Chapter 13: The Peripheral Nervous System

Sensory Receptors and Stimulus Classification

• Definition of Sensory Receptors: These are structures specialized to respond to changes in the environment, which are referred to as stimuli.

• Sensation vs. Perception:   - Sensation: This is the awareness of a stimulus.   - Perception: This is the interpretation of the meaning of the stimulus. Both sensation and perception occur within the brain.

• Classifications of Receptors: There are three primary ways to classify sensory receptors:   - By the type of stimulus they respond to.   - By their location in the body.   - By their structural complexity.

• Classification by Stimulus Type:   - Mechanoreceptors: These receptors respond to mechanical forces such as touch, pressure, vibration, and stretch.   - Thermoreceptors: These are sensitive to changes in temperature.   - Photoreceptors: These respond to light energy; a primary example is the retina of the eye.   - Chemoreceptors: These respond to chemicals in solution, such as those related to smell, taste, or changes in blood chemistry.   - Nociceptors: These are sensitive to pain-causing stimuli. Examples include extreme heat or cold, excessive pressure, and inflammatory chemicals.

Classification of Receptors by Location and Structure

• Classification by Body Location:   - Exteroceptors: These respond to stimuli arising from outside the body. They include receptors in the skin for touch, pressure, pain, and temperature, as well as most special sense organs.   - Interoceptors (Visceroceptors): These respond to stimuli arising from internal viscera and blood vessels. They are sensitive to chemical changes, tissue stretch, and temperature changes. While they sometimes cause discomfort, individuals are usually unaware of their workings.   - Proprioceptors: These respond to stretch in skeletal muscles, tendons, joints, ligaments, and connective tissue coverings of bones and muscles. Their primary function is to inform the brain of the body's movements.

• Classification by Receptor Structure:   - Simple Receptors of the General Senses: These are modified dendritic endings of sensory neurons found throughout the body. They monitor most types of general sensory information.   - Receptors for Special Senses: These are all housed in complex sense organs and relate to vision, hearing, equilibrium, smell, and taste.

Simple Receptors of the General Senses

• General Senses Information: General senses include tactile sensations (touch, pressure, stretch, vibration), temperature, pain, and muscle sense.

• Receptor Categories:   - Nonencapsulated (Free) Nerve Endings:     - These are abundant in epithelia and connective tissues.     - Most are nonmyelinated, small-diameter group $C$ fibers.     - The distal terminals often have knoblike swellings.     - They respond primarily to temperature, pain, or light touch.   - Encapsulated Nerve Endings:     - Almost all are mechanoreceptors.     - Their terminal endings are encased in a connective tissue capsule.

• Specific Examples of Nonencapsulated Endings:   - Thermoreceptors:     - Cold receptors: Activated by temperatures from $10$ to $40\,^\circ C$; located in the superficial dermis.     - Heat receptors: Activated from $32$ to $48\,^\circ C$; located in the deeper dermis.     - Pain Threshold: Outside of these specific temperature ranges, nociceptors are activated and the stimulus is interpreted as pain.   - Nociceptors: Pain receptors triggered by extreme temperatures, pinch, or the release of chemicals from damaged tissue.   - Tactile (Merkel) Discs: Function as light touch receptors located in the deeper layers of the epidermis.   - Hair Follicle Receptors: Free nerve endings that wrap around hair follicles.

• Specific Examples of Encapsulated Endings:   - Tactile (Meissner’s) Corpuscles: Small receptors for discriminative touch. Found just below the skin, primarily in sensitive, hairless areas like fingertips.   - Lamellar (Pacinian) Corpuscles: Large receptors that respond to deep pressure and vibration only when first applied (they سپس turn off). Located in the deep dermis.   - Bulbous Corpuscles (Ruffini endings): Respond to deep and continuous pressure.   - Muscle Spindles: Spindle-shaped proprioceptors that respond to muscle stretch.   - Tendon Organs: Proprioceptors located in tendons that detect stretch.   - Joint Kinesthetic Receptors: Proprioceptors that monitor joint position and motion.

Sensory Processing and Neural Integration

• Foundations of Survival:   - Sensation: Awareness of changes in the environment.   - Perception: Conscious interpretation of stimuli.

