Nervous System Chapter 38 Pages 731-757

Nerve Cell Neurons or nerve cells - receive, process, transmit information Glia - which assist neuron function. Provide nutrients Regulate the composition of the extracellular fluid (brain & spinal cord) Regulate communication between neurons Speed up the movement of electrical signals within neurons

Functions of Nerve Cells Localized in separate parts of the cell A neuron must perform four functions: Receive information from the environment Process the information and produce electrical signals Conduct electrical signals to a junction where it meets another cell Transmit information to other neurons, muscles, or glands

Structures of Nerve Cells Typical neurons have four distinct parts that carry out the functions: Dendrites A cell body An axon Synaptic terminals

Dendrites – respond to stimuli Dendrites are branched tendrils protruding from the cell body Perform the “receive information” function Branches provide a large surface area for receiving signals Dendrites of sensory neurons respond to specific stimuli- pressure, odor, light, body temperature, blood pH, or position of a joint Dendrites of neurons in the brain and spinal cord respond to chemicals or neurotransmitters, released by other neurons

Cell Body – processes dendrite signals Performing the “process information” function Electrical signals travel down the dendrite to the cell body, which integrates incoming information If incoming signals are strong enough, a large, rapid electrical signal called an action potential is produced Contains other organelles - nucleus, endoplasmic reticulum, and Golgi apparatus

Axon – conducts action potentials In a typical neuron, a long, thin axon extends outward from the cell body and conducts action potentials from the cell body to synaptic terminals at the axon’s end Single axons may stretch from our spinal cord to our toes, a distance of about 3 feet Axons are typically bundled together into nerves, much like wires are bundled within an electrical cable

Synapses – transmit signals between cells The site where a neuron communicates with another cell is a synapse A synapse consists of: The synaptic terminal - a swelling at the end of an axon of the “sending” neuron A dendrite or cell body of a “receiving” neuron, muscle, or gland cell A small gap separates the two cells Contain neurotransmitters that are released in response to an action potential reaching the terminal The plasma membrane of the receiving neuron has receptors that bind the neurotransmitters and stimulate a response in this cell So the output of the first cell becomes the input to the second

A Neuron Dendrites: Receive signals from other neurons 2 Synaptic terminals: Transmit signals from other neurons 1 Cell body: Integrates signals; coordinates the neuron’s metabolic activities 3 An action potential starts here 4 neurotransmitters dendrite synaptic terminal Axon: Conducts the action potential 5 receptors Synaptic terminals: Transmit signals to other neurons 6 synapse Dendrites (of other neurons): Receive signals 7

How is information carried? Information is carried within a neuron by electrical signals and is transmitted between neurons by neurotransmitters released from one neuron and received by a second An unstimulated, inactive neuron maintains a constant electrical voltage difference, or potential, across its plasma membrane, called a resting potential The voltage inside the cell is always negative and ranges from about – 40 to –90 millivolts (mV)

Produce and Transmit Information If the membrane potential becomes less negative, it reaches a threshold level and triggers an action potential During action potential, the membrane potential rises rapidly to +50 mV inside the cell, then returns to resting potential The action potential signal flows down the axon to the synaptic terminals with no change in voltage from the cell body to the synaptic terminals

Electrical Events During an Action Potential 4 action potential threshold resting potential 3 1 2 5 less negative more negative time (milliseconds)

Myelin Myelin speeds up the conduction of action potentials The thicker an axon, the faster the action potential moves Neurons increase the rate of action potential conduction by covering portions of the axon with a fatty insulation called myelin Formed by glial cells that wrap themselves around the axon, leaving nodes between myelin segments In myelinated neurons, action potentials “jump” from node to node, traveling at a rate of 10 - 330 ft/ second

A Myelinated Axon An action potential jumps from node to node, greatly speeding up conduction down the axon Schwann cell node myelin myelin sheath axon axon

