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Biology 30: Unit A The Nervous and Endocrine Systems Learning Objectives Formation and transmission of action potentials along an axon All-or-none Threshold Intensity response level of response The Action Potential What is it? An action potential occurs when there is a reversal of charge along the axon membrane of a neuron Resting Potential As with all cells, the neuron contains a plasma membrane Looking closely at the membrane, we see that there is a high concentration of sodium (Na+) outside the membrane and a high concentration of potassium (K+) inside the membrane Resting Potential Diffusion The plasma membrane of the neuron is selectively permeable – ions cannot just simply pass through the membrane, they need assistance. This assistance is met through the use of Na+ and K+ gated ion channels Diffusion There is a larger number of K+ ion channels along the membrane than Na+ channels More K+ is leaving the cell than Na+ entering which gives the membrane a more positive charge outside and a more negative charge inside The Resting Membrane The resting membrane is also called a polarized membrane because it is charged by unequal distribution of positive ions along the membrane The membrane is considered to be resting when it shows an electrical charge of -70mV The Action Potential There are 4 stages to an action potential: Depolarization Repolarization Hyperpolarization Refractory Period Depolarization This is the process by which there is a rush of sodium ions coming into the cell which causes a change in the charge of the cell. The inside of the cell membrane is now more positive than the outside Repolarization As soon as the charge changes, the Na+ gates close and the K+ gates open This eventually allows the charges to reverse again and bring the membrane back to its original state Hyperpolarization K+ gates take longer to close though which causes even more ions to move out of the cell leading to an even more positive extracellular environment The Na+/K+ pump uses active transport to bring the proper amount of K+ back into the cell Refractory Period This is the time it takes to for the cell to reach its resting membrane state This can last 1 – 10ms How does an Action Potential Move? A single action potential does not move on its own down the axon of a nerve cell. There are multiple action potentials occurring one after another to transmit the information There is a kind of domino effect How does an Action Potential Move? Remember that the nerve impulse goes from one end of the cell to the other and never backwards This If is all due to the action potential you look behind the action potential, the axon is in the depolarization stage and therefore cannot begin a new action potential Axons and Impulses When an axon is myelinated, the impulses travel by saltatory conduction This means that action potentials only occur at the nodes of Ranvier This greatly increases the speed of the nerve impulse along the axon because there is less space for the action potentials to happen Threshold Level Before an action potential can occur, it needs to reach the membrane’s threshold level This is the minimum level of stimulation required to get a response If the stimulus is too low, there will be no action potential All – or – None Response Once a membrane has reached its threshold level, the action potential either will happen completely or not at all It doesn’t matter how much you increase the stimulus, the action potential will give the exact same response All – or – None Response Threshold Levels Con’t Not all threshold levels are the same for every nerve cell This allows for us to sense many different stimuli in our environment Example: warm vs. hot objects Learning Objectives Transmission Main of a signal across a synapse chemicals and transmitters involved in synaptic transmission Synaptic Transmission The area between the end plate of one neuron and the cell bodies or dendrites of another neuron is called a synapse This is where neurons transmit information to one another through the use of chemicals called neurotransmitters Synaptic Transmission The presynaptic neuron carries the nerve impulse to the synapse The postsynaptic neuron carries the nerve impulse away from the synapse Synaptic Transmission Neurotransmitters are released from the presynaptic neuron into the synaptic cleft These neurotransmitters diffuse across the synapse to receptor sites on the postsynaptic neuron This creates a depolarization effect leading to an action potential in this neuron Synaptic Transmission Diffusion is a slow process so that is why certain neurons have very few synapses (ones involved in a reflex arc) while others have many more (problem solving neurons) Neurotransmitters What is a neurotransmitter? What is an example of a neurotransmitter and what does it do? Talk to the person beside you. Neurotransmitters These are the chemicals that are released from the presynaptic neuron They are the messengers that carry information from one neuron to the next There are many different kinds of neurotransmitters that each have their own ‘job’ to do Excitatory Neurotransmitters Acetylcholine is an excitatory neurotransmitter This means that it opens sodium ion channels stimulating an action potential One major effect of acetylcholine is the contraction of skeletal muscles Cholinesterase Cholinesterase acetylcholine This is an enzyme that destroys allows the sodium channels to close and the neuron to enter the repolarization stage Other Excitatory Neurotransmitters Norepinephrine wakefulness Dopamine Voluntary movements and emotion Inhibitory Neurotransmitters These neurotransmitters prevent action potentials from occurring by opening potassium gates leading to a state of hyperpolarization Inhibitory Neurotransmitters Norepinephrine Wakefulness Serotonin Sleep GABA Motor behaviour Inhibitory Neurotransmitters These neurotransmitters allow you to prioritize information This allows you to focus on important information and ignore other stimuli around you Summation Remember back to our action potential lecture and think about threshold levels. Each neuron needs to reach a certain voltage before an action potential can occur Some neurotransmitters will not achieve this threshold level when acting alone Summation To reach the threshold level, there needs to be a summation of the neurotransmitters Just as the word sum means, you add the reactions of two or more neurotransmitters to achieve the appropriate threshold level