Do both ends of a muscle contract?

Question

I was under the impression that both ends of a muscle contract. For instance, the fibers of the biceps run parallel to the humerus so I thought they pulled toward the middle.

But now I'm confused because it sounds like the contraction of the fibers doesn't necessarily have to parallel the motion of the muscle itself. In other words, just because the fibers contract doesn't mean that the muscle lengthens or shortens in a particular direction.

Can someone clarify if muscles contract in both directions?

Or perhaps a better question is do muscle contract in a particular direction and is this the same direction that the fibers contract or does the muscle as a whole move in a different direction in comparison to the direction the fibers are contracting?

Answer 1

The muscular fibers (or more exactly the actin and myosin filaments) are contracted towards each other. This makes them "move to the middle" and you build up a force on both sides of the muscle. See this image (from the Wikipedia), which illustrates this process:

There is also an animated picture available, which illustrates this process even better (from here):

The process of the contraction applies a equal force to both endpoints of the muscle.

Answer 2

Muscles do contract in both directions. To understand this better you need to know few things:

1. The smallest unit of a muscle is called myofiber or a muscle fiber. It is basically a multinucleated elongated cell. Each muscle fiber is covered by a layer called endomysium

2. A bunch of myofibers arranged linearly is called a fascicle. The fascicles are covered by a sheath called perimysium.

3. A muscle as you see in a cadaver or a picture is a group of fascicles surrounded by epimysium.

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A single muscle fiber is made up of numerous myofibrils which are basically the sarcomere arranged side-by-side.

The junction between the neuron and muscle fiber is called motor end plate. A single motor neuron innervates a group of muscle fibers. The whole assembly of muscle fibers innervated by the axon from one neuron is called a motor unit

The Key:

The key to understand muscle contraction on a macro level is in the fact:

Typically, an axon makes a single point of synaptic contact with a skeletal muscle fiber, midway along the length of the muscle fiber.¹

So when an action potential arrives at the neuromuscular junction (another name for end plate), the muscle starts depolarising and contracting from the center towards the periphery. In other words both the ends of the muscle contract toward the center.

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tl;dr:

Every skeletal muscle contracts toward its center, such that the proximal and distal ends act in opposite directions causing tension generation. The proximal end pulls the tendon down and the distal end pulls the tendon up thus generating tension.

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^{1: Medical Physiology A Cellular and Molecular Approach, 2nd Edition, Boron and Boulpaep p.217}

Answer 3

The above answers are correct, I just want to add a distinction between the different muscle types.

There are 3 types of muscle tissues:

1. Skeletal muscle (locomotion)

   • Striated
   • Voluntary

The skeletal muscle has visible striations under the microscope, because the myofilaments are aligned: they are all parallel. During contraction, the thin filaments slide over the thick filaments - at the end of the contraction the Z lines will be closer together. The filaments stay the same length, they just slide over one another, shortening the muscle. The muscle doesn’t contract by “going” in a particular direction, it shorts everywhere and the alignment of the filaments enables the muscle to shorten in only one dimension. The muscle is connected to the bone by tendons. When it shortens, it pulls on the tendons which pull on the bone, bringing it closer. A skeletal muscle cell can be up to 30cm long (Sartorius muscle).

2. Cardiac muscle (in the heart)

   • Striated
   • Involuntary

Cardiac muscle cells are very similar to skeletal muscle cells, however the fibers are interconnected via intercalated discs, which enables them to work in an even more synchronized way.

3. Smooth muscle (in the vascular system, the gastrointestinal tract, etc.)

   • Not striated
   • Involuntary

Smooth muscle filaments aren’t as organized as the ones in skeletal muscle. Therefore, the shortening can occur in all directions within the cell. It results in the shortening of the cell in more than 1 dimension.

Also, many organs such as the stomach have more than one layer of smooth muscle, providing constriction in many different “directions” (longitudinal layer, circular layer and oblique layer).