Chapter 4: The Cytology of Neurons Principles of Neural Science by Eric R. Kandel Fundamental Neuroscience by Duane E. Haines The World of the Cell by Wayne M. Becker 楊定一 (Ding-I Yang) 圖資大樓851室 分機7386

An Overall View The Structural and Functional Blueprint of Neurons is Similar to Epithelial Cells Membranous Organelles Are Selectively Distributed Throughout the Neuron The Cytoskeleton Determines the Shape of the Neuron The Neurons That Mediate the Stretch Reflex Differ in Morphology and Transmitter Substance (sensory neurons and motor neurons)

An Overall View (continued) Pyramidal Neurons in the Cerebral Cortex Have More Extensive Dendritic Trees Than Spinal Motor Neurons Glial Cells Produce the Insulating Myelin Sheath Around Signal-Conducting Axons

Common Features of Neurons That Differ from Other Tissues Neurons are highly polarized The cell function of neurons are compartmentalized, contributing to the processing of electrical signals -cell body (soma): RNA/proteins synthesis -dendrites: thin processes to receive synaptic input from other neurons -axons: another thin process to propagate electric impulse -terminals: for synaptic output

Common Features of Neurons That Differ from Other Tissues (continued) Neurons are excitable due to specialized protein structures, including ion channels and pumps, in the membrane. Although polarity (epithelial and other non-neuronal secretory cells) and excitability (muscle) are not unique to neurons, they are developed to a higher degree allowing signal to be conducted over long distance.

Neurons Develop from Epithelial Cells Axon arises from “apical surface”; dendrites arise from “basolateral surface”. Plasmalemma: external cell membrane of a neuron cytoplasm = cytosol (aqueous phase and cytoskeletal matrix) + membranous organelles (vacuolar apparatus, mitochondria, and peroxisomes) Most of the cytosolic proteins are common to all the neurons. However, certain enzymes involved in the synthesis or degradation of neurotransmitters are specifically synthesized in selected neurons. For example, acetylcholinesterase is only found in cholinergic neurons.

Membranous Organelles in the Neurons Rough endoplasmic reticulum (rough ER) Smooth endoplasmic reticulum (smooth ER) Golgi apparatus Nuclear envelop Mitochondria (energy) and peroxisomes (detoxification)

Selective Distribution of Membranous Organelles in Neurons A sharp functional boundary at the axon hillock, certain organelles are absent in axon protein biosynthetic machinery (ribosomes, rough ER, Golgi complex). lysosomes Axons are rich in synaptic vesicles endocytic intermediates involved in synaptic vesicle traffic synaptic vesicle precursor membranes Mitochondria and smooth ER (Ca2+ regulation) are present in all neuronal compartment including axon.

Fig.4-2. Endoplasmic reticulum in a pyramidal cell showing a basal pole. A single dendrite emerges from the cell body. Golgi Dendrite Golgi ER Nucleus

Selective Distribution of Membranous Organelles in Neurons The cytoplasm of the cell body extends into the dendritic tree without any functional boundary. However, concentrations of some organelles such as rough ER, Golgi, and lysosomes progressively diminish into dendrites.

Fig.4-3. Golgi complex appears as a network of filaments that extend into dendrites (arrow) but not into the axon dendrite axon

The Cytoskeletal Structures of Neurons The Cytoskeleton Determines the Shape of the Neuron Microtubules: developing and maintaining the neuron’s processes Neurofilaments: bones of the cytoskeleton; the most abundant fibrillar components of the axon; on average 3-10 times more abundant than microtubules in an axon Microfilaments: short polymers concentrated at the cell’s periphery lying underneath plasmalemma. This matrix plays important roles in the formation of pre- and post-synaptic morphological specializations

Microtubules subunits: a- and b-tubulin 25-28 nm in diameter polar, dynamic structure tubulin is a GTPase; microtubules grow by addition of GTP-bound tubulin dimers at plus end. microtubule-associated protein (MAP) mostly to stabilize or enhance microtubule assembly axon: tau (causing microtubules to form tight bundles in axon) and MAP3 dendrite: MAP2 13

