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Anatomy of the Nervous System

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1. Overview and Divisions

• Somatic nervous system - voluntary control of skeletal muscle, conscious sensation
• Autonomic nervous system (ANS) - involuntary control of viscera, glands, smooth muscle
  • Sympathetic division (thoracolumbar outflow)
  • Parasympathetic division (craniosacral outflow)

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2. The Brain

• Anterior fossa - frontal lobes
• Middle fossa - temporal lobes, pituitary
• Posterior fossa - cerebellum, brainstem

Major Brain Regions

Meninges and Coverings

1. Dura mater - outermost, thick fibrous layer; forms major infoldings (falx cerebri, tentorium cerebelli)
2. Arachnoid mater - middle, web-like; separated from the pia by the subarachnoid space containing CSF
3. Pia mater - innermost, closely adherent to brain surface

Ventricular System

• Two lateral ventricles (one per hemisphere) - C-shaped, follow the curve of the caudate nucleus, corpus callosum, and fornix
• Third ventricle - midline, between the thalami
• Fourth ventricle - posterior, between brainstem and cerebellum
• CSF flows from lateral ventricles -> third -> cerebral aqueduct -> fourth -> subarachnoid space

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3. The Spinal Cord

Internal Structure of the Spinal Cord

• Dorsal (posterior) horn - sensory processing; receives afferent input via dorsal root filaments
• Intermediate zone - interneurons, autonomic nuclei
• Ventral (anterior) horn - lower motor neurons; axons exit via ventral root filaments

• Dorsal (posterior) columns - fine touch, vibration, proprioception (ascending)
• Lateral columns - corticospinal tract (descending motor), spinothalamic tracts (ascending pain/temperature)
• Ventral (anterior) columns - mixed ascending/descending tracts

Rexed's Laminae

Spinal Cord Enlargements

• Cervical enlargement (C4-T1) - gives rise to the brachial plexus (arm innervation)
• Lumbosacral enlargement (L2-S3) - gives rise to the lumbosacral plexus (leg innervation)

Spinal Nerve Roots

• 31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal
• Each spinal nerve forms from a dorsal root (sensory, has dorsal root ganglion) and a ventral root (mostly motor), which join to form a mixed spinal nerve
• Cervical nerves C1-C7 exit above their corresponding vertebral pedicle; C8 exits between C7 and T1; all nerves below C8 exit below their named vertebra
• The dorsal root ganglion contains cell bodies of sensory neurons and is sensitive to pressure and heat

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4. Dermatomes and Cutaneous Nerve Distribution

• C5-C8, T1: upper limb
• T4: nipple line
• T10: umbilicus
• L1: inguinal region
• L4-S1: lower limb
• S2-S4: perineum/saddle area

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5. The Autonomic Nervous System (ANS)

Sympathetic Division ("Thoracolumbar")

• Presynaptic neurons originate in the lateral horn (intermediolateral cell column) of T1-L2 spinal cord
• Presynaptic axons travel to:
  • Paravertebral ganglia (sympathetic trunk, bilateral chains flanking the vertebral column)
  • Prevertebral ganglia (celiac, aorticorenal, superior/inferior mesenteric ganglia)
• Postsynaptic fibers distribute to viscera, blood vessels, sweat glands, and skin
• The adrenal medulla is a modified sympathetic ganglion innervated by presynaptic fibers directly - releases epinephrine (EPI) and norepinephrine (NE) into the bloodstream

Parasympathetic Division ("Craniosacral")

• Presynaptic neurons originate in:
  • Brainstem (midbrain, pons, medulla) - traveling with cranial nerves III, VII, IX, and X
  • Sacral spinal cord (S2-S4)
• Presynaptic fibers synapse in terminal ganglia located in or near the wall of target organs
• CN III (ciliary ganglion) - pupillary constriction, lens accommodation
• CN VII (pterygopalatine + submandibular ganglia) - lacrimal, sublingual, submandibular glands
• CN IX (otic ganglion) - parotid gland
• CN X (vagus) - heart, lungs, esophagus, stomach, liver, pancreas, intestines (up to the splenic flexure)
• S2-S4 - descending colon, rectum, bladder, reproductive organs

Sympathetic vs. Parasympathetic: Opposing Actions

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6. Cranial Nerves (Brief Overview)

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Summary: Organization at a Glance

Nervous System
├── Central Nervous System (CNS)
│   ├── Brain
│   │   ├── Cerebrum (Frontal, Parietal, Temporal, Occipital lobes)
│   │   ├── Diencephalon (Thalamus, Hypothalamus)
│   │   ├── Brainstem (Midbrain, Pons, Medulla)
│   │   └── Cerebellum
│   └── Spinal Cord
│       ├── Cervical (C1-C8)
│       ├── Thoracic (T1-T12)
│       ├── Lumbar (L1-L5)
│       └── Sacral/Coccygeal (S1-S5, Co1)
│
└── Peripheral Nervous System (PNS)
    ├── Somatic NS
    │   ├── 12 Cranial Nerve Pairs
    │   └── 31 Spinal Nerve Pairs
    └── Autonomic NS
        ├── Sympathetic (T1-L2) - "Fight or Flight"
        └── Parasympathetic (CN III,VII,IX,X + S2-S4) - "Rest & Digest"

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