Introduction Nervous system

Introduction Nervous system - Functional Organization of...

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Unformatted text preview: Functional Organization of the Nervous System: Nervous Neurons and Supporting Cells Neurons Terms CNS brain and CNS spinal cord Neurons and Supporting Cells Neurons Terms PNS nerves/ganglia outside the CNS (cranial/spinal nerves) CNS Neurons and Supporting Cells Neurons Terms Sensory (afferent) neuron transmit from sensory receptor to CNS CNS Dorsal root Dorsal Neurons and Supporting Cells Neurons Terms Motor (efferent) neuron transmit from CNS to effector organ (muscle) (muscle) Neurons and Supporting Cells Neurons Terms Nerve cable-like collection of many axons; may be mixed sensory/motor may Terms (continued) Terms Somatic motor nerve stimulates contraction of skeletal muscle contraction Terms (continued) Terms Autonomic motor nerve stimulates contraction of smooth/cardiac muscle, as well as glandular secretion smooth/cardiac Terms (continued) Terms Ganglion groups of neuron cell bodies found outside the CNS Terms (continued) Terms Neuron nervous cell Terms (continued) Terms Neuroglia (Glial cells) supporting cells of the nervous system Neurons (definitions) Neurons Perikaryon cell body of a neuron Neurons (definitions) Neurons Dendrites extensions of the cell body that transmit signals toward the neuron transmit Neurons (definitions) Neurons Axons extensions of the cell body that transmit signals away from the neuron transmit Neurons (definitions) Neurons Axon Hillock initial portion of the axon closest to the cell body closest Neurons (definitions) Neurons Axon axon axon Collaterals terminal branches of the Neurons (definitions) Neurons Myelin Sheath A white fatty material, composed chiefly of lipids and lipoproteins, that encloses certain axons and nerve fibers nerve Neurons (definitions) Neurons Myelin Sheath A white fatty material, composed chiefly of lipids and lipoproteins, that encloses certain axons and nerve fibers lipoproteins, Typical Neuron Dendrites Axon Cell Body Myelin Sheath Axon Hillock Axon Collaterals Structural Classification of Neurons Structural 3 different types of neurons based on their different structural configuration structural Bipolar has two processes; one at either Bipolar end (i.e. retina of eye) end Structural Classification of Neurons Structural 3 different types of neurons based on their structural configuration Multipolar most common type; has several dendrites and one axon (i.e. a typical motor neuron) axon Structural Classification of Neurons Structural 3 different types of neurons based on their structural configuration Pseudounipolar has a single, short process that branches like a T to form a pair of longer processes (i.e. a typical sensory neuron) neuron) Neuron of the retina of the eye Typical Sensory Neuron Typical Motor Neuron Supporting Cells (Neuroglia) Supporting Schwann cells form the myelin sheath around peripheral axons (neurolemmocytes) around Supporting Cells (Neuroglia) Supporting Satellite cells support ganglia in the PNS (ganglionic gliocytes) (ganglionic Supporting Cells (Neuroglia) Supporting Oligodendrocytes form the myelin sheath around CNS axons around Supporting Cells (continued) Supporting Microglia phagocytize pathogens and debris in the CNS debris Supporting Cells (continued) Supporting Astrocytes help regulate the external environment of neurons in CNS/ contribute to the blood-brain barrier to Ventricles of the Brain Ventricles Supporting Cells (continued) Supporting Ependymal cells form the epithelial lining of brain cavities (ventricles) and the central canal of the spinal cord (ventricles) The true Schwann cell Myelination of Axons Myelination Myelination Myelination Schwann These These Process Cells supporting cells myelinate by wrapping themselves repeatedly around axons themselves Myelination of Axons Myelination Myelination Myelination Schwann One Process Cells Schwann cell myelinates one region of the axon one Myelination of Axons Myelination Myelination Myelination Schwann Process Cells Successive Successive wrappings push the cytoplasm and organelles of the Schwann cell off to one side and can be likened to twisting a tube of toothpaste to push its contents toward the open end push Myelination of Axons Axons Myelination Myelination Schwann These These Process Cells cells only myelinate in the peripheral nervous system the Is an Oligodendrocyte Myelination of Axons Cont. Myelination Oligodendrocytes