Chapter Outline 12.1 Basic Structure and Functions of the Nervous System A. Overall Function of the N.S. & Basic Processes Used B. Classification of the Nervous System 12.2 Histology of Nervous System A. Neuroglia Cells B. Neurons C. Types of Neurons 12.4 The Action Potential A. Electrically Active Cell Membranes B. Membrane Potentials That Act As Signals C. Graded Potentials (See Text section 12.5) * D. Action Potentials 12.5 Communication Between Neurons A. Basic Concepts and Terms B. Synapses Neurotransmitter Systems 12.3 The Functions of the Nervous System– How the Parts Work Together

12.4 The Action Potentials … E. Action Potentials (AP) 1. Basic Concepts a. Cells: Initiated by: Threshold: Membrane depolarized by 15 to 20 mV Myelinated and Unmyelinated (see diagram next slide) Graded Potential Action Potential Neurotransmitter Dendrites Subthreshold stimulus is not detected

Non-Myelinated Axon Myelinated Axon

12.4 The Action Potentials … D. Action Potentials (AP) … 1. Basic Concepts … e. Start & End Locations: Type of Gated Channel: Ions involved h. All or None: Graded Potential Action Potential GP Neurotransmitter Dendrites

2. OVERVIEW OF MECHANISMS a. Graded Potential  GP Action Potential Neurotransmitter Section of Membrane Dendrites 2. OVERVIEW OF MECHANISMS a. Graded Potential  b. At Axon Hillock: Small Section of Membrane Depolarization– Na+ in Repolarization—K+ out Hyperpolarization– more K+ out Return to Resting State– Na-K Pump c. Adjacent chunks of membrane: stimulated one after another, each following above actions d. When end is reached: Neurotransmitter carries message across to next neuron

The big picture Action Potentials … 1 2 3 3 4 2 1 1 4 Resting state Depolarization 3 Repolarization 3 4 Hyperpolarization Membrane potential (mV) 2 Action potential Threshold 1 1 4 Time (ms) Figure 11.11 (1 of 5)

Action Potentials … Unmyelinated Axons … 3. Types of Voltage-gated Channels a. Na+ Channel has two Voltage gates ACTIVATION INACTIVATION Function: For Na+ to enter cell: b. K+ Channel has one Voltage gate Is Activating and:

4. Initiating Action Potential– Details a. Resting State of Channels 4. Initiating Action Potential– Details a. Resting State of Channels Na+ activation gate: closed Na+ inactivation gate open K+ activation gate: closed Takes 2 milli seconds Na+ Sodium channel Potassium channel Activation gates K+ 1 Resting state Inactivation gate

Action Potentials … b. Depolarizing Phase a. Initiated by: b. At threshold: c. Na+ flows in and membrane becomes: d. End Point-- + 30mVolts: e. Restoration to Resting State: Na+ 2 Depolarization K+

Repolarization c. Repolarizing Phase a. K+ Activation Gate Threshold: b. K+: c. Potential: is restored at: d. K+ gates: slowly closing Na+ K+ 3 Repolarization

Action Potentials … d. Hyperpolarization a. K+ gates: b. Excess K+: c. Undershoot to: K+ channels: Na channels: Na+ K+ 4 Hyperpolarization

Action Potentials … e. Resting State Restored Na+-K+ Pump: Leakage Channels

Generation of an Action Potential

The AP is caused by permeability changes in the plasma membrane 3 Action potential Membrane potential (mV) Na+ permeability 2 Relative membrane permeability K+ permeability 1 1 4 Time (ms) Figure 11.11 (2 of 5)

5. Propagation of the Action Potential– a. Nonmyelinated Axon Moves in: one direction: Continuously Local Current generated: Na+ that have entered the axon move horizontally towards the more negative adjacent section of the membrane: Na+ Channels of adjacent section of membrane: 5. Positive Feedback Cycle:

APs self-propagate – once started, all voltage channels open like dominoes (all or none) at 2 ms Voltage at 0 ms Voltage at 4 ms Recording electrode (a) Time = 0 ms. Action potential has not yet reached the recording electrode. (b) Time = 2 ms. Action potential peak is at the recording electrode. (c) Time = 4 ms. Action potential peak is past the recording electrode. Membrane at the recording electrode is still hyperpolarized. Resting potential Peak of action potential Hyperpolarization

Propagation of an Action Potential in Unmyelinated axons

1. Location of channels: 2. AP generate at: 3. 30 times faster Action Potentials … Propagation of an Action Potential … b. Myelinated Axon = Saltatory Conduction 1. Location of channels: 2. AP generate at: 3. 30 times faster SHOW REST OF VIDEO

Unmyelinated axon Stimulus Voltage-gated ion channel Stimulus Node of Ranvier Myelin sheath 1 mm Myelin sheath

Propagation of an Action Potential in Myelinated axons

Action Potentials … 6. Characteristics of Action Potentials a. Refractory Period: i. Absolute Refractory Period = Na+ channels are: ii. Relative Refractory Period: Na+ Channels: Stimulus: ** Diagram on next slide

