Warm-Up What is an electrochemical gradient? In what organelles do we find these in a cell?

Learning Goals to determine how an impulse travels down an axon of a neuron to identify how release of neurotransmitters can stimulate an action potential in a neighbouring neuron

© Kristen Cinnamon, SCI5952 THE NERVE IMPULSE Purpose of the nerve impulse The nerve impulse is responsible for carrying a message through the nervous system via neurons Neurons send messages electrochemically, meaning that chemicals (ions) cause an electrical impulse. The nerve impulse relies on the electrical potential difference between the axon and the surrounding fluid

© Kristen Cinnamon, SCI5952 Electrical Potential Difference The inside of the axon contains a fluid similar to cytoplasm = axoplasm Membrane of the axon = axomembrane The electrical potential difference is the difference in charge between the axoplasm and the surrounding tissue fluids, on either side of the axomembrane

© Kristen Cinnamon, SCI5952 Electrical Potential Difference Charge Charge refers to the positive or negative charge given to ions after gaining or losing electrons Electrons are negative An ion is a charged atom Ex: An atom gains an electron it is negatively charged = negative ion (Cl - ) An atom loses an electron it is positively charged = positive ion (Na + )

Electrical Potential Difference Potential difference is measured in millivolts (mV) Also called an imbalance in voltage © Kristen Cinnamon, SCI5952

Resting Potential Refers to the electrical potential difference (polarity) that exists between axoplasm and the surrounding fluid when the axon is not conducting an impulse (the axon is at rest) Resting potential measures -65 mV The - 65 mV potential difference means that the inside of the axon, the axoplasm, is more negative compared to the outside Since the inside of the axon is more negative, it must have less positively charged ions present than the outside © Kristen Cinnamon, SCI5952

Polarity Polarity refers to a charge difference Specifically in neurons, there is a charge different between the axoplasm and surrounding tissue fluid, therefore we can say that the charge is polar © Kristen Cinnamon, SCI5952 Think of it like the north and south poles. They are opposite of one another, therefore polar, similar to the opposite charges between the axomembrane of a neuron.

© Kristen Cinnamon, SCI5952 Sodium-Potassium Pump Is responsible for maintaining the uneven distribution of ions across the axomembrane Always working unless a neuron is transmitting Main player in conducting a nerve impulse The ions involved are sodium ions (Na + ) and potassium ions (K + ) The axomembrane is more permeable to K + ions than Na + ions For every 2 K + ions that enter the axon, 3 Na + ions are actively pushed out of the axon

Sodium-Potassium Pump This creates a more negative environment inside the axon Na + ions will naturally leak back into the axon, but are quickly pushed back out via active transport Cl - also plays a role in conducting the nerve impulse Ions use channels to travel across the membrane called voltage-gated channels, meaning they can open or close depending on the voltage across the membrane – Sodium gate allows Na + ions to pass through the membrane – Potassium gate allows K + ions to pass through the membrane © Kristen Cinnamon, SCI5952

Sodium-Potassium Pump

© Kristen Cinnamon, SCI5952 Sodium-Potassium Pump

© Kristen Cinnamon, SCI5952 The Action Potential Occurs when a neuron is excited and sends a nerve impulse down the axon It is an electrochemical change that takes place across the axomembrane so it can be referred to as a change in polarity across the axomembrane A reversal of the resting potential occurs and the electrical potential difference goes from -65 mV to +40 mV

© Kristen Cinnamon, SCI5952 The Action Potential The action potential is an all-or-nothing phenomenon, which means that a stimulus must change the polarity of the membrane to a certain level in order for an action potential to occur The change in polarity of the axomembrane must reach a threshold Both voltage-gated channels are required to open for an action potential to occur

Steps during an action potential 1.Sodium gates open – Na + flows into the axon – Membrane potential changes from -65 mV to +40 mV – Called a depolarization due to the change in charges inside the axon from negative to positive – Now the outside of the neuron and the inside of the axon are both positive; therefore, there is no polarity anymore © Kristen Cinnamon, SCI5952

2.Potassium gates open – K + flows to the outside of the axon – As K + leaves the action potential changes from +40 mV back to -65 mV – Called a repolarization because the inside of the axon becomes negative again – Inside of the axon now resumes a negative charge © Kristen Cinnamon, SCI5952 Steps during an action potential

© Kristen Cinnamon, SCI5952 Propagation of an Action Potential The action potential travels down the axon The electrical current will affect the permeability of adjacent areas of the membrane allowing the process to reoccur like a domino affect A wave of depolarization and repolarization occurs Each preceding portion of the axon undergoes an action potential Once the action potential has moved on from an area of the axon, a refractory period occurs During a refractory period the sodium gates of the previous portion are unable to open, ensuring the action potential cannot travel backwards

© Kristen Cinnamon, SCI5952 The action potential always moves from the cell body down the axon towards the axon terminals/bulbs Voltage-gated channels are concentrated in the Nodes of Ranvier of myelinated axons The action potential jumps from one node to the next causing the action potential to occur quickly (200 meters per second) This is called salutatory conduction Reaction Time: the time it takes for a nerve impulse to travel from a receptor cell to the CNS and back to an effector Anywhere from 1 millisecond in a myelinated fibre to 120 milliseconds in an unmyelinated fibre Propagation of an Action Potential

VIDEO NVE NVE © Kristen Cinnamon, SCI5952

Transmission across a Synapse The action potential travels down the axon towards axon bulbs Axon bulbs lie close to the dendrites or cell bodies of another neuron, but do not touch The synapse is the junction between two neurons, including: – The membrane of the axon bulb, called the presynaptic membrane – The membrane of the dendrites, called the postsynaptic membrane – The gap in between, called the synaptic cleft

Transmission across a Synapse © Kristen Cinnamon, SCI5952

Transmission across a Synapse Neurotransmitter molecules are chemicals responsible for transmitting the nerve impulse across the synapse Neurotransmitters are contained in vesicles within the axon bulbs When an action potential reaches the axon bulb, calcium (Ca+) enters the bulbs via gated channels and triggers the synaptic vesicles to merge with the presynaptic membrane Once fused, the vesicles release the neurotransmitters across the synaptic cleft The postsynaptic membrane picks up the neurotransmitter signals and the nerve impulse continues in the next neuron

© Kristen Cinnamon, SCI5952

Neurotransmitter Molecules An impulse travels from one neuron to the next across the synaptic cleft with the aid of neurotransmitters The two most common neurotransmitter molecules are: Acetylcholine(Ach) and Norepinephrine (NE) Neurotransmitter molecules can either enhance or inhibit the transmission of nerve impulses After the neurotransmitters are released they initiate the impulse in the next neuron, they are then picked up by the membrane of the receiving neuron and destroyed to prevent the dendrites from being constantly stimulated

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Review Explain what the resting membrane potential is, and why it is significant to the functioning of neurons. Identify three factors that contribute to the resting membrane potential of a neuron. Summarize how the sodium-potassium pump contributes to the separation of charge and the resulting electrical potential difference across the membrane of a neuron. Draw diagrams that summarize the changes that occur in an axon as a nerve impulse is transmitted. Explain the importance of repolarization in the transmission of a nerve impulse. Tetrodotoxin is a neurotoxin found in puffer fish. This large molecule blocks the sodium channels in neurons. Infer the effect tetrodotoxin would have on the propagation of an action potential in a neuron.