Transcript SBI4U - 9.2
Chapter 9.2: Electrochemical Impulse Pages 418-426 Recap • • • • The Importance of the Nervous System CNS (Brain and Spinal Cord) PNS (Somatic and Autonomic Nerves) Anatomy of a Nerve Cells (Dendrites, Axon, Cell Body, Nodes of Ranvier etc) • Types of Neurons (SIM) • Reflex Arc (5 steps) The Impulse 18th century: • Galvani realized that the leg muscle of a dead frog could be made to twitch electrical stimulation • Lead to scientists questioning how electric current is generated in the body • Electrical vs. Neural transmission Current in Wire, Current in Nerves There are differences between current in wire and those that travel through nerves: 1.Current in a wire travels much faster 1.The cytoplasmic core of a nerve cell offers great resistance to the movement of electric current 1.Electric currents diminish as they move through a wire Current in Wire, Current in Nerves 1. Nerve impulses remain as strong at the end as they were at the beginning of the impulse 1. Nerves cellular energy to generate current Electric conductor An external force to push the current through the conductor Figure 2. page 418 Placed an electrode inside a large nerve cell - 70 mV + 40 mV • The resting membrane normally had a potential near – 70 mV and registered + 40 mV when the nerve became excited • The voltage difference across a nerve cell membrane during the resting stage is called the resting potential • The reversal of potential is described as an action potential – the voltage difference across a nerve cell membrane when the nerve is excited How do nerve cells become charged? • Molecular level nerve cells have a rich supple of + and – ions both inside and outside of the cell • negative ions stay inside the cell • positive ions move and cause an unequal concentration 50x more permeable to potassium • The membranes become charged due to the movement of positive ions • Potassium is high inside, and sodium is high outside • As potassium diffuses out, sodium diffuses in The Resting Membrane Rapid diffusion of K+ ions nerve cells loses a greater number of positive ions than it gains Exterior membrane is + compared to interior The resting membrane: • Charged • Called a polarized membrane unequal distribution of + ions • Difference between + ions inside nerve membrane relative to outside = -70 mV When the nerve gets excited… • Nerve cell membrane more permeable to Na+ than K+ • Na+ ions rush into nerve cell through diffusion and charge attraction • Sodium inflow causes charge reversal depolarization • A sodium-potassium pump restores the original polarity of the nerve membrane repolarization When the nerve gets excited… Resting membrane Depolarization Repolarization Sodium-Potassium Pump (Repolarization) Active Transport • Nerve cells conducting an impulse cannot be activated until resting membrane condition is restored • Once depolarization is complete, nerves must repolarize before next action potential takes place • Time required to become repolarized refractory period (1-10 ms) Movement of the Action Potential • For the impulse to be conducted on the axon, the impulse moves from zone of depolarization to adjacent areas Action Potential • The flow of positively charged ions from the area of the action potential toward the adjacent regions of the resting membrane causes a depolarization in the adjoining area • This creates an electric disturbance, which causes adjacent sodium channels to open • The result is a wave of action potential that moves along the cell membrane • The wave of action potential and depolarization is followed by a wave of repolarization Action Potential Threshold Levels: All or None • Minimum level of a stimulus required to produce a response Figure 8. page 422 Threshold Levels: All or None • Increasing the intensity of the stimulus above critical threshold does NOT produce an increased response • Neurons fire maximally or none at all all-ornone response Detecting Intensity 1. The more intense the stimulus the greater the frequency of impulses Warm rod: lower frequency of impulse sent to brain Hot rod: higher frequency 2. Different threshold levels of neurons Synaptic Transmisson • Synapse: regions between neurons, or between neurons and effectors • A single neuron may branch off and join with many different neurons • Involves neurotransmitters: chemicals release from vesicles to synapses • Presynaptic neuron: neuron that carries impulses to the synapse • Postsynaptic neuron: neuron that carries impulses away from the synapse Synaptic Transmisson • Neurotransmitters released from presynaptic neuron, diffuse across the synaptic cleft, create a depolarization of dendrites of the postsynaptic neuron Acetylcholine • An example of an excitatory neurotransmitter • Acts on many postsynaptic neurons by opening the sodium ion channels Causes depolarization Cholinesterase • An enzyme that breaks down acetylcholine to reverse depolarization • Once acetylcholine is destroyed, Na+ close neuron begins recovery phase Any Questions?