Ciguatera Toxicity

Excerpt

Ciguatera poisoning is the most common nonbacterial seafood poisoning globally. The condition arises after consuming fish contaminated with the lipophilic, heat-stable xenobiotic ciguatoxin. The toxin is produced by single-celled dinoflagellates in the Gambierdiscus and Fukuyoa genera. Dinoflagellates are dominant phytoplankton members and can be found in all oceans between 35° north and 35° south latitudes. Ciguatera poisoning is endemic in tropical and subtropical waters.

The ciguatoxin is initially introduced into the flesh of herbivorous fish through the consumption of ciguatoxin-producing dinoflagellates. The toxin accumulates through predation until it reaches the top-tier predatory fish in sufficient concentrations. Consumption of contaminated fish harms humans. The fish that most commonly cause ciguatera toxicity are barracuda, grouper, moray eel, amberjack, sea bass, sturgeon, parrotfish, surgeonfish, and red snapper.

Ciguatoxin is a potent poison that may cause gastrointestinal, neurologic, cardiovascular, and dermatologic symptoms. While uncomfortable, ciguatera poisoning is rarely fatal. Understanding the symptomatology of this condition is essential to clinicians everywhere. Symptom onset may be delayed for several days. Thus, travelers may consult for the condition after moving to a different location. Contaminated fish may also be marketed in nonendemic regions.

Anatomy of the Synapse

The typical motor neuron has 3 major parts: the soma, axon, and dendrites (see Image. Anatomy of Neurons). The soma is the neuron's body, containing most cellular organelles. The axon is an outgrowth extending from the soma to a peripheral nerve. The dendrites are the soma's branching projections extending to surrounding areas in the spinal cord. Dendrites can be 1 mm or shorter in length.

Presynaptic terminals are the axonal or dendritic tips resembling small round knobs that release neurotransmitters at the synaptic cleft. Transmitter vesicles and mitochondria lie in the presynaptic terminals. Transmitter vesicles contain the neurotransmitters diffusing through the synaptic cleft to reach the next (postsynaptic) neuron. The mitochondria provide the energy that enables neurotransmitter synthesis, release, and reuptake at the presynaptic terminal.

The presynaptic membrane covers the presynaptic terminal and contains most of the neurons' voltage-gated calcium channels. Neuronal depolarization opens these calcium channels, increasing presynaptic calcium concentration. Calcium ions bind to release sites on the inner surface of the presynaptic membrane, stimulating transmitter vesicle transport toward the membrane and the vesicles' eventual exocytosis. The vesicles then release neurotransmitters in the synaptic cleft.

Postsynaptic surfaces may be found on the dendrites, bodies, and axons. The postsynaptic membrane has receptors that neurotransmitters may stimulate or inhibit. Some receptors act as ion channels, allowing cation or anion entry on neurotransmitter activation. Cations like sodium and calcium excite neurons, while anions like chloride do the opposite. However, an increased concentration of potassium, another cation, can inhibit neurons. The effects of ion channels are short-lived but important for quick activities like sensation and motion. Meanwhile, other postsynaptic receptors act as second-messenger system activators, enabling long-term neuronal functions such as memory.

Nerve Fiber Classification

Spinal nerve fibers are either myelinated or unmyelinated (see Image. Unmyelinated and Myelinated Nerve Fibers). Myelination increases neuronal signal transmission rates.

Type A fibers are myelinated and further divided into Aα, Aβ, Aγ, and Aδ. Type Aα fibers have the largest diameter of all spinal nerve fibers and are the fastest signal transmitters. Type Aα fibers provide motor impulses to the skeletal muscles and serve as the afferent pathways for muscle spindles. Type Aβ fibers are thinner than type Aα but also innervate the skeletal muscles and muscle spindles. Types Aγ and Aδ are even thinner and slower than Aα and Aβ types and serve mostly sensory functions.

Type C fibers are unmyelinated and have the smallest diameters. Thus, these spinal nerve fibers transmit signals slowly. Half of the peripheral nerve sensory fibers and all postganglionic autonomic fibers are of the C type.

Spinal nerve fibers may also be classified into Groups Ia, Ib, II, III, and IV. Group Ia fibers are type Aα fibers in the muscle spindles' annulospiral tips. Group Ib fibers are type Aα fibers supplying the Golgi tendon organs.

Group II fibers innervate cutaneous tactile receptors and the muscle spindles' flower-spray endings. These fibers are either type Aβ or Aγ. Group III fibers are of the Aδ type, carrying the sensations of temperature, crude touch, and pricking pain (fast pain). Group IV fibers are type C fibers that transmit "slow pain" (ie, visceral pain), itch, temperature, and crude touch sensations.