(Transcribed from Dr. Glasser’s lecture, 19 May 2000 by Brian Buschman)
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Before getting into the material from this lecture there was one chemical effecter that he forgot to mention in the last lecture, organophosphates. This class of chemicals bind to and then destroy ACh esterase. They are used as insecticides and as agents of war. Serin was the first and there was a period of a number of minutes between the time of binding and cleavage when an antidote could be administered to reverse the action. Thanks to Sadam Hussein there are new and improved organophosphates that act in less then a min so unless you took the antidote before being exposed you will experience effects from excess ACh stimulation and die.
A single neuron receives more then a thousand synapses. Each may be either EPSP or IPSP. The depolarizations migrate to the initial segment where the AP begins. Because the signal is carried to the initial segment as a diffusion of ions the signal is greatly weakened by the time it reaches the initial segment. Because of this a distant synapse will have a smaller effect then a proximal one.
It takes many, many depolarizations for threshold to be reached at the initial segment. Their depolarizations can be added together either through temporal or spatial summation. Spatial summation is when two or more EPSPs or IPSPs occur at about the same time on different locations in the dendrite system. As the ions flow to the initial segment together the depolarization is a function of the sum of the potentials created by the different stimuli. Temporal summation the result of summation of currents generated at a single spot across the course of a small time. This is possible because each depolarization lasts for a short time period.
The initial segment only requires about a +12 mV depolarization to initiate an AP while it requires about +15mV to do so in the axon.
When the AP begins the Na+ will take the membrane potential (Vm) to spike potential and then inactivate the Na+ channels for a period of time. During this time regardless how much depolarization you have it is impossible to start a new AP. This is called the absolute refractory period.
At the low point on the graph when K+ has hyperpolarized the membrane the Na+ channels are reactivated again. They can begin a new AP but require a greater depolarization due to the hyperpolarization. This is called the relative refractory period.
All depolarizations that trigger APs last for a given period of time which allows them to trigger multiple APs. If the depolarization is large enough to trigger a new AP during the relative refractory period it will create a firing pattern with a high frequency and a small interspike interval. If the depolarization is large enough to trigger a new AP but not large enough to do it during the relative refractory period then it will generate a firing pattern with a lower frequency and a larger interspike interval.
A single AP really means nothing in the nervous system. It’s all about firing rates.
Divergence is how a neuron is able to divide it’s axon to synapse with neurons in different places or even itself.
The whole spike potential of the AP takes about 2 ms while the relative refractory period takes about 20-30 ms.
First we have a few new terms that we need to know about the direction of AP propagation:
Normally you can initiate an AP at the initial segment and it will only cause an orthodromic AP. If you start it towards the middle or the end it will propagate both orhodromically and antidromically.
When you fire an AP Ca2+ will be allowed to flow in to trigger the release of the NT. When this happens it takes time for the Ca2+ pump to remove the Ca2+. If another AP is fired before all the Ca2+ is pumped back out then when the new Ca2+ is allowed in then there will result in a greater amount of Ca2+ in the terminal membrane. This will cause a larger amount of NT to be released.
This says that neurons that are used frequently will send larger signals then ones that are seldom used. It is thought that this mechanism plays a large role in short term memory.
Pre-synaptic facilitation is one of the two main types of axoaxonic synapses. It will cause a decrease in PK+, which will cause the resting potential to be higher. IT is higher because with less PK+ the resting Vm will be closer to the Na+ resting potential therefore being slightly less negative. At the same time when the AP fires it will cause the spike potential to be closer to the actual Na+ resting potential and therefore be higher. When the spike potential is higher then more Ca2+ will be allowed in at the terminal membrane and there will be a larger release of NT. The effects of pre-synaptic facilitation last for about a minute so it will effect many, many APs.
Pre-synaptic inhibition is a second type of axoaxonic synapse. It causes the opening of Cl- channels to allow Cl- to hyperpolarize the membrane near the distal part of the axon. This increases the Vm requiring much more signal for the next segment to depolarize to threshold. It is extremely inhibitatory and functions in analgesia.
Some dendrites interact with each other via regions that have vesicles that they release onto other dendrites. When one releases the NT onto the other then the receiver will release NT back onto the first. This is only known to function in olfaction which we will discuss later.
These are axodendritic synapses that release NT in the normal way but when the dendrite receives the NT it releases NO on CO back onto the terminal membrane. This is a way for the dendrite to as for more NT.
A skeletal muscle call is very long, multinucleated and just plain big. IT is innervated at only one point called the neuromuscular junction. The neuron which innervates this is called a lower motor neuron or an a-motor neuron. A lower motor neuron will innervate multiple muscle fibers. The group of muscle fibers innervated by a single neuron and that neuron itself are together called a motor unit. Motor units are often found to be interdigating.
There are two ways to increase the strength of a muscle contraction:
1) Recruit more motor units.
2) Increase the force of contraction of the given motor units.
Intrafusal fibers are fibers that are associated with a reflex.
Extrafusal fibers are normal muscle fibers that are innervated by an a-motor neuron.
We will talk about these more in the next lecture.
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