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Biology 220 Anatomy & Physiology I. Unit V ELECTROPHYSIOLOGY. Chapter 11 pp. 396-424. E. Gorski/ E. Lathrop-Davis/ S. Kabrhel. Plasma Membrane. Membrane potential = electrical voltage difference across plasma membrane of cell - PowerPoint PPT Presentation
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Unit VELECTROPHYSIOLOGY
Chapter 11 pp. 396-424
E. Gorski/ E. Lathrop-Davis/ S. Kabrhel
Biology 220Anatomy & Physiology I
Plasma MembraneMembrane potential = electrical voltage difference
across plasma membrane of cell• caused by differences in ion concentrations
maintained by plasma membrane proteinsMembrane structure• phospholipids• integral proteins - form channels
Fig. 11.6, p. 397
Review chapter 3 (pp. 68-80): membrane structure, transport
Membrane ChannelsTwo major classes of channels:1. Leakage channels (non-gated or passive channels)
° always open° more K+ than Na+ channels
- allow influx of Na+, efflux of K+
° found in cell body and dendrites2. Gated channels
° open/close based on environment° found in cell body, dendrites, axon hillock,
unmyelinated axons and myelinated axons (nodes of Ranvier)
Fig. 11.6, p. 397
Types of Gated Channels• chemically gated* -- respond to neurotransmitters, hormones, ions (e.g,. H+, Ca2+)° found in cell bodies and dendrites
• voltage gated* -- respond to change in membrane potential° found in axon hillock, axon
• mechanically gated -- respond to mechanical change (vibration, pressure, stretch; e.g., stretch or touch receptors)
Resting Membrane Potential (RMP)Intracellular environment different from extracellular environment in ionic composition
Inside:more K+, protein (anion)
Outside:more Na+, Cl-
Fig. 11.8, p. 398
Negative inside compared to outside; RMP = -70 mV
Resting Membrane Potential (RMP) • cell membrane with a potential (difference in voltage
across membrane) is polarized• in neuron, at rest:
° inside: more K+, protein (anion)- K+ diffuses out of cell through open K+/ Na+
channels ° outside: more Na+, Cl-
- Na+ diffuses into cell through open K+/ Na+
channels° ion gradients (necessary for passive moment of
ions) maintained by Na+/K+ pump (active transport system)
See also Chapter 3, pp. 81-83
Sodium-Potassium (Na+/K+) Pump
Fig. 3.9, p.76Active transport (requires ATP)
• uses transport protein in membrane (Na+/K+ Pump)
• moves 3 Na+ out of cell; 2 K+ into cell
Diffusion
Diffusion
• sets up and maintains ion gradients necessary for diffusion
Types of PotentialsGraded Potential• magnitude varies with stimulus
° more depolarization with stronger stimulus° decays away from point of stimulus
Action Potential• magnitude stays the same• once started, passes along axon as nerve impulse
Graded Potential• magnitude varies with stimulus --> allows graded
responses• localized• short-lived• membrane may be:
° hyperpolarized (more negative than resting potential; caused by influx of Cl- efflux of K+), or
° depolarized (less negative than resting; caused by influx of Na+)
• at receptor = receptor potential• at synapse = synaptic potential
Depolarization and Hyperpolarization
Fig. 11.9, p. 399
Graded Potential• depolarization starts at area
of stimulus• spreads by ions moving on
either side of membrane (not from outside to inside)
• larger stimulus opens more channels
• if membrane reaches threshold (~ -50 to -55 mV), action potential (AP) will be initiated
Fig. 11.10, p. 400
Action Potential• membrane potential goes from -70 mV to +30 mV
then back to -70 mV (after hyperpolarization)• all-or-none principle:
° either start and pass AP, or don’t° continues once started
• passed through membrane of excitable cells (neurons and muscles)° called nerve impulse when passed through axon
Action Potential (con’t)• long-distance communication• propagation is unidirectional (one direction away
from point of stimulation)• includes depolarization, repolarization and
undershoot (hyperpolarization)° depolarization: -70 mV to +30 mV
- based on influx of Na+
° repolarization: +30 mV to -70 mV- based on efflux of K+
° undershoot (hyperpolarization): -70 mV to -90mV - potassium permeability continues
Events of an Action Potential
Fig. 11.12, p. 402
AP: Depolarizationstimulus
chemically-gated Na+ channels open
Na+ influx
graded depolarization of dendrite or cell body
spreads to axon hillock, if threshold reached voltage-gated Na+ channels open (#2 in figure)
Na+ influx (positive feedback)
timed gates on Na+ channels close, K+ channels open for repolarization
See Fig. 11.12, p. 402
AP: RepolarizationNa+ channels closed, gated K+ channels open
K+ leaves cell taking + charge with it repolarization (#3)
goes past normal resting potential (hyperpolarization) (#4)
