THE NERVOUS SYSTEM

1. THE NERVOUS SYSTEM

2. OBJECTIVES • Understand various functions of the nervous system • Know anatomical features of a neuron and function(s) of each • Apply terminology used in class to describe stimulus-response pathways in the human body • Know the characteristics of a reflex and explain how reflexes differ from other neural circuits

3. OBJECTIVES • Understand factors contributing to a neuron’s RMP and the EP of each ion • Describe the movement of ions across the membrane of a neuron at rest as well as during an action potential • Describe the phases of an action potential and what contributes to each • Know the types of protein transporters that play a role in action potentials

4. THE NERVOUS SYSTEM Functions: • Sense and respond to changing conditions • Awareness of sights, sounds, hunger, fear and emotion • Active throughout your life, without rest, until your die

5. THE NEURON Neuron: • An excitable cell • Basic unit of communication in nearly all animal nervous systems • Astounding uniformity • Neurons function same way in most species

6. THE NEURON Neurons Work Collectively To: • Detect, integrate, and interpret info. regarding internal and external conditions • Select and regulate appropriate response to stimuli Input stimulus Receptors (Sensory Neurons) Motor Neurons Effectors Muscles; Glands Integrators (CNS)

7. INPUT stimulus RECEPTORS sensory neurons INTEGRATORS interneurons (e.g., the brain) motor neurons EFFECTORS muscle cells, gland cells OUPUT response NERVOUS SYSTEM

8. REFLEXES The CNS Integrates Information & Activates Neurons Involved in Response: • Reflexes: • Rapid responses connecting sensory neurons to motor neurons in a way that bypasses the brain

9. REFLEXES Sensory receptor Spinal cord Motor neuron Effector cells Sensory neuron When reflexes occur, sensory information bypasses the brain

10. THE NEURON Neuron Anatomy: • Dendrite • Fibers, often heavily branched, receive incoming signal • Cell Body • Command center of neuron • Contains nucleus • Axon • Fiber(s) extending from cell body • Contains axon hillock, region from which AP is fired

11. dendrites INPUT ZONE cell body TRIGGER ZONE axon axon endings CONDUCTING ZONE OUPUT ZONE NEURON ANATOMY

12. dendrites dendrite dendrite axon cell body axon cell body NEURON ANATOMY

13. THE SYNAPSE Synapses: • Tiny Spaces Between Adjoining Nerve Cells • A single neuron may have thousands • USUALLY A One Way Valve • Unidirectional transmission of signal • Site of signal Conversion • Electrical impulse converted to chemical signal

14. THE NEURON MEMBRANE Neuron Membrane: • A thin membrane • Similar to plasma membrane in composition • Lipid bilayer • Embedded proteins • Surrounds entire neuron

15. THE NEURON MEMBRANE Lipid Bilayer: • Impermeable to ions and many polar molecules • Results in disequilibruimin charge across membrane • Embedded proteins allow passage of some materials

16. THE NEURON MEMBRANE TRANSPORT PROTEINS: • Channel Proteins • Open channels (leak) • Gated channels • Ligand-gated • Voltage-gated • Carrier Proteins • Change conformation

17. INTERSTITIAL FLUID lipid bilayer of a neuron’s plasma membrane trigger zone CYTOPLASM THE NEURON MEMBRANE

18. ACTIVE & PASSIVE TRANSPORT • Passive Transport: • Includes simple and facilitated diffusion • No energy required • Ions move down electrochemical gradient • Active Transport (Pumps): • Energy required • Ions move against electrochemical gradient

19. ELECTROPHYSIOLOGY • Remember ions are charged! • When they move, they carry a charge with them! • Na+ • K+ • Cl-

20. ELECTROPHYSIOLOGY • Neurons at rest have a large difference in charge across the membrane • Resting Membrane Potential (RMP)

21. ELECTROPHYSIOLOGY Outside of cell Inside of cell Separation of charge creates the resting potential

22. ELECTROPHYSIOLOGY Resting Membrane Potential (RMP): • Describes charge inside neuron with respect to outside • Measured in millivolts (mV)

23. ELECTROPHYSIOLGY Two factors affect RMP: • Concentration of ions in & out of cell • Permeability of axon (membrane) to ions

