Sonoma State University
Department of Biology - Hanes
Animal Physiology
Chapter 6: Communication Along & Between
Neurons
Eckert & Randall - Chapter #6
Application
Discussion
Reflection
The input and output of the nervous system is pretty well understood, and it is also simple. The more complex and not well understood part of nervous systems is the processing of information. The processing is dependent upon circuitry. There are 1011 nerves in the human nervous system and one neuron may have more than 1000 inputs from other cells. We also know that in some instances there are parallel pathways that process information. It is like a computer with a number of processors. Yet all neurons conduct electrically, most often with action potentials which are essentially all alike. Qualitative information is interpreted by the neurons that are active and quantitative information is interpreted by the frequency of action potentials.
Sensory nerves produce receptor potentials at
their sensory ends wich are graded (continuous in
voltage with the strength of the stimulus). These receptor potentials
diminish with distance just like a wire would conduct electrically
(passive electrotonic conduction). This part of the
nerve cell does not have voltage gated sodium gates. The first sodium
gates are located at the first node of Ranvier. If the graded
depolarization is sufficient to meet the threshold voltage of these
sodium gates, an action potential will be produced. The frequency of
action potentials is increased as the receptor potential is
increased. ( See Fig 6-1 p 164)
At the other end of the nerve cell, the arrival of action potentials causes a release of neurotransmitters from their vesicles. Each arriving action potential releases about the same number of vesicles. Neurotransmitters diffuse across the synaptic space and come into contact with receptor proteins on the post-synaptic neuron. These receptors are chemically controlled gates which open, allow ion flow and change the voltage of the post-synaptic cell. These voltage changes are also graded and are called post-synaptic potentials (psp's). In order to cause an action potential in the post-synaptic neuron, this graded and decrementing psp must be sufficient to reach threshold of the first sodium gates found at the first node of Ranvier. Thus passing from nerve to nerve involves alternating graded and all-or-none voltages. The action potentials are used to carry information relatively long distances. The graded potentials for very short distances.
Transmission of Information within a single Neuron
I. Passive spread. (Fig 6-2 p 157) (Fig 6-3 p 158)
If a cell is "electrotonically" connected to another cell (by a gap junction), the signal will be passed to the next cell, although usually at a reduced voltage. Many cells communicate in this way , smooth muscle, heart, and some brain cells especially in invertebrates.
II. Propagation of Action Potentials
Many small nerve do not produce action potentials. They produce passive, graded potentials and secrete neurotransmitter in amounts relative to their voltages. Later we will discuss neurons of the eye that work in this manner.
The propagation of action potentials can best be illustrated by Figs 6-4 p 160.
All along the axons of non-myelinated nerve axons and at the Nodes of myelinated axons are sodium channels of a different sort that are called sodium gates. We will see later that these are very special. For now, we can characterize them electrically as when a particular voltage (threshold voltage) is reached, the gates open to Na+ sufficient to bring the cell voltage close to the sodium equilibrium potential. When this happens, another gate on the molecule closes due to the reversed voltage and excludes Na+ from the channel, which brings the voltage back to a resting level. Thus, while the voltage change is dramatic, it lasts a very short time (~ 1 msec). Each action potential is just like the next: same voltage, same time (this is only a slight lie). This is called a spike, or action potential, or all-or-none response, or non-decrementing. Can you visualize why the axon hillock is so important? The graded response will set off action potentials only if it is great enough at the hillock. Do you see how the soma can act as an integrator of incoming signals?
In myelinated nerves, the Na+ gates are located only at the Nodes. When the gates at one node fire, they produce electrical currents around that area which are great enough to cause the next node to be depolarized beyond its threshold voltage, thus passing on the voltage. This is a leaping kind of voltage change called saltatory conduction from the latin meaning "to jump". Nodes produce enough field effect so that there is about 5X the voltage needed to set off the next node (a 5X safety factor). Voltage changes that are quick, like action potentials, can be detected quite a distance away. The ECG and EEG are based on this. They are picking up voltages away from the cells, and this voltage is called the field effect. If one cell is firing, it can be detected if the electrode is close enough to it. If you took out a nerve cell and placed two electrodes on it about 2 cm apart and connected these to an oscilloscope, what pattern would be produced as the nerve cell fired? This would be a field effect and would be in the microvolt range. What if one electrode were inside the cell and the other outside? This would be a membrane voltage and in the millivolt range.
