Lec 2 - Nervous S structure

Lec 2 - Nervous S structure - Nervous System Structure and...

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Unformatted text preview: Nervous System Structure and Cellular Components Bio 334 Gross Organization of the Cerebral Cortex 1 Functional Organization of the Cerebral Cortex Functional Organization of the Cerebral Cortex 2 Cell Types of the Brain The brain is made of two types of cells: neurons and glia. •  Neurons –  ~100 billion neurons in the human brain –  Neurons carry out most (all?) of the information processing in the nervous system –  Enormous diversity of structure and function, as we will see shortly •  Glia –  Outnumber neurons by 10 to 1 –  Main roles are in maintaining homeostasis of extracellular space in the brain, insulating neurons, and providing both structural and nutrient support to neurons –  Typically divided into only three types (although this probably reflects our relative ignorance of glial cell function): astrocytes, oligodendrocytes, and microglia The “Neuron Doctrine” is a recent concept •  Prior to the late 1800’s it was thought that nerves and the brain consisted of a continuous network of fused cells. One variant of this was “hydraulic” models of neural function, before it became clear that the nervous system operates using electrical signals. •  The most noted proponent of the idea that neurons are separate cells (the “neuron doctrine”) that must communicate in some way at contact points was Santiago Ramon y Cajal. •  Cajal used the Golgi stain (invented by Camillo Golgi in 1873) to study neuroanatomy, and he concluded that nerve cells are in fact not continuous with each other. •  Golgi vigorously disputed the neuron doctrine, although Golgi and Cajal shared the Nobel Prize in 1906. Cajal, of course, was right. 3 Golgi and Cajal: Pioneers of Neuronal Structure A Golgi-stained neuron in the cerebral cortex Santiago Ramon y Cajal Camillo Golgi The Basic Parts of a Neuron •  Cell body, where the nucleus is located •  synonyms: soma (plural: somata), perikaryon (plural: perikarya) •  Neurites: thin processes that radiate out from the soma •  Two types of neurites: dendrites, axon •  Generally speaking, dendrites receive incoming signals from other neurons, and axons carry output signals to other neurons (although there are exceptions to both rules) •  Also generally speaking, a neuron cell body gives rise to only a single axon (which might subsequently branch profusely) but many dendrites 4 Neurons come in many shapes and sizes Just a few examples: Purkinje cell from cerebellum Internal structure of the prototypical neuron The plasma membrane is the external membrane that separates the intracellular space from the extracellular space. As we will see in subsequent lectures, the characteristics of the neuronal plasma membrane are crucial for the generation and transmission of electrical signals in the brain. The cell body contains an aqueous cytosol (a potassium-rich salt solution) and membrane-bound organelles: • nucleus • rough endoplasmic reticulum • smooth endoplasmic reticulum • Golgi apparatus • mitochondria. 5 Gene Transcription in Neurons Mature neurons are terminally differentiated. That is, there is no further cell division in mature neurons. Chromosomes therefore do not replicate and function only for gene transcription, the synthesis of RNA from the DNA template of the gene. Messenger RNA molecules (mRNA transcripts) are then exported from the nucleus to direct protein translation, the synthesis of protein molecules encoded by the mRNA. In this regard, neurons are no different from other cells, but there is a special set of genes that are transcribed to yield unique sets of neuronal proteins. Protein Translation Rough endoplasmic reticulum (ER) contains globular structures, ribosomes, which bind mRNA and translate the instructions to synthesize protein molecules. Soluble cytosolic proteins are translated on free ribosomes, which are not associated with the ER. Proteins to be inserted in the membrane are translated on ribosomes bound to the ER. Because neurons have many kinds of special membrane proteins used for signaling, the rough ER is a major site of protein synthesis in neurons. 6 Protein Translation Because neurons have many kinds of special membrane proteins used for signaling, the rough ER is a major site of protein synthesis in neurons. In fact, stains that bind to the ribosomal RNA can be used as a marker for neurons in the brain, because neurons have so much rough ER in the cell bodies. These pictures show examples of neuronal somata stained in this way (called a Nissl stain, after the 19th century neuroanatomist who invented it). Sorting of membrane proteins in the Golgi apparatus Derived from the ER, membrane-bound vesicles containing newly synthesized proteins fuse with the membranes of the Golgi apparatus. Vesicular transport then shuttles the proteins through different subcompartments of the Golgi apparatus. Both soluble and membrane-bound proteins bud off from the Golgi complex. Vesicle composition targets the proteins for different regions of the neuron. Separate types of proteins are thus sent to dendrites, axons, or synaptic terminals, which leads to the specialization of these subcellular compartments for receipt, transmission, or sending of signals. 7 A Major Source of Neuron Energy – the Mitochondrion The mitochondria convert pyruvic acid and O2 into ATP via the Krebs cycle. As we shall see, chemical energy stored in ATP fuels many critical neuronal functions, including maintaining the ion gradients across the plasma membrane that are required for electrical signaling. The brain is approximately 2% of the body mass, but it uses approximately 20% of the body’s oxygen regardless of the activity level. Neuron whose mitochondria have been labeled with a fluorescent dye. Each spot is a mitochondrion. Neuron Cytoskeleton As we have seen, neurons have a wide variety of shapes, with elaborate neurites that extend over substantial distances to receive and send signals. To maintain the shape of the cell, an internal scaffold is necessary to provide stiffness. Three major types of scaffolding structures (“bones”) in neurons are: • microtubules: polymers of the protein tubulin, together with microtubule-associated proteins (MAPs) • neurofilaments: consist primarily of proteins from the cytokeratin family • microfilaments: two intertwined polymer strands of the protein actin 8 The cytoskeleton is dynamic For example, actin filaments form short polymers and constantly undergo polymerization and depolymerization in microfilaments using ATP & ADP. The cytoskeleton is dynamic Microtubules also undergo constant remodeling by addition and subtraction of tubulin molecules (a GTP-dependent process). 