nerve fiber classification

Nerve Fiber ClassificationNerve Fiber Classification Nerve fibers are classified according to:Nerve fibers are classified according to:

DiameterDiameter Degree of myelinationDegree of myelination Speed of conductionSpeed of conduction

SynapsesSynapses A junction that mediates information transfer A junction that mediates information transfer

from one neuron:from one neuron: To another neuronTo another neuron To an effector cellTo an effector cell

Presynaptic neuron – conducts impulses Presynaptic neuron – conducts impulses toward the synapsetoward the synapse

Postsynaptic neuron – transmits impulses Postsynaptic neuron – transmits impulses away from the synapseaway from the synapse

SynapsesSynapses

Figure 11.17

Types of SynapsesTypes of Synapses Axodendritic – synapses between the axon of Axodendritic – synapses between the axon of

one neuron and the dendrite of anotherone neuron and the dendrite of another Axosomatic – synapses between the axon of Axosomatic – synapses between the axon of

one neuron and the soma of anotherone neuron and the soma of another Other types of synapses include:Other types of synapses include:

Axoaxonic (axon to axon)Axoaxonic (axon to axon) Dendrodendritic (dendrite to dendrite)Dendrodendritic (dendrite to dendrite) Dendrosomatic (dendrites to soma)Dendrosomatic (dendrites to soma)

Electrical SynapsesElectrical Synapses Electrical synapses:Electrical synapses:

Are less common than chemical synapsesAre less common than chemical synapses Correspond to gap junctions found in other cell typesCorrespond to gap junctions found in other cell types Are important in the CNS in:Are important in the CNS in:

Arousal from sleepArousal from sleep Mental attentionMental attention Emotions and memoryEmotions and memory Ion and water homeostasisIon and water homeostasis

Chemical SynapsesChemical Synapses Specialized for the release and reception of Specialized for the release and reception of

neurotransmittersneurotransmitters Typically composed of two parts: Typically composed of two parts:

Axonal terminal of the presynaptic neuron, which Axonal terminal of the presynaptic neuron, which contains synaptic vesicles contains synaptic vesicles

Receptor region on the dendrite(s) or soma of the Receptor region on the dendrite(s) or soma of the postsynaptic neuronpostsynaptic neuron

Synaptic CleftSynaptic Cleft Fluid-filled space separating the presynaptic Fluid-filled space separating the presynaptic

and postsynaptic neuronsand postsynaptic neurons Prevents nerve impulses from directly passing Prevents nerve impulses from directly passing

from one neuron to the nextfrom one neuron to the next Transmission across the synaptic cleft: Transmission across the synaptic cleft:

Is a chemical event (as opposed to an electrical Is a chemical event (as opposed to an electrical one)one)

Ensures unidirectional communication between Ensures unidirectional communication between neuronsneurons

Synaptic Cleft: Information Synaptic Cleft: Information TransferTransfer

Nerve impulses reach the axonal terminal of the Nerve impulses reach the axonal terminal of the presynaptic neuron and open Capresynaptic neuron and open Ca2+2+ channels channels

Neurotransmitter is released into the synaptic Neurotransmitter is released into the synaptic cleft via exocytosis in response to synaptotagmincleft via exocytosis in response to synaptotagmin

Neurotransmitter crosses the synaptic cleft and Neurotransmitter crosses the synaptic cleft and binds to receptors on the postsynaptic neuronbinds to receptors on the postsynaptic neuron

Postsynaptic membrane permeability changes, Postsynaptic membrane permeability changes, causing an excitatory or inhibitory effectcausing an excitatory or inhibitory effect

Synaptic vesiclescontaining neurotransmitter molecules

Axon of presynapticneuron

Synapticcleft

Ion channel(closed)

Ion channel (open)

