Chapter 12: Neural Tissue

1. Chapter 12: Neural Tissue BIO 210 Lab Instructor: Dr. Rebecca Clarke

2. Nervous System • Includes all neural tissue in the body • Organs • Brain and spinal cord • Sensory receptors • Nerves

3. Anatomical Divisions of the Nervous System • Central nervous system (CNS) • Brain and spinal cord • Peripheral nervous system (PNS) • All other neural tissue outside CNS, i.e., receptors, nerves, ganglia

4. Functions of the CNS • Acts as control center for nervous system • Integrates, processes, coordinates: • Sensory data: • Convey info about conditions inside/outside body • Motor commands: • Control/adjust activities of peripheral organs, e.g., skeletal muscles • Higher functions of brain: • Intelligence, memory, learning, emotion • Provides short-term control over activities of other systems

5. Functions of the PNS • Links CNS with other systems and with sense organs • Delivers sensory information from receptors to CNS • Carries motor commands from CNS to peripheral tissues and systems

6. Neural Tissue • Consists of 2 kinds of cells: • Neurons: • Send and receive signals • Perform all communication, information processing, and control functions of NS • Neuroglia (glial cells): • Support and protect neurons • Preserve physical and biochemical structure of neural tissue • Essential to survival and function of neurons

7. Structure of a Neuron • Neuron: basic functional unit of nervous system Figure 12–1

8. Multipolar Neuron • Common in the CNS • Large cell body (soma): • = perikaryon + nucleus • Several short, branched dendrites • Single, long axon

9. Major Organelles of the Cell Body • Large nucleus with nucleolus • Cytoplasm (perikaryon) • Mitochondria (produce energy) • RER and ribosomes (produce neurotransmitters) • Cytoskeleton (supports dendrites and axon) • LACK centrioles • Can’t divide

10. Nissl Bodies • Dense areas of RER and ribosomes • Make neural tissue appear gray • Neuron cell body = gray matter

11. Dendrites • Highly branched processes • Extend from cell body • Dendritic spines: • Many fine processes • Receive information from synaptic end bulbs/axonal terminals of axons of other neurons • 80–90% of neuron surface area

12. Axon • Long process that extends from cell body • Carries electrical signal (action potential) away from cell body • Axon structure is critical to function

13. Structures of the Axon (1 of 3) • Axoplasm: • Cytoplasm of axon • Axolemma: • Specialized cell membrane • Covers axoplasm

14. Structures of the Axon (2 of 3) • Axon hillock: • Cone-shaped, broad region of cell body • Attaches to… • Initial segment: • Base of axon that attaches to axon hillock of cell body

15. Structures of the Axon (3 of 3) • Collaterals: • Branches of a single axon • Enable axon to communicate with other cells • Telodendria: • Fine extensions of distal axon • Synaptic (axonal) terminals/end bulbs/knobs: • Tips of telodendria

16. Myelin/Myelin Sheath • Glossy white, multilayer, lipid wrap around axon • Insulates axon from contact with extracellular fluid (like insulation on electrical wires) • Increases speed of action potential along axon • Regions of CNS with many myelinated neurons  white matter of CNS

17. Myelin/Myelin Sheath • Formed by: • Schwann cells (PNS neuroglia) • Oligodendrocytes (CNS neuroglia)

18. Schwann Cells • Wrap around axons of PNS •  Myelin sheath (like oligodendrocytes in CNS) • Last layer of wrap (outer surface) = neurilemma • Loose sheath • Contains nucleus and cell organelles • Internodes = wrapped areas of axon • Nodes (of Ranvier) = gaps between internodes Figure 12–5a

19. Schwann Cells • 1 Schwann cell sheaths 1 segment of axon • Many Schwann cells sheath entire axon • Axons branch at nodes  collaterals

20. Oligodendrocytes • Wrap around CNS axons • Can myelinate portions of several adjacent axons • Process different from Schwann cells • Schwann cells can myelinate only one segment of a single axon Figure 12–4

21. Endoneurium • Connective tissue layer • Surrounds neurilemma of nerve fiber

22. 4 Structural Classifications of Neurons • Multipolar neuron • Unipolar neuron • Bipolar neuron • Anaxonic neuron