Neurotransmitters Neurons use chemicals to communicate at synapses A synapse is where the synaptic terminal of one neuron meets the dendrite of another. They do not actually touch at a synapse A tiny gap or synaptic cleft, separates the first or presynaptic neuron, from the second or postsynaptic neuron The presynaptic neuron sends neurotransmitter chemicals across the gap to the postsynaptic neuron There are many types of neurotransmitters A synaptic terminal contains vesicles, each full of neurotransmitter molecules When an action potential is initiated, it travels down an axon until it reaches its synaptic terminal

Across the Synapse The inside of the terminal becomes positively charged and triggers a cascade of changes that cause the vesicles to release neurotransmitters into the synapatic cleft The outer surface of the plasma membrane of the postsynaptic neuron is packed with receptor proteins that are specialized to bind the neurotransmitter released by the presynaptic neuron The neurotransmitter molecules diffuse across the gap and bind to these receptors

Synapses produce excitatory or inhibitory postsynaptic potentials The binding of neurotransmitter molecules to receptors on a postsynaptic neuron opens ion channels in the neuron’s plasma membrane Depending on which channels are associated with the receptors, ions such as Na+, K+, Ca2+, or Cl– may move through these channels causing a small, brief change in voltage, called a postsynaptic potential or PSP If the postsynaptic neuron becomes more negative, its resting potential moves farther away from threshold, reducing the likelihood of firing an action potential This change in voltage is an inhibitory postsynaptic potential (IPSP) If the postsynaptic neuron becomes less negative, then its resting potential will move closer to threshold, and it will be more likely to fire an action potential This voltage change is an excitatory postsynaptic potential (EPSP)

Signaling in Neurons

Neurotransmitter action is brief Some neurotransmitters are rapidly broken down by enzymes in the synaptic cleft acetylcholine, the transmitter that stimulates skeletal muscle cells Many other neurotransmitters are transported back into the presynaptic neuron

How Do Neurons Produce and Transmit Information? Summation of postsynaptic potentials determines the activity of a neuron The dendrites and cell body of a single neuron receive EPSPs and IPSPs from the synaptic terminals of presynaptic neurons The voltages of all the PSPs that reach the postsynaptic cell body are added up, a process called integration If the excitatory and inhibitory postsynaptic potentials added together raise the electrical potential inside the neuron above threshold, the postsynaptic cell produces an action potential

Nervous System Must be able to perform four operations: Determine the type of stimulus Determine and signal the intensity of a stimulus Integrate information from many sources Initiate and direct appropriate responses

Determine the type of Stimulus The nature of a stimulus is determined by connections between the senses and the brain All nervous systems interpret what a stimulus is by monitoring which neurons are firing action potentials For example, the brain interprets action potentials that occur in the axons of the eye and travel to the visual areas of the brain as the sensation of light Therefore, you distinguish the sound of music from the taste of coffee, or the bitterness of coffee from the sweetness of sugar, because these different stimuli result in action potentials in different axons that connect to different areas of the brain

Intensity is coded for by frequency of action potentials Because all action potentials are the same size and duration, no information about the strength or intensity of a stimulus can be encoded in a single action potential Intensity is coded in two ways: First, the intensity can be signaled by the frequency of action potentials in a single neuron—the more intense the stimulus, the faster the neuron fires action potentials Second, most nervous systems have many neurons that respond to the same input Stronger stimuli excite more of neurons, whereas weaker stimuli excite fewer neurons that fire at the same time A gentle touch may cause a single touch receptor in the skin to fire action potentials very slowly; a hard poke may cause several touch receptors to fire, some very rapidly

Signaling Stimulus Intensity sensory neuron 1 Sensory neuron 1 fires slowly; sensory neuron 2 is silent sensory neuron 2 sensory neuron 1 sensory neuron 2 (a) Gentle touch sensory neuron 1 Sensory neurons 1 and 2 both fire sensory neuron 2 sensory neuron 1 sensory neuron 2 (b) Hard poke time

The nervous system processes information from many sources The brain is bombarded by sensory stimuli from inside and outside the body The brain evaluates inputs, determines which are important, and decides how to respond A number of neurons may funnel their signals to fewer neurons Many sensory neurons may converge onto a small number of brain cells Some brain cells act as “decision-making” cells, adding up the PSP that result from the synaptic activity of the sensory neurons Depending on their strength (and other factors, as hormones or metabolic activity), they produce appropriate outputs