Axon terminal of presynaptic neuron

PostsynapticmembraneMitochondrion

Ion channel closed

Ion channel open

Neurotransmitter

Receptor

Postsynapticmembrane

Degradedneurotransmitter

Na+Ca2+

1

2

34

5

Action

potential

Figure 11.18

Synaptic Cleft: Information Synaptic Cleft: Information TransferTransfer

Termination of Neurotransmitter Termination of Neurotransmitter EffectsEffects Neurotransmitter bound to a postsynaptic Neurotransmitter bound to a postsynaptic

neuron: neuron: Produces a continuous postsynaptic effectProduces a continuous postsynaptic effect Blocks reception of additional “messages” Blocks reception of additional “messages” Must be removed from its receptorMust be removed from its receptor

Removal of neurotransmitters occurs when Removal of neurotransmitters occurs when they:they: Are degraded by enzymesAre degraded by enzymes Are reabsorbed by astrocytes or the presynaptic Are reabsorbed by astrocytes or the presynaptic

terminals terminals Diffuse from the synaptic cleftDiffuse from the synaptic cleft

Synaptic DelaySynaptic Delay Neurotransmitter must be released, diffuse Neurotransmitter must be released, diffuse

across the synapse, and bind to receptorsacross the synapse, and bind to receptors Synaptic delay – time needed to do this (0.3-Synaptic delay – time needed to do this (0.3-

5.0 ms) 5.0 ms) Synaptic delay is the rate-limiting step of Synaptic delay is the rate-limiting step of

neural transmissionneural transmission

Postsynaptic PotentialsPostsynaptic Potentials Neurotransmitter receptors mediate changes in Neurotransmitter receptors mediate changes in

membrane potential according to:membrane potential according to: The amount of neurotransmitter releasedThe amount of neurotransmitter released The amount of time the neurotransmitter is bound The amount of time the neurotransmitter is bound

to receptorsto receptors The two types of postsynaptic potentials are: The two types of postsynaptic potentials are:

EPSP – excitatory postsynaptic potentials EPSP – excitatory postsynaptic potentials IPSP – inhibitory postsynaptic potentialsIPSP – inhibitory postsynaptic potentials

Excitatory Postsynaptic Excitatory Postsynaptic PotentialsPotentials

EPSPs are graded potentials that can initiate an EPSPs are graded potentials that can initiate an action potential in an axonaction potential in an axon Use only chemically gated channelsUse only chemically gated channels NaNa++ and K and K++ flow in opposite directions at the same flow in opposite directions at the same

timetime Postsynaptic membranes do not generate Postsynaptic membranes do not generate

action potentialsaction potentials

Excitatory Postsynaptic Potential Excitatory Postsynaptic Potential (EPSP)(EPSP)

Figure 11.19a

Inhibitory Synapses and IPSPsInhibitory Synapses and IPSPs Neurotransmitter binding to a receptor at Neurotransmitter binding to a receptor at

inhibitory synapses: inhibitory synapses: Causes the membrane to become more permeable Causes the membrane to become more permeable

to potassium and chloride ions to potassium and chloride ions Leaves the charge on the inner surface negativeLeaves the charge on the inner surface negative Reduces the postsynaptic neuron’s ability to Reduces the postsynaptic neuron’s ability to

produce an action potentialproduce an action potential

Inhibitory Postsynaptic (IPSP)Inhibitory Postsynaptic (IPSP)

Figure 11.19b

SummationSummation A single EPSP cannot induce an action A single EPSP cannot induce an action

potentialpotential EPSPs must summate temporally or spatially EPSPs must summate temporally or spatially

to induce an action potentialto induce an action potential Temporal summation – presynaptic neurons Temporal summation – presynaptic neurons

transmit impulses in rapid-fire ordertransmit impulses in rapid-fire order

SummationSummation Spatial summation – postsynaptic neuron is Spatial summation – postsynaptic neuron is

stimulated by a large number of terminals at stimulated by a large number of terminals at the same timethe same time

IPSPs can also summate with EPSPs, IPSPs can also summate with EPSPs, canceling each other outcanceling each other out

SummationSummation

Figure 11.20

NeurotransmittersNeurotransmitters Chemicals used for neuronal communication Chemicals used for neuronal communication

with the body and the brainwith the body and the brain 50 different neurotransmitters have been 50 different neurotransmitters have been

identifiedidentified Classified chemically and functionallyClassified chemically and functionally

