The Nervous System PDF - PHED 2217

No class Monday The Nervous System PHED 2217: Systematic Approach to Integrated Human Physiology General Functions of the Nervous System Collect Information Process and Evaluate Information Initiate Response to Information Or...

No class Monday The Nervous System PHED 2217: Systematic Approach to Integrated Human Physiology General Functions of the Nervous System Collect Information Process and Evaluate Information Initiate Response to Information Organization of the Nervous System Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Structural organization Functional organization Brain Sensory nervous system Motor nervous system Central detects stimuli and initiates and transmits nervous transmits information from information from the CNS system (CNS) Spinal cord receptors to the CNS to effectors Nerves Peripheral Somatic sensory Visceral sensory Somatic motor Autonomic motor nervous system (PNS) Ganglia Sensory input Sensory input that Motor output Motor output that is that is consciously is not consciously that is consciously not consciously or perceived from perceived from or voluntarily is involuntarily receptors (e.g., blood vessels and controlled; effector controlled; effectors eyes, skin, ears) internal organs is skeletal muscle are cardiac muscle, (e.g., heart) smooth muscle, and glands Synapses Synapse Where neuron functionally connected to neuron or effector Two types: chemical and electrical Synapses Chemical synapse Most common Composed of presynaptic neuron, signal producer Composed of postsynaptic neuron, signal receiver Between axon and any portion of postsynaptic neuron most commonly with a dendrite Knob almost touches the postsynaptic neuron narrow fluid filled gap, the synaptic cleft Transmission at Chemical Synapse Neurotransmitter molecules released from synaptic knob Released from synaptic vesicles into cleft Diffusion of neurotransmitter across cleft Binding of some neurotransmitters to receptors Synaptic delay time between neurotransmitter release and binding Single postsynaptic neuron often stimulated by more than one neuron Transmission at Electrical Synapse Much less common Presynaptic and postsynaptic neuron physically bound together Gap junctions present No delay in passing electrical signal In limited regions of brain and eyes Nervous Tissue Two cell types in nervous tissue Glial cells nonexcitable cells that primarily support and protect neurons Neurons basic structural unit of the nervous system excitable cells that transmit electrical signals Nervous Tissue: Glial Cells General Characteristics Nonexcitable cells found in CNS and PNS Smaller than neurons Far outnumber neurons Half volume of nervous system Physically protect and nourish neurons Provide physical scaffolding for nervous tissue help guide migrating neurons to their destination Critical for normal function at neural synapses Glial Cells: Oligodendrocytes Large cells with slender extensions Processes ensheathing portions of axons of different neurons Processes repeatedly wrapping around axon Insulate axons in a myelin sheath Prevent passage of ions through axonal membrane Allow for faster action potential propagation through CNS Glial Cells: Nerolemmocytes Also known as Schwann cells Ensheathe PNS axons to form myelin sheath Allows for faster action potential propagation Nervous Tissue—Glial Cells: Myelination Myelination Process by which part of an axon wrapped in myelin Myelin, insulating covering around axon consists of repeating layers of glial cell plasma membrane has high proportion of lipids gives glossy appearance and insulates axon Completed by neurolemmocytes (PNS) Completed by oligodendrocytes (CNS) Nervous Tissue—Glial Cells: Myelination 3 The overlapping Neurolemmocyte inner layers of the 1 starts to wrap around neurolemmocyte a portion of an axon. plasma membrane Axon form the myelin sheath. Cytoplasm of the neurolemmocyte Neurolemmocyte Myelin sheath Nucleus Direction of wrapping 4 Eventually, the neurolemmocyte cytoplasm and nucleus are pushed 2 Neurolemmocyte to the periphery of cytoplasm and the cell as the myelin plasma membrane sheath is formed. begin to form consecutive layers around the axon as Myelin sheath wrapping continues. Neurolemmocyte nucleus Neurilemma Nervous Tissue—Glial Cells: Myelination Unmyelinated axons Associated with neurolemmocytes No myelin sheath covers them Axon in depressed portion of neurolemmocyte Not wrapped in repeated layers Nervous Tissue: Neurons Two cell types in nervous tissue Glial cells nonexcitable cells that primarily support and protect neurons Neurons basic structural unit of the nervous