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...
* The cell membranes of two cells are linked together via **gap junctions**
* Current flows **directly** from one neuron to another via gap junctions– form large pores (compared to ion channels) between cells made up of connexin proteins
Pore is approx 1 nm in diameter. Allows passage of small molecular weight substances like intracellular metabolites (a few hundred daltons), but not proteins (typically 5-500 kilodaltons in diameter)
In contrast to gap junctions/electrical synapses, for chemical synapses current flow does not occur directly from the presynaptic cell to postsynaptic cell.
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]. Cx36 KO mouse has no obvious behavioral phenotype other than retinal deficits[^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.
c elegans: 959 total cells in adult hermaphrodite. 302 are neurons, 58 are glia. Every cell in worm expresses 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.
* 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
Electrical synapses and synchronization of activity is characteristic of cells that stimulate pulses of pituitary hormones (e.g oxytocin/vasopressin secretion).
Inferior olivary nucleus: source of climbing fiber input to cerebellar cortex. ultrastructure and electrophysiology (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 evidence between neurons. 2-8Hz synchronous oscillations. [^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.
The retina has widespread electrical coupling. Extensive between the amacrine cells (interneurons) that synthesize GABA, acetylcholine as neurotransmitter), 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 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.
<figcaption class="big">Synchronous spikes between two crayfish neurons</figcaption>
<img src="figs/Neuroscience5e-Fig-05.02-1R_copy_2f541cc.jpg" height="400px"><figcaption>Neuroscience 6e Fig. 5.3, 5e Fig. 5.2; from Fushpan and Potter, 1959 </figcaption></figure>
Note:
In Crayfish an action potential in one neuron can spread quickly to the next in fraction of a millisecond.
--
## Electrical synapses
<figure>
<figcaption class="big">Synchronous spikes between a pair of mammalian hippocampal neurons</figcaption>
<img src="figs/Neuroscience5e-Fig-05.02-2R_copy_3cd5bb0.jpg" height="400px"><figcaption>Neuroscience 6e Fig. 5.3, 5e Fig. 5.2; from Beierlein et al. 2000 </figcaption></figure>
Note:
In hippocampal neurons, gap junctions can make neurons fire in synchrony
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:
---
## Acetylcholine (ACh) shown to be the vagus factor
* 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
- Curare used as a paralyzing poison by South American indigenous peoples for hunting that causes respiratory asphixiation (diaphragm muscle paralysis) in prey
- Curare is a plant alkaloid that is a competitive and reversible inhibitors of nicotinic acetylcholine receptor (nAChR)
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...
---
## 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>
<div class="fragment fade-in" data-fragment-index="2"><figcaption class="big">specific receptors on post-synaptic cell</figcaption><img src="figs/Neuroscience5e-Box-05A-0R-3_copy_7293cc0.jpg" height="400px"><figcaption>Neuroscience 5e Box 5A</figcaption></div>
Note:
Criteria depicted here
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 demonstrated 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
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>
* tetanus toxin cleaves the synaptobrevin SNARE protein within spinal cord interneurons. This results in less inhibition of spinal cord motor neurons giving rise to muscle hyperexcitation and "tetanic" contractions
Thus two definitions for tetanus from wordnet, one referring to the bacterial toxin and one referring to a hyperexcitable phenotype in muscle tissue:
tetanus (wn, noun)
: an acute and serious infection of the central nervous system caused by bacterial infection of open wounds; spasms of the jaw and laryngeal muscles may occur during the late stages
: a sustained muscular contraction resulting from a rapid series of nerve impulses
Thus in animal physiology when discussing sustained excitation of muscle tissue, it may be referred as "tetanic" stimulation or a muscle "in tetanus".. even when there is no tetanus toxin.
<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