Chemical NeurotransmittersChemical Neurotransmitters Acetylcholine (ACh)Acetylcholine (ACh) Biogenic aminesBiogenic amines Amino acidsAmino acids PeptidesPeptides Novel messengers: ATP and dissolved gases Novel messengers: ATP and dissolved gases

NO and CONO and CO

Neurotransmitters: AcetylcholineNeurotransmitters: Acetylcholine First neurotransmitter identified, and best First neurotransmitter identified, and best

understoodunderstood Released at the neuromuscular junctionReleased at the neuromuscular junction Synthesized and enclosed in synaptic vesiclesSynthesized and enclosed in synaptic vesicles

Neurotransmitters: AcetylcholineNeurotransmitters: Acetylcholine Degraded by the enzyme acetylcholinesterase Degraded by the enzyme acetylcholinesterase

(AChE)(AChE) Released by:Released by:

All neurons that stimulate skeletal muscleAll neurons that stimulate skeletal muscle Some neurons in the autonomic nervous systemSome neurons in the autonomic nervous system

Neurotransmitters: Biogenic Neurotransmitters: Biogenic AminesAmines

Include:Include: Catecholamines – dopamine, norepinephrine (NE), Catecholamines – dopamine, norepinephrine (NE),

and epinephrineand epinephrine Indolamines – serotonin and histamineIndolamines – serotonin and histamine

Broadly distributed in the brainBroadly distributed in the brain Play roles in emotional behaviors and our Play roles in emotional behaviors and our

biological clockbiological clock

Synthesis of CatecholaminesSynthesis of Catecholamines

Enzymes present in the Enzymes present in the cell determine length of cell determine length of biosynthetic pathwaybiosynthetic pathway

Norepinephrine and Norepinephrine and dopamine are synthesized dopamine are synthesized in axonal terminalsin axonal terminals

Epinephrine is released Epinephrine is released by the adrenal medullaby the adrenal medulla

Figure 11.21

Neurotransmitters: Amino AcidsNeurotransmitters: Amino Acids Include:Include:

GABA – Gamma (GABA – Gamma ()-aminobutyric acid )-aminobutyric acid GlycineGlycine AspartateAspartate GlutamateGlutamate

Found only in the CNSFound only in the CNS

Neurotransmitters: PeptidesNeurotransmitters: Peptides Include:Include:

Substance P – mediator of pain signalsSubstance P – mediator of pain signals Beta endorphin, dynorphin, and enkephalinsBeta endorphin, dynorphin, and enkephalins

Act as natural opiates; reduce pain perceptionAct as natural opiates; reduce pain perception Bind to the same receptors as opiates and Bind to the same receptors as opiates and

morphinemorphine Gut-brain peptides – somatostatin, and Gut-brain peptides – somatostatin, and

cholecystokinincholecystokinin

Neurotransmitters: Novel Neurotransmitters: Novel MessengersMessengers

ATPATP Is found in both the CNS and PNSIs found in both the CNS and PNS Produces excitatory or inhibitory responses Produces excitatory or inhibitory responses

depending on receptor typedepending on receptor type Induces CaInduces Ca2+2+ wave propagation in astrocytes wave propagation in astrocytes Provokes pain sensationProvokes pain sensation

Neurotransmitters: Novel Neurotransmitters: Novel MessengersMessengers

Nitric oxide (NO) Nitric oxide (NO) Activates the intracellular receptor guanylyl Activates the intracellular receptor guanylyl

cyclasecyclase Is involved in learning and memoryIs involved in learning and memory

Carbon monoxide (CO) is a main regulator of Carbon monoxide (CO) is a main regulator of cGMP in the braincGMP in the brain

Functional Classification of Functional Classification of NeurotransmittersNeurotransmitters

Two classifications: excitatory and inhibitoryTwo classifications: excitatory and inhibitory Excitatory neurotransmitters cause depolarizations Excitatory neurotransmitters cause depolarizations

