CHAPTER 12 NERVOUS SYSTEM CELLS: SUPPLEMENT 2

Transcription

1 CHAPTER 12 NERVOUS SYSTEM CELLS: SUPPLEMENT 2 PowerPoint by John McGill Supplemental Notes by Beth Wyatt Physiology of Nerve Impulses Generalizations Basic events 1

2 Generalizations about Nerve Impulses Stimulus initiates the nerve impulse (NI). NI is electrochemical. Electrical part of the NI flows in wavelike fashion along plasma membrane. NI does not lose strength from start to finish NI speed varies due to axon diameter and myelin sheath. GENERALIZATIONS: PRIOR TO AN IMPULSE STIMULUS: PRIOR TO AN IMPULSE (not on supplement) Sodium Potassium pump uses ATP to pump sodium out of the cell and potassium into the cell 2

3 Sodium Potassium Pump STIMULUS INITIATES NI Stimulus Causes NI to Begin Indicates a Need for Communication GENERALIZATIONS: STIMULUS INITIATES NI 3

4 GENERALIZATIONS: NI FLOWS IN WAVELIKE FASHION NI FLOWS IN WAVELIKE FASHION along the plasma membrane. Changes That Occur Involve Small Sections of the P.M. At a Time That Progress GENERALIZATIONS: ELECTROCHEMICAL ELECTROCHEMICAL The nervous impulse is Considered to be ELECTRICAL as it TRAVELS ALONG P. MEMBRANE (Like Electricity) Electrical Events Involve Changes in Ions, I.e. a difference in charge is voltage, voltage is a measurement for electricity. 4

5 GENERALIZATIONS: ELECTROCHEMICAL ELECTROCHEMICAL The nervous impulse CHEMICAL AS it TRAVELS ACROSS SYNAPSE due to the release of NEUROTRANSMITTERS) Synapse = Space B/T Neuron and Next Structure Chemical Events Involves the Release of Chemicals (Neurotransmitters) Allows NI to Cross the Synapse GENERALIZATIONS: DOESN T LOSE STRENGTH DOESN T LOSE STRENGTH FROM START TO FINISH (DUE TO ELECTRO- CHEMICAL NATURE) If NI s Were purely Electrical, They would be Stopped at Synapses; However at Synapses NI s Become Chemical 5

6 GENERALIZATIONS: SPEED & DIAMETER SPEED VARIES; DEPENDS ON DIAMETER OF NEURON S NERVE FIBERS (AXON) The Larger the Diameter of the Axon, the Faster the Speed of the NI (More Surface Area) GENERALIZATIONS: MYELIN SHEATH SPEED VARIES; DEPENDS ON PRESENCE OF MYELIN SHEATH: MYELINATED NERVE FIBERS CONDUCT USING SALTATORY CONDUCTION (FASTER) Saltatory Conduction: NI Leaps Along Nodes of Ranvier (Microscopic Gaps in Myelin Sheath, Myelin Sheath Acts as an Insulator) 6

7 GENERALIZATIONS: SALTATORY CONDUCTION MYELINATED NERVE FIBERS CONDUCT USING SALTATORY CONDUCTION (FASTER) Saltatory Conduction: NI Leaps Along Nodes of Ranvier (Microscopic Gaps in Myelin Sheath, Myelin Sheath Acts as an Insulator) BASIC EVENTS IN NERVE IMPULSE TRANSMISSION CONDUCTION ALONG PLASMA MEMBRANE (ELECTRICAL) RESTING MEMBRANE POTENTIAL (POLARIZATION) STIMULUS ACTION POTENTIAL NERVE IMPULSE REVERSE POLARIZATION DEPOLARIZED REPOLARIZATION REFRACTORY PERIODS 7

8 BASIC ELECTRICAL EVENTS: RESTING POTENTIAL CONDUCTION ALONG PLASMA MEMBRANE (ELECTRICAL) RESTING MEMBRANE POTENTIAL (POLARIZATION) Neuron Resting (Not Conducting a Nerve Impulse) RMP for Neurons: The OUTER Surface of the P. Membrane Contains a Slight Excess of POSITIVE Ions (Means INNER SURFACE is LESS POSITIVE/NEGATIVE Compared to OUTER SURFACE) Major Mechanism That Maintains the RMP: Na+/K+ Pumps Membrane Potential Difference in Ion Concentration on Each Side of PM Means a Difference in Electrical Charges (Polarization) Measured Like Electricity (Millivolts) BASIC EVENTS: STIMULUS & ACTION POTENTIAL STIMULUS: Often* Leads to Action Potential Stimulus Often Causes NI to Begin (*Depends on Strength of Stimulus) ACTION POTENTIAL NERVE IMPULSE REVERSE POLARIZATION DEPOLARIZATION Neuron Active (Conducting a Nerve Impulse) 8

