TLL 157 Neuron

Cell Body

The cell body of neurons, often referred to as the soma or perikaryon, contains a single, rounded nucleus surrounded by cytoplasm rich in organelles.

• Nucleus (blue) / Nucleolus (yellow) / Nuclear Envelope (purple) - dispersed chromatin (euchromatin) and prominent nucleolus
• Golgi Apparatus (#1,#2,#3; yellow) - highly-developed stacks of flattened sacs and nearby vesicles
  • Critical role in protein modifications, sorting, and packaging in vesicles for transport to specific locations, including organelles and synapses
  • Multiple Golgi apparatuses are usually distributed around the nucleus
• Mitochondria (red) - numerous to support the high levels of cellular and electrical activity in neurons
• Endoplasmic Reticulum (cyan) - abundant throughout the cell body
  • Rough Endoplasmic Reticulum (RER) - cisternae with bound ribosomes responsible for the synthesis of proteins destined for insertion into membranes, delivery to other organelles or secretory vesicles
    • Nissl Substance - clusters of RER and polyribosomes that are a distinguishing feature of neurons
  • Smooth Endoplasmic Reticulum (SER) - cisternae that lack ribosomes involved in lipid synthesis and Cs²⁺ storage
• Ribosomes - numerous free ribosomes scattered throughout the cytoplasm
  • Cell body is responsible for synthesizing most proteins needed in the axon and its terminals
• Lysosomes (#1,#2; orange) - break down waste materials and cellular debris
• Lipofuscin Granules (#1,#2; gray) - remnants from the incomplete breakdown of cellular debris that accumulate with aging
• Glycogen Granules - large numbers to fuel the high levels of cellular activity
• Cytoskeleton - networks of filaments that maintain its shape
  • Microtubules (blue) - long, hollow tubules also involved in the transport of cellular components
  • Neurofilaments (red) - intermediate filaments that provide structural support
  • Microfilaments - thin, flexible filaments closer to the cell membrane

Dendrites

Dendrites (#1,#2) are branching processes that increase the surface area for synapses with other neurons. Electrical signals travel along dendrites toward the cell body, where they are integrated to determine whether to fire an action potential down its axon.

• Dendritic Spines - small protrusions that further increase the sites for synapses
  • Spine Apparatus - specialized structure of smooth endoplasmic reticulum (SER) involved in Ca²⁺ signaling and plasticity
• Golgi Outposts - additional Golgi apparatuses usually located in the larger dendrites
  • Allow packaging of locally synthesized proteins into vesicles destined for the cell membrane or synapses
• Mitochondria (red) - meet local energy demands and sustain electrical activity
• Endoplasmic Reticulum (cyan) - both smooth endoplasmic reticulum (SER) and rough endoplasmic reticulum (RER)
• Cytoskeleton - longitudinal arrays of filaments that maintain their structural integrity
  • Microtubules (blue) - long hollow tubules also involved in the transport of cellular components
    • Motor proteins (kinesin and dynein) attach and crawl along microtubules to transport proteins, vesicles, and organelles
  • Neurofilaments - intermediate filaments

Axon

The axon is a single, long fiber that sends signals to other neurons or target cells. The action potential moves down the length of the axon, resembling the motion of a wave.

• Axon Hillock (yellow-green) - region of the cell body where the axon originates
  • Cone-shaped region where the soma narrows and transitions into the axon
  • Integrates incoming signals from synapses on the dendrites and cell body
  • When excitatory signals outweigh inhibitory signals and reach a certain threshold, it will fire an action potential down the axon
• Axon (yellow) - depending on the type of neuron, varies significantly in length, from a few microns to over a meter
  • Contains mitochondria and smooth endoplasmic reticulum (SER) but is devoid of rough endoplasmic reticulum (RER)
  • Cytoskeleton - longitudinal arrays of filaments
    • Microtubules - motor proteins (kinesin and dynein) attach and crawl along microtubules to transport proteins, vesicles, and organelles
    • Neurofilaments - intermediate filaments that provide structural support
  • Some axons are wrapped in a myelin sheath to speed up the transmission of electrical signals along the axon
• Axon Terminals - swellings at the end of axons that form synapses with other cells
  • Synaptic Vesicles - small vesicles that store neurotransmitters
  • Mitochondria - needed to meet local energy demands
  • Smooth Endoplasmic Reticulum (SER) - helps control the local concentrations of Ca²⁺ ions

Although a neuron has only one axon, it can branch extensively, enabling a single neuron to communicate with numerous cells.

Synapses

Synapses (#1,#2,#3,#4) are specialized junctions that transmit signals between a neuron and other cells, such as another neuron, muscle cell, or gland cell.

Chemical synapses consist of three main parts:

• Presynaptic Terminal - end of the axon of the neuron sending a signal
  • Synaptic Vesicles (purple) - small vesicles filled with neurotransmitters
• Synaptic Cleft - small gap between the two cells
• Postsynaptic Membrane - region of the cell membrane of the target cell that receives the signal

When an action potential travels down the axon, synaptic vesicles in the presynaptic terminal fuse with the cell membrane and release neurotransmitters into the synaptic cleft, they bind to receptors on the postsynaptic membrane, either exciting or inhibiting the next cell, depending on the type of neurotransmitter and receptor involved.

Chemical synapses between neurons are also defined by their location:

• Axo-Somatic - axon terminates on the cell body of another neuron
  • Stronger influence on whether the neuron will generate an action potential
• Axo-Dendritic (#1,#2) - axon terminates on a dendrite of another neuron
  • Most common type of synapse
• Axo-Axonic - axon terminates on the axon of another neuron

(Less common are electrical synapses, where electrical signals are transmitted directly between adjacent cells through gap junctions.)