(e.g., glutamate)(e.g., glutamate) Inhibitory neurotransmitters cause Inhibitory neurotransmitters cause

hyperpolarizations (e.g., GABA and glycine)hyperpolarizations (e.g., GABA and glycine)

Functional Classification of Functional Classification of NeurotransmittersNeurotransmitters

Some neurotransmitters have both excitatory Some neurotransmitters have both excitatory and inhibitory effects and inhibitory effects Determined by the receptor type of the Determined by the receptor type of the

postsynaptic neuron postsynaptic neuron Example: acetylcholineExample: acetylcholine

Excitatory at neuromuscular junctions with skeletal Excitatory at neuromuscular junctions with skeletal musclemuscle

Inhibitory in cardiac muscleInhibitory in cardiac muscle

Neurotransmitter Receptor Neurotransmitter Receptor MechanismsMechanisms

Direct: neurotransmitters that open ion channelsDirect: neurotransmitters that open ion channels Promote rapid responses Promote rapid responses Examples: ACh and amino acidsExamples: ACh and amino acids

Indirect: neurotransmitters that act through Indirect: neurotransmitters that act through second messengerssecond messengers Promote long-lasting effectsPromote long-lasting effects Examples: biogenic amines, peptides, and dissolved Examples: biogenic amines, peptides, and dissolved

gasesgases

Channel-Linked ReceptorsChannel-Linked Receptors Composed of integral membrane proteinComposed of integral membrane protein Mediate direct neurotransmitter action Mediate direct neurotransmitter action Action is immediate, brief, simple, and highly Action is immediate, brief, simple, and highly

localizedlocalized Ligand binds the receptor, and ions enter the Ligand binds the receptor, and ions enter the

cellscells Excitatory receptors depolarize membranesExcitatory receptors depolarize membranes Inhibitory receptors hyperpolarize membranesInhibitory receptors hyperpolarize membranes

Channel-Linked ReceptorsChannel-Linked Receptors

Figure 11.22a

G Protein-Linked ReceptorsG Protein-Linked Receptors Responses are indirect, slow, complex, Responses are indirect, slow, complex,

prolonged, and often diffuseprolonged, and often diffuse These receptors are transmembrane protein These receptors are transmembrane protein

complexes complexes Examples: muscarinic ACh receptors, Examples: muscarinic ACh receptors,

neuropeptides, and those that bind biogenic neuropeptides, and those that bind biogenic aminesamines

G Protein-Linked Receptors: G Protein-Linked Receptors: MechanismMechanism

Neurotransmitter binds to G protein-linked Neurotransmitter binds to G protein-linked receptorreceptor

G protein is activated and GTP is hydrolyzed G protein is activated and GTP is hydrolyzed to GDPto GDP

The activated G protein complex activates The activated G protein complex activates adenylate cyclase adenylate cyclase

G Protein-Linked Receptors: G Protein-Linked Receptors: MechanismMechanism

Adenylate cyclase catalyzes the formation of Adenylate cyclase catalyzes the formation of cAMP from ATPcAMP from ATP

cAMP, a second messenger, brings about cAMP, a second messenger, brings about various cellular responsesvarious cellular responses

(a)

(b)

Enzymeactivation

GTPGDP

Proteinsynthesis

Changes inmembranepermeabilityand potential

cAMP

PPi

ATP

Blocked ion flow

Channel closed Channel open

Ions flow

ReceptorG protein

Ion channelAdenylatecyclase

Nucleus

Neurotransmitter (ligand)released from axon terminalof presynaptic neuron

Activation ofspecific genes

GTP

GTP

1

2

3

34

5

5

Figure 11.22b

Neurotransmitter Receptor MechanismNeurotransmitter Receptor Mechanism

G Protein-Linked Receptors: G Protein-Linked Receptors: EffectsEffects

G protein-linked receptors activate G protein-linked receptors activate intracellular second messengers including intracellular second messengers including CaCa2+2+, cGMP, diacylglycerol, as well as cAMP, cGMP, diacylglycerol, as well as cAMP

