Abstract

Previously we have reported a near-infrared optical response in the region occupied by a peripheral nerve that is distal to the site of electrical stimulation of that peripheral nerve. This “intermediate” signal is vascular in nature but its biological origin not been elucidated. In the present study, an animal model of the signal has been created and our human studies expanded to directly investigate the contribution of non-artifactual vascular motion induced by muscle contraction to the biological origin of this signal. Under non-invasive conditions during stimulation of the exposed sciatic nerve of the Sprague-Dawley rat, optical responses are robust. These signals can be abolished both pharmacologically and surgically using methods that eliminate muscle motion while leaving the electrophysiological health of the nerve intact. In human studies, signals that are elicited on stimulation of nerves containing motor axons, both within and outside the predicted imaging volume of the spectrometer, have similar temporal characteristics of those previously observed. Moreover, stimulation of sensory nerves alone does not elicit an optical response. These results strongly suggest that the intermediate signals are derived from stimulus-induced muscle contraction (whether via an innervating nerve or by direct stimulation) causing translational vascular motion within the optically interrogated region.

© 2010 OSA

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References

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  1. Y. Tong, J. M. Martin, A. Sassaroli, P. R. Clervil, P. R. Bergethon, and S. Fantini, “Fast optical signals in the peripheral nervous system,” J. Biomed. Opt. 11(4), 044014 (2006).
    [CrossRef] [PubMed]
  2. D. K. Chen, Y. Tong, A. Sassaroli, P. R. Bergethon, and S. Fantini, “Fast optical response to electrical activation in peripheral nerves - art. no. 643104,” Multimodal Biomedical Imaging II 6431, 43104–43104 43100 (2007).
  3. S. Fantini, D. K. Chen, J. M. Martin, A. Sassaroli, and P. R. Bergethon, “Near-infrared signals associated with electrical stimulation of peripheral nerves,” Proc SPIE 7174 (2009).
  4. J. Steinbrink, F. C. Kempf, A. Villringer, and H. Obrig, “The fast optical signal--robust or elusive when non-invasively measured in the human adult?” Neuroimage 26(4), 996–1008 (2005).
    [CrossRef] [PubMed]
  5. D. K. Chen, M. K. Erb, Y. Yu, A. Sassaroli, P. R. Bergethon, and S. Fantini, “Spectral and Spatial Features of Diffuse Optical Signals in Response to Peripheral Nerve Stimulation,” submitted.
  6. S. Virmani, D. K. Tempe, V. Datt, A. S. Tomar, A. Banerjee, H. S. Minhas, and S. Goel, “Effect of muscle relaxants on heart rate, arterial pressure, intubation conditions and onset of neuromuscular block in patients undergoing valve surgery,” Ann. Card. Anaesth. 9(1), 37–43 (2006).
    [PubMed]
  7. A. Esmaoglu, A. Akin, A. Mizrak, Y. Turk, and A. Boyaci, “Addition of cisatracurium to lidocaine for intravenous regional anesthesia,” J. Clin. Anesth. 18(3), 194–197 (2006).
    [CrossRef] [PubMed]
  8. S. Fantini, M. A. Franceschini, J. B. Fishkin, B. Barbieri, and E. Gratton, “Quantitative-Determination of the Absorption-Spectra of Chromophores in Strongly Scattering Media - a Light-Emitting-Diode Based Technique,” Appl. Opt. 33(22), 5204–5213 (1994).
    [CrossRef]
  9. S. Fantini, M. A. Franceschini, and E. Gratton, “Semi-Infinite-Geometry Boundary-Problem for Light Migration in Highly Scattering Media - a Frequency-Domain Study in the Diffusion-Approximation,” J. Opt. Soc. Am. B 11(10), 2128–2138 (1994).
    [CrossRef]
  10. V. Quaresima, M. A. Franceschini, S. Fantini, E. Gratton, and M. Ferrari, “Difference in leg muscles oxygenation during treadmill exercise by a new near infrared frequency-domain oximeter,” Photon Propagation in Tissues Iii, Proceedings Of 3194, 116–120532 (1998).
  11. H. L. Liu, D. A. Boas, Y. T. Zhang, A. G. Yodh, and B. Chance, “A simplified approach to characterize optical properties and blood oxygenation in tissue using continuous near infrared,” Optical Tomography, Photon Migration, and Spectroscopy of Tissue and Model Media: Theory, Human Studies, and Instrumentation, Proceedings of, Pts 1 and 2 2359, 496–502880 (1995).
  12. C. Chen, N. Yamaguchi, and F. Varin, “Dose-dependency of pharmacokinetic/pharmacodynamic parameters after intravenous bolus doses of cisatracurium,” Br. J. Anaesth. 101(6), 788–797 (2008).
    [CrossRef] [PubMed]
  13. F. Buchthal and H. Schmalbruch, “Contraction times and fibre types in intact human muscle,” Acta Physiol. Scand. 79(4), 435–452 (1970).
    [CrossRef] [PubMed]
  14. R. Close, “Dynamic Properties of Fast and Slow Skeletal Muscles of the Rat during Development,” J. Physiol. 173, 74–95 (1964).
    [PubMed]