• Somatosensory System: This part of the sensory system serves the body wall and limbs. It receives inputs from exteroceptors, proprioceptors, and interoceptors.

• Levels of Neural Integration: Input is relayed toward the head and processed along three basic levels:   - Receptor Level: Sensory receptors.   - Circuit Level: Processing in ascending pathways.   - Perceptual Level: Processing in cortical sensory areas.

• Pathways of Three Neurons:   - First-order sensory neurons: Conduct impulses from the receptor level to spinal reflexes or to second-order neurons in the Central Nervous System (CNS).   - Second-order sensory neurons: Transmit impulses to third-order sensory neurons.   - Third-order sensory neurons: Conduct impulses from the thalamus to the somatosensory cortex at the perceptual level.

Structure and Classification of Nerves and Ganglia

• Definition of a Nerve: A cordlike organ of the Peripheral Nervous System (PNS). It is a bundle of myelinated and nonmyelinated peripheral axons enclosed by connective tissue.

• Nerve Classification by Origin: Can be either spinal or cranial nerves.

• Connective Tissue Coverings:   - Endoneurium: Loose connective tissue enclosing axons and their myelin sheaths (Schwann cells).   - Perineurium: Coarse connective tissue that bundles fibers into fascicles.   - Epineurium: A tough fibrous sheath surrounding all fascicles to form the nerve.

• Nerve Classification by Direction of Impulse:   - Mixed nerves: Contain both sensory and motor fibers; impulses travel both to and from the CNS. These are the most common.   - Sensory (afferent) nerves: Impulses travel only toward the CNS.   - Motor (efferent) nerves: Impulses travel only away from the CNS.

• Types of Fibers in Mixed Nerves:   - Somatic afferent: Sensory from muscle to brain.   - Somatic efferent: Motor from brain to muscle.   - Visceral afferent: Sensory from organs to brain.   - Visceral efferent: Motor from brain to organs.

• Ganglia: These contain neuron cell bodies associated with nerves in the PNS.   - Dorsal root ganglia: Associated with afferent nerve fibers; contain cell bodies of somatic sensory neurons.   - Autonomic ganglia: Associated with efferent nerve fibers; contain autonomic motor neurons (visceral).

Regeneration of Nerve Fibers

• PNS Regeneration: Mature neurons are amitotic. However, if the soma (cell body) of a damaged nerve remains intact, peripheral axons may regenerate in the PNS.

• CNS Limitation: Regeneration does not occur in the CNS.   - Growth Inhibition: CNS oligodendrocytes possess growth-inhibiting proteins.   - Scarring: Astrocytes at the injury site form scar tissue.   - Clinical Treatments: Potential treatments involve neutralizing growth inhibitors, blocking inhibitory protein receptors, or destroying scar tissue components.

The Twelve Pairs of Cranial Nerves

• General Characteristics: Twelve pairs attach to the brain. Two attach to the forebrain, while the rest attach to the brain stem. Most are mixed, though two pairs are purely sensory. They are numbered in Roman numerals ($I$ - $XII$) and named from rostral to caudal.

• Mnemonics for Names:   - "On occasion, our trusty truck acts funny—very good vehicle anyhow."   - "Oh once one takes the anatomy final, very good vacations are heavenly."

