Comprehensive Notes on Neurons and the Nervous System

Neuron Structure and Function

  • Neurons are interconnected cells with various parts, not physically touching each other.

Functions of the Nervous System

  • Sensory Input: Sensory receptors detect stimuli.
    • Five senses: taste, hearing, sight, smell, touch.
  • Integration: The nervous system processes sensory information using chemicals and electrical signals.
  • Motor Output: Activates muscles or glands.

General Organization of the Nervous System

  • Central Nervous System (CNS)
    • Brain and spinal cord.
  • Peripheral Nervous System (PNS)
    • All nerves outside the CNS, including those in the neck and shoulders.

Organization of the Peripheral Nervous System

  • Sensory Division
    • External environment sensing
    • Internal environment sensing
  • Motor Division
    • Autonomic Nervous System (ANS): Involuntary
      • Sympathetic Division: Fight, flight, or freeze (defense mechanisms)
      • Parasympathetic Division: Rest and digest
        • Associated with acetylcholine.
    • Somatic Nervous System: Voluntary

Impulses and the Nervous System

  • Sensory Division: Afferent - Takes impulses to the central nervous system, away from the sense origin.
  • Motor Division: Efferent - Takes impulses away from the central nervous system to muscles.
    • Mnemonic: "E" for exit from the central nervous system.

Nervous Tissue Cell Types

  • Neurons: Conduct impulses using electrical signals.
    • Vary in shape and size.
  • Schwann Cells: Supporting cells that do not conduct impulses.
    • Act as insulators, speeding up impulse travel.

Neurons: Basic Functional Units

  • Specialized cells with basic properties:
    • Informational processing
    • Transmission
    • Integration
    • Storage
    • Regulation of behavior
  • Different nerve cell types exist, including those in the optic nerve (rods).

Neuron Anatomy

  • Cell Body (Soma):
    • Contains the nucleus and most organelles.
    • Damage leads to cell death, appearing as gray matter.
  • Dendrites: Branching processes from the cell body.
    • Carry nerve impulses to the cell body (one-way system).
    • Usually numerous.
  • Axon: Branching processes from the cell body.
    • Carries impulses away from the cell body.
    • Can be very long (meter+).
    • Most neurons have many short processes that contact thousands of other neurons.
  • Myelin Sheath: Protective sheath around some axons.
    • Made of cell membrane wrappings from support cells (Schwann cells).
  • Node of Ranvier: Gaps of exposed axon between Schwann cells.
  • Axon Bulbs: Ends of axons that transmit messages.
    • Do not physically touch other cells or dendrites.
    • Transmit impulses via chemicals (neurotransmitters).
      • Examples: serotonin, dopamine, norepinephrine, adrenaline, acetylcholine.

Information Transmission in Neurons

  • One-way system: Dendrites to cell body through the axon to the receiver.
  • Stimuli received by dendrites, integrated in the cell body, sent through the axon.
  • Axon carries information away from the cell body as an electrical signal.
  • The axon terminal releases a chemical message at the axon bulbs to continue or stop the impulse.

Types of Neurons

  • Motor neuron
  • Interneuron
  • Sensory neuron

Schwann Cells Details

  • Support cells, not neurons.
  • Do not conduct impulses; act as insulators.
  • Cell membrane wraps around the axon to protect and insulate it.
  • The myelin sheath is white and fatty in nature (white matter).

Myelin and Impulse Speed

  • Myelin acts like electrical insulation.
  • Gaps between Schwann cells are called the Node of Ranvier.
  • Nodes of Ranvier speed up impulse rate.
  • Impulses with nodes are up to 200 times faster than unmyelinated axons.

Neuron Interconnections

  • Neurons are interconnected but not physically touching.
  • Dendrites, cell body, and axon can be activated by different neurons.
  • During an electrical impulse, physical changes occur at the nodes of Ranvier, where sodium, potassium, and sodium-potassium gates are active and exposed.
  • The signal jumps through the myelinated area to the next area where an electrical stimulus can be active.