1. Overview of the Nervous System
Lecture 10
Overview of the Nervous System

2. Outline Organization of Nervous System
Constituent Cells of Nervous System
Electrical Signals in Neurons
Source of Resting Membrane Potential
Gated Ion Channels
Qualitative Description of Action Potential
Graded Potential
Action Potential
Refractory Period

3. Outline Organization of Nervous System
Constituent Cells of Nervous System
Electrical Signals in Neurons
Source of Resting Membrane Potential
Gated Ion Channels
Qualitative Description of Action Potential
Graded Potential
Action Potential
Refractory Period

4. Organization of the Nervous System
Central nervous system
Brain and spinal cord
Peripheral nervous system
Afferent neurons
Sensory neurons
Efferent neurons
Send response to effector cells
Somatic motor division
Control skeletal muscle
Autonomic division
Controls smooth and cardiac muscle and exocrine/endocrine
Two components:
Sympathetic
Parasympathetic
Commonly exert antagonistic control over a single target

5. Fig 8.1 – Organization of the nervous system Silverthorn 2nd Ed

6. Outline Organization of Nervous System
Constituent Cells of Nervous System
Electrical Signals in Neurons
Source of Resting Membrane Potential
Gated Ion Channels
Qualitative Description of Action Potential
Graded Potential
Action Potential
Refractory Period

7. Cells of the Nervous System
Neurons
Basic signaling units of nervous system
Consist of:
Cell body
Axons – carry outgoing information
Dendrites – receive incoming signals
Glial Cells
Support cells
Outnumber neurons by 10-50X
Provide physical support for neural tissues
Direct growth of neural tissue during repair and development
Insulate axons creating myelin

8. Fig 8.2 – Model neurons Silverthorn 2nd Ed

9. Fig 8.6 – Formation of myelin Silverthorn 2nd Ed

10. Outline Organization of Nervous System
Constituent Cells of Nervous System
Electrical Signals in Neurons
Source of Resting Membrane Potential
Gated Ion Channels
Qualitative Description of Action Potential
Graded Potential
Action Potential
Refractory Period

11. Resting Membrane Potential
Nernst Equation
GHK Equation

12. Electrical Signals in Neurons
If membrane permeability to ion changes:
Membrane potential changes
To substantially change Vm:
Only small # of ions need to cross membrane
Changes in Vm do not alter ion concentrations inside and outside cell

13. Depolarization At rest: Depolarization:
Membrane potential mostly due to K+
Membrane almost impermeable to Na+
Depolarization:
Cell becomes permeable to Na+
Na+ rushes in, membrane potential drops
Moves towards +60mV of Na+

14. Gated Ion Channels How do cells change their membrane potential?
Open or close existing channels in membrane
Four major types of selective ion channels:
Na+, K+, Ca+, Cl-
Ions channel can be:
Normally open
Normally closed
Mechanically gated – sense pressure
Chemically gated – respond to neurotransmitters
Voltage gated – important in initiation and conduction of electrical signals

15. Outline Organization of Nervous System
Constituent Cells of Nervous System
Electrical Signals in Neurons
Source of Resting Membrane Potential
Gated Ion Channels
Qualitative Description of Action Potential
Graded Potential
Action Potential
Refractory Period

16. Action Potentials When ion channels open:
Ions move in or out depending on electro-chemical gradient
Resulting influx changes membrane potential
Two types of electrical signals:
Graded potentials:
Variable strength signals that travel short distances
Action potentials:
Large uniform depolarizations that travel rapidly over long distances without losing strength

17. Fig 8.7 – Graded potentials decrease in strength as they spread out from the point of origin Silverthorn 2nd Ed

18. Graded Potentials
Amplitude directly proportional to strength of triggering event
Begins on membrane at point where ions enter from ECF
e.g. where neurotransmitter combines with receptors on dendrite to open Na+ channels
Strength depends on how much charge enter cell
Travels until:
It dies out OR
Reaches trigger zone
IF - graded potential reaching trigger zone exceeds threshold, then AP
IF NOT – dies out

19. Fig 8.8 – Subthreshold and suprathreshold graded potentials in a neuron Silverthorn 2nd Ed

20. Action Potentials All action potentials are identical in strength
Do not diminish in strength as they travel

21. How Are APs Generated? Start at resting membrane potential
Graded potential exceeding threshold reaches trigger zone
Voltage gated Na+ channels open suddenly
Sharp depolarization of cell  Cell reaches peak positive voltage
Voltage gated K+ channels open slowly, Na+ channels close

22. How Are APs Generated?
K+ moves out of cell  Reduces membrane potential
K+ continues to leave, hyperpolarizes cell
Voltage gated K+ channels close, some K+ enters through leak channels
Cell returns to resting membrane potential

23. Fig 8.9 – the action potential Silverthorn 2nd Ed

24. Na+ Channel Dynamics Use a two-step process for opening and closing:
Activation gate:
Closed at resting membrane potential
Opens when cell depolarizes, allowing Na+ to enter
Inactivation gate:
Open at resting membrane potential
Closes when cell depolarizes, but has 0.5 ms delay
Both reset when cell repolarizes

25. Fig 8.10 a – Model of the voltage-gated Na+ channel Silverthorn 2nd Ed

26. Fig 8.10 b – Model of the voltage-gated Na+ channel Silverthorn 2nd Ed

27. Fig 8.10 c – Model of the voltage-gated Na+ channel Silverthorn 2nd Ed

28. Fig 8.10 d – Model of the voltage-gated Na+ channel Silverthorn 2nd Ed

29. Fig 8.10 e – Model of the voltage-gated Na+ channel Silverthorn 2nd Ed

30. Fig 8. 11 – Ion movements during the action potential
Fig 8.11 – Ion movements during the action potential Silverthorn 2nd Ed

31. Refractory Period
Double gating of Na+ channel leads to refractory period
Absolute refractory period
Once an AP has begun, for about 1 ms, a 2nd AP can’t be generated
Relative refractory period
After Na+ channel gates have been reset, but before Vm has returned to normal, a STRONG graded potential can start a 2nd AP
Graded potential opens Na+ channels, but Na+ entry is offset by continuing K+ loss through K+ channels that are still open

32. Fig 8.12 Refractory periods Silverthorn 2nd Ed

33. Features of APs Stimulus intensity is coded by AP frequency
Conduction of APs:
Travel from trigger zone to axon terminal
Refractory period  APs travel in only one direction
Speed of conduction:
Depends on neuron diameter
 diameter   speed
Resistance of membrane to current leak
Myelination increases speed of conduction

34. Fig 8.14 – Action potentials along an axon Silverthorn 2nd Ed

35. Fig 8.15 – Conduction of action potentials Silverthorn 2nd Ed

36. Myelination Nodes of Ranvier AP Propagation Saltatory Conduction
Membrane resistance lowest at these points
AP Propagation
Starts at trigger zone
AP flows to 1st Node of Ranvier
Node has high density of voltage gated Na+ channels
Na+ re-entry boosts strength of AP
Saltatory Conduction
“Leapfrogging” of APs

37. Fig 8.17 – Saltatory conduction Silverthorn 2nd Ed

38. Summary Organization of Nervous System
Constituent Cells of Nervous System
Electrical Signals in Neurons
Source of Resting Membrane Potential
Gated Ion Channels
Qualitative Description of Action Potential
Graded Potential
Action Potential
Refractory Period

39. Poem of the Day
I Chop Some Parsley While Listening to Art Blakey’s Version of “Three Blind Mice”
Billy Collins

40. Due Dates
Tuesday, October 5th
HW5