Mosby items and derived items © 2007, 2003 by Mosby, Inc.Slide 1 Chapter 12 Nervous System Cells.

Slide 1Mosby items and derived items © 2007, 2003 by Mosby, Inc.

Chapter 12Chapter 12Nervous System CellsNervous System Cells


IntroductionIntroduction

• The function of the nervous system, along The function of the nervous system, along with the endocrine system, is to communicatewith the endocrine system, is to communicate

• The nervous system is made up of the brain, The nervous system is made up of the brain, the spinal cord, and the nerves (Figure 12-1)the spinal cord, and the nerves (Figure 12-1)


Organization of the Nervous SystemOrganization of the Nervous System

• Organized to detect changes in internal and external Organized to detect changes in internal and external environments, evaluate the information, and initiate an environments, evaluate the information, and initiate an appropriate responseappropriate response

• Subdivided into smaller “systems” by location (Figure 12-2)Subdivided into smaller “systems” by location (Figure 12-2) Central nervous system (CNS)Central nervous system (CNS)

• Structural and functional center of entire nervous systemStructural and functional center of entire nervous system

• Consists of the brain and the spinal cordConsists of the brain and the spinal cord

• Integrates sensory information, evaluates it, and initiates Integrates sensory information, evaluates it, and initiates an outgoing responsean outgoing response

Peripheral nervous system (PNS)Peripheral nervous system (PNS)

• Nerves that lie in “outer regions” of nervous systemNerves that lie in “outer regions” of nervous system

• Cranial nerves—originate from brainCranial nerves—originate from brain

• Spinal nerves—originate from spinal cordSpinal nerves—originate from spinal cord



• Afferent and efferent divisionsAfferent and efferent divisions

Afferent division—consists of all incoming Afferent division—consists of all incoming sensory pathwayssensory pathways

Efferent division—consists of all outgoing Efferent division—consists of all outgoing motor pathwaysmotor pathways



• ““Systems” according to the types of organs Systems” according to the types of organs they innervatethey innervate

Somatic nervous system (SNS)Somatic nervous system (SNS)

• Somatic motor division—carries information to the Somatic motor division—carries information to the somatic effectors (skeletal muscles)somatic effectors (skeletal muscles)

• Somatic sensory division—carries feedback information Somatic sensory division—carries feedback information to somatic integration centers in the CNSto somatic integration centers in the CNS



• ““Systems” (cont.)Systems” (cont.)

Autonomic nervous system (ANS)Autonomic nervous system (ANS)

• Efferent division of ANS—carries information to the autonomic Efferent division of ANS—carries information to the autonomic or visceral effectors (smooth and cardiac muscles and glands)or visceral effectors (smooth and cardiac muscles and glands)

Sympathetic division—prepares the body to deal with immediate Sympathetic division—prepares the body to deal with immediate threats to the internal environment; produces “fight-or-flight” threats to the internal environment; produces “fight-or-flight” responseresponse

Parasympathetic division—coordinates the body’s normal resting Parasympathetic division—coordinates the body’s normal resting activities; sometimes called the “rest-and-repair” divisionactivities; sometimes called the “rest-and-repair” division

• Visceral sensory division—carries feedback information to Visceral sensory division—carries feedback information to autonomic integrating centers in the CNSautonomic integrating centers in the CNS


Cells of the Nervous System Cells of the Nervous System

• GliaGlia Glial cells support the neuronsGlial cells support the neurons Five major types of glia (Figure 12-3):Five major types of glia (Figure 12-3):

• AstrocytesAstrocytes Star-shaped, largest, and most numerous type of gliaStar-shaped, largest, and most numerous type of glia Cell extensions connect to both neurons and capillariesCell extensions connect to both neurons and capillaries Astrocytes transfer nutrients from the blood to the neuronsAstrocytes transfer nutrients from the blood to the neurons Form tight sheaths around brain capillaries, which, with tight Form tight sheaths around brain capillaries, which, with tight

junctions between capillary endothelial cells, constitute the blood-junctions between capillary endothelial cells, constitute the blood-brain barrier (BBB)brain barrier (BBB)

