Transcript Neural Phys

Electricity Definitions
 Voltage (V) – potential energy generated by separated
charges
 Current (I) – flow of charges between points
 Resistance (R) – hindrance to charge flow
 Insulator –high electrical resistance
 Conductor –low electrical resistance
Biological Currents & Resting Potential (Vr)
 Flow of ions rather than electrons
 Generated by different [Na+], [ K+], [ Cl], [anionic
proteins] and charged phospholipids
 Ion gradients
 Differential permeability to Na+ and K+
 Sodium-potassium pump
5 mM
Ca2+
1.8 mM
150 mM
Electrochemical Gradient
 Electrical current created & voltage across the membrane changes
when channels open
[Hi]
 Ions flow down their chemical gradient
 from high [] to low []
+
 Ions flow down their electrical gradient
[Lo]
+
 toward opposite charge
 Electrochemical gradient
 The combined potentials of the electrical and chemical gradients
taken together
Electrochemical Gradients & Nernst Equation
 Potential established by equilibrium of ion flow
 down concentration gradient balanced by repulsion of charges
 Vr is established when rate of K+ moving out = K+ moving in
 Nernst equation relates chemical equilibrium to electrical potential
 EK = [2.3RT/zF](log[Ko]/[Ki]) = 0.061V[log(.005M/.150M)] = -90mV
[Hi]
-
K+
[Lo]
+
Ion Channels
 Passive channels
 always slightly open
 Ligand gated channels
 opened/closed by a specific ligand
 Voltage-gated channels
 opened/closed by change in membrane polarity
 Mechanically-gated channels
 opened/closed by physical deformation
Operation of a Ligand Gated Channel
Operation of a Voltage-Gated Na+ Channel
Changes in Membrane Potential
 Depolarization – the inside of the membrane becomes
less negative
 Repolarization – the membrane returns to its resting
membrane potential
 Hyperpolarization – the inside of the membrane
becomes more negative than the resting potential
Graded Potentials
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Short-lived, local changes in membrane potential
Intensity decreases with distance
Magnitude varies directly with the strength of stimulus
If sufficiently strong enough can initiate action potentials
Action Potentials (APs)
 A brief reversal of membrane potential with a total
amplitude of ~100 mV
 Only generated by muscle cells & neurons
 Propagated by voltage-gated channels
 Don’t decrease in strength over distance
Action Potential: Resting State
 Na+ & K+ channels closed
 Some leakage of Na+ & K+
 Each Na+ channel has two voltage-regulated gates
 Activation gates – closed in the resting state
 Inactivation gates – open in the resting state
Figure 11.12.1
Action Potential: Depolarization Phase
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Na+ permeability increases; Vr reverses
Na+ gates opened; K+ gates closed
Threshold – critical level of depolarization (-55mV)
At threshold, depolarization becomes self-generating
Action Potential: Repolarization Phase
 Change in polarity closes Na inactivation gates
 As Na gates close, voltage-sensitive K+ gates open
 K+ leaves & Vr is restored
Action Potential: Hyperpolarization
 K gates remain open, allowing excessive efflux of K+
 causes hyperpolarization
 neuron refractory while hyperpolarized
Phases of the Action Potential
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1 – resting state
2 – depolarization phase
3 – repolarization phase
4 – hyperpolarization
Action Potential Propagation (T = 0ms)
 Na+ influx depolarizes patch of axonal membrane
 Positive ions in axoplasm move toward negative region of the
membrane
Action Potential Propagation (Time = 1ms)
 + Extracellular ions diffuse to the area of greatest - charge
 Creates current that depolarizes adjacent membrane in forward
direction
 Impulse propagates away from its point of origin
refractory
refractory
Refractory Periods
 Absolute - from opening to closing of Na+ activation gates
 Relative – after closing Na activation gates till K gates are
closed
Threshold and Action Potentials
 Threshold
 ~ 20 mV depolarization
 Graded potentials
 subthreshold stimuli that don’t transit to AP
 threshold stimuli are relayed into AP
 All-or-none phenomenon –
 AP either happens completely, or not at all
 Graded potentials occur along receptive zones of
neurons due to presence of only ligand-gated channels
 AP begins at axon hillock due to presence of voltagegated channels
Conduction Velocities of Axons
 Conduction velocities vary widely among neurons
 Rate of impulse propagation is determined by:
 Axon diameter – the larger the diameter, the faster the
impulse
 Presence of a myelin sheath – myelination dramatically
increases impulse speed
Saltatory Conduction
 Voltage-gated Na+ channels are located at the nodes of Ranvier
 Action potentials occur at the nodes and jump from one node to
the next because that is only place current can flow through the
axonal membrane
 Much faster than conduction along unmyelinated axons
Synapses
 Junction for information transfer from one neuron to
another neuron or effector cell
 Presynaptic neuron – conducts impulses toward the
synapse
 Postsynaptic neuron – transmits impulses away from the
synapse
Synapses
 Morphological Types
 Axodendritic –axon to dendrite
