Neurons and Nervous Systems
34 Neurons and Nervous Systems Chapter 34 Neurons and Nervous Systems
Key Concepts 34.1 Nervous Systems Consist of Neuronsand Glia 34.2 Neurons Generate and TransmitElectrical Signals 34.3 Neurons Communicate with OtherCells at Synapses Chapter 34 Neurons and Nervous Systems
Key Concepts 34.4 The Vertebrate Nervous System HasMany Interacting Components 34.5 Specific Brain Areas Underlie theComplex Abilities of Humans Chapter 34 Opening Question
How can a small brain tumor so dramaticallyaffect personality and behavior? Concept 34.1 Nervous Systems Consist of Neurons and Glia
Nervous systems have two categories ofcells: Neurons, or nerve cells, are excitabletheygenerate and transmit electrical signals,called action potentials. Glia, or glial cells, provide support andmaintain extracellular environment. Concept 34.1 Nervous Systems Consist of Neurons and Glia
Most neurons have four regions: Cell bodycontains nucleus andorganelles Dendrites carries signals, called nerveimpulses or action potentials, to the cellbody Axongenerates action potentials andconducts them away from the cell body Axon terminalsynapse at tip of axon;releases neurotransmitters VIDEO 34.1 Growing neurons Concept 34.1 Nervous Systems Consist of Neurons and Glia
Neurons pass information at synapses: The presynaptic neuron sends themessage The postsynaptic neuron receives themessage Figure 34.1 A Generalized Neuron Concept 34.1 Nervous Systems Consist of Neurons and Glia
Glial cells, or glia, outnumber neurons in thehuman brain. Glia do not transmit electrical signals butcan release neurotransmitters. Glia also give support during development,supply nutrients, remove debris, andmaintain extracellular environment. Important in neuroplasticitysynapsemodification Concept 34.1 Nervous Systems Consist of Neurons and Glia
Astrocytes are glia that contribute to thebloodbrain barrier, which protects thebrain. The blood-brain barrier is permeable to fat- soluble compounds like alcohol andanesthetics. Microglia provide the brain with immunedefenses since antibodies cannot enter thebrain. Concept 34.1 Nervous Systems Consist of Neurons and Glia
Oligodendrocytes are glia that insulateaxons in the brain and spinal cord. Schwann cells insulate axons in nervesoutside of these areas. The glial membranes form a nonconductivesheathmyelin. Myelin-coated axons are white matter andareas of cell bodies are gray matter. Multiple sclerosis is a demyelinatingdisease. Figure 34.2 Wrapping Up an Axon (Part 1) Figure 34.2 Wrapping Up an Axon (Part 2) Concept 34.1 Nervous Systems Consist of Neurons and Glia
Neurons are organized into neuralnetworks. Afferent neurons carry sensory informationinto the nervous system from sensorycells that convert stimuli into actionpotentials. Efferent neurons carry commands toeffectors such as muscles, glandsmotorneurons are effectors that carrycommands to muscles. Interneurons store information andcommunicate between neurons. Concept 34.1 Nervous Systems Consist of Neurons and Glia
Networks vary in complexity. Nerve netsimple network of neurons Ganglianeurons organized into clusters,sometimes in pairs, in simple animals Brainthe largest pair of ganglia, found inanimals with complex behavior requiringmore information-processing Figure 34.3 Nervous Systems Vary in Size and Complexity (Part 1) Figure 34.3 Nervous Systems Vary in Size and Complexity (Part 2) Figure 34.3 Nervous Systems Vary in Size and Complexity (Part 3) Concept 34.2 Neurons Generate and Transmit Electrical Signals
Neurons generate changes in membranepotentialthe difference in electricalcharge across the membrane. These changes generate nerve impulses, oraction potentials. An action potential is a rapid, large changein membrane potential that travels alongan axon and causes release of chemicalsignals. Concept 34.2 Neurons Generate and Transmit Electrical Signals
Voltage is a measure of the difference inelectrical charge between two points. Electrical current in solution is carried byions. Major ions in neurons: Sodium (Na+) Potassium (K+) Calcium (Ca2+) Chloride (Cl) Different concentrations and charges insideand out produce the membrane potential. See Concept 2.5 Concept 34.2 Neurons Generate and Transmit Electrical Signals
