Transmission of Nerve Impulses

1. Transmission of Nerve Impulses

2. Neurophysiology • The measure of the potential energy of separated electrical charges is called voltage (V) or potential. • Current (I) is the flow of electrical charge from one point to another. • Resistance (R) is hindrance to flow • Ohms Law: V = I * R

3. Neurophysiology • In the body, electrical charges are provided by ions, cellular plasma membranes provide resistance to flow. The membranes contain passive (open) and active (gated) channels.

4. Resting Potential The Neuron is Polarized • The Sodium Potassium pump is pumping Na+ out of the neuron and K+ into the neuron • K+ is leaking out of the neuron • Therefore the inside of the cell is negatively charged and the extra-cellular fluid is positively charged

5. Resting membrane potential • A resting neuron exhibits a resting membrane potential, which is -70 mV, which is due to differences in sodium and potassium ion concentrations inside and outside of the cell. • The ionic difference is the result of • Greater permeability of the membrane to potassium than to sodium • Sodium-potassium pump: 3 sodium out for 2 potassium in

7. Initiation of a Nerve Impulse • Generally the Sodium Gates on a neuron are CLOSED. • A Nerve Impulse STARTS when a Sensory Input, Disturbs a Neuron's Plasma Membrane, causing Sodium Gates to OPEN allowing Na+ ions to flow INSIDE the Cell Membrane. • This causes the inside of the cell to become more positive than the extracellular fluid

8. Action Potential • A large, but brief depolarization signal that underlies long distance neural communication.

9. Signal Conduction • A signal has to travel to the end of the axon if a neuron is to communicate with another cell

10. Unmyelinated Signal Conduction • Action potential versus a nerve signal • Example of dominos • Nerve signal cannot travel backwards • Because the membrane behind the nerve signal is in its refractory period and cannot be restimulated • Signal does not grow weaker as it travels • i.e. – fuse burns just as hot at the beginning as it does at the end

11. Myelinated Signal Conduction • Signal jumps from node to node • At the node of Ranvier the gates open to allow another action potential to occur • Signals weaken as they travel, but when they get to the node of Ranvier the signal is boosted back to its original strength

12. Neural Integration and Coding • Neural Integration • Information and decision making • Neural coding • Interpretation and passing on of information • Stronger excitation = more recruitment of additional neurons • Refractory period – creates a limit on how often a neuron can fire

13. Repolarization • As the impulse passes, the Potassium Gates OPEN, allowing positively charged K+ ions to FLOW OUT.  The inside of the axon resumes a NEGATIVE CHARGE. • Once again NEGATIVELY Charged on the INSIDE and POSITIVELY Charged on the OUTSIDE.

14. Refractory Period • after a nerve impulse is period when the neuron is unable to conduct a nerve impulse • is a very short period during which the sodium-potassium pump continues to return sodium ions to the outside and potassium ions to the inside of the axon, THUS RETURNING THE NEURON TO RESTING POTENTIAL.

15. The Synapse • The Small Gap or Space between the axon of one neuron and the dendrites or cell body on the next neuron or the point where impulses are passed from one cell to another • Neurons that transmit impulses to other neurons DO NOT actually touch one another.  • The synapse ensures one-way transmission of impulses.  • A nerve impulse CANNOT go backward across a Synapse.

16. Neurotransmitters • Chemicals contained in tiny vesicles within the axon terminals • Are used to signal other neurons • When an impulse reaches the Axon Terminal, neurotransmitters are discharged into the Synaptic Cleft. • molecules of the neurotransmitter diffuse across the gap and attach themselves to SPECIAL RECEPTORS on the membrane of the neuron receiving the impulse • When the neurotransmitter becomes attached to the cell membrane of the adjacent nerve cell, it changes the permeability of that membrane (like a stimulus).