Amphid neurons are one type of chemosensory neurons that are found in the anterior head region of nematode Caenorhabditis elegans (C. elegans).
Amphid Neurons: are sensory neurons or sensilla; specifically 12 sensory 11 of which are chemosensory neurons (ADF, ADL, ASE, ASG, ASH, ASI, ASJ, ASK, AWA, AWB, AWC) which C.elegans use to sense chemicals. 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.
C. elegans have 11 pairs of bilaterally symmetric amphid chemosensory neurons (ADF, ADL, ASE, ASG, ASH, ASI, ASJ, ASK, AWA, AWB, AWC). The twelfth, AFD, is a thermosensory neuron. The cell bodies of amhpid neurons, with cillliated dendrites that extend to the tip of the animals nose, 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: In response to stimuli, C. elegan neurons propagate an action potential and release biogenic amines such as dopamine, tyramine, and octopamine as neurotransmitter. C. elegan 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 repellant 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 repellant 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.
1. Bargmann CI. (2006) Chemosensation in C. elegans, WormBook, pages: 1-29. PMID: 18050433 [PubMed-indexed for MEDLINE} 2. Hilliard, C. Bargmann, P. (2002) C. elegans Responds to Chemical Repellents by Integrating Sensory Inputs from Head to Tail, Current Biology, vol: 12 pages 730-734 M 3. Gray, Jesse M. Hill, Joseph J. (2004) A Circuit for Navigation in Caenorhabditis elegans, PNAS
[www.wormbook.org/chapters/www_monoamines/monoamines.html| Biogenic amine neurotransmitters in C. elegans http://www.wormatlas.org/ward_buildv0.1/discussion.html The Nerve Ring of the Nematode Caenorhabditis elegans: Sensory Input and Motor Output http://www.genetics.org/cgi/content/full/173/1/131/FIG1 Searching for Neural Left/Right Asymmetry: Genomewide Analysis of Nematode Receptor http://www.wormatlas.org/handbook/hypodermis/Amphidimagegallery.htm]