Membrane potentials can be measured in allcells with electrodes. Resting potential is the membranepotential of a resting, or inactive, neuron. The resting potential of a membrane isbetween 60 and 70 millivolts (mV). The inside of the cell is negative at rest. Anaction potential allows positive ions to flowin briefly, making the inside of the cellmore positive. ANIMATED TUTORIAL 34.1 The Resting Membrane Potential Figure 34.4 Measuring the Membrane Potential (Part 1) Figure 34.4 Measuring the Membrane Potential (Part 2) Concept 34.2 Neurons Generate and Transmit Electrical Signals
Ion channels and ion transporters in themembrane create the resting and actionpotentials. Sodiumpotassium pumpmoves Na+ions from inside, exchanges for K+ fromoutsideestablishes concentrationgradients The Na+K+ pump is an antiporter, orsodiumpotassium ATPase, as it requiresATP. LINK The energetics of the sodiumpotassium pump are described in Concept 5.3 Figure 34.5 Ion Transporters and Channels (Part 1) Concept 34.2 Neurons Generate and Transmit Electrical Signals
Potassium channels are open in the restingmembrane and are highly permeable to K+ionsallow leak currents K+ ions diffuse out of the cell along theconcentration gradient and leave behindnegative charges within the cell. K+ ions diffuse back into the cell because ofthe negative electrical potential. These two forces acting on K+ are itselectrochemical gradient. Figure 34.5 Ion Transporters and Channels (Part 2) Concept 34.2 Neurons Generate and Transmit Electrical Signals
The equilibrium potential is the membranepotential at which the net movement of anion ceases. The Nernst equation calculates the valueof the equilibrium potential by measuringthe concentrations of an ion on both sidesof the membrane. APPLY THE CONCEPT Neurons generate and transmit electrical signals Concept 34.2 Neurons Generate and Transmit Electrical Signals
Some ion channels are gatedopen andclose under certain conditions: Voltage-gated channels respond tochange in voltage across membrane Chemically-gated channels depend onmolecules that bind or alter channelprotein Mechanically-gated channels respond toforce applied to membrane Concept 34.2 Neurons Generate and Transmit Electrical Signals
Gating provides a means for neurons tochange their membrane potentials inresponse to a stimulus. The membrane is depolarized when Na+enters the cell and the inside of the neuronbecomes less negative. If gated K+ channels open and K+ leaves,the cell becomes more negative inside andthe membrane is hyperpolarized. Figure 34.6 Membranes Can Be Depolarized or Hyperpolarized Concept 34.2 Neurons Generate and Transmit Electrical Signals
Graded membrane potentials are changesfrom the resting potential. Graded potentials are a means ofintegrating inputthe membrane canrespond proportionally to depolarization orhyperpolarization. Concept 34.2 Neurons Generate and Transmit Electrical Signals
Voltage-gated Na+ and K+ channels areresponsible for action potentialssudden,large changes in membrane potential. At rest most of these channels are closed. Local depolarization by gated channels indendrites produces a graded potential. It spreads to the axon hillock, where Na+voltage-gated channels are concentrated. Concept 34.2 Neurons Generate and Transmit Electrical Signals
The membrane in the axon hillock mayreach its threshold5 to 10 mV aboveresting potential. Many voltage-gated Na+ channels(activation gates) open quickly and Na+rushes into the axon. The influx of positive ions causes moredepolarization, the membrane potential isbriefly positive, and an action potentialoccurs. Concept 34.2 Neurons Generate and Transmit Electrical Signals
The axon quickly returns to resting potentialdue to two things: Voltage-gated K+ channels open slowlyand stay open longerK+ moves out Voltage-gated Na+ channels (inactivationgates) close Voltage-gated Na+ channels cannot openagain during the refractory perioda fewmilliseconds. ANIMATED TUTORIAL 34.2 The Action Potential Figure 34.7 The Course of an Action Potential (Part 1) Figure 34.7 The Course of an Action Potential (Part 2) Concept 34.2 Neurons Generate and Transmit Electrical Signals
An action potential is an all-or-none event positive feedback to voltage-gated Na+channels ensures the maximum actionpotential. An action potential is self-regeneratingbecause it spreads to adjacent membraneregions. Concept 34.2 Neurons Generate and Transmit Electrical Signals
