Chapter 12: Neural Tissue

1. Chapter 12: Neural Tissue

2. Neural Tissue • 3% of body mass • Cellular, ~20% extracellular space • Two categories of cells: • Neurons: conduct nervous impulses - cells that send and receive signals • Neuroglia/glial cells: “nerve glue” • Supporting Cells • Protect neurons

3. Organs of the Nervous System • Brain and spinal cord • Sensory receptors of sense organs (eyes, ears, etc.) • Nerves connect nervous system with other systems

5. Nervous Systems • Central Nervous System (CNS) • Spinal cord, brain • Functions: • integrate, process, coordinate sensory input and motor output • Peripheral Nervous System (PNS) • All neural tissue outside of CNS • Functions: Carry info to/from the CNS via nerves • Nerves: • Bundle of axons (nerve fibers) with blood vessels and CT • Carry sensory information and motor commands in PNS • Cranial nerves brain • Spinal nerves spinal cord

6. Division of PNS • Sensory/Afferent Division: carries sensory information • Sensory receptors  CNS • Somatic afferent division - From skin, skeletal muscles, and joints • Visceral afferent division - From internal organs

7. Division of PNS 2. Motor/Efferent Division: carries motor commands • CNS  effectors • Somatic Nervous System: Controls skeletal muscle contractions - “voluntary nervous system” • To skeletal muscles  contractions • Autonomic Nervous System (ANS) • “involuntary nervous system” • To smooth and cardiac muscle, glands  contractions • Sympathetic Division: stimulating effect - “fight or flight” • Parasympathetic Division: relaxing effect • “rest and digest” **Tend to be Antagonistic to Each Other**

8. Receptors and Effectors • Receptors: • detect changes or respond to stimuli • neurons and specialized cells • complex sensory organs (e.g., eyes, ears) • Effectors: • respond to efferent signals • cells and organs

9. What would damage to the afferent division of the PNS affect? ability to learn new facts ability to experience motor stimuli ability to experience sensory stimuli ability to remember past events

10. The structure of a typical neuron, and the function of each component.

11. Histology of Nervous System • Neuron/Nerve Cell • Function: conduct nervous impulses (message) • Characteristics: • Extreme longevity • Amitotic - Direct cell division by simple cleavage of the nucleus without spindle formation or appearance of chromosomes - exceptions: hippocampus, olfactory receptors • High metabolic rate: need O2 and glucose

12. The Structure of Neurons Figure 12–1

13. The Structure of Neurons • Large soma/perikaryon(cytoplasm) • Large nucleus, large nucleolus (rRNA) • Many mitochondria, ribosomes, RER, Golgi • Increase ATP, increase protein synthesis to produce neurotransmitters • Nissl bodies: visible RER and ribosomes, gray • Neurofilaments: internal structure • Neurofibrils, neurotubules • No centrioles • 2 types of processes (cell extension): • Dendrite • Axon

14. Regions of a Neuron • Dendrites: • Receive info • Carry a graded potential toward soma • Contain same organelles as soma • Short, branched • End in dendritic spines

15. Regions of a Neuron • Axon: • single, long • Carry an action potential away from soma • Release neurotransmitters at end to signal next cell • Long ones = “nerve fibers” • Contains: • Neurofibrils and neurotubules (abundant) • Vesicles of neurotransmitter • Lysosomes, mitochondria, enzymes • No nissl bodies, no golgi (no protein synthesis in axon)

16. Regions of a Neuron 2. Axon • Connects to soma at axon hillock • Covered in axolemma (membrane) --- Axoplasm (cytoplasm) • May branch: axon collaterals • End in synaptic terminals or knobs • May have myelin sheath: protein+lipid • Function: • Protection, Insulation, and Increase speed of impulse • CNS: myelin from Oligodendrocytes • PNS: myelin from Schwann cells

17. Axoplasmic Transport • Move materials between soma and terminal • Large molecules synthesized in the cell body, such as vesicles and mitochondria are unable to move via simple diffusion • Large molecules are transported by motor proteins called kinesins, which walk along neurotubule tracks to their destinations. • Anterograde transport = soma  terminal • neurotransmitters from soma • Retrograde transport = terminal  soma • Recycle breakdown products from used neurotransmitters • Some viruses use retrograde transport to gain access to CNS (polio, herpes, rabies)