system excitable cells that transmit electrical signals Special Characteristics of Neurons Excitability responsive to stimulation type dependent on its location Conductivity electrical charges propagated along membrane can be local and short-lived or self-propagating Secretion release neurotransmitters in response to electrical charges given neuron releasing only one type of neurotransmitter Extreme longevity most formed before birth still present in advanced age Amitotic mitotic activity lost in most neurons not always the case (e.g., occasionally in hippocampus) Components of Neurons Cell body enclosed by plasma membrane contains cytoplasm surrounding a nucleus neuron’s control center conducts electrical signals to axon Components of Neurons Dendrites short processes branching off cell body may have one or many receive input and transfer it to cell body more dendrites = more input possible Components of Neurons Axon makes contact with other neurons, muscle cells, or glands first part, a triangular region, axon hillock Cytoplasm → axoplasm plasma membrane → axolemma at extreme tips, expanded regions, synaptic knobs knobs containing numerous synaptic vesicles contain neurotransmitter Functional Classification of Neurons Functional classification Sensory neurons (afferent neurons) neurons of the sensory nervous system conduct input from somatic and visceral receptors most unipolar, few bipolar cell bodies usually in posterior root ganglia, outside CNS Motor neurons (efferent neurons) neurons of the motor nervous system conduct motor output to somatic and visceral effectors all multipolar most cell bodies in CNS Functional Classification of Neurons Functional classification (continued) Interneurons (association neurons) entirely within the CNS receive stimulation from many other neurons receive, process, and store information “decide” how body responds to stimuli facilitate communication between sensory and motor neurons 99% of neurons generally multipolar Functional Classification of Neurons Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Posterior root ganglion Spinal cord Cell body of sensory Sensory neuron input Skin receptors Sensory neuron Motor Interneuron output Motor neuron Skeletal muscle  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Plasma membrane of entire neuron Receptive segment Chemically Chemically Chemically gated cation gated K+ gated Cl– channel channel channel (b) Cell body Dendrites Initial segment Voltage-gated Voltage-gated Axon hillock Na channel + K+ channel Na+/K+ Na+ leak K+ leak (c) pump channel channel Conductive segment Voltage-gated Voltage-gated Na channel + K+ channel Entire neuron Axon (d) Transmissive segment Voltage-gated Ca2+ pump Ca2+ channel (a) Synaptic bulb (e) Receptive Segment Reception of neurotransmitter triggers postsynaptic potential Neurotransmitter binds to chemically gated ion channels and opens them Ions diffuse across membrane changing its electrical potential The voltage change is a graded potential: it can vary in size (from a few mV to many mV) It is a local potential: it starts at dendrites or soma and does not go far The direction of the potential depends on what type of ion channel opens If Na+ channels open, Na+ diffuses in and membrane becomes less negative If Cl– channels open, Cl– diffuses in and membrane becomes more negative If K+ channels open, K+ diffuses out and membrane becomes more negative When a cell is less negative than RMP it is depolarized; when it is more negative it is hyperpolarized It is a short-lived potential (lasting only milliseconds) Postsynaptic Potentials in the Receptive Segment Excitatory postsynaptic potentials (EPSPs) are depolarizations caused by cation entry Postsynaptic Potentials in the Receptive Segment Inhibitory postsynaptic potentials (IPSPs) are hyperpolarizations caused by cation exit or anion entry Several Presynaptic Neurons with a Postsynaptic Neuron Initial Segment Summation of EPSPs and IPSPs occurs at axon hillock Voltage changes from the dendrites and soma are added The sum may or may not reach threshold membrane potential for initiating an action potential Threshold is the minimum voltage change required Typically, threshold is about –55 mV Generally, multiple EPSPs must be added to reach threshold If threshold is reached at the axon hillock (initial segment) Voltage-gated channels open, and an action potential is generated Initial Segment Summation occurs across space and time Spatial summation Multiple locations on cell’s receptive regions receive neurotransmitter simultaneously and generate postsynaptic potentials Temporal summation A single presynaptic