These These cells myelinate axons by sending out cytoplasmic extensions, like tentacles, that encircle the axons the Myelination of Axons Cont. Myelination Oligodendrocytes One One oligodendrocyte can myelinate several axons at one time by sending out several extensions at Myelination of Axons Cont. Myelination Oligodendrocytes Extensions Extensions will successively wrap themselves around the axon at various points along its length around Myelination of Axons Cont. Myelination Oligodendrocytes Oligodendrocytes Oligodendrocytes only myelinate axons in the central nervous system central Myelination of Axons Cont. Myelination Whether myelin is laid down by Schwann cells or Whether oligodendrocytes, there are always gaps in between the myelin along the length of the axon myelin Myelination of Axons Cont. Myelination These gaps are called Nodes of Ranvier and will play an These Nodes important role in the transmission of nervous impulses important Myelination of Axons Cont. Myelination Myelinated axons allow nervous impulses to travel at faster Myelinated speeds down axons compared to the rate of travel down unmyelinated axons unmyelinated Regeneration of a Cut Axon Regeneration More prevalent in the PNS than CNS Process of regeneration of a cut axon Cut axon of peripheral nerve begins to degenerate Cut distally (at the end farthest from the cell body) distally Degenerated area is phagocytosed by Schwann Degenerated cells cells Schwann cells line up to form a regeneration tube Schwann regeneration Schwann cells then secrete chemicals that attract Schwann the growing axon tip the Regeneration tube also guides the regenerating Regeneration axon axon Astrocytes Astrocytes Most abundant glial cells in the Most central nervous system central Large and star-shaped cells Take up K+ (potassium) from the extracellular environment extracellular K+ is a byproduct of active neurons during conduction of nerve impulses nerve Thus, astrocytes are important in Thus, maintaining ionic environment for neurons for Astrocytes Astrocytes Also, astrocytes take up Also, specific neurotransmitter chemicals that are released from the ends of axons from Astrocytes break down Astrocytes these neurotransmitters into glutamine glutamine Glutamine is used to Glutamine resynthesize the neurotransmitters neurotransmitters Astrocytes and the Blood-Brain Barrier Astrocytes Occurs through the interaction of astrocytes with Occurs the capillaries in the brain the Astrocytes send out vascular processes that Astrocytes vascular almost entirely surround brain capillaries almost These capillaries in the brain have their These endothelial cells connected via tight junctions which forms a strong, impermeable barrier which Astrocytes and the Blood-Brain Barrier Astrocytes This barrier prevents the brain from obtaining This molecules from the blood via filtering mechanisms molecules Instead, molecules must move via diffusion and/or Instead, active transport (assisted by protein carriers for instance) instance) Results in a very selective blood-brain barrier Electrical Activity in Axons Electrical The Resting Membrane Potential Refers to the difference which exists across a Refers cell membrane separating a more negatively charged internal environment compared to the external environment external May see it written or referred to as rmp May rmp Results from a membrane which tends to trap Results large, negatively charged ions within the cell while allowing very little diffusion of positively charged ions out of the cell charged The Resting Membrane Potential The Effect of the Na/K Pump Active Transport Pump requires carrier Active proteins for the movement of Na and K; also, requires an input of energy (ATP) requires Pumps 3 Na+ out of the cell (so more concentrated outside the cell) concentrated Pumps 2 K+ into the cell (more concentrated inside the cell) inside This is a major contributor to a cell’s resting This membrane potential membrane The Na/K Pump In neurons, contributions from this pump mechanism, help to maintain a rmp of -70mV Excitability/Irritability Excitability/Irritability Refers to neurons’ (and muscles’) ability to Refers alter their resting membrane potential in response to stimulation response They do so by altering their cell membrane They permeability to certain ions once they are stimulated stimulated Excitability/Irritability Excitability/Irritability If stimulation causes positive charges to If flow into the cell, creating a more positive internal