Chapter 11: Fundamentals of the Nervous System and Nervous Tissue

Chapter 11: Fundamentals of the Nervous System and Nervous Tissue

Nervous System

• Master controlling and communicating system of the body

• Cells communicate by electrical signals and chemical signals

• Rapid and specific• Usually cause an immediate response• Neurons = nerve cells

Functions

• 3 overlapping functions1. Sensory input – NS uses its millions of sensory

receptors to monitor changes• Sensory input2. Integration – NS process and interprets sensory

input and decides what should be done – integration

3. Motor Output – response by activating effector organs – muscles and glands

Figure 11.1

Sensory input

Motor output

Integration

Divisions

• Central NS – brain and spinal cord

• Dorsal body cavity• Integrating and command

center of NS• Interprets sensory input and

dictates motor responses based on reflexes – current and past experience

Divisions

• Peripheral NS – • Part of NS outside the CNS• Consists of nerves – bundles

of axons – extend brain and spinal cord

• Spinal nerves – carry impulses to and from the brain

• Cranial nerves – impulses to and from the brain

Divisions of PNS

• 2 Functions – • Sensory, or afferent, division – “carrying

towards”– Nerve fibers (axons) and convey impulses to CNS– Somatic afferent fibers – transmit impulses from

skeletal muscle– Visceral afferent fibers – transmit impulses from

visceral organs

Divisions of PNS• Motor, or efferent, division – “carrying away”• Impulses from CNS to effector organs• Activate muscles to contract and glands to secrete• Effect response1. Somatic NS – somatic motor nerve fibers conduct impulses

from CNS to skeletal muscle - voluntary NS

2. Automatic NS (ANS) – visceral motor nerve fibers- Regulate activity of smooth muscle, cardiac muscle and glands- “a law unto itself”- Cannot control pumping of heart or food through digestive tract- 2 functional subdivisions – sympathetic and parasympathetic NS –

work in opposition to each other

Figure 11.2

Central nervous system (CNS)Brain and spinal cordIntegrative and control centers

Peripheral nervous system (PNS)Cranial nerves and spinal nervesCommunication lines between theCNS and the rest of the body

Parasympatheticdivision

Conserves energyPromotes house-keeping functionsduring rest

Motor (efferent) divisionMotor nerve fibersConducts impulses from the CNSto effectors (muscles and glands)

Sensory (afferent) divisionSomatic and visceral sensorynerve fibersConducts impulses fromreceptors to the CNS

Somatic nervoussystem

Somatic motor(voluntary)Conducts impulsesfrom the CNS toskeletal muscles

Sympathetic divisionMobilizes bodysystems during activity

Autonomic nervoussystem (ANS)

Visceral motor(involuntary)Conducts impulsesfrom the CNS tocardiac muscles,smooth muscles,and glands

StructureFunctionSensory (afferent)division of PNS Motor (efferent) division of PNS

Somatic sensoryfiber

Visceral sensory fiber

Motor fiber of somatic nervous system

Skin

Stomach Skeletalmuscle

Heart

BladderParasympathetic motor fiber of ANS

Sympathetic motor fiber of ANS

Histology of Nervous Tissue

• Highly cellular• Less than 20 % of CNS extracellular space• Densely packed and tightly intertwined• 2 principal cells– 1. Supporting cells – neuroglia

• Small cells that surround and wrap neurons– 2. Neurons – excitable nerve cells that transmit

signals

Neuroglia

• “nerve glue”• Glial cells• 6 types – each own unique function• Supportive scaffold for neurons• Produce chemicals that guide young neuron’s

growth• Wrap around and insulate neuronal process to

speed up action potential conduction

Neuroglia in CNS

• Astrocytes• Microglia• Ependymal cells• Oligoderocytes• Most have branching processes (extensions)

and a central cell body• Distinguished by smaller size and darker

staining nucleus

CNS Neuroglia - Astrocytes• “star cells”• Most abundant and most versatile• Radiating processes cling to neurons and synaptic endings• Cover nearby capillaries• Support and branch neurons• Anchor them to nutrient supply• Role in making exchanges – capillaries – neurons• “mopping up” leaked K ions and recapturing released

neurotransmitters• Connected by gap junctions• Signal each other with Ca

Figure 11.3a

(a) Astrocytes are the most abundantCNS neuroglia.

