The squid giant synapse is a chemical synapse

Chemical synapse

Chemical synapses are specialized junctions through which neurons signal 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 system. They are crucial to the biological computations that underlie...

 found in squid

Squid

Squid are cephalopods of the order Teuthida, which comprises around 300 species. Like all other cephalopods, squid have a distinct head, bilateral symmetry, a mantle, and arms. Squid, like cuttlefish, have eight arms arranged in pairs and two, usually longer, tentacles...

. 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

John Zachary Young FRS , generally known as "JZ" or "JZY", was an English zoologist and neurophysiologist, described as "one of the most influential biologists of the 20th century .....

 in 1939. It lies in the stellate ganglion

Ganglion

In anatomy, a ganglion is a biological tissue mass, most commonly a mass of nerve cell bodies. Cells found in a ganglion are called ganglion cells, though this term is also sometimes used to refer specifically to retinal ganglion cells....

 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 barrier) 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 and Andrew Huxley

Andrew Huxley

Sir Andrew Fielding Huxley, OM, FRS is an English physiologist and biophysicist, who won the 1963 Nobel Prize in Physiology or Medicine for his experimental and mathematical work with Sir Alan Lloyd Hodgkin on the basis of nerve action potentials, the electrical impulses that enable the activity...

 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 (Theodore Holmes Bullock

Theodore Holmes Bullock

Theodore Holmes “Ted” Bullock is one of the founding fathers of neuroethology. During a career spanning nearly seven decades, this American academic was esteemed both as a pioneering and influential neuroscientist, examining the physiology and evolution of the nervous system across organizational...

 and Hagiwara (1957), Hagiwara and Ichiji Tasaki

Ichiji Tasaki

Dr. Ichiji Tasaki was a Japanese biophysicist and physician involved in research relating to the electical impulses in the nervous system. He is credited with discovering the insulating function of the myelin sheath...

 (1958), Takeuchi and Takeuchi (1962). Classical experiments later on demonstrated that, in the absence of action potentials, transmission could occur (Bloedel et al. (1966), Katz and Miledi (1967), Kusano et al. (1967). The calcium

Calcium

Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...

 hypothesis for synaptic transmission was directly demonstrated in this synapse by showing that at the equilibrium potential for calcium (+60 mV, calcium does not enter the presynaptic terminal) no transmitter is released (Bernard Katz

Bernard Katz

Sir Bernard Katz, FRS was a German-born biophysicist, noted for his work on nerve biochemistry. He shared the Nobel Prize in physiology or medicine in 1970 with Julius Axelrod and Ulf von Euler...

 and R. Miledi 1967). Thus, calcium entry and not the change in the transmembrane electric field is responsible for voltage clamp

Voltage clamp

The voltage clamp is used by electrophysiologists to measure the ion currents across the membrane of excitable cells, such as neurons, while holding the membrane voltage at a set level. Cell membranes of excitable cells contain many different kinds of ion channels, some of which are voltage gated...

 studies that determined the relationship between the time course and amplitude of the calcium current and the amount of transmitter release directly determined (Llinas et al. 1981, Augustine et al. 1985) 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

Calyx of held

The Calyx of Held is a particularly large synapse in the mammalian auditory central nervous system, named by H. Held in his 1893 article Die centrale Gehörleitung, due to its flower-petal-like shape....

.