Expression of the Genes for Tau and MAP2C in a Nonneuronal Cell Line Sf9 is an insect cell line that is non-neuronal. Sf9 cells expressing tau protein Sf9 cells expressing MAP2C protein normal Sf9 cells

Neurofilaments cytokeratin family including glial fibrillary acidic protein (GFAP) 10 nm in diameter stable polymers neurofibrillary tangle in Alzheimer’s disease patients 1 neurofilament  32 monomer 8 protofilaments in each neurofilament 4 monomers in each protofilament

Microfilaments subunits: b- and g-actin monomer 3-5 nm in diameter polar, dynamic structure ATP With actin-binding proteins, actin filaments form a dense network lying underneath the plasmalemma. This matrix plays a key role in the formation of pre- and postsynaptic morphologic specializations.

In dendrites, microtubules with opposite polarities are mixed. Microtubules and actin filament act as tracks for intracellular protein and organelle movement In axon, all the microtubules are arranged with the plus end pointing away from the cell body, minus end facing the cell body. In dendrites, microtubules with opposite polarities are mixed. microtubule a-tubulin b-tubulin (G) GTP 25-28 nm dynamic but more stable in mature axons and dendrites neurofilament cytokeratins GFAP etc (F) none 10 nm stable and polymerized microfilament b-actin g-actin (G) ATP 3-5 nm dynamic, ～ ½ of the actin in neurons can be unpolymerized

The neurons that mediate the stretch reflex differ in morphology and transmitter substance Sensory neurons convey information about the state of muscle contraction. The cell bodies are round with large diameter (60-120 mm) located in dorsal root ganglia. The pseudo-unipolar neuron bifurcates into two branches from cell body. The peripheral branch projects to muscle. The central branch project to spinal cord, where it forms synapses on dendrites of motor neurons. Motor neurons convey central motor commands to the muscle fiber. Unlike sensory neurons which have no dendrites, motor neurons have several dentritic trees.

When excited, the sensory neuron releases excitatory amino acid neurotransmitter L-glutamate that depolarizes the motor neurons. Orange: sensory axons enter the spinal cord and Green: dendrites of motor neurons

The sensory neuron conducts information from the periphery to the central nervous system Fig. 4-8A: The axon of the sensory neuron bifurcates into a central and a peripheral branch. Sc, Schwann cells; Nuc, nucleolus; N, nucleus. Fig. 4-8B: Motor neuron. Left, many dendrites typically branch from the cell bodies of spinal motor neurons, as shown by five spinal motor neurons in the ventral horn of a kitten. Right, “synaptic bouton”, a knob-like enlargement on the cell membrane where nerve endings from presynaptic neurons attach. den den

Dendrites of Motor Neurons Dorsal root ganglion sensory neurons have no dendrites, but motor neurons have several dendritic trees that arise directly from the cell body. Short specialized dendritic extensions, or spines, serve to increase the area of the neuron available for synaptic inputs. Dendrites are functional extensions of the cell body with protein synthesis. The mRNA is transported along dendrites and appears to concentrated at the base of dendritic spines.

Extensive dendritic structure of a cat spinal motor neuron

The Morphological Characteristics of Motor Neurons Axon hillock: where each motor neuron gives rise to its only one axon. Synaptic boutons: the knob-like terminals of the axons of presynaptic neurons. Trigger zone: axon hillock and initial segment (unmyelinated) of the axon where incoming signals from other neurons are integrated and the action potential is generated. Recurrent collateral branches: the branches of the axon project back to the motor neuron and modify its own activity.