18. Synapse • Site where neuron communicates with another cell: • neuron or effector • Presynaptic cell sends message along axon to axon terminal • Postsynaptic cell receives message as neurotransmitter Neurotransmitter = chemical, transmits signal from pre- to post- synaptic cell across synaptic cleft Synaptic knob = small, round, when postsynaptic cell is neuron, synapse on dendrite or soma Synaptic terminal = complex structure, at neuromuscular or neuroglandular junction

19. Structural Classification of Neurons • Anaxonic neurons: • Dendrites and axon look same • Brain and special sense organs • Bipolar neurons: • 1 dendrite, 1 axon • Soma in middle • Rare: special sense organs, • relay from receptor to neuron • Unipolar neurons: • 1 long axon, dendrites at one end, • soma off side (T shape) • Most sensory neurons • Multipolar neurons: • 2 or more dendrites • 1 long axon • 99% of all neurons • Most CNS

20. A tissue sample shows unipolar neurons. Are these more likely to be sensory neurons or motor neurons? sensory neurons motor neurons

21. Functional Classification of Neurons • Sensory/Afferent Neurons • Transmit info from sensory receptors to CNS • Mostly unipolar neurons • Soma in peripheral sensory ganglia • Ganglia = collection of cell bodies in PNS • Somatic Sensory Neurons - Receptors monitor outside conditions • Visceral Sensory Neurons • Receptors monitor internal conditions

22. Functional Classification of Neurons • Motor/Efferent Neurons • Transmit commands from CNS to effectors • Mostly multipolar neurons • Somatic Motor Neurons • Innervate skeletal muscle • Innervation = distribution of sensory/motor nerves to a specific region/organ • Conscious control or reflexes • Visceral/Autonomic Motor Neurons - Innervate effectors on smooth muscle, cardiac muscle, glands, and adipose

23. Functional Classification of Neurons • Interneurons/Association Neurons • Distribute sensory info and coordinate motor activity • Between sensory and motor neurons • In brain, spinal cord, autonomic ganglia • Most are multipolar

24. The locations and functions of neuroglia.

25. Neuroglia • Neuroglia = supporting cells • Neuroglia in CNS • Outnumber neurons 10:1 • Half mass of brain • Neuroglia Cell in the CNS • Ependymal cells • Astrocytes • Oligodendrocytes • Microglia

26. Neuroglia Cells of the CNS • Ependymal Cells • Line central canal of spinal cord and ventricles of the brain • Secrete cerebrospinal fluid (CSF) • Have cilia to circulate CSF • CSF: cushion brain, nutrient and gas exchange • Astrocytes • Most abundant CNS neuroglia • Varying functions: • Blood brain barrier • Processes wrap capillaries • Control chemical exchange between blood and interstitial fluid of the brain • Framework of CNS • Repair damaged neural tissue • Guide neuron development in embryo • Control interstitial environment: - Regulate conc. Ions, gasses, nutrients, neurotransmitters

27. Neuroglia Cells of the CNS • Oligodendrocytes • Wide flat processes wrap around local axons = myelin sheath • 1 cell contributes myelin to many neighboring axons • Lipid in membrane insulates axon for faster action potential conductance • Gaps on axon between processes/myelin = nodes of Ranvier, necessary to conduct impulse • White, myelinated axons = “white matter” • Microglia • Phagocytic • Wander CNS • Engulf debris, pathogens • Important CNS defense • No immune cells or antibodies

28. Neuroglia of the CNS Figure 12–4

29. Neuroglia in PNS • Satellite Cells • Surround somas in ganglia • Isolate PNS cells • Regulate interstitial environment of ganglia • Ganglia = mass of neuronal soma and dendrites • Schwann cells • Myelinate axon in PNS • Whole cells wraps axon, many layers • Neurilemma: bulge of schwann cell, contains organelles • Nodes of Ranvier between cells