Second messengers:Second messengers: Open or close ion channelsOpen or close ion channels Activate kinase enzymesActivate kinase enzymes Phosphorylate channel proteins Phosphorylate channel proteins Activate genes and induce protein synthesisActivate genes and induce protein synthesis

Neural Integration: Neuronal Neural Integration: Neuronal PoolsPools

Functional groups of neurons that:Functional groups of neurons that: Integrate incoming informationIntegrate incoming information Forward the processed information to its Forward the processed information to its

appropriate destinationappropriate destination

Neural Integration: Neuronal Neural Integration: Neuronal PoolsPools

Simple neuronal poolSimple neuronal pool Input fiber – presynaptic fiberInput fiber – presynaptic fiber Discharge zone – neurons most closely associated Discharge zone – neurons most closely associated

with the incoming fiberwith the incoming fiber Facilitated zone – neurons farther away from Facilitated zone – neurons farther away from

incoming fiberincoming fiber

Simple Neuronal PoolSimple Neuronal Pool

Figure 11.23

Types of Circuits in Neuronal Types of Circuits in Neuronal Pools Pools

Divergent – one incoming fiber stimulates ever Divergent – one incoming fiber stimulates ever increasing number of fibers, often amplifying increasing number of fibers, often amplifying circuitscircuits

Figure 11.24a, b

Types of Circuits in Neuronal Types of Circuits in Neuronal Pools Pools

Convergent – opposite Convergent – opposite of divergent circuits, of divergent circuits, resulting in either resulting in either strong stimulation or strong stimulation or inhibition inhibition

Figure 11.24c, d

Types of Circuits in Neuronal Types of Circuits in Neuronal PoolsPools

Reverberating – chain of neurons containing Reverberating – chain of neurons containing collateral synapses with previous neurons in collateral synapses with previous neurons in the chainthe chain

Figure 11.24e

Types of Circuits in Neuronal Types of Circuits in Neuronal Pools Pools

Parallel after-discharge – incoming neurons Parallel after-discharge – incoming neurons stimulate several neurons in parallel arraysstimulate several neurons in parallel arrays

Figure 11.24f

Patterns of Neural ProcessingPatterns of Neural Processing Serial ProcessingSerial Processing

Input travels along one pathway to a specific Input travels along one pathway to a specific destinationdestination

Works in an all-or-none mannerWorks in an all-or-none manner Example: spinal reflexesExample: spinal reflexes

Patterns of Neural ProcessingPatterns of Neural Processing Parallel ProcessingParallel Processing

Input travels along several pathwaysInput travels along several pathways Pathways are integrated in different CNS systemsPathways are integrated in different CNS systems One stimulus promotes numerous responsesOne stimulus promotes numerous responses

Example: a smell may remind one of the odor Example: a smell may remind one of the odor and associated experiencesand associated experiences

Development of NeuronsDevelopment of Neurons The nervous system originates from the neural The nervous system originates from the neural

tube and neural cresttube and neural crest The neural tube becomes the CNSThe neural tube becomes the CNS There is a three-phase process of differentiation:There is a three-phase process of differentiation:

Proliferation of cells needed for developmentProliferation of cells needed for development Migration – cells become amitotic and move Migration – cells become amitotic and move

externallyexternally Differentiation into neuroblastsDifferentiation into neuroblasts

Axonal GrowthAxonal Growth Guided by:Guided by:

Scaffold laid down by older neuronsScaffold laid down by older neurons Orienting glial fibersOrienting glial fibers Release of nerve growth factor by astrocytesRelease of nerve growth factor by astrocytes Neurotropins released by other neuronsNeurotropins released by other neurons Repulsion guiding moleculesRepulsion guiding molecules Attractants released by target cellsAttractants released by target cells

N-CAMsN-CAMs N-CAM – nerve cell adhesion molecule N-CAM – nerve cell adhesion molecule Important in establishing neural pathwaysImportant in establishing neural pathways Without N-CAM, neural function is impairedWithout N-CAM, neural function is impaired Found in the membrane of the growth coneFound in the membrane of the growth cone