Voltage V potential energy generated by separated charges

1
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

2
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
Ca21.8 mM
150 mM
3
Electrochemical Gradient

• Electrical current created voltage across the
  membrane changes when channels open
• Ions flow down their chemical gradient
• from high to low
• Ions flow down their electrical gradient
• toward opposite charge
• Electrochemical gradient
• The combined potentials of the electrical and
  chemical gradients taken together

4
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(logKo/Ki)
  0.061Vlog(.005M/.150M) -90mV

5
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

6
Operation of a Ligand Gated Channel
7
Operation of a Voltage-Gated Na Channel
8
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

9
Graded Potentials

• 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

10
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
• Dont decrease in strength over distance

11
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
12
Action Potential Depolarization Phase

• Na permeability increases Vr reverses
• Na gates opened K gates closed
• Threshold critical level of depolarization
  (-55mV)
• At threshold, depolarization becomes
  self-generating

13
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

14
Action Potential Hyperpolarization

• K gates remain open, allowing excessive efflux of
  K
• causes hyperpolarization
• neuron refractory while hyperpolarized

15
Phases of the Action Potential

• 1 resting state
• 2 depolarization phase
• 3 repolarization phase
• 4 hyperpolarization

16
Action Potential Propagation (T 0ms)

• Na influx depolarizes patch of axonal membrane
• Positive ions in axoplasm move toward negative
  region of the membrane

17
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
18
Refractory Periods

• Absolute - from opening to closing of Na
  activation gates
• Relative after closing Na activation gates till
  K gates are closed

19
Threshold and Action Potentials

• Threshold
• 20 mV depolarization
• Graded potentials
• subthreshold stimuli that dont 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
  voltage-gated channels

20
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

21
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

22
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

23
Synapses

• Morphological Types
• Axodendritic axon to dendrite
• Axosomatic axon to soma
• Axoaxonic (axon to axon)

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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

25
Synapse Structure

• Synaptic cleft
• Space between pre- and postsynaptic neurons
• Halts action potential
• Transmission of signal occurs by neurotransmitter

Figure 11.19
26
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

27
Neurotransmitters

• gt50 identified
• Classified chemically and functionally
• Acetylcholine (ACh)
• Biogenic amines
• Amino acids
• Peptides
• Dissolved gases NO and CO

28
Neurotransmitters Acetylcholine

• 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

29
Neurotransmitters Biogenic Amines

• Broadly distributed in the brain
• Behaviors and circadian rythyms
• Catecholamines dopamine, norepinephrine (NE),
  and epinephrine
• Indolamines serotonin and histamine

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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
31
Neurotransmitters Amino Acids

• Found only in CNS
• Include
• GABA Gamma (?)-aminobutyric acid
• Glycine
• Aspartate
• Glutamate

32
Neurotransmitters Peptides

• Tachykinin substance P mediator of pain
  signals
• ?-endorphin, dynorphin, enkephalins natural
  opiates that block pain
• somatostatin cholecystokinin communicate
  between gut and CNS

33
Neurotransmitters Gases

• Nitric oxide (NO)
• Activates the intracellular receptor guanylyl
  cyclase
• Involved in learning and memory
• Vascular smooth muscle

34
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

35
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

36
Termination of Neurotransmitter Effects

• Degradation by enzymes (acetylcholinesterase)
• Absorption by astrocytes or the presynaptic
  terminals
• Diffusion from the synaptic cleft

37
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

38
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

39
Inhibitory Synapses and IPSPs

• Receptor activation increases permeability to K
  and Cl-
• Makes charge on the inner surface more negative
• Reduces postsynaptic neurons ability to produce
  an action potential

40
Summation

• EPSPs summate to induce an action potential
• Summation of IPSPs and EPSPs cancel each other
  out

41
Neural Integration Neuronal Pools

• Functional groups of neurons that
• Integrate incoming information
• Forward the processed information to its
  appropriate destination

42
Types of Circuits in Neuronal Pools

• Divergent one incoming fiber stimulates ever
  increasing number of fibers, often amplifying
  circuits

Figure 11.25a, b
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Types of Circuits in Neuronal Pools

• Convergent resulting in either strong
  stimulation or inhibition

Figure 11.25c, d
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Types of Circuits in Neuronal Pools

• Reverberating chain of neurons containing
  collateral synapses with previous neurons in the
  chain

Figure 11.25e
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Types of Circuits in Neuronal Pools

• Parallel after-discharge incoming neurons
  stimulate several neurons in parallel arrays

Figure 11.25f
46
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

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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