brain spinalCord peripheralNervousSystem neuroanatomy

Spinal Cord and Peripheral Nervous System

Spinal Cord Anatomy

• CNS = brain + spinal cord.
  • Brain includes brainstem, forebrain structures
• PNS = spinal nerves + cranial nerves.
• Spinal cord ends around L1 vertebra.
• Nerves extend downward as the cauda equina.

Spinal Nerve Organization

• Cervical (C1–C8)
• Thoracic (T1–T12)
• Lumbar (L1–L5)
• Sacral (S1–S5)
• Coccygeal (Co1–Co3)

Spinal Cord and Reflexes

• Spinal Nerves branch from the spinal cord. Each has two roots:
  • Dorsal root → carries afferent sensory fibers into spinal cord. Cell bodies sit in the dorsal root ganglion.
  • Ventral root → carries efferent motor fibers out to muscles. Motor neuron cell bodies are in the spinal cord’s gray matter.
• Together, dorsal + ventral roots form a mixed spinal nerve.

Simple Reflex Arc

• Example: Triceps reflex (deep tendon reflex).
  • A stretch receptor in the muscle sends signal via sensory neuron → spinal cord.
  • Synapses directly onto a motor neuron → causes contraction.
• Reflexes are fast, involuntary, and involve minimal circuitry.

Cranial Nerves

• 12 pairs, some sensory, some motor, some mixed:
  • I. Olfactory – smell
  • II. Optic – vision
  • III. Oculomotor – eye movement, pupil control
  • IV. Trochlear – eye movement
  • V. Trigeminal – facial sensation, jaw muscles
  • VI. Abducens – eye movement
  • VII. Facial – taste, facial expression
  • VIII. Auditory (vestibulocochlear) – hearing, balance
  • IX. Glossopharyngeal – taste, throat & larynx muscles
  • X. Vagus – internal organs
  • XI. Accessory – neck muscles
  • XII. Hypoglossal – tongue movements

Peripheral Nervous System (PNS) Divisions

1. Somatic Nervous System – voluntary control of skeletal muscles.
2. Autonomic Nervous System – involuntary control of organs, glands, smooth muscle.
   • Subdivisions:
     • Sympathetic (“fight or flight”): pupil dilation, bronchodilation, ↑ HR, ↓ digestion, orgasm, stress hormone release.
     • Parasympathetic (“rest and digest”): pupil constriction, salivation, ↓ HR, ↑ digestion, bladder relaxation, sexual arousal.
   • Neurotransmitters:
     • Sympathetic: preganglionic ACh, postganglionic norepinephrine.
     • Parasympathetic: preganglionic ACh, postganglionic ACh.
3. Enteric Nervous System – controls digestion independently, but modulated by sympathetic/parasympathetic inputs.

Meninges

• Protective layers surrounding brain and spinal cord:
  1. Dura mater (outer, tough)
  2. Arachnoid membrane (middle, web-like) → filled with CSF in subarachnoid space
  3. Pia mater (inner, thin, hugs brain surface)
• Infections:
  • Meningitis = meninges infection → swelling, pressure on brain.
  • Encephalitis = infection of brain tissue itself.

Blood Supply

• Supplied via Circle of Willis: anterior, middle, and posterior cerebral arteries, plus basilar and internal carotid arteries.
• Veins drain into dural venous sinuses → internal jugular vein.
• Critical for preventing ischemia (strokes).

Cerebrospinal Fluid (CSF) and Ventricular System

• Produced by the choroid plexus inside ventricles.
• Pathway: lateral ventricles → 3rd ventricle → cerebral aqueduct → 4th ventricle → central canal & subarachnoid space → absorbed into dural sinuses via arachnoid villi.
• Functions: cushions brain, regulates chemical environment, removes waste, spreads neuromodulators, supports neurogenesis.
• Disorders:
  • Hydrocephalus = CSF buildup → enlarged ventricles, compressed brain tissue.