Action Potential 2021 PDF

Action Potentials Nerve response to stimuli Receptors Sensory neurons contain receptors on their ends, often linked to collagen fibre networks that have ion channels nearby These sensory receptors detect changes in the environment and trigger a wave of depol...

Action Potentials Nerve response to stimuli Receptors Sensory neurons contain receptors on their ends, often linked to collagen fibre networks that have ion channels nearby These sensory receptors detect changes in the environment and trigger a wave of depolarization, called an action potential This is the electrochemical message transmitted through nerve fibres  Cutaneous Receptors Mechanoreceptors From Neuron to Neuron The Neuron Transmits signals sent in the form of electrochemical impulses in in ONE direction The axon sends a wave of depolarization along its length The wave of depolarization is primarily the movement of Na+ and K+ ions across the axon’s membrane The Neuron at Rest The outside of the neuron is more positively charged than the inside (outside: ↑ Na+, ↓K+; inside: ↓Na+, ↑ K+) A resting membrane potential of -70 mV exists All-or-None Response The All-or-None Response Sensory neurons can be stimulated by chemicals, light, heat, disruption of the membrane, or by electrical currents A minumum level of the stimulus must exist before the neuron fires an impulse; this is called the threshold level Once the threshold level is reached, going above it does not change the strength of the response, it is uniform along the length of the axon No such thing as a mild or strong response, it will either respond or not; this is called the all-or-none response How Can We Tell The Difference Between Warm and Hot? The more intense a stimulus, the greater the frequency of impulses warm 🡪 slower frequency hot 🡪 higher frequency Each neuron has a different threshold level (more neurons firing 🡪 greater number of impulses to the brain 🡪 greater intensity of the response) Depolarization When a nerve cell becomes excited by a stimulus, the gates of the Na+ channels in the membrane open and sodium rushes into the axon Because the gates of the K+ ion are closed, this inflow of Na+ creates a charge reversal (more positive inside) where the membrane potential becomes approximately + 30 mV This is called depolarization The depolarizaton of one part of the axon causes neighbouring Na+ channels to open, and this depolarization continues along the length of the axon Repolarization Depolarization occurs for a split second in any region of the axon Immediately after it occurs, Na+ channels close shut, and K+ channels open to allow for K+ to move outside of the axon This occurs to allow for the membrane to be restored to its resting potential Repolarization A sodium-potassium pump in the membrane also helps to restore the resting membrane potential ATP is used to actively transport 3Na+ out for every 2K+ in Nerves conducting an impulse can’t be activated again until the membrane is repolarized 🡪 the time in between is called the refractory period Action Potential and the Wave of Depolarization https://www.youtube.com/watch?v=iBDXOt_u HTQ&t=150s Movement of the Action Potential The wave of depolarization moves along the entire length of a neuron and the strength does not weaken The wave of depolarization is followed by a wave of repolarization Graph: Action Potential Graph: Action Potential https://www.youtube.com/watch?v=b2ctEsGEp e0 Overview of the Action Potential Summary of Steps Resting membrane -70 mV Stimulus causes sodium channels to open, 🡪 sodium rushes into axon 🡪 depolarization occurs (+30 mV) Very quickly, sodium channels close, potassium channels open 🡪 K+ diffuses out of axon to help repolarize the membrane Na+/K+ pump helps to bring the membrane back to resting potential (3Na+ out / 2K+ in) Meanwhile, wave of depolarization travels down the axon, maintaining its strength the whole way Homework Read section 9.2 and answer questions # 1- 8, and 10 on page 426. Synaptic Transmission Tiny gaps between neurons are called synapses spanning a distance of approximately 20 nm. Message is carried from the pre-synaptic neuron across the gap to the post-synaptic neuron What are Neurotransmitters? Chemicals released by nerve endings that diffuse across the synaptic cleft and bind to receptors on the post – synaptic membrane They are contained in spherical, membrane bound vesicles 🡪 once released they diffuse across the cleft 🡪 after binding to receptors they can be degraded or re-used. Approximately 100 different types, many are amino acid derivatives or peptides (e.g: glutamate, endorphins) What are Neurotransmitters? They can excite or inhibit the post-synaptic neuron – Excitatory: Cause Na+ channels to open in the post synaptic membrane which initiating a wave of depolarization – Inhibitory: Prevent an action potential from occurring by opening K+ channels (hyperpolarization) therefore no wave of depolarization occurs – Some neurotransmitters can both excite or inhibit, depending on the receptor it binds to Excitatory and Inhibitory Neurotransmitters The interaction of excitatory and inhibitory neurotransmitters is what allows you to throw a ball… As the triceps muscle receives excitatory signals and contracts, the biceps muscle receives inhibitory signals and relaxes This way the two muscles do not pull against each other How Do Neurotransmitters Work? Acetylcholine Acetylcholine – excitatory neurotransmitter Used at the neuromuscular junction (motor neurons release this to activate muscles) Degraded by the enzyme acetylcholinesterase Drugs or substances that interfere with cholinergic systems can have dangerous effects 🡪 convulsions, paralysis Making Connections (HW) 1. Research how pain relievers (like Advil) work. 2. The action of many psychoactive drugs can be explained in terms of neurotransmitters. LSD and mescalin, both hallucinogenic drugs, are though the interact with the receptor sites of serotonin. Research how.