• MicrogliaMicroglia Small, usually stationary, cellsSmall, usually stationary, cells In inflamed brain tissue, they enlarge, move about, and carry on In inflamed brain tissue, they enlarge, move about, and carry on

phagocytosisphagocytosis


Cells of the Nervous SystemCells of the Nervous System

Five major types of glia (cont.):Five major types of glia (cont.):

• Ependymal cellsEpendymal cells

Resemble epithelial cells and form thin sheets that line Resemble epithelial cells and form thin sheets that line fluid-filled cavities in the CNSfluid-filled cavities in the CNS

Some produce fluid; others aid in circulation of fluidSome produce fluid; others aid in circulation of fluid

• OligodendrocytesOligodendrocytes

Smaller than astrocytes with fewer processesSmaller than astrocytes with fewer processes

Hold nerve fibers together and produce the myelin sheathHold nerve fibers together and produce the myelin sheath



Five major types of glia (cont.):Five major types of glia (cont.):

• Schwann cellsSchwann cells

Found only in PNSFound only in PNS

Support nerve fibers and form myelin sheaths Support nerve fibers and form myelin sheaths (Figure 12-4)(Figure 12-4)

Gaps in the myelin sheath are called nodes of RanvierGaps in the myelin sheath are called nodes of Ranvier

Neurilemma is formed by cytoplasm of Schwann cell Neurilemma is formed by cytoplasm of Schwann cell (neurilemmocyte) wrapped around the myelin sheath; (neurilemmocyte) wrapped around the myelin sheath; essential for nerve regrowthessential for nerve regrowth

Satellite cells are Schwann cells that cover and support cell Satellite cells are Schwann cells that cover and support cell bodies in the PNSbodies in the PNS



• NeuronsNeurons

Excitable cells that initiate and conduct impulses that make Excitable cells that initiate and conduct impulses that make possible all nervous system functionspossible all nervous system functions

Components of neurons (Figure 12-5)Components of neurons (Figure 12-5)

• Cell body (perikaryon)Cell body (perikaryon)

Ribosomes, rough endoplasmic reticulum (ER), Golgi apparatusRibosomes, rough endoplasmic reticulum (ER), Golgi apparatus

– Provide protein molecules (neurotransmitters) needed for Provide protein molecules (neurotransmitters) needed for transmission of nerve signals from one neuron to anothertransmission of nerve signals from one neuron to another

– Neurotransmitters are packaged into vesiclesNeurotransmitters are packaged into vesicles

– Provide proteins for maintaining and regenerating nerve fibersProvide proteins for maintaining and regenerating nerve fibers

Mitochondria provide energy (ATP) for neuron; some are Mitochondria provide energy (ATP) for neuron; some are transported to end of axontransported to end of axon


Cells of the Nervous SystemCells of the Nervous System Components of neurons (cont.)Components of neurons (cont.)

• DendritesDendrites Each neuron has one or more dendrites, which branch from Each neuron has one or more dendrites, which branch from

the cell bodythe cell body

Conduct nerve signals to the cell body of the neuronConduct nerve signals to the cell body of the neuron

Distal ends of dendrites of sensory neurons are receptorsDistal ends of dendrites of sensory neurons are receptors

Dendritic spines—small knob-like protrusions on dendrites of some Dendritic spines—small knob-like protrusions on dendrites of some brain neurons; serve as connection points for axons of other neuronsbrain neurons; serve as connection points for axons of other neurons

• AxonAxon A single process extending from the axon hillock, sometimes A single process extending from the axon hillock, sometimes

covered by a fatty layer called a myelin sheath (Figure 12-6)covered by a fatty layer called a myelin sheath (Figure 12-6)

Conducts nerve impulses away from the cell body of the neuronConducts nerve impulses away from the cell body of the neuron

Distal tips of axons are telodendria, each of which terminates in a Distal tips of axons are telodendria, each of which terminates in a synaptic knobsynaptic knob



Components of neurons (cont.)Components of neurons (cont.)

