Overview
This lecture provides an overview of neuron structure, including the main parts of a neuron and the different structural types found in the nervous system.
Basic Neuron Structure
- Neurons consist of a soma (cell body) and processes called neurites.
- Neurites are divided into dendrites and axons.
- Dendrites are short, branched processes often covered in spines, increasing surface area.
- The axon is usually long and unbranched until its end, where it forms axon terminals.
- The axon hillock is the region where the axon leaves the soma.
- The axon initial segment (trigger zone) is the first part of the axon.
- Axons depend on transport systems to move substances between soma and axon terminals.
- Large axons are often wrapped in myelin, with gaps called nodes of Ranvier.
- Axon terminals form synapses with target cells, which can be neurons, muscle cells, gland cells, or occasionally capillaries (for hormone secretion).
Neuron Development and Structural Types
- In the central nervous system, neurons originate from neural stem cells, which become neuroblasts.
- Neuroblasts migrate to their final location and extend an axon tipped by a growth cone.
- In the peripheral nervous system, neurons originate from neural crest cells.
- Early-stage neurons have only one process and are called unipolar neurons; these are mainly found during development.
- Bipolar neurons have one axon and one dendrite.
- Multipolar neurons have one axon and multiple dendrites; this is the most common neuron type in adults.
- Pseudounipolar neurons have one short process from the soma that splits into two long axons (peripheral and central axons).
Key Terms & Definitions
- Soma — the cell body of a neuron
- Neurite — a projection from the neuron (either dendrite or axon)
- Dendrite — branched process that receives signals
- Axon — long process transmitting signals away from the soma
- Axon Hillock — area where the axon leaves the soma
- Axon Initial Segment/Trigger Zone — first part of the axon where impulses begin
- Myelin — insulating sheath around axons
- Nodes of Ranvier — gaps in the myelin sheath
- Synapse — junction between an axon terminal and a target cell
- Neural Stem Cell — precursor that can become any neural cell in CNS
- Neuroblast — precursor that can become only a neuron
- Growth Cone — structure at the tip of a growing axon
- Unipolar, Bipolar, Multipolar, Pseudounipolar Neurons — neuron types defined by number and arrangement of processes
Action Items / Next Steps
- Review the different neuron structural types and their characteristics.
- Prepare to learn about the functions of dendrites and axons in the next lecture.