III. Speed of Propagation
Larger cells produce more current flow with an action potential, so more field effect, and faster conduction - Table 6-1 p 166. Why? Increased temperature and increased myelination also speed up nerve transmission. Mammalian nerve cells conduct from about 0.5 M/sec (unmyelinated) to 120 M/sec (large myelinated). This is about 60 miles/hr. Note the arrangement of the oligodendrocyte forming myelin sheaths p 164.
Nerve cells have been classified by their speed of conduction. With early oscilloscopes, one could see that not all nerve cells in a nerve conducted impulses at the same rate. The fast ones were classified as "A" and slower ones "B" and "C". Better oscilloscopes found a number of conduction velocities in "A" so then the classification system was "A alpha", "A beta", "A gamma", "A delta", B and C. Now you will find other classification systems using roman numerals etc. There is still some use as in alpha-motor neurons and gamma-efferent nerves. Pain nerves were found to be fast and slow (A delta & C).
Transmission of Information between Neurons
Chemical transmission is usually one way (rectifying), although there are instances in which both cells have neurotransmitter and receptors. The synaptic delay is at least one millisecond and can be hundreds of milliseconds. Presynaptic membrane has voltage controlled Ca++ gates. When the A.P. arrives the voltage change opens these gates allowing Ca++ to enter. The calcium combines with intracellular chemicals that cause the vesicles of about 40nm diameter to exocytose and release neurotransmitter into the synaptic space. One study estimated the number of neurotransmitter molecules in an alpha motor neuron at 1 X 104 per vesicle. The membrane migrates to the side of the bouton and invaginates to form empty vesicles which are later refilled. The neurotransmitter diffuses across the synapse (about 20 nm) and binds to a receptor molecule embedded in the postsynaptic membrane. This, in turn, opens an ion channel (either to Na+, K+, Cl-, or Ca++) which, depending upon the ion, causes depolarization or hyperpolarization. Remember that opening an ion channel causes the voltage to go toward the equilibrium potential of that ion. The neurotransmitter diffuses away from the receptor and is either enzymatically destroyed or reabsorbed by the presynaptic cell, depending on the transmitter. (Fig 6-11 p 169)
Neurotransmitters may be a number of types of chemicals. Acetylcholine is rather unique, others are modified or unmodified amino acids such as epinephrine, norepinephrine, Gamma amino butyric acid, glutamine, glycine; short polypeptides as Substance P, antidiuretic hormone, CCK, and Enkephalin.
Fast chemical transmissions are produced when the neurotransmitters directly affect the receptors that are part of the same protein that is the gate. Slow chemical transmissions also occur at synapses where the receptor is a separate protein than the gate. The receptor is a G-protein and affects the gates via a series of reactions.
Electrotonic transmission is also called ephaptic transmission. It is by direct electrical stimulation via gap junctions. Usually these are functionally symmetrical (either cell can stimulate the other equally), but sometimes rectification can occur if one cell is much larger than the other. The larger cell can contact a higher percentage of the smaller cell's membrane; it produces more current per voltage change and has more volume to dissipate a return signal. These can be detected electrically by a very short synaptic delay (in the microsecond range), their symmetry, and their graded responses. They help to coordinate the firing of several cells to fire all at once. Fig 6-9 p 168
Subsynaptic and Motor-End-Plate Morphology
See Fig 6-13 p 171. The motor end-plate has been studied extensively as a post-synaptic membrane because it is visible and large. The muscle cell membrane beneath the nerve is folded into upside down "Y"-shaped clefts called junctional folds. Between the openings of the folds are finger-like projections of Schwann cell. Acetylcholine esterase is embedded in the membrane of the folds. This quickly hydrolyses acetylcholine so that it does not remain around receptors. The esterase is blocked by parathion or eserine (physostigmine) and can be visualized by reacting with radioactive isotopes of these chemicals. Acetylcholine receptors are also present in the subsynaptic membrane near the openings of the folds and these are associated with ion channels that can allow Na+ to enter and K+ to leave with about equal ease. The receptors are bound non-competitively by alpha-bungarotoxin, a poison from the krait, a snake in Australia. Fig 6-13 p 179 says it all. Receptors can be located using an isotope of this poison. Receptors can also be blocked competitively by tubocurare and succinylcholine. What are the differences between non-competitive and competitive binding?