9 Neuron: Axon and Synapse The axon does not contain rough endoplasmic reticulum and virtually no free ribosomes (this is why the Nissl stain marks only neuronal cell bodies). The membrane protein composition of the axon is different from the soma. The synaptic terminal does not contain microtubules, but does contain: • synaptic vesicles • specialized proteins for exocytosis • a high concentration of mitochondria • special subtypes of ion channels There is fast and slow axonal transport of proteins down and up the axon. Slow transport is 1-10 mm/day and fast transport is up to 1000 mm/day. Transport from the soma to the synapse uses kinesin “walking down” the microtubules using ATP. Transport from the synapse towards the soma uses dynein in a manner similar to kinesin. 10 Neuron: Dendrites Dendrites are covered with synapses at points with specific membrane proteins including receptors and channels. Dendritic spines are specialized to isolate specific synaptic inputs. Unlike axons, dendrites have polyribosomes directly under spines. Synaptic input can modulate protein translation at these ribosomes. N, soma; S, spine; SA, spine apparatus; ↑ polyribosome. 200 nm Glia: Oligodendrocytes Oligodendrocytes myelinate axons in the central nervous system. One oligodendrocyte myelinates many neurons. Schwann cells myelinate axons in the peripheral nervous system. Schwann cells myelinate only one axon. 11 Glia Oligodendrocytes Electron micrograph of a cross-section of optic nerve, showing the myelin sheath formed by oligodendrocytes Glia – Astrocytes & Microglia Astrocytes form tight junctions on blood vessels to form the blood-brain barrier. Astrocytes maintain K+ concentration in the extracellular space. Some astrocytes take up neurotransmitters in the extracellular space. Microglia are phagocytes that the CNS mobilizes after injury or infection. Derived from the same myeloid progenitor cells (from the bone marrow) as macrophages of the immune system. 12 Evolutionary Perspective •  Complex structure of the brain can be better appreciated in evolutionary context, by looking at simpler nervous systems •  Indeed, as we will see, the basis for nervous system function (that is, electrical signaling) predates the evolution of nervous systems human brain is really just .... a folded tube. 13 Simpler brains also help make the subdivisions clearer forebrain hindbrain midbrain fish picture: www.bio.umass.edu/biology/kunkel/fish Why does the nervous system use electrical signals? •  Speed! Ideal for rapidly coordinating actions of diverse organ systems in ways that make sense for the organism as a whole. •  But electrical signals evolved before there were nervous systems, and in fact before there were multi-cellular organisms 14 •  aramecia swim by beating of cilia P •  irection of ciliary beating is controlled by intracellular D concentration of calcium ions (Ca2+): direction reverses when internal Ca2+ rises. •  mutant called “pawn” can only swim forward. Why? A http://micro.magnet.fsu.edu/moviegallery/pondscum/protozoa/paramecium/index.html •  Like neurons, single-celled animals such as paramecium are “electrically excitable” •  That is, they can produce active changes in membrane potential in response to stimulation 15 The simplest true nervous system is the nerve net found in cnidarians. An example is Hydra. www.fishpondinfo.com/hydra1.jpg • n a nerve net, action potentials propagate equally in all directions I •  o neuron can be said to control the activity of any other (no N specialization of function; no hierarchy) •  owever, the nerve net does speed responses, so that Hydra can H effectively capture prey 16 Four Important Trends in the Evolution of Nervous Systems •  Evolution from radial symmetry to bilateral symmetry of the body plan •  Evolution of specialized function of individual neurons •  Cephalization (increasing importance of the brain) •  Hierarchical organization (corollary: increasing importance of forebrain) Echinoderms: An intermediate example between diffuse nerve net and centralized nervous system •  Like Hydra, echinoderms such as starfish have bodies with radial symmetry •  In a starfish, each arm is innervated by a neural connective that radiates from a ring of nerve fibers surrounding the mouth at the center of the body •  This marks the beginning of the evolutionary trend toward more centralized organization, with central clumps of neurons giving rise to nerve fibers that radiate outward to innervate the body 17 Bilateral symmetry and the emergence of centralized nervous systems head tail •  A segmented annelid, such as the leech, has a true central nervous system, with the cell bodies of neurons clustered in centrally located ganglia (singular: ganglion), one in each body segment •  The ganglia are connected by a central nerve trunk, running longitudinally along the midline •  Leech neurons also are specialized for particular functions: some are sensory neurons, some are motor neurons, and still others are interneurons, which are interposed between sensory and motor neurons Cephalization: increasing size and importance of the ganglia at the head end of the animal •  Most bilaterally symmetrical animals locomote parallel to the long axis of the body •  So, the leading end (head) comes into contact with environmental stimuli (e.g., food) first, which favors evolution of larger and more elaborate cephalic ganglia (for example, in insects) •  As ganglia at the head grew in size and complexity, the individual ganglia fused into a single mass that could be called a brain •  In fact, there are many individual clusters of neurons, called ganglia or nuclei, in mammalian brains, which are derived from the more primitive organization of neurons into chains of ganglia in invertebrates 18 Hierarchical organization •  With increasing cephalization, nervous systems become increasingly hierarchical •  In vertebrate nervous systems, the brain (the cephalic ganglia) became the overseer and initiator of behavior, while the spinal cord (the segmental ganglia) coordinates reflex actions and provides circuitry for executing commands descending from the brain 19 ...
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