Capillary

Neuron

Astrocyte

CNS Neuroglia - Microglia

• Small oviod cell with long thorny processes• Processes touch neurons – monitor health • When neurons injuries or in trouble – migrate

towards them• Transform into macrophages – phagocytize

foreign debris• Protective role

Figure 11.3b

(b) Microglial cells are defensive cells inthe CNS.

NeuronMicroglialcell

CNS Neuroglia – Epedymal Cells

• “wrapping garment”• Shape – squamous columnar• Many ciliated• Line central cavities of brain and spinal cord• Permeable barrier between cerebral spinal

fluid and tissue fluid of CNS• Cilia – circulated fluid

Figure 11.3c

Brain orspinal cordtissue

Ependymalcells

Fluid-filled cavity

(c) Ependymal cells line cerebrospinalfluid-filled cavities.

CNS Neuroglia – Oligodendrocytes

• Fewer processes• Line up along thicker neuron fibers and wrap

processes around them• Covering sheaths – myelin sheaths

Figure 11.3d

(d) Oligodendrocytes have processes that formmyelin sheaths around CNS nerve fibers.

Nervefibers

Myelin sheathProcess ofoligodendrocyte

Neuroglia in PNS

1. Satellite Cells –surround neuron cell bodies located in PNS

• Thought to have same functions as astrocytes2. Schwann Cells – surrond and form myelin

sheaths in PNS• Function similar to oligodendrocytes• Vital to regeneration of damaged nerves

Figure 11.3d

(d) Oligodendrocytes have processes that formmyelin sheaths around CNS nerve fibers.

Nervefibers

Myelin sheathProcess ofoligodendrocyte

Neurons

• Nerve cell• Structural unit of NS• Highly specialized cells• Conduct messages – nerve impulses• Large complex cells• Cell body and processes• Plasma membrane – electrical signaling • Cell-cell interactions

Neurons

• Special characteristics – 1. extreme longevity – can function optimally over a

lifetime ~100 years2. amitotic – loose ability to divide

• Cannot be replaced if destroyed• Exceptions – olfactory epithelium and hippocampel regions

– stem cells• Cannot survive for more than a few minutes without oxygen

3. High metabolic rate – require continuous and abundant oxygen and glucose

Figure 11.4b

Dendrites(receptive regions)

Cell body(biosynthetic centerand receptive region)

Nucleolus

NucleusNissl bodies

Axon(impulse generatingand conducting region)

Axon hillockNeurilemma

Terminalbranches

Node of RanvierImpulsedirection

Schwann cell(one inter-node)

Axonterminals(secretoryregion)

(b)

Neuron – Cell Body• Spherical nucleus surrounded by cytoplasm• Perikaryon or soma – cell body• Ranges in diameter from 5 to 140 um• Major biosynthetic center of the neuron• Usual organelles• Clustered and free ribosomes and rough ER – most active and

developed in the body• Rough ER – Nissl bodies – chromatophilic substance• Golgi – well developed and form arci or complete circle around

nucleus• Mitochondria - scattered

Figure 11.4b

Dendrites(receptive regions)

Cell body(biosynthetic centerand receptive region)

Nucleolus

NucleusNissl bodies

Axon(impulse generatingand conducting region)

Axon hillockNeurilemma

Terminalbranches

Node of RanvierImpulsedirection

Schwann cell(one inter-node)

Axonterminals(secretoryregion)

(b)

Neuron – Cell Body

• Neurofibrils – bundles of intermediate filaments• Maintain cell shape and integrity• Pigment inclusions – black melanin, red iron containing

pigment, gold brown pigment• Lipofuscin – ageing pigment – accumulates in neurons of

elderly• Most cell bodies in CNS are protected by bones of skull

and vertebral column• Clusters of cell bodies in CNS – nuclei• Clusters of cell bodies in PNS - ganglia

Neurons - Processes

• Arm like• Extend from cell bodies• Bundles of processes – – Tracts in CNS– Nerves in PNS

Neurons - Processes

• 2 types – 1. dendrites - short , tapering diffusely

branching extensions• Main receptive or input regions• SA for receiving signals• Convey incoming messages toward cell body• Usually not AP but short distance signals

called graded potentials

Neurons - Processes2. Axon – single• Arises from axon hillock then narrows to form slender

processes• Some short or absent• others – long – up to 3 to 4 ft• Long axon = nerve fiber• Axon branches – axon collaterals• Branches profusely at its end• 10000 or more terminal branches – telodendria• Knob like distal ends – axon terminals, synaptic knobs, boutons