gated K+ channels close
Na+/K+ pump returns Na+/K+ levels to resting
See Fig. 11.12, p. 402
AP: Refractory PeriodTime during which
neuron membrane does not respond normally to additional stimuli
• Absolute refractory period: time in which a new AP cannot be started
• Relative refractory period: time in which new AP can only be started by stronger stimulus
Fig. 11.15, p. 405
Comparison of Graded andAction Potentials
Graded Potential Action PotentialCharacteristic
Variable Always the same (all-or-none)Amplitude
Variable (depends on stimulus)
Rapid membrane changeDuration
Chemically- or mechanically-gated
Voltage-gatedChannels
Localized (short-distance) Transmitted along axon as impulse
Propagation
None (allows summation) Absolute (no new APs); Refractory (only with stronger stimulus)
Refractory period
Depolarization or hyperpolarization
Depolarization, followed by repolarization and hyperpolarization
Membrane voltage change
Dendrites, perikaryon Axon hillock, axonLocation
Impulse Conduction• action potential (AP) passed through the axon as
an impulse• “Rapid, transient, self-propagating reversal in
membrane potential” Raven & Wood, 1976
• two types of conduction:° continuous° saltatory
direction is one-way due to absolute refractory period shown in light blue
Continuous Impulse Conduction• involves passage of AP
along entire membrane• occurs in unmyelinated
axons and muscle fibers• depolarization/
repolarization occurs in step-wise manner as Na+ and K+ channels open and close in adjacent parts of membrane
Fig. 11.13, p. 404
Saltatory Conduction• AP passed from one node of Ranvier to the next• occurs in myelinated fibers• saves ATP (Na+/K+ pump only used at nodes)• faster
Fig. 11.16, p. 406
Fig. 11.14, p. 405
Stimulus Intensity• affects number of impulses sent per unit time• does not affect velocity of conduction• also affects number of neurons involved
Factors Influencing Impulse Conduction: Intrinsic Factors
1. Fiber diameter° larger (thicker) is faster (because of lower
resistance)2. Degree of myelination
° myelinated fibers are quicker because of saltatory conduction
° multiple sclerosis – degenerative autoimmune disease in which myelin sheaths of CNS are destroyed by person’s own antibodies
Factors Influencing Impulse Conduction: Intrinsic Factors
3. Three groups of fibers:° A° B° C
• Groups based on:° diameter° degree of myelination
Fiber Types - Group A
• Group A (fastest): ° largest diameter (thickest)° thick myelin sheath° conduction velocities of 15-150 m/s° include somatic motor and some somatic
sensory (from skin, skeletal muscles and joints - touch, pressure, hot/cold, stretch, tension)
Fiber Types - Group B & C• Group B (intermediate):
° intermediate diameter° thin myelin sheath° conduction velocities of 3-15 m/s
• Group C (slowest): ° small diameter° no myelin sheath (continuous conduction)° conduction velocities of 1 m/s, or less
• Group B & C include:° autonomic NS motor fibers to viscera° sensory fibers from viscera° small somatic fibers from skin (pain, some
pressure and light touch receptors)
Factors Influencing Speed of Conduction: Extrinsic Factors
Factors other than axon itself1. temperature – due to general influence of heat on
chemical reactions° warmer goes faster° colder goes slower
2. pH° decreased pH < 7.35 (increased H+)
decreased excitability (depression)° increased pH > 7.45 (decreased H+)
increased excitability
Extrinsic Factors (con’t)3. excessive or prolonged pressure (interrupts blood flow)4. inhibitory chemicals reduce membrane permeability to
Na+ (harder to depolarize) ° alcohol, sedatives, anesthetics
5. excitatory chemicals cause easier depolarization° caffeine, nicotine
6. Ca2+ levels° low Ca2+ - increases excitability° high Ca2+ -decreases excitability
Synapses• junctions between neurons at which information is
passed from one neuron (presynaptic neuron) to another (postsynaptic neuron)
• junction between neuron and effector (muscle or gland) usually called neuroeffector junction (NEJ)° neuromuscular junction (NMJ)
- neuron to muscle° neuroglandular junction (NGJ)
- neuron to gland
Structure of Distal End of Axon• telodendria – terminal branches of axon
° allow axon to contact more than one cell or one cell in several places
• axonal terminals = synaptic end bulbs° contain neurotransmitter (or gap junctions)
• synaptic cleft – space between presynaptic and postsynaptic membranes
See Figure 11.18, p. 409
See Fig. 11. 17, p. 408
Types of SynapsesDefined by:1. location: where signal comes from (e.g., axon) to
where it goes (e.g., dendrite, muscle)2. how signal is transferred
a. based on location:° neuron-neuron° neuron-muscle° neuron-gland
