Peripheral NS Flashcards
What is a nerve in the PNS?
A cordlike organ of the PNS consisting of a bundle of myelinated and nonmyelinated peripheral axons enclosed by connective tissue
What is the endoneurium?
Loose connective tissue that wraps around individual axons and their myelin or neurilemma sheaths.
What is the perineurium, and what does it do?
Coarse connective tissue that bundles nerve fibers into fascicles (groups of axons).
What is the epineurium?
The epineurium surrounds the fascicles and runs between the fascicular groups.
How are nerves classified based on impulse transmission?
Mixed nerves – Contain both sensory & motor fibers (impulses travel to and from CNS).
Sensory (afferent) nerves – Impulses only toward CNS (rarely pure).
Motor (efferent) nerves – Impulses only away from CNS (rarely pure).
What is the primary function of the olfactory nerve (CN I)?
Purely sensory (afferent)
Detects and transmits smell signals from nasal mucosa to the brain
Trace the pathway of the olfactory nerve (CN I).
Olfactory receptors in nasal mucosa →Olfactory bulb (synapse) →Olfactory tract → Primary olfactory cortex (temporal lobe).
What condition results from damage to CN I, and what causes it?
Anosmia (total/partial loss of smell).
Causes: Fracture of ethmoid bone. Lesions of olfactory fibers (e.g., trauma, infections, neurodegenerative diseases).
What is the primary function of the optic nerve (CN II)?
Purely sensory (afferent)
Transmits visual signals from the retina to the brain.
Describe the visual pathway involving CN II.
Retina → Optic nerve →Optic chiasm (partial decussation) → Optic tract →Thalamus (lateral geniculate nucleus) →Occipital cortex (visual processing).
What visual deficits occur from damage to CN II or its pathway?
Anopsia : Optic nerve damage Blindness in the affected eye.
- Beyond the chiasm (e.g., optic tract):
What are the motor and parasympathetic functions of CN III?
Motor: Innervates 4/6 extrinsic eye muscles (medial, superior/inferior rectus; inferior oblique).
Levator palpebrae (raises eyelid).
Parasympathetic: Constricts pupil (sphincter pupillae). Controls lens accommodation (ciliary muscle).
Where does CN III originate and travel?
Origin: Ventral midbrain (nuclei).
Exits skull: Superior orbital fissure → innervates eye muscles.
What are the signs of oculomotor nerve (CN III) palsy?
Eye deviation: “Down and out” (external strabismus) due to unopposed lateral rectus (CN VI) and superior oblique (CN IV).
Ptosis (drooping eyelid), has double vision and trouble focusing on close things
What is the primary function of the trochlear nerve (CN IV)?
Motor only. Innervates the superior oblique muscle (moves eye downward and laterally; intorsion)
What are the effects of trochlear nerve (CN IV) damage?
Diplopia (double vision):
Impaired inferolateral eye rotation
Describe the pathway of the trochlear nerve (CN IV).
Origin: Dorsal midbrain (trochlear nucleus)
Crosses midline before innervating the contralateral superior oblique muscle
Enters orbit via superior orbital fissure
What is the primary function of the abducens nerve (CN VI)?
Motor only
Innervates the lateral rectus muscle (abducts the eye)
Trace the pathway of the abducens nerve (CN VI).
Origin: Pons (abducens nucleus)
Exits skull: Superior orbital fissure
Innervates: Lateral rectus muscle
What happens in abducens nerve (CN VI) palsy?
Eye cannot abduct (move laterally)
Internal strabismus: Eye deviates medially at rest
What are the functions of the trigeminal nerve (CN V) divisions?
V1 (Ophthalmic): Sensory (forehead, cornea, nasal cavity)
V2 (Maxillary): Sensory (cheek, upper lip, maxillary teeth)
V3 (Mandibular):
Sensory: Lower jaw, tongue (general sensation)
Motor: Muscles of mastication (masseter, temporalis
Describe the trigeminal nerve (CN V) pathway.
fibers extends from pons to face
What is trigeminal neuralgia?
Excruciating stabbing pain. Triggered by mild stimuli (brushing teeth, breeze)
Cause: by a loop of artery or vein
that compresses the trigeminal nerve near its exit from the brain stem.
What are the motor, sensory, and parasympathetic functions of CN VII?
Motor: Controls facial expression (smiling, eye closure, blinking). dampens loud noises in inner ear.
Parasympathetic: Stimulates lacrimal (tear) and salivary glands (submandibular/sublingual).
Sensory: Taste from anterior 2/3 of tongue.
Describe the pathway of the facial nerve (CN VII).
Pons to lateral aspect of the face
Branches: 5 major branches (temporal, zygomatic, buccal, mandibular, cervical) to facial muscles.
What is Bell’s palsy, and how does it present?
