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The squid giant synapse is a
chemical synapse Chemical synapses are biological junctions through which neurons' signals can be sent to each other and to non-neuronal cells such as those in muscles or glands. Chemical synapses allow neurons to form circuits within the central nervous syste ...
found in squid. It is the largest chemical junction in nature.


Anatomy

The squid giant synapse (Fig 1) was first recognized by
John Zachary Young John Zachary Young FRS (18 March 1907 – 4 July 1997), generally known as "JZ" or "JZY", was an English zoologist and neurophysiologist, described as "one of the most influential biologists of the 20th century". Biography Young went to schoo ...
in 1939. It lies in the stellate ganglion on each side of the midline, at the posterior wall of the squid’s muscular mantle. Activation of this synapse triggers a synchronous contraction of the mantle musculature, causing the forceful ejection of a jet of water from the mantle. This water propulsion allows the squid to move rapidly through the water and even to jump through the surface of the water (breaking the air-water interface) to escape predators. The signal to the mantle is transmitted via a chain consisting of three giant neurons organized in sequence. The first is located in the ventral magnocellular lobe, central to the eyes. It serves as a central integrating manifold that receives all sensory systems and consists of two symmetrical neurons (I). They, in turn, contact secondary neurons (one in each side) in the dorsal magnocellular lobe and (II) and in turn contact the tertiary giant axons in the stellate ganglion (III, one in each side of the mantle). These latter are the giant axons that the work of
Alan Hodgkin Sir Alan Lloyd Hodgkin (5 February 1914 – 20 December 1998) was an English physiologist and biophysicist who shared the 1963 Nobel Prize in Physiology or Medicine with Andrew Huxley and John Eccles. Early life and education Hodgkin was bo ...
and
Andrew Huxley Sir Andrew Fielding Huxley (22 November 191730 May 2012) was an English physiologist and biophysicist. He was born into the prominent Huxley family. After leaving Westminster School in central London, he went to Trinity College, Cambridge ...
made famous. Each secondary axon branches at the stellate ganglion and contacts all the tertiary axons; thus, information concerning relevant sensory input is relayed from the sense organs in the cephalic ganglion (the squid’s brain) to the contractile muscular mantle (which is activated directly by the tertiary giant axons).


Electrophysiology

Many essential elements of how all chemical synapses function were first discovered by studying the squid giant synapse. Early electrophysiological studies demonstrated the chemical nature of transmission at this synapse by making simultaneous intracellular recording from the presynaptic and postsynaptic terminals in vitro (, , ). Classical experiments later on demonstrated that, in the absence of action potentials, transmission could occur (, , ). The
calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar t ...
hypothesis for synaptic transmission was directly demonstrated in this synapse by showing that at the
equilibrium potential In a biological membrane, the reversal potential is the membrane potential at which the direction of ionic current reverses. At the reversal potential, there is no net flow of ions from one side of the membrane to the other. For channels that are pe ...
for calcium, no transmitter is released . Thus, calcium entry and not the change in the transmembrane electric field ''per se'' is responsible for transmitter release (Llinás et al. 1981, ). This preparation continues to be the most useful for the study of the molecular and cell biological basis for transmitter release. Other important new mammalian preparations are now available for such studies such as the
calyx of Held The Calyx of Held is a particularly large synapse in the mammalian auditory central nervous system, so named after Hans Held who first described it in his 1893 article ''Die centrale Gehörleitung''Held, H. "Die centrale Gehörleitung" Arch. Anat ...
. image:RL Squid Synapse 2.jpg, 1000px, center, Fig 2. Upper left image: Enlarged picture of the squid stellate ganglion showing the giant synapse. Intracellular dye injection was used to stain the presynaptic axon green and the postsynaptic axon red. The presynpatic fiber has seven branches, each for one giant tertiary axon. Only the last postsynaptic axon on the right is colored. Lower left image: A) Simultaneous intracellular recording from the presynaptic fiber (pre) and the postsynaptic axon (post). The synaptic action potential releases a transmitter substance (glutamate) that acts on the postsynaptic receptors and activates the postsynaptic action potential. B & C) Synaptic transmission can be evoked with either a square voltage pulse (B) or an artificial action potential wave form (C) these are delivered to a command amplifier as shown in D. D. Diagram of a command amplifier (CO) and current injection amplifier (I) with a feedback control via presynaptic voltage (Pre V). The response to these stimuli is recorded as a current (Im) and displayed in green in E and F. E) Synaptic transmission and calcium current (ICa, green) evoked by a square voltage pulse (Pre). F) Calcium current (green) and postsynaptic potential (Post) evoked by an artificial action potential (Pre). Note that in F the calcium curren starts during the down swing of the presynaptic action potential (modified from ). Middle Image: Left, Voltage clamp records illustrating the relation between transmembrane voltage in mV (square wave at the bottom of each record) Calcium current amplitude in nA (middle record) and postsynaptic potential in mV. Time mark one ms. The voltage steps are generates from a holding potential of -170mV. (Llinás et al. 1981). Right image: Relation between voltage and current for the “on” (red plot) and “tail” (white plot) calcium current. Voltage in mV current in nA (modified from ).


See also

*Squid giant axon


References

* * * * * * * * * * * * {{DEFAULTSORT:Squid Giant Synapse Cephalopod zootomy Squid, Giant synapse