Movement: An Overview

  • Published1 Apr 2012
  • Reviewed1 Apr 2012
  • Author
  • Source BrainFacts/SfN

From the stands at sports events, we marvel at the perfectly placed serves of professional tennis players and the lightning-fast double plays executed by big league baseball infielders. But in fact, each of us in our daily activities performs a host of complex, skilled movements — such as walking upright, speaking, and writing — that are just as remarkable.

Diagram of reflexes and movement
The stretch reflex (top) occurs when a doctor taps a muscle tendon to test your reflexes. This sends a barrage of impulses into the spinal cord along muscle spindle sensory fibers, activating motor neurons to the stretched muscle. This series of events cause a contraction, completing the stretch reflex. Flexion withdrawal (bottom) occurs when your bare foot encounters a sharp object. Your leg is immediately lifted (flexion) from the source of potential injury, but the opposite leg responds with increased extension so that you can maintain your balance. The latter event is called the crossed extension reflex.
Illustration by Lydia V. Kibiuk, Baltimore, MD; Devon Stuart, Harrisburg, PA

The stretch reflex (top) occurs when a doctor taps a muscle tendon to test your reflexes. This sends a barrage of impulses into the spinal cord along muscle spindle sensory fibers, activating motor neurons to the stretched muscle. This series of events cause a contraction, completing the stretch reflex. Flexion withdrawal (bottom) occurs when your bare foot encounters a sharp object. Your leg is immediately lifted (flexion) from the source of potential injury, but the opposite leg responds with increased extension so that you can maintain your balance. The latter event is called the crossed extension reflex.

What’s more, movement also reflects our mood and state of mind. For example, posture and patterns of movement can indicate whether we are happy or sad. Facial expressions such as a smile and a frown have a universal meaning.

These and all of our actions are made possible by a finely tuned and highly complex central nervous system, which controls the actions of hundreds of muscles. Through new experiences — and the formation of new neural connections — the nervous system can adapt to changing movement requirements to accomplish these everyday marvels. With practice, these movements can be performed even more skillfully.

To understand how the nervous system performs such feats, we have to start with the muscles, the body parts that produce movement under the control of the brain and spinal cord. Most muscles attach to points on the skeleton and cross one or more joints. The close relationship of these muscles to the skeleton gives them their name—skeletal muscles. Activation of a given muscle can open or close the joints that it spans, depending on whether it is a joint flexor (closer) or an extensor (opener). Flexors and extensors work in opposition to each other, causing the contraction of some muscles and the lengthening of others. For example, bending the elbow involves contraction of the biceps and lengthening of the triceps. Muscles that move a joint in an intended direction are called agonists, and those that oppose this direction of movement are antagonists. Skilled movements at high speed are started by agonists and stopped by antagonists, thus ensuring that the joint or limb is returned to the desired position.

Each skeletal muscle is made up of thousands of individual muscle fibers, and each muscle fiber is controlled by one alpha motor neuron in either the brain or the spinal cord. Furthermore, each single alpha motor neuron controls many muscle fibers (ranging from a few to 100 or more); an alpha motor neuron and all the muscle fibers it contains form a functional unit referred to as a motor unit. Motor units are the critical link between the brain and muscles. If the motor neurons die, which can happen in certain diseases, such as amyotrophic lateral sclerosis (ALS), a person is no longer able to move.

Some muscles act on soft tissue, such as the muscles that move the eyes and tongue and those that control facial expressions. These muscles also are under control of the central nervous system. They operate in much the same way as those that attach to bone.

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