• CytoskeletonCytoskeleton Microtubules and microfilaments, as well as neurofibrils (bundles Microtubules and microfilaments, as well as neurofibrils (bundles

of neurofilaments)of neurofilaments) Allow the rapid transport of small organelles (Figure 12-7)Allow the rapid transport of small organelles (Figure 12-7)

– Vesicles (some containing neurotransmitters), mitochondria Vesicles (some containing neurotransmitters), mitochondria

– Motor molecules shuttle organelles to and from the far ends of a Motor molecules shuttle organelles to and from the far ends of a neuron neuron

• Functional regions of the neuron (Figure 12-8)Functional regions of the neuron (Figure 12-8) Input zone—dendrites and cell bodyInput zone—dendrites and cell body Summation zone—axon hillockSummation zone—axon hillock Conduction zone—axonConduction zone—axon Output zone—telodendria and synaptic knobs of axonOutput zone—telodendria and synaptic knobs of axon



• Classification of neuronsClassification of neurons

Structural classification—classified according to Structural classification—classified according to number of processes extending from cell body number of processes extending from cell body (Figure 12-9)(Figure 12-9)

• Multipolar—one axon and several dendritesMultipolar—one axon and several dendrites

• Bipolar—only one axon and one dendrite; least Bipolar—only one axon and one dendrite; least numerous kind of neuronnumerous kind of neuron

• Unipolar (pseudounipolar)—one process comes off Unipolar (pseudounipolar)—one process comes off neuron cell body but divides almost immediately into two neuron cell body but divides almost immediately into two fibers: central fiber and peripheral fiberfibers: central fiber and peripheral fiber



• Classification of neurons (cont.)Classification of neurons (cont.)

Functional classification (Figure 12-10)Functional classification (Figure 12-10)

• Afferent (sensory) neurons—conduct impulses to spinal Afferent (sensory) neurons—conduct impulses to spinal cord or braincord or brain

• Efferent (motor) neurons—conduct impulses away from Efferent (motor) neurons—conduct impulses away from spinal cord or brain toward muscles or glandular tissuespinal cord or brain toward muscles or glandular tissue

• InterneuronsInterneurons



• Reflex arcReflex arc A signal conduction route to and from the CNS, A signal conduction route to and from the CNS,

with the electrical signal beginning in receptors with the electrical signal beginning in receptors and ending in effectorsand ending in effectors

Three-neuron arc—most common; consists of Three-neuron arc—most common; consists of afferent neurons, interneurons, and efferent afferent neurons, interneurons, and efferent neurons (Figure 12-11)neurons (Figure 12-11)

• Afferent neurons—conduct impulses to the CNS from Afferent neurons—conduct impulses to the CNS from the receptorsthe receptors

• Efferent neurons—conduct impulses from the CNS to Efferent neurons—conduct impulses from the CNS to effectors (muscle or glandular tissue)effectors (muscle or glandular tissue)



• Reflex arc (cont.)Reflex arc (cont.) Two-neuron arc—simplest form; consists of Two-neuron arc—simplest form; consists of

afferent and efferent neuronsafferent and efferent neurons

SynapseSynapse

• Where nerve signals are transmitted from one neuron Where nerve signals are transmitted from one neuron to anotherto another

• Two types: electrical and chemical; chemical synapses Two types: electrical and chemical; chemical synapses are typical in the adultare typical in the adult

• Chemical synapses are located at the junction of the Chemical synapses are located at the junction of the synaptic knob of one neuron and the dendrites or cell synaptic knob of one neuron and the dendrites or cell body of another neuronbody of another neuron


Nerves and TractsNerves and Tracts

• Nerves—bundles of peripheral nerve fibers held Nerves—bundles of peripheral nerve fibers held together by several layers of connective tissue together by several layers of connective tissue (Figure 12-12)(Figure 12-12)

Endoneurium—delicate layer of fibrous connective Endoneurium—delicate layer of fibrous connective tissue surrounding each nerve fibertissue surrounding each nerve fiber

Perineurium—connective tissue holding together Perineurium—connective tissue holding together fascicles (bundles of fibers)fascicles (bundles of fibers)

Epineurium—fibrous coat surrounding numerous Epineurium—fibrous coat surrounding numerous fascicles and blood vessels to form a complete nervefascicles and blood vessels to form a complete nerve

