Comprehensive Study Guide to the Peripheral Nervous System

Functional Divisions and Structural Components of the Peripheral Nervous System

The Peripheral Nervous System (PNS) is organized into functional divisions comprising sensory and motor components. Sensory inputs and motor outputs connected to the Central Nervous System (CNS) are further subdivided into somatic or visceral categories. Sensory inputs are specifically categorized as either general or special. Structurally, the PNS includes sensory receptors, which detect stimuli from internal or external environments, and nerves, which consist of bundles of peripheral axons. Ganglia are defined as clusters of peripheral neuronal cell bodies located within the PNS. Motor endings serve as axon terminals for motor neurons and function to innervate effectors, which include muscle fibers and glands.

Classification of Peripheral Sensory Receptors

Peripheral sensory receptors are classified based on three primary criteria: location, structure, and the type of stimulus (function). Classification by location includes exteroceptors, which are sensitive to stimuli from outside the body and are located at or near body surfaces to monitor touch, pressure, pain, temperature, vision, hearing, and equilibrium. Interoceptors (visceroceptors) receive stimuli from internal viscera, such as the digestive tube, bladder, and lungs, monitoring changes in chemical concentration, tissue stretching, and temperature. Proprioceptors are located in skeletal muscles, tendons, joints, and ligaments to monitor the degree of stretch and send data regarding body movement to the CNS. Functional classification by stimulus type includes mechanoreceptors (mechanical forces such as touch, pressure, stretch, vibration, and itch), baroreceptors (blood pressure), thermoreceptors (temperature changes), chemoreceptors (chemicals in solution), photoreceptors (light), and nociceptors (harmful stimuli resulting in pain).

Structural Classification of General Sensory Receptors

General sensory receptors are widely distributed and monitor touch, pressure, vibration, stretch, pain, temperature, and proprioception. These are structurally divided into free nerve endings and encapsulated nerve endings. Free nerve endings are located in most body tissues, particularly connective tissues like ligaments, tendons, dermis, and epithelia; they function as nociceptors, thermoreceptors, and potentially mechanoreceptors. Modified free nerve endings include epithelial tactile complexes (Merkel discs), which are exteroceptors for light pressure located in the basal layer of the epidermis. Hair follicle receptors are exteroceptors that detect hair deflection. Encapsulated receptors include Tactile (Meissner’s) corpuscles (light pressure and low-frequency vibration in hairless skin), Lamellar (Pacinian) corpuscles (deep pressure, stretch, and high-frequency vibration in the dermis, hypodermis, and tendons), and Bulbous corpuscles or Ruffini endings (deep pressure and stretch in the dermis and joint capsules). Specific proprioceptors include muscle spindles (skeletal muscle stretch), tendon organs (tendon stretch), and joint kinesthetic receptors (mechanoreceptors and nociceptors in synovial joint capsules).

Overview and Profiles of the Cranial Nerves

Cranial nerves attach to the brain and pass through specific foramina of the skull, numbered from $I$ to $XII$. Nerves $I$ and $II$ attach to the forebrain, while all others attach to the brain stem. They primarily serve head and neck structures, except for the Vagus nerve ($X$), which extends into the abdomen. The nerves are ordered as follows: $I$ Olfactory, $II$ Optic, $III$ Oculomotor, $IV$ Trochlear, $V$ Trigeminal, $VI$ Abducens, $VII$ Facial, $VIII$ Vestibulocochlear, $IX$ Glossopharyngeal, $X$ Vagus, $XI$ Accessory, and $XII$ Hypoglossal. The mnemonic provided is ‐OH, OH OH! TO TOUCH AND FEEL VERY GOOD VELVET AH!‐

Olfactory Nerve ($I$) provides special visceral sensory function for smell via receptor cells in the nasal cavity, passing through the cribriform foramina of the ethmoid bone. Optic Nerve ($II$) provides special somatic sensory function for vision, originating in the retina and passing through the optic canals of the sphenoid bone. Oculomotor Nerve ($III$) provides somatic motor function to four extrinsic eye muscles (superior rectus, medial rectus, inferior rectus, and inferior oblique) and visceral motor function for pupil constriction and lens shape, passing through the superior orbital fissure. Trochlear Nerve ($IV$) provides somatic motor function to the superior oblique muscle, passing through the superior orbital fissure. Trigeminal Nerve ($V$) is the largest cranial nerve, with three divisions: Ophthalmic ($V_1$), Maxillary ($V_2$), and Mandibular ($V_3$), the latter of which innervates mastication muscles.