Neurons - Processes

• Axon Cont• Axon – conducting region• Generates nerve impulses and transmits them away

from cell body along plasma membrane – axolemma• Nerve impulses from axon hillock to axon to axon

terminal – secretory region• Depends on • 1. cell body to renew necessary proteins and membrane

components• 2. efficient transport mechanisms to distribute

Neurons - Processes

• Axon cont• Anterograde movement – movement toward an

axon terminal • Retrograde movement – movement in the

opposite direction

• Viruses and bacteria toxins – damage neural tissues – use retrograde axonal transport to reach cell body – polio, rabies, herpes, tetanus

Neurons – Myelin Sheath and Neurilemma

• Myelin Sheath – whitish, fatty (protein-lipid)• Protects and insulates fibers• Increases the speed of transmission of nerve

impulses• Myelintaed fibers – conduct fast • Unmyelinated fibers – slower• Dendrites always unmyelinated• Formed by Schwann cells – indent to receive an

axon, then wrap around them

Figure 11.5a

(a) Myelination of a nervefiber (axon)

Schwann cellcytoplasmAxon

NeurilemmaMyelin sheath

Schwann cellnucleus

Schwann cellplasma membrane

1

2

3

A Schwann cellenvelopes an axon.

The Schwann cell thenrotates around the axon, wrapping its plasma membrane loosely around it in successive layers.

The Schwann cellcytoplasm is forced from between the membranes. The tight membrane wrappings surrounding the axon form the myelin sheath.

Neurons – Myelin Sheath and Neurilemma

• Neurilemma – exposed part of plasma membrane• Gaps in sheaths – nodes of Ranvier – myelin sheath

gaps occur at regular intervals• ~1 mm apart along axon

• Regions of brain and spinal cord – • White matter – dense collections of myelinated fibers• Gray matter – nerve cell bodies and unmyelinated

fibers

Classification of Neurons

• Structural – grouped according to number of processes extending from cell body

• 3 major groups – multipolar, bipolar, and unipolar

Structural Classification

1. Multipolar – 3 or more processes• 1 axon and the rest dendrites• Most common• 99 % of neurons• Major type in CNS


2. Bipolar Neurons – 2 processes• Axon and dendrite• Extend from opposite sides of the cell• Rare – found in special sense organs• Neurons in retina of eye – olfactory mucosa


3. Unipolar Neurons – single short process• Emerges and divides – T-like proximal and

distal• Distal – peripheral process• Proximal – central process• Pseudounipolar neurons – originate as bipolar

– fuse during development, chiefly in ganglia of PNS

Table 11.1 (1 of 3)

Table 11.1 (2 of 3)

Functional Classification• Groups neurons according to direction in which nerve

impulse travels relative to CNS1. Sensory, or afferent, neurons – transmit impulses from

sensory receptors in skin or internal organs toward the CNS

• Almost always unipolar• Cell bodies – sensory ganglia outside CNS• Distal parts - receptor sites• Peripheral process – very long• Big tow – 1 meter till spinal cord• Receptive endings are naked

Functional Classification

2. Motor, or efferent, neurons – carry impulses away from CNS to effector organs (muscles/glands)

• Multipolar• Cell bodies located in CNS, except for some in

ANS

Functional Classification

3. Interneurons, association neurons – • In between• In neural pathways • Shuttle signals through CNS where integration

occurs• Most confined in CNS• 99 % of neurons in body• Multipolar• Diversity in size and fiber branching patterns

Table 11.1 (3 of 3)

Membrane Potentials

• Neurons – highly irritable or excitable response to stimuli

• Stimulation impulse generated and conducted along length of axon

• Action Potential – nerve impulse always the same regardless of source or type of stimulus

Membrane Potential – Basic Principals

• Human body – electrically neutral same number of + and –

• Areas where 1 type of charge predominates – regions that are + or –

• Opposite charges attract – energy must be used to separate them

• Coming together – liberates energy


• Voltage – measure of potential energy – volts or mV

• Measured between 2 points• Called potential difference or simply potential

between 2 points• Greater difference in charge – higher voltage


• Current – flow of electrical charge from one point to another

• Amount of charge depends on 2 factors – voltage and resistance


• Resistance – hindrance to charge flow provided by substances through which the current must pass

• High resistance – insulators• Low resistance - conductors


• Ohm’s Law – • Current is directly proportional to voltage• Greater voltage (potential difference) the

greater the current

Role of Membrane Ion Channels

• Membrane proteins – ion channels• Channels selective as to type of ion it allows to

pass• K+ ion channel only allow K+

Membrane Ion Channels

• Leakage, or nongated, channels – open part of protein – molecular gate – changes shape to open and close to change channel in response to specific signals – “gated” channels

• Chemically gated, or ligand gated, channels – • Opens when appropriate chemical binds• Voltage gated channels – open and close in

response to changes in membrane potential• Mechanically gated channels – open in response to

physical deformation of receptor

Figure 11.6

(b) Voltage-gated ion channels open and close in responseto changes in membrane voltage.

