The Peripheral Nervous System

The Peripheral Nervous System

Overview of the Peripheral Nervous System (PNS)

  • Definition: The PNS comprises all the nervous elements outside the brain and spinal cord, and plays a crucial role in connecting the central nervous system (CNS) to limbs and organs.

  • Major Components:

    • Spinal Nerves: 31 pairs of spinal nerves originating from the spinal cord.

    • Plexuses: Networks formed from the ventral rami of most spinal nerves (except T2-T12).

    • Cranial Nerves: Twelve pairs of nerves originating from the brain, primarily the brainstem.

Structural Organization of the Nervous System

  • Central Nervous System (CNS)

    • Brain

    • Spinal cord

  • Peripheral Nervous System (PNS)

    • Divided into:

    • Sensory (Afferent) Division: Transmits sensory information to the CNS.

    • Motor (Efferent) Division: Transmits motor commands from the CNS to effectors, further divided into:

      • Somatic Nervous System: Controls voluntary movements.

      • Autonomic Nervous System (ANS): Controls involuntary functions, further subdivided into:

      • Sympathetic Division: Prepares body for stress-related activities.

      • Parasympathetic Division: Restores the body to a state of calm.

Spinal Nerves

  • General Overview

    • Total of 31 pairs of spinal nerves each named according to the level of the vertebral column from which they emerge:

    • 8 Cervical Nerve Pairs: C1 - C8

    • 12 Thoracic Nerve Pairs: T1 - T12

    • 5 Lumbar Nerve Pairs: L1 - L5

    • 5 Sacral Nerve Pairs: S1 - S5

    • 1 Coccygeal Nerve Pair: Co1

  • Cauda Equina: Refers to the collection of lumbar, sacral, and coccygeal nerve roots that descend from the spinal cord before reaching their respective intervertebral foramina.

Organization of Spinal Nerves

  • Spinal nerves contain mixed nerves (both sensory and motor fibers).

  • Structure: Each spinal nerve arises from the spinal cord by:

    • Ventral (anterior) root: Carries motor information from the CNS to the periphery.

    • Dorsal (posterior) root: Contains the dorsal root ganglion which houses the cell bodies of sensory neurons.

  • Ramus (Branches):

    • Dorsal Ramus: Supplies posterior body structures.

    • Ventral Ramus: Supplies anterior and lateral body structures, with fibers that can form networks (plexuses).

Plexuses Formed by Interconnections of the Spinal Nerves

  • Function: Plexuses redistribute nerve fibers to innervate specific areas, reducing the number of nerves required for each body part.

  • Major Plexuses:

    • Cervical Plexus (C1-C4): Innervates innovations to neck and diaphragm (notably the Phrenic Nerve responsible for breathing).

    • Brachial Plexus (C5-T1): Innervates upper limb; major nerves include: Axillary, Musculocutaneous, Median, Ulnar, Radial.

    • Lumbar Plexus (L1-L4): Primarily innervates lower limbs; notable nerve: Femoral Nerve.

    • Sacral Plexus (L4-S4): Supplies the pelvis and lower limbs; notable nerves include: Sciatic, Common Fibular, and Tibial Nerves.

Dermatome and Myotome Definitions

  • Dermatome: A designated area of skin that is mainly supplied by a single spinal nerve root, indicating its sensory innervation.

  • Myotome: Refers to skeletal muscles innervated by a specific spinal nerve root, indicating its motor function.

Cranial Nerves Overview

  • Total Count: Twelve pairs of cranial nerves that emerge from the brain, mostly the brainstem.

  • Classification by Function:

    • Mixed Nerves: Contain motor and sensory fibers.

    • Sensory Nerves: Primarily carry sensory information.

- Motor Nerves: Predominantly transmit motor impulses with few sensory fibers.

List of Cranial Nerves

  1. Olfactory Nerve (I): Responsible for sense of smell.

    • Production of olfactory nerve fibers from sensory neurons in nasal mucosa.

  2. Optic Nerve (II): Responsible for vision, connecting retina to the brain.

  3. Oculomotor Nerve (III): Controls eye movement and pupil response.

  4. Trochlear Nerve (IV): Controls the superior oblique muscle of the eye.

  5. Trigeminal Nerve (V): Has three branches supplying sensation and motor function in the face.

  6. Abducens Nerve (VI): Controls lateral eye movement.

  7. Facial Nerve (VII): Involved in facial expressions and taste sensation from anterior two-thirds of the tongue; autonomic functions for salivary glands.

  8. Vestibulocochlear Nerve (VIII): Responsible for hearing and balance; sensory only.

  9. Glossopharyngeal Nerve (IX): Involved in taste from the posterior third of the tongue and motor function in swallowing.

  10. Vagus Nerve (X): Supplies autonomic control to heart, lungs, and digestive tract; widespread innervation.

  11. Accessory Nerve (XI): Involved in neck movement (sternocleidomastoid and trapezius).

  12. Hypoglossal Nerve (XII): Controls tongue movement.

Reflex Arc Components

  • A reflex arc consists of five components:

    1. Receptor: Detects the stimulus.

    2. Sensory Nerve Fiber: Transmits afferent impulses to the CNS.

    3. Integration Center: Can be monosynaptic or polysynaptic within the CNS.

    4. Motor Nerve Fiber: Conducts efferent impulses from integration center to effector.

    5. Effector: The muscle or gland responding to the motor impulse.

There are 31 pairs of spinal nerves and 12 pairs of cranial nerves.