2008 (1)

C. Chen, N. Yamaguchi, and F. Varin, “Dose-dependency of pharmacokinetic/pharmacodynamic parameters after intravenous bolus doses of cisatracurium,” Br. J. Anaesth. 101(6), 788–797 (2008).
[CrossRef] [PubMed]

2006 (3)

Y. Tong, J. M. Martin, A. Sassaroli, P. R. Clervil, P. R. Bergethon, and S. Fantini, “Fast optical signals in the peripheral nervous system,” J. Biomed. Opt. 11(4), 044014 (2006).
[CrossRef] [PubMed]

S. Virmani, D. K. Tempe, V. Datt, A. S. Tomar, A. Banerjee, H. S. Minhas, and S. Goel, “Effect of muscle relaxants on heart rate, arterial pressure, intubation conditions and onset of neuromuscular block in patients undergoing valve surgery,” Ann. Card. Anaesth. 9(1), 37–43 (2006).
[PubMed]

A. Esmaoglu, A. Akin, A. Mizrak, Y. Turk, and A. Boyaci, “Addition of cisatracurium to lidocaine for intravenous regional anesthesia,” J. Clin. Anesth. 18(3), 194–197 (2006).
[CrossRef] [PubMed]

2005 (1)

J. Steinbrink, F. C. Kempf, A. Villringer, and H. Obrig, “The fast optical signal--robust or elusive when non-invasively measured in the human adult?” Neuroimage 26(4), 996–1008 (2005).
[CrossRef] [PubMed]

1998 (1)

V. Quaresima, M. A. Franceschini, S. Fantini, E. Gratton, and M. Ferrari, “Difference in leg muscles oxygenation during treadmill exercise by a new near infrared frequency-domain oximeter,” Photon Propagation in Tissues Iii, Proceedings Of 3194, 116–120532 (1998).

1994 (2)

1970 (1)

F. Buchthal and H. Schmalbruch, “Contraction times and fibre types in intact human muscle,” Acta Physiol. Scand. 79(4), 435–452 (1970).
[CrossRef] [PubMed]

1964 (1)

R. Close, “Dynamic Properties of Fast and Slow Skeletal Muscles of the Rat during Development,” J. Physiol. 173, 74–95 (1964).
[PubMed]

Akin, A.

A. Esmaoglu, A. Akin, A. Mizrak, Y. Turk, and A. Boyaci, “Addition of cisatracurium to lidocaine for intravenous regional anesthesia,” J. Clin. Anesth. 18(3), 194–197 (2006).
[CrossRef] [PubMed]

Banerjee, A.

S. Virmani, D. K. Tempe, V. Datt, A. S. Tomar, A. Banerjee, H. S. Minhas, and S. Goel, “Effect of muscle relaxants on heart rate, arterial pressure, intubation conditions and onset of neuromuscular block in patients undergoing valve surgery,” Ann. Card. Anaesth. 9(1), 37–43 (2006).
[PubMed]

Barbieri, B.

Bergethon, P. R.

Y. Tong, J. M. Martin, A. Sassaroli, P. R. Clervil, P. R. Bergethon, and S. Fantini, “Fast optical signals in the peripheral nervous system,” J. Biomed. Opt. 11(4), 044014 (2006).
[CrossRef] [PubMed]

D. K. Chen, M. K. Erb, Y. Yu, A. Sassaroli, P. R. Bergethon, and S. Fantini, “Spectral and Spatial Features of Diffuse Optical Signals in Response to Peripheral Nerve Stimulation,” submitted.

Boyaci, A.

A. Esmaoglu, A. Akin, A. Mizrak, Y. Turk, and A. Boyaci, “Addition of cisatracurium to lidocaine for intravenous regional anesthesia,” J. Clin. Anesth. 18(3), 194–197 (2006).
[CrossRef] [PubMed]

Buchthal, F.