The nerve is also modified above the openings of the folds. It has active-zones in its membranes, which are thickened rods. There are vesicles of neurotransmitter lined up along-side the rod and an accumulation of vesicles surrounding it. It is here that exocytosis occurs readily.
Synaptic currents and Potentials
Channels in the post synaptic membranes of skeletal muscle have been studied extensively because they are large. These channels, when stimulated by ACh, permit the passage of Na+ and K+. The reversal potential in mammals is about -15 mV. A reversal potential is the voltage at which there is no net current flow into or out of the cell even though the channels are open. It is a function of the relative permeability of the channel to the ions that pass through it. It is done with a voltage clamp machine that holds the voltage of a cell steady while measuring the current flow into or out of the cell Fig 6-18. 6-19 pp 184-185. One simply makes the cell less and less negative starting at the resting potential until the current flow (usually + charge inward) starts to become + charge outward. The equilibrium potential of Na+ is about +40 mV and that of K+ is about -90 mV. These could also be called reversal potentials for Na+ and K+. A reversal potential of -15 mV is nearly half-way between these two so one can say that Na+ and K+ are about equally permeable to the channels.
It is not the neurotransmitter that determines whether or not a cell will be stimulated or inhibited, but rather the type of channel that the receptor molecule is attached to. Skeletal muscle is stimulated by ACh because the receptor is of a type called nicotinic. Some smooth muscle, as in the pyloric sphincter or even cardiac muscle, is inhibited by ACh. It has a receptor type called muscarinic that is attached to channels that open to K+ or sometimes Cl-. When neurotransmitter causes inhibition directly to the post-synaptic cell, it is called post-synaptic inhibition. Fig 6-22 p 181. Pre-synaptic inhibition occurs when a bouton ends upon an excitatory bouton. The first bouton will cause inhibition in the second by opening channels to either K+ or Cl-, and thus reduce the size of the voltage delivered to the end of the second bouton. This reduces the amount of Ca++ entrance and the number of neurotransmitter vesicles released. Pre-synaptic inhibition is like using a scalpel instead of a chain saw as only one input to a cell is reduced in its effectiveness and not necessarily the whole cell.
The Acetylcholine Receptor
We know a lot about this particular receptor because it can be obtained in quantity from fish electric organs. Cellular probes (antibodies with attachments) tell us that they exist on the post-synaptic or muscle cell membrane only under the bouton of an acetylcholine secreting nerve. If skeletal muscle is denervated, it will have ACh receptors all over its membrane and at a density similar to what is normally under the nerve. After a nerve reconnects to the muscle cell, the ACh receptors are again found only under the nerve.
There are a number of known neurotransmitters and even more putative ones. Some that we know are:
* Acetylcholine (ACh)
* Catecholamines - Epinephrine (Epi), Norepinephrine (NEpi), Dopamine
* Modified amino acids - Gamma-amino butyric acid (GABA), Serotonin (5-HT)
* Amino acids - Glutamate, Aspartate
* Small peptides - Substance P, Enkephalins, Gastric inhibitory peptide., Anti-diuretic Hormone? (ADH), Oxytocin?, etc.
