* Two general classes– chemical and electrical synapses
* Chemical– neurons talk to each other by release of neurotransmitters
* Electrical– direct, passive flow of current between neurons
Note:
Thus far we’ve discussed how neurons generate action potentials that propagate down axons with high fidelity over cm’s to to meters of space and the ion channels in the membrane that underly voltage dependent excitability.
But is through synapses that neurons actually talk with one another and it is also through synapses that the nervous system effects behavior function enabling us to interact with the world around us– in other words there are synapses between pairs of neurons that form the basis of inter-neuronal communication as well as synapses on muscle fibers that neurons use to get our muscles to contract.
Now there are two general classes of synapses, chemical...
<figure><img src="figs/Neuroscience5e-Fig-05.02-1R_copy_2f541cc.jpg" height="300px"><figcaption>Neuroscience 6e Fig. 5.3, 5e Fig. 5.2; from Fushpan and Potter, 1959 </figcaption></figure>
<figure><img src="figs/Neuroscience5e-Fig-05.02-2R_copy_3cd5bb0.jpg" height="300px"><figcaption>Neuroscience 6e Fig. 5.3, 5e Fig. 5.2; from Beierlein et al. 2000 </figcaption></figure>
In hippocampal neurons gap junctions can make neurons fire in synchrony
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## Electrical Synapses: putative functions
* Synchronization of the electrical activity of large populations of neurons
* the large populations of neurosecretory neurons that synthesize and release biologically active peptide neurotransmitters and hormones are extensively connected by electrical synapses
inferior olivary nucleus: source of climbing fiber input to cerebellar cortex. ultastructure adn ephys (Llinas 1974) found electrical coupling between pairs of neurons in cat inferior olive. Same thing demonstrated later in guinea pig, rat, mouse. Also dye coupling. 2-8Hz synchronous oscillasions. [^Connors:2004]
thalamic reticular nucleus (thin interneuron layer) of dorsal thalamus. Spatially localized coupling (cells 40 um apart). [^Connors:2004]
hippocampus. between pyramidal neurons and also interneurons. [^Connors:2004]
in neocortex only rarely found between pyramidal neurons, often between interneurons. 'Late spiking' L1 interneurons make electrical synapse with other neurons of the same class 83% of time but with other interneuron types only 2% of time. Maybe necessary for gamma frequency rhthyms.
retina has widespread electrical coupling. Extensive between amacrine cells, scoptopic vision impaired in Cx36 KO mice from loss in rods and cones and between amacrine cells and bipolar cells.
Cx36 in both olfactory epithelium and olfactory bulb. between granule cells. between mitral cells in same glomerulus.
Early in development, first postnatal week in rat electrical coupling extensive between motor neurons in spinal cord. Declines during first postnatal week but still present in adult.
gap junction proteins:
connexins (chordates), innexins (invertebrates). Similar topologies but dissimilar gene/amino acid sequences. Also pannexins in
connexins : 20 isoforms in humans and mice. 40 connecxin orthologues across species. Cx36 36kDa protein, hexamer possibly only forming hemichannels homotypically, specific to neurons. [^Connors:2004]
50% of mammalian connexins widely expressed in CNS. Some strong in astrocytes (Cx26,30,43) or oligodendrocytes (Cx29,32,47) [^Connors:2004]
gap junctions first found and studied in invertebrates. Innexins for gap junctions in drosophila, c elegans molluscs, annelids, playhelminthes. Mammalian pannexin genes are similar to innexins and Px1 and Px2 mRNA is present in pyramidal neurons and interneurons of the hippocampus.
gap junctions may be sensitive to Ca2+ influx, at least at high concentrations. But are very sensitive to small intracellular (but not extracellular) pH changes and intracellular pH changes occur doing neuronal activity.
Carbenoxolone (from licorice root) not very specific for Cx36.
Quinine selectively blocks Cx36,50,45. Mefloquine is a derivative that is 100x more potent.