Axon diameter and myelination by glial cellsincrease the speed of action potentials inaxons. The nodes of Ranvier are regularly spacedgaps where the axon is not covered bymyelin. Action potentials are generated at the nodesand the positive current flows down theinside of the axon. Concept 34.2 Neurons Generate and Transmit Electrical Signals
When positive current reaches the nextnode, the membrane is depolarized another axon potential is generated. Action potentials appear to jump from nodeto node, a form of propagation calledsaltatory conduction. Figure 34.8 Saltatory Action Potentials (Part 1) Figure 34.8 Saltatory Action Potentials (Part 2) Concept 34.3 Neurons Communicate with Other Cells at Synapses
Neurons communicate with other neurons ortarget cells at synapses. In a chemical synapse neurotransmittersfrom a presynaptic cell bind to receptors ina postsynaptic cell. The synaptic cleftabout 25 nanometerswideseparates the cells. Concept 34.3 Neurons Communicate with Other Cells at Synapses
In an electrical synapse, cells are joinedthrough gap junctions. Gap junctions are made of proteins(connexins) that create channels. Ions flow through the channelsthe actionpotential spreads through the cytoplasm. These action potentials are fast but do notallow for complex integration of inputs. Concept 34.3 Neurons Communicate with Other Cells at Synapses
The neuromuscular junction is a chemicalsynapse between motor neurons andskeletal muscle cells. An action potential causes voltage-gatedCa+ channels to open in the presynapticmembrane, allowing Ca+ to flow in. The presynaptic neuron releasesacetylcholine (ACh) from its axon terminals(boutons) when vesicles fuse with themembrane. ANIMATED TUTORIAL 34.3 Synaptic Transmission INTERACTIVE TUTORIAL 34.1 Neurons: Electrical and Chemical Conduction APPLY THE CONCEPT Neurons communicate with other cells at synapses Figure 34.9 Chemical Synaptic Transmission Concept 34.3 Neurons Communicate with Other Cells at Synapses
The postsynaptic membrane of the musclecell is the motor end plate. ACh diffuses across the cleft and binds toACh receptors on the motor end plate. These receptors allow Na+ and K+ to flowthrough, and the increase in Na+depolarizes the membrane. If it reaches threshold, more Na+ voltage- gated channels are activated and anaction potential is generated. LINK The physical and molecular interactions of muscle contraction are described in Concept 36.1 Figure 34.10 Chemically Gated Channels Concept 34.3 Neurons Communicate with Other Cells at Synapses
The postsynaptic cell must sum theexcitatory and inhibitory input. Summation occurs at the axon hillock, thepart of the cell body at the base of theaxon. Spatial summation adds up messages atdifferent synaptic sites. Temporal summation adds up potentialsgenerated at the same site, over time. Figure 34.11 The Postsynaptic Neuron Sums Information Concept 34.3 Neurons Communicate with Other Cells at Synapses
Neurotransmitters are cleared from the cleftafter release in order to stop their action inseveral ways: Diffusion Reuptake by adjacent cells Enzymes present in the cleft may destroythem Example: Acetylcholinesterase acts on ACh. Concept 34.3 Neurons Communicate with Other Cells at Synapses
There are many types of neurotransmitters,and each may have multiple receptorsubtypes. For example, ACh has two: Nicotinic receptors are ionotropic andmainly excitatory Muscarinic receptors are metabotropic andmainly inhibitory The action of a neurotransmitter depends onthe receptor to which it binds. Concept 34.3 Neurons Communicate with Other Cells at Synapses
Synapses can be fast or slow: Neurotransmitters binding to anionotropic receptor, or ion channel,cause a change in ion movement response is fast and short-lived Metabotropic receptors induce signalingcascades in the postsynaptic cell that leadto changes in ion channels.Cell responses are generally slower andlonger-lived. Vertebrate nervous systems:
Concept 34.4 The Vertebrate Nervous System Has Many Interacting Components Vertebrate nervous systems: Brain, spinal cord, and peripheral nervesthat extend throughout the body. Central nervous system (CNS)brain andspinal cord Peripheral nervous system (PNS) cranial and spinal nerves that extend orreside outside of the brain and spinal cord,and connect the CNS to all tissues Figure 34.12 Organization of the Nervous System The afferent part of the PNS carries sensory information to the CNS.
