Which contains only motor fibers?

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Step 1: Identify the anatomical structures shown in the image. The image depicts spinal nerve roots and their relationship to the spinal cord. The labels a, b, c, and d correspond to different nerve fiber bundles.

Step 2: Recall the functional classification of spinal nerve roots. The dorsal (posterior) root contains sensory (afferent) fibers, while the ventral (anterior) root contains motor (efferent) fibers.

Step 3: Determine which labels correspond to dorsal and ventral roots. In the image, label 'a' points to the dorsal root ganglion area, indicating sensory fibers, and label 'd' points to the ventral root, which contains only motor fibers.

Step 4: Understand that the spinal nerve (label 'b') is a mixed nerve containing both sensory and motor fibers, and label 'c' represents the spinal nerve trunk after the roots merge.

Step 5: Conclude that the structure containing only motor fibers is the ventral root, which corresponds to label 'd' in the image.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Spinal Nerve Roots

Spinal nerves are formed by the combination of dorsal (posterior) and ventral (anterior) roots. The dorsal root contains sensory fibers that carry information to the spinal cord, while the ventral root contains motor fibers that transmit signals from the spinal cord to muscles.

Motor Fibers

Motor fibers are nerve fibers that carry impulses from the central nervous system to effector organs such as muscles, causing movement. These fibers are found exclusively in the ventral root of the spinal nerve.

Sensory vs Motor Fiber Distribution

Understanding the distinction between sensory and motor fiber distribution is crucial. The dorsal root (labeled 'a' in the image) contains only sensory fibers, the ventral root (labeled 'd') contains only motor fibers, and the spinal nerve (labeled 'b') contains both sensory and motor fibers.

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