1. Structure and functions of cells of the nervous system
Chapter 2
Structure and functions of cells of the nervous system

2. Review Basic Genetics Sex Chromosomes and Sex-linked Traits
Genes
Chromosomes
Made up of 4 nucleotide bases
Adenine-thymine, guanine-cytosine
Replication
Duplication errors
Sex Chromosomes and Sex-linked Traits
Structural and Operator genes

3. Review
Cells of the Nervous
System
Neurons
Basic structure

4. Cells of the Nervous System
Neurons
Multipolar
Unipolar
Bipolar
Glial cells
Various types
Provide a wide variety of supportive functions

5. Cells of the Nervous System
Types of Neurons
Multipolar Neuron – neuron with one axon and many dendrites attached to its soma; most common type in CNS.
Figure 2.1

6. Cells of the Nervous System
Figure 2.2
Types of Neurons
Bipolar Neuron – neuron with one axon and one dendrite attached it its soma.
sensory systems (vision and audition)
Unipolar Neuron – neuron with one axon attached to its soma; the axon divides, with one branch receiving sensory information and the other sending the information into the central nervous system.
somatosensory system (touch, pain, etc)

8. Copyright © 2006 by Allyn and Bacon

9. Figure 2.5 The Principal Internal Structures of a Multipolar Neuron

10. Inside the Cell Body
From DNA (nucleus) to protein synthesis (cytoplasm)
Transcriptional and translational processes take place in the cell body

11. Genetic Code and Genetic Expression Mechanism of gene expression
Strand of DNA unravels
Messenger RNA (mRNA) synthesized from DNA
mRNA leaves nucleus and attaches to ribosome in the cell’s cytoplasm
Ribosome synthesizes protein according to 3-base sequences (codons) of mRNA

12. Cells of the Nervous System
Internal Structure
Membrane – a structure consisting principally of lipid molecules that defines the outer boundaries of a cell and also constitutes many of the cells organelles, such as the Golgi apparatus
Figure 2.20
Embedded in the membrane are a variety of proteins that have special functions---we’ll discuss some of these later

13. Cells of the Nervous System
Internal Structure
Cytoplasm – the viscous, semi-liquid substance contained in the interior of the cell; contains organelles
Mitochondria – an organelle that is responsible for extracting energy from nutrients; ATP (adenosine triphosphate.
Figure 2.5
Back to the rest of the cell…covered membrane and nucleus
Bulk of the cell consists of cytoplasm

14. Cells of the Nervous System
Internal Structure
Endoplasmic Reticulum – parallel layers of membrane in the cytoplasm; stores and transports chemicals through the cell; 2 types
Rough ER – contains ribosomes; produces proteins secreted by the cell
Smooth ER – site of synthesis of lipids; provides channels for the segregation of molecules involved in various cellular processes

15. Cells of the Nervous System
Internal Structure
Golgi Apparatus – special form of smooth ER; some complex molecules are assembled here; also acts as a packaging plant, where products of a secretory cell are wrapped
Exocytosis – the secretion of a substance by a cell through means of vesicles; the process by which neurotransmitters are secreted
Lysosomes – an organelle containing enzymes that break down waste products; produced by Golgi apparatus.
Exocytosis – container migrates to cell membrane, fuses with it and then bursts

16. Cells of the Nervous System
Internal Structure
Cytoskeleton – formed of microtubules and other protein fibers giving the cell its shape.
Microtubule – a long strand of bundles of 13 protein filaments arranged around a hollow core; part of the cytoskeleton and involved in transporting substances from place to place within the cell.
Axoplasmic Transport – active process by which substances are propelled along microtubules; 2 types
Anterograde axoplasmic transport – movement from the soma to the terminal buttons; accomplished by kinesin and ATP; fast (500 mm/day)
Retrograde axoplasmic transport – movement from the terminal buttons to the cell body; accomplished by dynein; about ½ as fast as antergrade transport
Transport within a neuron -- Axons can be extremely long and sometimes the terminal buttons need items only produced in the cell body
Need a way to get these items transported thru the axoplasm (cytoplasm of axons)
Kinesis – protein molecule with ‘legs’