Na+Na+

Closed Open

Receptor

(a) Chemically (ligand) gated ion channels open when theappropriate neurotransmitter binds to the receptor,allowing (in this case) simultaneous movement of Na+ and K+.

Na+

K+

K+

Na+

Neurotransmitter chemicalattached to receptor

Chemicalbinds

Closed Open

Membranevoltagechanges

Membrane Ion Channels

• Ion gated channels – open – ions diffuse across membrane

• Creating electrical currents and voltage changes

• Voltage (V) = current (I) * resistance (R)

• Ions move along concentration gradients – high low concentration

• Electrical gradients – toward an area of opposite charge• Together make – electrochemical gradient

Resting Membrane Potential

• Potential difference measured with a voltmeter• Membrane ~ 70 mV• Negative on cytoplasmic side (inside) negative

relative to outside• Resting membrane potential• Membrane said to be polarized• Value varies -40mv -90 mV• Generated by differences on ionic makeup

Figure 11.7

Voltmeter

Microelectrodeinside cell

Plasmamembrane

Ground electrodeoutside cell

Neuron

Axon


• Cytosol – lower concentration of Na and higher concentration of K

• Na balanced by Cl• K important role


• Resting membrane – impermeable to amniotic proteins

• Slightly permeable to Na• 75X more permeable to K• K ions diffuse out more easily than Na diffuses in• Cell becomes negative inside• Na – K pump – ejects 3 Na out and 2 k in • Stabilizes membrane

Figure 11.8

Finally, let’s add a pump to compensate for leaking ions.Na+-K+ ATPases (pumps) maintain the concentration gradients, resulting in the resting membrane potential.

Suppose a cell has only K+ channels...K+ loss through abundant leakagechannels establishes a negativemembrane potential.

Now, let’s add some Na+ channels to our cell...Na+ entry through leakage channels reducesthe negative membrane potential slightly.

The permeabilities of Na+ and K+ across the membrane are different.

The concentrations of Na+ and K+ on each side of the membrane are different.

Na+

(140 mM )K+

(5 mM )

K+ leakage channels

Cell interior–90 mV



K+

Na+

Na+-K+ pump

K+

K+K+

K+

Na+

K+

K+K

Na+

K+K+ Na+

K+K+

Outside cell

Inside cell Na+-K+ ATPases (pumps) maintain the concentration gradients of Na+ and K+

across the membrane.

The Na+ concentration is higher outside the cell.

The K+ concentration is higher inside the cell.

K+

(140 mM )Na+

(15 mM )

Membrane Potential - Signal

• Changes in membrane potential – communication signal

• Can be produced by 1. anything that alters ion concentration on 2

sides of the membrane2. anything that changes membrane

permeability to any ion

Membrane Potential - Signal

• Changes in Potential – 2 types of signals1. Graded signals – incoming signal operating

over short distances2. action potentials – long distance signals

Membrane Potential - Changes

• Depolarization – reduction in membrane potential

• Inside less negative than resting potential• -70 to -65 mV• Hyperpolarization – membrane potential

increases• More negative that resting potential• -70 to -75 mV

Figure 11.9a

Depolarizing stimulus

Time (ms)

Insidepositive

Insidenegative

Restingpotential

Depolarization

(a) Depolarization: The membrane potentialmoves toward 0 mV, the inside becoming less negative (more positive).

Figure 11.9b

Hyperpolarizing stimulus

Time (ms)

Restingpotential

Hyper-polarization

(b) Hyperpolarization: The membranepotential increases, the inside becomingmore negative.

Graded Potentials

• Short lived, localized changes in membrane potential that can be either depolarizations or hyperpolarizations

• Changes cause current flows that decrease in magnitude with distance

• “graded” – magnitude varies directly with stimulus strength

• Stronger stimulus – more voltage changes and farther the flow

Graded Potential

• Triggered by a change in neuron’s environment causes gated ion channels to opensensory receptor excited – heat, light, etc. – resulting graded potential – receptor potential or generator potential

Figure 11.10a

Depolarized regionStimulus

Plasmamembrane

(a) Depolarization: A small patch of the membrane (red area) has become depolarized.

