Lecture #13 – Animal Nervous Systems

1. Lecture #13 – Animal Nervous Systems

2. Key Concepts: • Evolution of organization in nervous systems • Neuron structure and function • Neuron communication at synapses • Organization of the vertebrate nervous systems • Brain structure and function • The cerebral cortex • Nervous system injuries and diseases???

3. All animals except sponges have some kind of nervous system • Increasing complexity accompanied increasingly complex motion and activities • Nets of neurons  bundles of neurons  cephalization

4. First split was tissues; next was body symmetry; echinoderms “went back” to radial symmetry

5. Derived radial symmetry and nerve network

6. Cephalization • The development of a brain • Associated with the development of bilateral symmetry • Complex, cephalized nervous systems are usually divided into 2 sections • Central nervous system (CNS) integrates information, exerts most control • Peripheral nervous system (PNS) connects CNS to the rest of the body

8. Critical Thinking • What is the functional advantage of cephalization???

9. Critical Thinking • What is the functional advantage of cephalization??? • All the sensory, processing, eating and many feeding structures are located at the advancing end of the animal

10. Cephalization • The development of a brain • Associated with the development of bilateral symmetry • Complex, cephalized nervous systems are usually divided into 2 sections • Central nervous system (CNS) integrates information, exerts most control • Peripheral nervous system (PNS) connects CNS to the rest of the body

11. PNS  CNS  PNS

12. Specialized neurons support different sections • Sensory • Transmit information from the sensory structures that detect the both external and internal conditions • Interneurons • Analyze and interpret sensory information, formulate response • Motor • Transmit information to effector cells – the muscle or endocrine cells that respond to input

13. Critical Thinking • Which type of neuron would have the most branched structure??? • Sensory neurons • Interneurons • Motor neurons

14. Critical Thinking • Which type of neuron would have the most branched structure??? • Sensory neurons • Interneurons • Motor neurons • Interneurons have the most connections of all neurons • They make “all the connections”

15. Neuron structure is complex 100 billion nerve cells in the human brain!

18. Cell body Dendrites Axons Axon hillock Myelin sheath Synaptic terminal Basic Neuron Structure

19. Cell Body • Contains most cytoplasm and organelles • Extensions branch off cell body

20. Dendrites • Highly branched extensions • Receive signals from other neurons

21. Axons • Usually longer extension, unbranched til end • Transmits signals to other cells

22. Axon Hillock • Enlarged region at base of axon • Site where axon signals are generated • Signal is sent after summation

23. Myelin Sheath • Insulating sheath around axon • Also speeds up signal transmission

24. Synaptic Terminal • End of axon branches • Each branch ends in a synaptic terminal • Actual site of between-cell signal generation

25. Synapse • Site of signal transmission between cells • More later…

26. Supporting Cells - Glia • Maintain structural integrity and function of neurons • 10 – 50 x more glia than neurons in mammals • Major categories • Astrocytes • Radial glia • Oligodendrocytes and Schwann cells

27. Glia – Astrocytes • Structural support for neurons • Regulate extracellular ion and neurotransmitter concentrations • Facilitate synaptic transfers • Induce the formation of the blood-brain barrier • Tight junctions in capillaries allow more control over the extracellular chemical environment in the brain and spinal cord

28. Glia – Radial Glia • Function mostly during embryonic development • Form tracks to guide new neurons out from the neural tube (neural tube develops into the CNS) • Can also function as stem cells to replace glia and neurons (so can astrocytes) • This function is limited in nature; major line of research

29. Glia – Oligodendrocytes (CNS) and Schwann Cells (PNS) • Form the myelin sheath around axons • Cells are rectangular and tile-shaped, wrapped spirally around the axons • High lipid content insulates the axon – prevents electrical signals from escaping • Gaps between the cells (Nodes of Ranvier) speed up signal transmission

30. The nerve signal is electrical! • To understand signaling process, must understand the difference between resting potential and action potential

31. Resting Potential • All cells have a resting potential • Electrical potential energy – the separation of opposite charges • Due to the unequal distribution of anions and cations on opposite sides of the membrane • Maintained by selectively permeable membranes and by active membrane pumps • Charge difference = one component of the electrochemical gradient that drives the diffusion of all ions across cell membranes