18. Cells of the Nervous System
Supporting Cells
Glia (glial cells) - Supporting cells that “glue” the nervous system together; 3 most important types are:
Astrocytes
Oligodendrocytes
Microglia

19. Glial Cells Astrocytes – largest glia, many functions Myelin producers
Oligodendrocytes (CNS)
Schwann cells (PNS)
Microglia – involved in response to injury or disease

20. Astrocytes Provide support to neurons Clean up debris
phagocytosis.
Provide nutrients and other substances
Regulate chemical composition of the extracellular fluid
Some of astrocyte’s processes are wrapped around blood vessels; other processes are wrapped around parts of neuron
Astrocytes receive glucose from capillaries and break it down to lactate
Lactate released into extracellular fluid and then taken up by neurons
Astrocyte = star cell

21. Astrocytes and the Blood-Brain-Barrier
‘Selectively permeable’
Some substance can pass through the BBB
BBB is not uniform
Area postrema (medulla)
Compromised
Normal
Figure 2.12

22. Glial Cells
Oligodendrocytes
Myelinate axons in the CNS
Support axons and produce the myelin sheath
A sheath that surrounds axons and insulates them, preventing messages from spreading between adjacent axons
The sheath is not continuous (the bare portions are called nodes of Ranvier)
A given oligodendrocyte produces up to 50 segments of myelin

23. Oligodendrocyte Figure 2.10

24. Glial Cells: Oligodendrocytes
Myelin
80% lipid
20% protein
Nodes of Ranvier
1-2 μm
A given oligodendrocyte can give up to 50 coatings of myeline
Figure 2.10

25. Glial Cells: Schwann Cells
Peripheral cells
Located in the PNS
Can aid in the removal of dead or dying neurons
Can then guide axonal sprouting
CNS: axonal sprouts are hindered by glial scars (gliosis)
A given oligodendrocyte can give up to 50 coatings of myeline
Figure 2.11

26. Glial Cells: Microglia
10-20% of glial cells are microglia
Cells originate in the periphery
Phagocytosis- breakdown dying neurons, protect from invading microorganisms
Primarily gray matter
Hippocampus, olfactory telencephalon, basal ganglia, substantia nigra
Phagocytosis

27. Microglia and Aging Reactive microglia present in aging rats
6 month
24 month
Lucin and Wyss-Coray (2009)
Reactive microglia present in aging rats
Stress also shown to activate microglia

28. Microglia Activation and Alzheimer’s Disease
Cagnin et al. (2001)
The Lancet
[11C]-PK11195:
Peripheral BZP binding site present on activated microglia
AD:
entorhinal, temporoparietal, and cingulate cortex

29. The Cell Membrane Lipid bilayer Proteins embedded in the bilayer
Selectively permeable to very few ions
Proteins embedded in the bilayer
Channel proteins
Selective for ion type
Receptor proteins
Signalling devices
The Cell Membrane

30. Neuronal Charge: Simple Design
Measuring membrane voltage
Requires:
ONE recording electrode inside the cell (intracellular)
ONE recording electrode outside the cell (extracellular)
Figure 2.15

31. The Ionic Basis of the Resting Membrane Potential
Membrane potential: The voltage across the neuronal membrane at any given time.
Resting Membrane Potential: The voltage when a neuron is at rest (without synaptic input)
At rest (RMP) mV
During an action potential
-65 to +30 mV

32. Resting Membrane Potentials
The RMP is entirely dependent upon
The types of ions
Where they are found (distribution across the membrane)
It is because these ions are unequally distributed across the membrane, that the inside of the cell sits more negative in reference to the external environment.
65 mV

33. IONS Concentrations at Rest
IONS OF INTEREST
substance
symbol
-anions A–
potassium K+
sodium
Na+
chloride
Cl–
IONS Concentrations at Rest
Uneven distribution of ions across the membrane