Concept 34.4 The Vertebrate Nervous System Has Many Interacting Components The afferent part of the PNS carries sensoryinformation to the CNS. The efferent part of the PNS carriesinformation from the CNS to muscles andglands. Efferent pathways can be divided into twodivisions: The voluntary division, which executesconscious movements The involuntary, or autonomic, division,which controls physiological functions Concept 34.4 The Vertebrate Nervous System Has Many Interacting Components
Autonomic Nervous System (ANS)theoutput of the CNS that controls involuntaryfunctions ANS has two divisions that work inoppositionone will increase a functionand the other will decrease it Sympathetic division prepares the bodyfor emergenciesfight or flight Parasympathetic division slows the heart,lowers blood pressure and increasesdigestionrest and digest Figure 34.13 The Autonomic Nervous System Concept 34.4 The Vertebrate Nervous System Has Many Interacting Components
Autonomic efferent pathways begin withpreganglionic neurons with cell bodies inthe CNS. Axons of preganglionic neurons synapse ona second neuron outside the CNS in acollection of neurons called a ganglion. The second neuron is postganglionicitsaxon leaves the ganglion and synapses inthe target organs. Example: Pacemaker cells in the heart.
Concept 34.4 The Vertebrate Nervous System Has Many Interacting Components Sympathetic postganglionic neurons arenoradrenergicuse norepinephrine astheir neurotransmitter. Postganglionic neurons of theparasympathetic division are mostlycholinergicrelease acetylcholine. Target cells respond in opposite ways toacetylcholine and norepinephrine. Example: Pacemaker cells in the heart. 60 Sympathetic and parasympathetic divisions have different anatomy.
Concept 34.4 The Vertebrate Nervous System Has Many Interacting Components Sympathetic and parasympathetic divisionshave different anatomy. The sacral region contains preganglionicneurons of the parasympathetic region. The thoracic and lumbar regions containsympathetic preganglionic neurons. 61 Anatomy of the spinal cord:
Concept 34.4 The Vertebrate Nervous System Has Many Interacting Components Anatomy of the spinal cord: Gray matter is in the center and containscell bodies of spinal neurons White matter surrounds gray matter andcontains axons that conduct information upand down the spinal cord Spinal nerves extend from the spinal cord 62 Each spinal nerve has two roots.
Concept 34.4 The Vertebrate Nervous System Has Many Interacting Components Each spinal nerve has two roots. One spinal root connects to the dorsalhorn, the other to the ventral horn Afferent (sensory) axons enter through thedorsal root Efferent (motor) axons leave through theventral root 63 The knee-jerk reflex is monosynaptic:
Concept 34.4 The Vertebrate Nervous System Has Many Interacting Components Spinal reflexafferent information convertsto efferent activity without the brain. The knee-jerk reflex is monosynaptic: Stretch receptors send axon potentialsthrough dorsal horn to ventral horn viasensory axons At synapses with motor neurons in theventral horn, action potentials are sent toleg muscles, causing contraction 64 Flexors bend or flex the limb Extensors straighten or extend the limb
Concept 34.4 The Vertebrate Nervous System Has Many Interacting Components Most spinal circuits are more complexlimbmovement is controlled by antagonisticmuscle sets. Flexors bend or flex the limb Extensors straighten or extend the limb Coordination of relaxation and contraction isdone by interneuronsthey makeinhibitory synapses in a polysynaptic reflex See Concept 36.3 ANIMATED TUTORIAL 34.4 Information Processing in the Spinal Cord 65 Figure 34.14 The Spinal Cord Coordinates the Knee-jerk Reflex The hindbrain becomes the medulla, the pons, and the cerebellum.
