Organization of the Nervous System & Neuron Structure

Macro-Organization of the Nervous System

• The entire nervous system can be parsed into two overarching categories:
  • Central Nervous System (CNS): Brain + spinal cord (briefly alluded to but not the focus of this clip).
  • Peripheral Nervous System (PNS): All nerves that branch away from the CNS and reach the body’s periphery.
• Within the PNS we further divide information flow into two one-way “highway lanes,” inspired by the speaker’s northbound/southbound highway analogy:
  1. Sensory (Afferent) Division – “Northbound”
  • Carries signals from body → brain.
  • Examples cited: touch (skin receptors to brain), smell (olfactory epithelium to brain), and any other sense (vision, taste, hearing, proprioception, etc.).
  • Conceptual simplification: considers only the direction of information, ignoring feedback loops for introductory clarity.
  1. Motor (Efferent) Division – “Southbound”
  • Sends commands from brain → body.
  • Governs both overt muscular movement and subtler regulation of organs, glands, blood vessels, etc.
  • Sub-branches: • Somatic Nervous System (SNS):
    • Targets skeletal muscle.
    • Entirely voluntary—e.g., raising a hand.
    • Largely mastered in Anatomy & Physiology I via the neuromuscular junction.
      • Autonomic Nervous System (ANS):
    • Regulates viscera (heart, smooth muscle, glands).
    • Involuntary/automatic—e.g., aldosterone secretion, basal heart rate (you cannot “decide” to set HR to 40\,\text{bpm}).

Course Road-Map Context

• First ~60 % of the syllabus covers foundational neuroanatomy and neurophysiology.
• Sequence mentioned: lecture block → practice exam → real exam (end of next week).
• Two weeks from the recorded date the class starts an entire unit (Unit 2) devoted to the sensory division.
• Somatic motor material is mostly review from earlier coursework and will not be revisited in detail.

Nervous Tissue: Cell Types and Morphology

• Neurons are the primary information-processing & communicating cells; visually distinctive (“big splashy splatter starburst” with a long tail).
• Glia ("glue", family name): multiple non-neuronal cell types that support, nourish, insulate, and protect neurons.
  • Often less obvious on standard histology slides.

Canonical Neuron Anatomy

• Cell Body (Soma):
  • Contains nucleus, organelles; metabolic & synthetic hub.
• Dendrites:
  • Tree-like extensions (Greek "dendron" = tree).
  • Input surface—receive chemical/electrical signals from other neurons or sensory receptors.
• Axon:
  • Single, elongated projection—can span microns to over a meter.
  • Output cable—propagates action potentials away from soma toward synaptic terminals.
  • Information travels as a wave-like cascade of voltage changes (class demonstrated “stadium wave” analogy).
• Axon Hillock / Initial Segment: funnel-like region where summed dendritic inputs trigger (or fail to trigger) an action potential.
• Myelin Sheath:
  • Lipid-rich insulating layers wrapped around many axons (“bubble-wrap” metaphor).
  • Produced by glial cells (Schwann cells in PNS, oligodendrocytes in CNS).
  • Increases conduction velocity and prevents short-circuiting.
  • Clinical link: Demyelinating disorders (e.g., Multiple Sclerosis) strip away myelin → conduction failure → tremors, progressive neurological deficits.

Functional & Clinical Connections

• Sensory and motor pathways must remain insulated and directional, analogous to separated highway lanes; damage or misrouting leads to sensory loss, paralysis, or dysautonomia.
• Voluntary vs. involuntary distinction has ethical and therapeutic implications—targeted drugs or neuroprosthetics must respect whether pathways are consciously controlled.
• Understanding glial roles is crucial; historically ignored (“just support cells”), but modern research implicates glia in neurotransmission, immune defense, and disease etiology.

Recurring Analogies & Mnemonics

• Highway (northbound/southbound): afferent vs. efferent.
• Bubble-wrap myelin: visualization of axon insulation.
• Stadium wave: illustrates sequential depolarization along an axon.

Key Terminology Recap

• Peripheral Nervous System (PNS)
• Sensory/Afferent vs. Motor/Efferent
• Somatic vs. Autonomic
• Neuron, Soma, Dendrite, Axon, Axon Hillock
• Myelin, Glia

Numbers & Equations Mentioned

• Desired heart rate example: 40\,\text{bpm} (used to illustrate involuntary control via ANS).