The Peripheral Nervous System and Drug Effects

The Peripheral Nervous System (PNS)

  • The Peripheral Nervous System (PNS) lies outside the Central Nervous System (CNS) and contains all the nerves.

  • Nerves are classified based on their origin:

    • Cranial nerves: Arise from the brain.

    • Spinal nerves: Arise from the spinal cord.

  • All nerves function to carry signals to and from the CNS.

    • Example: Your eyes send messages via cranial nerves to the brain for reading this text. Your brain directs finger muscles via the spinal cord and spinal nerves to proceed to the next chapter.

  • Neuron Components in the PNS:

    • The cell body and dendrites of neurons are located either in the CNS or in ganglia.

    • Ganglia: Collections of nerve cell bodies situated outside the CNS.

    • The axons of neurons project from the CNS and constitute the spinal cord; therefore, nerves (both cranial and spinal) are primarily composed of axons.

Cranial Nerves

  • Humans possess 12 pairs of cranial nerves, each attached to the brain and conventionally referred to by Roman numerals.

  • Cranial nerves can be categorized by their fiber composition:

    • Sensory nerves: Contain only sensory fibers.

    • Motor nerves: Contain only motor fibers.

    • Mixed nerves: Contain both sensory and motor fibers.

  • These nerves are largely responsible for innervating the head, neck, and facial regions of the body.

  • The Vagus Nerve (Roman numeral X):

    • An exception, as it has branches extending not only to the pharynx and larynx but also to most of the internal organs.

    • It originates from the brainstem, specifically the medulla oblongata, which communicates with the hypothalamus.

    • The medulla oblongata and hypothalamus are crucial for controlling internal organ functions.

Spinal Nerves

  • Humans have 31 pairs of spinal nerves that emerge from either side of the spinal cord.

  • The roots of the spinal nerve physically separate the axons of sensory neurons from those of motor neurons, forming an arrangement resembling the letter 'Y'.

    • Posterior (dorsal) root: Contains sensory fibers that direct sensory receptor information inward, toward the spinal cord. The cell body of a sensory neuron is located in the posterior root ganglion (also known as a dorsal root ganglion).

    • Anterior (ventral) root: Contains motor fibers that conduct impulses outward or away from the spinal cord, destined for effectors.

  • The anterior and posterior roots merge to form a spinal nerve.

  • All spinal nerves are classified as mixed nerves because they contain both sensory and motor fibers.

  • Each spinal nerve innervates a specific region of the body where it is located (e.g., the intercostal muscles of the rib cage are innervated by thoracic nerves).

Divisions of the Peripheral Nervous System

  • The PNS is broadly divided into two main systems:

    • The Somatic System

    • The Autonomic System

The Somatic System

  • This system comprises nerves that serve the skin, skeletal muscles, and tendons.

  • Sensory Nerves: Carry sensory information from external sensory receptors to the CNS.

  • Motor Nerves: Convey motor commands from the CNS to skeletal muscles.

  • Voluntary vs. Involuntary Actions: Not all somatic motor actions are voluntary; some are automatic.

  • Reflexes: Automatic responses to a stimulus within the somatic system that occur quickly without conscious thought.

    • Example: Blinking your eyes in response to a flash of light is a reflex.

  • The Reflex Arc: This pathway illustrates the detailed route of nerve signals to and from the CNS.

    • Scenario: Touching a sharp pin.

      1. Sensory receptors in the skin generate nerve signals.

      2. These signals travel along sensory fibers through the posterior (dorsal) root ganglia toward the spinal cord.

      3. Sensory neurons entering the spinal cord posteriorly transmit signals to numerous interneurons.

      4. Some interneurons synapse with motor neurons, whose short dendrites and cell bodies are located in the spinal cord.

      5. Nerve signals then travel along these motor fibers to an effector (e.g., a muscle).

      6. The muscle contracts, causing you to withdraw your hand from the pin.

      7. Simultaneously, some interneurons carry nerve signals to the brain, leading to awareness of the stimulus (pain), along with other reactions like wincing or crying out. Pain is not felt until the brain receives and interprets this information.

The Autonomic System

  • Part of the PNS, the autonomic system regulates the activities of cardiac muscle, smooth muscles, internal organs, and glands.

  • It is organized into two divisions that generally elicit opposite responses:

    • Sympathetic Division

    • Parasympathetic Division

  • Shared Features of the Autonomic Divisions:

    1. They typically function in an involuntary manner.

    2. They innervate all internal organs.

    3. Each impulse pathway involves two neurons and one ganglion:

      • The first neuron has its cell body within the CNS and a preganglionic fiber that extends into the ganglion.

      • The second neuron has its cell body within the ganglion and a postganglionic fiber that projects out of the ganglion.

  • Role in Homeostasis: Reflex actions, such as those controlling blood pressure and breathing rate, are vital for maintaining homeostasis. These reflexes initiate when sensory neurons in internal organs send messages to the CNS and are completed by motor neurons within the autonomic system.

Sympathetic Division

  • Origin and Ganglia: Most preganglionic fibers originate from the middle portion of the spinal cord and terminate almost immediately in ganglia located near the spinal cord.

  • Function (Fight or Flight): This division is crucial during emergency situations, preparing the body for