Cell Biology of Neurons Sept 8, 2006

The nervous system is composed of billions of processing units (neurons) whose cellular processes (the dendrites and axons) form an elaborate and complex meshwork of circuits and pathways. Signals are transmitted between neurons via specialized cell-cell contacts known as synapses.

Neurons are highly compartmentalized cells

Neurons are highly compartmentalized cells

Neurons are highly compartmentalized cells

Neuronal form is highly variable

The Neuronal Cell Body

Neuronal cell bodies:. -typical assortment of organelles Neuronal cell bodies: -typical assortment of organelles -point of origin for processes -transcriptional powerhouses -provide virtually all of the protein constituents for the entire cell 100 µm toe 6 feet

Nucleus Rough endoplasmic reticulum Plasma membrane Smooth endoplasmic reticulum Lysosome Peroxisome Cytosol Mitochondrion Golgi complex Ribosomes

The plasma membrane bounds the cell, separating the ECF from the ICF. It controls the passage of substances into and out of the cell. The nucleus contains DNA and specialized proteins enclosed by a double-layered membrane. DNA codes for the synthesis of structural and enzymatic proteins, and is the blueprint for cell replication.

The cytoplasm consists of the organelles and the cytosol. Organelles are separate, membrane-bound compartments. Organelles include the: endoplasmic reticulum Golgi complex lysosome peroxisome mitochondrion The cytosol is a semiliquid, gel-like mass.

The sER packages new proteins in transport vesicles. ER lumen Rough ER Smooth ER The rER is an extensive, continuous membranous network of fluid-filled tubules and flattened sacs that is partially covered with ribosomes. rER synthesizes proteins for secretion and membrane construction. The sER packages new proteins in transport vesicles. Ribosomes

The Golgi complex packages vesicles and targets them to the appropriate cellular destination. Set of stacked, flattened membranous sacs. It modifies, packages, and distributes products.

Peroxisomes and Lysosomes Peroxisomes house oxidative enzymes that detoxify various waste products. Lysosomes serve as the intracellular digestive system. They are membranous sacs containing hydrolytic enzymes that destroy foreign substances and cellular debris.

Mitochondria are the energy organelles Mitochondria are the energy organelles. They are enclosed by a double membrane. The inner membrane is folded into cristae. Mitochondria are the major site of ATP production. They contain enzymes for the citric acid cycle (matrix) and the electron transport chain (inner membrane cristae).

The Cytosol functions for intermediary metabolism, protein synthesis, and nutrient storage. Enzymes in the cytosol regulate degradation, synthesis and transformation of small organic molecules (simple sugars, amino acids, fatty acids), capturing energy for cellular activities and raw materials for maintenance of cellular structure, function and growth. Ribosomes (free in the cytosol) are the site of protein synthesis. Glycogen and fat are stored in the cytosol (inclusion bodies).

The axon hillock and initial segment

The Myelin Sheath

The Axon

Axonal arborizations can be quite complex

The Synapse

Symmetric and asymmetric synapses sp Dend Symmetric and asymmetric synapses

Dynamic Polarization Axo-dendritic Axo-somatic

Dynamic Polarization: Exceptions Spinal cord afferent terminals contacted by interneurons- “pre-synaptic inhibition” Axo-axonic

Dynamic Polarization: Exceptions Olfactory bulb and retina Dendro-dendritic Dendro-somatic

Dendritic arbors: complex geometries

The Dendrite T.E.M. Freeze fracture sp

Polyribosomes are sometimes associated with dendritic spines

Spine morphology is affected by synaptic activity

The Cytoskeleton (Intermediate filaments) Abnormalities associated with degenerative diseases - Alzheimers, Down’s, ALS, etc. Intracellular “rapid” transport - severely disrupted by Colchicine (depolymerize microtubules

The Cytoskeleton is a complex protein network in the cytosol. The cytoskeleton functions as an integrated whole and links other parts of the cell together. Its three types of elements are microtubules, microfilaments, and intermediate filaments. neurofilaments

Axoplasmic transport Dendrites have limited capabilities for local protein synthesis, but axons have essentially none Thus, both types of processes require transport of proteins produced in the cell body Multiple transport systems, transport different types of material to different intracellular compartments, at different rates

Endoplasmic reticulum Nucleus Golgi complex Secretory vesicle Microtubular “highway” Axon Debris Axon terminal Lysosome Cell body

Neurocytology & Tract-tracing Widely used techniques for studying neurons and circuits: Visualization of neurons Nissl staining, Golgi methods, intracellular dye injections, immunohistochemistry Degeneration and reactive changes in the neuron after lesion Wallerian degeneration Axonal transport methods Autoradiography, HRP, Lectins, Biocytin, Dextrans, Fluorescent Tracers

Neuronal cell bodies: Nissl method

The Golgi method cerebellar Purkinje cell

Intracellular injection of Lucifer Yellow Biolistics (“gene-gun”) Intracellular injection of Lucifer Yellow

Immunohistochemistry L7 protein reveals cerebellar Purkinje cells PEP-19 antiserum reveals the calyx of Held

Tract-Tracing Anterograde Degeneration: Reduced silver method and Anterograde Wallerian degeneration Retrograde Anterograde Degeneration: Reduced silver method and electron microscopy

Anterograde Tract-tracing Autoradiography Collateral projections Labeled terminals Anterograde transport Uptake by Cell body Radioactively labeled amino acid

Retrograde Tract-Tracing HRP, Dextran Retrograde transport Uptake by terminals HRP

Tract-tracing: Fluorescent tracers Superior olivary neuron

Combining techniques at the LM and EM level