Concept 34.4 The Vertebrate Nervous System Has Many Interacting Components The embryonic neural tube develops intothe hindbrain, midbrain, and forebrain. The hindbrain becomes the medulla, thepons, and the cerebellum. Together the pons, medulla, and midbrainare known as the brainstem. All information between the spinal cord andthe brain passes through the brainstem. See Figure 33.13 67 Low to mid-brainstem activity is involved with balance, coordination.
Concept 34.4 The Vertebrate Nervous System Has Many Interacting Components Many sensory axons have branches in thebrainstem that form synapses with thereticular systema network of neurons inthe brainstem. Low to mid-brainstem activity is involvedwith balance, coordination. 68 The embryonic forebrain develops the diencephalon and telencephalon.
Concept 34.4 The Vertebrate Nervous System Has Many Interacting Components The embryonic forebrain develops thediencephalon and telencephalon. The diencephalon consists of the: Thalamusthe final relay station forsensory information Hypothalamusregulates physiologicalfunctions such as hunger and thirst LINK The roles of the hypothalamus in homeostatic regulation and sensory integration are detailed in Concepts 29.5 and 30.3 69 Amygdalainvolved in fear and fear memory
Concept 34.4 The Vertebrate Nervous System Has Many Interacting Components Structures in primitive regions of thetelencephalon form the limbic system responsible for basic physiological drives. Amygdalainvolved in fear and fearmemory Hippocampustransfers short-termmemory to long-term memory 70 Figure 34.15 The Limbic System Concept 34.4 The Vertebrate Nervous System Has Many Interacting Components
The cerebrum is the dominant structure inmammals, with left and right cerebralhemispheres. Cerebral cortexa sheet of gray mattercovering each hemisphere, folded intoconvolutions 72 Figure 34.16 The Human Cerebrum (Part 1) Figure 34.16 The Human Cerebrum (Part 2) Regions of the cerebral cortex have specific functions.
Concept 34.4 The Vertebrate Nervous System Has Many Interacting Components Regions of the cerebral cortex have specificfunctions. Association cortex is made up of areasthat integrate or associate sensoryinformation or memories. 75 Each cerebral hemisphere consists of four lobes: Temporal lobe
Concept 34.4 The Vertebrate Nervous System Has Many Interacting Components Each cerebral hemisphere consists of four lobes: Temporal lobe Frontal lobe Parietal lobe Occipital lobe 76 Receives and processes auditory and visual information
Concept 34.4 The Vertebrate Nervous System Has Many Interacting Components Temporal lobe: Receives and processes auditory and visualinformation Association areas of the temporal lobeinvolve: Identification Object naming Recognition Agnosia: Disorder of the temporal lobe 77 Association areas involve: Feeling, planning Personality
Concept 34.4 The Vertebrate Nervous System Has Many Interacting Components Frontal Lobe: Primary motor cortex is anterior to theparietal lobe and controls muscles inspecific body areas. Association areas involve: Feeling, planning Personality 78 Figure 34.17The Body Is Represented in Primary Motor and Primary Somatosensory Cortexes Primary somatosensory motor cortex behind the primary motor cortex
Concept 34.4 The Vertebrate Nervous System Has Many Interacting Components Parietal lobe: Primary somatosensory motor cortexbehind the primary motor cortex Receives touch and pressure information The entire body surface is mapped, morearea for fine discriminations in touch. 80 Receives and processes visual information Association areas involve:
Concept 34.4 The Vertebrate Nervous System Has Many Interacting Components Occipital lobe: Receives and processes visual information Association areas involve: Making sense of the visual world Translating visual experience intolanguage 81 Concept 34.5 Specific Brain Areas Underlie the Complex Abilities of Humans
The lateralization of language functionsshows that 97 percent occurs in the leftbrain hemisphere. An aphasia is a deficit in the ability to use orunderstand words; occurs after damage tothe left hemisphere. 82 Concept 34.5 Specific Brain Areas Underlie the Complex Abilities of Humans
Language areas: Brocas areain frontal lobe; damageresults in slow or lost speech; still can readand understand language Wernickes areain temporal lobe.Damage results in inability to speaksensibly; written or spoken language notunderstood. Still can produce speech 83 Figure 34.18 Imaging Techniques Reveal Active Parts of the Brain Learningmodification of behavior by experience.