• Nerve Details ($I$–$XII$):   - I: Olfactory nerves: Purely sensory nerves for smell. They run from the nasal mucosa to the olfactory bulbs through the cribriform plate of the ethmoid bone. Damage results in anosmia.   - II: Optic nerves: Purely sensory nerves arising from the retinas. They pass through optic canals, partially cross at the optic chiasma, and continue as optic tracts to the thalamus, ending at the visual cortex in the occipital lobe.   - III: Oculomotor nerves: Extend from the ventral midbrain through superior orbital fissures. Functions: Raising eyelids, directing eyeballs, constricting the iris (parasympathetic), and controlling lens shape.   - IV: Trochlear nerves: Originate in the dorsal midbrain and enter orbits via superior orbital fissures to innervate the superior oblique muscle. Primarily motor; directs the eyeball.   - V: Trigeminal nerves: The largest cranial nerves, extending from the pons to the face. Three divisions: Ophthalmic ($V_1$), Maxillary ($V_2$), and Mandibular ($V_3$). Convey sensory impulses from the face and supply motor fibers for mastication (chewing).   - VI: Abducens nerves: Fibers from the inferior pons enter orbits via superior orbital fissures. Primarily motor, innervating the lateral rectus muscle.   - VII: Facial nerves: Fibers from the pons travel via internal acoustic meatuses and stylomastoid foramina. Chief motor nerves of the face (five branches). Motor: Facial expression, lacrimal and salivary gland secretion. Sensory: Taste from the anterior two-thirds of the tongue.   - VIII: Vestibulocochlear nerves: Sensory fibers for hearing (cochlear) and equilibrium (vestibular) pass from the inner ear to the pons-medulla border. Small motor component for receptor sensitivity adjustment.   - IX: Glossopharyngeal nerves: Fibers from the medulla leave via the jugular foramen. Motor: Swallowing (tongue and pharynx) and parotid salivary gland secretion. Sensory: Taste/sensory from the pharynx and posterior tongue; impulses from carotid chemoreceptors and baroreceptors.   - X: Vagus nerves: The only cranial nerves extending beyond the head and neck. Exit via the jugular foramen. Motor: Parasympathetic regulation of heart, lungs, and abdominal viscera. Sensory: Impulses from thoracic/abdominal viscera, baroreceptors, chemoreceptors, and taste buds of the posterior tongue.   - XI: Accessory nerves: Formed from ventral rootlets of the spinal cord (not brain). Enter the cranium via the foramen magnum and exit via the jugular foramen. Innervate the trapezius and sternocleidomastoid muscles.   - XII: Hypoglossal nerves: Fibers from the medulla exit via the hypoglossal canal. Innervate muscles of the tongue for swallowing and speech.

Spinal Nerves: Structure and Distribution

• Overview: There are $31$ pairs of mixed nerves, named for their point of exit from the spinal cord:   - $8$ pairs of cervical nerves ($C_1$–$C_8$).   - $12$ pairs of thoracic nerves ($T_1$–$T_{12}$).   - $5$ pairs of lumbar nerves ($L_1$–$L_5$).   - $5$ pairs of sacral nerves ($S_1$–$S_5$).   - $1$ pair of coccygeal nerves ($Co_1$).

• Cervical Exit Rule: Although there are only $7$ cervical vertebrae, there are $8$ pairs of cervical nerves. The first $7$ exit superior to their named vertebrae; the $8^{th}$ pair exits inferior to vertebra $C_7$. All subsequent spinal nerves exit inferior to the vertebra for which they are named.

• Spinal Roots:   - Ventral roots: Contain motor (efferent) fibers from ventral horn motor neurons innervating skeletal muscles.   - Dorsal roots: Contain sensory (afferent) fibers from sensory neurons in dorsal root ganglia.

• Branching into Rami: Almost immediately after exiting the intervertebral foramen, nerves divide into:   - Dorsal ramus: Smaller branch supplying the posterior body trunk.   - Ventral ramus: Larger branch supplying the rest of the trunk and limbs.   - Meningeal branch: Tiny branch reentering the vertebral canal to innervate meninges and blood vessels.

• Root vs. Ramus Distinctions:   - Roots: Medial to the spinal nerves; purely sensory or motor.   - Rami: Distal to/branches of the spinal nerves; can carry both sensory and motor (mixed).

Nerve Plexuses and Innervation of the Limbs

• Nerve Plexuses: Except for $T_2$–$T_{12}$, all ventral rami form interlacing networks called plexuses (Cervical, Brachial, Lumbar, Sacral).

• Functional Significance of Plexuses: Fibers crisscross so each branch contains fibers from several nerves, and fibers from a single ventral ramus travel to the periphery via several routes. This ensures that limb muscle damage to one spinal nerve does not cause complete paralysis.

• Cervical Plexus ($C_1$–$C_4$):   - Innervates skin of the neck, ear, back of the head, and shoulders.   - Phrenic nerve: The most important nerve of this plexus; receives fibers from $C_3$, $C_4$, and $C_5$. It is the motor and sensory nerve of the diaphragm.