F. Buchthal and H. Schmalbruch, “Contraction times and fibre types in intact human muscle,” Acta Physiol. Scand. 79(4), 435–452 (1970).
[CrossRef] [PubMed]

Chen, C.

C. Chen, N. Yamaguchi, and F. Varin, “Dose-dependency of pharmacokinetic/pharmacodynamic parameters after intravenous bolus doses of cisatracurium,” Br. J. Anaesth. 101(6), 788–797 (2008).
[CrossRef] [PubMed]

Chen, D. K.

D. K. Chen, M. K. Erb, Y. Yu, A. Sassaroli, P. R. Bergethon, and S. Fantini, “Spectral and Spatial Features of Diffuse Optical Signals in Response to Peripheral Nerve Stimulation,” submitted.

Clervil, P. R.

Y. Tong, J. M. Martin, A. Sassaroli, P. R. Clervil, P. R. Bergethon, and S. Fantini, “Fast optical signals in the peripheral nervous system,” J. Biomed. Opt. 11(4), 044014 (2006).
[CrossRef] [PubMed]

Close, R.

R. Close, “Dynamic Properties of Fast and Slow Skeletal Muscles of the Rat during Development,” J. Physiol. 173, 74–95 (1964).
[PubMed]

Datt, V.

S. Virmani, D. K. Tempe, V. Datt, A. S. Tomar, A. Banerjee, H. S. Minhas, and S. Goel, “Effect of muscle relaxants on heart rate, arterial pressure, intubation conditions and onset of neuromuscular block in patients undergoing valve surgery,” Ann. Card. Anaesth. 9(1), 37–43 (2006).
[PubMed]

Erb, M. K.

D. K. Chen, M. K. Erb, Y. Yu, A. Sassaroli, P. R. Bergethon, and S. Fantini, “Spectral and Spatial Features of Diffuse Optical Signals in Response to Peripheral Nerve Stimulation,” submitted.

Esmaoglu, A.

A. Esmaoglu, A. Akin, A. Mizrak, Y. Turk, and A. Boyaci, “Addition of cisatracurium to lidocaine for intravenous regional anesthesia,” J. Clin. Anesth. 18(3), 194–197 (2006).
[CrossRef] [PubMed]

Fantini, S.

Y. Tong, J. M. Martin, A. Sassaroli, P. R. Clervil, P. R. Bergethon, and S. Fantini, “Fast optical signals in the peripheral nervous system,” J. Biomed. Opt. 11(4), 044014 (2006).
[CrossRef] [PubMed]

V. Quaresima, M. A. Franceschini, S. Fantini, E. Gratton, and M. Ferrari, “Difference in leg muscles oxygenation during treadmill exercise by a new near infrared frequency-domain oximeter,” Photon Propagation in Tissues Iii, Proceedings Of 3194, 116–120532 (1998).

S. Fantini, M. A. Franceschini, J. B. Fishkin, B. Barbieri, and E. Gratton, “Quantitative-Determination of the Absorption-Spectra of Chromophores in Strongly Scattering Media - a Light-Emitting-Diode Based Technique,” Appl. Opt. 33(22), 5204–5213 (1994).
[CrossRef]

S. Fantini, M. A. Franceschini, and E. Gratton, “Semi-Infinite-Geometry Boundary-Problem for Light Migration in Highly Scattering Media - a Frequency-Domain Study in the Diffusion-Approximation,” J. Opt. Soc. Am. B 11(10), 2128–2138 (1994).
[CrossRef]

D. K. Chen, M. K. Erb, Y. Yu, A. Sassaroli, P. R. Bergethon, and S. Fantini, “Spectral and Spatial Features of Diffuse Optical Signals in Response to Peripheral Nerve Stimulation,” submitted.

Ferrari, M.

V. Quaresima, M. A. Franceschini, S. Fantini, E. Gratton, and M. Ferrari, “Difference in leg muscles oxygenation during treadmill exercise by a new near infrared frequency-domain oximeter,” Photon Propagation in Tissues Iii, Proceedings Of 3194, 116–120532 (1998).

Fishkin, J. B.

Franceschini, M. A.

Goel, S.

S. Virmani, D. K. Tempe, V. Datt, A. S. Tomar, A. Banerjee, H. S. Minhas, and S. Goel, “Effect of muscle relaxants on heart rate, arterial pressure, intubation conditions and onset of neuromuscular block in patients undergoing valve surgery,” Ann. Card. Anaesth. 9(1), 37–43 (2006).
[PubMed]

Gratton, E.

