Neurons (PPT 5)

Importance of Myelination

-Large myelinated fibers (skeletal muscles) = 120 m/sec -Small unmyelinated fibers (digestive tract) = 0.7 m/sec -Optic nerve (myelinated) = 3 mm -Optic nerve (unmyelinated) = 300 mm (to reach the same velocity) Dont memorize these

Categories of Graded Potential

1. Post-synaptic potentials - neurons  2. Receptor potentials - from special sense neurons  3. End-plate potentials - neurons communicating with skeletal muscles 4. Pacemaker potentials - cardiac cells that generate potentials on their own; in the lungs as well 5. Slow-wave potentials - GI tract

Current and Resistance

Conductors - decrease resistance • In a cell, ICF & ECF • Difference in ECF & ICF will reduce the resistance  Insulators - increase resistance  • In a cell, lipids • Lipids are insulators because ions cannot flow freely across the lipid membrane

Contiguous Conduction

• AP travels along the entirety of the axon  • More energy-consuming than myelination/saltatory conduction • Why? Because more ion exchange has to occur for this to happen; thus, need to utilize the Na/K ATP pumps  • Have to reestablish the ion gradient to return to the action potential  • Depolarization of the first patch of membrane

Absolute Refractory Period

• Absolute refractory period occurs first  • Two reasons for period: 1. Voltage-gated Na channels are already open - more stimuli is not going to cause them to open any more than they already are 2. Inactivation gates of the Na channels are closed  • Red ball and chain = inactivation gate • Open at rest • Blue = activation gate  • Closed at rest

Comparing Graded and Action Potentials (2)

• Action potential is going to propagate and not diminish - magnitude remains the same from axon hillock to axon terminal  Summation:  1. Graded: if two different stimuli are close enough together, they can add or subtract from one another 2. AP: no summation; all or nothing Refractory period: period where it is impossible/difficult to stimulate a potential  1. Graded: none; only depends upon how long channel is open  2. AP: relative and absolute

Potential Propagation

• Action potential: 1. Contiguous conduction • Unmyelinated axons only • Slow 2. Saltatory conduction • Myelinated axons • Fast

Graded Potential

• Active area - depolarized region • Inactive area - resting region • Example above is of a dendrite • Channel of the dendrite is in green  • Stimulant -> ion channel opens -> ions flow into the cell (positive charge) -> depolarization of the membrane = active area  • Other areas that have not changed = resting area(s) • Eventually the charges are going to spread into the inactive area

Nodes of Ranvier

• Area between the two sheets that are not wrapped in myelin = Nodes of Ranvier • Nodes are exposed to the ECF • Area has a high [conc] of voltage-gated K channels

Magnitude of the Current

• As we spread/move further from the initial site of change in potential, there is less and less change in potential that is occurring • Why? 1. Electrical resistance - ions cannot move easily against lipids 2. Current loss - potassium diffuses across the membrane through leak channels • Only a few chemicals coming in..ion channels open briefly and allowing very few ions to flow through -> small change in potential

Absolute Refractory Period (2)

• At peak potential: the activation gate is still open, but the inactivation gate closes  • Peak potential breaks up the first part and the second part of the absolute refractory period

Neuron

• Axon hillock - high concentration of channels that help propagate the action potential  • Axon terminal - connects to cells or other neurons

Conducting and Output Zones

• Axon: conducting zone • Stimulation is not needed at the axon; once I passes the axon hillock it doesn't't need any more stimulate (unless something is wrong with the nerve) • Propagates on its own  • Axon terminal: output zone • Highly branched extensions of the axon that can communicate with other cells • Release NTs

Current

• By convention, we look at the flow of positive charges when talking about current • Amount of current depends upon: 1. Difference in potential of the 2 areas 2. Resistance of the material through which the charges are moving

Saltatory Conduction

• Current only flow at the nodes of Ranvier because that's the only located where the channels are In contiguous conduction you have to flow through every spot/portion of the membrane • Wastes energy • Saltatory faster because it jumps from area to area and conserves energy

Relative Refractory Period 2

• K is flowing out of the cell, so strong stimulus needed

Comparing Graded and Action Potentials

• Magnitude of the graded potential can vary depending on the stimulus/triggering event

Trigger Zone

• Must hit the threshold (-50 mV) in order to create an action potential - only then will the graded potential be converted into the action potential  • A high number of voltage-gated Na channels make it easier to reach threshold

Contiguous Conduction (2)

• Na comes into the axon hillock and depolarization occurs • Positive charges move into the next path of membrane, and that next patch of membrane begins to depolarize as well Bottom windows: 1. Peak 2. Threshold reached 3. Has not reached threshold yet = moving along..contiguous conduction

Structure of a Nerve

• Nerves are made of many, many neurons

Refractory Period (more info)

• Refractory periods due to the periods of the ion channels • Unidirectional movement of ions due to the refractory period • If you have an AP that occurs during the relative RP, it does not change the refractory periods of the next AP • Why? Because this does not affect the ion channel behaviors

Action Potential Frequencies

• Shows the signals are dependent upon the frequency of action potentials

Input Zone (2)

• Signals generate changes to membrane potential - aka a change in ion permeability • Known as a graded potential • Graded potentials are NOT voltage-gated

Refractory Period

• Since an action potential spreads through diffusion, it would be reasonable to think that it would not be unidirectional • Why doesn't it: refractory period • Refractory periods only occur in APs; NOT graded potentials • During the refractory period, new action potentials cannot be initiated  = one action potential must be over before another AP can occur at the same time = APs are not undergoing summation  • Two periods: 1. Absolute 2. Relative

Relative Refractory Period

• Stimulus must be of a very large magnitude  • Activation gate is closed, but able to be opened if the stimulus overcomes the hyperpolarization  Absolute Refractory Period • Na voltage gated channels: 1st half  1. Activation gate: open 2. Inactivation gate: closed  2nd half 1. Activation gate: open 2. Inactivation gate: closed

Input Zone

• Where the information comes into the cell • Info received by the dendrite, which are extension of the cell body • Dendrites increase the surface area of the cell body in order to enhance communications