Portage Learning>BIOD151 >1.4: Action Potentials, Study notes of Nursing

Download Portage Learning>BIOD151 >1.4: Action Potentials and more Study notes Nursing in PDF only on Docsity! Portage Learning>BIOD151 >1.4: Action Potentials Resting potential - how neurons are able to conduct neural/ electrical impulses Neurons: specialized to conduct electrical impulses Polarized: at rest, the plasma membrane has a different charge on the inside than the outside Resting potential: at rest, this charge is about -70mv (NEGATIVE) bc inside is more negative than outside Maintained by the sodium-potassium pump (exchanges Na and K within the same pump) gates allow to flow freely - active transport via THE SAME integral protein - Carries ions across the plasma membrane - Three sodium (Na+) ions pumped OUT - Two potassiums (K+) ions pumped IN - The 3:2 ratio makes the inside more negative compared to the outside of the cell. MUST BE MAINTAINED TO KEEP THAT DIFFERENCE IN CHARGE Sodium and potassium gates (different than pump) (normally closed and allow only sodium or only potassium) - enable action potentials (opposite of resting potential) - Special protein-lined channels with gates in the membrane - Allow sodium or potassium to pass through - Gates are voltage-activated and respond to changes in shape Action Potentials: rapid change in polarity across membrane Depolarization: the membrane potential becomes more + (inside more positive than outside) sodium rushes in - Once a cell body decides its going to move, it must travel the ENTIRE LENGTH of the axon!!! - self-propagating along the axon (once it starts it makes chain reaction) - ALL-OR-NOTHING response!! - The cell body has to decide when its going to send a signal and when its not. If the signal doesn’t meet the threshold, its fails - failed initiations. Action Potential is not responsible for determining intensity of sensation (number, and HOW FAST) Determining the intensity of sensation is not based on the action potential because the action potential is ALL OR NOTHING (how hard or sold were touched) - number of neurons stimulated (light tough very few neurons stimulated, hard touch more neurons stimulated) - How fast they’re being stimulated — Frequency with which neurons are stimulated (being touched a long time and its deep pressure, lots of neurons are firing quickly — brain saying large intense response) 1.4: Action Potentials Resting Potential Neurons are specialized to conduct electrical impulses called action potentials. The nerve impulse is an electrochemical charge moving along an axon created by the movement of unequally distributed ions on either side of an axon’s plasma membrane. At rest, the plasma membrane is said to be polarized, meaning that one side has a different charge than the other side. When the axon is not conducting an impulse, this difference in electrical charge is called resting potential, or the resting state of a neuron, and is equal to about -70mV (millivolts). The charge is negative because the charge on the inside of the axon's cell membrane is 70 millivolts less than the outside of the membrane (Figure 1.13). Figure 1.13 Resting State of a neuron is -70mv, meaning there is a net negative charge inside the cell. The resting potential is maintained by a sodium- potassium pump (Figure 1.14), which uses active transport to carry ions across the plasma membrane. The pump works by using an integral carrier protein that, for every three sodium (Na+) ions pumped out, two potassium (K+) ions are pumped in. The pump must keep in constant operation because the Na+ and K+ ions will naturally diffuse back to where they originated. Because the Figure 1.15 The resting potential becomes an action potential if the membrane becomes depolarized. Once an action potential occurs, it continues through the entire length of the axon. The action potential is due to special protein-lined channels in the membrane, which can open to allow either sodium or potassium ions to pass through. These channels have gates, called sodium gates and potassium gates. These channels and their gates are voltage activated, as proteins respond to changes in voltage with changes in shape. Follow along with Figure 1.16 and Figure 1.17 below to see the phases of an action potential step by step. • Phase 1 : Resting Potential: During the resting phase, both sodium and potassium gates are closed. • Phase 2 : Depolarization: The sodium gates open, and sodium rushes into the axon during the depolarization phase of the action potential. Voltage travels to zero and then on up to +40 mV. • Phase 