Kempf, F. C.

J. Steinbrink, F. C. Kempf, A. Villringer, and H. Obrig, “The fast optical signal--robust or elusive when non-invasively measured in the human adult?” Neuroimage 26(4), 996–1008 (2005).
[CrossRef] [PubMed]

Martin, J. M.

Y. Tong, J. M. Martin, A. Sassaroli, P. R. Clervil, P. R. Bergethon, and S. Fantini, “Fast optical signals in the peripheral nervous system,” J. Biomed. Opt. 11(4), 044014 (2006).
[CrossRef] [PubMed]

Minhas, H. S.

S. Virmani, D. K. Tempe, V. Datt, A. S. Tomar, A. Banerjee, H. S. Minhas, and S. Goel, “Effect of muscle relaxants on heart rate, arterial pressure, intubation conditions and onset of neuromuscular block in patients undergoing valve surgery,” Ann. Card. Anaesth. 9(1), 37–43 (2006).
[PubMed]

Mizrak, A.

A. Esmaoglu, A. Akin, A. Mizrak, Y. Turk, and A. Boyaci, “Addition of cisatracurium to lidocaine for intravenous regional anesthesia,” J. Clin. Anesth. 18(3), 194–197 (2006).
[CrossRef] [PubMed]

Obrig, H.

J. Steinbrink, F. C. Kempf, A. Villringer, and H. Obrig, “The fast optical signal--robust or elusive when non-invasively measured in the human adult?” Neuroimage 26(4), 996–1008 (2005).
[CrossRef] [PubMed]

Quaresima, V.

V. Quaresima, M. A. Franceschini, S. Fantini, E. Gratton, and M. Ferrari, “Difference in leg muscles oxygenation during treadmill exercise by a new near infrared frequency-domain oximeter,” Photon Propagation in Tissues Iii, Proceedings Of 3194, 116–120532 (1998).

Sassaroli, A.

Y. Tong, J. M. Martin, A. Sassaroli, P. R. Clervil, P. R. Bergethon, and S. Fantini, “Fast optical signals in the peripheral nervous system,” J. Biomed. Opt. 11(4), 044014 (2006).
[CrossRef] [PubMed]

D. K. Chen, M. K. Erb, Y. Yu, A. Sassaroli, P. R. Bergethon, and S. Fantini, “Spectral and Spatial Features of Diffuse Optical Signals in Response to Peripheral Nerve Stimulation,” submitted.

Schmalbruch, H.

F. Buchthal and H. Schmalbruch, “Contraction times and fibre types in intact human muscle,” Acta Physiol. Scand. 79(4), 435–452 (1970).
[CrossRef] [PubMed]

Steinbrink, J.

J. Steinbrink, F. C. Kempf, A. Villringer, and H. Obrig, “The fast optical signal--robust or elusive when non-invasively measured in the human adult?” Neuroimage 26(4), 996–1008 (2005).
[CrossRef] [PubMed]

Tempe, D. K.

S. Virmani, D. K. Tempe, V. Datt, A. S. Tomar, A. Banerjee, H. S. Minhas, and S. Goel, “Effect of muscle relaxants on heart rate, arterial pressure, intubation conditions and onset of neuromuscular block in patients undergoing valve surgery,” Ann. Card. Anaesth. 9(1), 37–43 (2006).
[PubMed]

Tomar, A. S.

S. Virmani, D. K. Tempe, V. Datt, A. S. Tomar, A. Banerjee, H. S. Minhas, and S. Goel, “Effect of muscle relaxants on heart rate, arterial pressure, intubation conditions and onset of neuromuscular block in patients undergoing valve surgery,” Ann. Card. Anaesth. 9(1), 37–43 (2006).
[PubMed]

Tong, Y.

Y. Tong, J. M. Martin, A. Sassaroli, P. R. Clervil, P. R. Bergethon, and S. Fantini, “Fast optical signals in the peripheral nervous system,” J. Biomed. Opt. 11(4), 044014 (2006).
[CrossRef] [PubMed]

Turk, Y.

A. Esmaoglu, A. Akin, A. Mizrak, Y. Turk, and A. Boyaci, “Addition of cisatracurium to lidocaine for intravenous regional anesthesia,” J. Clin. Anesth. 18(3), 194–197 (2006).
[CrossRef] [PubMed]

Varin, F.

C. Chen, N. Yamaguchi, and F. Varin, “Dose-dependency of pharmacokinetic/pharmacodynamic parameters after intravenous bolus doses of cisatracurium,” Br. J. Anaesth. 101(6), 788–797 (2008).
[CrossRef] [PubMed]

Villringer, A.

