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Physiology of Nerve Impulses

Anatomy Project Chapter 7: part 1
by

Christi Harrison

on 14 January 2013

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Transcript of Physiology of Nerve Impulses

(the study of how nerve impulses work) Physiology of Nerve Impulses Neurons have 2 main functional properties: These 6 steps described explain propagation of a nerve impulse along UNMYELINATED FIBERS 1. irritability- ability to respond to a stimulus and convert it into a nerve impulse
2. conductivity- ability to transmit the impulse to other neurons, muscles, or glands First, the resting membrane is polarized This means... Second, the stimulus initiates local depolarization Third, depolarization and generation of an action potential is activated This means... inactive (neuron is polarized)
fewer positive ions in the face of the neuron's plasma membrane than on it's outer face
positive ion inside the cell is potassium (K+)
positive ions outside the cell are sodium (Na+)
As long as the inside remains more negative than the outside, the neuron will stay inactive
Membrane is relatively impermeable to both ions Now we will consider these functional abilities A stimulus (light, sound, pressure, chemicals released etc.) changes the permeability of a local "patch" of the membrane
Sodium ions diffuse rapidly into the cell.
Because of this the polarity of the membrane is changed (outside becomes more negative than the inside of the membrane) at this site. If the stimulus is strong enough, depolarization causes the membrane polarity to be completely reversed and an action potential is initiated Fourth, Propagation of the action potential This means... Sixth, Initial ionic conditions are restored This means... Step 2 repeats rapidly along the entire membrane
This happens because the permeability of the first membrane patch causes permeability changes in the adjacent membrane, then that membranes causes changes in the one adjacent to that, and so on. Fifth, Repolarization takes place This means... Potassium ions diffuse out of the cell as the membrane permeability changes again
This restores the negative charge on the inside of the membrane
This occurs in the same direction as depolarization
*Until repolarization occurs, a neuron cannot conduct another impulse This means... The ionic conditions of the resting state are restored later by the activity of the sodium-potassium pump (which uses ATP)
Three sodium ions are ejected for every two potassium ions carried back into the cell So here is Irritability... And now here is
conductivity... How does the impulse travel from one neuron to another? The impulse doesn't! Instead, a neurotransmitter chemical transmits the signal. Now we will talk about how... Transmission of the Signal at Synapses First, Action Potential Arrives This means... when the action potential reaches an axon terminal, the electrical change opens calcium channels
Second, the vesicle fuses with plasma membrane This means... Calcium causes vesicles containing the neurotransmitter chemical to fuse with the axonal membrane.
Pore like openings form and release the transmitter Third, the neurotransmitter is released into the synaptic cleft This means... the neurotransmitter molecules diffuse across the synapse Fourth, the neurotransmitter binds to receptor on the receiving neuron's membrane Fifth, the ion channel opens This means... eventually there will be a nerve impulse in the neuron beyond the synapse Sixth, the ion channel closes This means... The neurotransmitter is broken down and released either by diffusion, reuptake, or by enzymatic breakdown
This limits each nerve impulse to a period shorter than a blink of an eye Keep in mind not only is the neuron receiving impulses, they are also sending them out, all at the same time!
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