Chapter 12 Cont

What is a relay synapse?

Also called a neuromuscular synapse. An action potential in the presynaptic cell always causes an action potential in the post-synaptic cell.

Graded Potentials( Local Potential)

Are post-synaptic potentials where a local change in the membrane potential decreases with distance.Controlled by ligands. The spot that is important is the axon hillock(trigger zone). Sodium that is coming into the cell depolarizes it. The concentration gradient is causing the sodium to come in via diffusion and will dissipate throughout the neuron. It gets weaker through the cell body( faded lines in pic).

Chemical synapses can be excitatory

Excitatory post-synaptic potential (EPSP) : a depolarizing stimulus(Na+) brings the membrane potential of the post-synaptic neuron closer to the threshold.

Action Potential Current Pattern

If injected current does not depolarize the membrane threshold, no action potentials will be generated. If injected current depolarizes the membrane beyond threshold action potentials will be generated. The action potential firing rate increases as the depolarizing current increases. The higher the increase of injected current you have makes it easier to pash past or through the relative refractory period. There were always be a small space in between each action potential.

Trigger Zone( Axon hillock)

If the membrane in this area is depolarized above a certain threshold, then the nerve will generate a nerve impulse (an action potential) that will travel (or propagate) down its axon.

Chemical synapses can be inhibitory.

Inhibitory post-synaptic potential (IPSP): a hyperpolarizing stimulus takes the membrane potential of the post- synaptic neuron further from the threshold. Ex: K+ flowing out of the cell when a potassium channel is opened.

What is threshold

Is the millivoltage required to open the first voltage gated sodium channel. This value is the threshold for voltage gated sodium channels.

Relative Refractory Period

it is more difficult to elicit another action potential. (i.e. It would require a greater depolarization than normal.) Slide 29

Depolarization

membrane potential becomes less negative (more positive) If the membrane potential becomes less negative than the resting membrane potential, then the cell membrane is said to be this. This is done by selectively altering permeability (opening or closing ion channels). Ex: Sodium ions entering the cell....attracted to the negative charges on the inner surface.

Why is the trigger zone( Axon Hillock) important

Contains those ion channels on the receptive part of our multi-polar neuron. We have ligand gated channels all over the cell body and dendrites. What could change is at the Axon Hillock( trigger zone). There's a different gating at the Axon Hillock(trigger zone) you have VOLTAGE GATED ION CHANNELS. The Axon Hillock(trigger zone) is where we switch from ligand gated to voltage gated ion channels that line the rest of the axon. This is also where you find the FIRST VOLTAGE GATED SODIUM CHANNELS!!! If we get a high enough voltage it will open up the voltage gated sodium channels. And we get a huge response with an ACTION POTENTIAL. The AXON is not receptive because it has voltage gated channels( not receptive from other cells).

Temporal summation

Still a graded potential. The amount of depolarization is greater when 2 or more EPSPs are elicited in quick succession. The amount of hyperpolarization is greater when 2 or more IPSPs are elicited in quick succession. Sodium channels are excitatory and will come into the cell and change the membrane potential( depolarization). This is not likely to reach threshold. If it fires again by opening another sodium channel you can add to it making the action potential closer to threshold. Same thing happens for Potassium( inhibitory).

Spatial Summation

Still a graded potential. When 2 or more postsynaptic potentials are elicited simultaneously from separate synapses, the affects add (or cancel). Because post synaptic potentials add, inhibitory post synaptic potentials can cancel out excitatory post synaptic potentials. A FIRES THEN B FIRES= add them together( A+B) and they cancel each other out.

Myelination Cont.

The Na+ current spreads passively from node to node, so the membrane potential decreases with distance. Because the nodes of Ranvier contain voltage-gated channels, the action potential is regenerated at each node.

Removal of a Neurotransmitter from the Synaptic Cleft

The synapse must be reset to terminate the signal. This means that the neurotransmitter must be removed from the synaptic cleft. They can be returned to axon terminals for reuse or transported into glial cells. These can also be inactivated by enzymes in the synaptic cleft. Then are diffused out of the synaptic cleft. Acetylcholinesterase is needed to remove this from the synaptic cleft. It does this by binding to the receptors.

Gated Ion Channels

These channels open or close in response to a stimulus Example: neurotransmitter Channels only allow specific ions to pass through the membrane. Ions move down their electrochemical gradient Only relatively small numbers of ions move across

Action Potential

Through spatial and temporal summation, the combination of all the excitatory and inhibitory post-synaptic potentials will determine if the neuron fires this. This will only happen if the trigger zone is depolarized to the threshold value. The trigger zone is the important part. If we hit the threshold we have created this. It is going to travel all the way down the axon. Only thing stopping this is something pathological.

A nerve impulse (action potential) can only be generated at the trigger zone and propagated along the axon. Why?

Voltage-gated Na+ and K+ channels are necessary to generate an action potential. These channels are only found at the trigger zone and along the axon.

Axon Hillock( trigger zone)

Where you change from ligand gated to voltage gated channels. Also where you find the first voltage gated sodium channels

Incoming signals

are integrated at the trigger zone( axon hillock). All of the incoming excitatory and inhibitory signals that the neuron is receiving (This may be a thousand signals all coming at once.) are integrated into 1 response - either the neuron fires an action potential or it does not.

Nerve and Muscle Cells

are said to have excitable membranes, because their plasma membranes exhibit voltage changes in response to stimulation. Because there are positive charges along the outside and negative charges along the inside, the resting cell membrane is electrically polarized.

Absolute Refractory Period

it is impossible to elicit another action potential. You can have one coming behind but not right on top of each other. Slide 29

Hyperpolarization

membrane potential becomes more negative. This is done by selectively altering permeability (opening or closing ion channels). If the membrane potential becomes more negative than the resting membrane potential, then the cell membrane is said to be this. Ex: The opening up of potassium channels could cause this

Repolarization

membrane potential returns to resting value. Via ion movement through membrane channels and the activities of ion pumps, like the Sodium-Potassium pump.

Myelination

thick cholesterol derived stuff that clogs up and insulates the axon. Speeds up the continuation of the action potential down the axon. Schwaan Cells= PNS Oligodendrocytes= CNS In these axons, the action potential propagates by saltatory( jumping across something) conduction.