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Complete Quiz Prep Guide: ANS, Plexuses, Special Senses

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1. AUTONOMIC NERVOUS SYSTEM (ANS)

Overview

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Sympathetic vs. Parasympathetic: Key Comparison

Sweat glands are an important exception: they are sympathetically innervated but use acetylcholine as their postganglionic transmitter.

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Origins of Autonomic Fibers

Sympathetic (Thoracolumbar)

• Preganglionic cell bodies in the lateral horn (intermediolateral cell column) of T1-L2/L3 spinal cord
• Axons exit via ventral roots, travel through white rami communicantes to reach the paravertebral sympathetic chain (sympathetic trunk) running from C1 to coccyx
• From the chain, fibers may:
  1. Synapse at that level in a paravertebral ganglion
  2. Travel up or down the chain before synapsing
  3. Pass through the chain without synapsing, continue as splanchnic nerves to prevertebral ganglia

Parasympathetic (Craniosacral)

• Cranial outflow (CN III, VII, IX, X): preganglionic fibers originate in brainstem nuclei
• Sacral outflow (S2-S4): preganglionic fibers form the pelvic splanchnic nerves (nervi erigentes)

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Major Autonomic Ganglia

Sympathetic Ganglia

Parasympathetic Ganglia (Head)

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Autonomic Pathways by Region

Head and Neck

• Sympathetic: From superior cervical ganglion via plexuses on the carotid arteries. Supplies dilator pupillae, superior tarsal muscle (Müller's muscle), sweat glands and vasomotors of the face.
• Parasympathetic: Via CN III (ciliary ganglion), CN VII (pterygopalatine and submandibular ganglia), CN IX (otic ganglion).

Thorax

• Cardiac: Sympathetic from superior/middle/inferior cervical and T1-T4 ganglia (increases HR, increases conduction velocity, increases contractility). Parasympathetic from CN X (vagus, decreases HR).
• Pulmonary: Sympathetic (bronchodilation via β2), Parasympathetic CN X (bronchoconstriction).
• Sympathetic preganglionic for head/neck arise from T1-T5.

Abdomen

• Greater splanchnic nerve: T5-T9 → celiac ganglion
• Lesser splanchnic nerve: T10-T11 → superior mesenteric and aorticorenal ganglia
• Least splanchnic nerve: T12 → renal plexus
• Parasympathetic: CN X (vagus) supplies everything up to the splenic flexure of the colon

Pelvis

• Sympathetic: From inferior mesenteric ganglion (L1-L2) via the hypogastric plexus - mediates ejaculation in males, bladder relaxation, urethral sphincter contraction
• Parasympathetic: Pelvic splanchnic nerves (S2-S4) - mediate erection, bladder contraction (detrusor), relaxation of urethral sphincter, supply descending colon, sigmoid, rectum

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Horner Syndrome

1. Ptosis - drooping of the upper eyelid (paralysis of the superior tarsal/Müller's muscle)
2. Miosis - constricted pupil (paralysis of the dilator pupillae)
3. Anhidrosis - absence of sweating on the ipsilateral face

• 1st-order neuron: Hypothalamus → ipsilateral ciliospinal center of Budge (C8-T2) - lesion here causes central Horner (e.g., stroke, syrinx)
• 2nd-order neuron: Exits spinal cord → travels over the apex of the lung → around the subclavian artery → ascends to the superior cervical ganglion - lesion here causes preganglionic Horner (e.g., Pancoast tumor, carotid dissection)
• 3rd-order neuron: Superior cervical ganglion → travels with the internal carotid artery to the eye - lesion here causes postganglionic Horner (e.g., carotid dissection, cavernous sinus pathology)

Anhidrosis is present with 1st- and 2nd-order lesions but absent in 3rd-order lesions because sudomotor fibers travel with the external carotid, not the internal carotid. - Guyton & Hall Medical Physiology; Adams and Victor's Principles of Neurology, 12th Ed.

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2. CERVICAL PLEXUS AND NECK ANATOMY

Cervical Plexus (C1-C4)

• Lesser occipital nerve (C2) - posterior scalp
• Great auricular nerve (C2, C3) - skin over parotid, ear, mastoid
• Transverse cervical nerve (C2, C3) - anterior neck
• Supraclavicular nerves (C3, C4) - skin over clavicle and upper chest/shoulder

• Ansa cervicalis (C1-C3) - innervates infrahyoid (strap) muscles: omohyoid, sternohyoid, sternothyroid, thyrohyoid
• Phrenic nerve (C3, C4, C5 - "C3, 4, 5 keeps the diaphragm alive") - sole motor supply to the diaphragm

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3. BRACHIAL PLEXUS

Structure

Cord Contents and Major Branches

• Lateral cord (C5-C7): Musculocutaneous nerve, lateral root of median nerve, lateral pectoral nerve
• Medial cord (C8-T1): Ulnar nerve, medial root of median nerve, medial cutaneous nerves of arm and forearm, medial pectoral nerve
• Posterior cord (C5-T1): Radial nerve, axillary nerve, thoracodorsal nerve, subscapular nerves (STAR mnemonic: Subscapular, Thoracodorsal, Axillary, Radial)

