054340%UOTTAWA@ICNUCEVM.CNUCE.CNR.IT (Matthew Simpson) (06/18/91)
Dear Friends,
Could anyone here direct me to the literature which
would tell me how far along a neural pathway an
impulse will travel as intensity varies?
I am interested in research which indicates that
impulses, resulting from stimuli of varying intensity,
will travel different distances along the auditory
pathway.
More simply put, do louder sounds travel further along
auditory pathways than sounds which are more quiet?
Thank you for your time and attention.
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======brp@dino.berkeley.edu (Bruce Raoul Parnas) (06/18/91)
In article <9106171949.AA20716@genbank.bio.net> 054340%UOTTAWA@ICNUCEVM.CNUCE.CNR.IT (Matthew Simpson) writes: >Dear Friends, >More simply put, do louder sounds travel further along >auditory pathways than sounds which are more quiet? Any sound which we perceive, whether loud or soft, must travel into the auditory cortex, hence all the way along the pathway. The intensity of the sound doesn't affect whether the signal is propagated. If it is transduced at the cochlea, it will be transmitted. Some neurons are level dependent, i.e. their discharge probability is a function of input intensity, but many are not. These will convey the signal (almost) independent of its intensity. bruce (brp@bandit.berkeley.edu)
slehar@park.bu.edu (Steve Lehar) (06/18/91)
The reason why the brain uses neural spiking, and encodes signal magnitude as spiking frequency is exactly to avoid the degredation with distance that is experienced by the alternative method of neural signaling, i.e. the density of ions of a particular charge. The ions, injected at the site of neural input must diffuse passively along the neuron, which works ok as long as they don't have to diffuse too far. When you get one of those neurons with an extremely long axon however, there may be little or no charge left by the time the signal gets to the end, so the signal decays with distance. In a spiking neuron, the diffusion must only travel the distance from the dendrites to the axon hillock. There, the ions either have enough charge density to trigger an action potential, or they don't. Once the action potential is triggered, it is guaranteed to travel the whole length of the axon, and since each spike is a complete depolarization of the membrane, there is no distinction between "weak spikes" and "strong spikes", all spikes are essentially the same. ========[ end of quick answer- beginning of more detail ]============= Here is a simplistic explaination designed to clarify the dynamics of neural firing without delving into deep technicalities. The sodium pump constantly and steadily pumps sodium (+) ions from inside the cell to outside, until a negative charge is built up inside the cell relative to the outside. There are a few passive channels around that allow some of the charge to leak back in at a rate proportional to the potential difference across the membrane, so that even though the pumps run continuously, the charge can never build up too great, but settles at some equilibrium value, where the rate at which the pumps pump it out is exactly balanced by the rate at which it flows back in through the passive channels. Electrically gated channels are also scattered about, and these will open if the membrane is DE-polarized, i.e. if the potential begins to break down, the electrically gated channels will make it break down even more. This creates an unstable situation, because a little local depolarization near an electrically gated channel, say, from a chemically gated channel that has just locked on to a transmitter molecule, will create a larger local depolarization. The electrically gated channel has a refactory period, so that it can only allow a little gulp of positive ions back into the cell before it slams shut again to recover. That gulp of ions diffuses outward, and what happens next depends critically on the density of electrically gated channels in the local viscinity. If the next one is too far away, then the charge will not be strong enough to trigger it, and the charge diffuses slowly in space and time. If enough of these events occur however, and close enough in time, then the total positive charge in the cell will become high enough to trigger even the more remote channels. Now the axon hillock is richly endowed with electrically gated channels in close proximity to each other, so that if a single one of these were to open, it will set off a cascade of channel openings that will