The nervous system produces outputs to muscles and glands Action potentials from the decision-making neurons may travel to other parts of the brain, the spinal cord, or the sympathetic and parasympathetic nervous system Ultimately, the output of the nervous system will stimulate activity in the muscles or glands that produce behaviors The same principles of connectivity and intensity coding for sensory inputs are used for the brain’s outputs Which muscles or glands are activated is determined by their connections to the brain or spinal cord How hard a muscle contracts is determined by how many neurons connect to it and how fast those neurons fire action potentials

How are behaviors controlled? Most behaviors are controlled by pathways composed of four elements: Sensory neurons respond to a stimulus, either internal or external Interneurons receive signals from sensory neurons, hormones, or neurons that store memories; based on this input, interneurons often activate motor neurons Motor neurons receive information from sensory neurons or interneurons and activate muscles or glands Effectors, usually muscles or glands, perform the response directed by the nervous system

Reflexes are simple behaviors Simple behaviors, such as reflexes, may be controlled by activity in as few as two or three neurons—a sensory neuron, a motor neuron, and an interneuron in between, usually stimulating a single muscle In humans, simple reflexes such as the knee-jerk or pain- withdrawal reflexes are produced by neurons in the spinal cord

Complex behaviors Complex behaviors are organized by interconnected neural pathways in which several types of sensory input converge on a set of interneurons By integrating the postsynaptic potentials from multiple sources, the interneurons “decide” what to do and stimulate motor neurons to direct the appropriate activity in muscles and glands Hundreds, or even millions of neurons, mostly in the brain, may be required to perform complex actions such as playing the piano

Simple nervous systems In the animal kingdom, there are two nervous system designs. A diffuse nervous system Cnidarians (Hydra, jellyfish, and their relatives) Radially symmetrical cnidarians have no “front end,” so there is no evolutionary pressure to concentrate the senses in one place Cnidarian nervous systems are composed of a network of neurons, called a nerve net, woven through the animal’s tissues, with a cluster of neurons, called a ganglion, but nothing like a real brain

Nervous System Organization ring of ganglia diffuse network of neurons (a) Hydra

More Complex A centralized nervous system, in more complex organisms Most animals are bilaterally symmetrical, with head and tail ends The head is usually the first part of the body to encounter food, danger, and potential mates. It is advantageous to have sense organs concentrated there Sizable ganglia evolved that integrate the information gathered by the senses and direct appropriate actions Over evolutionary time, the major sense organs of became localized in the head, and the ganglia became centralized into a brain This process, called cephalization, reached a peak in the vertebrates

Nervous System Organization brain nerve cords cerebral ganglia (brain) (b) Flatworm (c) Octopus

Nervous system divided into 2 parts The central nervous system (CNS) - brain and spinal cord The peripheral nervous system (PNS) - neurons that lie outside the CNS and axons that connect them with CNS The cell bodies of neurons of the PNS are often located in ganglia alongside the spinal cord or in ganglia near target organs, such as ganglia in the head and neck that control the salivary glands

Organization and Functions of the Vertebrate Nervous System

The PNS links the CNS to the Body Nerves of the PNS – Connect the brain and spinal cord with muscles, glands, sensory organs, and digestive, respiratory, urinary, reproductive, and circulatory systems Contain axons of sensory neurons, bringing sensory information to the CNS from all parts of the body These nerves also contain the axons of motor neurons that carry signals from the CNS to glands and muscles The motor portion of the PNS consists of 2 parts: The somatic and Autonomic nervous systems

Somatic Nervous System Controls voluntary movement Motor neurons of the somatic nervous system form synapses with skeletal muscles and control voluntary movement Lifting a cup of coffee or adjusting your iPod The cell bodies of somatic motor neurons are located in the spinal cord, and their axons go directly to the muscles they control

Autonomic Nervous System Controls involuntary actions Motor neurons of the autonomic nervous system innervate the heart, smooth muscles, and glands, and produce involuntary actions It is controlled by the hypothalamus, medulla, and pons—parts of the brain It consists of two divisions that innervate the same organs, but with opposing actions: The sympathetic division The parasympathetic division