Cx36 KO mouse has no obvious behavioral phenotype other than retinal deficits[^Connors:2004].
c elegans: 959 total cells in adult hermaphrodite. 302 are neurons, 58 are glia. Every cell in worm expresss innexins, most of the 20+ isoforms are expressed in nervous system and every neuron is believed to form gap junctions. 7000 synapses. 6393, 890 electrical junctions. 1410 NMJ.
<div><figcaption class="big">chemical synapse, type 1</figcaption><img src="figs/image2_1bf4990.png" height="220px"><figcaption>[SynapseWeb, Kristen M. Harris, PI](https://synapseweb.clm.utexas.edu)</figcaption></div>
<div><figcaption class="big">chemical synapse, type 2</figcaption><img src="figs/image3_5af29bc.png" height="220px"><figcaption>[SynapseWeb, Kristen M. Harris, PI](https://synapseweb.clm.utexas.edu)</figcaption></div>
<div><figcaption class="big">synaptic vesicles</figcaption><img src="figs/image4_b39a9f7.png" height="220px"><figcaption>[SynapseWeb, Kristen M. Harris, PI](https://synapseweb.clm.utexas.edu)</figcaption></div>
<div><figcaption class="big">synaptic cleft</figcaption><img src="figs/image5_a67adf4.png" height="220px"><figcaption>[SynapseWeb, Kristen M. Harris, PI](https://synapseweb.clm.utexas.edu)</figcaption></div>
## The discovery of the neurotransmitter acetylcholine
* Otto Loewi– wanted to figure out how stimulation of vagus nerve caused the heart to slow down
* Vagus nerve (cranial nerve X) has both sensory and motor axons. Regulates heartbeat
* Loewi transfered a solution generated from one heart to slow down another heart even without stimulation
* Demonstrated a diffusible substance was released upon stimulation
Note:
The vagus nerve is responsible for such varied tasks as heart rate, gastrointestinal peristalsis, sweating, and quite a few muscle movements in the mouth, including speech (via the recurrent laryngeal nerve). It also has some afferent fibers that innervate the inner (canal) portion of the outer ear (via the auricular branch, also known as Alderman's nerve) and part of the meninges.
The vagus nerve (/ˈveɪɡəs/ vay-gəs), historically cited as the pneumogastric nerve, is the tenth cranial nerve or CN X, and interfaces with parasympathetic control of the heart and digestive tract. The vagus nerves are paired; however, they are normally referred to in the singular.
The vagus nerve supplies motor parasympathetic fibers to all the organs except the suprarenal (adrenal) glands, from the neck down to the second segment of the transverse colon. The vagus also controls a few skeletal muscles, notable ones being:
* Cricothyroid muscle
* Levator veli palatini muscle
* Salpingopharyngeus muscle
* Palatoglossus muscle
* Palatopharyngeus muscle
* Superior, middle and inferior pharyngeal constrictors
* Muscles of the larynx (speech).
*This means that the vagus nerve is responsible for such varied tasks as heart rate, gastrointestinal peristalsis, sweating, and quite a few muscle movements in the mouth, including speech (via the recurrent laryngeal nerve).*
*It also has some afferent fibers that innervate the inner (canal) portion of the outer ear (via the auricular branch, also known as Alderman's nerve) and part of the meninges. This explains why a person may cough when tickled on the ear, such as when trying to remove ear wax with a cotton swab.[citation needed]*
*Afferent vagus nerve fibers innervating the pharynx and back of the throat are responsible for the gag reflex.*
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## The discovery of acetylcholine
<div><img src="figs/Neuroscience5e-Fig-05.04-1R_copy_a5a415a.jpg" height="300px"><figcaption>Neuroscience 5e Fig. 5.4</figcaption></div>
<div class="fragment fade-in" data-fragment-index="1"><img src="figs/Neuroscience5e-Fig-05.04-2R_copy_87d0da2.jpg" height="300px"><figcaption>Neuroscience 5e Fig. 5.4</figcaption></div>
Free acetylcholine acts on **muscarinic receptors** which **hyperpolarize** the cells of the SA node and slow the conduction of the action potential through the AV node. This slows heart rate. Acetylcholine also decreases Ca2+ influx which lowers the heart's force of contraction.