• Brachial Plexus ($C_5$–$T_1$):   - Structure: Roots (five ventral rami) → Trunks (upper, middle, lower) → Divisions (anterior, posterior) → Cords (lateral, medial, posterior).   - Major Nerves: Axillary, musculocutaneous, median, radial, and ulnar.

• Lumbar Plexus ($L_1$–$L_4$):   - Innervates the thigh, abdominal wall, and psoas muscle.   - Femoral nerve: Innervates quadriceps and skin of the anterior thigh/medial leg.   - Obturator nerve: Passes through the obturator foramen to innervate adductor muscles.

• Sacral Plexus ($L_4$–$S_4$):   - Serves the buttock, lower limb, pelvic structures, and perineum.   - Sciatic nerve: The longest and thickest nerve in the body. Composed of two nerves—the tibial and common fibular. It innervates hamstrings, adductor magnus, and most leg/foot muscles.

Clinical Homeostatic Imbalances of the Peripheral Nerves

• Phrenic Nerve: Irritation causes hiccups. Severing both phrenic nerves or destruction of the $C_3$–$C_5$ region causes diaphragm paralysis and respiratory arrest, requiring mechanical respirators.

• Brachial Plexus Injuries: Common from pulling the upper limb too hard or blows to the shoulder.

• Median Nerve: Injury impairs the pincer grasp (thumb and index finger). Common in carpal tunnel syndrome or wrist-slashing attempts.

• Ulnar Nerve: Damage results in sensory loss/paralysis and clawhand (hyperextension of knuckles, flexion of distal joints). Striking the "funny bone" (medial epicondyle) affects this nerve.

• Radial Nerve: Trauma leads to wrist drop (inability to extend hand). Can be caused by improper crutch use or "Saturday night paralysis" (compressing blood supply by draping an arm over a chair while intoxicated).

• Lumbar Plexus: Compression leads to gait problems, pain, or numbness of the anterior thigh.

• Sciatica: Stabbing pain radiating over the sciatic nerve course. If transected, hamstrings are paralyzed and footdrop (permanent plantar flexion) occurs.

Innervation of the Thorax, Abdominal Wall, Back, and Dermatomes

• Intercostal Nerves: Ventral rami of $T_1$–$T_{12}$ supply muscles of the ribs, anterolateral thorax, and abdominal wall.

• Specific Thoracic Nerves: includes a tiny $T_1$ and the $T_{12}$ subcostal nerve.

• Back Innervation: Handled by dorsal rami, with each branch innervating a strip of muscle and skin in line with its emergence.

• Dermatomes:   - Definition: An area of skin innervated by cutaneous branches of a single spinal nerve.   - All spinal nerves except for $C_1$ participate in dermatomes.   - Clinical Use: Affected dermatomes help ascertain the extent of spinal cord injuries.   - Note: Most dermatomes overlap; therefore, destruction of a single nerve typically does not cause total numbness.

Questions & Discussion

• Question 1: Fill in the blank: ______ are stimulated when sound waves vibrate hair cells in the inner ear.   - Answer: a) Mechanoreceptors.

• Question 2: An intensely painful stimulus is distinguished from a mildly painful one by:   - Answer: a) a higher frequency of action potentials initiated.

• Question 3: Ganglia associated with efferent nerve fibers mostly contain cell bodies of:   - Answer: c) autonomic motor neurons.

• Question 4: The perineurium:   - Answer: c) binds groups of fibers into bundles.

• Question 5: Differences in regeneration capability between CNS and PNS neurons can mainly be attributed to the fact that:   - Answer: d) both a and b are true (PNS supporting cells promote cleanup/regrowth; CNS oligodendrocytes contain growth-inhibiting proteins).

• Question 6: Which cranial nerve is the exception and travels to the thoracic and abdominal cavities?   - Answer: d) Vagus.

• Question 7: Which of the following cranial nerves has both sensory and motor functions?   - Answer: a) Facial.

• Question 8: The dorsal roots along the spinal cord contain ______ neurons, while the dorsal rami contain ______ neurons.   - Answer: b) sensory; mixed.