Figure 11.10b

(b) Spread of depolarization: The local currents (black arrows) that are created depolarize adjacent membrane areas and allow the wave of depolarization to spread.

Action Potential

• Brief reversal of membrane potential with a total amplitude (change in voltage) of about 100 mV (-70 mV to +30mV)

• Depolarization phase followed by a repolarization phase and often a short period of hyperpolarization

• Few milliseconds in duration• Does not decrease in strength with distance

Action Potential

• Also called a nerve impulse• Transmission identical in skeletal muscle cells

and neurons• Typically generated only in axons

Actionpotential

1 2 3

4

Resting state Depolarization Repolarization

Hyperpolarization

The big picture

1 1

2

3

4

Time (ms)

ThresholdMem

bran

e po

tent

ial (

mV)

Figure 11.11 (1 of 5)

Generation of an Action Potential

1. Resting State – all gated Na and K channels are closed• Only leakage channels are open, maintaining resting

membrane potential• Na – 2 gates – voltage sensitive activation gate and

inactivation gate• Depolarization opens and then inactivates the Na

channels• Both gates must be open for Na to enter• K – single voltage gate closed at resting and opens

slowly


2. Depolarization Phase – Na Channels open• Axon membrane depolarized by local currents,

voltage gates Na channels open and Na rushes into cell

• Influx of Na – positive charge – opens more Na channels

• Depolarization at site reaches threshold• Membrane potential becomes less and less

negative and then overshoots to about +30 mV


3. Repolarizing phase: Na channels are inactivating and K channels open

• Rising phase of the AP self limiting – slow inactivation gates of Na channel begin to close

• Membrane permeability to N declines• Voltage gated K channels open – K rushes out of

cell• Initial negativity of resting neuron is restored -

Repolarization


• 4. Hyperpolarization – some K channels remain open and Na channels reset

• Increased K permeability lasts longer than needed to restore resting state

• Excessive K permeability – after hyperpolarization – undershoot – AP curve dips slightly

• Na channels begin to reset

Actionpotential

Time (ms)

1 1

2

3

4

Na+ permeability

K+ permeability

The AP is caused by permeability changes inthe plasma membrane

Mem

bran

e po

tent

ial (

mV)

Rela

tive

mem

bran

e pe

rmea

bilit

y

Figure 11.11 (2 of 5)

Propagation of an Action Potential

• AP transmitted along the axon’s entire length• AP generated by an influx of Na• Local currents that depolarizes adjacent

membrane areas in forward directions (away from the origin of the nerve impulse) which opens voltage gated channels and triggers an AP

• Region where AP originated has just generated AP – Na gates inactivated, so no AP generated there

• AP propagates away from the point of origin

Propagation of an Action Potential

• All or none phenomenon – must reach threshold values if an axon is to “fire”

• Threshold – membrane potential at which outward current created by K movement is exactly equal to the inward current created by Na movement

• Typically when membrane has been depolarized by 15 to 20 mV

Figure 11.12a

Voltageat 0 ms

Recordingelectrode

(a) Time = 0 ms. Action potential has not yet reached the recording electrode.

Resting potentialPeak of action potentialHyperpolarization

Figure 11.12b

Voltageat 2 ms

(b) Time = 2 ms. Action potential peak is at the recording electrode.

Figure 11.12c

Voltageat 4 ms

(c) Time = 4 ms. Action potential peak is past the recording electrode. Membrane at the recording electrode is still hyperpolarized.

All-or-none Phenomenon

• Either happens completely or doesn't happen at all

Refractory Period

• Absolute Refractory Period - opening of Na channels until Na channels begin to reset to original state

• Ensures each AP is separate• Relative Refractory Period – interval following

absolute refractory period – Na channels return to resting state

• Some K channels open, repolarization is occurring

Figure 11.14

Stimulus

Absolute refractoryperiod

Relative refractoryperiod

Time (ms)

Depolarization(Na+ enters)

Repolarization(K+ leaves)

After-hyperpolarization

Conduction Velocity• Conduction velocities vary widely• Neural pathways – transmit rapidly • Internal organs – transmit slowly• Rate of propagation depends on 2 factors-1. Axon Diameter – axons vary in diameter- Larger the diameter the faster it conducts impulses- Larger – less resistance1. Degree of Myelination – unmyelinated axons channels

immediately adjacent to each other – conduction slow – continuous conduction

- Myelin sheaths – rate AP propagation myelin acts as an insulator- saltatory conduction

Figure 11.15

Size of voltage

Voltage-gatedion channel

StimulusMyelinsheath

Stimulus

Stimulus

Node of Ranvier

Myelin sheath

(a) In a bare plasma membrane (without voltage-gatedchannels), as on a dendrite, voltage decays becausecurrent leaks across the membrane.

