Peripheral nervous system in humans

Main

The peripheral nervous system (PNS) is one of the two main components of the nervous system, along with the central nervous system (CNS). The PNS consists of nerves and ganglia that lie outside the brain and spinal cord, serving as a relay between the CNS and the rest of the body. Its main function is to connect the CNS to the limbs and organs, transmitting motor and sensory information. The PNS is essential for:

• Receiving sensory input from the environment and the body's internal state

• Conveying motor commands to muscles and glands

• Regulating involuntary functions like heart rate and digestion

The PNS has two main divisions:

• Somatic nervous system: Controls voluntary movements and receives sensory information from the skin, muscles, and joints

• Autonomic nervous system: Regulates involuntary functions and has two subdivisions:

  • Sympathetic nervous system: Responsible for "fight or flight" responses

  • Parasympathetic nervous system: Responsible for "rest and digest" functions

Terms

• Nervous system: The complex network of neurons and fibers that transmits messages between different parts of the body.

• Central nervous system (CNS): The part of the nervous system consisting of the brain and spinal cord.

• Nerves: Bundles of nerve fibers (axons) that conduct information to and from the CNS.

• Ganglia: Clusters of nerve cell bodies found in the PNS.

• Afferent (sensory) fibers: Nerve fibers that carry sensory information from receptors to the CNS.

• Efferent (motor) fibers: Nerve fibers that carry motor commands from the CNS to effectors (muscles and glands).

Analogy

The PNS is like a network of roads connecting a central city (the CNS) to the surrounding towns and villages (the limbs and organs). Just as roads allow goods and people to travel to and from the city, the nerves of the PNS allow information to flow between the CNS and the rest of the body. For example, when you touch a hot stove, sensory nerves in your hand send a message to your brain, which then sends a command back through motor nerves to pull your hand away.

Misconception

A common misconception is that the PNS is under voluntary control, while the CNS controls involuntary functions. In reality, the PNS includes both voluntary (somatic) and involuntary (autonomic) divisions. For instance, while you can consciously control the movement of your arm (somatic), you cannot control your heart rate (autonomic) through the PNS.

History

• 4th century BC: Aristotle recognized the distinction between nerves and tendons.

• 2nd century AD: Galen described the cranial and spinal nerves.

• 1664: Willis published a detailed description of the peripheral nerves.

• 1800s: The autonomic nervous system and its functions were discovered.

• 1900s: Advances in microscopy and staining techniques allowed for detailed study of PNS structure.

• Late 20th century: Discoveries in molecular biology and genetics further expanded our understanding of PNS development and function.

• 21st century: Ongoing research continues to unravel the complexities of the PNS, including its role in pain, regeneration, and disease.

How to use it

1. To diagnose peripheral neuropathy, a doctor may test your reflexes, sensation, and muscle strength, which all depend on proper PNS function.

2. Understanding the anatomy of the PNS is crucial for surgeons operating on peripheral nerves to avoid causing damage.

3. Researchers studying the PNS can gain insights into how the nervous system develops, adapts, and regenerates, potentially leading to new therapies for nerve injuries and disorders.

Facts

• The human PNS contains 43 pairs of nerves: 12 cranial nerve pairs and 31 spinal nerve pairs.

• The sciatic nerve, running from the lower back to the feet, is the longest and widest nerve in the human body.

• Some PNS nerve fibers can transmit signals at speeds up to 120 meters per second.

• The total length of all nerves in the human PNS is estimated to be around 45 miles (72 kilometers).

• Unlike the CNS, the PNS has a remarkable capacity for regeneration after injury, although the process is slow and often incomplete.