Peripheral Nervous System & Special Senses: Study Notes (Chapters 14-16)

Chapter 14: Peripheral Nervous System (PNS)

I. Overview of PNS Organization

The Peripheral Nervous System (PNS) connects the central nervous system (CNS) to limbs and organs. It is divided into Sensory (Afferent) and Motor (Efferent) divisions, each with specialized functions.

Division	Type	Function
Sensory (Afferent)	Somatic - General	Touch, pain, pressure, vibration, temperature, proprioception
Sensory (Afferent)	Somatic - Special	Hearing, equilibrium, vision
Sensory (Afferent)	Visceral - General	Stretch, pain, temperature, chemical changes in viscera
Sensory (Afferent)	Visceral - Special	Taste, smell
Motor (Efferent)	Somatic	Motor innervation of skeletal muscle
Motor (Efferent)	Autonomic (ANS)	Motor innervation of smooth muscle, cardiac muscle, glands (Sympathetic & Parasympathetic)

II. Sensory Receptor Classification

Sensory receptors are classified by location and stimulus type, allowing the body to detect and respond to various environmental and internal changes.

• Exteroceptors: Sensitive to external stimuli (touch, pressure, pain, temperature)

• Interoceptors: Receive stimuli from internal viscera (chemical changes, stretch, temperature)

• Proprioceptors: Located in muscles, tendons, joints, ligaments; monitor body position and movement

Classification by Stimulus Type:

• Mechanoreceptors: Touch, pressure, stretch, vibration, itch

• Baroreceptors: Blood pressure

• Thermoreceptors: Temperature changes

• Chemoreceptors: Chemicals in solution

• Photoreceptors: Light (in the eye)

• Nociceptors: Harmful stimuli causing pain

III. Cranial Nerves (12 Pairs)

The cranial nerves are twelve pairs of nerves that emerge directly from the brain, each with specific sensory and/or motor functions.

Number	Name	Function	Pathway
I	Olfactory	Special visceral sensory - smell	Cribriform foramina of ethmoid bone
II	Optic	Special somatic sensory - vision	Optic canals of sphenoid bone
III	Oculomotor	Somatic motor - 4 extrinsic eye muscles; Visceral motor - pupil constriction, lens shape	Superior orbital fissure
IV	Trochlear	Somatic motor - superior oblique muscle	Superior orbital fissure
V	Trigeminal	Sensory from face (3 divisions: V1, V2, V3); Motor to mastication	V1: Superior orbital fissure; V2: Foramen rotundum; V3: Foramen ovale
VI	Abducens	Somatic motor - lateral rectus muscle	Superior orbital fissure
VII	Facial	Special visceral sensory - taste (anterior 2/3 tongue); Motor to facial muscles; Visceral motor - lacrimal and salivary glands	Internal acoustic meatus
VIII	Vestibulocochlear	Special somatic sensory - hearing (cochlear) and equilibrium (vestibular)	Internal acoustic meatus
IX	Glossopharyngeal	Special visceral sensory - taste (posterior 1/3 tongue); Motor to pharynx; Visceral motor - parotid gland	Jugular foramen
X	Vagus	General visceral sensory from thoracic/abdominal viscera; Motor to pharynx/larynx; Parasympathetic to heart, lungs, abdomen	Jugular foramen
XI	Accessory	Somatic motor - trapezius and sternocleidomastoid	Foramen magnum & Jugular foramen
XII	Hypoglossal	Somatic motor - tongue muscles	Hypoglossal canal

IV. Spinal Nerves & Major Nerve Plexuses

Spinal nerves are distributed throughout the body and form complex networks called plexuses, which innervate specific regions.

• Distribution: 8 cervical (C1–C8), 12 thoracic (T1–T12), 5 lumbar (L1–L5), 5 sacral (S1–S5), 1 coccygeal (Co1)

• Dorsal root: Sensory fibers; cell bodies in dorsal root ganglion

• Ventral root: Motor fibers; cell bodies in anterior gray column

• Dorsal ramus: Innervates back muscles and skin

• Ventral ramus: Forms nerve plexuses (except T2–T12)

Major Nerve Plexuses:

• Cervical Plexus (C1–C4): Phrenic nerve (innervates diaphragm), cutaneous nerves, anterior neck muscles

• Brachial Plexus (C5–T1): Musculocutaneous (biceps), Median (forearm muscles), Ulnar (hand muscles), Axillary (deltoid), Radial (posterior arm)

• Lumbar Plexus (L1–L4): Femoral nerve (anterior thigh), Obturator nerve (adductor muscles)

• Sacral Plexus (L4–S4): Sciatic nerve (largest), Tibial nerve (posterior leg), Common fibular (peroneal) nerve (anterolateral leg), Gluteal nerves, Pudendal nerve (perineum)

V. Autonomic Nervous System (ANS) Overview

The ANS is the General Visceral Motor division that regulates involuntary functions such as heart rate, blood pressure, digestion, and urination. It differs from the somatic motor system in several key ways.

Feature	Somatic Motor	Autonomic Motor
Number of neurons	One motor neuron	Two motor neurons (preganglionic & postganglionic)
Myelination	Well myelinated (FAST)	Thin/none (SLOWER)
Synapse location	At muscle	In a ganglion

II. Sympathetic Division ("Fight or Flight")

The sympathetic division prepares the body for stressful or emergency situations.

