment that could eventually involve are mixed, creating the perception of basilar membrane. When the mem- auditory cortex, neurons are able to neurogenesis with the goal of replacing flavor. Recent research suggests that brane moves up and down, micro- process harmony, rhythm, and melody, damaged hair cells. Your tongue’s receptors, called taste buds, The olfactory bulb is a structure in the people can identify odors as quickly scopic hair-like stereocilia extending and combine the types of auditory transform information about tastes and send forebrain responsible for processing smell as 110 milliseconds after their first from the hair cells bend against an information into a voice or instrument TASTE AND SMELL them to the brain to be processed into your information. This series of images shows sniff. Interestingly, the size of the favorite flavors. In this image of a mouse the olfactory bulbs from a zebrafish at overlying structure called the tectorial that you can recognize. The senses of taste (gustation) and tongue, the axons that connect to these three stages of development. olfactory bulbs and the way neurons membrane. This bending opens small Although sound is processed on smell (olfaction) are closely linked and receptors are highlighted in red. are organized can change over time. channels in the stereocilia that allow both sides of the brain, the left side is help you navigate the chemical world. As mentioned above, the olfactory ions in the surrounding fluid to rush typically responsible for understanding Just as sound is the perception of air they enable people to detect hazardous glossopharyngeal, and vagus nerves bulbs in rodents and primates (includ- in, converting the physical movement and producing speech. Someone with pressure waves and sight is the percep- substances they might inhale or ingest. — to taste regions in the brainstem. ing humans) are one of the few brain into an electrochemical signal. Hair damage to the left auditory cortex tion of light, smell and taste are your The cells processing taste and smell are The impulses are then routed through regions able to generate new neurons cells stimulated in this way then excite (particularly a region called Wernicke’s perceptions of tiny molecules in the air exposed to the outside environment, the thalamus to the gustatory cortex in (neurogenesis) throughout life. the auditory nerve, which sends its area), as from a stroke, is able to hear and in your food. Both of these senses leaving them vulnerable to damage. the frontal lobe, and insula where electrical signals on to the brainstem. a person speak but no longer under- contribute to how food tastes, and Because of this, taste receptor cells regu- specific taste perceptions are identified. Combining Taste and Smell The next stop for sound process- stands what is being said. both are important to survival, because larly regenerate, as do olfactory receptor Taste and smell are separate senses ing is the thalamus, the brain’s relay neurons. In fact, olfactory neurons are From Molecules to Smell with their own receptor organs. Yet, station for incoming sensory informa- the only sensory neurons that are con- Odors enter the nose on air we notice their close relationship when tion, which then sends the information tinually replaced throughout our lives. currents and bind to special- our nose is stuffed up by a cold and into the auditory part of the cerebral ized olfactory cells on a small patch of everything we eat tastes bland. It seems cortex. Several thousand hair cells are From Molecules to Taste mucus membrane high inside the nasal like our sense of taste no longer works, positioned along the length of the bas- Our ability to taste foods cavity. Axons of these sensory neurons but the actual problem is that we ilar membrane. Each hair cell responds depends on the molecules set enter the two olfactory bulbs (one for detect only the taste, not taste and most strongly to just a narrow range free when we chew or drink. These each nostril) after crossing through smell combined. Taste sense itself is of sound frequencies, depending on molecules are detected by taste (or tiny holes in the skull. From there, the rather crude, distinguishing only five how far along the cochlea it is located. gustatory) cells within taste buds information travels to the olfactory basic taste qualities, but our sense of Thus, each nerve fiber connecting with located on the tongue and along the cortex. Smell is the only sensory smell adds great complexity to the the hair cells is tuned to very specific roof and back of the mouth. We have system that sends sensory information flavors we perceive. Human studies frequencies and carries this informa- between 5,000 and 10,000 taste buds directly to the cerebral cortex without have shown that taste perceptions are tion into the brain. but start to lose them around age 50. first passing through the thalamus. particularly enhanced when people are Each taste bud consists of 50 to 100 We have around 1,000 different exposed to matching combinations of Chittka, et. al. PLoS Biology, 2005. Making Sense of Sound sensory cells that are receptive to one of types of olfactory cells, but can iden- familiar tastes and smells. For example, On the way to the cortex, the at least five basic taste qualities: sweet, tify about 20 times as many smells. sugar tastes sweeter when combined brainstem and thalamus use sour, salty, bitter, and umami (Japanese The tips of olfactory cells are equipped with the smell of strawberries, than the information from both ears to for “savory”). Contrary to common with several hair-like cilia that are when paired with the smell of peanut compute a sound’s direction and belief, all tastes are detected across the receptive to a number of different odor butter or no odor at all. Taste and location. The frequency map of the tongue and are not limited to specific molecules, and many cells respond to smell information appear to converge basilar membrane is maintained Sound waves — vibrations in the air caused by the sound’s source — are picked up by the outer regions. When taste receptor cells are the same molecules. A specific smell in several central regions of the brain. ear and funneled down the auditory canal to the ear drum. There, the malleus (hammer) transfers throughout, even in the primary vibrations to the incus (anvil) and then onto the stapes. Hair cells in the cochlea convert the infor- stimulated, they send signals through will therefore stimulate a unique There are also neurons in the inferior auditory cortex in the temporal lobe, mation in these vibrations to electrical signals, which are sent to the brain via the cochlear nerve. three cranial nerves — the facial, combination of olfactory cells, cre- frontal lobe that respond selectively to 22 Brain Facts | society for neuroscience society for neuroscience | Brain Facts 23  2 Senses & Perception Senses & Perception 2 specific taste and smell combinations. you to continue feeling pain, which pathway). From there, they are relayed a person feels. They also explain why Some of our sensitivity to taste helps you protect a damaged part of to several areas of the cerebral cortex some people develop chronic pain that and smell is lost as we age, most likely the body. Prostaglandins, for example, that monitor the state of the body and does not respond to regular treatment. because damaged receptors and senso- enhance the sensitivity of receptors to transform pain and itch messages into Research shows that endorphins act at Hadjab, et al. The Journal of Neuroscience, 2013. ry neurons are no longer replaced by tissue damage, making you feel pain conscious experience. Once aware, the multiple types of opioid receptors in new ones. Current research is getting more intensely. They also contribute to brain has to opportunity to change the brain and spinal cord, which has closer to understanding how stem cells a condition called allodynia, in which how it responds to these messages. important implications for pain ther- give rise to the neurons that mediate even soft touch can produce pain, as on apy, especially for people who suffer smell or taste. With this knowledge, badly sunburned skin. A long-lasting Pain Management from intense chronic pain. For exam- stem cell therapies might one day be injury may lead to nervous system Why do different people, ple, opioid drugs can now be delivered used to restore taste or smell to those