  The detection of physiological activity at the microscopic level is key for
understanding the function of biosystems and relating this to physical
structure. Current sensing methods often rely on invasive probes to stimulate
and detect activity, bearing the risk of inducing damage in the target system.
In recent years, a new type of biosensor based on color centers in diamond
offers the possibility to passively, noninvasively sense and image living
biological systems. Here, we use such a sensor for the \textit{in-vitro}
recording of the local magnetic field generated by tightly focused, high
intensity pulsed laser optogenetic neuromuscular stimulation of the extensor
digitorum longus muscles. Recordings captured a compound action potential
response and a slow signal component which we seek to explain using a detailed
model of the biological system. We show that our sensor is capable of recording
localized neuromuscular activity from the laser stimulation site without
photovoltaic or fluorescence artifacts associated with alternative techniques.
Our work represents an important step towards selective induction of localized
neurobiological activity while performing passive sensing and imaging with
diamond sensors, motivating further research into mapping of neural activity
and intra-cellular processes.
