Overview
This lecture explains the structure and main components of neurons, describing their organelles, function, and the roles of different neuron types in the nervous system.
Basic Structure of a Neuron
- Neurons have three main parts: dendrites (branching extensions), a soma (cell body), and an axon (long extension).
- The axon hillock, or trigger zone, is where the axon attaches to the soma and initiates signals.
- Axon terminals are branches at the end of the axon that connect with other neurons.
Functional Roles of Neuron Parts
- Dendrites are usually the input region, receiving signals.
- The soma integrates signals and makes decisions for output.
- Axons are generally the output region, sending signals to other cells, although some signaling can occur in reverse.
Types of Neurons
- Multipolar neurons have multiple extensions and are common in the brain and spinal cord.
- Bipolar neurons have two poles, and pseudo-unipolar neurons have a single extension.
Organelles Within Neurons
- The soma contains all organelles: nucleus, nucleolus, rough ER (missile bodies), smooth ER, mitochondria, Golgi apparatus, lysosomes, and peroxisomes.
- Dendrites contain most organelles except the nucleus.
- Axons lack rough ER, Golgi apparatus, and the nucleus.
Protein and Neurotransmitter Processing
- Proteins are made in the soma, processed in the Golgi, and transported to the axon terminals.
- Microtubules and motor proteins (kinesins/dyneins) transport vesicles containing neurotransmitters down the axon.
- Lysosomes digest materials, sometimes forming lipofuscin (aging pigment) as neurons age.
Cytoskeletal Elements
- Microtubules provide transport “railways” and resist compressive forces.
- Actin underlies membranes, shapes the cell, and enables structural changes.
- Keratin resists tensile forces within the neuron.
Myelination and Node of Ranvier
- Myelin, formed by Schwann cells (PNS) or oligodendrocytes (CNS), increases signal speed and conductivity.
- Nodes of Ranvier are gaps between myelin segments that enable signal propagation.
Key Terms & Definitions
- Dendrites — Branching neuron extensions that receive input signals.
- Soma — The cell body of a neuron; integrates incoming signals.
- Axon — Long projection transmitting signals away from the soma.
- Axon Hillock (Trigger Zone) — Region where the axon meets the soma; initiates signals.
- Axon Terminals — Branching ends of the axon that communicate with other neurons.
- Missile Bodies — Specialized rough ER in neurons, essential for protein/neurotransmitter synthesis.
- Lipofuscin — Aging pigment from lysosomal digestion in neuron soma.
- Microtubules — Cytoskeletal components for transport within the neuron.
- Myelin — Fatty substance surrounding axons, increasing the speed of nerve impulses.
- Node of Ranvier — Gaps between myelin sheaths on axons.
- Multipolar/Bipolar/Pseudo-unipolar neurons — Types of neurons classified by shape and number of extensions.
Action Items / Next Steps
- Review the differences between neuron parts and their functions.
- Study diagrams of neuron structures and organelle locations.
- Prepare for questions on neuron types and signal processing for upcoming lessons.