IS: initial segment AH: axon hillock

Motor neuron can receive signal inputs from… Excitatory input from primary sensory neurons Recurrent collateral branches of its own Recurrent excitatory input from other motor neuron Both excitatory and inhibitory input from interneurons driven by descending fibers from brain that control and coordinate movement Inhibitory input from Renshaw cells (an interneuron in spinal cord using L-glycine as neurotransmitters)

The difference between sensory neurons and motor neurons no dendrites L-glutamate pseudo-unipolar has few if any boutons on its cell body; primary input from sensory receptors at the terminal of peripheral branch extensive dendritic structures acetylcholine multipolar receive inputs throughout its dendrites and cell body, with inhibitory synapses on the cell body close to trigger zone and excitatory ones located farther out along the dendrites

The information flow from sensory to motor neurons is… Divergent- each sensory neuron contact 500-1000 motor neurons with 2-6 synapses on each motor neuron Convergent- each motor neuron receives input from many sensory neurons; more than 100 sensory neurons are required to reach firing threshold of action potential

Pyramidal neurons in cerebral cortex have more extensive dendritic trees than spinal motor neurons Motor neurons are the major excitatory projection neurons in spinal cord. Pyramidal cells are the excitatory projection neurons in the cerebral cortex using L-glutamate as neurotransmitter. Pyramidal cells have not one but two dendritic trees emerging from opposite sides of the cell body: basal dendrites (the same side that gives rise to axon) and apical dendrites The Schaffer collaterals (CA3 pyramidal cell axons) form en passant synapses with CA1 dendrites.

Pyramidal neurons in cerebral cortex have more extensive dendritic trees than spinal motor neurons Hippocampus (for processing memory formation) is divided into two major regions, CA1 and CA3. The cell bodies of pyramidal cells are situated in a single continuous layer, the stratum pyramidale. The axons of pyramidal neurons run in the stratum radiatum.

Fig. 4-15 Pyramidal cells in the CA3 region of the hippocampus form synapses on the dendrites of CA1 cells in the stratum radiatum Left: Golgi-stained CA1 pyramidal cells with dendrites extending downward 350 mm into stratum radiatum. Right: Three micrographs show synapses formed on this CA1 cell by CA3 cells. A. Axons of two CA3 neurons form synapses on a dendrite 50 mm from CA1 neuron’s cell body. B. A single CA3 axon forms synapses on dendrites 259 mm from the cell body. C. A single CA3 axon form synapses on two dendrites 263 mm from the cell body. CA3 CA3 CA3 CA1 CA3

The spines on the CA1 pyramidal cells have only excitatory synapse. Four types of spines in the dendrites of pyramidal cells in CA1 region: thin, stubby, mushroom, branched. The neck of the spine restricts diffusion between the head and the rest of dendrites. Each spine may function as a separate biochemical region.

Glial Cells Produce the Insulating Myelin Sheath Around Signal-Conducting Axons Myelin has a biochemical composition of 70% lipid and 30% protein that is similar to plasma membrane. Peripheral nerve is myelinated by Schwann cells. Each internodal (node of Ranvier) segment represents a single Schwann cells. The expression of myelin genes is regulated by the contact between the axon and the myelinating Schwann cells.

Glial Cells Produce the Insulating Myelin Sheath Around Signal-Conducting Axons In CNS, the central branch of dorsal root ganglion cell axons and motor neurons are myelinated by oligodendrocyte. Unlike Schwann cells, each oligodendrocyte ensheathes several axon processes. Expression of myelin genes by oligodendrocyte depends on the presence of astrocyte, the other major type of glial cells in CNS.

Shiverer mutant mice: an animal model for demyelination diseases The shiverer mice have tremors and frequent convulsions, often died at young ages. Five out of six exones of myelin basic protein (MBP) are deleted in shiverer mice, with only 10% of MBP as compared to normal mice. As a result, myelination is incomplete in these mutant mice. Transgenic shiverer mice expressing normal MBP gene has improved myelination. Despite occasional tremors, these mice do not have convulsions and live a normal life span.

Charcot-Marie-Tooth Disease This disease is characterized by progressive muscle weakness, greatly decreased conduction in peripheral nerves, as well as cycles of demyelination and remyelination. Duplication of peripheral myelin protein (PMP22) gene on chromosome 17 causing over-production of this Schwann cell protein.

An Overall View Four distinctive compartments in nerve cells Cell body – protein synthesis Axon – projection over long distances to target cells Dendrites – receiving signal from other neurons Nerve terminals – release of neurotransmitters at synapses with targets