• Tracts—within the CNS, bundles of nerve fibers are Tracts—within the CNS, bundles of nerve fibers are called tracts rather than nervescalled tracts rather than nerves



• White matterWhite matter

PNS—myelinated nervesPNS—myelinated nerves

CNS—myelinated tractsCNS—myelinated tracts

• Gray matterGray matter

Made up of cell bodies and unmyelinated fibersMade up of cell bodies and unmyelinated fibers

CNS—referred to as nucleiCNS—referred to as nuclei

PNS—referred to as gangliaPNS—referred to as ganglia



• Mixed nervesMixed nerves

Contain sensory and motor neuronsContain sensory and motor neurons

Sensory nerves—nerves with predominantly Sensory nerves—nerves with predominantly sensory neuronssensory neurons

Motor nerves—nerves with predominantly Motor nerves—nerves with predominantly motor neuronsmotor neurons


Repair of Nerve FibersRepair of Nerve Fibers

• Mature neurons are incapable of cell division; Mature neurons are incapable of cell division; therefore, damage to nervous tissue can be therefore, damage to nervous tissue can be permanentpermanent

• Neurons have limited capacity to repair Neurons have limited capacity to repair themselvesthemselves

• Nerve fibers can be repaired if the damage is Nerve fibers can be repaired if the damage is not extensive, the cell body and neurilemma not extensive, the cell body and neurilemma are intact, and scarring has not occurredare intact, and scarring has not occurred


Repair of Nerve FibersRepair of Nerve Fibers• Stages of repair of an axon in a peripheral motor neuron (Figure Stages of repair of an axon in a peripheral motor neuron (Figure

12-13)12-13) Following injury, distal portion of axon and myelin sheath degeneratesFollowing injury, distal portion of axon and myelin sheath degenerates Macrophages remove the debrisMacrophages remove the debris Remaining neurilemma and endoneurium form a tunnel from the point of injury Remaining neurilemma and endoneurium form a tunnel from the point of injury

to the effectorto the effector New Schwann cells grow in the tunnel to maintain a path for regrowth of the New Schwann cells grow in the tunnel to maintain a path for regrowth of the

axonaxon Cell body reorganizes its Nissl bodies to provide the needed proteins to extend Cell body reorganizes its Nissl bodies to provide the needed proteins to extend

the remaining healthy portion of the axonthe remaining healthy portion of the axon Axon “sprouts” appearAxon “sprouts” appear When “sprout” reaches tunnel, its growth rate increasesWhen “sprout” reaches tunnel, its growth rate increases The skeletal muscle cell atrophies until the nervous connection is reestablishedThe skeletal muscle cell atrophies until the nervous connection is reestablished

• In CNS, similar repair of damaged nerve fibers is unlikelyIn CNS, similar repair of damaged nerve fibers is unlikely


Nerve ImpulsesNerve Impulses• Membrane potentialsMembrane potentials

All living cells maintain a difference in the concentration of ions All living cells maintain a difference in the concentration of ions across their membranesacross their membranes

Membrane potential—slight excess of positively charged ions on Membrane potential—slight excess of positively charged ions on outside of the membrane and slight deficiency of positively outside of the membrane and slight deficiency of positively charged ions on inside of membrane (Figure 12-14)charged ions on inside of membrane (Figure 12-14)

Difference in electrical charge is called potential because it is a Difference in electrical charge is called potential because it is a type of stored energytype of stored energy

Polarized membrane—a membrane that exhibits a membrane Polarized membrane—a membrane that exhibits a membrane potentialpotential

Magnitude of potential difference between the two sides of a Magnitude of potential difference between the two sides of a polarized membrane is measured in volts (V) or millivolts (mV); polarized membrane is measured in volts (V) or millivolts (mV); the sign of a membrane’s voltage indicates the charge on the the sign of a membrane’s voltage indicates the charge on the inside surface of a polarized membraneinside surface of a polarized membrane


Nerve ImpulsesNerve Impulses• Resting membrane potential (RMP)Resting membrane potential (RMP)