9 BASIC EVENTS: STIMULUS & ACTION POTENTIAL continued ACTION POTENTIAL NERVE IMPULSE REVERSE POLARIZATION DEPOLARIZATION Neuron Active (Conducting a Nerve Impulse) A Reverse in Polarization Occurs: The INNER SURFACE Now Contains a Slight Excess of POSITIVE Ions (Means the OUTER SURFACE is LESS POSITIVE ( NEGATIVE ) in Relation to the INNER SURFACE) BASIC EVENTS: ACTION POTENTIAL Mechanism: Stimulus Causes Stimulus Gated Na+ Channels (At the Point of Stimulation) to Open ---->Na+ Diffuses Into the Cell If ENOUGH Stimulus Gated Na+ Channels Open and ENOUGH Na+ Ions Diffuse Into the Cell*, then Voltage Gated Channels Open, and EVEN MORE Na+ Ions Diffuse Into the Cell (Reverse Polarization) * Depends Upon the Strength of the Stimulus Na+ Channels Close Automatically Once a Maximum Number of Na+ Ions Move Into the Cell (Voltage Peaks, All-or-None) 9

10 BASIC EVENTS: ACTION POTENTIAL How the Action Potential is Conducted: Along Small Sections of the Neuron at a Time (Wavelike) (Reverse in Polarization Sets Up an Electrical Current Flow) A Neuron Can Conduct More Than 1 NI at a Time BASIC EVENTS: REPOLARIZATION REPOLARIZATION A Return to Polarization (RMP) Mechanism: 1) K+ channels Open* ---> K+ Diffuses Out of the Cell (*Reaching the Peak in Voltage Causes Them to Open) 2) RMP Restored by the Na+/K+ Pump Repolarization Occurs Along Sections of the Neuron Just Behind the Action Potential (Wavelike) 10

11 BASIC EVENTS: REFRACTORY PERIODS REFRACTORY PERIODS ABSOLUTE REFRACTORY PERIOD Occurs Where the Action Potential is Occurring This Area of P. Membrane TOTALLY RESISTS STIMULATION BASIC EVENTS: REFRACTORY PERIODS REFRACTORY PERIODS RELATIVE REFRACTORY PERIOD Occurs Where Repolarization is Ocurring This Area of P. Membrane PARTIALLY RESISTS STIMULATION 11

12 BASIC EVENTS: REFRACTORY PERIODS The Refractory Periods are Important Because They Prevent The Action Potential From Moving Backwards Keep NI s From Moving on Top of One Another Electrical Neuron Impulse: Summary Neuron maintains resting potential (polarization) with active transport by the Na K pump. Stimulus triggers Na gate to open and Na goes INTO neuron, K comes OUT. If action potential is reached wave of depolarization occurs. Repolarization takes place. Refractory period begins with Action Potential and ends once repolarization is complete. 12

13 Wave of Depolarization CONDUCTION ACROSS SYNAPSES (CHEMICAL) LOCATION OF SYNAPSES BETWEEN NEURONS (PRE & POSTSYNAPTIC) BETWEEN MOTOR NEURON AND EFFECTOR 13

14 SYNAPSES -Structure STRUCTURE OF CHEMICAL SYNAPSES SYNAPTIC KNOBS SYNAPTIC CLEFT PLASMA MEMBRANE OF NEXT STRUCTURE; EITHER: Another neuron An effector Bulges Located at the Terminal Ends of Telodendria Store Neurotransmitters SYNAPTIC KNOBS 14

15 SYNAPTIC CLEFT Space Between Neuron and Next Structure PLASMA MEMBRANE OF NEXT STRUCTURE EITHER: POSTSYNAPTIC NEURON CELLS OF EFFECTOR Plasma Membrane Contains Receptors to Which Neurotransmitters Bind 15

16 PLASMA MEMBRANE OF NEXT STRUCTURE Nerve impulse Ca influx Vesicles move to Presynaptic Membrane Release neurotransmitters (NT) by exocytosis NT s diffuse across the synaptic cleft NT s bind with protein receptors on the post synaptic membrane Causing Na channels/gates to open Continuation of the nervous impulse Steps of Synaptic Transmission 16