Amphid neurons are chemosensory neurons located in the anterior head region of the nematode Caenorhabditis elegans.
Amphid Neurons: are sensory neurons. There are 12 sensory neurons, 11 of which are chemosensory neurons (ADF, ADL, ASE, ASG, ASH, ASI, ASJ, ASK, AWA, AWB, AWC) which C.elegans use to sense chemicals. The twelfth, AFD, is a thermosensory neuron. As a result of what these tiny animals sense, they can either decide to move toward the chemical (as with chemical attractants), away from the chemical (as with chemical repellants), or not at all. Phasmid neurons are another type of chemosensory neurons that share many of the functions expressed by amphid neurons; however, only the amphid neurons (ASH 5) are required for chemotaxis.
[[Image:Amphid Dendrites.jpg|thumb|right|This image shows the dendritic extensions of the amphid neurons in the head region. Amphid sensilla image gallery
[[Image:Amphid neurons.jpg|thumb|right|This image in A. shows all of the amphid neurons in the head region of C. elegans. Ortis, Christopher Etchberger, John F. (2006)
C. elegans have 11 pairs of bilaterally symmetric amphid chemosensory neurons. Amhpid neurons have ciliated dendrites that extend to the tip of the animal's nose. The cell bodies are located outside of the nerve ring. The nerve ring, or nerve bundle, is where the axons of amphid neurons reside and make synaptic connections with neighboring cells.
- Neurotransmitter: Amphid neurons (ASE, ASG, ASH, and ASI) express tyramine as their neurotransmitter. One of the functions of tyramine is to inhibit head oscillations in absence of food.
In response to stimuli, the C. elegans can use more than one type of sensory neuron at a time. For example, when a "drop test" was used to drop chemical repellent on the tail of C. elegans, the animal sensed the touch with the dendrites of its mechanosensory phasmid neurons, located in the posterior region of the animal, and moved forward. However, when the chemical repellent reached the anterior head region of the animal, the dendrites of the amphid neurons sensed the chemical causing the animal to move backward. When food is available, the amphid chemosensory neurons act on the phasmid motor neurons and cause the C. elegans move their heads toward the food source. When there is a mutation in the gene responsible for normal amphid neuron function and there is no food available, head oscillations still occur. When food is unavailable the amphid neurons release tyramine that inhibit the actions of phasmid motor neurons which, thereby, prevents head oscillation.
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