30. Neuroglia in PNS • Schwann Cells cont. • Some hold bundles of unmyelinated axon • Vital to repair of peripheral nerve fibers after injury • Guide growth to original synapse

31. Which type of neuroglia would occur in larger than normal numbers in the brain tissue of a person with a CNS infection? astrocytes microglial cells ependymal cells oligodendrocytes

32. Neural Responses to Injuries Figure 12–6 (1 of 2)

33. Neural Responses to Injuries Figure 12–6 (2 of 2)

34. KEY CONCEPT • Neurons perform all communication, information processing, and control functions of the nervous system • Neuroglia preserve physical and biochemical structure of neural tissue, and are essential to survival and function of neurons

35. How the resting potential is created and maintained.

36. 5 Main Membrane Processes in Neural Activities Figure 12–7 (Navigator)

37. Neurophysiology • Neurons: conduct electrical impulse • Requires transmembrane potential = electrical difference across the cell membrane • Cells: positive charge outside (pump cations out) and negative charge inside (protein) Voltage = measure of potential energy generated by separation of opposite charges Current = flow of electrical charges (ions) Cell can produce current (nervous impulse) when ions move to eliminate the potential difference (volts) across the membrane Resistance = Restricts ion movement (current) • High resistance = low current • Membrane has resistance, restricts ion flow/current

38. Neurophysiology • Ohm’s Law: current = voltage ÷ resistance • Current is highest when voltage is high and resistance is low • Cell voltage set at -70mV but membrane resistance can be altered to create current • Membrane resistance depends on permeability to ions: open or closed ion channels • Cell must always have some resistance or ions would equalize, voltage = zero • No current generated = no nervous impulse

39. Membrane Ion Channels • Allow ion movement (alter resistance) • Each channel is specific to one ion type • Passive Channels (leaky channels) • Active Channels • Chemically regulated/ligand-gated • Voltage regulated channels • Mechanically regulated channels

40. Membrane Ion Channels • Passive Channels (leaky channels) - Resting Potential • Always open, free flow • Sets resting membrane potential at -70mV

41. Active Channels: Gated Channels Figure 12–10

42. Membrane Ion Channels • Active Channels • open/close in response to signal • Chemically regulated/ligand-gated • Open in response to chemical binding • Located on any cell membrane • Dendrites and soma

43. Membrane Ion Channels 2. Active Channels B.Voltage regulated channels - open/close in response to shift in transmembrane potential - excitable membrane only: conduct action potentials - axolemma, sarcolemma

44. Membrane Ion Channels 2. Active Channels C.Mechanically Regulated Channels - Open in response to membrane distortion - On dendrites of sensory neurons for: - touch, pressure, vibration

45. Membrane Ion Channels • When channel opens, ions flow along electrochemical gradient: • Diffusion (high conc. to low) • Electrical attraction/repulsion

46. Sodium-Potassium Pump

47. Sodium-Potassium Pump • Uses ATP to move 3 Na+ out and 2 K+ in • 70% of neurons use ATP for this • Runs anytime the cell is not conducting an impulse • Creates high [K+] inside and high [Na+] outside • When Na+ cell opens • Na+ flows into cell: • Favored by diffusion gradient • Favored by electrical gradient Open channel = decr. Resistance = incr. ion flow/current • When K+ channel opens • K+ flows out of cell: • Favored by diffusion gradient only • Electrical gradient repels K+ exit - Thus less current than Na+

48. Channels open = resistance low = ions move until equilibrium potential: depends on • Diffusion gradient • Electrical gradient • Equilibrium Potential

49. Electrical vs. Chemical Gradients • The electrical gradient opposes the chemical gradient • K+ inside and outside of the cell are attracted to the negative charges on the inside of the cell membrane, and repelled by the positive charges on the outside of the cell membrane • indicated in white on the next slide • Chemical gradient is strong enough to overpower the electrical gradient, but this weakens the force driving K+ out of the cell • Net driving force indicated in grey on the next slide • The Electrochemical Gradient

50. Electrochemical Gradients Figure 12–9c, d