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Neural Circuits

Neural Circuits. 100 billion neurons @ 1000 synapses / neuron = 100 trillion (10 14 ) connections In the understanding of neural circuits there is plenty of room for complexity and uncertainty. Neural Circuits. basic “wiring” patterns not complete circuits in the electrical sense

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Neural Circuits

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  1. Neural Circuits 100 billion neurons @ 1000 synapses / neuron = 100 trillion (1014) connections • In the understanding of neural circuits there is plenty of room for complexity and uncertainty.

  2. Neural Circuits • basic “wiring” patterns • not complete circuits in the electrical sense • typical pattern is an arc sensory afferents  integration in CNS  motor efferents • integration • The CNS not only links sensory and motor information, but also interprets the various inputs and determines what sort of motor signal(s) to send out. This “interpreting” typically requires no conscious perception or thought. • Some signals are amplified, but > 99% of information received is filtered out and ignored.

  3. Neural Circuits • Interpretation occurs at neuronal pools (“relay stations”) where signals pass from one set of cells to another. • These relay stations are anatomically distinct. For example: • cerebral cortex • gray matter of spinal cord • nuclei • ganglia Guyton, Medical Physiology

  4. What happens to signals at neuronal pools? • divergence • convergence Fig. 12.30

  5. Diverging Circuits - Facilitation • discharge zone • receiving cells stimulated above threshold and fire action potentials • facilitated zone • receiving cells stimulated below threshold (subliminal stimulation) • Because of facilitation, it will be easier for an additional stimulus to cause the receiving cell(s) to fire action potentials. Fig. 12.29

  6. Diverging Circuits – Lateral Inhibition • lateral inhibition • stimulation in the central pathway • inhibition in lateral pathway • due to the presence of an inhibitory interneuron (#2) • Input fiber stimulates cell #2, which then releases an inhibitory neurotransmitter from its axon terminal, which inhibits cell #3. • allows for a sharp dividing line between ON and OFF portions of the pathway Inhibitory interneuron Guyton, Medical Physiology

  7. Nerve Fiber Classification • Nerve fibers are classified by speed of conduction. • increased speed of conduction  fiber diameter myelination • insulation • saltatory conduction

  8. Two Classification Schemes“ABC” scheme: for both sensory and motor neurons Table 2-1 Ganong • Types A - A (myelinated fibers) • motor neurons to skeletal muscle: A and A • Type B (myelinated) • Type C (unmyelinated)

  9. Two Classification Schemes“I,II,III,IV” scheme: sensory neurons only Table 2-2 Ganong • Types I, II and III (myelinated) • Type IV (unmyelinated)

  10. The “Ferrari Principle” • Why not have all fast fibers? Why are some fibers slow? What’s the cost for speed? • 2/3rds of PNS fibers are type IV fibers. • small, unmyelinated • type C fibers • dull pain, temperature, itch, tickle

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