 Axosomatic –axon to soma
 Axoaxonic (axon to axon)
Conductance Synapses Types
 Chemical :
 release and reception of neurotransmitters
 presynaptic membrane with synaptic vesicles
 postsynaptic membrane with receptors
 Electrical :
 less common
 gap junctions
 important in CNS for:
 Control of mental arousal
 Emotions and memory
 Ion and water homeostasis
Synapse Structure
 Synaptic cleft
 Space between pre- and postsynaptic neurons
 Halts action potential
 Transmission of signal occurs by neurotransmitter
Figure 11.19
Synaptic Events
 APs reach terminal of presynaptic neuron & open Ca2+
channels
 Neurotransmitter released into synaptic cleft
 Neurotransmitter crosses cleft & binds receptors on
postsynaptic membrane
 Postsynaptic membrane permeability changes, causing
an excitatory or inhibitory effect
Neurotransmitters
 >50 identified
 Classified chemically and functionally
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Acetylcholine (ACh)
Biogenic amines
Amino acids
Peptides
Dissolved gases NO and CO
Neurotransmitters: Acetylcholine
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1st neurotransmitter identified
Released at neuromuscular junctions
Synthesized and enclosed in synaptic vesicles
Degraded by enzyme acetylcholinesterase (AChE)
Released by:
 All neurons that stimulate skeletal muscle
 Some neurons in the autonomic nervous system
Neurotransmitters: Biogenic Amines
 Broadly distributed in the brain
 Behaviors and circadian rythyms
 Catecholamines – dopamine, norepinephrine (NE), and
epinephrine
 Indolamines – serotonin and histamine
Synthesis of Catecholamines
 Enzymes present in the
cell determine length of
biosynthetic pathway
 Norepinephrine and
dopamine are synthesized
in axonal terminals
 Epinephrine is released by
the adrenal medulla
Figure 11.22
Neurotransmitters: Amino Acids
 Found only in CNS
 Include:
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GABA – Gamma ()-aminobutyric acid
Glycine
Aspartate
Glutamate
Neurotransmitters: Peptides
 Tachykinin & substance P – mediator of pain signals
 -endorphin, dynorphin, & enkephalins – natural opiates
that block pain
 somatostatin & cholecystokinin – communicate between
gut and CNS
Neurotransmitters: Gases
 Nitric oxide (NO)
 Activates the intracellular receptor guanylyl cyclase
 Involved in learning and memory
 Vascular smooth muscle
Functional Classification of Neurotransmitters
 Excitatory neurotransmitters cause depolarization
(e.g., glutamate)
 Inhibitory neurotransmitters cause hyperpolarization
(e.g., GABA and glycine)
 Some can be either
 Determined by receptor on postsynaptic neuron
 i.e. acetylcholine
 Excitatory at skeletal neuromuscular junctions
 Inhibitory in cardiac muscle
Neurotransmitter Receptor Mechanisms
 Direct:
 Directly activate (open) ion channels
 Promote rapid responses
 Examples: ACh and amino acids
 Indirect:
 Bind receptors and act through second messengers
 Promote long-lasting effects
 Examples: biogenic amines, peptides, and dissolved gases
Termination of Neurotransmitter Effects
 Degradation by enzymes (acetylcholinesterase)
 Absorption by astrocytes or the presynaptic terminals
 Diffusion from the synaptic cleft
Postsynaptic Potentials
 Neurotransmitter receptors mediate changes in
membrane potential according to:
 # of receptors activated  the amount of neurotransmitter
released
 The length of time the receptors are stimulated
 The two types of postsynaptic potentials are:
 EPSP – excitatory postsynaptic potentials
 IPSP – inhibitory postsynaptic potentials
Excitatory Postsynaptic Potentials (EPSPs)
 Graded potentials that
initiate action potentials
 Use only ligand gated
channels
 Na+ and K+ flow in
opposite directions at the
same time
Inhibitory Synapses and IPSPs
 Receptor activation increases
permeability to K+ and Cl Makes charge on the inner
surface more negative
 Reduces postsynaptic
neuron’s ability to produce
an action potential
Summation
 EPSPs summate to induce an action potential
 Summation of IPSPs and EPSPs cancel each other out
Neural Integration: Neuronal Pools
 Functional groups of neurons that:
 Integrate incoming information
 Forward the processed information to its appropriate
destination
Types of Circuits in Neuronal Pools
 Divergent – one incoming fiber stimulates ever
increasing number of fibers, often amplifying circuits
Figure 11.25a, b
Types of Circuits in Neuronal Pools
 Convergent –
resulting in either
strong stimulation
or inhibition
Figure 11.25c, d
Types of Circuits in Neuronal Pools
 Reverberating – chain of neurons containing collateral
synapses with previous neurons in the chain
Figure 11.25e
Types of Circuits in Neuronal Pools
 Parallel after-discharge – incoming neurons stimulate
several neurons in parallel arrays
Figure 11.25f
Patterns of Neural Processing
 Serial Processing
 Input travels along one pathway to a specific destination
 Works in an all-or-none manner
 Example: spinal reflexes
Patterns of Neural Processing
 Parallel Processing
 Input travels along several pathways
 Pathways are integrated in different CNS systems
 One stimulus promotes numerous responses
 Example: a smell may remind one of the odor and
associated experiences