Nervous System Overview

Introduction to the Nervous System

• The nervous system is crucial for all bodily functions.
• Controls organs, physiological and psychological reactions.
• Connects with the endocrine system.
• Fundamental to existence: "There’s no you without it."

Structure and Organization

• Two main parts:
  • Central Nervous System (CNS): Brain and spinal cord.
  • Peripheral Nervous System (PNS): All nerves branching from the brain and spinal cord.

Functions of the Nervous System

• Sensory Input: Detection of stimuli (e.g., spider on skin).
• Integration: Processing of sensory input (deciding response).
• Motor Output: Response to stimuli (e.g., removing spider).

Divisions of the Peripheral Nervous System

• Sensory (Afferent) Division: Sends sensory information to the CNS.
• Motor (Efferent) Division: Sends commands from CNS to muscles and glands.
  • Somatic Nervous System: Controls voluntary movements.
  • Autonomic Nervous System: Controls involuntary actions (e.g., heart rate).
    • Sympathetic Division: Mobilizes body for action (fight or flight).
    • Parasympathetic Division: Relaxes body (rest and digest).

Nervous Tissue Composition

• Composed mainly of nervous tissue with very little extracellular space.
• Neurons: Main signaling cells; respond to stimuli and transmit signals.
• Glial Cells: Support and protect neurons; outnumber neurons by 10 to 1.

Types of Glial Cells

• CNS:
  • Astrocytes: Anchor neurons to blood supply and manage nutrient exchange.
  • Microglial Cells: Immune defense in the CNS.
  • Ependymal Cells: Produce and circulate cerebrospinal fluid.
  • Oligodendrocytes: Form myelin sheath around CNS neurons.
• PNS:
  • Satellite Cells: Support neuron cell bodies in the PNS.
  • Schwann Cells: Insulate axons with myelin sheath in the PNS.

Neuron Structure and Function

Common Features

• Long-lived cells, usually amitotic (cannot divide).
• High metabolic demand; require significant glucose and oxygen.

Basic Structure

• Soma (Cell Body): Contains nucleus and organelles.
• Dendrites: Receive signals from other neurons (input).
• Axon: Transmits impulses away from the cell body (output).

Neuron Types Based on Processes

• Multipolar Neurons: Most common; multiple processes (e.g., CNS).
• Bipolar Neurons: Rare; one axon and one dendrite (e.g., retina).
• Unipolar Neurons: Single process; primarily in sensory receptors.

Neuron Types Based on Function

• Sensory Neurons (Afferent): Transmit impulses to CNS (mostly unipolar).
• Motor Neurons (Efferent): Transmit impulses away from CNS (mostly multipolar).
• Interneurons: Connect sensory and motor neurons (mostly multipolar); most abundant.

Example of Nervous System Function

• Response to Spider on Knee:
  1. Sensory input from unipolar sensory neurons activated by the spider.
  2. Signal travels to CNS via interneurons.
  3. Response generated by motor neurons to remove the spider or remain calm.

Conclusion

• The nervous system is vital for processing sensory input and generating motor responses.
• Future lessons will explore how nerve cells communicate chemically and electrically.

Acknowledgments

• Thanks to viewers and supporters of Crash Course.
• Episode credits: Written by Kathleen Yale; edited by Blake de Pastino; consulted by Dr. Brandon Jackson.