Chapter 14: The Peripheral Nervous System – Anatomy & Physiology Study Notes

Peripheral Nervous System (PNS)

Overview and Functional Divisions

The Peripheral Nervous System (PNS) consists of all neural structures outside the brain and spinal cord. It connects the central nervous system (CNS) to limbs and organs, enabling the body to sense and respond to stimuli.

• Functional Divisions: The PNS is divided into sensory (afferent) and motor (efferent) divisions.

• Both divisions are further subdivided into somatic (body wall and limbs) and visceral (internal organs) components.

• Sensory division includes general (touch, pain, temperature) and special sensory (vision, hearing, taste, smell, equilibrium) information.

Nerves in the PNS allow the CNS to receive information and initiate action.

Basic Divisions of the Nervous System

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

• Efferent (Motor) Division: Transmits motor commands from the CNS to effectors (muscles and glands).

• Somatic: Voluntary control (skeletal muscles).

• Visceral: Involuntary control (smooth muscle, cardiac muscle, glands; regulated by the autonomic nervous system).

Organization of the PNS

Structural Components

• Sensory receptors: Detect stimuli from inside or outside the body.

• Nerves: Bundles of peripheral axons; cranial and spinal nerves are mostly mixed nerves (contain both sensory and motor fibers).

• Ganglia: Clusters of peripheral neuronal cell bodies.

• Motor endings: Axon terminals of motor neurons that innervate effectors.

Somatic vs. Visceral Motor Components

• Somatic motor: Innervates skeletal muscle.

• Visceral motor: The autonomic nervous system (ANS), which has two divisions: parasympathetic and sympathetic.

Peripheral Sensory Receptors

Categories and Functions

Peripheral sensory receptors are specialized structures that detect sensory stimuli and transmit electrical signals to the CNS.

• Free nerve endings: Monitor general sensory information (touch, pain, temperature, itch).

• Complete receptor cells: Specialized epithelial cells or small neurons that monitor special sensory information (vision, hearing, taste, equilibrium).

Classification of Sensory Receptors

• By Location:

  • Exteroceptors: Sensitive to stimuli from outside the body; located at or near body surfaces; include receptors for touch, pressure, pain, temperature, and most special sense organs.

  • Interoceptors: Receive stimuli from internal viscera (digestive tube, bladder, lungs); monitor chemical concentration, tissue stretching, and temperature.

  • Proprioceptors: Monitor degree of stretch in skeletal muscles, tendons, joints, and ligaments; send information about body movement to the CNS.

• By Stimulus Detected:

  • Mechanoreceptors: Respond to mechanical forces (touch, pressure, stretch, vibration, itch); baroreceptors monitor blood pressure.

  • Thermoreceptors: Respond to temperature changes.

  • Chemoreceptors: Respond to chemicals (taste, smell, blood components such as $H^+$, $Na^+$).

  • Photoreceptors: Respond to light (located in the eye).

  • Nociceptors: Respond to harmful stimuli that result in pain.

• By Structure:

  • General sensory receptors: Widely distributed; nerve endings of sensory neurons that monitor general somatic senses (touch, stretch, pressure, pain, vibration, temperature, proprioception).

  • Divided into two main groups: free nerve endings and encapsulated nerve endings.

Free Nerve Endings

• Abundant in epithelia and underlying connective tissue.

• Respond to pain, temperature, and itch; monitor affective senses (emotional response).

• Two modified types:

  • Epithelial tactile complexes (Merkel discs): Wrap around tactile epithelial cells; slowly adapting mechanoreceptors for light touch.

  • Hair follicle receptors: Wrap around hair follicles; rapidly adapting mechanoreceptors for light touch.

Table: Free Nerve Endings

Structural Class	Functional Class	Body Location
Free nerve endings	Exteroceptors, mechanoreceptors (pain, temperature, pressure, itch)	Most body tissues, especially connective tissues and epithelia
Merkel discs	Exteroceptors, mechanoreceptors (light pressure, slowly adapting)	Basal layer of epidermis
Hair follicle receptors	Exteroceptors, mechanoreceptors (hair movement, rapidly adapting)	In and surrounding hair follicles

Encapsulated Nerve Endings

• Consist of end fibers of sensory neurons enclosed in a capsule of connective tissue.