23. Multipolar Neuron • 2 or more dendrites • 1 Long axon • Most common type in CNS • Includes all skeletal muscle motor neurons • In somatic nervous system (SNS) Figure 12–3)

24. Unipolar Neuron • 1 Long axon • Fused dendrite and axon • Cell body to one side • Found in sensory neurons of PNS Figure 12–3

25. Bipolar Neuron • Small cell • 1 Dendrite • Branches extensively at distal tip  dendritic spines • 1 Axon • Cell body between axon and dendrite • Rare • Found in special sense organs, e.g., retina of eye Figure 12–3

26. Anaxonic Neuron • Small • All cell processes look alike • Found in brain and sense organs Figure 12–3

27. Functional Classifications of Neurons • Sensory Neuron – carries sensory (afferent) information from sensory receptors to the CNS • Somatic sensory neurons – monitor outside world and our position in it • Visceral sensory neurons – monitor internal conditions • Interneuron – between the sensory and motor neurons; information processing • Motor Neuron – carries motor (efferent) information from the CNS to the effector • Somatic motor neuron – carries info to skeletal muscles • Visceral motor neuron – carries info to smooth m., cardiac m., glands, and adipose tissue

28. Neuroglia • Supporting cells • Half the volume of the nervous system • Many types of neuroglia in CNS and PNS

29. Neuroglia of the CNS • Ependymal cells • Microglia • Astrocytes • Oligodendrocytes

30. Neuroglia of the CNS: Ependymal Cells • Columnar epithelial cells • Form ependyma • Line fluid (CSF)-filled cavities in: • Spinal cord (central canal) • Brain (ventricles) • Have slender processes on “nonpassageway” side of cell • Assist in: • Producing, circulating and monitoring composition of CSF Figure 12–4

31. Cerebrospinal fluid (CSF) • Surrounds: • Brain • Spinal cord •  Protective cushion • Transports dissolved gases, nutrients, wastes, etc.

32. Neuroglia of the CNS: Microglia • Smallest, rarest neuroglia in CNS • Many fine-branched processes • Phagocytic macrophage • Migrates through neural tissue; wandering police force/janitorial service • Cleans up cellular debris, waste products, pathogens Figure 12–4

33. Neuroglia of the CNS: Astrocytes • Star-shaped • Largest and most numerous neuroglia in CNS • Have “feet” on end of many processes • Wide range of functions Figure 12–4

34. Functions of Astrocytes • Maintain blood-brain barrier • “Feet” extensions wrap around capillaries • Create 3-D framework for CNS • Repair damaged neural tissue • Structural repairs that stabilize tissue/prevent further injury • Guide neuron development in embryonic brain • Control interstitial environment • Regulate ion, nutrient, dissolved gas concentrations and transport • Control blood flow through capillaries • Absorb and recycle NTs

35. Neuroglia of the CNS: Oligodendrocytes • Smaller cell body • Fewer processes • Processes • Contact other neuron cell bodies • Wrap around axons to form myelin sheath  myelinated axon • Many oligodendrocytes on each axon • Each oligodendrocyte myelinates segments of several axons Figure 12–4

36. Neuroglia of the PNS • Schwann cells • Satellite cells

37. Neuroglia of the PNS:Schwann Cells • Wrap around peripheral axons of PNS •  Myelin sheath (like oligodendrocytes in CNS) Figure 12–5a

38. Neuroglia of the PNS:Satellite Cells • Surround ganglia (collections of neuron cell bodies) • Regulate environment around neurons (like astrocytes in CNS)

39. White Matter and Gray Matter • White matter of the CNS • Myelinated axons • Gray matter of the CNS • Neuron cell bodies, unmyelinated axons, and neuroglia

40. Terminology

41. Neural Responses to Injuries • Wallerian degeneration • Axon distal to injury degenerates • Schwann cells • Form path for new growth • Wrap new axon in myelin

42. Neural Responses to Injuries • Nerve Regeneration in CNS • Limited by chemicals released by astrocytes that • Block growth • Produce scar tissue

43. Neural Responses to Injuries

44. Neural Responses to Injuries

45. Neural Responses to Injuries

46. Neural Responses to Injuries