• Activation: Fear, rage, exercise

• Effects: Increased heart rate, blood flow to muscles, dilated pupils, decreased digestive/urinary activity

• Anatomy: Originates from lateral horn of gray matter (T1–L2); trunk ganglia on both sides of vertebral column

• Adrenal Medulla: Secretes norepinephrine and epinephrine (adrenaline)

III. Parasympathetic Division ("Rest and Digest")

The parasympathetic division conserves energy and promotes maintenance activities.

• Activation: When body is at rest

• Effects: Decreased heart rate, increased digestion, energy conservation

• Anatomy: Originates from brain stem and sacral spinal cord; short postganglionic fibers; neurotransmitter is acetylcholine

• Divisions: Cranial outflow (head, neck, thorax, abdomen); Sacral outflow (distal digestive tract, pelvic organs)

IV. Visceral Sensory Neurons

Visceral sensory neurons monitor internal organ conditions and transmit information to the CNS.

• Cell bodies: Located in dorsal root ganglion

• Visceral pain: Results from chemical irritation or inflammation, not cutting

Chapter 16: Special Senses

I. Taste (Gustation)

Taste is detected by taste buds located on the tongue, which contain specialized epithelial cells.

• Location: Taste buds (50–100 epithelial cells per bud)

• Cell Types: Gustatory epithelial cells, basal epithelial cells (replaced every 7–10 days)

• Papillae Types: Fungiform, vallate, foliate

• Taste Qualities: Sweet, sour, salty, bitter, umami

• Neural Pathway: CN VII and IX (some CN X) → solitary nucleus (medulla) → thalamus → gustatory area of cerebral cortex (insula)

Additional info: The "taste map" is a myth; all taste qualities can be detected across the tongue.

II. Smell (Olfaction)

Olfaction is the sense of smell, detected by olfactory sensory neurons in the nasal epithelium.

• Olfactory Epithelium: Pseudostratified columnar with olfactory sensory neurons, supporting cells, basal cells

• Mechanism: Airborne chemicals dissolve in nasal mucus, bind to cilia, and trigger neural signals

• Neural Pathway: Olfactory nerve → olfactory bulbs (mitral cells) → olfactory tract → limbic system & primary olfactory cortex

III. Vision (Sight)

Vision is the most complex special sense, with 70% of all sensory receptors located in the eyes.

A. External Structures

• Eyebrows: On superciliary arches

• Eyelids (palpebrae): Contain tarsal plates and glands

• Conjunctiva: Transparent mucous membrane

• Lacrimal apparatus: Produces and drains tears

B. Extrinsic Eye Muscles (6 muscles)

• Lateral rectus, medial rectus, superior rectus, inferior rectus, superior oblique, inferior oblique

C. Three Layers of the Eye

1. Fibrous Layer (External): Sclera (white, supports shape), cornea (transparent, refracts light)

2. Vascular Layer (Middle): Choroid (pigmented, prevents light scatter), ciliary body (lens shape), iris (controls pupil size)

3. Retina (Inner Layer): Photoreceptor cells (rods & cones), bipolar cells, ganglion cells; specialized regions: ora serrata, macula lutea, fovea centralis, optic disc

D. Photoreceptors

Feature	Rod Cells	Cone Cells
Sensitivity	More sensitive to light	Operate in bright light
Function	Dim light vision	High acuity, color vision
Location	Throughout retina	Concentrated in macula/fovea

• Properties: Considered neurons; vulnerable to damage; cannot regenerate if destroyed; continuously renew outer segments

E. The Lens

• Thick, transparent, biconvex disc

• Held by ciliary zonule

• Lens fibers added throughout life

F. Visual Pathway

• Light activates photoreceptors → bipolar cells → ganglion cells → optic nerve → optic tracts → lateral geniculate nucleus (thalamus) → optic radiation → primary visual cortex

• Partial decussation at optic chiasm allows depth perception

• Secondary pathways: To midbrain (circadian rhythms, reflexes)

Chapter 16: Special Senses (continued)

IV. Hearing and Equilibrium

A. External Ear

• Auricle (pinna): Directs sounds

• External acoustic meatus: Contains hairs, glands

• Tympanic membrane: Boundary between external and middle ear

B. Middle Ear

• Tympanic cavity: Air-filled space in temporal bone

• Ear Ossicles: Malleus (attaches to eardrum), Incus (between malleus and stapes), Stapes (vibrates against oval window)

• Other structures: Oval window, round window, pharyngotympanic tube (links middle ear and pharynx), tensor tympani and stapedius muscles

C. Internal Ear (Labyrinth)

• Bony Labyrinth: Semicircular canals, vestibule, cochlea

• Membranous Labyrinth: Semicircular ducts, utricle and saccule, cochlear duct

D. Cochlea

• Structure: Spirals around modiolus (pillar of bone); contains osseous spiral lamina

• Three Chambers: Scala vestibuli (upper), cochlear duct (middle, contains receptors), scala tympani (lower)

• Spiral Organ Components: Supporting cells, inner hair cells (main receptors), outer hair cells (tune/amplify signal)

E. Vestibule and Semicircular Canals (Equilibrium)

• Utricle and Saccule: Contain macula (sensory epithelium), monitor head position when still, hair cells synapse with vestibular nerve, tips embedded in otolith membrane

• Semicircular Canals: Three canals (anterior, posterior, lateral); each contains a semicircular duct; ampulla contains crista ampullaris (hair cells detect rotational acceleration)

Study Tips

• Create comprehensive charts and diagrams to visualize structures

• Practice labeling pathways and structures

• Focus on clinical correlations and real-world applications

• Self-check with practice questions and flashcards