Lecture Notes on the Peripheral Nervous System

The Peripheral Nervous System (PNS)

  • Peripheral Nervous System (PNS)

    • Definition: The PNS links the central nervous system (CNS) to the body and the external environment.

    • Function: The PNS detects sensory stimuli and delivers information to the CNS as sensory input.

    • Processing: The CNS processes this input and transmits impulses through the PNS to muscle cells and glands as motor output.

Divisions of the PNS

  • Somatic Sensory Division

    • Detects both internal and external stimuli.

    • General Sense: Detect stimuli from the skin.

    • Special Sensory: Detect stimuli from special sense organs.

  • Visceral Sensory Division

    • Relays internal information (e.g., blood pressure) from organs in the abdominopelvic and thoracic cavities.

  • Sensory Division

    • Composed of sensory (afferent) neurons.

    • Sends sensory stimuli to the CNS.

  • Motor Division

    • Composed of motor (efferent) neurons.

    • Carries out the motor functions of the nervous system.

    • Somatic Motor Division: Controls voluntary motor functions, triggering skeletal muscle contractions.

    • Visceral Motor Division (Autonomic Nervous System - ANS): Maintains various aspects of homeostasis with involuntary motor functions in the body.

    • Two subdivisions:

      • Sympathetic Division

      • Parasympathetic Division

Functions of the PNS Integrated with CNS

  • Sensory Function:

    • Sensory neurons detect stimuli and transmit it along sensory neurons (either spinal or cranial) to the cerebral cortex.

    • In the cortex, sensory information is interpreted, integrated, and an appropriate motor response is selected and initiated.

  • Motor Division:

    • The ANS is further divided into sympathetic and parasympathetic nervous systems.

    • Sympathetic Nervous System (

    • Known as the "fight or flight" division, activates during emergency situations.

    • Vital for maintenance of homeostasis during physical work and emotional responses.

    • Effects:

      • Increases heart rate.

      • Increases diameter of airways and pupils.

      • Blood flow redirected away from GI tract and kidneys to the organs involved in the activity (e.g., skeletal muscles).

      • Liver enhances conversion of glycogen to glucose for ATP production.

    • Parasympathetic Nervous System (

    • Known as the "rest and digest" division, maintains body functions during rest.

    • Also called craniosacral division (linked to cranial nerves and pelvic nerves from sacral plexus).

    • Effects:

      • Decreases heart rate, reduces diameter of airways and pupils.

      • Promotes salivation, lacrimation, urination, digestion, and defecation.

      • Liver converts absorbed glucose to glycogen stores.

Overview of Peripheral Nerves and Associated Ganglia

  • Peripheral Nerves:

    • Main organs of PNS composed of axons of many neurons bound together by connective tissue.

    • Contact (innervate) majority of structures in the body.

    • Types of nerves:

    • Mixed Nerves: Contain both sensory and motor neurons.

    • Sensory Nerves: Contain only sensory neurons (some involved in muscle stretch and tension).

  • Spinal Nerves:

    • Originate from the spinal cord, innervating structures below the head and neck.

    • All 31 pairs of spinal nerves are mixed nerves.

    • Anterior Root: Contains motor neurons from the anterior horn.

    • Posterior Root: Contains sensory neurons from the posterior horn.

    • Posterior Root Ganglion: Swollen area in the posterior root housing cell bodies of sensory neurons.

  • Connective Tissue Layers:

    • Epineurium: Outermost layer of connective tissue; holds motor and sensory axons together.

    • Fascicles: Small groups of bundled axons surrounded by connective tissue (perineurium).

    • Each individual axon within a fascicle is surrounded by its own connective tissue (endoneurium).

  • Cranial Nerves:

    • Attach to the brain and mostly innervate structures in the head and neck.

    • They are not formed by the fusion of sensory and motor roots like spinal nerves, allowing for purely sensory, mixed, and mostly motor nerves.

Cranial Nerve Functions Summary

  • Mnemonic Device for Cranial Nerve Functions:

    1. Olfactory I (S) - Say

    2. Optic II (S) - Say

    3. Oculomotor III (M) - Marry

    4. Trochlear IV (M) - Money

    5. Trigeminal V (B) - But

    6. Abducens VI (M) - My

    7. Facial VII (B) - Brother

    8. Vestibulocochlear VIII (S) - Says

    9. Glossopharyngeal IX (B) - Big

    10. Vagus X (B) - Brains

    11. Accessory XI (M) - Matters

    12. Hypoglossal XII (M) - Most

The Spinal Nerves

  • Spinal Nerve Structure:

    • Consists of short mixed nerves.

    • Divides into two mixed nerves that carry somatic motor and sensory information:

    • Posterior Ramus: Travels to posterior side of body.

    • Anterior Ramus: Travels to anterior side of body and/or to an upper or lower limb.

  • Cervical Plexus:

    • Formed from anterior rami of C1-C5 and small contribution from hypoglossal nerve.

    • Innervates skin of the neck and sections of the head, chest, and shoulders.

    • Major motor branch is the Phrenic Nerve (C4, contributions from C3 and C5) which innervates the diaphragm.