Otto Loewi (Austrian)– on the discovery of vagus nerve substance:
>"In the night of Easter Saturday, 1921, I awoke, turned on the light, and jotted down a few notes on a tiny slip of paper. Then I fell asleep again. It occurred to me at six o'clock in the morning that during the night I had written down something most important, but I was unable to decipher the scrawl. That Sunday was the most desperate day in my whole scientific life. During the next night, however, I awoke again, at three o'clock, and I remembered what it was. This time I did not take any risk; I got up immediately, went to the laboratory, made the experiment on the frog's heart, described above, and at five o' clock the chemical transmission of nervous impulse was conclusively proved."
</div>
Note:
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## Acetylcholine (ACh) shown to be the vagus factor
* curare used as a paralyzing poison by South American indigenous peoples for hunting that causes respiratory asphixiation (diaphragm muscle paralysis) in prey
* alkaloid arrow poisons that are competitive and reversible inhibitors of nicotinic acetylcholine receptor (nAChR)
* ACh action has same pharmacology as vagus nerve substance in that it is sensitive to curare (a plant poison that kills by preventing muscle contraction). Competes with curare for receptor binding
2. Must be released in response to a depolarization and be Ca²⁺ dependent
3. Must have specific receptors localized on the post-synaptic cell
* Note– It does not have to function uniquely as a neurotransmitter (it may have other functions). e.g. glutamate, glycine, ATP
Note:
There are a few criteria that define a neurotransmitter...
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## Criteria that define a neurotransmitter
<div><figcaption class="big">present in presynaptic cell</figcaption><img src="figs/Neuroscience5e-Box-05A-0R-1_copy_bd28d1f.jpg" height="400px"><figcaption>Neuroscience 5e Box 5A</figcaption></div>
It depends on how you count, but maybe 30 - 100 different molecule types, with 10 of them doing 99% of the work. More than 100 different neurotransmitters have been identified.
There are two main broad categories of neurotransmitters: "Small molecule" neurotransmitters (glutamate, GABA, acetylcholine, biogenic amines (dopamine, serotonin, noradrenaline, and histamine)) and neuropeptides (opioid peptides, substance P). ATP/purines and unsaturated fatty acids like endocannabinoids (anandamide, 2-AG) also can act as neurotransmitters.
motor unit is a motor neuron’s axon terminals and all the skeletal muscle fibers it innervates (10 for extraocular muscles, 1000 for thigh muscles). Motor pool is a bunch of motor units of same fiber type.
A presynaptic action potential releases a lot of ACh, opening channels in the muscle cell. The resulting depolarization in the muscle cell at the neuromuscular junction is called an end plate potential (EPP).
Muscle fibers are excitable cells. They are multinucleated myocytes. They too generate action potentials.
End plate potentials evoked by motor neuron stimulation almost are almost always above threshold and result in an action potential along the muscle fiber.
It is the synaptic potential at the neuromuscular junction.
motor unit is a motor neuron’s axon terminals and all the skeletal muscle fibers it innervates (10 for extraocular muscles, 1000 for thigh muscles). Motor pool is a bunch of motor units of same fiber type.
<figure><img src="figs/Neuroscience5e-Fig-05.06-2Rc_copy_864df54.jpg" height="400px"><figcaption>Neuroscience 6e Fig. 5.5; from Fatt and Katz *J Physiol* 1952</figcaption></figure>
* in the absence of stimulation there is spontaneous postsynaptic membrane transients called minature EPPs. Small amplitude.
* Bath in low calcium and stimulate you get small subthreshold EPPs that are about the same size as the MEPPs.
* Examination of the muscle membrane potential at high gain reveals small, spontaneous depolarizations. These are miniature end plate potentials (MEPPs)
This work was on frog neuromuscular junc in 1950s but subsequent investigations have demosntrated these synaptic properties for all chemical synapses studied to date.
