MARION COPELAND AND LYNNETTE LACEK FIRST YEAR MEDICAL STUDENTS PENN STATE HERSHEY COLLEGE OF MEDICINE The Action Potential and Neurotransmission

Review ● What are the major components of the nervous system? (different categories) ● What are the main structures of a nerve cell? ● What happens at the nerve axon terminal?

Review

Learning Objectives 1. Students will review the properties of the neuronal membrane and explain the role of inward and outward flow of ions (current) in creating membrane potentials. 1. Students will understand how the depolarization of the neuronal membrane contributes to generating an action potential. Students will understand that the action potential is an all or nothing response.

The Membrane jd ❖ There is a lot of stuff in the membrane of cells ➢ Peripheral Proteins ➢ Globular Protein ➢ Integral Protein ➢ Cholesterol ❖ Important to know: ➢ Integral Protein ■span the membrane from the Extracellular fluid to the cytoplasm Integral Proteins are Important in the nerve and sending signals GATED ION CHANNEL= one type that we are going to talk about today

Why do we need proteins in the membrane? ●The cell membrane is impermeable to charged molecules ●Proteins allow the cell a way to let these charged molecules inside ● The proteins help pick and choose which molecules are allowed into the cell ●Proteins that do this are called transporters and channels

Lets Watch the Channel Function Before we do : what does the membrane channel do?

What Drives the Movement of Ions Across the Membrane Two forces move ions across the membrane 1.Concentration Gradient : Diffusion a.Ions move from high concentration to low concentration 2.Voltage or potential difference across the membrane a.You can calculate this driving force with an equation ●The combination of these two forces drives ions across the membrane through the channel proteins ●This is known as the Electrochemical gradient

NEURON AT REST ❖ Key Players: Sodium (Na+) Potassium (K+) ❖ Sodium (Na+) is in high concentrations outside of the neuron ❖ K+ is high inside the neuron When the channel opens what ions moves where?

Membrane Potential ●When the cell is at rest the concentration of the ions on either side of the membrane creates a resting membrane potential ●This is -70mV ●At rest the ion channels are closed = no ion movement

Action Potential 1 Cell receives an excitatory stimulus Na+ Channels Open reaches Threshold Potential (1) Na+ Rushes into the cell causing depolarization (1,2,3) K+ channels begin to open and K+ leaves out of the cell (2) Na+ Channel gates close no more Na+ can enter the cell (3) K+ channels fully open - K+ leaves the cell (4) K+ channels close and Na+ Channels Reset

Action Potential 2 ★ Depolarization : when the membrane potential is increasing ○Becoming MORE POSITIVE ○WHY? ★ Repolarization : When the membrane potential is decreasing ○Becoming MORE NEGATIVE ○WHY? ★ Hyperpolarization: When the membrane potential dips below the resting potential ○Why?

Refractory Periods Absolute Refractory Period ●Sodium (Na+) Channels Inactivate when the membrane becomes depolarized ●CANNOT be activated again and another Action Potential cannot be generated until membrane potential goes negative to de-inactivate

Why is this important? Action Potential is initiated at the Axon Hillock ●Signal travels down axon all the way to the axon terminals

What do you remember ? What does the letter A represent? A.Hyperpolarization B.Threshold potential C.Resting Membrane Potential D.Depolarization

What do you remember ? Letter C represents __________ and the ______ gate is open. A.Hyperpolarization, Na+ B.Repolarization, K+ C.Hyperpolarization, K+ D.Depolarization, Na+

What do you remember ? The cell receives a depolarizing stimulus at C what will happen next? A.The cell will depolarize and the Na+ channels will open. B.The cell will do nothing with this new stimulus. C.The stimulus will cause hyperpolarization. D.The stimulus will add to the current action potential creating a stronger signal

Learning Objectives 1. Students will review the importance of the schwann cell and their role in myelin production and synaptic transmission. 1. Students will be introduced to the synapse and the different types of synapses in the nervous system.