Release of Neurotransmitters Fig 6-24 p 182
Quantal release of neurotransmitters. If one can stick an electrode directly beneath a neuro-muscular junction and treat the preparation with a poison that blocks acetylcholine esterase, one can measure with an oscilloscope a strong end-plate potential (EPP) as the nerve is stimulated. If the Ca++/Mg++ ratio is also reduced, the EPP is reduced until it reaches a minimum value. Then it may sometimes not show up, or produce the minimum value. It is no longer a graded response. Also, even without stimulating the nerve, one can see occasional brief voltage changes that look exactly like these minimal EPP's. Fatt and Katz (my two favorite authors) called the unstimulated responses miniature end-plate potentials (MEPPS). By a statistical analysis of MEPS and EPPS, Fatt and Katz proved a high likelihood that acetylcholine is released in packets and that a each packet has a similar effect on the voltage change in the post-synaptic cell. Normally about 100 to 300 packets are released with each nerve stimulation at a myo-neural junction. Nerve to nerve transmission, however, may release only one or a few packets with each pre-synaptic action potential. Later researchers used the myo-neural junction to estimate that there are 10,000 to 40,000 ACh molecules per vesicle, two ACh molecules are needed to open each channel, and that about 2000 channels are opened with each nerve stimulus to a skeletal muscle.
Depolarization-release coupling. Depolarizing the pre-synaptic terminal end of a nerve increases the number of vesicles released, even if there is no action potential. The amplitude of membrane depolarization is directly related to the number of vesicles released although under most circumstances, each action potential produces the same depolarization. By using aequorin (a dye that gives off light in the presence of Ca++) filled cells, it was also found that the amount of Ca++ entry to the pre-synaptic cell is also directly related to the membrane depolarization.
If a series of AP's is close enough together in time, the
excitatory post-synaptic potential (EPSP) will
increase with each AP in a stair-step fashion or, if slower, a second
AP may still cause more EPSP than the first. This phenomenon is
called potentiation. It is caused by an accumulation
of Ca++ in the pre-synaptic cell so that more neurotransmitter is
released with a second stimulus that is close in time. After a series
of stimuli, a pre-synaptic nerve can run low on vescicles and thus
produce a reduced response in the post-synaptic cell. The increased
response of a post-synaptic cell to repeated stimuli from one input
is called temporal summation and is mostly due to
the opening of more gates before previously opened ones are
completely shut - so there is an increase in ion permeability and
membrane voltage.
There is also a phenomenon called spatial summation.
This occurs if AP's arrive on two separate presynaptic cells and
cause an EPSP greater than either one alone. This is, of course,
caused by more channels being opened in the post-synaptic cell.
Synaptic responses may also be plastic. There are many different shapes and kinds of boutons that may release different amounts of neurotransmitter. There are also post-synaptic anatomical specializations as spines and membrane specializations that can enhance the efficacy of post-synaptic cells. Chemicals such as ACh, Epi, GABA, Octopamine, and enkephalins can act as neuromodulators, which enhance or suppress the efficiency of synapses. They may act by influencing the ease of opening for ion channels (Ca++?). An example of this can be found in the effect of enkephalin on the moderation of pain. Enkephalin inhibits the release of Substance P (a neurotransmitter of pain cells in the spinal chord). This inhibition is completely blocked by the drug Naloxone which can bring a heroin addict down instantly or test for the usefulness of acupuncture in pain moderation.
If you are getting weary at this point, just read the pictures after page 190.
What accounts for delays in reflexes?
For what logical reason might there be so many types of
neurotransmitters?
How does the brain determine the type of input (pain, touch,
pressure, light, etc.)?
How can the brain amplify very light sensory input?
1. Differentiate between axons and dendrites anatomically and
functionally.
2. Draw & describe the association of Schwann cells to neurons.
Describe step-by-step what happens to channels, currents, etc...
during saltatory conduction.
3. List as many items as you can that differentiate A.P. Na+ channels
and subsynaptic channels.
4. What influences the speed of nerve conduction?
5. List, in order, as many events as you can from a neuron detecting
ACh secreted onto it to its own release of ACh.
6. How would you experimentally differentiate chemical from
electrical conductance?
7. How can a nerve cell be inhibited? Ions, transmitters,
anatomy?
8. What is the distribution of the following on cell membranes: ACh
receptors, Cholinesterase, Ca++ channels, A.P. Na+ channels, synaptic
channels, Na+-K+ pumps?
9. What do the following chemicals do? Bungarotoxin, TTX, Ouabain,
Parathion, Tubocurare, Physostigmine, TEA, Aequorin, Naloxone.
10. What evidence do we have that the release of ACh is quantal?
So how would you put all of this information into the functioning of the brain?