If you measure the amplitudes of these small low calcium EPPs and plot their distribution, e.g. this histogram here you can see a certain statistical distribution that indicates these amplitudes fall into discrete steps or quanta showing that the smallest amplitude ones that are about the same size as the spontaneous MEPPs must be result of neurotransmitter release from single synaptic vesicles.
Poisson statistics used to analyse independent occurence of unitary events. Red curve shows what the distribution would expected to be if neurotransmitter release is quantal, made up of discrete message packets (vesicles) made of multiples of MEPP amplitudes (e.g. 0.4 mV)
* The **MEPP is the quantal event of neurotransmission**. It represents the postsynaptic response to the release of a single vesicle of neurotransmitter
* The EPP is the result of the synchronized release of many vesicles. It is the sum of many MEPPs
(Experiments by Heuser and Reese, 1973). HRP enzyme forms dense reaction product, can be visualized easily in electron microscopy.
Clathrin has a unique three arm structure that forms little geodesic dome coverings around membrane segments and dynamin forms a ring that pinches or 'buds' off the vesicle.
<figure><img src="figs/Neuroscience5e-Fig-05.09-2R_copy_4977b31.jpg" height="300px"><figcaption>Neuroscience 6e Fig. 5.8, 5e Fig. 5.9</figcaption></figure>
<img src="figs/Neuroscience5e-Fig-05.10-0_copy_a76faf6.jpg" height="400px"><figcaption>Neuroscience 6e Fig. 5.9, 5e Fig. 5.10; from Augustine and Eckert *J Physiol* 1984</figcaption></div>
<div style="width:400px; float:left"><figcaption class="big">microinjection of Ca²⁺ into presynaptic terminal</figcaption><img src="figs/Neuroscience5e-Fig-05.11-2R_copy_13a54e8.jpg" height="300px"><figcaption>Neuroscience 6e Fig. 5.10; from Smith et al. *J Physiol* 1993, Miledi *Proc R Sci Lon B* 1973</figcaption></div>
<div style="width:450px; float:left; margin: 0 25px"><figcaption class="big">microinjection of Ca²⁺ chelator BAPTA into presynaptic terminal</figcaption><img src="figs/Neuroscience5e-Fig-05.11-3R_copy_6d4bfd9.jpg" height="300px"><figcaption>Neuroscience 6e Fig. 5.10; from Adler et al *J Neurosci* 1991</figcaption></div>
<div style="float:left; margin:0 20px"><figcaption class="big">Molecular model of a synaptic vesicle</figcaption><img src="figs/Neuroscience5e-Fig-05.13-1R_copy_f29479f.jpg" height="400px"><figcaption>Neuroscience 5e Fig. 5.13; from Takamori *Cell* 2006</figcaption></div>
Just know there are is a calcium sensitive protein called synaptotagmin and that there are proteins like SNAREs that help dock and pinch membranes together
<figure><img src="figs/Neuroscience5e-Fig-05.14-2R_copy_0df493d.jpg" height="400px"><figcaption>Neuroscience 6e Fig. 5.12, 5e Fig. 5.14</figcaption></figure>
>Cleavage of the SNARE proteins inhibits release of acetylcholine.[45] Hence, botulinum toxins A, B, and E specifically cleave SNAREs, preventing "neurosecretory vesicles" from docking/fusing with the interior surface of the plasma membrane of the nerve synapse, and so block release of neurotransmitter. In inhibiting acetylcholine release, nerve impulses are blocked, causing the flaccid (sagging) paralysis of muscles characteristic of botulism[45]
<figure><figcaption class="big">SNARE protein sites cleaved by tetanus and botulinum toxins</figcaption><img src="figs/Neuroscience5e-Box-05B-2-0_copy_0d09c20.jpg" height="400px"><figcaption>Neuroscience 5e Box 5B, see also Clinical Application 6e p. 99-100</figcaption></figure>
* Dermatologists have been using botulinum toxin (or Botox) for cosmetic purposes
* When injected locally into a particular muscle or surrounding area, Botox causes a paralysis of that muscle due to a blockade of ACh release from the incoming motor nerve fibers. This leads to a reduction of wrinkle lines, although effective for only a few months
