Comprehensive PNS and Cranial Nerve System Overview for Students

Peripheral Nervous System (PNS)

All neural structures outside the brain and spinal cord.

Parts of the PNS

1. Sensory receptors 2. Transmission lines (nerves and their structure/repair) 3. Motor endings and motor activity 4. Reflex activity

Function of sensory receptors

Respond to environmental changes (stimuli) and trigger nerve impulses.

Classification of sensory receptors

By stimulus type, body location, and structural complexity.

Mechanoreceptors

Respond to touch, pressure, vibration, stretch.

Thermoreceptors

Respond to temperature changes.

Photoreceptors

Respond to light energy.

Nociceptors

Respond to pain-causing stimuli.

Exteroceptors

Respond to stimuli outside the body.

Interoceptors

Respond to internal stimuli from viscera and blood vessels.

Proprioceptors

Inform the brain about body movements by sensing stretch in muscles, tendons, and joints.

Two receptor categories

Simple receptors (general senses) and special sense receptors.

Nonencapsulated nerve endings

Free nerve endings in epithelia/connective tissue, respond to temperature, pain, light touch.

Tactile (Merkel) discs

Light touch receptors in epidermis.

Encapsulated dendritic endings

Mechanoreceptors with terminal endings in connective tissue capsules.

Tactile (Meissner's) corpuscles

Discriminative touch receptors.

Somatosensory system

Receives inputs from exteroceptors, proprioceptors, and interoceptors.

Three levels of neural integration

Receptor level, circuit level, perceptual level.

Pain perception

Warns of actual or impending tissue damage.

Pain threshold vs tolerance

Threshold same for all; tolerance varies.

Referred pain

Pain perceived in an area different from its origin.

Structure of a nerve

Bundles of myelinated and unmyelinated peripheral axons.

Connective tissue coverings

Endoneurium, perineurium, epineurium.

Nerve types

Spinal and cranial.

Ganglia

Contain neuron cell bodies.

PNS axon injury

Axons may regenerate if soma is intact.

CNS fibers regeneration

Oligodendrocytes bear growth-inhibiting proteins.

Wallerian degeneration

Degeneration of axon fragments distal to injury.

Macrophages

Clean debris to promote regeneration.

Number of cranial nerves

12 pairs.

General roles of cranial nerves

Sensory, motor, or mixed; mostly arise from brain stem.

Cranial nerve numbering

I-XII, rostral to caudal.

I

Olfactory - Smell

II

Optic - Vision

III

Oculomotor - Eyelid movement, eye movement, pupil constriction

IV

Trochlear - Eye movement (superior oblique)

V

Trigeminal - Facial sensation, chewing

VI

Abducens - Lateral eye movement

VII

Facial - Facial expression, taste (anterior 2/3), salivary & lacrimal glands

VIII

Vestibulocochlear - Hearing and balance

IX

Glossopharyngeal - Taste (posterior 1/3), swallowing

X

Vagus - Parasympathetic control of thoracic/abdominal viscera

XI

Accessory - Trapezius and sternocleidomastoid muscles

XII

Hypoglossal - Tongue movement

Spinal nerve pairs

31 (mixed nerves)

Body parts supplied

All except head and part of neck

Cervical nerves

8 pairs (C1-C8)

Roots vs rami

Roots = medial, form spinal nerves; Rami = lateral branches, carry both sensory & motor fibers

Cervical plexus

C1-C4: phrenic nerve (diaphragm)

Brachial plexus

Upper limb (axillary, median, ulnar, radial, musculocutaneous)

Lumbar plexus

Thigh and abdominal wall (obturator nerve)

Sacral plexus

Lower limb (sciatic nerve — longest & thickest)

Hilton's Law

A nerve serving a muscle also innervates its joint and overlying skin

Neuromuscular junction

Site where motor neurons innervate skeletal muscle

Neurotransmitter

Acetylcholine (ACh)

Effect of ACh

Depolarization → action potential → contraction

Inborn reflex

Rapid, involuntary, predictable response

Learned reflex

Acquired through practice (e.g., driving)

Reflex arc components

Receptor → sensory neuron → integration center → motor neuron → effector

Somatic vs autonomic reflexes

Somatic = skeletal muscle; Autonomic = visceral effectors

Stretch reflex

Maintains muscle tone

Reciprocal inhibition

Antagonistic muscles are inhibited

Stretch reflexes

Monosynaptic and ipsilateral

Function of ANS

Subconscious control of smooth muscle, cardiac muscle, and glands

Divisions of ANS

Parasympathetic (rest-and-digest) & sympathetic (fight-or-flight)

Anatomical origins of ANS

Parasympathetic = craniosacral; Sympathetic = thoracolumbar

Fiber lengths in ANS

Parasympathetic = long pre, short post; Sympathetic = short pre, long post

Ganglia locations in ANS

Parasympathetic near target organs; sympathetic near spinal cord

Neurotransmitters in ANS

Preganglionic = ACh; postganglionic = ACh or norepinephrine

Receptor effects in ANS

Nicotinic: always stimulatory; Muscarinic: inhibitory or excitatory depending on organ

Sympathetic tone

Partial constriction of blood vessels to regulate blood pressure

Unique sympathetic roles

Thermoregulation, renin release, metabolic effects

Parasympathetic dominance

Slows heart rate, promotes digestion

Main ANS control center

Hypothalamus

Biofeedback

Conscious control over some autonomic functions

Sympathetic effects

Vasoconstriction, bronchodilation, increased heart rate, stress response

Parasympathetic effects

GI stimulation, energy conservation

Vagus nerve

90% of preganglionic parasympathetic fibers

Adrenal medulla

Releases norepinephrine and epinephrine

Sympathetic pathways

Trunk ganglia, collateral ganglia, ascending/descending

Temperature regulation

Sympathetic dilation of skin vessels, sweat gland activation