Root Branches (directly from roots)

• Dorsal scapular nerve (C5): rhomboids + levator scapulae
• Long thoracic nerve (C5, C6, C7): serratus anterior

Trunk Branches (directly from superior trunk)

• Suprascapular nerve (C5, C6): supraspinatus + infraspinatus
• Nerve to subclavius (C5, C6)

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Clinical Correlations

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4. LUMBAR AND SACRAL PLEXUSES

Lumbar Plexus (T12-L4)

Sacral Plexus (L4-S4)

The lumbosacral trunk (L4-L5) links the two plexuses. The sciatic nerve (the largest nerve in the body) receives contributions from both plexuses. - Neuroanatomy through Clinical Cases, 3rd Ed.

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5. EYE AND VISUAL SYSTEM

Chambers of the Eye

Layers of the Retina (from inside to outside)

1. Internal limiting membrane (border between retina and vitreous)
2. Nerve fiber layer (axons of ganglion cells, converge at optic disc)
3. Ganglion cell layer (3rd-order neurons)
4. Inner plexiform layer (synapses between bipolar and ganglion cells)
5. Inner nuclear layer (bipolar, horizontal, amacrine cell bodies)
6. Outer plexiform layer (synapses between photoreceptors and bipolar cells)
7. Outer nuclear layer (rod and cone cell bodies)
8. External limiting membrane
9. Photoreceptor layer (inner and outer segments of rods and cones)
10. Retinal pigment epithelium (RPE) - supports photoreceptors, phagocytoses shed outer segments
11. Choroid - vascular supply to outer retinal layers

Key concept: Light must pass through all inner layers before hitting photoreceptors. Signal flow is: photoreceptor → bipolar cell → ganglion cell → optic nerve. - Gray's Anatomy for Students

Macula, Fovea, and Optic Disc

Ciliary Ganglion and Ciliary Nerves

• Parasympathetic root: From the Edinger-Westphal nucleus via CN III (preganglionic). Synapses in the ciliary ganglion → postganglionic short ciliary nerves → sphincter pupillae (miosis) and ciliary muscle (accommodation - lens rounds up for near vision)
• Sympathetic root: From superior cervical ganglion via nasociliary nerve - passes through the ganglion without synapsing → dilator pupillae (mydriasis), superior tarsal muscle
• Sensory root: Nasociliary branch of V1 (ophthalmic CN V) - sensory from cornea, iris

Visual Field Defects and Blind Spot Anatomy

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6. OLFACTORY PATHWAY

• Receptor neurons: Bipolar neurons in the olfactory epithelium (superior nasal cavity). Their unmyelinated axons pass through the cribriform plate of the ethmoid bone as CN I (olfactory nerve), the only CN whose fibers don't pass through the brainstem.
• They synapse in the olfactory bulb (glomeruli) on mitral and tufted cells.
• From the olfactory bulb, axons form the olfactory tract that splits into:
  • Lateral olfactory stria → primary olfactory cortex (piriform cortex, amygdala, entorhinal cortex)
  • Medial olfactory stria → septal nuclei, contralateral olfactory bulb
• Olfaction is the only sensory system that does not relay through the thalamus first.

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7. TASTE PATHWAYS AND CRANIAL NERVE INNERVATION

The chorda tympani carries both taste from anterior 2/3 tongue AND parasympathetic fibers to the submandibular ganglion (for submandibular and sublingual gland secretion). It is a branch of CN VII that joins with the lingual nerve (V3). - Gray's Anatomy for Students

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8. EAR AND VESTIBULAR SYSTEM

Overview of Inner Ear Structures

• Cochlea - hearing
• Vestibular apparatus - balance

Vestibular Apparatus Components

Static vs. Kinetic Equilibrium

• Static equilibrium (utricle + saccule): Detect head tilt relative to gravity. Hair cells sit in the macula (sensory epithelium), covered by the otolithic membrane with calcium carbonate crystals (otoliths/otoconia). Gravity displaces the otoconia, bending hair cells.
• Kinetic (dynamic) equilibrium (semicircular canals): Detect head rotation. Each canal is oriented in a different plane (3 canals = 3 planes of space). Hair cells sit in the crista ampullaris within the ampulla. Endolymph lags behind rotation, bending the cupula (gelatinous mass without otoliths), deflecting the hair cell stereocilia.

Hair cell deflection toward the kinocilium = depolarization (increased firing); away = hyperpolarization. Both utricle/saccule and semicircular canals send signals via CN VIII (vestibulocochlear nerve), superior and inferior vestibular branches, to vestibular nuclei in the brainstem. - Langman's Medical Embryology; Guyton & Hall Medical Physiology

• Vestibulo-ocular reflex (VOR) - stabilizes gaze during head movement (via MLF to extraocular muscles)
• Vestibulospinal reflex - adjusts postural tone
• Nausea/vomiting via connections to CN X dorsal nucleus
• Nystagmus occurs when the VOR is disrupted

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Quick Reference Summary Table

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