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Nervous System
Part 1

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Be sure to check these outlines also:
Overview of Nervous System       Nervous System Part 2

Overview of neurons and their role in the nervous system

	Overview of nervous system Central nervous system (CNS) is brain and cord Peripheral nervous system (PNS) is nerves and related structures
	Review of nerve reflex arc (pattern of information/control like a feedback loop) Receptor (sensitive organ or tip of neuron) Sensory neuron (carries info toward CNS) Interneuron (may be none or many) Motor neuron (carries info away from CNS) Effector (always a muscle or gland)

Typical neuron

	Excitable cell (capable of an "impulse" or voltage fluctuation)
	Types of neurons Functional categories Sensory (afferent) neuron Interneuron (association neuron) Motor (efferent) neuron Structural categories Multipolar Bipolar     GA Unipolar
	Cell body (soma, perikaryon) Conducts impulses toward axon Mitochondria replicate here (some move to extensions) Ribosomes (Nissl bodies), ER, Golgi apparatus     GA  Manufacture/remanufacture neurotransmitters
	Cell extensions (nerve fibers; neurites) Dendrites (lit. "tree branches") Conduct impulses toward axon May have dendritic spines--bumps that connect with other neurons Axon (just one) Conducts impulses away from cell body Axon hillock - tapered origin of the axon where impulses add together; if of sufficient size, then will travel down the axon Axon may be myelinated: covered with myelin sheath Schwann cells or oligodendrocytes Electrical insulation Gaps called nodes [of Ranvier] Allows rapid conduction of impulse from node to node Synaptic terminal Store, then release, neurotransmitter molecules
	Cytoskeleton Neurofibrils (intermediate filaments; neurofilaments), microtubules and microfilaments     GA Axonal transport system Transport system of "railways" and "motor molecules" that shuttle vesicles and mitochondria from cell body to end of axon and bring vesicles back to cell body

Explore optional animated overviews of nervous system cells :  click here

Nerves and tracts

	Nerve: bundle of nerve fibers in PNS     pp Connective tissue component     GA Endoneurium - around individual axons Perineurium - around fascicles (bundles of axons) Epineurium - around whole nerve
	Tract: bundle of nerve fibers in CNS Lack connective tissue component
	Gray and white matter     pp White matter (white substance) Myelinated nerves and tracts (myelin is white) Where information is "passing through" Gray matter (gray substance) Unmyelinated nerves, tracts, cell bodies Where information is processed Nucleus = area of gray matter in the CNS Ganglion = area of gray matter in PNS

Click on each image to see a larger, more detailed view.

Nerve impulses

Definitions Potential = Gradient of electrical potential energy (difference in electrical charge) between two points Voltmeter = Detects electric potential - measured in volts (V) or millivolts (mV)     pp Hint: Print several copies of the figure at FIG and have them ready in class to take notes  Membrane potential = Electrical gradient maintained across living cell membranes Cell voltmeters are arranged so that the sign (- or +) tells the charge on the inside surface of the plasma membrane Resting membrane potential (RMP) = Membrane potential during rest (in an excitable cell) = -70 mV

Mechanisms Two ions at play here sodium [Na+] and potassium [K+] For the sake of our story, these two ions are the ONLY two ions that can move, all other positive ions and all negative ions are impermeant Na+/K+ pump makes sure that       ANIM Na+ is concentrated outside the cell (remember, cells HATE Na+) K+ is concentrated inside the cell (cells LOVE K+) Several gated channels are at play here, too: Stimulus-gated channels are triggered by sensory stimuli or nerve stimuli (neurotransmitters) Voltage-gated channels are triggered by a fluctuation in voltage (membrane potential) The minimum voltage needed to trigger a voltage-gated channel is called the "threshold potential" (-59 mV) All of these channels are specific (either allow Na+ or K+ through --not both)     ANIM RMP There's more Na+ outside the cell than K+ inside, thus there is an imbalance of too many positive ions on the outside Produces an RMP of -70 mV Membrane is "polarized" --that is, it has a negative pole and positive pole Local potentials     pp Fluctuations from RMP caused by activation of stimulus-gated channels in the dendrites/cell body Decremental conduction = they "poop out" Summation = they can add together Depolarization = if Na+ gates open, Na+ rushes into cell and increases voltage (less negative inside) Hyperpolarization = if K+ gates open, K+ rushes out of cell and decreases voltage (more negative inside) Synaptic potential = local potential triggered by chemical signal at a synapse (junction) Receptor potential = local potential triggered by a sensory stimulus Action potentials If local potential reaches axon, and is a depolarization large enough to reach the threshold potential (-59 mV), then voltage-gated channels in the axon will open --causing an action potential Nondecremental conduction Nonsummating (all-or-none events) Voltage-gated Na+ channels open first, and Na+ rushes in to depolarize the membrane to +30 mV Then voltage-gated K+ channels open (they're slow, like your A & P prof, OK?) and K+ rushes out to repolarize the membrane back toward RMP  Peak of +30 mV triggers the next section of axon to open its voltage-gated channels, and the process repeats --and keeps repeating all the way to the terminals at end of axon Velocity of conduction Proportional to diameter of fiber Saltatory (node to node) conduction by myelinated fibers increases velocity (compared to ordinary point-to-point conduction in nonmyelinated fibers) Explore optional animations of regular and saltatory conduction: click here Myelin disorders Disrupt saltatory conduction and thus change speed of nerve signaling, causing coordination problems Example: multiple sclerosis (MS) Montel Williams (see Climbing Higher) Teri Garr (see Speedbumps: Flooring it Through Hollywood) Action potentials are all-or-none events Refractory period Absolute refractory period - no new action potential can begin Relative refractory period - a new action potential can begin ONLY if there is a very large depolarization the threshold potential is is very high during this phase this means the frequency of action potentials can be higher than normal if the stimulus is very large

