NEUROCYTOLOGY Sept. 2, 2009 Jennifer LaVail, Ph.D. (

NEUROCYTOLOGY Objectives 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?

Neuron? Epithelial cell? Glial cell?

The neuron The neuron has the same biochemistry and complement of organelles as other cells of the body. What makes a neuron special?

Cell types But, neurons are remarkable for their polarity. Structurally the two most distinctive features of the neuron are its polarized shape and localized secretory function.

Parts of the neuron Action potential Polarized shape -

Parts of the neuron Action potential The axon requires unusual modifications of the cytoskeleton and specialized intracellular transport mechanisms. Among all the cells in our body, only neurons have 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 function have to be transported to the axon. Where is the cell body?

The soma Electron micrograph of a neuronal cell body Cytoplasm = pink; nucleus = yellow What is special about this neuron cell body?

Nissl bodies 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.

Neuronal polarity

Dendrites Input region of neuron Form of energy varies, e.g., chemical, stretch, thermal, neurochemical. Integrates excitatory and inhibitory synaptic inputs. Dendrites pyramidal-Purkinje

Principal structures of a neuron Cell Body Dendrites Axon hillock Initial Segment Axon Synaptic terminals Blowfly neuron processes Pyramidal neuron, rat cerebral cortex

Motor end plate 1 Motoneuron axons grow long distances to muscle endplateso

The structure of chemical synapses Neuromuscular junction in Lateral rectus muscle Motor end plate 2

Vesicle release

Chemical Synapse Voltage-sensitive calcium channels Synaptic cleft nm

Axon terminal

Synaptic components: Synaptic vesicles Presynaptic membrane Synaptic cleft Postsynaptic membrane CNS synapses synapse

Synapse em-2 Two large synaptic terminals containing mitochondria contacting a dendrite A synaptic terminal containing dense cored vesicle indicating the presence of catecholamine transmitters

Synapse em3 A large dendrite and spine contacted by many synaptic boutons

Distinguishing characteristics of axons and dendrites Characteristic AxonDendrite Ontogenydevelops earlydevelops later Lengthseveral µm to >1 mlimited Size/Shape0.1 to 20 µm0.1 – 5 µm uniform diametertapered Axon hillockpresentabsent RERabsentpresent Cytoplasm vol.100 x greaterrelatively little Number per neuron1 > 1 Excitabilityconducts APpassive spread Myelinpresent on axonsrare 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.

giraffe Neuron cell body ~100 µm Axon 7 ft 2.13 x 10 7

Differentiating neuronal cells (actin, microtubules and DNA) Torsten Wittman Motile embryonic neuron Embryonic 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 mm/day or much slower, 1-2 mm/day. Bidirectional movement depends on balance of cytoplasmic motors. Axonal transport

Real time assays of viral transport

Anterograde transport of HSV

Axonal transport of virus HSV transport

Cell with myelin 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.

Myelin

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

RELATIVE SIZES Structure(mm) µmnm Visual Cortex (thickness)5 Cortical column (thickness) Neuron cell body Red blood cell 7 Neuromuscular junction 2-4 Sympathetic axon CNS synaptic bouton Dense core synaptic vesicle100 Synaptic vesicle 50 Smooth endoplasmic reticulum 5 sizes