Threshold elevated, only exceptionally strong stimuli cause APs Absolute refractory period Relative refractory period Nerve cannot fire, Threshold elevated, only exceptionally strong stimuli cause APs Depolarization (Na+ enters) Repolarization (K+ leaves) After-hyperpolarization Stimulus Time (ms) Figure 11.14

Action Potentials … b. Coding for Stimulus Intensity Strong stimuli: Intensity determined by: AP Frequency Stimulus Strength

Action Potentials … c. Conduction Velocity Diameter of Axon: Myelinated or not: Summary of GP vs AP

Action Potentials … 7. Multiple Sclerosis (MS) STUDENTS DO: Text page 488

12. 5 Communication Between Neurons A 12.5 Communication Between Neurons A. Graded Potentials - see previous section =

Nerve Impulses– within and between neurons Neurotransmitters Synapse

A. Types of Synapses Chemical: Basic Components (Text p. 502) Electrical: Chemical: Basic Components (Text p. 502) presynaptic element neurotransmitter in vesicles synaptic cleft receptor proteins postsynaptic element neurotransmitter elimination or re-uptake

12.5 Communication between Neurons– The Synapse … B. Chemical Synapses– details Drug (legal & illegal) Affects–: Addiction is due to:

Not common or well understood B. Chemical Synapses– details … 2. Parts of Presynaptic and Postsynaptic Neurons involved Axodendritic synapses Dendrites Axosomatic synapses Cell body Axoaxonic synapses (a) Axon Not common or well understood Axon Axosomatic synapses Cell body (soma) of postsynaptic neuron (b)

Action potential arrives at axon terminal. B. The Chemical Synapse … 3. Steps of Presynaptic Neuron communication with Postsynaptic Neuron Presynaptic neuron Presynaptic neuron a Postsynaptic neuron Action potential arrives at axon terminal. Mitochondrion Ca2+ Ca2+ Ca2+ Ca2+ Synaptic cleft Axon terminal Synaptic vesicles Postsynaptic neuron Figure 11.17, step 1

The Chemical Synapse Ca2+ Ca2+ Ca2+ b Figure 11.17, step 2 Presynaptic neuron Presynaptic neuron Postsynaptic neuron Ca2+ Mitochondrion Ca2+ Ca2+ Ca2+ Synaptic cleft Axon terminal Synaptic vesicles Postsynaptic neuron b Voltage-gated Ca2+ channels open:. Figure 11.17, step 2

The Chemical Synapse c Synaptic vesicles Figure 11.17, step 3 Presynaptic neuron Presynaptic neuron Postsynaptic neuron Synaptic vesicles Mitochondrion Ca2+ Ca2+ Ca2+ Ca2+ Synaptic cleft c Ca2+ entry causes: Axon terminal Postsynaptic neuron

The Chemical Synapse d Figure 11.17, step 4 Presynaptic neuron Postsynaptic neuron Mitochondrion Ca2+ Ca2+ Ca2+ Ca2+ Synaptic cleft Axon terminal Synaptic vesicles Postsynaptic neuron d Neurotransmitter: Figure 11.17, step 4

Bound Neurotransmitter Graded potential e Binding of neurotransmitter to Receptors: Graded Potential produced: f Figure 11.17, step 5

g Neurotransmitter effects terminated: by Reuptake Enzymatic degradation Diffusion away from synapse g Neurotransmitter effects terminated: by Figure 11.17, step 6

Example: DRUGS THAT BLOCK RE-UPTAKE OF NEUROTRANSMITTER–Antidepressants & other drugs that block re-uptake of Serotonin, Norepinephrine, and Dopamine

4. Types of Neurotransmitter-Receptor Systems = specific neurotransmitters and the: Body Location of & functions of Cells responsive to specific neurotransmitters # of Different Receptors on Neurons per Neurotransmitter: # of Different Neurotransmitters per Neuron

d. Neurotransmitter Classification based on Chemical Structure Acetylcholine Amino Acid based: iii) Purines (nucleic acid bases): iv) Lipid-derived:

4. Mechanisms for Neurotransmitters Action Ion Channel-linked receptors - Direct action: Ion flow blocked Ions flow Ligand Closed ion channel Open ion channel (a) Channel-linked receptors open in response to binding of ligand (ACh in this case).

b. G Protein-Linked Receptors Indirect action: Eventually Channel Activates: Neurotransmitter (1st messenger) binds and activates receptor. 1 Closed ion channel Open ion channel Adenylate cyclase Receptor G protein cAMP changes membrane permeability by opening or closing ion channels. 5a cAMP activates specific genes. 5c GDP 5b cAMP activates enzymes. Receptor activates G protein. 2 G protein activates adenylate cyclase. 3 Adenylate cyclase converts ATP to cAMP (2nd messenger). 4 Nucleus Active enzyme (b) G-protein linked receptors cause formation of an intracellular second messenger (cyclic AMP in this case) that brings about the cell’s response.

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