Chapter 39

1. Chapter 39 Neural Signaling

2. Neural signaling process • Reception of information by a sensory receptor • Transmission by afferent neuron to the central nervous system • Integration by CNS interneurons • Efferent neuron transmission • Action by effectors

3. Stimulus response

4. Glial cells • Support and nourish neurons • Microglia are phagocytic cells • Astrocytes • Some are phagocytic • Others help regulate composition of the CNS extracellular fluid • May induce and stabilize synapses

5. Oligodendrocytes • Glial cells that form myelin sheaths around axons in the CNS • Schwann cells • Form sheaths around axons in the peripheral nervous system (PNS)

6. Structure of a typical neuron • A cell body contains the nucleus and most of the organelles • Many branched dendrites extend from the cell body • Single long axon extends from the cell body and forms branches called axon collaterals

7. Structure of a multipolar neuron

8. Dendrites receive stimuli and send signals to the cell body • Axon transmits signals into its terminal branches that end in synaptic terminals • Many axons are surrounded by an insulating myelin sheath formed of Schwann cells

9. Nodes of Ranvier • Gaps in the sheath between successive Schwann cells • Nerve • Several hundred axons wrapped in connective tissue • Ganglion • Mass of neuron cell bodies

10. Nerve structure

11. Neuron resting potential • In a resting neuron, the inner surface of the plasma membrane is negatively charged compared with the outside • Potential difference of about -70 millivolts (mV) across the membrane

12. Differences in concentrations of specific ions—Na+ (sodium) and K+ (potassium)—inside the cell relative to the extracellular fluid • Selective permeability of the plasma membrane to these ions • Ions pass through specific passive ion channels

13. K+ leaks out more readily than Na+ can leak in • Cl- (chlorine) ions accumulate along the inner surface of the plasma membrane • Gradients that determine the resting potential are maintained by ATP

14. Resting potential

15. Sodium-potassium pumps • Continuously transport three sodium ions out of the neuron for every two potassium ions transported in

16. Voltage-activated ion channels

17. Depolarized membrane • Stimulus caused the membrane potential to become less negative • Hyperpolarized membrane • Membrane potential becomes more negative than the resting potential

18. Graded potential • Local response that varies in magnitude depending on the strength of the applied stimulus • Fades out within a few mm of its point of origin

19. Action potential • Wave of depolarization that moves down the axon • Voltage across the membrane declines to a critical point • Voltage-activated ion channels open • Na+ flows into the neuron • Action potential is generated

20. Action potential

21. Action potential is an all-or-none response • No variation exists in the strength of a single impulse • Membrane potential either exceeds threshold level, leading to transmission of an action potential, or it does not

22. Repolarization • As the action potential moves down the axon, repolarization occurs behind it • During depolarization, the axon enters a refractory period • Time when it cannot transmit another action potential

23. Depolarization Resting state

24. Return to resting state Repolarization

25. Continuous conduction • Takes place in unmyelinated neurons • Involves the entire axon plasma membrane

26. Saltatory conduction • More rapid than continuous conduction • Takes place in myelinated neurons • Depolarization skips along the axon from one node of Ranvier to the next

27. Saltatory conduction

28. Synapses • Junction between two neurons or between a neuron and effector • Most synapses are chemical • Transmission depends on release of neurotransmitter from synaptic vesicles in the synaptic terminals of a presynaptic neuron

29. Neurotransmitters • Acetylcholine • Triggers contraction of skeletal muscle • Glutamate • Main excitatory neurotransmitter in the brain • GABA • Inhibitory neurotransmitter

30. Biogenic amines • Norepinephrine • Serotonin • Dopamine • Play important roles in regulating mood • Dopamine is important in motor function

31. Neuropeptides • Endorphinsm • Enkephalins • Nitric oxide (NO) • Gaseous neurotransmitter that transmits signals from the postsynaptic neuron to the presynaptic neuron

32. Synaptic transmission • Calcium ions cause synaptic vesicles to fuse with the presynaptic membrane and release neurotransmitter into the synaptic cleft • Neurotransmitter combines with specific receptors on a postsynaptic neuron

33. Synaptic transmission

34. Neurotransmitter receptors • Many are proteins that form ligand-gated ion channels • Others work through a second messenger such as cAMP

35. Excitatory and inhibitory signals • Excitatory postsynaptic potential (EPSP) • Bring the neuron closer to firing • Inhibitory postsynaptic potential (IPSP) • Move the neuron farther away from its firing level

36. A postsynaptic neuron integrates incoming stimuli and “decides” whether or not to fire • Each EPSP or IPSP is a graded potential • Varies in magnitude depending on the strength of the stimulus applied

37. The mechanism of neural integration is summation • Process of adding and subtracting incoming signals • By summation of several EPSPs, the neuron may be brought to critical firing level

38. Temporal summation • Repeated stimuli cause new EPSPs to develop before previous EPSPs have decayed • Spatial summation • Postsynaptic neuron stimulated at several different places

39. Convergence • Single neuron is controlled by converging signals from two or more presynaptic neurons • Permits the CNS to integrate incoming information from various sources

40. Divergence • Single presynaptic neuron stimulates many postsynaptic neurons • Allows widespread effect • Reverberation • Axon collateral synapses with an interneuron

41. Reverberation