(b) In an unmyelinated axon, voltage-gated Na+ and K+

channels regenerate the action potential at each pointalong the axon, so voltage does not decay. Conduction is slow because movements of ions and of the gatesof channel proteins take time and must occur beforevoltage regeneration occurs.

(c) In a myelinated axon, myelin keeps current in axons(voltage doesn’t decay much). APs are generated onlyin the nodes of Ranvier and appear to jump rapidlyfrom node to node.

1 mm

Conduction Velocity

• Nerve fibers classified according to diameter, degree of myelination, and conduction

• Group A fibers – mostly somatic sensory and motor fibers deriving the skin, skeletal muscles, and joints

• Largest diameter • Thick myelin sheaths• Impulse speed – 150 m/s

Conduction Velocity

• Group B fibers – lightly myelinated fibers of intermediate diameter

• Impulse speed – 15 m/s

• Group C fibers – smallest diameter• Unmyelinated• Incapable of saltatory conduction• Impulse speed – 1 m/s

Multiple Sclerosis (MS)• An autoimmune disease that mainly affects young

adults• Symptoms: visual disturbances, weakness, loss of

muscular control, speech disturbances, and urinary incontinence

• Myelin sheaths in the CNS become nonfunctional scleroses

• Shunting and short-circuiting of nerve impulses occurs

• Impulse conduction slows and eventually ceases

Multiple Sclerosis: Treatment

• Some immune system–modifying drugs, including interferons and Copazone:– Hold symptoms at bay– Reduce complications– Reduce disability

Nerve Fiber Classification

• Nerve fibers are classified according to:– Diameter– Degree of myelination– Speed of conduction

Synapse

• To clasp or join• Junction that mediates information transfer

from one neuron to another• Axodendritic Synapses – synapse between axon

endings of one neuron and dendrites of another neuron

• Axosomatic Synapses – between axon endings of one neurons and cell bodies of other neurons

Figure 11.16

Dendrites

Cell body

Axon

Axodendriticsynapses

Axosomaticsynapses

Cell body (soma) ofpostsynaptic neuron

Axon

(b)

Axoaxonic synapses

Axosomaticsynapses

(a)

Synapse

• Presynaptic neuron – neuron conducting impulses toward the synapse

• Postsynaptic neuron – neuron transmitting electrical signal away from the synapse

Electrical Synapse

• Less common variety• Gap junctions found between certain body cell• Contain protein channels – connexons –

initially connect the cytoplasm of adjacent neurons and adjacent neurons – allow ions and molecules to pass directly

• Electrically coupled• Transmission rapid

Chemical Synapses

• Specialized for release and reception of neurotransmitter

• 2 parts –1. axon terminal – contains tiny membrane bound sacs –

synaptic vesicles – contain thousands of neurotransmitters

2. neurotransmitter receptor region on membrane of dendrite or cell body of postsynaptic region

- Always separated by a synaptic cleft – fluid filled space – 30- 50 nm

Information Transfer Across Chemical Signals

1. Action potential arrives at axon terminal2. Voltage gated Ca channels opens and Ca enters axon

terminal3. Ca entry cause neurotransmitter-containing vesicles to

release their contents by exocytosis4. Neurotransmitter diffuses across synaptic cleft and

binds to specific receptors on the postsynaptic membrane

5. Binding of neurotransmitter opens ion channels, resulting in graded potentials

6. Neurotransmitter effects are terminated

Figure 11.17

Action potentialarrives at axon terminal.

Voltage-gated Ca2+

channels open and Ca2+

enters the axon terminal.

Ca2+ entry causesneurotransmitter-containing synapticvesicles to release theircontents by exocytosis.

Chemical synapsestransmit signals fromone neuron to anotherusing neurotransmitters.

