Spinal nerves

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The spinal nerves are a collection of thirty-one pairs of mixed nerves which arise from the spinal cord. They form the largest component of the peripheral nervous system (PNS), transmitting afferent (sensory) and efferent (motor/autonomic) information between much of the periphery and central nervous system. Neurons within each spinal nerve can be functionally categorized as somatic (related pertaining to skin, skeletal muscle, tendons and joints), or visceral (pertaining to internal organ/smooth muscle, cardiac muscle and glands). Thus, neurons within a typical spinal nerve can be classed as belonging to one of four functional modalities: 

• Somatic afferent/efferent, or
• Visceral afferent/efferent

The spinal nerves can be subdivided as follows: eight pairs of cervical nerves, twelve thoracic, five lumbar, five sacral and one coccygeal. Each spinal nerve arises from the spinal cord within the vertebral canal as anterior and posterior rootlets (6-8 of each) which combine to form the anterior and posterior roots of the spinal cord. The anterior roots of the spinal cord contain efferent (i.e. motor/autonomic) nerve fibers while the posterior roots contain the processes of afferent (i.e. sensory) neurons. 

The spinal nerves exit the vertebral canal through the intervertebral foramen, or via the sacral foramina in the sacral region, and give off two primary branches: a small posterior ramus and a larger anterior ramus. These branches supply cutaneous, motor and autonomic innervation to muscles and skin of the neck, trunk and limbs. 

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Summary