Sympathetic Division The neurons of the sympathetic division release the neurotransmitter norepinephrine onto their target organs, preparing the body for stressful or energetic actions, “fight or flight” During these activities, it directs some of the blood supply from the digestive tract to the muscles of the arms and legs The heart rate accelerates, the pupils of the eyes open wider, and the air passages in the lungs expand

Parasympathetic Division The neurons of the parasympathetic division release acetylcholine onto their target organs The parasympathetic division controls maintenance activities that can be carried out at leisure, often called “rest and digest” Under parasympathetic control, the digestive tract becomes active, the heart rate slows, and air passages in the lungs constrict, because the body requires less blood flow and less oxygen

The Autonomous Nervous System PARASYMPATHETIC DIVISION SYMPATHETIC DIVISION constricts pupil eye dilates pupil stimulates salivation and tears salivary and lacrimal glands inhibits salivation and tearing cranial cranial constricts airways lungs relaxes airways cervical reduces heartbeat cervical heart increases heartbeat stimulates pancreas to release insulin and digestive enzymes liver stimulates glucose production and release pancreas stimulates secretion of epinephrine and norepinephrine from adrenal medulla thoracic kidney thoracic stomach stimulates digestion inhibits digestion spleen kidney lumbar dilates blood vessels in gut small intestine large intestine lumbar rectum urinary bladder sacral sacral stimulates bladder to contract relaxes bladder sympathetic ganglia uterus stimulates sexual arousal external genitalia stimulates orgasm

Central Nervous System Receives and processes sensory information, generates thoughts, directs responses The brain and spinal cord are protected from physical damage in 3 ways: The skull surrounds the brain, and vertebrae protect the spinal cord The triple connective tissue layer of meninges lies between the bone and spinal cord Between the meninges layers is the cerebrospinal fluid that cushions the brain and spinal cord, and nourishes the cells

Extra Brain Protection Protected from damaging chemicals by the blood–brain barrier Capillary system is less permeable than in the rest of the body and selectively transports needed materials into the brain while keeping many dangerous substances out The blood–brain barrier keeps water-soluble substances from diffusing from the blood into the brain, but many lipid-soluble substances can still diffuse across the capillary walls

Spinal Cord Controls reflexes, conducts information to and from the brain Nerves carrying axons of sensory neurons emerge from the dorsal part of the spinal cord, and nerves carrying axons of motor neurons emerge from the ventral part Merge to form the spinal nerves that innervate the body Resemble tree roots tree that merge into a single trunk, the branches are called the dorsal and ventral roots of the spinal nerves

Swellings on each dorsal root - dorsal root ganglia, contain the cell bodies of sensory neurons In the center of the spinal cord is a butterfly-shaped area of gray matter Contain cell bodies of motor neurons that control voluntary muscles and the autonomic nervous system, plus interneurons that communicate with the brain and other parts of the spinal cord

The Spinal Cord gray matter dorsal root contains the cell contains the bodies of motor neurons and interneurons dorsal root contains the axons of sensory neurons white matter contains myelinated axons dorsal root ganglion contains the cell bodies of sensory neurons spinal nerve ventral root contains the axons of motor neurons

White Matter The gray matter is surrounded by white matter - contains myelin-coated axons of neurons that extend up or down the spinal cord These axons carry sensory signals from internal organs, muscles, and the skin up to the brain Axons also extend downward from the brain, carrying signals that direct the motor portions of the peripheral nervous system If the spinal cord is severed, body parts innervated by motor and sensory neurons located below the injured area are paralyzed and feel numb, though the motor and sensory neurons, the spinal nerves, and the muscles remain intact

The Spinal Cord gray matter dorsal root contains the cell contains the bodies of motor neurons and interneurons dorsal root contains the axons of sensory neurons white matter contains myelinated axons dorsal root ganglion contains the cell bodies of sensory neurons spinal nerve ventral root contains the axons of motor neurons

Reflexes The neuronal circuits for many reflexes reside in the spinal cord The simplest type of behavior is the reflex, an involuntary movement of a body part in response to a stimulus In vertebrates, many reflexes are produced by the spinal cord and peripheral neurons, and do not include the brain

