Key Event Description
A description of the biological state being observed or measured, the biological compartment in which it is measured, and its general role in the biology should be provided. For example, the biological state being measured could be the activity of an enzyme, the expression of a gene or abundance of an mRNA transcript, the concentration of a hormone or protein, neuronal activity, heart rate, etc. The biological compartment may be a particular cell type, tissue, organ, fluid (e.g., plasma, cerebrospinal fluid), etc. The role in the biology could describe the reaction that an enzyme catalyses and the role of that reaction within a given metabolic pathway; the protein that a gene or mRNA transcript codes for and the function of that protein; the function of a hormone in a given target tissue, physiological function of an organ, etc. Careful attention should be taken to avoid reference to other KEs, KERs or AOPs. Only describe this KE as a single isolated measurable event/state. This will ensure that the KE is modular and can be used by other AOPs, thereby facilitating construction of AOP networks.
In bivalves, the muscle involved in valve movement is the anterior byssus retractor muscle (ABRM). This muscle and other muscles can undergo a catch state of contraction, which is characterized by a very slowly decaying force in the absence of stimulation. When contraction of the ABRM (and other catch-capable muscles) is initiated by the neurotransmitter acetylcholine, it contracts actively and remains in the contracted state with minimal energetic investment (Güth et al. 1984; Butler et al. 1998) even after the acetylcholine stimulation has ceased and internal calcium stores are returned to basal levels(Ishii et al. 1989). This unique physiology allows the muscle to maintain a closed valve without depleting energy reserves. In mollusks, the catch state is terminated by serotonin.