32. Neuron Function – Resting Potential • Neuron resting potential is ~ -70mV • At resting potential the neuron is NOT actively transmitting signals • Maintained largely because cell membranes are more permeable to K+ than to Na+; more K+ leaves the cell than Na+ enters • An ATP powered K+/Na+ pump continually restores the concentration gradients; this also helps to maintain the charge gradient

33. Resting Potential Ion Concentrations • Cell membranes are more permeable to K+ than to Na+ • There is more K+ inside the cell than outside • There is more Na+ outside the cell than inside • Both ions follow their [diffusion] gradients

34. Critical Thinking • If both ions follow their diffusion gradients, what is the predictable consequence???

35. Critical Thinking • If both ions follow their diffusion gradients, what is the predictable consequence??? • A dynamic equilibrium where both charge and concentration were balanced

36. Resting Potential Ion Concentrations • A dynamic equilibrium is predictable, but is prevented by an ATP powered K+/Na+ pump

37. Neuron Function – Resting Potential • Neuron resting potential is ~ -70mV • At resting potential the neuron is NOT actively transmitting signals • Maintained largely because cell membranes are more permeable to K+ than to Na+; more K+ leaves the cell than Na+ enters • An ATP powered K+/Na+ pump continually restores the concentration gradients; this also helps to maintain the charge gradient

38. Resting Potential Ion Concentrations • ATP powered pump continually transfers 3 Na+ ions out of the cytoplasm for every 2 K+ ions it moves back in to the cytoplasm • This means that there is a net transfer of + charge OUT of the cell

39. Resting Potential Ion Concentrations • Thus, the membrane potential is maintained • Cl- and large anions also contribute to the net negative charge inside the cell

40. REVIEW Neuron Function – Resting Potential • Neuron resting potential is ~ -70mV • At resting potential the neuron is NOT actively transmitting signals • Maintained largely because cell membranes are more permeable to K+ than to Na+; more K+ leaves the cell than Na+ enters • An ATP powered K+/Na+ pump continually restores the concentration gradients; this also helps to maintain the charge gradient • Cl-, other anions, and Ca++ also affect resting potential

41. Gated Ion ChannelsWhy Neurons are Different • All cells have a membrane potential • Neurons can change their membrane potential in response to a stimulus • The ability of neurons to open and close ion gates allows them to send electrical signals along the extensions (dendrites and axons) • Gates open and close in response to stimuli Only neurons can do this!

42. Gated Ion ChannelsWhy Neurons are Different • Gated ion channels manage membrane potential • Stretch gates – respond when membrane is stretched • Ligand gates – respond when a molecule binds (eg: a neurotransmitter) • Voltage gates – respond when membrane potential changes

43. Gated Ion ChannelsWhy Neurons are Different • Hyperpolarization = inside of neuron becomes more negative • Depolarization = inside of neuron becomes more positive • Either can occur, depending on stimulus • Either can be graded – more stimulus = more change in membrane potential • Depolarization eventually triggers an action potential = NOT graded

44. Depolarizationeventually triggers an action potential – action potentials are NOT graded

45. Action Potentials ARE the Nerve Signal • Triggered whenever depolarization reaches a set threshold potential • Action potentials are all-or-none responses of a fixed magnitude • Once triggered, they can’t be stopped • There is no gradation once an action potential is triggered • Action potentials are brief depolarizations • 1 – 2 milliseconds • Voltage gated ion channels control signal

46. Critical Thinking • If the action potential is of a fixed magnitude, how do we sense different levels of a stimulus???

47. Critical Thinking • If the action potential is of a fixed magnitude, how do we sense different levels of a stimulus??? • They can occur with varying frequency • Frequency is part of the information • They can occur from a large number of nearby neurons

48. Action Potentials ARE the Nerve Signal • Triggered whenever depolarization reaches a set threshold potential • Action potentials are all-or-none responses of a fixed magnitude • Once triggered, they can’t be stopped • There is no gradation once an action potential is triggered • Action potentials are brief depolarizations • 1 – 2 milliseconds • Voltage gated ion channels control signal

49. Fig. 48.13; p. 1019, 7th Ed.

50. Voltage Gate Activity • Resting Potential – Na+ and K+ activation gates closed; Na+ inactivation gate open on most channels • Depolarization – Na+ activation gates begin to open – Na+ begins to enter cell • Rising Phase – threshold is crossed, Na+ floods into the cell, raising the membrane potential to ~ +35mV