Schwann Cells ●How cells and structures help action potential travel down the nerve axon? ○ Myelin ○ Oligodendrocyte sSchwann Cells ○ Nodes of Ranvier

What happens when the signal reaches the Axon terminal? ●Once the action potential reaches the Axon terminal, the “message” must be transmitted to another structure. ●What structures do nerves send signals to? ○ Muscles, other nerves, blood vessels ●The synapse is the structure that allows a nerve to pass an electrical or chemical signal to another neuron

Components of the Synapse ●Presynaptic membrane ●Synaptic Cleft ●Post Synaptic membrane ●Neurotransmitters ●Receptors

Neurotransmission ●Synaptic vesicles hold neurotranmitters in the presynaptic neuron ●Voltage-gated Ca 2+ channel is closed at rest ●Receptors and enzymes on the postsynaptic membrane are unoccupied

Neurotransmission 1.Action potential reaches Axon terminal 2.Voltage-gated Ca 2+ channel opens, allowing Ca 2+ to flow into the presynaptic neuron 3.Ca 2+ binds to synaptic vesicles containing neurotranmitters, ushering them towards the presynaptic membrane

Neurotransmission 4.Synaptic vesicles fuse with the presynaptic membrane, releasing neurotransmitters into the synaptic cleft 5.Neurotransmitter binds to receptor on postsynaptic membrane, causing a response 6.Neurotransmitter binds and is degraded by an enzyme on the postsynaptic membrane

Neurotransmission 7.Some molecules of neurotransmitter are reabsorbed into the presynaptic neuron 8.Other molecules of neurotransmitter diffuse away from the synaptic cleft

Learning Objectives 1. Students will be able to identify several types of excitatory and inhibitory neurotransmitters, including glutamate, GABA, norepinephrine, glycine, and acetylcholine. 1. Students will understand the concept of a receptor and be introduced to different types.

Receptors ★ Where : Postsynaptic Cell Membrane ○What part of the neuron receives these signals? ★ What: Integral Membrane proteins that are specific for a certain ligand ★ How: Based on their fit, think of a lock and key model ★ Why: different molecules and different receptor will cause different downstream effects in the cell ★ How do they elicit an action potential?

Response to Neurotransmitters Excitatory Inhibitory ●Opening of ligand-gated sodium channels on the postsynaptic membrane. ●Allows sodium ions to flow into the cell, making the interior less negative ●Exitatory Postsynaptic Potential (EPSP) ●Opening of potassium channels on the postsynaptic membrane ●Allows potassium ions to flow out of the cell, making the interior more negative ●Inhibitory Postsynaptic Potential (IPSP)

Neurotransmitters Exitatory Inhibitory ●Acetylcholine ●Norepinephrine ●Glutamate ●Aspartate ●GABA ●Glycine ●Serotonin

Summary ●Explain to someone sitting next to you what creates the membrane potential

Summary-Membrane Potential ●Created by the different concentrations of the ions (Na+ and K+) on different sides of the membrane.

Summary ●Talk with a different partner and summarize how a gated channel works.

Summary - Gated Channel ●Ligand binds the receptor causing a conformational change ( a change in its structure) that allows the ions to pass through

Summary ●Explain to someone sitting next to you the role of neurotransmitters.

Summary-Neurotransmitter Neurotransmitters are released into the synaptic cleft to transmit the “message” to the next nerve. They bind to receptors on the postsynaptic membrane and either signal an exitatory or inhibitory response

Summary Talk with a neighbor about the difference between exitatory and inhibitory neurotransmitters

Summary- Neurotransmitters ●Excitatory neurotransmitters bind to receptors and cause the opening of Sodium channels which elicits an action potential in the postsynaptic neuron ●Inhibitory neurotransmitters bind to receptors and cause the opening of Potassium channels which hyerpolarize the cell membrane of the postsynaptic neuron

Learning Objectives … Again 1.Properties of the neuronal membrane and the role of ion movement in creating the membrane potential 2.Understand how an action potential is generated 3.Review the importance of the schwann cell and their role in myelin production and synaptic transmission. 4.Understand the components of a synapse and how neurotransmission occurs. 5.Identify types of excitatory and inhibitory neurotransmitters, including glutamate, GABA, norepinephrine, glycine, and acetylcholine. 6.Understand the role of a receptor and the different types