Lecture 1: Review of Cell Structure PDF

Neurons are specialized cells Intracellular (within the cell) electrical signal – very fast Intercellular (between cells) chemical signal Presynaptic neuron Postsynaptic neuron Neurons are...

Neurons are specialized cells Intracellular (within the cell) electrical signal – very fast Intercellular (between cells) chemical signal Presynaptic neuron Postsynaptic neuron Neurons are specialized cells  The structure of a typical neuron What does a neuron look like? How are the functions of a neuron different than those of a typical cell? What structures does a neuron need to carry out its functions?  The structure of a typical neuron Information flow is from the dendrites, through the cell body, along the axon to the nerve terminal, which forms the presynaptic side of the synapse with the target cell.  The structure of a typical neuron Neurons have many different arrangements of dendrites (blue) and axons (red).  The structure of a typical neuron What does a neuron look like? How are the functions of a neuron different than those of a typical cell? What structures does a neuron need to carry out its functions? Neurons are specialized Cell structure – long axons and complicated dendrites Requires cytoskeleton and transport Generation of action potential – ion channels, transporters Cell signaling - receptors in the cell membranes Exocytosis – release of neurotransmitters Endocytosis – recycling neurotransmitter vesicles High energy – metabolism  The cell membrane is a fluid mosaic leaflet { Fluid - lipids and proteins can move around within the bilayer => the lipid bilayer is dynamic Mosaic - has structures within the bilayer => protein complexes, groups of specific lipids within the bilayer  The LIPID BILAYER is made mostly of PHOSPHOLIPIDS Phospholipids are amphipatic lipid molecules with a hydrophilic and a hydrophobic domain. => Have a polar head group and two non-polar tails. => The hydrophillic (POLAR) head is composed of phosphate and glycerol with an attached functional group (choline is shown here). => The hydrophobic (NON-POLAR) fatty acid tails are made up of two long carbon chains. The lipid bilayer is a barrier to most molecules The membrane permits the passage of small hydrophobic molecules. Small uncharged molecules can pass the membrane at very slow rates. Large uncharged polar molecules cross the membrane very rarely. Water soluble ions cannot cross the membrane at all. The lipid bilayer is a barrier to most molecules The membrane permits the passage of small hydrophobic molecules. Small uncharged molecules can pass the membrane at very slow rates. Large uncharged polar molecules cross the membrane very rarely. Water soluble ions cannot cross the membrane at all. The regulation of the flow of ions across the membrane generates the electrical signals used by neurons Many different kinds of proteins are present on the cell membrane Proteins get their variation in form and function due to the differences in amino acid sequence Amino acid side chains have variable functional groups that fall into four main categories How will the different chemistry of amino acids affect how they interact with ions? Proteins will try to minimize their energy by forming noncovalent bonds Spontaneous folding of the initial peptide chain is due to the protein finding its minimum energy structure  Common protein secondary structures One common folding pattern or motif is an α-helix. In this shape the amino acids form a distinct helical patter leading to the formation of a rigid cylinder. This motif is abundant in the protein keratin which makes up skin, hair, nails and horns.  Common protein secondary structures Another motif is the β-sheet, a banding pattern in which the protein strands associate via hydrogen bonds producing a flat surface. Transmembrane proteins commonly have multiple alpha helices or are beta barrels Many proteins interact with membranes. The center of the lipid bilayer is hydrophobic. How do proteins interact with the hydrophobic interior of the lipid bilayer? From gene to protein DNA template 3ʹ 5ʹ strand A C C A A A C C G A G T T G G T T T G G C T C A 3ʹ 5ʹ TRANSCRIPTION => from nucleic acid (DNA) to nucleic acid (RNA) mRNA U G G U U U G G C U C A 5ʹ 3ʹ Codon => from nucleic acid TRANSLATION (RNA) to amino acid (protein) Protein Trp Phe Gly Ser Amino acid Proteins are synthesized in the cytoplasm or on the rough ER and