Synaptic transmission

	Types of synapses Electrical - gap junctions Chemical - neurotransmitters & receptors
	Parts of a synapse Presynaptic neuron = the neuron that gets the signal first and is about to transmit it to a second neuron Synaptic cleft = narrow space separating two adjoining neurons Postsynaptic neuron = the neuron that gets the signal after having received it from the presynaptic neuron
	Mechanism of synaptic transmission       ANIM Action potential reaches the presynaptic terminal High voltage of action potential triggers opening of voltage-gated Ca++ channels in presynaptic terminal Ca++ flows in and triggers the cytoskeleton to move vesicles containing neurotransmitter to surface and undergo exocytosis (release of contents) Neurotransmitter diffuses across synaptic cleft Neurotransmitter molecules bind to receptors in postsynaptic plasma membrane (according to lock-and-key model) Neurotransmitter-receptor binding causes a change in potential (voltage) in the postsynaptic membrane        ANIM If it's depolarization, then it can also be called "excitation" or "facilitation" because it gets the postsynaptic cell closer to the threshold potential and therefore closer to the action potential or "excitation" If it's hyperpolarization, then it can also be called "inhibition" because it inhibits the chances of an action potential in the postsynaptic neuron Transmission must be "turned off" or it will continue forever       ANIM Reuptake of neurotransmitter into presynaptic terminal May be transported into nearby glial cells (which may release them again for reuptake by presynaptic cells) Breaking of neurotransmitter molecule by enzymes Loss of some neurotransmitter from synapse by diffusion
	Other concepts of synaptic transmission Direct and indirect signal transduction Signal transduction refers to ANY mechanism by which a chemical or other stimulus is interpreted by the cell to cause a change    pp Direct mechanisms involve a receptor that is part of the ion channel that responds to the neurotransmitter Indirect mechanisms are second-messenger systems that may involve separate receptors that activate G-protein and cAMP to get ion channels to respond Many drugs are targeted at G-protein-coupled receptors (GPCRs)—about 25% of the top-selling drugs and more than half of all currently used drugs. There is always a low-level "baseline" amount of transmitter in every synapse (that is, you don't ever really start a synaptic transmission with an empty synaptic cleft) This is often referred to as "the balance of chemicals" in your brain --whether you have sufficient baseline amounts of transmitter in certain neural pathways to allow for normal function of those pathways     pp Can be altered by therapeutic chemicals    FIG reuptake inhibitors - block return of transmitter to presynaptic neuron, thus getting baseline amounts in synapse back up to normal     ANIM enzyme inhibitors - block the breakdown of transmitter molecules at the synapse, increasing the amount of active transmitter present in the synapse Postsynaptic cells integrate signals from thousands of presynaptic neurons to determine whether or not the signal will continue along the neural pathway  Excitatory postsynaptic potential (EPSP) = depolarization in postsynaptic membrane Inhibitory postsynaptic potential (IPSP) = hyperpolarization in postsynaptic membrane Summation Occurs at axon hillock Types Temporal - add up potentials over narrow window of time Spatial - add up potentials coming from different locations Neural pathways can: Converge (come together) Diverge (split apart)
	Neurotransmitters and receptors Receptors Can be many different receptors for the same neurotransmitter Neurotransmitters Small molecule transmitters Acetylcholine Amines Amino acids Very small transmitters (e.g. NO [nitric oxide]) Large molecule transmitters Neuropeptides Can also be classified as "excitatory" or "inhibitory" but the same transmitter may have different effects in different locations

Notice that a synaptic terminal may have a synapse that influences its output of neurotransmitter.  With this arrangement, a neural network can operate a "gateway" in which information can be "cut off" or "enhanced" before moving further along.

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