Neuron and Nervous System Notes

Facts

  • A typical neuron makes about ten thousand connections to neighboring neurons.
  • Given the billions of neurons, there are as many connections in a single cubic centimeter of brain tissue as there are stars in the Milky Way galaxy. - David Eagleman

Science

  • AI brain implant helps a woman speak for the first time in 18 years.

Neuron

  • A neuron is a specialized cell that forms the basis of the nervous system; it is also referred to as a nerve cell.
  • Neurons are able to transmit and gather electrochemical signals throughout the body (up to several feet or a few meters).
  • These signals are referred to as action potentials, nerve impulses, or spikes.

Structure of a Neuron

  • The main parts of a neuron include:
    • Cell Body
    • Nucleus
    • Dendrite
    • Axon
    • Myelin Sheath
    • Nodes of Ranvier
    • Axon Terminals

The Cell Body

  • The cell body contains:
    • The Nucleus (contains DNA)
    • Endoplasmic Reticulum
    • Mitochondria
    • Golgi Apparatus
    • Ribosomes
    • Secretory bodies that contain the neurotransmitters.

The Dendrites

  • Dendrites are like small branches, which allow the neuron to connect to more than one neuron.
  • They allow the neuron to talk to other cells or perceive the environment.
  • Dendrites receive stimuli from the environment or from other neurons.
  • The dendrite converts the information into a neural impulse that is transmitted to the cell body.
  • Etymology: "Dendr" = tree, branch; "Ite" = little (Greek).

The Axon

  • The axon is the longest fiber of the neuron.
  • It carries the neural impulse away from the cell body to the next connecting neuron, brain, or other body part (effectors).
  • Axons can be very long, such as those that run from your toes to your spine.

The Myelin Sheath

  • Myelin is made of a fatty substance that insulates the axon.
  • This insulation prevents the electrical current from leaving the axon.
  • The insulation acts to increase the rate of transmission of signals.

Nodes of Ranvier

  • Named after L.A. Ranvier (1835 – 1922), a French histologist and pathologist.
  • A gap (Node of Ranvier) exists between each myelin sheath cell along the axon.
  • The signals "jump" from one gap to the next.

Multiple Sclerosis (MS)

  • Multiple sclerosis (MS) is a chronic, disabling disease of the central nervous system.
  • The disease causes inflammation, destruction, and scarring of the myelin sheath that covers nerve fibers in the brain and spinal cord.
  • As a result, electrical signals from the brain are slowed or blocked from reaching the eyes, muscles, and other parts of the body.
  • The cause of MS remains uncertain; most theories suggest that it is an autoimmune disorder triggered by a virus in genetically susceptible individuals.
  • There is no cure for MS.

Nervous System Organization

  • The nervous system is organized into:
    • Central Nervous System (CNS)
    • Peripheral Nervous System (PNS)
  • The PNS is further divided into:
    • Sensory Division
    • Motor Division
      • Somatic Nervous System
      • Autonomic Nervous System
        • Sympathetic Division