Axons

Examples of Axons in the following topics:

• 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 seen as tiny circles within each perineurium.

• Organization of Motor Neuron Pathways

  • The axons of these cells pass from 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.

• Spinal Cord Grey Matter and Spinal Roots

  • The grey matter of the spinal cord contains neuronal cell bodies, dendrites, axons, and nerve synapeses.
  • Axons of these sensory neurons travel into the spinal cord via the dorsal roots.
  • The axons of dorsal root ganglion neurons are known as afferents.
  • These neurons are of the pseudo-unipolar type, meaning that they have an axon with two branches that act as a single axon, often referred to as a distal and proximal processes.
  • The dendrite receives information from another neuron's axon at the synapse, and the axon sends information to the next neuron's dendrites.

• Capacity of Different Tissues for Repair

  • Human axon growth rates can reach 2 mm/day in small nerves and 5 mm/day in large nerves.
  • In the later stages of regeneration the remaining endoneurial tube directs axon growth back to the correct target.
  • 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.
  • It is important to note that central nervous system axons have been proven to regrow in permissible environments; therefore, the primary problem to central nervous system axonal regeneration is crossing or eliminating the inhibitory lesion site.

• Somatic Sensory Pathways to the Cerebellum

  • The axon runs via the fila radicularia (nerve rootlets) to the dorsal horn of the gray matter.
  • There it makes a synapse with the dendrites of two neurons that send their axons bilaterally to the ventral border of the lateral funiculi (transmit the contralateral corticospinal and spinothalamic tracts).
  • This is in contrast with the dorsal spinocerebellar tract (C8 - L2/L3), which only has one unilateral axon that has its cell body in Clarke's nucleus (only at the level of C8 - L2/L3).
  • Axons first cross midline in the spinal cord and run in the ventral border of the lateral funiculi.
  • These axons ascend to the pons where they join the superior cerebellar peduncle to enter the cerebellum.