environment than the rmp it is termed DEPOLARIZATION DEPOLARIZATION If stimulation causes a return to the rmp it If is termed REPOLARIZATION REPOLARIZATION If stimulation causes negative charges to If flow into the cell, creating a more negative internal environment than the rmp it is termed HYPERPOLARIZATION HYPERPOLARIZATION Action Potentials Action Impulses travel along the axon of a neuron to reach a second cell Depolarization of the axon membrane occurs as Na+ gates open and positive ions come rushing into the axon This causes an increase in the rmp from –70 mV to +30mV (threshold for neurons) along the axon membrane As depolarization occurs, a positive feedback loop is created as more channels open for Na+ Action Potentials (continued) Action An acceleration in depolarization occurs as these additional Na+ gates open and more sodium ions flow into the cell This phenomenon, or characteristic of the Na+ channels/gates, is referred to as voltage regulation Next, after a slight delay, K+ channels open, allowing K+ to flow out of the cell Action Potentials (continued) Action This movement of positive ions OUT of the cell works to bring the membrane potential back to rmp (­70mV) Recall, this is termed REPOLARIZATION Recall, this is termed These changes in the membrane potential constitute the Action Potential All Or None Law All Or None Law Statement of the fact that neurons transmit signals to their maximum extent when exposed to a stimulus of threshold strength The length of time the Na+ and K+ gates stay open is independent of the strength of the depolarization stimulus, but rather are dependent on the frequency of the stimulus As long as the threshold is reached (+30mV), the action potential will propagate in an a ll­ or­none fashion Conduction of Nerve Impulses Conduction of Nerve Impulses Depolarization of the axon membrane occurs once the threshold is reached This occurs as Na+ rush into the axon making the rmp more positive Na+ ions flow to adjacent regions of the axon that have a rmp of –70 mV Get a depolarization of the adjacent region as the threshold is reached there Conduction of Nerve Impulses Conduction of Nerve Impulses Action potential continues on as more voltage gated channels open down the length of the axon So, each action potential is produced by depolarization that results from the preceding action potential This means that each action potential is actually a completely different, individual event Conduction Down an Unmyelinated Axon Unmyelinated Action potentials are produced along the entire length of the axon The action potential is a repeating type of situation (like the concept of a ‘wave’ at a football game) Subsequent action potentials have the same amplitude as the first meaning, they are conducted without decrement Conduction is relatively slow b/c action potentials must be produced at every point along the axon’s length Therefore, there are actually more action potentials created down an unmyelinated axon than a myelinated axon Sequence down an unmyelinated axon: Conduction Down a Myelinated Axon Myelinated Myelin serves as insulation for the axon It prevents movement of Na+ or K+ in or out of the axon This explains why myelin is not present continuously along an axon’s length because A.P.s would not be able to travel along the axon due to myelin’s impermeability Conduction Down a Myelinated Axon Myelinated There are ‘interruptions’ along the axon’s length where the myelin sheath is broken = nodes of Ranvier These regions are high in their number of Na+ and K+ gated channels This means that action potentials only occur at the Nodes of Ranvier in myelinated axons Conduction Down a Myelinated Axon Myelinated The process of action potentials ‘jumping’ from node to node is called SALTATORY CONDUCTION But, even though it seems like the action potential is leaping or jumping, the same depolarization sequence is occurring The action potential at one node depolarizes the membrane at the next node to threshold and so on Sequence down a myelinated axon: The Synapse The The functional connection between a neuron and a second cell Neuromuscular junction (neuron­muscle synapse; myoneural junction) Neuron­Neuron synapses Axodendritic synapse (most common) Axoaxonic synapse Dendrodendritic synapse Presynaptic vs. Postsynaptic Neurons Presynaptic vs. Postsynaptic Neurons Presynaptic neuron before the synapse Postsynaptic neuron after the synapse Most commonly, the connection is between a presynaptic axon and a postsynaptic