J. Steinbrink, F. C. Kempf, A. Villringer, and H. Obrig, “The fast optical signal--robust or elusive when non-invasively measured in the human adult?” Neuroimage 26(4), 996–1008 (2005).
[CrossRef] [PubMed]

Virmani, S.

S. Virmani, D. K. Tempe, V. Datt, A. S. Tomar, A. Banerjee, H. S. Minhas, and S. Goel, “Effect of muscle relaxants on heart rate, arterial pressure, intubation conditions and onset of neuromuscular block in patients undergoing valve surgery,” Ann. Card. Anaesth. 9(1), 37–43 (2006).
[PubMed]

Yamaguchi, N.

C. Chen, N. Yamaguchi, and F. Varin, “Dose-dependency of pharmacokinetic/pharmacodynamic parameters after intravenous bolus doses of cisatracurium,” Br. J. Anaesth. 101(6), 788–797 (2008).
[CrossRef] [PubMed]

Yu, Y.

D. K. Chen, M. K. Erb, Y. Yu, A. Sassaroli, P. R. Bergethon, and S. Fantini, “Spectral and Spatial Features of Diffuse Optical Signals in Response to Peripheral Nerve Stimulation,” submitted.

Acta Physiol. Scand. (1)

F. Buchthal and H. Schmalbruch, “Contraction times and fibre types in intact human muscle,” Acta Physiol. Scand. 79(4), 435–452 (1970).
[CrossRef] [PubMed]

Ann. Card. Anaesth. (1)

S. Virmani, D. K. Tempe, V. Datt, A. S. Tomar, A. Banerjee, H. S. Minhas, and S. Goel, “Effect of muscle relaxants on heart rate, arterial pressure, intubation conditions and onset of neuromuscular block in patients undergoing valve surgery,” Ann. Card. Anaesth. 9(1), 37–43 (2006).
[PubMed]

Appl. Opt. (1)

Br. J. Anaesth. (1)

C. Chen, N. Yamaguchi, and F. Varin, “Dose-dependency of pharmacokinetic/pharmacodynamic parameters after intravenous bolus doses of cisatracurium,” Br. J. Anaesth. 101(6), 788–797 (2008).
[CrossRef] [PubMed]

J. Biomed. Opt. (1)

Y. Tong, J. M. Martin, A. Sassaroli, P. R. Clervil, P. R. Bergethon, and S. Fantini, “Fast optical signals in the peripheral nervous system,” J. Biomed. Opt. 11(4), 044014 (2006).
[CrossRef] [PubMed]

J. Clin. Anesth. (1)

A. Esmaoglu, A. Akin, A. Mizrak, Y. Turk, and A. Boyaci, “Addition of cisatracurium to lidocaine for intravenous regional anesthesia,” J. Clin. Anesth. 18(3), 194–197 (2006).
[CrossRef] [PubMed]

J. Opt. Soc. Am. B (1)

J. Physiol. (1)

R. Close, “Dynamic Properties of Fast and Slow Skeletal Muscles of the Rat during Development,” J. Physiol. 173, 74–95 (1964).
[PubMed]

Neuroimage (1)

J. Steinbrink, F. C. Kempf, A. Villringer, and H. Obrig, “The fast optical signal--robust or elusive when non-invasively measured in the human adult?” Neuroimage 26(4), 996–1008 (2005).
[CrossRef] [PubMed]

Photon Propagation in Tissues Iii, Proceedings Of (1)

V. Quaresima, M. A. Franceschini, S. Fantini, E. Gratton, and M. Ferrari, “Difference in leg muscles oxygenation during treadmill exercise by a new near infrared frequency-domain oximeter,” Photon Propagation in Tissues Iii, Proceedings Of 3194, 116–120532 (1998).

Other (4)

H. L. Liu, D. A. Boas, Y. T. Zhang, A. G. Yodh, and B. Chance, “A simplified approach to characterize optical properties and blood oxygenation in tissue using continuous near infrared,” Optical Tomography, Photon Migration, and Spectroscopy of Tissue and Model Media: Theory, Human Studies, and Instrumentation, Proceedings of, Pts 1 and 2 2359, 496–502880 (1995).

D. K. Chen, M. K. Erb, Y. Yu, A. Sassaroli, P. R. Bergethon, and S. Fantini, “Spectral and Spatial Features of Diffuse Optical Signals in Response to Peripheral Nerve Stimulation,” submitted.