Ca2+

Synapticvesicles

Axonterminal

Mitochondrion

Postsynapticneuron

Presynapticneuron

Presynapticneuron

Synapticcleft

Ca2+

Ca2+

Ca2+

Neurotransmitterdiffuses across the synapticcleft and binds to specificreceptors on thepostsynaptic membrane.

Binding of neurotransmitteropens ion channels, resulting ingraded potentials.

Neurotransmitter effects areterminated by reuptake throughtransport proteins, enzymaticdegradation, or diffusion awayfrom the synapse.

Ion movementGraded potential

Reuptake

Enzymaticdegradation

Diffusion awayfrom synapse

Postsynapticneuron

1

2

3

4

5

6

Figure 11.17, step 1



Ca2+

Synapticvesicles

Axonterminal

Mitochondrion

Postsynapticneuron

Presynapticneuron

Presynapticneuron

Synapticcleft

Ca2+

Ca2+

Ca2+

Postsynapticneuron

1



Voltage-gated Ca2+




Ca2+

Synapticvesicles

Axonterminal

Mitochondrion

Postsynapticneuron

Presynapticneuron

Presynapticneuron

Synapticcleft

Ca2+

Ca2+

Ca2+

Postsynapticneuron

1

2



Voltage-gated Ca2+





Ca2+

Synapticvesicles

Axonterminal

Mitochondrion

Postsynapticneuron

Presynapticneuron

Presynapticneuron

Synapticcleft

Ca2+

Ca2+

Ca2+

Postsynapticneuron

1

2

3



Voltage-gated Ca2+





Ca2+

Synapticvesicles

Axonterminal

Mitochondrion

Postsynapticneuron

Presynapticneuron

Presynapticneuron

Synapticcleft

Ca2+

Ca2+

Ca2+


Postsynapticneuron

1

2

3

4




5


Reuptake



Neurotransmitter effects are terminatedby reuptake through transport proteins,enzymatic degradation, or diffusion awayfrom the synapse.

6

Figure 11.17


Voltage-gated Ca2+





Ca2+

Synapticvesicles

Axonterminal

Mitochondrion

Postsynapticneuron

Presynapticneuron

Presynapticneuron

Synapticcleft

Ca2+

Ca2+

Ca2+



Neurotransmitter effects areterminated by reuptake throughtransport proteins, enzymaticdegradation, or diffusion awayfrom the synapse.


Reuptake



Postsynapticneuron

1

2

3

4

5

6

Postsynaptic Potentials and Synaptic Integration

• Excitatory Synapses and EPSPs – • Neurotransmitter binding causes depolarization

on postsynaptic membrane• Single type of chemically gated ion channel opens• Allows Na and K to diffuse together• Na influx greater than K, depolarization occurs• Local graded depolarization• Funtion – trigger AP distally

Figure 11.18a

An EPSP is a localdepolarization of the postsynaptic membranethat brings the neuroncloser to AP threshold. Neurotransmitter binding opens chemically gated ion channels, allowing the simultaneous pas-sage of Na+ and K+.

Time (ms)(a) Excitatory postsynaptic potential (EPSP)

Threshold

Stimulus

Mem

bran

e po

tent

ial (

mV)

Postsynaptic Potentials and Synaptic Integration

• Inhibitory Synapses and IPSPs – • Binding of neurotransmitters reduces

postsynaptic neurons ability to fire• Most induce hyperpolarization by making

membrane more permeable to K or Cl• Larger depolarization currents are required to

induce AP

Figure 11.18b

An IPSP is a localhyperpolarization of the postsynaptic membraneand drives the neuron away from AP threshold. Neurotransmitter binding opens K+ or Cl– channels.

Time (ms)(b) Inhibitory postsynaptic potential (IPSP)

Threshold

Stimulus

Mem

bran

e po

tent

ial (

mV)

Summation

• EPSPs can add together or summate to influence the activity of postsynaptic neuron

• Temporal Summation – one or more presynaptic neurons transmit impulses in rapid fire order and bursts of neurotransmitter are released in quick succession

• Spatial Summation – postsynaptic neurons stimulated at the same time by a large number of terminals from same or different neurons

Figure 11.19a, b

Threshold of axon ofpostsynaptic neuron

Excitatory synapse 1 (E1)

Excitatory synapse 2 (E2)

Inhibitory synapse (I1)

Resting potential

E1 E1 E1 E1

(a) No summation:2 stimuli separated in time cause EPSPs that do notadd together.

(b) Temporal summation:2 excitatory stimuli closein time cause EPSPsthat add together.