34. Ions of Interest: Resting Membrane Potential
Figure 2.18

35. Membrane Potentials: The Pressures
Membrane (lipid bilayer) is only selectively permeable to K+, Na+, Cl- (not permeable to A-)
Figure 2.18

36. Membrane Potentials: The Pressures
Figure 2.20
Two passive processes- Require NO energy
One active process- Energy consuming

37. The Movement of Ions: Passive Processes
1) Diffusion
Dissolved ions distribute evenly
Ions flow down concentration gradient
Diffusion of ions:
Channels permeable to specific ions
Concentration gradient across the membrane

38. The Movement of Ions: Passive Processes
2) Electrical (Electrostatic) Processes
Opposite charges attract
Like charges repel
Attract
Repel
Cation
Anion

39. The Movement of Ions: Active Processes
Sodium-Potassium Transporter (also known as the Na+/K+ pump or Na+/K+-ATPase)
Active mechanism in the membrane that extrudes 3 Na+ out and transports 2 K+ in.
Figure 2.20

40. Channel Proteins (summarized) How Ions are Transferred Across the Membrane
1. Active
2. LEAK
3. Needs voltage to open (passive diffusion)
3. Voltage-Gated (open or closed)
1. Na+/K+-Pump
2. Non-Gated (always open)

41. An Action Potential
Figure 2.17
Action potentials require a threshold level of depolarization to occur + + + 4

42. Action Potential Summary

43. An Action Potential
Temporal and sequential importance of ion transfer across the membrane.
Dependent on voltage-gated (dependent) channels
Figure 2.21

44. Summary: Things to think about
Membrane potentials
Lipid bilayer
Ion types (cations and anions contributing)
Distribution of ions across the membrane
Membrane proteins
Channels
Pumps/transporters:
Passive vs active movement of ions
Action potentials
Threshold
Temporal explanation of ion movement across the membrane.

45. Communication Within a Neuron
Conduction of the Action Potential
All-or-None Law – Principle that once the action potential begins, it proceeds without decrement to the terminal buttons.
Figure 2.23
Studies have looked at conduction of action potential in giant squid axon…attach electrical stimulator to electrode at one end of axon and placed recording electrodes at different points along the axon

46. Communication Within a Neuron
Conduction of the Action Potential
Rate Law – principle that variations in the intensity of a stimulus or other information being transmitted in an axon are represented by variations in the rate at which that axon fires.
Figure 2.24
If action potential is all or none how do we get variability …i.e., in the strength of a muscle contraction, the intensity of a stimulus
A single action potential is not the basic element of information…info is represented by an axon’s rate of firing

47. Communication Within a Neuron
Rate Law
A single action potential is not the basic element of information
Variable information is represented by an axon’s rate of firing
A high rate of firing causes a strong muscular contraction
Strong stimulus (bright light) casus a high rate of firing in axons of the eyes

48. Communication Within a Neuron
Cable Properties – passive conduction of electrical current, in a decremental fashion, down an axon.
Figure 2.25
APs are not the only kind of electrical signal that occur in neurons
When a message is sent across synapse, a small electrical signal is produced in receiving neuron
The transmission of weak subthreshold depolarizations is passive (no ion channels open or close)

49. Communication Within a Neuron
Saltatory Conduction – conduction of action potentials by myelinated axons. The action potential appears to jump from one node of Ranvier to the next.
No flow of Na+
Passive conductance until it reaches the nodes of Ranvier ….signal get smaller but still big enough to trigger AP at the node
Figure 2.26

50. Factors Influencing Conduction Velocity
Saltatory conduction
High density of Na+ V-D at Nodes of Ranvier
2 advantages of Saltatory Conduction
Economical
Much less Na+ enters cell (only at nodes of Ranvier) mush less has to be pumped out.
Speed
Conduction of APs is faster in myelinated axons because the transmission between the nodes is very fast.

51. Communication Within a Neuron
Multiple sclerosis
Autoimmune degradation of myelin in PNS
Without myelin the spread of + charge is diminished