Action Potentials and Neurons, Exams of Nursing

Download Action Potentials and Neurons and more Exams Nursing in PDF only on Docsity! Action Potentials BIOD-151 Portage Learning 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 Repolarization: everything goes back to normal (more positive inside again) potassium rushes out • Phase 1: Resting Potential: During the resting phase, both sodium and potassium gates are closed. -70mv • 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 (more negative!!!!) and then returns to -70mV as the resting state begins. P Graphic representation of the steps of an action potential. How do action potentials move? Figure 1.14 The resting potential is maintained by a sodium-potassium pump, which continually pumps sodium outside the cell and potassium inside. Action Potentials: Electrical Propagation If the axon is stimulated to conduct a nerve impulse, there is a rapid change in the polarity. This change in polarity is called the action potential. 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. First, the membrane potential becomes more positive (called depolarization), indicating that the inside of the membrane is now more positive than the outside. Then the potential returns to normal (called re-polarization), indicating that the inside of the axon is negative again (Figure 1.15). 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 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 rushes into the muscle cell, triggering an action potential, which reaches the sarcoplasmic reticulum. The sarcoplasmic reticulum is a specialized type of smoother ER found within striated muscle tissue. Calcium ions are released from the sarcoplasmic reticulum of the muscle cell, causing the muscle to contract. (see Figure 1.21) Figure 1.20 The NMJ Step 1: The action potential reaches the presynaptic motor neuron, releasing acetylcholine into the synapse.