Muscle Contraction - Introduction

Tutorial 31: Muscle Contraction

The skeletal muscles control our body movements. They are attached to adjacent bones via tendons, and function to induce movement at the joint formed where the two bones meet. Muscles can dynamically contract, their muscle fibers shortening, causing movement in one direction or the other. The placement of the muscle across the joint determines the direction of the movement. Flexion occurs when contraction of a muscle reduces the angle formed by the two bones, causing the distal bone or limb to bend in and move closer into the body. Extension occurs when contraction of a muscle increases the angle formed by the two bones, causing the distal bone to straighten or move farther away from the body. Extensor muscles work against gravity in controlling such movements as standing up from a sitting position. Muscles that induce the same type of movement at a joint are called synergistic, whereas muscles that act in opposition at a joint are called antagonistic. Some muscle contraction, isometric contraction, increases the tension within fibers but does not cause shortening of the fibers.

Our ability to adjust body movements in response to a changing environment is mediated by the proprioception system. This system is sensitive to body position and movement and provides the motor systems of the body with ongoing information/feedback that is used in the execution of subsequent movement. This is the system that keeps us upright when we stumble over a rock. The position sense that allows us such control is provided primarily via the afferent input of two sensory receptors located in skeletal muscle, the muscle spindles and Golgi tendon organs. In addition to position sense, this proprioceptive input is used by the motor system to control spinal reflexes and to provide ongoing feedback. This feedback provides the nervous system with information concerning the length of muscles and their force of contraction.

Skeletal muscles are composed of two distinct fiber types. Intrafusal muscle fibers are embedded within the extrafusal fibers, and contain proprioceptive sensory receptors called muscle spindles. Extrafusal muscle fibers produce movements upon contraction, and the contraction of these fibers is controlled by alpha motor neurons. The proprioceptive, muscle spindles of intrafusal fibers use both sensory and motor neurons to execute their duties. The sensory neuron originating from the muscle spindle conveys a signal when a muscle is stretched. The motor neuron then stimulates the contraction of both intrafusal and extrafusal muscle fibers. The contraction of intrafusal muscle fibers is induced by stimulation of gamma motor neurons.

Motor spinal reflexes are involuntary movements that typically protect the body from damage. Involuntary movements are unaffected by such internal behavioral phenomena as motivation and memory, but are automatic and rapid responses to external conditions. There are two reflexes that help to protect the skeletal muscle system from damage under extreme and opposite conditions. The first reflex, utilizing the muscle spindle receptor, protects the muscles under conditions of extensive stretch when fibers are in danger of tearing. The muscle spindle reflex also helps to maintain necessary muscle tone by increasing muscle contraction. The second reflex, utilizing the Golgi tendon organ, protects the muscles under conditions of excessive contraction when fibers are in danger of pulling away from their attachments. The Golgi tendon organ prevents muscle damage under such conditions by braking or preventing additional muscle contraction.

Tutorial 31 illustrates and describes the motor reflexes and the structures involved.

Suggestions for further study

Tutorial 31: Muscle Contraction

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