Membrane potential maintained by a nonconducting neuron’s Membrane potential maintained by a nonconducting neuron’s plasma membrane; typically –70 mVplasma membrane; typically –70 mV

The slight excess of positive ions on a membrane’s outer surface The slight excess of positive ions on a membrane’s outer surface is produced by ion transport mechanisms and the membrane’s is produced by ion transport mechanisms and the membrane’s permeability characteristicspermeability characteristics

The membrane’s selective permeability characteristics help The membrane’s selective permeability characteristics help maintain a slight excess of positive ions on the outer surface of the maintain a slight excess of positive ions on the outer surface of the membrane (Figure 12-15)membrane (Figure 12-15)

Sodium-potassium pump (Figure 12-16)Sodium-potassium pump (Figure 12-16)

• Active transport mechanism in plasma membrane that transports Na+ Active transport mechanism in plasma membrane that transports Na+ and K+ in opposite directions and at different ratesand K+ in opposite directions and at different rates

• Maintains an imbalance in the distribution of positive ions, resulting in Maintains an imbalance in the distribution of positive ions, resulting in the inside surface becoming slightly negative with respect to its outer the inside surface becoming slightly negative with respect to its outer surfacesurface


Nerve ImpulsesNerve Impulses• Local potentialsLocal potentials

Local potentials—slight shift away from the resting membrane in Local potentials—slight shift away from the resting membrane in a specific region of the plasma membrane (Figure 12-17)a specific region of the plasma membrane (Figure 12-17)

Excitation—when a stimulus triggers the opening of additional Excitation—when a stimulus triggers the opening of additional Na+ channels, allowing the membrane potential to move toward Na+ channels, allowing the membrane potential to move toward zero (depolarization)zero (depolarization)

Inhibition—when a stimulus triggers the opening of additional K+ Inhibition—when a stimulus triggers the opening of additional K+ channels, increasing the membrane potential (hyperpolarization)channels, increasing the membrane potential (hyperpolarization)

Local potentials are called graded potentials because the Local potentials are called graded potentials because the magnitude of deviation from the resting membrane potential is magnitude of deviation from the resting membrane potential is proportional to the magnitude of the stimulusproportional to the magnitude of the stimulus


Action PotentialAction Potential• Action potential—the membrane potential of a neuron that Action potential—the membrane potential of a neuron that

is conducting an impulse; also known as a nerve impulseis conducting an impulse; also known as a nerve impulse

• Mechanism that produces the action potential Mechanism that produces the action potential (Figures 12-18 and 12-19)(Figures 12-18 and 12-19) An adequate stimulus triggers stimulus-gated NaAn adequate stimulus triggers stimulus-gated Na++ channels to open, allowing Na channels to open, allowing Na++

to diffuse rapidly into the cell, producing a local depolarizationto diffuse rapidly into the cell, producing a local depolarization

As threshold potential is reached, voltage-gated NaAs threshold potential is reached, voltage-gated Na++ channels open and more Na channels open and more Na++ enters the cell, causing further depolarizationenters the cell, causing further depolarization

The action potential is an all-or-none responseThe action potential is an all-or-none response

Voltage-gated NaVoltage-gated Na++ channels stay open for only about 1 millisecond before they channels stay open for only about 1 millisecond before they automatically closeautomatically close

After action potential peaks, membrane begins to move back toward the resting After action potential peaks, membrane begins to move back toward the resting membrane potential when Kmembrane potential when K++ channels open, allowing outward diffusion of K channels open, allowing outward diffusion of K++; ; process is known as repolarizationprocess is known as repolarization

Brief period of hyperpolarization occurs and then the resting membrane potential Brief period of hyperpolarization occurs and then the resting membrane potential is restored by the sodium-potassium pumpsis restored by the sodium-potassium pumps


Action PotentialAction Potential

• Refractory period (Figure 12-20)Refractory period (Figure 12-20) Absolute refractory period—brief period (lasting Absolute refractory period—brief period (lasting

approximately half a millisecond) during which approximately half a millisecond) during which a local area of a neuron’s membrane resists a local area of a neuron’s membrane resists restimulation and will not respond to a stimulus, restimulation and will not respond to a stimulus, no matter how strongno matter how strong