• Capsules amplify the stimulus or filter out irrelevant stimuli.

• All are mechanoreceptors; four main types:

  • Tactile (Meissner's) corpuscles: Spiral nerve endings in dermal papillae of sensitive, hairless skin; rapidly adapting receptors for fine, discriminative touch.

  • Lamellar (Pacinian) corpuscles: Single nerve ending in dermis and hypodermis; rapidly adapting receptors for deep pressure.

  • Bulbous corpuscles (Ruffini endings): Array of nerve endings in dermis; slowly adapting receptors for pressure and touch; best for monitoring continuous pressure.

  • Proprioceptors: Monitor stretch in locomotory organs.

Table: Encapsulated Nerve Endings

Structural Class	Functional Class	Body Location
Tactile (Meissner's) corpuscles	Exteroceptors, mechanoreceptors (light pressure, discriminative touch, vibration, rapidly adapting)	Dermal papillae of hairless skin (fingertips, nipples, external genitalia)
Lamellar (Pacinian) corpuscles	Exteroceptors, interoceptors, mechanoreceptors (deep pressure, stretch, vibration, rapidly adapting)	Dermis, hypodermis, periosteum, tendons, ligaments, joint capsules, fingers, soles, external genitalia
Bulbous corpuscles (Ruffini endings)	Exteroceptors, mechanoreceptors (deep pressure, stretch, slowly adapting)	Deep in dermis, hypodermis, joint capsules

Proprioceptors

• Monitor stretch in locomotory organs; three main types:

• Muscle spindles: Measure changing length (stretch) of a muscle; embedded in perimysium between muscle fascicles.

• Golgi tendon organs: Located near muscle-tendon junction; monitor tension within tendons during muscle contraction.

• Joint kinesthetic receptors: Located within joint capsules; monitor stretch in synovial joints; include lamellar corpuscles, bulbous corpuscles, free nerve endings, and tendon organ-like receptors.

Cranial Nerves

General Features

• 12 pairs, attach to the brain and pass through specific foramina of the skull.

• Numbered I to XII (Roman numerals).

• Cranial nerves I and II attach to the forebrain; others to the brain stem.

• Most serve head and neck structures; Vagus nerve (X) extends into the abdomen.

Table: Cranial Nerves and Functions

Number	Name	Type	Main Function
I	Olfactory	Sensory	Smell
II	Optic	Sensory	Vision
III	Oculomotor	Motor	Eye movement, pupil constriction, lens shape
IV	Trochlear	Motor	Eye movement (superior oblique muscle)
V	Trigeminal	Both	Facial sensation, chewing muscles
VI	Abducens	Motor	Eye movement (lateral rectus muscle)
VII	Facial	Both	Facial expression, taste (anterior tongue)
VIII	Vestibulocochlear	Sensory	Hearing, equilibrium
IX	Glossopharyngeal	Both	Taste (posterior tongue), swallowing, blood pressure
X	Vagus	Both	Visceral organ control, taste (epiglottis)
XI	Accessory	Motor	Head and shoulder movement
XII	Hypoglossal	Motor	Tongue movement

Mnemonic for Cranial Nerves

• Names: "On, On, On, They Traveled And Found Voldemort Guarding Very Ancient Horcruxes"

• Functions: "Some Say Money Matters, But My Brother Says Big Brains Matter Most" (S = Sensory, M = Motor, B = Both)

Examples and Clinical Applications

• Olfactory Nerve (I): Lesion causes anosmia (loss of smell).

• Optic Nerve (II): Lesion affects vision.

• Trigeminal Nerve (V): Anesthesia for dental procedures targets maxillary and mandibular branches.

• Facial Nerve (VII): Lesion can cause Bell's palsy (facial paralysis).

Additional info:

• These notes cover the organization, structural components, and functional classification of the peripheral nervous system, including sensory receptors and cranial nerves, as outlined in a college-level Anatomy & Physiology course.