Information Highways

Information Highways: How the Brain and Body Communicate

There are two major information pathways by which information of the peripheral nervous system and the central nervous system are exchanged: the somatosensory and the motor control pathways. Some general functions of each system can be separated and described, but these 2 pathways influence one another in many ways. It is difficult to say, "this is how somatosensory information travels, and this is how motor information travels," because both information pathways have influences over one another. The following is a very basic summary of the information pathways for both sensory and motor systems. This should provide you with an understanding of how a normal brain communicates with the body, which in turn will help you better understand how a severed corpus callosum might interfere with certain aspects of motor control or visual interpretation.

Somatosensory Pathways

The somatosenses provide information about what is happening on the surface of our body and inside it. Cutaneous senses, or skin senses, respond to several different stimuli: pressure, vibration, heating, cooling, and events that cause tissue damage (such as pain). Organic senses arise from receptors in and around the internal organs, providing us with unpleasant sensations (such as stomach aches), or pleasurable ones (such as a cold drink on a hot summer day). Receptors located throughout our bodies detect environmental stimuli, and quickly send information to corresponding regions in the brain. All neural information is sent in the same manner, it is where, in the brain, the information is sent which determines how it will be interpreted and what type of corresponding information will be sent back as a response.

Somatosensory axons from the skin, muscles, or internal organs enter the central nervous system via spinal nerves. Somatosensory nerves located in the face and head primarily enter the brain through the cranial nerves.

Precisely localized information (such as fine touch) and imprecisely localized information (such as pain and temperature) are transmitted to the brain by different pathways. Axons that convey precisely localized information ascend throughout the dorsal columns in the white matter of the spinal cord to nuclei in the lower medulla. From there, axons actually cross to the hemisphere opposite the side of the body that the stimuli was received. Axons cross to the opposite side of the brain at the medulla, travel to the thalamus. The thalamus is divided into several nuclei, or groups of neurons of similar shape and function. Some of these nuclei receive the sensory information from the ascending pathways and project it out to the somatosensory cortex so that it can be interpreted.

In contrast, the axons that convey poorly localized information (pain, temperature), enter the spinal cord and immediately cross to the opposite side. From here these neurons ascend through the spinothalamic tract to the nuclei in the thalamus, subsequently being passed to the correct region of the brain for interpretation.

Motor Pathways

Information is sent to muscles in the body through motor pathways. This information allows you to flex your biceps, squeeze a tennis ball, correct your posture, move. There are two types of descending pathways: corticospinal pathways, which originate in the cerebral cortex, and noncorticospinal pathways, which originate in the brainstem. In general, the corticospinal pathways have greater influence over motor neurons that control muscles involved in fine, isolated movements, particularly those of the fingers and hands. The noncortoicospinal pathways are more involved with coordination of the large muscle groups used in things such as the maintenance of upright posture, balance, walking, and in head and body movements when turning toward a specific stimulus. Motor pathways may be excitatory (causing a muscle to contract), or inhibitory (preventing a muscle contraction).

What's the Point?

In general, the right hemisphere interprets information and controls actions of the left side of the body. The left hemisphere interprets information and controls actions of the right side of the body. If the connection between the hemispheres is severed, sensory information cannot pass to the correct region of the brain in order for corresponding response to be made. For example callosal apraxia is a form of limb apraxia caused by damage to the anterior corpus callosum. When a person hears a verbal request to perform a movement, let's say to raise both hands in the air, the meaning of the speech is analyzed by circuits in the left hemisphere. Then, a neural command activates the region of the brain that contains the memory of the movement, the prefrontal cortex. This information is passed to the part of the brain that controls the actual movement to be performed, the motor cortex. The left motor cortex controls the movements of the right hand, and the right motor cortex controls the movements of the left hand. In order for the right motor cortex to be activated so that the left hand can be raised, the analysis of the verbal command must be passed from the left hemisphere to the right side, through the corpus callosum. Thus, the right arm can perform the requested movement, but the left cannot.

Early after a split brain surgery, the patient shows a marked apraxia of the left hand to verbal command. This occurs because the right hemisphere, which controls the left hand, has poor language comprehension. Remarkably, this symptom recovers to a considerable degree. It is possible that the left hemisphere gains ipsilateral (same side) control of the left hand, and/or the right hemisphere acquires some basic language skill.

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