3 : Repolarization: The sodium gates close, and potassium gates open allowing potassium to rush out of the axon. This returns a negative voltage to the inside of the axon • Phase 4 : Afterpolarization, also called hyperpolarization. Potassium gates are slow to close, and there is an undershoot of the potential. The voltage drops below -70mV and then returns to -70mV as the resting state begins. Figure 1.16 Graphic representation of the steps of an action potential. Figure 1.17 Cell membrane during steps of the action potential The action potential travels along the length of an axon like a wave. It is self-propagating because the ion channels are prompted to open whenever the membrane potential decreases (depolarizes) in an adjacent area. An action potential is an all-or- nothing response, either occurring or not. Since no variation exists in the strength of a single impulse, intensity of a sensation (minor or major pain) is distinguished by the number of neurons stimulated and the frequency with which the neurons are stimulated. Chemical Transmission of an Action Potential: An impulse passing from one nerve cell to another always moves in only one direction. There is a very short delay in transmission of the nerve impulse from one neuron to another because neurons do not touch. There is a minute fluid-filled space, called a synapse, between the axon terminal of the sending (presynaptic) neuron and the dendrite of the receiving (postsynaptic) neuron (Figure 1.18). The transmission of nerve impulses is electrochemical in nature as chemicals called neurotransmitters allow the signal to jump the synaptic gap (Figure 1.18). The signal moves from electrical (through the neuron) to chemical (in the synapse) to electrical again once the signal reaches the next neuron. When a nerve impulse reaches the end of an axon, voltage-gated calcium channels open. As calcium ions (Ca2+) rushes in, it causes vesicles containing the neurotransmitters to fuse with the plasma membrane and release the neurotransmitter into the synapse. When the neurotransmitter released binds with a receptor on the next neuron, sodium ion (Na+) channels in the receiving dendrites open. Depolarization occurs in the next neuron, and the impulse is propagated forward to another neuron or to a target organ, always in one direction. Figure 1.18 Neurons propagate electrochemical responses. Electrical responses are send along the length of the neurons, while chemical responses are sent in the synapse. Transmission of an action potential travels in one direction: from the presynaptic neuron to the postsynaptic neuron, with the synapse separating the neurons. Neurotransmitters are released by the presynaptic neuron into the synapse. Once a neurotransmitter has been released into a synapse, it has only a short time to act. Some synapses contain enzymes that rapidly inactivate the neurotransmitter. For example, the enzyme acetylcholinesterase, or simply cholinesterase, breaks down the neurotransmitter acetylcholine. In other synapses, the synaptic ending rapidly reabsorbs the neurotransmitter. Some neurons repackage the neurotransmitters in synaptic vesicles while others chemically breakdown the neurotransmitters. The short existence of neurotransmitters in the synapse prevents continuous stimulation of postsynaptic membranes. Prevention of continuous stimulation is called inhibition. Types of Neurotransmitters Norepinephrine and epinephrine are neurotransmitters produced by the adrenal glands. Dopamine is a specialized brain neurotransmitter to help regulate emotional responses and muscle tone. Acetylcholine is a neurotransmitter found at neuromuscular junctions (NMJ) in the peripheral nervous system. The NMJ is located where a motor neuron ends on a muscle instead of another neuron. For a muscle to contract, the nervous system must work together with the muscular system (see Figure 1.19). Figure 1.19 Representation of a motor neuron (orang) synapsing on a muscle fiber (red) at the neuromuscular junction. The Neuromuscular Junction The nervous system interacts with the muscular system at neuromuscular junctions to enable muscular contraction. First, a nerve impulse must be sent to the muscle by the presynaptic motor neuron. A neuromuscular junction is a special type of synapse formed between a motor neuron and muscle tissue. Once the nerve impulse reaches the muscle fiber (at the neuromuscular junction), acetylcholine is released into the synapse (see Figure 1.20). Acetylcholine binds to receptors on the muscle fiber that cause sodium channels to open. Sodium