1. FUNCTIONAL ORGANIZATION OF THE NERVOUS SYSTEM
1. Overview: The Central and Peripheral Nervous Systems
The central nervous system
Brain
Spinal cord
The peripheral nervous system
Peripheral nerves
somatic portion

2. The nervous system consists of the central nervous system (CNS) and the peripheral nervous system (PNS).

3. autonomic portion
sympathetic nerves
parasympathelic nerves
Ganglia
2. Cells of the Nervous System
A. Neurons
Structures
Dendrites
Cell body - soma
Axon
Synaptic terminal

5. Classification
By structure
Multipolar
Bipolar
Unipolar
By function
Sensory neuron (afferent neuron)
Motor neuron (efferent neuron)
Interneuron

6. Pseudounipolar sensory neurons appear to have a single process
Pseudounipolar sensory neurons appear to have a single process*However, during development, the axon and dendrite fused, creating a single process called an axon*Bipolar sensory neurons have two relatively equal fibers extending off central cell body

7. Axonal transport
Antigrade transport
Fast: 400mm/day (organelles: synaptic vesicles, mitochondria, etc.)
Slow:1-2mm/day (structural proteins: actin, microtubules, etc.)
Kinesin: a protein motor
Retrograde transport
Dynein

8. Slow axonal transport moves slowly by axoplasmic flow from body out axon*Fast axonal transport uses microtubules as tracks and quickly moves substances back and forth between body and axon
Axonal transport of membranous organelles occurs in two forms: slow axonal transport, and fast axonal transport.

9. B. Glia (Glial cells)
Oligodendroglia (CNS)/Schwann cells (PNS)
Myelination
Nodes of Ranvier
astroglia
Fibrous astrocyte
Protoplasmic astrocyte
Microglia

11. In the CNS, oligodendrocytes form myelin around portions of several interneuron axons.
IAstrocytes contact both neurons and blood vessels, but do not form myelin*They may transfer nutrients between blood vessels and neurons

12. Structural map of the location and function of glial cells.
Map indicates which glial cells are found in PNS, and which in CNS, and functions for each*Note which glial cells produce the myelin sheath in each region, which is important in repair.
Structural map of the location and function of glial cells.

13. 3. The Membrane Potential and the Action Potential
A. Resting membrane potential (RMP)
RMP: -60 TO -90 mV
predominately diffusion potential (largely K+)
Ion channels of nervous membranes
Passive ion channels
found in all areas of the nerve cell
channel specificity
Chemical activated ion channels
located predominantly on dendrites and the soma

14. normally closed
also known as receptors
Voltage-activated ion channels
found in axons and soma
open at certain voltage
responsible for generating and propagating action potential
Electrochemical gradients across the cell membrane
e.g. K+
RMP = Equilibrium potential (E), can be mathematically calculated

15. Nernst equation:
E[ion]= RT/zF x ln([ion]o/[ion]i)
where z is the ion valance and F is the faraday constant
or EK = 58 x log10([K+]o/[ K+]i) at 20° C
e.g. EK = -75mV, ENa = +55 mV
Goldman equation:

16. B. The action potential
A temporary change in the membrane potential
Components of action potential
Resting potential membrane depolarization  threshold  action potention  repolariztion  hyperpolarization
Ion channels and the action potential
Na+ channel:
Resting state
Activation state
Inactivation state

17. K+ channel:
Resting state
Slow activation state
Phases of the action potential and corresponding movements of ions
Refractory periods
Absolute refractory period
Relative refractory period
Propagation of the action potential

18. Membrane potential begins at resting potential, rises to threshold & rapidly depolarizes and then repolarizes (period of hyperpolarization) back to resting*Membrane becomes very permeable to Na+ during depolarization, and to K+ during repolarization
Diagram indicates the membrane potential events and the changes in membrane ion permeability during an action potential.

