The term “contralateral” is a fundamental directional word in human anatomy and medicine, offering a precise way to describe the body’s structure and function. It specifically means “relating to the opposite side” of the body, which is defined by the imaginary vertical line that divides the body into symmetrical right and left halves. Understanding this concept is necessary for accurately communicating the location of body parts, the pathways of the nervous system, and the effects of injury or disease.
Defining Contralateral
The word “contralateral” is composed of two Latin-derived parts: the prefix ‘contra-‘ meaning “opposite,” and the root ‘-lateral’ meaning “side.” This combination denotes the opposite side of the body relative to a specific point of reference, such as a muscle, a nerve tract, or a brain region. For example, a structure on the right side of the body is contralateral to a structure on the left side, and vice versa. This concept is essential when describing how the central nervous system interacts with the limbs and trunk.
The Essential Contrast: Ipsilateral
To fully grasp the meaning of contralateral, it is helpful to consider its direct opposite, the term “ipsilateral.” The prefix ‘ipsi-‘ is derived from the Latin word meaning “self” or “same,” defining ipsilateral as “on the same side of the body.” These two terms are frequently used together to describe the bilateral organization of the body’s systems. Your left arm is ipsilateral to your left leg because both are located on the same side of the body’s midline. However, your left arm is contralateral to your right arm.
How Contralateral is Used in Medicine
The primary medical application of the term contralateral is in neurology, particularly concerning the control of movement and sensation. The brain’s main motor pathway, called the corticospinal tract, is organized in a contralateral fashion. This means the left cerebral hemisphere controls the movement of the right side of the body, and the right hemisphere controls the left side.
This crossed organization is due to a phenomenon called decussation, where the nerve fibers cross over from one side to the other in the brainstem. Consequently, a stroke that damages the motor cortex in the left side of the brain results in weakness or paralysis, known as hemiparesis or hemiplegia, on the contralateral right side of the body. Sensory pathways, which transmit information like pain and touch, also largely follow a contralateral route to the sensory cortex for processing.
Contralateral in Rehabilitation
The concept of contralateral organization also extends to rehabilitation and therapy. For instance, in some post-stroke therapies, the uninjured, or contralesional, side of the brain is targeted to help recover function in the affected limb. The term ensures medical professionals can precisely locate the site of a lesion and accurately predict the resulting symptoms on the opposite side of the body.