neuron repeatedly releases neurotransmitter and produces multiple EPSPs within a very short period of time Conductive Segment The axon conducts action potentials Action potential involves depolarization and repolarization Depolarization is gain of positive charge as Na+ enters through voltage-gated Na+ channels Repolarization is return to negative potential as K+ exits through voltage-gated K+ channels Action potential is propagated down axon to synaptic knob Voltage-gated channels open sequentially down axolemma Propagation is called an impulse or nerve signal Generation of an Action Potential—Depolarization Action potential: steps in depolarization 1. At RMP, voltage-gated channels are closed 2. As Na+ enters from adjacent region, voltage-gated Na+ channels open Generation of an Action Potential - Depolarization Action potential: steps in depolarization (continued ) 3. Na+ enters the axon causing the membrane to have a positive potential 4. Na+ channels close becoming inactive (unable to open) for a time Steps 1–4 repeat in adjacent regions and the impulse moves toward synaptic knob Generation of an Action Potential - Repolarization Action potential: steps in repolarization 5. Depolarization slowly opens K+ channels, and K+ diffuses out, causing negative membrane potential 6. K+ channels stay open for a longer time, so K+ exit makes cell more negative than RMP Generation of an Action Potential--Repolarization Action potential: steps in repolarization (continued ) 7. K+ channels eventually close and RMP is reestablished Steps 5–7 repeat in adjacent regions as the impulse moves toward synaptic knob Events of an Action Potential ON MIDTERM Know this diagram: be able to label and describe processes. Where chemically gated, inhibitory or excitatory, depolarize, depolarize, hyper polarize, what ion channels open Conductive Segment Refractory period Period of time after start of action potential when it is impossible or difficult to fire another action potential Absolute refractory period (about 1 ms) No stimulus can initiate another action potential Na+ channels are open, then inactivated Ensures propagation goes toward synaptic knob; doesn’t reverse direction Relative refractory period ( just after absolute) Another action potential is possible (Na+ channels have reset) but the minimum stimulus strength is now greater Some K+ channels are still open; cell is slightly hyperpolarized and further from threshold Conductive Segment Continuous vs. saltatory conduction Continuous conduction occurs on unmyelinated axons Charge opens voltage-gated channels, which allows charge to enter, which spreads to adjacent region and opens more channels, sequentially Conductive Segment Continuous vs. saltatory conduction (continued) Saltatory conduction occurs on myelinated axons Action potential occurs only at neurofibril nodes, which is where the axon’s voltage-gated channels are concentrated After Na+ enters at a node it starts a rapid positive current down the inside of the axon’s myelinated region The current becomes weaker with distance, but still strong enough to open voltage-gated channels at the next node Full action potential occurs at the node, and the process repeats: impulse seemingly jumping from node to node Saltatory conduction is much faster than continuous conduction and myelinated cells use less ATP to maintain resting membrane potential Transmissive Segment Activity at the synaptic knob Arrival of action potential opens voltage-gated Ca2+ channels Ca2+ diffuses into knob (Ca2+ pumps had established gradient) Ca2+ binds to proteins associated with synaptic vesicles and triggers exocytosis Vesicles fuse with membrane and neurotransmitter released into cleft (Subsequently, transmitter binds to postsynaptic receptors) Historically, it was believed that one neuron releases only one type of transmitter, but recent research indicates more options are possible Events of Neuron Physiology Events of Neuron Physiology Events of Neuron Physiology Events of Neuron Physiology Graded Potentials Versus Action Potentials Graded potentials Occur in neuron’s receptive region due to ion flow through chemically gated channels Can be positive or negative changes in charge Are graded: have larger potential change to stronger stimulus Are local (travel only a short distance) Action potentials Occur on neuron’s conductive region (axon) due to ion flow through voltage- gated channels Involve depolarization (Na+ in) then repolarization (K+ out) Are all or none once threshold is reached Propagate down entire axon to synaptic knob Velocity of Action Potential Propagation Conduction speed depends on axon