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Chapter 9 Nervous System

Chapter 9 Nervous System. 9.6-9.9. 9.6. Cell Membrane Potential. Resting Neuron. Cell membrane is electrically charged ( polarized ) Caused by unequal distribution of positive and negative ions between sides of the membrane Important to conduction of nerve impulse

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Chapter 9 Nervous System

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  1. Chapter 9 Nervous System 9.6-9.9

  2. 9.6 Cell Membrane Potential

  3. Resting Neuron • Cell membrane is electrically charged (polarized) • Caused by unequal distribution of positive and negative ions between sides of the membrane • Important to conduction of nerve impulse • Action potential – change in membrane polarization and return to resting state

  4. Distribution of Ions • More sodium ions (Na+) outside the cell and more potassium ions (K+) inside the cell • Cytoplasm has many large, negatively charged ions (can’t diffuse through the membrane) • K+ can travel through the membrane more easily than Na+ • It is even most difficult for Ca+

  5. Resting Potential • K+ diffuses out of the cell faster than Na+ can diffuse in • Results in slightly (+) charge outside the cell and slightly (-) charge inside the cell • Difference in electrical charge is potential difference (called resting potential in a resting nerve) • Concentration gradient maintained by Na+/K+ pumps

  6. Potential Changes • Nerve cells are excitable (detect changes in temp., light, or pressure) or react to neurotransmitters • Cause membrane to be depolarized (inside membrane becomes less negative) • Local potential changes are graded (greater stimulus = greater depolarization)

  7. Potential Changes • Threshold potential – occurs when neurons are depolarized sufficiently (-55 millivolts) • Action potential – results when threshold is reached and is the basis for the nerve impulse

  8. Action Potential • Threshold potential changes membrane permeability • Na+ channels open (diffuses inward) • Membrane loses negative charge (depolarized) • K+ also moves out, returning negative charge • Hyperpolarization – membrane potential becomes overly negative • Repolarization – return to resting potential

  9. 9.7 Nerve Impulses

  10. Nerve Impulses • An action potential in one region of them membrane causes a bioelectric current to flow to adjacent portions (triggers more action potentials) • Nerve impulse = wave of action potentials moving down an axon to the end

  11. Impulse Conduction • Unmyelinated axon conducts impulse over entire surface • Myelin insulates and prevents almost all ion flow • Myelin sheath would prevent the conductions of impulses at all if it were continuous, however it is interrupted by the nodes of Ranvier • Saltatory conduction makes the impulse jump from node to node (faster) • Bigger diameter = faster impulse

  12. All-or-None Response • If a neuron responds at all, it will respond completely • Impulses carried through an axon are of the same strength once threshold is reached • Greater intensity of stimulation = more impulses per second • Refractory period = for a very short time following a nerve impulse, a threshold stimulus will not trigger another impulse • Ensures the impulse proceeds in only on direction

  13. 9.8 Synaptic Transmission

  14. Excitatory and Inhibitory Actions • Excitatory neurotransmitters - increase postsynaptic permeability to Na+ and bring membrane closer to threshold, triggering nerve impulses • Inhibitory neurotransmitters – decrease the chance that a nerve impulse will occur • Thousands of synaptic knobs communicate with the dendrites of a single cell and may release either excitatory or inhibitory neurotransmitters • More excitatory neurotransmitters = threshold reached • More inhibitory neurotransmitter = threshold not reached

  15. Neurotransmitters • Acetylcholine – stimulates skeletal muscle contraction • Monoamines – epinephrine, norepinephrine, dopamine and seratonin • Amino acids – glycine, glutamic acid, aspartic acid, and GABA • Neuropeptides • Synthesized in cytoplasm and stored in synaptic vesicles

  16. Neurotransmitters • Action potential increases membrane’s permeability to Ca+ and it flows inward • Causes synaptic vesicles to fuse with the membrane and release contents into synaptic cleft • Some neurotransmitters and decomposed by enzymes while others are transported back into the synaptic knob for reuse (reuptake) • Prevents continuous stimulation

  17. 9.9 Impulse Processing

  18. Neuronal Pools • Groups of neurons that make hundreds of synaptic connections • Work together to perform a common function

  19. Facilitation • A single neuron may receive excitatory and inhibitory input • If the net effect is excitatory but subthreshold, an impulse is not triggered, but the neuron is more excitable to incoming stimulation than before

  20. Convergence • Nerve impulses from two or more axons arriving at the same neuron • Have an additive effect • Brings info from several sensory receptors, processes it, and allows it to respond in a special way

  21. Divergence • Impulses leaving a neuron pass into several other output neurons • Amplifies and impulse • From one neuron in the CNS to several in the skeletal muscles, allowing for contraction

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