Neuroscience Lecture 2: Neurons and Glia

Lecture 2: Neurons and Glia

• The lecture will cover the challenges of experimentally studying neurons.
• The lecture will cover the cellular and morphological characteristics of neurons.
• The lecture will cover methods to classify neurons according to different schemes.
• The lecture will cover other cell types that make up the nervous system.
• Reading material is Bear, chapter 2.

Visualizing a Neuron - Challenges

• Neurons are small, measuring 10 to 40 μm.
• This is less than 1/5 the size of a typical object the human eye can see.
• Magnification of 40-200x is often needed.
• Obtaining thin nervous tissue is required for visualization.

The Neuron Doctrine

• Histology (study of tissue structures) uses stains like Cresyl violet.
• Nissl staining targets Nissl bodies, similar to endoplasmic reticulum, in the cytoplasm.
• Golgi staining (AgNO3) reveals soma (cell body) and neurites (axons and dendrites).
• Santiago Ramon y Cajal improved Golgi's method and showed neurons communicate by contact but not continuity.
• Neurons adhere to the cell theory.
• Microscopes have different resolutions. Optical microscopes have a resolution around 0.1 μm, while electron microscopes have a resolution around 0.01 nm. A synaptic cleft is approximately 20nm (0.02μm).

Prototypical Neuron

• Parts of a typical neuron include the soma, axon, nerve terminal, dendrite, dendritic spines, nucleus, cellular membrane, and cytoskeleton.

Soma

• Mitochondria are the site of cellular respiration.
• Krebs cycle occurs, producing ATP for cellular energy.
• The human brain consumes 20% of the oxygen, despite representing only 2% of body mass.

Axon

• The axon transmits signals.
• Axons range in diameter from 0.1 to 10 μm.
• Branches (collaterals) typically fork at 90-degree angles.
• The axon hillock is where the axon emerges from the soma.
• Action potentials are initiated at the axon hillock.
• The axon terminal (nerve terminal) has unique characteristics compared to the axon, like the absence of endoplasmic reticulum (ER) and a large number of mitochondria.

Synapse

• Synaptic transmission can be either electric (gap junctions) or chemical (neurotransmitters).

Dendrites

• Dendrites, resembling antennae, receive and process information through synapses.
• They make up more than 95% of the cell surface area.
• Dendrites, in some neurons, possess dendritic spines which are small mushroom or spine-shaped structures.
• They receive signals from synaptic terminals, and the post-synaptic compartment contain synaptic receptors.
• Dendrites are related to memory and learning.
• Dendrites measure from 0.1 μm to 3 μm in diameter.

Cell Membrane & Cytoskeleton

• The cell membrane is a barrier around the cytoplasm.
• The membrane is roughly 5 nm thick.
• Different types of membrane proteins are found at various regions of the neuron.
• Presynaptic, postsynaptic, and axonal membranes are specific areas of cell membrane with specialized functions.
• The cytoskeleton is the internal scaffolding of the neuron's membrane and has a dynamic nature.
• Components of the cytoskeleton include microtubules, microfilaments (actin), and neurofilaments which provide support and stability.

Axoplasmic Transport

• Anterograde transport moves materials from the soma to the nerve terminal, using kinesin along microtubules and consuming ATP.
• Retrograde transport moves materials from the nerve terminal to the soma, using dyneins along microtubules and consuming ATP.
• In neuroscience, axoplasmic transport is useful for tracing neurons using anterograde and retrograde tracers that label nerve terminals and cells that innervate the targeted region.

Neuronal Classification

• The large number of neurons necessitates classification.
• Multiple schemes exist for organizing neurons, facilitating understanding of their roles.
• Classification by the number of neurites (axons and/or dendrites) - unipolar, bipolar, multipolar.
• Classification by cell morphology - oval/spherical cells, pyramidal cells.
• Classification by length of axon; Golgi type I (projection neurons), Golgi type II (local neurons).
• Classification by connectivity - sensory neurons, motor neurons, interneurons.
• Classification by neurotransmitter secreted - cholinergic, glutamatergic etc.
• Classification by molecular characteristics - transcription factors involved in neuronal development.

Glia

• Glial cells have roles in support, such as maintaining the ionic extracellular concentrations and influencing neurite growth, as well as regulating synaptic transmission.
• Astrocytes, the most numerous glial cell, star-shaped, have many projections, form most of the space between neurons and regulate aspects of neuron function, including axon growth and synaptic transmission.
• Myelinated glia (oligodendrocytes in the CNS, Schwann cells in the PNS) insulate axons and increase speed of electrical signals.
• Nodes of Ranvier are gaps in the myelin sheath where sodium channels are concentrated enabling saltatory transmission.
• Microglia are phagocytic cells of the immune system that eliminate cellular waste and dead cells.

Questions

• The student is assigned the task of identifying which neurons within a particular region (Nucleus3350) receive synaptic projections.
• They need to determine which type of tracing (anterograde or retrograde) is required.
• The features to look for would distinguish neurons that project to Nucleus3350 from other neurons.
• A student observes a neurite under a microscope and must distinguish whether the neurite is a dendrite or axon based on the distinguishing features.