Pulsed laser versus electrical energy for peripheral nerve stimulation

Abstract

Transient optical neural stimulation has previously been shown to elicit highly controlled, artifact-free potentials within the nervous system in a non-contact fashion without resulting in damage to tissue. This paper presents the physiologic validity of elicited nerve and muscle potentials from pulsed laser induced stimulation of the peripheral nerve in a comparative study with the standard method of electrically evoked potentials. Herein, the fundamental physical properties underlying the two techniques are contrasted. Key laser parameters for efficient optical stimulation of the peripheral nerve are detailed. Strength response curves are shown to be linear for each stimulation modality, although fewer axons can be recruited with optically evoked potentials. Results compare the relative transient energy requirements for stimulation using each technique and demonstrate that optical methods result in highly selective functional nerve stimulation. Adjacent stimulation and recording of compound nerve potentials in their entirety from optical and electrical stimulation are presented, with optical responses shown to be free of any stimulation artifact. Thus, use of a pulsed laser exhibits distinct advantages when compared to standard electrical means for excitation of muscle potentials in the peripheral nerve in the research domain and possibly for clinical diagnostics in the future.

Figure 1

Beer's Law depicting the exponential relationship of laser radiant exposure as a function of depth of laser penetration in a homogeneous tissue. In this graph, nerve tissue optical properties with Holmium:YAG laser irradiation are modeled using the threshold laser radiant exposure required for peripheral nerve excitation.

Figure 2

Experimental setup for optical and electrical simulation with electrical nerve and muscle recording techniques.

Figure 3

(a) CMAP recording from the rat gastrocnemius during optical stimulation (b) Upon topical administration of succinylcholine (c) 30 minutes following drug washout (Gain = 1).

Figure 4

(a) Measured CNAP following adjacent high intensity electrical stimulation (1.4 A/cm2) and recording at a distance of 10 mm demonstrate presence of electrical artifact. (b) The entire measured CNAP from optical stimulation (0.55 J/cm2) at the same site shows no artifact. Note the order of magnitude difference in the scale between electrical and optical recordings (Gain = 1).

Figure 5

Relationship between the laser radiant exposure [J/cm²] and peak amplitude of the evoked CNAP. The measured optical strength-response curve is similar to that observed in electrical stimulation. Notice a linear relationship exists between this peak ENG amplitude (Gain = 1) graphed as a function of the laser radiant exposure.

Figure 6

Spatial targeting of transient optical nerve stimulation. (a) Threshold CMAP response from electrical stimulation of the main branch of the sciatic nerve proximal to the first branch point with 1.02 A/cm2. (b) Corresponding results from threshold optical stimulation (0.4 J/cm²) of specific target nerve fibers that innervates the gastrocnemius with no response from adjacent nerve fibers (quiet biceps femoris). The distance from the stimulation spot to recording electrodes was held constant for trials involving each stimulation modality (Gain = 1).

Figure 7

Rat sciatic nerve functional map generated with optical stimulation using 400 and 600μm fiber diameters. Colors and shading representing the 12 muscles mapped of the hind leg are labeled to the right of the generalized functional map (n=12) The main sciatic nerve was stimulated just proximal to the branch point of the n. fibularis superficialis, n. fibularis profundus, and n. tibialis, while mapped muscles include the semitendinosus, gastrocnemius (upper and lower), vastis lateralis, biceps femoris (upper and lower), semimembranosus, and the 5 hindpaw digit muscles.

Figure 8

Penetration depth as a function of infrared wavelength from 2 to 6.5 μm based on the absorption spectrum of the rat sciatic nerve as measured through FTIR spectroscopy. The shaded region represents the theoretical optimal penetration depth range specific to the tissue morphology of the rat sciatic nerve. The spectral range for which optical fibers may be employed is also depicted.