b. based on method of information transfer:° electrical synapses° chemical synapses
Synapse LocationsBased on locations, most common are:• axodentritic = axon (presynaptic) to dendrite
(postsynaptic)• axosomatic = axon (presynaptic) to cell body, or
soma (postsynaptic)• axoaxonic = axon (presynaptic) to axon or axon
hillock (less common than the other 2)Fig. 11.17, p. 408
Electrical Synapses• less common type of synapse• joined by gap junctions• cells said to be “electrically coupled”• very rapid transmission• excitatory only• allow bi-directional flow• importance:
° allow synchronization of neuronal firing (important to stereotypical behavior)
° important during development of nervous system (later, most replaced by chemical synapses)
• also present in visceral smooth muscle, cardiac muscle
• axonal terminal of presynaptic neuron releases neurotransmitter (NT) from synaptic vesicle into synaptic cleft
• postsynaptic membrane (of neuron or effector) contains receptors that recognize NT
• slower than electrical• unidirectional (one way)• inhibitory or excitatory• found at:
° most neuron-neuron synapses
° neuroeffector junctions
Chemical Synapses
Fig. 11.18, p. 409
Events at Chemical Synapse1. impulse within presynaptic neuron reaches axon
terminal, depolarizes membrane voltage-gated Na+ and Ca2+ channels open in presynaptic membrane --> Ca2+ enters cell
2. entrance of Ca2+ into cell signals synaptic vesicles to fuse with axonal plasma membrane for release of NT into synaptic cleft (exocytosis)
Fig. 11.18, p. 409
See A.D.A.M.Nervous System IICD
Events at Chemical Synapse (con’t)
3. NT diffuses across synaptic cleft4. NT binds to its specific receptor on postsynpatic
membrane5. ion channels open in postsynpatic membrane
allowing ion movement
Fig. 11.18, p. 409
See A.D.A.M.Nervous System IICD
Postsynaptic Potentials and Synaptic Integration
• transmission from presynaptic to postsynaptic neuron is excitatory or inhibitory depending on type of NT released° each presynaptic neuron releases either
excitatory NT or inhibitory NT• postsynpatic membranes normally dendrite or cell
body (soma or perikaryon)
Postsynaptic Potentials and Synaptic Integration
• reaction of receptors to NTs is graded, response depends on number of receptors involved (which depends on amount of NT released)° excitatory postsynaptic potentials (EPSPs)° inhibitory postsynaptic potentials (IPSPs)
Excitatory Synapses and EPSPs• binding of NT released by presynaptic membrane to receptor (on
postsynaptic membrane) causes opening of membrane channels that allow both Na+ and K+ to diffuse across postsynaptic membrane
• because more Na+ enters than K+ leaves --> net depolarization
• local graded excitatory postsynaptic potential (EPSP)
• if EPSP is sufficiently large, may spread to axon hillock leading to AP
Excitatory Synapses and EPSPs
Fig. 11.19, p. 410
Inhibitory Synapses and IPSPs
Fig. 11.19, p. 410
• binding of NT released by presynaptic membrane to receptor (on postsynaptic membrane) causes opening of membrane channels that allow K+ to diffuse out of post-synaptic cell, or Cl- to diffuse in, or both
• causes hyperpolarization
Modification of Synaptic EventsTemporal summation
• 1 or more presynaptic neurons fire before 1st EPSP fades
• if summed EPSP is large enough, then get AP
Fig. 11.20, p. 412
Modification of Synaptic EventsSpatial summation• large number of axonal terminals from different
neurons or the same neuron fire at the same time• if EPSP is large enough, then get AP
Fig. 11.20, p. 412
Spatial Summation EPSP and IPSP• IPSP and EPSP have opposite effects• if only IPSPs occur, postsynaptic membrane becomes
hyperpolarized° effects of IPSP may be temporally or spatially
summed• usually IPSPs prevent membrane from becoming as
depolarized as it would with only EPSPs
Fig. 11.20, p. 412
Synaptic Potentiation and Facilitation
• synaptic potentiation: presynaptic axonal terminal that has received repeated (in short period of time) or continuous stimulation contains more intracellular Ca2+ than normal triggers greater release of NT into synaptic cleft --> produces larger EPSP in postsynaptic cell(important in memory and learning processes)
• facilitation: postsynaptic neuron that has been partially depolarized is more likely to undergo AP,
Termination of NT Effects
1. removal from cleft by reuptake into astrocytes or presynaptic membrane (e.g., norepinephrine)
2. degradation of NT by enzymes present in postsynaptic membrane or synaptic cleft
• e.g., acetylcholine [ACh] degraded by the enzyme acetylcholinesterase - [AChE]
3. diffusion away from cleft
Functional Classification ofNeurotransmitters (NTs)
A. Based on effects° excitatory – cause depolarization (glutamate)° inhibitory – cause hyperpolarization (GABA)° effect of some depends on postsynaptic