Pain-withdrawal Reflex, an example The pain-withdrawal reflex involves neurons of both the central and peripheral nervous systems If you put your hand on a tack, tissue damage activates pain sensory neurons Action potentials in the axons of pain sensory neurons travel up the spinal nerve and enter the spinal cord through a dorsal root Within the gray matter of the cord, the pain sensory neuron stimulates an interneuron, which stimulates a motor neuron Action potentials in the axon of the motor neuron leave the spinal cord through a ventral root and travel in a spinal nerve to a skeletal muscle The action potential stimulates the muscle, which contracts, and you withdraw your hand from the tack

The Pain-withdrawal Reflex stimulus sensory neuron spinal cord motor dorsal root interneuron ventral root The motor neuron stimulates the effector muscle The effector muscle causes a withdrawal response A painful stimulus activates a pain sensory The signal is transmitted by the pain sensory neuron to the spinal cord transmitted to an interneuron and then to a motor neuron 4 3 2 1 5

Animation: Reflex Arcs

Many spinal cord interneurons also have axons that extend to the brain Action potentials in these axons inform the brain about stuck hands and may trigger more complex behaviors, such as shrieks and learning about the dangers of thumbtacks The brain sends action potentials down axons in the spinal cord white matter to interneurons and motor neurons in the gray matter, which modify spinal reflexes With enough training, or motivation, you can suppress the pain-withdrawal reflex

Some complex actions coordinated within the spinal cord The wiring for some complex activities resides within the spinal cord All the neurons needed for basic movements of walking and running are contained in the spinal cord The advantage of the semi-independent arrangement between brain and spinal cord increases speed and coordination, because messages do not travel up to the brain and back down, just to swing a leg forward while walking The brain’s role in these semi-automatic behaviors is to initiate, guide, and modify spinal motor neuron activity

The Brain All vertebrate brains consist of three major parts: The hindbrain - medulla, pons, and cerebellum The midbrain The forebrain - thalamus, hypothalamus, and cerebrum In the earliest vertebrates, these three anatomical divisions were also functional divisions: The hindbrain governed breathing and heart rate The midbrain controlled vision The forebrain dealt with the sense of smell

In nonmammalian vertebrates, the three divisions remain prominent However, in mammals—particularly humans—the brain regions are significantly modified Some have been reduced in size; others, especially the forebrain, are greatly enlarged

A Comparison of Vertebrate Brains optic lobe thalamus cerebellum cerebrum medulla midbrain cerebrum forebrain midbrain hindbrain cerebellum (a) Embryonic vertebrate brain midbrain cerebrum cerebellum (d) Horse brain (b) Shark brain cerebrum midbrain cerebrum cerebellum midbrain (inside cerebellum (c) Goose brain (e) Human brain

Hindbrain - Medulla In structure and function, the medulla is like an enlarged extension of the spinal cord Like the spinal cord, the medulla has neuron cell bodies at its center, surrounded by a layer of myelin-covered axons It controls automatic functions - breathing, heart rate, blood pressure, swallowing

Hindbrain - Pons Located above the medulla, Neurons influence transitions between sleep and wakefulness, affect the rate and pattern of breathing

Hindbrain - Cerebellum Coordinates body movements Receives information from command centers in the forebrain and position sensors in muscles and joints. By comparing information from these two sources, the cerebellum guides smooth, accurate motions and body position The cerebellum is also involved in motor learning as a result of practice of a repeated activity

The Cerebral Cortex The thin outer layer of each cerebral hemisphere, with billions of neurons packed in a highly organized way into a sheet just a few mm. thick Folded into convolutions - raised, wrinkled ridges that increase its surface area to over two square yards Neurons in the cortex receive sensory information, process it, direct voluntary movements, create memories, and allow us to be creative and even envision the future The cortexes in each hemisphere communicate with each other through a band of axons, the corpus callosum

Midbrain Clusters of neurons that contribute to movement, arousal, emotion The midbrain is small in humans, and has an auditory relay center and clusters of neurons that control reflex movements of the eyes For example, if you are sitting in class and someone races through the door, centers in your midbrain are alerted and direct your gaze to the new, and potentially interesting or threatening, visual stimulus