Relative refractory period—time during which the Relative refractory period—time during which the membrane is repolarized and is restoring the membrane is repolarized and is restoring the resting membrane potential; the few milliseconds resting membrane potential; the few milliseconds after the absolute refractory period; membrane will after the absolute refractory period; membrane will respond only to a very strong stimulusrespond only to a very strong stimulus


Action PotentialAction Potential• Conduction of the action potentialConduction of the action potential

At the peak of the action potential, the plasma membrane’s polarity At the peak of the action potential, the plasma membrane’s polarity is now the reverse of the RMPis now the reverse of the RMP

The reversal in polarity causes electrical current to flow between The reversal in polarity causes electrical current to flow between the site of the action potential and the adjacent regions of the site of the action potential and the adjacent regions of membrane and triggers voltage-gated Namembrane and triggers voltage-gated Na++ channels in the next channels in the next segment to open; this next segment exhibits an action potential segment to open; this next segment exhibits an action potential (Figure 12-21)(Figure 12-21)

This cycle continues to repeatThis cycle continues to repeat

The action potential never moves backward, as a consequence of The action potential never moves backward, as a consequence of the refractory periodthe refractory period

In myelinated fibers, action potentials in the membrane only occur In myelinated fibers, action potentials in the membrane only occur at the nodes of Ranvier; this type of impulse conduction is called at the nodes of Ranvier; this type of impulse conduction is called saltatory conduction (Figure 12-22)saltatory conduction (Figure 12-22)

Speed of nerve conduction depends on diameter and on the Speed of nerve conduction depends on diameter and on the presence or absence of a myelin sheathpresence or absence of a myelin sheath


Synaptic TransmissionSynaptic Transmission

• Two types of synapses (junctions) Two types of synapses (junctions) (Figure 12-23):(Figure 12-23):

Electrical synapses occur where cells joined by Electrical synapses occur where cells joined by gap junctions allow an action potential to simply gap junctions allow an action potential to simply continue along postsynaptic membranecontinue along postsynaptic membrane

Chemical synapses occur where presynaptic cells Chemical synapses occur where presynaptic cells release chemical transmitters (neurotransmitters) release chemical transmitters (neurotransmitters) across a tiny gap to the postsynaptic cell, possibly across a tiny gap to the postsynaptic cell, possibly inducing an action potential thereinducing an action potential there


Synaptic TransmissionSynaptic Transmission• Structure of the chemical synapse (Figure 12-21)Structure of the chemical synapse (Figure 12-21)

Synaptic knob—tiny bulge at the end of a terminal branch of Synaptic knob—tiny bulge at the end of a terminal branch of a presynaptic neuron’s axon that contains vesicles housing a presynaptic neuron’s axon that contains vesicles housing neurotransmittersneurotransmitters

Synaptic cleft—space between a synaptic knob and the Synaptic cleft—space between a synaptic knob and the plasma membrane of a postsynaptic neuronplasma membrane of a postsynaptic neuron

Arrangements of synapsesArrangements of synapses

• Axodendritic—axon signals postsynaptic dendrite; commonAxodendritic—axon signals postsynaptic dendrite; common

• Axosomatic—axon signals postsynaptic soma; commonAxosomatic—axon signals postsynaptic soma; common

• Axoaxonic—axon signals postsynaptic axon; may regulate Axoaxonic—axon signals postsynaptic axon; may regulate action potential of postynaptic axonaction potential of postynaptic axon

Plasma membrane of a postsynaptic neuron—has protein Plasma membrane of a postsynaptic neuron—has protein molecules that serve as receptors for the neurotransmittersmolecules that serve as receptors for the neurotransmitters


