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Nervous System Overview

Jul 7, 2025

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Overview

This lecture introduces the anatomy, organization, and main functions of the human nervous system, focusing on its structure, types of cells, and pathways of neural communication.

Functions of the Nervous System

  • The nervous system controls all organs, responses, and most body functions.
  • Main functions: sensory input (detecting stimuli), integration (processing information), and motor output (initiating responses).

Organization of the Nervous System

  • Central nervous system (CNS): consists of the brain and spinal cord, serves as the main control center.
  • Peripheral nervous system (PNS): made up of all nerves branching from the CNS, enables communication with the rest of the body.
  • PNS splits into sensory (afferent) and motor (efferent) divisions.
  • Sensory (afferent): carries signals from receptors to CNS.
  • Motor (efferent): sends commands from CNS to muscles/glands.
  • Motor division includes somatic (voluntary) and autonomic (involuntary) systems.
  • Autonomic system has sympathetic (activating) and parasympathetic (calming) divisions.

Nervous Tissue and Cell Types

  • Nervous tissue is densely packed; less than 20% is extracellular space.
  • Two main cell types: neurons (signal transmission) and glial cells (support/protection).
  • Glial cells outnumber neurons about 10:1 and have several types:
    • Astrocytes: anchor neurons, manage exchange with capillaries.
    • Microglial cells: immune defense in CNS.
    • Ependymal cells: circulate cerebrospinal fluid.
    • Oligodendrocytes: create myelin sheath in CNS.
    • Satellite cells: support neurons in PNS.
    • Schwann cells: form myelin sheath in PNS.

Neuron Structure and Classification

  • Neurons are long-lived, irreplaceable, and have high metabolic needs.
  • Basic neuron structure: soma (cell body), dendrites (receive messages), axon (transmits messages).
  • Classified by shape:
    • Multipolar: multiple processes; most common.
    • Bipolar: one axon, one dendrite; rare, sensory organs.
    • Unipolar: single process; mainly sensory neurons.
  • Classified by function:
    • Sensory (afferent): carry signals to CNS; mostly unipolar.
    • Motor (efferent): carry signals from CNS; mostly multipolar.
    • Interneurons: connect sensory and motor neurons within CNS; mostly multipolar.

Neural Pathways Example (Spider Scenario)

  • Sensory receptors detect spider and activate unipolar neurons.
  • Signal travels via Schwann cell-insulated axons to CNS.
  • Interneurons process and relay signal to motor neurons or brain.
  • Motor output leads to a physical response (e.g., kicking or brushing spider away).

Key Terms & Definitions

  • Neuron — a nerve cell that transmits electrical and chemical signals.
  • Glial cells (Neuroglia) — supporting cells in the nervous system.
  • CNS (Central Nervous System) — brain and spinal cord; main control center.
  • PNS (Peripheral Nervous System) — nerves outside CNS; links body to CNS.
  • Afferent (Sensory) Division — carries sensory info to CNS.
  • Efferent (Motor) Division — sends commands from CNS to effectors.
  • Somatic Nervous System — controls voluntary muscle movements.
  • Autonomic Nervous System — controls involuntary functions.
  • Myelin Sheath — insulating layer around axons for faster signal transmission.

Action Items / Next Steps

  • Review neuron structure and glial cell functions.
  • Prepare for next lesson on how nerve cells communicate using chemistry and electricity.

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