A nerve is an enclosed, cable-like bundle of nerve fibers, which are the long extensions, or axons, of individual nerve cells called neurons. These bundles exist only outside the brain and spinal cord, forming the structures of the peripheral nervous system. While organs or bones can be counted precisely, the nervous system operates at two scales: the macroscopic bundles of the nerves themselves and the microscopic billions of cells that make them up. Therefore, a single, definitive number for “nerves” is impossible to provide, as the count depends entirely on whether one is tallying the major named trunks or the countless microscopic branches that pervade every tissue.
Understanding the Impossible Count
The largest counts exist at the cellular level, as the brain alone contains an estimated 86 billion neurons. These cells are the source of the axons that bundle together to form the macroscopic structures we call nerves. A single, named nerve is not a single wire but a massive cable of thousands of individual fibers.
For instance, the sciatic nerve, the largest and longest nerve, is comprised of a bundle of axons that can number in the tens of thousands. These major nerves continually split and branch into countless smaller filaments that reach all tissues. The most common estimate refers to the hundreds of peripheral nerves that have been named and mapped by anatomists, but any final count remains subjective.
Structural Divisions of the Nervous System
The complexity of the nervous system is divided into two structural parts: the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists solely of the brain and the spinal cord, serving as the command and integration centers. Within the CNS, bundles of axons are referred to as tracts, not nerves.
The PNS is the location of the structures referred to as nerves, which extend from the CNS to the rest of the body. The PNS is organized into two primary groups based on their connection point. The Cranial Nerves are 12 pairs that emerge directly from the brain or brainstem, managing functions primarily in the head and neck.
The Spinal Nerves consist of 31 pairs that emerge from the spinal cord. These nerves are grouped and named according to the region of the vertebral column they exit. The pairs include:
- 8 cervical
- 12 thoracic
- 5 lumbar
- 5 sacral
- 1 coccygeal pair of nerves
Each spinal nerve contains both sensory and motor fibers, acting as a mixed pathway for information traveling to and from the spinal cord.
Functional Categories of Nerve Fibers
Nerve fibers are categorized by the type of information they carry and the direction of the signal. The Somatic Nervous System is concerned with voluntary activities and conscious sensation. This system governs the skeletal muscles, allowing for purposeful movement, and includes sensory pathways for touch, pain, and temperature.
The Autonomic Nervous System handles involuntary functions that maintain the body’s internal balance. This system controls smooth muscle in organs, cardiac muscle, and glands. The autonomic system is subdivided into the sympathetic and parasympathetic divisions, which often exert opposing effects.
The sympathetic division is associated with the “fight or flight” response, mobilizing energy reserves and increasing heart rate. Conversely, the parasympathetic division is responsible for “rest and digest” functions, conserving energy and regulating processes like digestion and a slower heart rate. Signals are classified by direction: Afferent (Sensory) fibers transmit information toward the CNS, while Efferent (Motor) fibers transmit commands away from the CNS to muscles or glands.