dendrite A space exists b/w the pre­ and postsynaptic structures Synaptic Cleft Synaptic Cleft Chemicals (neurotransmitters) are released here They stimulate action potentials in postsynaptic neurons/cells Therefore, transmission of impulses is actually more chemical than ‘electrical’ Chemical Synapses Chemical Synapses Presynaptic endings (called axon terminals) release neurotransmitters Axon terminals are separated from the postsynaptic cell by a synaptic cleft Neurotransmitter is contained in synaptic vesicles located in the presynaptic neuron These vesicles fuse w/ the axon membrane and, through exocytosis, release their contents into the synaptic cleft Chemical Synapses (continued) Chemical Synapses Fusion of the synaptic vesicles w/ the axon membrane, and release of neurotransmitter is initiated by an inward diffusion of Ca2+ that occurs as the action potential reaches the axon terminal The amount of neurotransmitter released depends on the FREQUENCY of the action potentials reaching the axon terminal, not the strength of the signal Chemical Synapses (continued) Chemical Synapses Once released, neurotransmitter crosses the cleft and enters the postsynaptic cell It does so by binding to receptor proteins in the postsynaptic membrane There is a high specificity between neurotransmitters and their receptor proteins This binding between neurotransmitter and receptor leads to opening of ion channels in the postsynaptic membrane Result of Ion Channel Opening Result If opening of the channels causes a depolarization of the post­synaptic cell membrane (movement closer to the threshold level), it is called an EPSP (excitatory postsynaptic potential) In this situation, the inside of the postsynaptic membrane becomes less negative and more likely to propagate an action potential Result of Ion Channel Opening Result If opening of channels causes a hyperpolarization of the post­synaptic cell membrane (movement away from the threshold), it is called an IPSP (inhibitory postsynaptic potential) In this situation, the inside of the postsynaptic membrane becomes more negative and less able to propagate an action potential Acetylcholine as a Acetylcholine as a Neurotransmitter In the CNS, it is used as an excitatory neurotransmitter Somatic motor neurons (neuromuscular junctions) also use it as an excitatory neurotransmitter Autonomic motor neurons can use ACh as either excitatory or inhibitory Variation is due to the type of receptor found at the postsynaptic cell ACh Receptors ACh Nicotinic ACh receptors get their name from the fact that they are also activated by the drug nicotine Muscarinic ACh receptors get their name from the fact that they are also activated by muscarine Mechanisms of ACh/Receptor Binding Mechanisms of ACh/Receptor Binding Ligand­Operated Channels Seen with nicotinic ACh receptors This is the most direct mechanism The ion channel runs through the receptor itself ACh binds to its receptor in the postsynaptic membrane Once ACh is bound, a channel opens through the receptor and allows for the inward diffusion of Na+ This causes depolarization of the postsynaptic membrane Constitutes an excitatory postsynaptic potential (EPSP). Ligand­Operated Channel Mechanisms of ACh/Receptor Binding Mechanisms of ACh/Receptor Binding G­protein­Operated Channels Seen with muscarine ACh receptors These receptors do not contain their own ion channels Instead, channels are found in a different location in the postsynaptic membrane ACh binds the muscarine receptor and this activates protein complexes known as G­ proteins The complex contains 3 subunits (alpha, beta, gamma) and the alpha subunit dissociates from the other two, leaving a beta­ gamma subunit G-Proteins (continued) G-Proteins Depending on the case, one of the subunits will diffuse through the membrane and bind an ion channel located somewhere nearby This activates the channel which causes it to open Allows for ion diffusion either into or out of the post­synaptic cell G-Protein Operated Channels G-Protein Acetylcholinesterase Acetylcholinesterase Enzyme present to inactivate ACh and control activity in the postsynaptic cell Present on the postsynaptic cell membrane Necessary to contain amounts of ACh and keep it from building up ...
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This note was uploaded on 10/10/2011 for the course ZOO 3733 taught by Professor Sa during the Spring '07 term at University of Central Florida.

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