Lecture Notes: Somatic Motor Neuron Model

Classification

• Multipolar Neurons
  • Have many projections off the soma (cell body).
  • Classified as motor neurons or interneurons.
  • Interneurons are in central nervous system (CNS) gray matter.

Myelination

• Myelin presence indicates white matter.
• Since myelin is present, this neuron is a motor neuron, not an interneuron.

Parts of the Motor Neuron

Soma (Cell Body)

• Contains the nucleus.
• Nissl Bodies: Analogous to rough ER, produce proteins.

Dendrites

• Thickened projections off of the soma.
• Receive information from other neurons.
• Contain ligand gates (mechanical gates are for sensory neurons).

Synaptic End Bulbs

• Belong to a presynaptic neuron.
• Do not touch the soma or dendrites due to a synaptic cleft.

Axon Hillock

• Cone-shaped area off of the soma.
• Contains the trigger zone with sodium voltage gates.
• Action potential begins here if threshold (-55 mV) is reached.

Axon

• Covered by myelin sheath.
• Myelin formed by Schwann cells in the peripheral nervous system (PNS).
• Schwann Cells: Multiple cells wrap around the axon, forming the myelin sheath.
• Neurolemma: Outer layer of Schwann cell's myelin sheath.
• Node of Ranvier: Indentations on axon lacking myelin, allowing action potential to "jump" (saltatory conduction).

Structural Support

• Neurofibrils: Provide structure and support for the neuron shape.

Conduction

• Myelin blocks voltage gates; action potential jumps between nodes, speeding conduction (saltatory conduction).
• Endoneurium: Connective tissue layer protecting delicate Schwann cells and myelin.

Summary

• Motor neuron, somatic type, targets skeletal muscle.
• Function: Sends information to glands and muscles to prompt action.
• Structure: Multipolar, distinguished by soma, Nissl bodies, dendrites, axon hillock, myelinated axon, and nodes of Ranvier.

Key Terms

• Soma: Another term for cell body.
• Nissl Bodies: Blue rough areas for protein production.
• Dendrites: With ligand gates for receiving signals.
• Axon Hillock: Trigger zone for action potential.
• Schwann Cells: Form myelin sheath in PNS.
• Neurolemma: Outer membrane of Schwann cells.
• Node of Ranvier: Indentations for saltatory conduction.