19. Sodium cannot diffuse into neuron with channel closed
Sodium cannot diffuse into neuron with channel closed*Resting membrane potential is maintained
Depolarizing stimulus arrival opens the activation gate in the channel.
Entry of sodium ions depolarizes the membrane potential
Inactivation gate finally closed*Membrane potential has reached +30 mvolts

20. Once the two gates have reset to their original positions, the absolute refractory period for the neuron is finished
This is a positive feedback loop since depolarization is enhanced as more sodium ions enter cell, allowing more entry*Repolarization as potassium leaves breaks the loop

21. During the relative refractory period, a larger-than-normal stimulus can initiate a new action potential*During this period, the Na+ channel gates are resetting to their resting positions, and the K+ channels are open

22. There are many action potentials taking place along an axon at any point in time
Twelve electrodes have been placed along the axon; the recordings from them are shown below the axon
In the distal parts of the axon, local current flow from the active region causes new sections of the membrane to depolarize
Voltage-gated Na+ channels open and Na+ enters the axon

23. Velocity of the action potential Unmyelinated axon: diameter
Myelinated nerve fibers
Diameter of the axon
Distance between nodes of Ranvier -
saltatory conduction
Action potentials appear to jump from one node of Ranvier to the next when local current flow moves rapidly through the myelinated sections between the nodes*Voltage-gated Na+ channels are scarce between the nodes
Since there are few Na+ channels in the demyelinated regions, the action potentials are not conducted as strongly between the nodes

24. Subthreshold graded potential does not fire action potential when blood K+ conc. is in the normal range (normokalemia).
Above-threshold (suprathreshold) stimulus will fire an action potential when K+ concentration is normal.
Increased blood K+ conc. brings membrane closer to threshold. Now a subthreshold stimulus can trigger action potential.
Decreased blood K+ concentration hyperpolarizes membrane and makes the neuron less likely to fire an action potential.

25. Communication between nerve cells: synaptic transmission
Electrical synapses
Gap junctions
Connexon
Connexin
Communication between adjacent cells
By passing molecules between cells
Found between
axons/soma
axons/dendrites
dendrites/dendrites
soma/soma

26. Provide a rapid communication between cells
Synchronize the activity of many adjoining cells
Chemical synapses
Mediate communication between distant cells by transmitter-receptor interaction
Components of chemical synapses
Presynaptic cell
Postsynaptic cell
Synaptic cleft

27. Action potential invading synaptic terminal  activation of voltage-sensitive Ca2+ channels  Ca2+ in the terminal  release of neurotransmitters  coupling of the transmitter with the receptor  change in postsynaptic potential
The axon terminal contains mitochondria and synaptic vesicles filled with neurotransmitter*The postsynaptic membrane has receptors for neurotransmitter that diffuses across the synaptic cleft

28. Postsynaptic potential
Single channel current
Synaptic current
Unitary postsynaptic potential
Summation of postsynaptic potential
Excitatory postsynaptic potentials (EPSPs)
Inhibitory postsynaptic potentials (IPSPs)
Termination of synaptic transmission
Reuptake
Degradation

29. Acetylcholine (ACh) is made from choline and acetyl CoA in the axon terminal*In the synaptic cleft ACh is rapidly broken down by the enzyme acetylcholinesterase*Choline is transported back into the axon terminal and is used to make more Ach

30. Many neurotransmitters create rapid, short-acting responses by opening ion channels*Some neurotransmitters create slower, longer-lasting responses by activating second messenger systems

31. The enzymes stop neurotransmitter activity by breaking down the molecule

32. Divergent pathways, such as the one shown, can be responsible for the widespread effects of the sympathetic nervous system, whose neurons show wide divergence
Convergent pathways, such as the one shown, allow input from many different source to influence the activity of the postsynaptic cell, by summation

33. A single neuron many have synapses with as many as 10,000 presynaptic neurons, creating an amazing capacity for summation

34. The 3 neurons are excitatory neurons
The 3 neurons are excitatory neurons*Their graded potentials separately are all below threshold*But if their graded potentials arrive at trigger zone together they sum to create a suprathreshold signal

35. The two excitatory potentials are diminished by summation with the inhibitory potential*No action potential is generated since the summed potentials are below threshold.

36. The same two subthreshold potentials must arrive at the trigger zone within a period of time shorter than the total destruction of their released neurotranmitters, so that their summation can create an action potential

37. Note that in this way, one target of the neuron can be selectively inhibited
In postsynaptic inhibition, all targets of the postsynaptic cell will be inhibited equally