membrane receptors- ACh and NE have different receptor types –
some that cause excitation and other types that causes inhibition
B. Based on mechanism of action° direct (channel-linked receptors)° indirect (G protein-linked receptors = second
messenger system)
Modes of Action: Direct Action
• open ion channels • immediate and localized
action• action depends on binding
of NT to receptors followed by channel activation, ion influx and membrane potential changes
• excitatory examples: aspartate, acetylcholine (ACh), glutamate, ATP*open Na+/K+, Ca2+
channels leading to depolarization
• inhibitory examples: gamma aminobutyric acid (GABA), glycine *open Cl- or K+ channels
leading to hyperpolarization
Fig. 11.22, p. 418
Modes of Action: Indirect Action• slower, longer-lasting effects• work through second messengers
° binding of NT with receptor activates G protein in membrane which works through cyclic AMP (cAMP = second messenger) to:- regulate ion channels (open or close)- activate kinase enzymes within cytoplasm
(activate proteins in cytoplasm)
Modes of Action: Indirect ActionExamples• Biogenic amines (dopamine, norepinephrine, epinephrine)• Peptides (endorphins, dynorphins, substance P)• ACh (at muscarinic receptors)
Fig. 11.22, p. 418
Structural Classes of Neurotransmitters
• Classified according to chemical structure:° Acetylcholine (ACh)° Biogenic Amines° Amino Acids° Peptides° Novel Messengers
Acetylcholine (ACh)
• first NT to be discovered• excitatory to skeletal muscles• excitatory/inhibitory to viscera• found in CNS and PNS (NMJ with skeletal muscle,
NEJ for parasympathetic nervous system)• formed from acetyl-CoA and choline• degraded by acetylcholinesterase (AChE)• Myasthenia gravis - autoimmune disorder of
skeletal muscle ACh receptors• Alzheimer’s disease - decreased ACh level in brain
that ultimately results in mental deterioration
Biogenic Amines• synthesized from amino acid tyrosine• found in CNS and PNS
° catecholamines - norepinephrine [NE], epinephrine, dopamine, - excitatory or inhibitory
° indolamines - seratonin and histamine- generally inhibitory
• play role in emotional behavior and help regulate biological clock; norepinephrine is main NT of sympathetic division of ANS
• schizophrenia - overproduction of dopamine• Parkinson’s disease - deficient dopamine in basal
ganglia
Amino Acids & Peptides
• Amino Acids° GABA = gamma amino butyric acid (principal
inhibitory NT in brain), ° glycine (generally inhibitory NT, in spinal cord),° glutamate (CNS, excitatory)
• Peptides (Neuropeptides)° strings of amino acids produced in CNS and PNS:
- endorphins and enkephalins (natural opiates)- substance P (mediator of pain signals)
° some also produced by nonneural tissues (e.g., cells of GI tract - somatostatin, cholecystokinin, vasoactive intestinal peptide [VIP])
Novel Messengers Neurotransmitters that don’t fit other categories• NO = nitric oxide
° involved in long-term synaptic potentiation (learning and memory)
° relaxation of intestinal smooth muscles ° responsible for brain damage in stroke patients
• ATP = adenosine triphosphate ° promotes synthesis and uptake of other NTs in CNS
and PNS• CO = carbon monoxide
° enhances neurotransmission in some circuits involved in logic
° regulator of cyclic GMP (second messenger)
Organization of Neurons:Types of Circuits
• Simple series circuit• Converging circuit• Diverging circuit• Reverberating (oscillatory) circuit• Parallel after-discharge circuit
Fig. 11.25, p. 421
Simple Series Circuit
• one presynaptic neuron goes to one postsynaptic neuron; e.g., simple reflex arc
presynaptic
postsynaptic
synapses
Converging Circuits
• several presynaptic axonal terminals go to single postsynaptic neuron (output)
• input from several pathways produces single result
• e.g., voluntary vs sub- conscious breathing; “happy baby”
Fig. 11.24, p. 420
Diverging Circuits • one presynaptic neuron --> several postsynaptic
neurons• e.g., single motor neuron from brain may go to
several motor neurons in spinal cord (thence to several muscle fibers)e.g., single sensory neuron to CNS may be part of reflex but also send info to brain
Fig. 11.24, p. 420
Reverberating (Oscillatory) Circuits • chain of neurons with synapses to neurons earlier
in circuit° sleep-wake cycle° breathing° possibly short-term memory° some motor activities (arm swinging)
Fig. 11.24, p. 420
Parallel After-Discharge Circuit • one presynaptic neuron fires to several
postsynaptic neurons arranged in parallel that eventually result in common output
• many different responses occur simultaneously° may be involved in problem solving
Fig. 11.24, p. 420