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Transcript
Biology 12 Name: ____________________ 12.2 Transmission of Nerve Impulses The nervous system uses the nerve impulse to convey information. The nerve impulse can be studied using excised axons and a voltmeter called an oscilloscope. • Voltage: measured in millivolts (mV); a measure of the electrical potential difference between two points • In a neuron, the two points are in the inside (axoplasm) and the outside of the axons • On a voltmeter, voltage is displayed as a trace (pattern) over time Resting Potential In an axon that is not conducting an impulse, the voltmeter records a potential difference across an axon membrane equal to -70mV. • This reading, known as the resting potential, shows that the inside of the axon is negative compared to the outside (there is polarity across the axonal membrane) • The resting potential is the potential difference across the membrane in a resting neuron The polarity of the resting axonal membrane is due to a difference in ion distribution on each side. • The concentration of Na+ is greater outside the axon than inside • The concentration of K+ is greater inside the axon than outside • This unequal distribution is maintained by carrier proteins called sodium-potassium pumps, which actively transport Na+ out of the axon and K+ into the axon o The pumps are always working because the membrane is permeable to Na+ and K+ o The membrane is more permeable to K+, therefore there are always more positive ions outside the membrane than inside o Negatively charged organic ions on the inside of the axon also contribute to the polarity across a resting axonal membrane Biology 12 Name: ____________________ Action Potential An action potential is a rapid change in polarity across an axonal membrane as the nerve impulse occurs. • An all-or-none phenomenon: if a stimulus causes the membrane to depolarize to a certain level (threshold), an action potential occurs • The strength of an action potential does not change, but an intense stimulus can cause an axon to fire (start an action potential) more often • Requires two gated channel proteins in the membrane: o One channel protein allows Na+ to pass into the axon o One channel protein allows K+ to pass out of the axon Action Potential: Sequence of Events Sodium Gates Open (Depolarization) • When an action potential begins, sodium channel gates open, and Na+ flows down its concentration gradient into the axon • As Na+ moves inside the axon, the membrane potential changes from -70 mV to +35 mV • This is called depolarization because the charge inside the axon changes from negative to positive Potassium Gates Open (Repolarization) • The potassium channel gates open, and K+ flows down its concentration gradient out of the axon • As K+ flows out of the axon, the action potential becomes more negative again (repolarization) o During this time, it briefly becomes slightly more negative that its original resting potential (hyperpolarization) Conduction of an Action Potential Action potentials in nonmyelinated axons • The action potential travels down an axon one small section at a time • When an action potential has moved on, the previous section undergoes a refractory period, during which the sodium gates are unable to open • The action potential cannot move backward; it always moves down an axon • When the refractory period is over, the sodium-potassium pump has restored the ion distribution by pumping Na+ out of the axon and K+ into the axon Biology 12 Name: ____________________ Action potentials in myelinated axons • The gated ion channels that produce an action potential are concentrated at the nodes of Ranvier • Ion exchange only occurs at these nodes, therefore the action potential travels faster than in nonmyelinated axons • The action potential “jumps” from node to node (saltatory conduction) Transmission Across a Synapse Every axon branches into endings that have a small swelling called an axon terminal • Each terminal lies close to the dendrite or cell body of another neuron or a muscle cell • This region of close proximity is called a synapse or chemical synapse o Membrane of the first neuron: presynaptic membrane o Membrane of the second neuron: postsynaptic membrane • Two neurons at a synapse do not physically touch each other; they are separated by a tiny gap called the synaptic cleft An action potential cannot cross a synapse. • Communication between two neurons at a chemical synapse is carried out by neurotransmitters (chemicals stored in the synaptic vesicles in axon terminals) • When an action potential arrives at an axon terminal: • Gated channels for Ca2+ open, and Ca2+ enters the terminal • Ca2+ interacts with contractile proteins, which contract and pull the synaptic vesicles to the presynaptic membrane • Rise in Ca2+ stimulates synaptic vesicles to merge with the presynaptic membrane, resulting in exocytosis • Neurotransmitter molecules are released into the synaptic cleft and diffuse across the synapse to the postsynaptic membrane, where they bind to specific receptors Biology 12 • Name: ____________________ Depending on the neurotransmitter, the postsynaptic neuron can either be excited (causing an action potential) or inhibited (stopping an action potential) Synaptic Integration A neuron may receive many excitatory and inhibitory signals since its dendrites and cell body can have synapses with many other neurons. • Excitatory signals: cause a depolarizing effect • Inhibitory signals: cause a hyperpolarizing effect Synaptic integration is the summing up of the excitatory and inhibitory signals in a postsynaptic neuron. • If the combined signals cause the membrane potential to rise above threshold, an action potential will occur Neurotransmitters Once a neurotransmitter has been released into a synaptic cleft and has initiated a response, it is removed from the cleft. • This prevents continuous stimulation (or inhibition) of postsynaptic membranes o In some synapses, the postsynaptic membrane contains enzymes that break down the neurotransmitter o In other synapses, the presynaptic membrane reabsorbs the neurotransmitter for repackaging in synaptic vesicles Many drugs that affect the nervous system act by interfering or enhancing the action of neurotransmitters. Drugs can: • Enhance or block the release of neurotransmitter • Mimic the neurotransmitter • Block the receptor for the neurotransmitter • Interfere with the removal of the neurotransmitter Example: • Sarin gas is a chemical weapon that inhibits acetylcholinesterase (AChE), an enzyme that is responsible for the breakdown of acetylcholine (ACh) o Leads to prolonged ACh activity (convulsive spasms)