Organization of the Nervous System

NS Divisions & Functions - The Grand Design

The nervous system is organized into two primary divisions for rapid information processing and control.

• Central Nervous System (CNS):

  • Components: Brain, Spinal Cord.
  • Function: Integration, command center.

• Peripheral Nervous System (PNS):

  • Components: Cranial nerves, Spinal nerves, Ganglia.
  • Function: Relays information to/from CNS.

• PNS Functional Divisions:

  • Somatic NS (SNS):
    • Voluntary control (skeletal muscles).
    • Sensory input (skin, special senses).
  • Autonomic NS (ANS):
    • Involuntary control (viscera, glands).
    • Subdivisions:
      • Sympathetic: "Fight or flight" (e.g., ↑ heart rate).
      • Parasympathetic: "Rest and digest" (e.g., ↓ heart rate).
      • Enteric (ENS): "Brain of the gut"; controls GI tract.

⭐ The ANS often provides dual innervation to organs, where sympathetic and parasympathetic inputs exert opposing effects to fine-tune physiological responses (e.g., heart rate).

CNS Structure & Function - Brain & Cord Blueprint

• Brain (Encephalon): Primary control center.

  • Cerebrum (Telencephalon): Largest part; higher functions.
    • Lobes: Frontal (executive, motor), Parietal (somatosensory, spatial), Temporal (auditory, memory), Occipital (visual).
    • Basal Ganglia: Modulates motor output. Limbic System (hippocampus, amygdala): Emotion, learning, memory.
  • Diencephalon:
    • Thalamus: Main sensory relay (except olfaction) to cortex.
    • Hypothalamus: Regulates homeostasis, ANS, endocrine system.
  • Brainstem: Connects cerebrum/cerebellum to spinal cord.
    • Midbrain (Mesencephalon): Visual/auditory reflexes, motor pathways (e.g., substantia nigra).
    • Pons: Relays information, involved in sleep, respiration, CN V-VIII.
    • Medulla Oblongata: Autonomic reflex center (cardiac, vasomotor, respiratory). CN IX-XII.

      ⭐ The Medulla Oblongata contains vital centers; lesions can be rapidly fatal.

  • Cerebellum: Coordinates voluntary movements, posture, balance, motor learning.

• Spinal Cord: Pathway for nerve signals, reflex mediation.

  • Extends from foramen magnum to conus medullaris (approx. L1-L2 in adults).
  • 31 pairs of spinal nerves (C8, T12, L5, S5, Co1).
  • Internal Structure:
    • Grey Matter (H-shape): Neuronal cell bodies.
      • Dorsal Horns: Sensory input.
      • Ventral Horns: Motor output.
      • Lateral Horns (T1-L2, S2-S4): Autonomic.
    • White Matter: Myelinated axons in tracts (ascending sensory, descending motor).

PNS & Cellular Level - Neuron & Nerve Net

• Peripheral Nervous System (PNS): Neural structures outside CNS; links CNS to entire body.
  • Somatic Nervous System (SNS):
    • Voluntary control; skeletal muscles.
    • 12 Cranial & 31 Spinal nerve pairs.
  • Autonomic Nervous System (ANS):
    • Involuntary; smooth/cardiac muscle, glands.
    • Sympathetic: "Fight or flight"; thoracolumbar outflow; NT: NE (most postganglionic).
    • Parasympathetic: "Rest and digest"; craniosacral outflow; NT: ACh.
    • Enteric: Intrinsic nervous system of gut.
• Neuron (Nerve Cell): Fundamental unit; processes & transmits info.
  • Structure: Soma (cell body with nucleus), Dendrites (receive signals), Axon (transmits action potentials).
  • Myelin Sheath: Fatty insulation; ↑ impulse speed (saltatory conduction).
    • PNS: Schwann cells (one cell/segment).
    • CNS: Oligodendrocytes (myelinates multiple axons).
  • Nodes of Ranvier: Myelin gaps; high ion channel density.
  • Synapse: Neuron-to-cell communication junction (e.g., NMJ).
• Nerve Net: Simplest diffuse nervous system (e.g., Hydra). Non-polarised, slow, multidirectional signals.

⭐ All preganglionic autonomic fibers (sympathetic & parasympathetic) release ACh. Postganglionic parasympathetic also release ACh. Most postganglionic sympathetic release NE (except sweat glands: ACh; adrenal medulla: Epi/NE to blood).

High‑Yield Points - ⚡ Biggest Takeaways

• CNS is brain and spinal cord; PNS is cranial/spinal nerves and ganglia.
• Somatic nervous system for voluntary control; Autonomic nervous system (ANS) for involuntary functions.
• ANS: Sympathetic (fight-or-flight), Parasympathetic (rest-and-digest), and Enteric (gut).
• Neurons are functional units; Glia (neuroglia) provide support, insulation, and nourishment.
• Afferent (sensory) pathways transmit signals to CNS; Efferent (motor) pathways transmit from CNS.
• Gray matter: neuron cell bodies, dendrites, unmyelinated axons; White matter: myelinated axons.