thickness and myelination Thicker fibers conduct faster than thin ones Thick axons offer less resistance to current flow down the axon Myelinated fibers conduct much faster than unmyelinated ones Current flow under myelin (between nodes) is very fast Velocity of Action Potential Propagation Nerve fiber groups Nerve fiber: an axon and its myelin sheath Group A: conduction velocity as fast as 150 m/sec Large diameter, myelinated fibers E.g., most somatic sensory neurons; all somatic motor neurons Group B conducts at 15 m/sec; Group C at 1 m/sec Small diameter and/or unmyelinated E.g., some visceral neurons; some somatic sensory neurons from skin Frequency of Action Potentials Impulse frequency (action potentials per second) varies Although impulse amplitude is constant, firing frequency varies with stimulus strength (up to a max) Bright lights cause faster firing frequency on the optic nerve than dim lights When motor nerves fire at faster frequency it causes muscle to generate more tension For neurons that can use different transmitter, the firing frequency can influence the type of transmitter released Resume here from last lec Classification of Neurotransmitters What are neurotransmitters? Small organic compounds synthesized by neurons Stored in vesicles in synaptic knobs Released by exocytosis when action potential triggers calcium entry into knob Trigger a physiologic response in target cell Approximately 100 different ones have been named and classified into groups Classification of Neurotransmitters Four main chemical classes of neurotransmitters Acetylcholine Structure differs substantially from other transmitters Biogenic amines (monoamines) An amino acid is slightly modified to synthesize the transmitter Catecholamines (e.g., dopamine) are made from tyrosine Indolamines (e.g., seratonin) are made from histidine or tryptophan Amino acids Include common transmitters glutamate, glycine, GABA Neuropeptides Chains of amino acids (2 to 40 amino acids long) including endorphins, substance P Classification of Neurotransmitters Neurotransmitters are also classified by function Classes by effect Excitatory transmitters cause EPSPs; inhibitory transmitters cause IPSPs But some transmitters can excite some targets and inhibit others depending on target cells’ receptors Classes by action Direct transmitters bind to receptors that are chemically gated channels (immediate postsynaptic potential) Indirect transmitters bind to receptors that involve G-proteins and second messengers in order to cause postsynaptic potential Features of Neurotransmitters Acetylcholine (ACh) is the best characterized transmitter Used in PNS to stimulate skeletal muscle; used in the CNS to increase arousal Synthesized from acetate and choline; stored in synaptic vesicles Action potential triggers its release into cleft Some ACh attaches (briefly) to postsynaptic receptor ACh is cleared from cleft by being broken down to acetate and choline by acetylcholinesterase Acetate and choline are taken up by presynaptic cell for recycling Features of Neurotransmitters Removal of neurotransmitter from the synaptic cleft can occur in varied ways Enzymes might degrade transmitter Presynaptic transporters might import transmitter (“reuptake”) Some transmitter diffuses away from synapse, reabsorbed by glia Some drugs have their effect by influencing transmitter removal E.g., selective serotonin reuptake inhibitors treat depression E.g., galantamine hydrobromide is an acetylcholinesterase inhibitor used to treat Alzheimer disease Ach Release, Removal from Synaptic Cleft, and Action ACh effect on target cell depends on receptor - Nicotinic receptor directly causes EPSP - Muscarinic receptor engages G protein and 2nd messenger o Indirectly leads to either EPSP or IPSP Neuromodulation Neuromodulators—chemicals that alter responses of local neurons Modulation Facilitation Modulation that causes greater response in postsynaptic neuron May increase amount of neurotransmitter in cleft or number of postsynaptic receptors Inhibition Modulation that causes weaker response May decrease amount of neurotransmitter in cleft or number of postsynaptic receptors Neuromodulation Nitric oxide Might be a transmitter or a modulator Is a short-lived, nonpolar gas Made and released by postsynaptic neurons in brain where it is believed to strengthen memory by affecting presynaptic cells Effects in the PNS include blood vessel dilation Endocannabinoids Influence same receptors that marijuana does Small, nonpolar molecules Made and released by postsynaptic neurons Have effects on presynaptic transmitter release Influence memory, appetite Questions?