Midbrain The midbrain contains neurons that produce dopamine Cluster of neurons, the substantia nigra, helps control movement Another cluster is an essential part of the “reward circuit” that is responsible for pleasurable sensations and addiction Contains a portion of the reticular formation that consists of dozens of interconnected clusters of neurons in the medulla, pons, and midbrain, which send axons to the forebrain

Reticular Formation These neurons receive input from every sense, part of the body and many areas of the brain Plays a role in sleep, wakefulness, emotion, muscle tone, and some movements and reflexes. It filters sensory inputs before they reach the conscious regions of the brain Activities of the reticular formation allow you to read and concentrate in the presence of a variety of distracting stimuli, such as music or smells Example - mother who wakes hearing the faint cry of her infant, but sleeps through loud traffic noise

Forebrain - Thalamus Complex relay station that channels all sensory information except olfaction, from all parts of the body to the cerebral cortex Signals traveling from the spinal cord, cerebellum, medulla, pons, and reticular formation pass through the thalamus

Forebrain - Hypothalamus Clusters of neurons that release hormones into the blood or control the release of hormones from the pituitary gland Other regions direct activities of the autonomic nervous system The hypothalamus, by hormone production and neural connections, maintains homeostasis by influencing body temperature, food intake, water balance, heart rate, blood pressure, the menstrual cycle, and circadian rhythms

Forebrain - Cerebrum Outer cerebral cortex Two cerebral hemisphere Outer cerebral cortex Corpus Collosum - clusters of neurons beneath the cortex near the thalamus Bundles of axons that interconnect the two hemispheres and connect the hemispheres to the midbrain and hindbrain

Structures of the interior cerebrum The amygdala - clusters of neurons - produce sensations of pleasure, fear, or sexual arousal The hippocampus plays an important role in the formation of long-term memory, particularly of places, and is required for learning All vertebrates have a hippocampus Birds, especially jays and nutcrackers, remember where they store seeds during the winter - have a larger hippocampus than other birds

The basal ganglia - deep within the cerebrum and substantia nigra in the midbrain Important in the overall control of movement The basal ganglia are essential to the decision to initiate a particular movement and to suppress other movements Parkinson’s disease & substantia nigra Huntington’s & basal ganglia

The Human Brain meninges skull FOREBRAIN (within dashed blue line) cerebral cortex corpus callosum cerebral cortex (gray matter) hypothalamus thalamus pituitary gland myelinated axons (white matter) corpus callosum thalamus MIDBRAIN basal ganglia cerebellum pons HINDBRAIN medulla hypothalamus spinal cord hippocampus substantia nigra (a) A lateral section of the human brain (b) A cross-section of the brain

Limbic System A diverse group of structures The hypothalamus, the amygdala, and the hippocampus, as well as nearby regions of the cerebral cortex, located in a ring between the thalamus and cerebral cortex Helps to produce emotions and emotional behaviors - fear, rage, calm, hunger, thirst, pleasure, sexual responses Other brain regions are involved in emotions, including other parts of the cerebral cortex, midbrain, hindbrain, and spinal cord

The Limbic System limbic region of cortex cerebral cortex corpus callosum thalamus olfactory bulb hypothalamus hippocampus amygdala

4 Anatomical Regions The cerebral cortex is divided into four anatomical regions: frontal, parietal, occipital, and temporal It can also be divided into functional areas: Primary areas are regions where signals originating in sensory organs such as the eyes and ears are received and converted into subjective impressions Nearby association areas interpret the sounds as speech or music, and the visual stimuli as recognizable objects or words

The Cerebral Cortex primary sensory area sensory association area visual auditory association area: language comprehension memory speech motor area higher intellectual functions auditory leg trunk arm hand face tongue Parietal Lobe Frontal Temporal Occipital premotor

Primary Sensory Areas The parietal lobe - interpret sensations of touch that originate in all parts of the body In an adjacent region of the frontal lobe, primary motor areas command movements in corresponding areas of the body by stimulating the motor neurons in the spinal cord that innervate muscles, allowing you to walk Like the primary sensory area, the primary motor area has adjacent association areas, including the motor association area which directs the motor area to produce movements

