Performing Crush Axotomy on the Facial Nerve in a Mouse Model

Begin with an anesthetized mouse placed on its side, with its right ear fixed and the area behind the ear shaved. 

Trace the posterior auricular vein and make a small incision to access underlying tissues. 

Dissect the tissue layers while preventing damage to nearby structures. 

Use a cotton swab to stop any bleeding. 

Locate the white facial nerve. 

Identify the ear canal, a pearly white structure, and the digastric muscle belly as key landmarks for accurately locating the facial nerve.

Dissect around these landmarks to expose the facial nerve trunk containing a bundle of axonal fibers that transmit signals to facial muscles.

Keep the exposed area open and, using forceps, gently crush the facial nerve trunk with constant pressure. 

Repeat the crush at the same site from a perpendicular angle. 

This controlled crushing induces mechanical stress on the axon fibers, resulting in their damage and disruption of neuronal signaling.

To determine the location of the first incision, trace the posterior auricular vein from the ear caudally to the area posterior of the ear protuberance. Then, using spring scissors, make a 4 millimeter incision 2 to 3 millimeters posterior to the protuberance. For teaching purposes, this incision will be enlarged to expose the underlying anatomy.

Now, make a blunt dissection of the subcutaneous fat and fascia. Blood vessels and muscle can be easily damaged by scissors. If bleeding occurs, apply pressure with a sterile cotton swab for at least 30 seconds. If significant fluid loss has occurred, inject up to 1/2 a milliliter of saline intraperitoneally with a 25 or 27-gauge needle.

Now, locate the facial nerve using the key landmarks. The spinal accessory nerve is superficial to the facial nerve and can be seen traveling from the caudal portion of the skull, to innervate the trapezius muscle. Identify the cartilaginous ear canal. It is pearly white and can be seen rostral to the facial nerve. The facial nerve wraps around the caudal portion of the ear canal.

Then, identify the muscle belly of the anterior digastric muscle that lies on top of and caudal to the facial nerve. Dissect under this structure to expose the facial nerve. It is a solid white structure adhered to a layer of underlying fascia. Identify the main branches of the facial nerve. Trace them dorsally to their origin from the stylomastoid foramen.

Use fine forceps to hold open the site while using the tip of the spring scissors to dissect along the facial nerve's path. The bony stylomastoid foramen can be felt by the forceps. Now, identify the facial nerve trunk. To crush the nerve, compress it with the fine forceps for 30 seconds with constant pressure. This will sever all the axons. Then repeat the compression at a perpendicular angle for another 30 seconds. Keep the pressure consistent between experiments.