Synaptic TransmissionSynaptic Transmission• Mechanism of synaptic transmission (Figure 12-25)—Mechanism of synaptic transmission (Figure 12-25)—

sequence of events is the following:sequence of events is the following: Action potential reaches a synaptic knob, causing calcium ions to Action potential reaches a synaptic knob, causing calcium ions to

diffuse into the knob rapidlydiffuse into the knob rapidly Increased calcium concentration triggers the release of Increased calcium concentration triggers the release of

neurotransmitter via exocytosisneurotransmitter via exocytosis Neurotransmitter molecules diffuse across the synaptic cleft and Neurotransmitter molecules diffuse across the synaptic cleft and

bind to receptor molecules, causing ion channels to openbind to receptor molecules, causing ion channels to open Opening of ion channels produces a postsynaptic potential, either Opening of ion channels produces a postsynaptic potential, either

an excitatory postsynaptic potential (EPSP) or an inhibitory an excitatory postsynaptic potential (EPSP) or an inhibitory postsynaptic potential (IPSP)postsynaptic potential (IPSP)

The neurotransmitter’s action is quickly terminated by either The neurotransmitter’s action is quickly terminated by either neurotransmitter molecules being transported back into the synaptic neurotransmitter molecules being transported back into the synaptic knob (reuptake) and/or metabolized into inactive compounds by knob (reuptake) and/or metabolized into inactive compounds by enzymes and/or diffused and taken up by nearby glia enzymes and/or diffused and taken up by nearby glia (Figure 12-26)(Figure 12-26)


Synaptic TransmissionSynaptic Transmission

• Summation (Figure 12-28)Summation (Figure 12-28)

Spatial summation—adding together the effects of Spatial summation—adding together the effects of several knobs being activated simultaneously and several knobs being activated simultaneously and stimulating different locations on the postsynaptic stimulating different locations on the postsynaptic membrane, producing an action potentialmembrane, producing an action potential

Temporal summation—when synaptic knobs Temporal summation—when synaptic knobs stimulate a postsynaptic neuron in rapid stimulate a postsynaptic neuron in rapid succession, their effects can summate over a brief succession, their effects can summate over a brief period of time to produce an action potentialperiod of time to produce an action potential


NeurotransmittersNeurotransmitters• Neurotransmitters—means by which neurons communicate with Neurotransmitters—means by which neurons communicate with

one another; there are more than 30 compounds known to be one another; there are more than 30 compounds known to be neurotransmitters, and dozens of others are suspectedneurotransmitters, and dozens of others are suspected

• Classification of neurotransmitters—commonly classified by Classification of neurotransmitters—commonly classified by these features:these features: Function—function of a neurotransmitter is determined by the postsynaptic Function—function of a neurotransmitter is determined by the postsynaptic

receptor; two major functional classifications are excitatory receptor; two major functional classifications are excitatory neurotransmitters and inhibitory neurotransmitters; can also be classified neurotransmitters and inhibitory neurotransmitters; can also be classified according to whether receptor directly opens a channel or instead uses a according to whether receptor directly opens a channel or instead uses a second messenger mechanism involving G proteins and intracellular second messenger mechanism involving G proteins and intracellular signals (Figures 12-29 and 12-30)signals (Figures 12-29 and 12-30)

Chemical structure—mechanism by which neurotransmitters cause a Chemical structure—mechanism by which neurotransmitters cause a change; there are four main classes; since the functions of specific change; there are four main classes; since the functions of specific neurotransmitters vary by location, they are usually classified according to neurotransmitters vary by location, they are usually classified according to chemical structurechemical structure


NeurotransmittersNeurotransmitters

• Small-molecule neurotransmitters Small-molecule neurotransmitters (Figure 12-31)(Figure 12-31)

AcetylcholineAcetylcholine

• Unique chemical structure; acetate (acetyl coenzyme-A) Unique chemical structure; acetate (acetyl coenzyme-A) with cholinewith choline

• Acetylcholine is deactivated by acetylcholinesterase, with Acetylcholine is deactivated by acetylcholinesterase, with the choline molecules being released and transported the choline molecules being released and transported back to presynaptic neuron to combine with acetateback to presynaptic neuron to combine with acetate

• Present at various locations; sometimes in an excitatory Present at various locations; sometimes in an excitatory role and at other times, an inhibitory onerole and at other times, an inhibitory one



• Small-molecule neurotransmitters (cont.)Small-molecule neurotransmitters (cont.) AminesAmines

• Synthesized from amino acid moleculesSynthesized from amino acid molecules

• Two categories: monoamines and catecholaminesTwo categories: monoamines and catecholamines

• Found in various regions of the brain; affecting learning, Found in various regions of the brain; affecting learning, emotions, motor control, etc.emotions, motor control, etc.

