chapter 11: fundamentals of the nervous system and nervous tissue
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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. - PowerPoint PPT PresentationTRANSCRIPT
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
Structural Classification
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
Structural Classification
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
Membrane Potential – Basic Principals
• 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
Membrane Potential – Basic Principals
• Current – flow of electrical charge from one point to another
• Amount of charge depends on 2 factors – voltage and resistance
Membrane Potential – Basic Principals
• Resistance – hindrance to charge flow provided by substances through which the current must pass
• High resistance – insulators• Low resistance - conductors
Membrane Potential – Basic Principals
• 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
Resting Membrane Potential
• Cytosol – lower concentration of Na and higher concentration of K
• Na balanced by Cl• K important role
Resting Membrane Potential
• 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
Cell interior–70 mV
Cell interior–70 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
Generation of an Action Potential
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
Generation of an Action Potential
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
Generation of an Action Potential
• 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
Action potentialarrives at axon terminal.
Chemical synapsestransmit signals fromone neuron to anotherusing neurotransmitters.
Ca2+
Synapticvesicles
Axonterminal
Mitochondrion
Postsynapticneuron
Presynapticneuron
Presynapticneuron
Synapticcleft
Ca2+
Ca2+
Ca2+
Postsynapticneuron
1
Figure 11.17, step 2
Action potentialarrives at axon terminal.
Voltage-gated Ca2+
channels open and Ca2+
enters the axon terminal.
Chemical synapsestransmit signals fromone neuron to anotherusing neurotransmitters.
Ca2+
Synapticvesicles
Axonterminal
Mitochondrion
Postsynapticneuron
Presynapticneuron
Presynapticneuron
Synapticcleft
Ca2+
Ca2+
Ca2+
Postsynapticneuron
1
2
Figure 11.17, step 3
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+
Postsynapticneuron
1
2
3
Figure 11.17, step 4
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.
Postsynapticneuron
1
2
3
4
Figure 11.17, step 5
Ion movementGraded potential
Binding of neurotransmitteropens ion channels, resulting ingraded potentials.
5
Figure 11.17, step 6
Reuptake
Enzymaticdegradation
Diffusion awayfrom synapse
Neurotransmitter effects are terminatedby reuptake through transport proteins,enzymatic degradation, or diffusion awayfrom the synapse.
6
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
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
Classification by Chemical Structure
• Biogenic Amines – • Catecholamines – dopamines, norepinephrine,
epinephrine• Indolamines – serotonin, histamine• Broadly distributed in brain• Play a role in emotional behavior and help
regulate biological clock
Classification by Chemical Structure
• Amino Acids – • occur in all cells, important in biochemical
reactions• Gamma-aminobutyric acie• Glycine• Aspartate• glutamate
Classification by Chemical Structure
• Peptides – • Neuropeptides – strings of amino acids• Substance P – mediator of pain• Endorphins – natural opaites• Gut-brain peptides – produced by nonneural
body tissues
Classification by Chemical Structure
• Purines – adenosine triphophate (ATP) neurotransmitter in PNS and CNS
• Produces a fast excitatory response • Adenosine – acts outside the cell• Potent inhibiter in brain
Classification by Chemical Structure
• 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
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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