Study Notes on Neurons and Glial Cells

Cells of the Nervous System: The Neuron and Glia

Types of Cells in the Nervous System

• Neuron

  • Major type of cell that transmits electrical information.

• Glia (Glial Cells)

  • Supporting cells of the nervous system.

  • Subtypes:

  • Astrocytes

  • Microglia

  • Oligodendrocytes

  • Schwann Cells

  • Other Cell Types: Ependymal Cells

Major Functions of Neurons and Glia

• Neuron

  • Function: Transmit electrical signals over short and long distances in the body. Neurons are electrically and chemically excitable.

• Glia

  • Function: Support neuronal function, contribute to maintenance of the nervous system.

Structure of Typical Neuron

• Composed of:

  • Dendrites

  • Cell Body (Soma)

  • Axon

  • Presynaptic Terminal

Function of Neurons

• Primary function is to send electrical signals throughout the body.

• Neurons are able to perform this function based on their structural characteristics, which influence their role and efficiency.

Milestones in Neuron Doctrine

• Camillo Golgi and Santiago Ramón y Cajal:

  • First clearly described neurons using the Golgi method, a silver staining technique.

  • They won the Nobel Prize in Physiology and Medicine in 1906, credited as the fathers of neurobiology.

Neuron Structure & Function

Dendrites

• Dendrites are tree-like branches that receive signals from other neurons.

• Complexity of dendritic branching determines the number of inputs a neuron can receive.

• Dendrites feature protrusions known as spines, which increase the surface area for communication and play a role in synaptic plasticity.

• Plasticity: Dendrites can change in shape based on environmental factors such as stress or drug exposure, which aids in memory formation.

Cell Body (Soma)

• The cell body contains:

  • The nucleus

  • Organelles such as endoplasmic reticulum, Golgi apparatus, mitochondria, ribosomes, and secretory vesicles.

• Responsible for processing incoming signals and making decisions on passing signals to other neurons:

  • Performs "cellular arithmetic" on incoming excitatory, inhibitory, and modulatory signals.

Axon Hillock

• Axonal structure:

  • Main output region, beginning at the axon hillock, transitioning from the cell body.

• Axons can branch to connect with multiple target cells.

• Axons can vary significantly in length, from the lengthy segments of the sciatic nerve down to short interneurons.

Myelin

• A fatty layer covering most vertebrate axons, enhancing action potential speed.

• Nodes of Ranvier: Gaps in myelin that regenerate action potentials, facilitating saltatory conduction—a rapid form of signal transmission.

• Regions rich in myelinated axons appear as 'white matter', contrasting with 'gray matter' where cell bodies reside.

Axon Characteristics

• Length: Varies by neuron type and function.

• Diameter: Affects the speed of action potential; larger diameters lead to faster signal transduction and are usually accompanied by thicker myelin.

• Transport Mechanisms:

  • Axoplasmic Transport: Movement of materials within the axon:

  • Anterograde Transport: From cell body to terminal (utilizes motor proteins like Kinesin).

  • Retrograde Transport: From terminal to cell body (utilizes motor proteins like Dynein).

Synapse

• Definition: A synapse is the junction at which two neurons communicate.

• Neuronal signals travel from the presynaptic neuron to the postsynaptic neuron

Presynaptic vs. Postsynaptic Neurons

• Presynaptic Neuron: Releases neurotransmitters into the synapse.

• Postsynaptic Neuron: Receives neurotransmitter signals.

• Communication at the synapse requires both electrical (action potential) and chemical (neurotransmitter) signals.

Types of Synapses

• Chemical Synapses:

  • Utilizes neurotransmitters for signaling.

  • Size: Approximately 15–40 nm across.

• Electrical Synapses:

  • Physically share cytoplasm, allowing for rapid communication.

  • Typically less than 5 nm apart, with separate membranes.

Variations in Neuron Structures

• Neurons share common structures (cell body, dendrites, axon), but overall designs can vary based on function:

  • Unipolar Neurons: One branch from the cell body; both dendrites and axon terminals.

  • Bipolar Neurons: One axon and one dendritic branch.

  • Multipolar Neurons: Multiple processes extending from the cell body with varied branching characteristics.

Glial Cell Types and Their Functions

Astrocytes

• Star-shaped cells responsible for maintaining the blood-brain barrier through endfeet that interact with blood vessels.

• Produce trophic factors for neuron survival, synapse formation, and maintenance, as well as regulate ion concentrations at the synapse.

Oligodendrocytes

• Present exclusively in the central nervous system (CNS), these cells myelinate multiple axons, thereby increasing conduction speeds.

• Each oligodendrocyte can myelinate up to 50 axons simultaneously.

• Highest metabolic rate of all cell types in the brain.

Schwann Cells

• Found only in the peripheral nervous system (PNS), these cells myelinate single segments of axons and aid in axon regeneration post-injury by guiding regrowth.

Microglia

• Act as immune cells within the CNS, constituting about 10-15% of brain cells.

• Act as scavengers to remove debris, dead cells, and foreign pathogens, particularly after CNS injury.

Ependymal Cells

• Line brain ventricles and produce cerebrospinal fluid (CSF).

• Contain cilia aiding in the movement of CSF and form part of the choroid plexus, which separates blood from CSF.

Key Takeaways

• The neuron structure is specialized and varies based on location and functional role.

• Multiple types of glial cells exist, each fulfilling unique supportive roles within the nervous system.

References

• Allen, N., Barres, B. (2009). Glia — more than just brain glue. Nature, 457, 675–677.

• Yuste, R. (2015). From the neuron doctrine to neural networks. Nat Rev Neurosci, 16, 487–497.

• Various scientific journals and articles as cited throughout the notes.