Time Time

E1 E1

Figure 11.19c, d

E1 + E2 I1 E1 + I1

(d) Spatial summation ofEPSPs and IPSPs:Changes in membane potential can cancel each other out.

(c) Spatial summation:2 simultaneous stimuli atdifferent locations causeEPSPs that add together.

Time Time

E1

E2 I1

E1

Synaptic Potential

• Repeated or continuous use of synapse enhances presynaptic neuron’s ability to excite the postsynaptic neuron – producing larger than expected postsynaptic potentials

Presynaptic Inhibition

• Release of excitatory neurotransmitter by one neuron is inhibited by the activity of another

Neurotransmitters

• Language of NS• Means by neurons communicate• More than 50 have been identified

Classification by Chemical Structure

• Acetylcholine – ACh – • 1st identified• Released at neuromuscular junctions• Released by all neurons that stimulate skeletal

muscle and some of ANS


• Biogenic Amines – • Catecholamines – dopamines, norepinephrine,

epinephrine• Indolamines – serotonin, histamine• Broadly distributed in brain• Play a role in emotional behavior and help

regulate biological clock


• Amino Acids – • occur in all cells, important in biochemical

reactions• Gamma-aminobutyric acie• Glycine• Aspartate• glutamate


• Peptides – • Neuropeptides – strings of amino acids• Substance P – mediator of pain• Endorphins – natural opaites• Gut-brain peptides – produced by nonneural

body tissues


• Purines – adenosine triphophate (ATP) neurotransmitter in PNS and CNS

• Produces a fast excitatory response • Adenosine – acts outside the cell• Potent inhibiter in brain


• Gases and Lipids – Nitric Oxide and Carbon monoxide

• NO – short lived toxic gas• Variety of functions – including memories• CO – airy messenger

Classification by Function

• Effects: Excitatory vs. Inhibitory• Excitatory – cause depolarization• Inhibitory – cause hyperpolarization

Classification by Function

• Actions: Direct vs. Indirect –• Direct – neurotransmitters that bind to and open

ion channels• Indirect – promote broader, long lasting effects by

acting through intracellular second messengers• Neuromodulator – chemical messenger released

by a neuron that does not directly cause EPSPs or IPSPs but instead effects the strength of synaptic transmission

Neurotransmitter Receptors

• Channel –linked receptors – ligand gated ion channels that direct transmitter action

• G-protein linker receptors – indirect, complex, slow and often prolonged

• Transmembrane protein receptors

Basic Concepts of Neural Integration

• Neural Integration – parts must be fused into a smoothly operating whole

• Neuronal Pool – functional groups of neurons that integrate incoming information received from receptors and forward to other destinations

Figure 11.21

Presynaptic(input) fiber

Facilitated zone Discharge zone Facilitated zone

Circuits • Pattern of synaptic connections in neuronal pools• Diverging Circuits – one incoming fibers triggers a response

in ever-increasing numbers of neurons further along pathway

• Converging Circuits – pool receives inputs from several presynaptic neurons

• Reverberating, or oscillating, circuits – incoming signal travels through a chain of neurons, sent continuously through circuit

• Parallel after discharge circuits – incoming fiber stimulates several neurons arranged in parallel arrays that eventually stimulate a common output

Figure 11.22a

Figure 11.22c, d

Figure 11.22e

Figure 11.22f

Neural Processing

• Serial Processing – whole system works in an all or nothing manner

• One neuron stimulates the next, which stimulates the next, etc

• Reflexes – rapid, automatic responses to stimuli

• Reflex arc – neural pathways of reflexes

Figure 11.23

1

2

3

4

5

Receptor

Sensory neuron

Integration center

Motor neuron

Effector

Stimulus

ResponseSpinal cord (CNS)

Interneuron

Neural Processing

• Parallel Processing – inputs are segregated into many different pathways and information delivered is dealt with simultaneously

• Higher level mental functioning

Developmental Aspects

• NS originates from a dorsal neural tube and neural crest formed from the surface of the ectoderm

• Neural tube becomes CNS• 3 phase differentiation process – 2nd month of

development1. proliferate2. neuroblasts migrate3. axons connect with function targets to

become neurons

Developmental Aspects

• Axon outgrowth and synapse formation guided by other neurons, glial cells, and chemicals

• Neurons that do not make the appropriate synapses die

• 2/3 of neurons formed in embryo undergo programmed cell death before birth

Documents

Chapter 11: Fundamentals of the Nervous System and Nervous Tissue