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
Ca 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
[Hi]
[Lo] + -

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](log[Ko]/[Ki]) = 0.061V[log(.005M/.150M)] = -90mV
[Hi]
[Lo] K+

5. Ion Channels Passive channels Ligand gated 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
Don’t 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

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
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
~ 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 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)

24. 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 >50 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

30. 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 neuron’s 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

43. Types of Circuits in Neuronal Pools
Convergent – resulting in either strong stimulation or inhibition
Figure 11.25c, d

44. Types of Circuits in Neuronal Pools
Reverberating – chain of neurons containing collateral synapses with previous neurons in the chain
Figure 11.25e

45. 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

47. 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