May 30, 2012

Health Minute With CASI – The Amazing Brain – Part 2

This month we will continue a discussion of the Central Nervous System which we began last month. The Central Nervous System includes the brain and spinal cord.

The Brain Stem is the central core of the brain stem consisting of mostly nerve fibers. It has three areas.
• Midbrain: The most anterior part of the brain stem that has the tubular structure of the spinal cord. It merges with the thalamus and hypothalamus. It contains many neurons and tracts.
• Pons: the enlarged area containing ascending and descending fiber tracts.
• Medulla: Continuation of the spinal cord in the brain that contains all ascending, descending fiber tracts connecting the brain and spinal cord. It has vital autonomic centers such as (respiration, heart, gastrointestinal function), as well as nuclei for the nerves VIII through XII. Crossing of the motor fibers occurs here.

Spinal Cord: The spinal cord is a long cylinder like structure about as big as the little finger. It occupies the upper two thirds of the vertebral canal. It is know as the main highway for ascending and descending fiber tracts that connects the brain to the spinal nervous, and mediates reflexes. Its nerve cell bodies, or gray matter, are arranged in a butterfly shape with anterior and posterior “horns.”

Pathways of the Central Nervous System

Crossed Representation: Is a notable feature of the nerve tracts, the left cerebral cortex receives sensory information from and controls motor function to the right side of the body, like the right cerebral cortex interacts with the left side of the body.

Sensory Pathways: Millions of sensory receptors are embroidered into the skin, mucous membranes, muscles, tendons, and viscera. They monitor conscious sensation, internal organ functions, body position and reflexes.

Spinothalamic Tract: Contains sensory fibers that transmit the sensations of pain, temperature, crude or light touch. The fibers enter the dorsal root of the spinal cord and synapse with a secondary sensory neuron. The second-order neuron will cross to the opposite side and ascend up the spinothalamic tract to the thalamus. Fibers carrying pain and temperature sensations ascend the lateral spinothalmic tract, those of crude touch form the anterior spinothalamic tract. At the thalamus, the fibers synapse with a third sensory neuron, which carries the message to the sensory cortex for full interpretation.

Posterior (Dorsal) Columns: These fibers conduct the sensations of position, vibration and finely localized touch.
• Position – Without looking, you know where body parts are in space and in relation to each other.
• Vibration – Feeling vibrating objects
• Finely Localized touch – Without looking, you can indentify familiar objects by touch.

These fibers enter the dorsal root and immediately proceed up the same side of the spinal cord to the brain stem. At the medulla, they will synapse with a secondary neuron and cross. Travel to the thalamus, synapse again and proceed to the sensory cortex, which allows the sensation to be in full discrimination. The sensory cortex is arranged in a specific pattern forming what we like to call “map” of the body. Example, you know you have a heart, but it is considered absent from the brain map. So, pain that is actually generating from the heart may be felt by “proxy” by another body part that may have the felt image. So, this experience may be felt in the left arm, chest or shoulder, which were its neighbors while in fetal development.

Follow up next month for more exciting information on the Central nercous System, including continuation of motor pathways, motor neurons, peripheral nervous system.