Two contrasting behaviors: The first is steady swimming at slow and intermediate speeds using low amplitude axial undulations (i.e. the side to side displacement of the body is small). The second is burst swimming and escape responses using high amplitude axial undulation. In the escape response, a fish bends into a C-like posture then whips its tail in the opposite direction, accelerating a mass of water behind the fish, which causes the fish to accelerate forward. The key is that body bending is small in steady swimming and large in bursts and escapes. Also, the escape response can be very fast, that is less than 1/10th of a second.

The toadfish is an important fish muscle specimen because it has been found that its swim bladder muscles are the fastest twitching muscles in the vertebrate world. The image by Robert Golder shown here is from an article at the Marine Biological Laboratory on this talented but ugly fish. Several articles at the St. Andrews University website of its Fish Muscle Research Group provide the latest ideas and theories in this important subject for mechanics, anatomy, genetics, and aquaculture.

The image for this learn node is from Dr. Emad Eskandar’s Harvard-MIT Health Sciences and Technology Neuroscience course handout (Motor Systems I). Page 2, With the image the handout explains:

We will begin our discussion from the bottom up starting with the physiology of the muscles and the spinal cord. An important concept to grasp is that of the motor unit. The following points should be kept in mind.
- A whole muscle is made up of many muscle fibers
- A muscle fiber is a single mutlinucleated cell
- Each muscle fiber is innervated by only one alpha motor neuron
- Each alpha motor neuron innervates numerous muscle fibers within a muscle
- A single neuron and all the muscle fibers it innervates are a motor unit
- The motor unit is the smallest division that the system can control individually

You can connect the muscle concepts above to another superb academic source by going to the Tufts Dental School course: Histology: Study of Cells, Tissues and Organs. Lecture 9: Muscle, on page 5 of the PDF summarizes the sequence of events of a muscle contraction. This Tufts muscle lecture can flex the most curious young mind � one that wants move past the medium learning fare.

To drop by and look over the shoulders of some scientists learning about muscle motor neurons from the transparent spinal cord of the zebrafish, click into The Journal of Neuroscience, where the articles are freely open online for the benefit of scientists, students and teachers.

For a look at the same information in non-academic, non-medical terms: How Muscles Work at HowStuffWorks.com.

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