Concept 34.5 Specific Brain Areas Underlie the Complex Abilities of Humans Learningmodification of behavior byexperience. Memorywhat the nervous system retains. Long-term potentiation (LTP) describeshow synapses become more responsive torepeated stimuli. 85 A conditioned reflex is a type of associative learning.
Concept 34.5 Specific Brain Areas Underlie the Complex Abilities of Humans Associative learning occurs when twounrelated stimuli become linked to aresponse. A conditioned reflex is a type ofassociative learning. Example: Salivary reflex in Pavlovs dog 86 We pay attention to anothers behavior
Concept 34.5 Specific Brain Areas Underlie the Complex Abilities of Humans Complex, or observational learning in humanshas a pattern of three elements: We pay attention to anothers behavior We retain a memory of what we observe We try to copy or use that information 87 Concept 34.5 Specific Brain Areas Underlie the Complex Abilities of Humans
Declarative memory is of people, places,and things that can be recalled anddescribed. Procedural memory is how to perform amotor task and cannot be described. 88 Immediateevents happening now Short-termlasts 10 to 15 minutes
Concept 34.5 Specific Brain Areas Underlie the Complex Abilities of Humans Types of memory: Immediateevents happening now Short-termlasts 10 to 15 minutes Long-termlasts from days to a lifetime Memories can be associated with specificbrain regions and neuronal properties. 89 Memories are transferred from short- to long-term.
Concept 34.5 Specific Brain Areas Underlie the Complex Abilities of Humans Memories are transferred from short- tolong-term. Hippocampal or limbic system damage mayprevent this transfer. Example: H.M. was unable to transfermemories to long-term storage afterremoval of the hippocampus. 90 Sleep research uses electroencephalogram (EEG):
Concept 34.5 Specific Brain Areas Underlie the Complex Abilities of Humans Sleep research uses electroencephalogram(EEG): Measures neuronal activity and recordschanges in electrical potential in entirebrain regions In birds and mammals, there are two mainsleep states: Rapid eye movement (REM) sleep Non-REM sleep 91 Concept 34.5 Specific Brain Areas Underlie the Complex Abilities of Humans
When awake, nuclei in the brainstem areactive and cells depolarize often. Neurons in the thalamus and cortex arenear threshold and sensitive to inputreflects a wakeful state. At sleep onset, activity slows in thebrainstemless neurotransmitter isreleased, cells are less excitable. Information processing slows andconsciousness is lostthe state of non- REM sleep 92 Figure 34.19 Stages of Sleep (Part 1) Cells in non-REM sleep fire in bursts, called slow-wave sleep.
Concept 34.5 Specific Brain Areas Underlie the Complex Abilities of Humans Cells in non-REM sleep fire in bursts, calledslow-wave sleep. During non-REM and REM transition: Brainstem nuclei become active again andfiring bursts cease Cortex can process information as cells atthreshold can depolarize Sensory and motor pathways are stillinhibited; without this feedback the cortexmay produce bizarre dreams. 94 After a period of REM sleep, we return to non-REM sleep.
Concept 34.5 Specific Brain Areas Underlie the Complex Abilities of Humans After a period of REM sleep, we return tonon-REM sleep. Repeated cycles occur in humans and othermammals. Hypotheses for sleep patterns includeimmune function, maintenance of neuralconnections, and for learning and memory. VIDEO 34.2 Brain waves of slow-wave and REM sleep cycles 95 Figure 34.19 Stages of Sleep (Part 2) Concept 34.5 Specific Brain Areas Underlie the Complex Abilities of Humans
Consciousness refers to being aware ofyourself, your environment, and eventsoccurring around you. Conscious experience requires a perceptionof self, using integration of informationfrom the physical and social environment,with information from past experience. 97 Answer to Opening Question
Charles Whitmans brain tumor pressed onthe hypothalamus and parts of the limbicsystem, including the amygdala. When neurons in the amygdala areactivated, intense emotions such as fearand rage may be felt. These strong emotions may have led to hisactions. 98 Figure 34.20 Source of the Fear Response