24. ELECTROPHYSIOLOGY Na+ K+ outside plasma membrane K+ Na+ inside • In Most Animal Neurons: • High Potassium (K+) inside • High Sodium (Na+) outside

25. ELECTROPHYSIOLOGY Cl-, Na+ Outside cell In Most Animal Neurons: • Membrane mostly permeable to K+ • K+ diffuses out of the cell, down its concentration gradient, through leak channels • Low permeability to Na+ & Cl- K+

26. ELECTROPHYSIOLOGY In Resting Neurons: • K+ diffuses out of cell • Down it’s concentration gradient • K+ moves out until it’s movement is opposed electrically • Equilibrium Potential (EP) of K+ Outside cell K+

27. ELECTROPHYSIOLOGY Equilibrium Potential (EP): • Voltage at which concentration gradient is matched and opposed by electrical gradient • Specific to each ion • Results in no net movement of ion

28. ELECTROPHYSIOLOGY OUT K+ K+ K+ In Neurons At Rest: • K+ is permitted to exit cell via leak channels • K+ carries a + charge • Leaving inside of cell more K+ _ _ _ _ IN _

29. ELECTROPHYSIOLOGY • Net movement of K+ ceases when inward pull of electric charge equals/exceeds outward pull of diffusion • For K+ this occurs at a voltage of –93mv • Na+ has an EP of +60mv

30. ELECTROPHYSIOLOGY • Since neurons are mostly permeable to K+, RMP tends to be near EP of K+ • RMP of human neurons = -70mV

31. NERVE CELL COMMUNICATION • Transmission of information within nervous system occurs by way of electrical impulses • Neurons “talk” with ea. other using electrical impulses

32. ELECTRICAL IMPULSES IN NEURONS The Action Potential (AP): • A messages that’s electrical in nature • If astimulus disturbs distribution of electrical charge across neuron’s membrane, an AP may be fired

33. ELECTRICAL IMPULSES IN NEURONS dendrites INPUT ZONE cell body TRIGGER ZONE (AXON HILLOCK) axon axon endings CONDUCTING ZONE OUPUT ZONE • Neurons Fire Action Potentials From Trigger Zone : • Trigger Zone is rich in v-gated Na+ channels • that open in response to adequate stimuli (threshold)

34. THE ACTION POTENTIAL +40 +40 0 0 1. Depolarization phase 2. Repolarization phase Membrane potential (mV) Membrane potential (mV) -40 -40 -60 -60 -80 -80 Resting potential 3. Undershoot Time

35. At the resting potential, voltage-gated Na+ channels are closed. Na+ Na+ Na+ + + + + + + + + + + + + + + + + + + + + + + – – – – – – – – – – – – – – – – – – – – – – – – – When the membrane is depolarized, conformational changes open voltage-gated channel Na+ – – – – – – – – – – – – – – – – – – – – – – – – + + + + + + + + + + + + + + + + + + + + + + Na+ Na+

36. THE ACTION POTENTIAL • When Na+ channels open, Na+ moves into cell for two reasons: • Electrical gradient: • Negative charge inside cell attracts positive sodium • Concentration gradient: • Na+ is more concentrated outside the neuron

37. THE ACTION POTENTIAL The Rising Phase: • Na+ movement into the neuron = a positive feedback mechanism • Na+ enters  V-gated Na+ channels open More Na+ enters!

38. THE ACTION POTENTIAL The Rising Phase • As neuron becomes more positive (less negative) inside, it’s “depolarized” • Na+ continues entering cell until v-gated Na+ channels shut • At +30mv

39. THE ACTION POTENTIAL Falling Phase • Around the time Na+ channels close, K+ channels open • K+ Delay • K+ flows out of cell, down concentration gradient, carrying positive charge with it

40. THE ACTION POTENTIAL The Falling Phase: • As K+ leaves, inside of neuron becomes more negative • Cell is “Repolarizing”, returning to the Resting Membrane Potential

41. THE ACTION POTENTIAL K+ channels open +40 +40 Na channels close +30 0 0 Influx Na+ Efflux K+ Membrane potential (mV) Membrane potential (mV) -40 -40 -60 -60 RMP -80 -80 Time Repolarization Depolarization