1. NERVOUS TISSUE

2. Histology 1- Neuron 2- Neuroglia: a- Astrocytes b- microglia
c- Schwann cell
d- Oligodendrocytes
e- Ependymal cells

3. 1- Neuron

4. Neuron:

5. Neurons: Roles generate impulses Integrate information (impulses)
Transmit impulses

6. Neurons

7. Neurons: close up

8. Neuron classification

9. Neuroglia 90% of nervous tissue cells
Do not have transmission properties
Provide structural support and protect the integrity of the nervous tissue
5 types of cells:
- Astrocytes
- Microglia
- Ependyma cells
- Oligodendrocytes
- Schwann cells

10. Astrocytes Provide physical support for neurons
Control movements of nutrients and wastes to and from neurons
Help recycle and process some neurotransmitters
Play a role in the Blood-Brain Barrier (BBB)
Play a role in synapse formation
Maintain constant brain ECF

11. Blood-brain barrier Protects the brain from “external” influences
Inability of some compounds to cross from the blood into the brain
Due to tight junctions between endocytes of he capillary wall
Maybe due to astrocytes
Glucose, gases can pass
Some medications and other compounds can not pass

12. Microglia Derived from macrophages Defense of the brain
Clean up old, sick and dead cells

13. Ependymal cells
Line the ventricles of the brain and form, with the blood vessels, the choroid plexuses
Secrete the cerebrospinal fluid (CSF)

14. Schwann cells Found in the peripheral nervous system (PNS) only
Form myelinated sheath

15. Myelin sheath and Schwann cell

16. Oligodendrocytes Present in the white matter of the CNS only
One cell sends cytoplasmic extensions containing myelin toward several neurons
Each extension wraps around a portion of the neuronal axon

17. Neuron physiology 1- Resting membrane potential 2- Action potential
3- Graded potential
- Excitatory potential
- Inhibitory potential

18. Neuron physiology 1- Resting membrane potential 2- Action potential
3- Graded potential
- Excitatory potential
- Inhibitory potential

19. Resting membrane potential
Due to:
a large number of sodium ions located outside of the cell
A large number of potassium ions located inside the cell
Large amount of proteins inside the cell gives it a negative charge while the outside is positively charged
The difference between the 2 sides of the cell membrane is -70 mV

20. Resting membrane potential

21. Resting potential
Na+ and K+ are constantly leaking across the cell membrane
The Na+/K+ pump re-establishes the membrane potential

22. Neuron physiology 1- Resting membrane potential 2- Action potential
3- Graded potential
- Excitatory potential
- Inhibitory potential

23. Action Potential (AP)
Wave of reversal of charges sweeping along the axonal membrane

24. Action Potential Initiated at the hillock of the neuron
The hillock “sums up” the information coming from the dendrites and neuronal body.
“Summing up” for a neuron means adding up all excitatory and inhibitory impulses. If the membrane potential (the sum) reaches threshold (-55 mV) or above), an AP is triggered

25. Action Potential
The difference in potential activates voltage gated channels  channels that open and close as a function of the current, regional membrane potential

26. Action Potential

27. Action Potential: depolarizing phase

29. Action Potential
All-or None effect: Once the threshold is reached, the AP is triggered. All APs are identical.

30. Action Potential: Refractory periods
Absolute refractory period: all the depolarizing phase + most of repolarizing phase:
Relative refractory period: end of repolarization until the Na/K pump reestablish resting state

31. Action Potential
Note: the AP travels always in 1 direction, from the soma toward the axonal bulb
The AP cannot travel toward the soma because of the presence of the refractory period: the voltage-gated sodium channels are still closed (because of the inner gate) while the neighboring channels are being activated and conduct the AP down the axon

32. Neuron physiology 1- Resting membrane potential 2- Action potential
3- Graded potential
- Excitatory potential
- Inhibitory potential

33. Graded Potentials
- They are small changes in membrane potential across the cell membrane
- They occur on the dendrites and the neuron soma
- They are controlled by ligand-gated channels

34. Some compounds such as novocaine and tetrodotoxin (TTX) from the puffer fish bind to voltage gated sodium channels.
What will be the consequences?

35. Ligand gated channels
The ligand binding to these channels are various neurotransmitters present in the NS
Ligand binding is specific and related to the receptor shape
Receptors for ligand A usually cannot bind to ligand B

36. Excitatory post synaptic potential (EPSP)
A sodium channel opens in response to the binding of a ligand on its receptor.
Sodium ions move inside the cell, therefore depolarizing it
The amount of sodium entering the cell is not enough for the membrane to reach threshold

37. EPSP
Any time a positively charged ions (cations) move into the cell, the membrane depolarizes. Any other ECF cations will have the same effect
Negatively charged ions (anions) from the ICF moving toward the ECF will have the same effect (excitation)

39. Inhibitory post synaptic potential (IPSP)
Ligand gated potassium channels open in response to the binding of a ligand on its receptor.
Potassium ions leave the cell  the membrane potential become more negative (-90mV)  it is moving away from the threshold it is even more difficult to depolarize the membrane  inhibition

40. IPSP
Any positive charged ions moving from the cell toward the IF will have an inhibitory effect
Any negatively charged ions moving from the IF toward the cell will also have an inhibitory effect (ex: chloride ions)

41. Summation
The impulses arriving at the neuron are summed at the hillock. If the result produce a voltage greater than the threshold, an AP will be triggered

42. Temporal summation
Two impulses must arrive at the same location at very close time intervals

43. Spatial summation
Two impulses arrive at different locations at the same time

44. EPSP & IPSP & summation

45. Graded potentials
Immediately after a graded potential, Na/K pumps reestablish resting membrane potential

46. Impulse conduction Saltatory conduction along the myelinated axons
Continuous conduction along the unmyelinated axons

47. Saltatory conduction + Myelinated axon Myelin sheath Node of Ranvier
Extracellular fl
uid
Direction of action potential propagation
Intracellular –
Axon hillock
Saltatory conduction

48. Continuous conduction
Unmyelinated axon +
Extracellular fluid
Resting
Plasma
membrane
Intracellular fluid –
Initiation
Site of
original
action
potential
Site A
Region of
depolarization
Site B
Axon hillock
Direction of action potential propagation
Propagation
Refractory
state
Site C
repolarization
(resting state)
Site D
continues
(a)
(b)
(c)
(d)
Continuous conduction
Along the axon
In one direction only (due to the refractory period)

49. Factors affecting the speed of AP transmission
State of myelination of the axon –
Myelinated axon transmit much
faster than unmyelinated axons
The size of the axon: the larger the axon, the faster the AP

51. Readings: Chp. 7: p. 167-195 Chp. 8, p. 197-213
Clinical connections: p. 180, p. 193, p. 210, p. 212.
Not expected: Toolbox, p.174, p. 178, p. 192,