Refining the Sensory and Motor Ratunculus of the Rodent Upper Extremity: Evaluation of the C7 Nerve Root Using fMRI and Direct Nerve Stimulation

Abstract

Background

Since the 1980s, the C7 nerve root has gained clinical relevance as a donor nerve in severe brachial plexus root avulsion injuries. Despite success with the cross-chest C7 nerve transfer, inducing injury on an otherwise normal side hinders global acceptance. By sacrificing the C7 nerve root, a predictable pattern of transient sequelae is seen, including extensor weakness and index and middle finger anesthesia. The purpose of this study is to observe cortical activity during direct stimulation of the C7 nerve root using blood oxygen level dependent functional magnetic resonance imaging (fMRI) in a rat model.

Methods

A total of 12 male Sprague-Dawley rats, weighing 200–250 g, were used in this study. Following an acclimation period of 1 week, 12 rats underwent exposure and dissection of the brachial plexus. Seven rats underwent placement of an implantable electrode (AISI 304, Plastics1, Roanoke, VA, USA) on the C7 nerve root, while five rats underwent electrode placement on the radial nerve. All animals then underwent fMRI during direct nerve stimulation. Ten consecutive coronal images were obtained during nerve stimulation, using a 9.4-T small-animal MRI scanner.

Results

Cortical activation is seen within a very specific area of the primary sensory region of the forelimb during C7 nerve root stimulation. The cortical activation seen during radial nerve stimulation includes that seen during C7 stimulation but extends several slices caudally.

Conclusions

The sensory representation of the C7 nerve root is seen in only a small area in the S1FL region compared to that seen in the terminal branches of the brachial plexus. However, this area shows a significant overlap with the S1FL area of activation seen during radial nerve stimulation. This makes sense as the C7 nerve root contributes some, but not all, sensory axons to the radial nerve. Mapping of the C7 cortical representation in the rat brain not only adds to the ongoing development of the motor and sensory ratunculus but also provides an important foundation to study subsequent C7 donor nerve models.

Keywords

C7 nerve root Rodent MRI Cortical plasticity

Get full access to this article

View all access options for this article.

References

Bandettini

Wong

Hinks

Tikofsky

Hyde

. Time course EPI of human brain function during task activation. Magn Reson Med. 1992;25:390–7.

Bertelli

Mira

Gilbert

Michot

Legagneuz

. Anatomical basis of rat brachial plexus reconstruction. Surg Radiol Anat. 1992;14(1):85–6.

Bertelli

Taleb

Saadi

Mira

Pecot-Dechavassine

. The rat brachial plexus and its terminal branches: an experimental model for the study of peripheral nerve regeneration. Microsurgery. 1995;16 (2):77–85.

Cho

Jones

Pawela

Kao

Schulte

. Cortical brain mapping of peripheral nerves using functional magnetic resonance imaging in a rodent model. J Reconstr Microsurg. 2008;24:551–7.

Cho

Pawela

Kao

Schulte

Runquist

. Refining the sensory and motor ratunculus of the rat upper extremity using fMRI and direct nerve stimulation. Magn Reson Med. 2007;58:901–9.

. Contralateral C7 root transfer over the last 20 years in China. Chin Med J. 2007;120(13):1123–6.

. Nerve transfer for treatment of root avulsion of the brachial plexus: experimental studies in a rat model. J Reconstr Microsurg. 1991;7(1):15–22.

Zhang

Chen

. Cervical nerve root transfer from contralateral normal side for treatment of brachial plexus root avulsions. Chin Med J. 1991;104:208.

Zhang

Chen

Yan

. Seventh cervical nerve root transfer from the contralateral healthy side for treatment of brachial plexus root avulsion. Br J Hand Surg. 1992;17:518–21.

10.

Kaas

. What, if anything, is SI? Organization of first somatosensory area of cortex. Physiol Rev. 1983;63:206–31.

11.

Kaas

. Plasticity of sensory and motor maps in adult mammals. Annu Rev Neurosci. 1991;14:137–68.

12.

Kaas

. Topographic maps are fundamental to sensory processing. Brain Res. 1997;44:107–12.

13.

Kaas

Merzenich

Killackey

. The reorganization of somatosensory cortex following peripheral nerve damage in adult and developing mammals. Ann Rev Neuroscience. 1983;6:325–56.

14.

Kaas

Nelson

Sur

Lin

Merzenich

. Multiple representations of the body within the primary somatosensory cortex of primates. Science. 1979;204(4392):521–3.

15.

Kwong

Belliveau

Chesler

Goldberg

. Dynamic magnetic resonance imaging of human brain activity during primary sensory stimulation. Proc Natl Acad Sci USA. 1992;89(12):5675–9.

16.

Hettinger

Pawela

. Detecting single cortical column activation under super high spatial resolution at 9.4 T using single-shot half k-space GR-EPI (Abstract). Presented at the 18th annual International Society for Magnetic Resonance in Medicine Meeting. Stockholm, Sweden. May 1–7. 2010.

17.

Lundborg

. Brain plasticity and hand surgery: an overview. Br J Hand Surg. 2000;25(3):242–52.

18.

Lundborg

. Nerve injury and repair—a challenge to the plastic brain. J Hand Surg. 2003;25:242–52.

19.

Merzenich

Kaas

Wall

Nelson

Sur

Felleman

. Topographical reorganization of somatosensory cortical areas 3b and 1 in adult monkeys following restricted deafferentation. Neuroscience. 1983;8(1):33–55.

20.

Merzenich

Nelson

Stryker

Cynader

Schoppmann

Zook

. Somatosensory cortical map changes following digit amputation in adult monkeys. J Compar Neurol. 1984;224(4):591–605.

21.

Ogawa

Tank

Menon

Ellermann

. Intrinsic signal changes accompanying sensory stimulation: functional brain mapping with magnetic resonance imaging. Proc Natl Acad Sci USA. 1992;89(13):5951–5.

22.

Parkins

Matloub

Yan

Hyde

Pawela

. A peripheral nerve repair model using fMRI in rats (Abstract). Presented at the 17th annual International Society for Magnetic Resonance in Medicine Meeting. Honolulu, HI. USA. April 18–24. 2009.

23.

Pauling

Coryell

. The magnetic properties and structure of hemoglobin, oxyhemoglobin and carbon monooxyhemoglobin. Proc Natl Acad Sci USA. 1936;22:210–6.

24.

Pawela

Biswal

Hudetz

Schulte

. A protocol for use of medetomidine anesthesia in rats for extended studies using task-induced BOLD contrast and resting state functional connectivity. Neuroimage. 2009;46(4):1137–47.