Unilateral facial paralysis . Loss of taste (anterior 2/3 tongue).
Dry eye or excessive tearing.
caused by swollen or inflamed facial nerve due HSV-1 inflammation/swelling of CN VII.
Treatment: Corticosteroids (70% recover fully).
What are the divisions and functions of CN VIII?
Cochlear division: Hearing (sound → brainstem).
Vestibular division: Balance/equilibrium (head position → brainstem).
Minor motor: Adjusts receptor sensitivity
What deficits result from CN VIII damage?
Lesions of cochlear nerve or cochlear receptors result in central, or nerve,
deafness. Damage to vestibular division produces dizziness, rapid involuntary
eye movements, loss of balance, nausea, and vomiting
What are the motor, sensory, and parasympathetic functions of CN X?
Parasympathetic (80% of fibers): Regulates heart rate, lungs, and digestive organs.
Somatic Motor: Innervates pharynx (swallowing) and larynx
Sensory: Baroreceptors (carotid sinus), chemorecptors .General sensation from pharynx, larynx, and visceral organs.
Describe the pathway of the vagus nerve (CN X).
Origin: Medulla (multiple nuclei).
Only cranial nerves that extend beyond head and neck
region
What happens if the vagus nerve is damaged?
Laryngeal damage: Hoarseness or loss of voice (vocal cord paralysis).
Swallowing difficulty (dysphagia)
maintains the normal state of visceral organ activity. Without the sympathetic nerves, which mobilize and accelerate vital
body processes (and shut down digestion), would dominate
What is the primary function of the accessory nerve (CN XI)?
Somatic motor only.
Innervates: Sternocleidomastoid (head rotation). Trapezius (shoulder shrug/neck extension)
Trace the pathway of CN XI.
formed from ventral root from Spinal root: C1-C5 spinal cord.
What are the signs of accessory nerve palsy?
Head turns toward lesion (sternocleidomastoid paralysis).
Shoulder droop (trapezius weakness).
What is the function of the hypoglossal nerve (CN XII)?
Motor only:
Controls intrinsic/extrinsic tongue muscles (speech, swallowing, chewing)
Describe the pathway of CN XII.
Origin: Medulla
How does hypoglossal nerve damage present?
Tongue deviates toward lesion (atrophy over time).
Bilateral lesion: Cannot protrude tongue. Speech/swallowing impaired.
What are the motor, sensory, and parasympathetic functions of CN IX?
Motor: Innervates stylopharyngeus (elevates pharynx during swallowing).
Sensory: Taste from posterior 1/3 of tongue. General sensation from pharynx, tonsils, posterior tongue.
Chemoreceptors (carotid body: O₂/CO₂ levels).
Baroreceptors (carotid sinus: blood pressure).
Parasympathetic: Stimulates parotid salivary gland.
Describe the pathway of CN IX.
Medulla
Damage to the Glossopharengeal Nerve
Injured or inflamed glossopharyngeal nerves impair swallowing and taste.
Three ways to classify receptors
type of stimulus, body location, and structural
complexity
What are sensory receptors, and what is their role?
Specialized cells that detect environmental changes (stimuli).
Convert stimuli into graded potentials → nerve impulses.
Sensation (awareness) and perception (interpretation) occur in the brain.
Name the 5 types of receptors based on stimulus.
Mechanoreceptors: Touch, pressure, vibration, stretch.
Thermoreceptors: Temperature changes.
Photoreceptors: Light (e.g., retina).
Chemoreceptors: Chemicals (smell, taste, blood pH).
Nociceptors: Pain (extreme heat/cold, pressure, chemicals)
Contrast exteroceptors, interoceptors, and proprioceptors.
Exteroceptors: Detect external stimuli (e.g., skin receptors for touch/temperature).
Interoceptors: Monitor internal conditions (e.g., visceral pain, blood pH, tissue stretch, temp change).
Proprioceptors: Respond to stretch. Sense body position/movement (muscles, tendons, joints).
Compare simple vs. special sense receptors.
Simple receptors: General senses (touch, pain, temperature).
Types:
Nonencapsulated (free nerve endings).
Encapsulated (e.g., Meissner’s corpuscles).
Special sense receptors: Complex organs (e.g., retina, cochlea).
Where are nonencapsulated nerve endings found, and what do they detect?
abundant in Epithelia, connective tissues.
Stimuli: Temperature (thermoreceptors). Pain (nociceptors). Light touch (e.g., Merkel discs).
Structure: Knob-like swellings on unmyelinated C fibers.
What are the 3 levels of sensory integration?
Receptor level: Sensory detection (receptors → graded potentials).
Circuit level: Signal relay to thalamus (ascending pathways).
Perceptual level: Cortical processing (interpretation in brain).