D. K. Chen, Y. Tong, A. Sassaroli, P. R. Bergethon, and S. Fantini, “Fast optical response to electrical activation in peripheral nerves - art. no. 643104,” Multimodal Biomedical Imaging II 6431, 43104–43104 43100 (2007).

S. Fantini, D. K. Chen, J. M. Martin, A. Sassaroli, and P. R. Bergethon, “Near-infrared signals associated with electrical stimulation of peripheral nerves,” Proc SPIE 7174 (2009).

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Figures (8)

Fig. 1
Fig. 1

Experimental setup for the animal model. A surgical window was created over the proximal course of the left sciatic nerve in Sprague-Dawley rats. A non-invasive optical probe delivering 690 and 830 nm light at 3 different source-detector separation distances was coupled to the PMT of a frequency-domain spectrometer and oriented perpendicularly to the course of the nerve. 60 x 600 ms epochs, each synchronized to the delivery of a 0.1 ms current stimulus to the exposed nerve (1), were averaged to generate an optical response. Simultaneously, electrophysiological responses to the stimulus were recorded from the plantar muscles of the left foot. (2) ground electrode, (3) active recording electrode, (4) reference electrode.

Fig. 2
Fig. 2

Experimental setup for the human studies. Myogenic components of the optical response were investigated in human subjects using 6 standard NCV/EMG tests (3 Motor Action Potential tests and 3 Sensory Nerve Action Potential tests). (a) Configuration of the Ag/AgCl electrodes and optical probe while making CMAP measurements of APB during stimulation of the median nerve at the elbow. The anatomical cross section illustrates the location of the median and ulnar nerves in the region of optical interrogation. The probe was positioned to interrogate the median nerve. The configuration of electrodes for investigation of the recurrent branch of the (b) median nerve and (c) ulnar nerve.Optical probe and electrode setup for the (d) median, (e) medial antebrachial cutaneous and (f) lateral antebrachial cutaneous nerves.

Fig. 3
Fig. 3

Typical results from middle source-detector separation in the animal model. (a) A plantar muscle action potential. (b) Signals generated in the lower limb of the Sprague-Dawley rat are faster than signals generated in human subjects.

Fig. 4
Fig. 4

Signals do not depend on the distance between source and detector. From left to right are examples of signals obtained from each source-detector separation in 1 rat. Across all 7 rats, there was no relationship between source-detector separation and response amplitude, suggesting that the response was not localized to an anatomical compartment with a specific depth from the surface of the tissue.

Fig. 5
Fig. 5

The effect of Cisatracurium Besylate on signals generated in the rat. (a) Signals from the middle source-detector separation in one of the rats prior to delivery of the drug. (b) Optical traces from the same rat following drug delivery. (c) Following a 15 minute washout period, both the electrophysiological and optical response returned.

Fig. 6
Fig. 6

Optical responses at the wrist during stimulation of mixed nerves in subject 2. (a) CMAP generated in APB (left) and the corresponding optical response (right) during stimulation of the median nerve at the elbow [c.f. Fig. 2, panel (a)]. (b) CMAP and corresponding optical trace during stimulation of the recurrent branch of the median nerve in the palm [c.f. Fig. 2, panel (b)]. (c) CMAP and corresponding optical response during stimulation of the ulnar nerve at the elbow [c.f. Fig. 2, panel (c)]. Differences in the latency-to-onset of the electrophysiological response between stimulation at the elbow and at the recurrent branch of the median nerve reflect the increased distance (~20 cm) over which the nerve impulse must travel before depolarizing the muscle.

Fig. 7
Fig. 7

Optical data during sensory nerve stimulation in subject 2. (a) Median nerve SNAP measured at the wrist during stimulation of the 2nd digit (top trace) and the corresponding optical data [c.f. Fig. 2(e) for the location of the probe]. (b) Medial antebrachial cutaneous SNAP and corresponding optical data [c.f. Fig. 2(f) for the location of the probe]. (c) Lateral antebrachial cutaneous SNAP and corresponding optical data [c.f. Fig. 2(g) for the location of the probe].

Fig. 8
Fig. 8

Photographs taken through a dissecting microscope of the exposed sciatic nerve of the rat. Panels (a) and (b) are nerves from separate surgeries on different rats at different stages of the nerve exposure. (a) Vessels accompanying the nerve in the surrounding fascia. (b) With the fascia further dissected away, the epineurial plexus of arterioles that supply the nerve can be directly visualized.

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