Amino acidsAmino acids

• Believed to be among the most common neurotransmitters of Believed to be among the most common neurotransmitters of the CNSthe CNS

• In the PNS, amino acids are stored in synaptic vesicles and In the PNS, amino acids are stored in synaptic vesicles and used as neurotransmittersused as neurotransmitters



• Small-molecule neurotransmitters (cont.)Small-molecule neurotransmitters (cont.)

Other small transmittersOther small transmitters

• Nitric oxide (NO) derived from an amino acidNitric oxide (NO) derived from an amino acid

• NO from a postsynaptic cell signals the presynaptic NO from a postsynaptic cell signals the presynaptic neuron, providing feedback in a neural pathwayneuron, providing feedback in a neural pathway



• Large-molecule neurotransmitters—neuropeptides (Figure Large-molecule neurotransmitters—neuropeptides (Figure 12-32)12-32)

Peptides made up of 2 or more amino acidsPeptides made up of 2 or more amino acids

May be secreted by themselves or in conjunction with a second or May be secreted by themselves or in conjunction with a second or third neurotransmitter; in this case, neuropeptides act as a third neurotransmitter; in this case, neuropeptides act as a neuromodulator, a “cotransmitter” that regulates the effects of the neuromodulator, a “cotransmitter” that regulates the effects of the neurotransmitter released along with itneurotransmitter released along with it

Neurotrophins (neurotrophic [nerve growth] factors) stimulate Neurotrophins (neurotrophic [nerve growth] factors) stimulate neuron development but also can act as neurotransmitters or neuron development but also can act as neurotransmitters or neuromodulatorsneuromodulators


Cycle of Life: Nervous System CellsCycle of Life: Nervous System Cells

• Nerve tissue developmentNerve tissue development

Begins in ectodermBegins in ectoderm

Occurs most rapidly in womb and in first 2 yearsOccurs most rapidly in womb and in first 2 years

• Nervous cells organize into body networkNervous cells organize into body network


Cycle of Life: Nervous System CellsCycle of Life: Nervous System Cells

• SynapsesSynapses

Form and reform until nervous system is intactForm and reform until nervous system is intact

Formation of new synapses and strengthening or Formation of new synapses and strengthening or elimination of old synapses stimulate learning and elimination of old synapses stimulate learning and memorymemory

• Aging causes degeneration of the nervous Aging causes degeneration of the nervous system, which may lead to senilitysystem, which may lead to senility


The Big Picture: Nervous System The Big Picture: Nervous System and the Whole Body and the Whole Body

• Neurons act as the “wiring” that connects structures Neurons act as the “wiring” that connects structures needed to maintain homeostasisneeded to maintain homeostasis

• Sensory neurons—act as receptors to detect Sensory neurons—act as receptors to detect changes in the internal and external environment; changes in the internal and external environment; relay information to integrator mechanisms in the relay information to integrator mechanisms in the CNSCNS

• Information is processed and a response is relayed to Information is processed and a response is relayed to the appropriate effectors through the motor neuronsthe appropriate effectors through the motor neurons


The Big Picture: Nervous System The Big Picture: Nervous System and the Whole Bodyand the Whole Body

• At the effector, neurotransmitter triggers a At the effector, neurotransmitter triggers a response to restore homeostasisresponse to restore homeostasis

• Neurotransmitters released into the Neurotransmitters released into the bloodstream are called hormonesbloodstream are called hormones

• Neurons are responsible for more than just Neurons are responsible for more than just responding to stimuli; circuits are capable of responding to stimuli; circuits are capable of remembering or learning new responses, remembering or learning new responses, generation of thought, etc.generation of thought, etc.

Mosby items and derived items © 2007, 2003 by Mosby, Inc.Slide 1 Chapter 12 Nervous System Cells.

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