Recording from Sensory Nerves While Stretching Muscles
Stretch receptors are specialized fibers that are in parallel to the fibers in muscle. The receptors stretch as the muscle is lengthened, and they generate action potentials. The frequency of these action potentials is proportional to the length of muscle stretch and the position of the muscle. Through sensory nerve fibers from the stretch receptors, the action potentials and their frequency provide feedback to the central nervous system that will modulate reflex responses and motor control of the muscle.
For these studies, the whole muscle and the nerve that innervates it are isolated from the organism. A muscle and its nerve, like the Soleus, is isolated and placed in the cuvette of an SI-MT or SI-HTB muscle research system. The muscle is positioned so that the myoneural junction (the place where the nerve innervates the muscle) is on the top side of the muscle. The ends of the muscle are attached to a transducer and a motor through the tissue holders, and the end of the nerve is in the bath solution in the cuvette.
Making Recordings - Suction Electrode
Recordings are made from the nerve as the muscle is stretched and relaxed using a device known as a suction electrode. Suction electrodes are specialized extracellular electrodes designed to hold onto the ends or sides of nerves and record the action potentials passing through those nerves. The tip of the suction electrode is a short, glass microcapillary pipette with an internal diameter that matches the diameter of the nerve. The conductors in a suction electrode are positioned so that the nerve can be kept alive in the bath solution and the action potentials can be recorded at the same time. The signals recorded by the suction electrode are amplified through a simple, extracellular amplifier and recorded on the same data acquisition unit as the muscle length and the muscle tension.
Materials Needed to Modify an SI-H System
To add this feature to an SI-MT or SI-HTB system, the following items are needed:
(1) Micromanipulator or micropositioner. The unit should be no larger than the KITE or the M3301. The M325 is ideal. The control should be right-handed, because the unit will be to the right of the microscope head.
(1) Micromanipulator rod. Since the base of the SI-MT and SI-MKB is non-ferrous, the micromanipulator needs to be mounted on a rod screwed into the base of the system in a position that allows the electrode to reach the center of the cuvette.
(1) Suction electrode. Suction electrodes are delicate and are best constructed by the user. They need to be made of fine wire so that electrode can flex when the muscle is stretched. See Constructing an Extracellular Suction Electrode.