then transported to their correct location In “course materials” have uploaded textbook chapter and links for information on neuron structure and cell function. Will add to this as we go along Lecture slides will be posted there as well  Genes can be EXPRESSED with different efficiencies Gene A can give rise to a lot of protein while gene B may yield only a little. Gene expression is regulated by both transcription and translation. Gene expression The first step in transcription involves the RNA polymerase binding to the promoter. This step is regulated by other proteins to control how much mRNA for a specific protein is made. Promoter Transcription unit 5ʹ 3ʹ 3ʹ 5ʹ Start point RNA polymerase In eukaryotes, the rate of transcription can be increased by the binding of an ACTIVATOR protein to a DNA region called the ENHANCER that may be located far away from the promoter site  Multiple transcription factor proteins can regulate expression of a gene Activation of signaling pathways can lead to changes in transcription and the expression of different proteins Different types of neurons have different dendritic morphologies Cerebellar Purkinje cell of the guinea pig alpha-motoneuron from the cat spinal cord Neostriatal spiny neuron from the rat Locust axonless interneuron The cytoskeleton Microtubules Vesicle ATP Receptor for motor protein Motor protein Microtubule (ATP powered) of cytoskeleton (a) Motor proteins “walk” vesicles along cytoskeletal fibers. Microtubule Vesicles 0.25 μm (b) SEM of a squid giant axon The microtubules in neurons are highly organized Are in overlapping segments In axons are all oriented with minus ends at cell body and plus ends at the axon terminus In dendrites, some segments have their minus end at the cell body and some have plus end at cell body Actin 0.25 µm Microvillus Plasma membrane Microfilaments (actin filaments) Intermediate filaments Actin Muscle cell 0.5 µm Actin filament Myosin filament Myosin head Chloroplast 30 µm (a) Myosin motors in muscle cell contraction (c) Cytoplasmic streaming in plant cells Cortex (outer cytoplasm): gel with actin network 100 µm Inner cytoplasm (more fluid) Extending pseudopodium (b) Amoeboid movement  Intermediate filaments INTERMEDIATE FILAMENTS (Neurofilaments) extend in a circular pattern around the nucleus of the cell span the cytoplasm from one cell-cell junction to another strengthening the entire epithelium form the nuclear lamina provide mechanical strength and resistance to shear stress in neurons Proteins and organelles must be transported throughout the cell The cytoskeleton acts as a super highway for transport Motor proteins move proteins, vesicles and organelles along cytoskeleton tracts  Neurons are high energy cells Glycolysis and the citric acid cycle provide the building blocks for many neurotransmitters Neurotransmitters are synthesized from amino acids, Acetyl CoA, and nucleotides Mitochondria produce most of the ATP used for cellular functions Are also important for producing compounds used for neurotransmitter synthesis Typical structure of four types of common neurons Axon hillock – beginning of the axon, where the action potential is initiated Endoplasmic reticulum does not extend into the axon Microtubules run the length of the axon, but do not extend into the axon terminus Mitochondria are found in the axon terminus  Although neurons are specialized cells, they must do all the basic functions of cells in general The properties of the cell membranes of neurons are extremely important as membranes are essential for generating the electrochemical gradient used in electrical signaling. Neurons express specific genes required for their function; genes are regulated by transcription factors that bind to specific DNA sequences. Transcription, translation, and protein trafficking are necessary to get proteins to their correct destination. Neurons use motor proteins moving along microtubules to transport proteins and organelles from the cell body to the ends of axons and dendrites. Glycolysis and acetylCoA provide the building blocks for the synthesis of neurotransmitters. Mitochondria produce ATP necessary for neuronal functions. Mitochondria may be found near the axon terminus.  Draw two neurons forming a synapse Include and label – Pre and post synaptic cells Synapse Dendrite Axon Axon hillock Axon terminus Cell body (soma) Endoplasmic reticulum (ER) Golgi Nucleus Microtubules Mitochondria