neurocytology sept. 2, 2009 jennifer lavail, ph.d. (

NEUROCYTOLOGY

Sept. 2, 2009

Jennifer LaVail, Ph.D.

(http://anatomy.ucsf.edu/Pages/lavaillab/index.html)

NEUROCYTOLOGYObjectives

To build on your understanding of the structure and diversity of the cells that make up the nervous system

1. Structure and function of neurons

2. Axoplasmic transport

3. Structure and function of synapses

4. Structure and function of glial cells

What kinds of cells are these?

What kinds of cells are these?

Neuron?Epithelial cell?Glial cell?

The neuron has the same biochemistry and complement of organelles as other cells of the body.

What makes a neuron special?

But, neurons are remarkable for their polarity.

Structurally the two most distinctive features of the neuron are its polarized shape and localized secretory function.

Action potential

Polarized shape -

Action potential

The axon requires unusual modifications of the cytoskeleton and specializedintracellular transport mechanisms.

Among all the cells inour body, only neuronshave axons.

Microtubule polarity

Fibroblasts have continuous µts. Neurons have multiple µts.Fibroblasts have unipolar µts. Axons have unipolar µts. Dendritic µts have mixed polarity.

Microtubule polarity

Soma

The neuron cell body is the primary location of protein synthesis.

Most of the proteins for axon maintenance and synaptic functionhave to be transported to the axon.

Where is the cell body?

Electron micrograph of a neuronal cell bodyCytoplasm = pink; nucleus = yellow

What is specialabout this neuroncell body?

Electron micrograph of Nissl bodies (stacks of RER) “protein synthesis machines”

DRG axon hillock

Neuron cell body also has polarity axon hillock - region with no ribosomes…beginning of the

axon.

DendritesInput region of neuron

Form of energy varies, e.g.,chemical, stretch, thermal, neurochemical.Integrates excitatory and inhibitorysynaptic inputs.

Dendrites pyramidal-Purkinje

Principal structures of a neuron

Cell BodyDendritesAxon hillockInitial SegmentAxonSynaptic terminals

Blowfly neuron

processes

Pyramidal neuron, rat cerebral cortex

Motoneuron axons grow long distances to muscle endplateso

The structure of chemical synapses

Neuromuscular junction in Lateral rectus muscle

Motor end plate 2

Chemical Synapse

Voltage-sensitivecalcium channels

Synaptic cleft15-100 nm

Synaptic components:

Synaptic vesiclesPresynaptic membraneSynaptic cleftPostsynaptic membrane

CNS synapses

synapse

Two large synaptic terminals containing mitochondria contacting a dendrite

A synaptic terminal containing dense cored vesicle indicating the presence of catecholamine transmitters

A large dendrite and spine contacted by many synaptic boutons

Distinguishing characteristics of axons and dendrites

Characteristic Axon Dendrite

Ontogeny develops early develops later

Length several µm to >1 m limited

Size/Shape 0.1 to 20 µm 0.1 – 5 µmuniform diameter tapered

Axon hillock present absent

RER absent present

Cytoplasm vol. 100 x greater relatively little

Number per neuron 1 > 1

Excitability conducts AP passive spread

Myelin present on axons rare but glial wrap >than 1 um

Major Point: Mature axon has no RER, ribosomes, Golgi, so no way to make proteins. Proteins have to be transported into and along axon.

Neuron cell body ~100 µm

Axon 7

ft

2.13 x

107

Differentiating neuronal cells (actin, microtubules and DNA)

Torsten Wittman

Motile embryonic neuronEmbryonic neurons move, axons grow.

The components of axonal transport

microtubules

cargo

motor

Cargo comprised of protein, peptides, proteoglycans and amino acids.Most are transported within organelles. (No synaptic vesicle transport).Rates are about 200-400 mm/day or much slower, 1-2 mm/day.Bidirectional movement depends on balance of cytoplasmic motors.

Axonal transport

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Real time assays of viral transport

QuickTime™ and aSorenson Video decompressorare needed to see this picture.

Anterograde transport of HSV

Axonal transport of virusHSV transport

The myelin sheath along a myelinated CNS axon is composed of sections of myelin made from a number of individual cells

Remember, however, not all axons are myelinated. Small diameter axons generally have no myelin covering.

In the PNS axons less than 1 µm in diameter are unmyelinated.

Axon PNS

RELATIVE SIZES

Structure (mm) µm nm

Visual Cortex (thickness) 5Cortical column (thickness) 0.25 250Neuron cell body 15-100Red blood cell 7Neuromuscular junction 2-4Sympathetic axon 0.2 200CNS synaptic bouton 0.5-2.0 500-2000Dense core synaptic vesicle 100Synaptic vesicle 50Smooth endoplasmic reticulum 5

sizes