Peripheral nervous system

The Peripheral Nervous System (PNS) comprises the nerves and ganglia outside of the brain and spinal cord, connecting the central nervous system (CNS) to the rest of the body. The main function of the PNS is to connect the CNS to the limbs and organs, essentially serving as a communication relay.

Nervous system diagram

Structure and Components

The PNS is divided into two main subsystems:

• Sensory (afferent) division: Conveys sensory information from peripheral tissues and organs to the CNS.
• Motor (efferent) division: Transmits motor instructions from the CNS to peripheral tissues and organs.

The motor division can be further categorized into:

• Somatic nervous system: Controls voluntary movements by innervating skeletal muscles.
• Autonomic nervous system (ANS): Controls involuntary functions by influencing organs, glands, and smooth muscles. The ANS is subdivided into:
  • Sympathetic division: Prepares the body for stress-related activities.
  • Parasympathetic division: Regulates rest-and-digest activities and conserves energy.

Nerves

Nerves are bundles of axons that transmit signals to and from the CNS. Based on their function, nerves can be classified as:

• Sensory nerves
• Motor nerves
• Mixed nerves (both sensory and motor functions)

Functions

• Sensory function: The sensory neurons of the PNS collect information regarding external stimuli, like temperature and touch, as well as internal stimuli, such as bodily pH or blood pressure.
• Motor function: Motor neurons transmit signals from the CNS to the effector organs, initiating a response.
• Integrative function: Certain reflexes, like the withdrawal reflex, are integrated within the peripheral nerves and do not need brain involvement.

Disorders and Injuries

The PNS is susceptible to various disorders, including:

• Neuropathies: Resulting from damage to peripheral nerves, leading to weakness, numbness, and pain.
• Guillain-Barré syndrome: An autoimmune disorder attacking the PNS, causing rapid-onset muscle weakness.
• Bell's palsy: Sudden weakness in facial muscles, often related to viral infections.

Traumatic injuries can lead to severed nerves, impacting normal function. However, unlike CNS neurons, peripheral nerve fibers can regenerate to some extent.

Clinical Significance

Understanding the PNS is crucial for numerous clinical procedures, such as nerve blocks for pain management. Moreover, many therapeutic interventions target the PNS to treat disorders like hypertension or heart failure.

See Also

• Central Nervous System
• Neuron
• Autonomic Nervous System
• Nerve Regeneration

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