flood the cell with positive charge in one great pulse. Now along the axon there are more ion pumps and electrically gated channels, (positioned at the nodes of Ranvier so that they have access to the extracellular environment) so that a similar event occurs all along the axon. You can see that a saturation event like this cannot occur half-way, either the system fires or it does not. At the output end of the neuron these spasms of depolarization trigger the release of pulses of transmitter which cause the injection of gulps of ions into the postsynaptic cell, thereby automatically performing a frequency - to - magnitude, or digital - to - analog conversion of the phasic pulsed signal into an "analog" magnitude of charge in the postsynaptic cell. -- (O)((O))(((O)))((((O))))(((((O)))))(((((O)))))((((O))))(((O)))((O))(O) (O)((O))((( slehar@park.bu.edu )))((O))(O) (O)((O))((( Steve Lehar Boston University Boston MA )))((O))(O) (O)((O))((( (617) 424-7035 (H) (617) 353-6741 (W) )))((O))(O) (O)((O))(((O)))((((O))))(((((O)))))(((((O)))))((((O))))(((O)))((O))(O)
brp@dino.berkeley.edu (Bruce Raoul Parnas) (06/19/91)
In article <SLEHAR.91Jun18084520@park.bu.edu> slehar@park.bu.edu (Steve Lehar) writes: > >The reason why the brain uses neural spiking, and encodes signal >magnitude as spiking frequency is exactly to avoid the degredation >with distance that is experienced by the alternative method of neural >signaling, i.e. the density of ions of a particular charge. > i think the question here is not one of active vs. electrotonic spread of electrical activity through nerves, but of information transfer. signals which are weak, i.e. produce only a few spikes, could lose their identity in background firing rates of subsequent neurons. thus, spikes still propagate rather than dissipating, but the signal is lost. it turns out that in the auditory system this is probably not the case. the preponderance of the noise is at the front end, and signals are actually refined by ensemble processing as they travel along. thus, any signal that is lucky enough to make it past the cochlea and get transduced, i.e. pulled out from the noise, will very likely make it to the auditory cortex. >(O)((O))((( slehar@park.bu.edu )))((O))(O) >(O)((O))((( Steve Lehar Boston University Boston MA )))((O))(O) bruce (brp@bandit.berkeley.edu)
tbd@neuro (Tristan Davies) (06/21/91)
>>More simply put, do louder sounds travel further along >>auditory pathways than sounds which are more quiet? > >Any sound which we perceive, whether loud or soft, must travel into the >auditory cortex, hence all the way along the pathway. The intensity of the >sound doesn't affect whether the signal is propagated. If it is transduced >at the cochlea, it will be transmitted. Some neurons are level dependent, Absolutely correct!! >i.e. their discharge probability is a function of input intensity, but many >are not. These will convey the signal (almost) independent of its intensity. > > >bruce >(brp@bandit.berkeley.edu) Thank you for the simplest, most elegant answer. I have an additional fact which y'all might find interesting. The range of sensitivity of an auditory neuron is measured by a *tuning curve*, which is a graph of sound frequency (x axis) vs. intensity to cause firing (i.e., threshold) (y-axis). When recording the activity of single auditory neurons, physiologists find that most neurons have a tuning curve which is roughly V-shaped, indicating that the neuron has the lowest threshold at a single frequency and its ability to respond to a pure tone decreases as the frequency of that tone is farther from the preferred frequency of the neuron. Get it? Here's the neat thing: some neurons have **circular tuning curves**! That is, they respond only to a narrow range of both frequency and intensity. While some neurons prefer loder noises, there are also neurons that prefer soft sounds, and will not fire in response to a loud sound, even if that sound is at the preferred frequency. Thus the loudness of a sound is probably encoded in *which* neurons fire more than the rate at which they fire. BTW, I have encountered a couple of these neurons during a lab rotation where I recorded from the inferior colliculus in bats, so I'm fairly certain they exist... Hope this helps! Tristan Davies Dept. of Neurobiology, Duke Univ. Go Blue Devils!!! e-mail: tbd@neuro.duke.edu "The brain is truly an impressive organ. It starts working the instant we get up in the morning and doesn't stop working until we get to the office." --paraphrased from an unknown source