Frontal Lobe Behind the bones of the forehead lies the association areas of the frontal lobe They are important in complex reasoning functions such as short-term memory, decision making, predicting the consequences of actions, controlling aggression, planning for the future, and working for delayed rewards

Damage to the Cortex Damage to the cortex from trauma, stroke, or a tumor results in specific deficits such as problems with speech, difficulty reading, or the inability to sense or move specific parts of the body Most brain cells of adults cannot be replaced, so if a brain region is destroyed, these deficits may be permanent Training sometimes allows undamaged regions of the cortex to take over and restore some of the lost functions

Animation: The Human Brain

How do neuroscientists learn about the functions of brain regions? The functions of different parts of the brain were discovered by examining the behaviors and abilities of people who suffered brain injuries In 1848, Phineas Gage was setting an explosive charge to clear rocks from a railroad line under construction when the gunpowder triggered prematurely The blast blew a 13-pound steel rod through Gage’s skull, severely damaging both of his frontal lobes Gage survived his wounds, but his personality changed radically Before the accident, Gage was conscientious, industrious, and well-liked; after his recovery, he became impetuous, profane, and incapable of working toward a goal

A Revealing Accident

Studies of other people with brain injuries have revealed that many parts of the brain are highly specialized One patient with damage to the left frontal lobe was unable to name fruits and vegetables, although he could name everything else Other victims of brain damage are unable to recognize faces, suggesting that the brain has regions specialized to recognize categories of things Modern neuroscience has powerful techniques for visualizing brain structure and activity, offering insights into the functioning of the human brain

Left Brain or Right Brain Specialized for different functions The structural brain symmetry does not extend to brain function In the 1950s, Roger Sperry of the California Institute of Technology studied people whose hemispheres had been separated by cutting the corpus callosum to prevent the spread of epilepsy from one hemisphere to the other Severing the corpus callosum prevented the two hemispheres from communicating with each other

Sperry made use of the fact that axons from the eyes, which are not severed by the surgery, follow a pathway that causes the left half of each visual field to be “seen” by the high hemisphere and the right half to be seen by the left hemisphere Sperry used a device that projected different images onto the left and right visual fields and thus sent different signals to each hemisphere When Sperry projected an image of a nude figure onto only the left visual field, the subjects would blush and smile, but would claim to have seen nothing The same figure projected onto the right visual field was readily described verbally

Specialization of Cerebral Hemispheres Left HEART Right LEFT HEMISPHERE Controls right side of body Input from right visual field, right ear, left nostril Centers for language, speech, reading, mathematics, logic RIGHT HEMISPHERE Controls left side of body Input from left visual field, left ear, right nostril Centers for spatial perception, music, creativity, recognition of faces and emotions retina optic nerve optic chiasma corpus callosum visual cortex

In right-handed people, the left hemisphere is dominant in speech, reading, writing, language, comprehension, mathematical ability, and logical problem solving The right side of the brain is superior in musical skills, artistic ability, recognizing faces, spatial visualization, and the ability to recognize and express emotions Recent experiments indicate that the left–right dichotomy is not as rigid as was once believed

Learning and Memory Involve biochemical and structural changes in specific parts of the brain Learning has two phases: working memory and long- term memory Remembering a telephone number long enough to dial it is working memory; if the number is called often enough, it becomes remembered as long-term memory The frontal and parietal lobes of the cerebral cortex and some of the basal ganglia deep in the cerebrum are the primary sites of working memory

Most working memory probably requires the repeated activity of a particular neural circuit in the brain, and as long as the circuit is active, the memory stays In contrast, long-term memory seems to be structural and the result of persistent changes in the expression of certain genes It may require the formation of new, long-lasting synaptic connections between specific neurons, or the long-term strengthening of existing, but weak, synapses For many memories, converting working memory into long-term memory seems to involve the hippocampus, which is believed to process new memories and transfer them to the frontal and temporal lobes of the cerebral cortex for permanent storage