Axons

Examples of Axons in the following topics:

• Nerve Impulse Transmission within a Neuron: Action Potential

  • Once the signal reaches the axon terminal, it stimulates other neurons.
  • The speed of conduction of an action potential along an axon is influenced by both the diameter of the axon and the axon's resistance to current leak.
  • A node of Ranvier is a natural gap in the myelin sheath along the axon .
  • Nodes of Ranvier are gaps in myelin coverage along axons.
  • The action potential is conducted down the axon as the axon membrane depolarizes, then repolarizes.

• Neurons

  • The cell body contains a specialized structure, the axon hillock, that integrates signals from multiple synapses and serves as a junction between the cell body and an axon: a tube-like structure that propagates the integrated signal to specialized endings called axon terminals .
  • Neurons usually have one or two axons, but some neurons, like amacrine cells in the retina, do not contain any axons.
  • Along these types of axons, there are periodic gaps in the myelin sheath.
  • A bipolar neuron has one axon and one dendrite extending from the soma.
  • Each multipolar neuron contains one axon and multiple dendrites.

• Introducing the Neuron

  • The cell body contains a specialized structure, the axon hillock, which serves as a junction between the cell body and the axon.
  • One neuron's axon will connect chemically to another neuron's dendrite at the synapse between them.
  • to neurons with very long axons that travel up the spine to the brain.
  • Some axons are covered with myelin, a fatty material that wraps around the axon to form the myelin sheath.
  • At these nodes, the signal is "recharged" as it travels along the axon.

• Axon Bundles

  • A bundle of axons is called a nerve in the peripheral nervous system and a tract in the central nervous system.
  • Underlying this layer of flat cells, the perineurium, forms a complete sleeve around a bundle of axons called fascicles.
  • Surrounding each axon is the endoneurium.
  • Describe bundles of axons in the central and peripheral nervous systems

• Characteristics of Nervous Tissue

  • A typical neuron consists of dendrites, the cell body, and an axon.
  • The axon is surrounded by a whitish, fatty layer called the myelin sheath.
  • The myelin sheath surrounds and insulates the axon.
  • Transmission electron micrograph of a myelinated axon.
  • The myelin layer (concentric) surrounds the axon of a neuron, showing Schwann cells.

• Structural Diversity of Neurons

  • A typical neuron consists of a cell body and neuronal processes such as dendrites and axon.
  • Axons arise from a cone-shaped area of the cell body called axon hillock.
  • Nerve impulses are generated in the axon and transmitted away from the cell body towards the synapse.
  • Multipolar neurons, the most common type, have one axon and two or more dendrites.
  • A signal propagating down an axon to the cell body and dendrites of the next cell

• Grey and White Matter

  • The basic pattern of the CNS is a central cavity surrounded by gray matter made up of neuronal cell bodies external to which is the white matter which is made up of myelinated axons.
  • A second major component of the central nervous system is white matter and it is composed of bundles of myelinated axons that connect various grey matter regions of the nervous system to each other and carry nerve impulses between neurons .
  • White matter only contains the myelinated axon tracts,  and not the cell bodies.  
  • Myelin is a lipid that forms a thin layer, known as the myelin sheath, around the axons.  
  • It acts as an electrical insulator and increases speed of transmission by allowing the signal to jump down the axon.

• Structure of a Nerve

  • A nerve is the primary structure of the peripheral nervous system and is composed of bundles of axons.
  • Each nerve contains many axons that are sometimes referred to as fibers.
  • Within a nerve, each axon is surrounded by a layer of connective tissue called the endoneurium.
  • The axons are bundled together into groups called fascicles.
  • Individual axons can also be resolved as tiny circles within each perineurium.

• Organization of Motor Neuron Pathways

  • The axons of these cells pass in the depth of the cerebral cortex to the midbrain and the medulla oblongata.
  • These axons synapse with lower motor neurons in the ventral horns of all levels of the spinal cord.
  • The remaining 10% of axons descend on the ipsilateral side as the ventral corticospinal tract.
  • These axons also synapse with lower motor neurons in the ventral horns.
  • The midbrain nuclei include four motor tracts that send upper motor neuronal axons down the spinal cord to lower motor neurons.

• Regeneration of Nerve Fibers

  • Such mechanisms may include generation of new neurons, glia, axons, myelin, or synapses.
  • When an axon is damaged, the distal segment undergoes Wallerian degeneration, losing its myelin sheath.
  • Human axon growth rates can reach 2 mm/day in small nerves and 5 mm/day in large nerves.
  • Glial scars rapidly form and the glia actually produce factors that inhibit remyelination and axon repair.
  • The axons themselves also lose the potential for growth with age.