Abducens Nerve ($VI$) innervates the lateral rectus muscle to abduct the eye through the superior orbital fissure. Facial Nerve ($VII$) handles special visceral sensory (taste on the anterior $2/3$ of the tongue), somatic sensory (ear skin), somatic motor (facial muscles via five branches), and visceral motor (lacrimal and salivary glands), entering via the internal acoustic meatus and exiting the stylomastoid foramen. Vestibulocochlear Nerve ($VIII$) provides special somatic sensory for hearing (cochlear branch) and equilibrium (vestibular branch). Glossopharyngeal Nerve ($IX$) manages taste on the posterior $1/3$ of the tongue, oxygen/blood pressure sensing (carotid body/sinus), pharynx elevation, and the parotid gland, passing through the jugular foramen. Vagus Nerve ($X$) is the longest nerve, providing parasympathetic innervation to the heart, lungs, and abdominal viscera, and somatic motor to the pharynx/larynx. Accessory Nerve ($XI$) innervates the trapezius and sternocleidomastoid, arising from spinal rootlets ($C_1$ to $C_5$). Hypoglossal Nerve ($XII$) innervates tongue muscles and exits via the hypoglossal canal.

Spinal Nerves and Nerve Plexuses

There are $31$ pairs of spinal nerves connected to the spinal cord: $8$ pairs of cervical ($C_1-C_8$), $12$ pairs of thoracic ($T_1-T_{12}$), $5$ pairs of lumbar ($L_1-L_5$), $5$ pairs of sacral ($S_1-S_5$), and $1$ pair of coccygeal ($Co_1$). Spinal nerves connect via a dorsal root (sensory, contains dorsal root ganglion) and a ventral root (motor). Because they contain fibers from both, they are termed ‐mixed‐ nerves. They branch into a dorsal ramus (innervating back muscles in a segmented pattern) and a ventral ramus. Ventral rami (except $T_2-T_{12}$) form nerve plexuses in the cervical, brachial, lumbar, and sacral regions. The mnemonic for plexus organization is ‐Real Tired? Drink Coffee Now!‐ representing Rami (ventral), Trunks, Divisions, Cords, and Nerves.

The Cervical Plexus ($C_1-C_4$) includes the phrenic nerve ($C_3, C_4, C_5$), which innervates the diaphragm. The Brachial Plexus ($C_5-C_8, T_1$) serves the upper limb via five main nerves: Musculocutaneous (biceps brachii, brachialis), Median (anterior forearm, lateral palm), Ulnar (intrinsic hand, medial hand skin), Radial (posterior upper limb), and Axillary (deltoid, teres minor). Injuries include ‐Hand of benediction‐ (median nerve), ‐Clawhand‐ (ulnar nerve), and ‐Wrist-drop‐ (radial nerve). The Lumbar Plexus ($L_1-L_4$) includes the Femoral nerve (anterior thigh) and Obturator nerve (adductors). The Sacral Plexus ($L_4-S_4$) includes the Sciatic nerve (the body’s largest nerve, comprising the tibial and common fibular nerves), gluteal nerves, and the Pudendal nerve (perineal muscles).

Dermatomes and PNS Disorders

A dermatome is an area of skin innervated by the cutaneous branches of a single spinal nerve. The upper limb is supplied by the brachial plexus, while the lower limb is supplied by the lumbar plexus (anterior) and sacral plexus (posterior). A significant disorder of the PNS is Shingles (herpes zoster), a viral infection stemming from childhood chicken pox. It is often triggered by stress, mostly affects individuals over the age of $50$, and can impact any peripheral nerve.