Function of the Spinal Cord
The spinal cord is part of the central nervous system. It is a bundle of nerves that passes through the bones of the vertebrae down the back. It sends sensations to the brain from the body, and returns motor commands to the various parts of the body. The spinal cord has a primary role in reflexes and in the autonomic nervous system.
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Geography
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A cross section of the spinal cord reveals a butterfly-shaped region of gray matter. Surrounding the gray matter is white matter. The gray matter is formed by nerve cells and the white matter by myelin-sheathed neuronal tracts, which contain sensory and motor neurons. All sensory tracts travel up toward the brain and are called "afferent pathways." All motor tracts travel down from the brain and are called "efferent pathways."
Benefits
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In addition to transmitting information to the brain and sending commands to the body, the spinal cord is also responsible for most reflexes. A reflex is a built-in response to dangerous stimuli, allowing the body to react faster than it would if the information had to travel to the brain, be thought about and then travel back to the body. Snatching back your hand from a source of pain instantly is an example of a reflex.
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Function
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The reflex arc is the pathway of nerves through the spinal cord which allows reflexes to work. The first stage of the reflex arc is stimulation of a receptor nerve. Receptor nerves can sense excessive heat, coolness, pressure or over-stretching of a muscle. That sensory neuron sends the impulse to the spinal cord. Depending on the reflex being stimulated, that sensory nerve connects directly with a motor or glandular nerve, or goes through an intermediate nerve and then to the motor or glandular nerve. That nerve signals the muscles or glands to react.
Types
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There are several types of reflexes that travel through the spinal cord. Visceral reflexes control heart muscle, glands and organs. Somatic reflexes control involuntary movement of the skeletal muscles. Innate reflexes are the ones an organism possesses at birth. Acquired reflexes are conditioned through repetition and involve complex learned motor patterns.
Considerations
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The innate somatic reflexes are the ones biologically programmed into on organism to protect it from harm. The stretch reflex regulates muscle length. If a muscle is overly stretched, the stretch reflex forces it to contract in an attempt to prevent the muscle from tearing. A similar reflex is the tendon reflex, which prevents a tendon from producing enough tension to break. The tendon reflex is what causes a person trying to hold up too much weight to suddenly loose strength and drop it. The withdraw reflex is the reflex that makes the body yank away from a painful stimulus, such as yanking back a foot when a toe is stubbed. The reflex that works with the withdraw reflex is the crossed extensor reflex. When the withdraw reflex is activated in the leg that experienced pain, the crossed extensor reflex is what makes the other leg stiffen to prevent the person from falling.
Significance
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The spinal cord gives rise to the autonomic nervous system (ANS). The ANS is responsible for the sympathetic and parasympathetic nervous system. The sympathetic nervous system is responsible for the "fight or flight" reaction in the body. It draws blood from the digestive system and sends it to the muscles, causes the heart to beat harder and the pupils to dilate. The parasympathetic nervous system has the opposite function, and is responsible for "rest and digest." It slows the heart rate, moves the blood to the digestive system and is also involved in producing erections in males.
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