Nervous System Fundamentals PDF

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PET3322 24 September 2020 Lecture 6- Nervous System Fundamentals Nervous System- Functions  Master controlling and communicating system o Cells communicate via chemical and electrical signals  Important elements: calcium, potassium, sodium  Generate electrical signal from nerve cell  Rapid and specific  Usually cause immediate response  Three overlapping functions o Sensory input: info gathered by receptors about internal and external changes o Integration: processing and interpretation of input  In the brain- CNS o Motor output: activation of effector organs produces response Organization**  Central nervous system (CNS) o Contains brain and spinal cord o Integrative and control centers  Peripheral nervous system (PNS) o Cranial nerves and spinal nerves o Communication lines between CNS and rest of the body  PNS two parts o Sensory (afferent) division  Somatic and visceral sensory nerve fibers  Conducts impulses from receptors to CNS o Motor (efferent) division  Motor nerve fibers  Conducts impulses from CNS to muscles and glands  Two types  Somatic nervous system o Voluntary o Conducts impulses from CNS to skeletal muscle  Autonomic nervous system o Involuntary o Conducts CNS impulses to cardiac muscle, smooth muscle, and glands o Two types  Sympathetic division  Mobilizes body systems during activity  “Fight or flight”  Parasympathetic division

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Conserves energy Promotes house-keeping functions during rest

Neurons  Structural units of nervous system (nerve cells)  Large/long, highly specialized, conduct impulses  Characteristics o Extreme longevity- last a person’s lifetime o High metabolic rate- requires continuous supply of oxygen and glucose  All have cell body and one or more processes Structure of Neurons  Dendrites: receptive regions, branched out from nucleus  Cell body: biosynthetic center and receptive region  Chromatophilic substance: rough ER  Axon: impulse generating and conducting region o Long, can be over 1m  Myelin sheath gap: “node of Ranvier” o Protective, check point  Axon terminals: secretory region Neurons- Cell Body  Biosynthetic center of neuron o Synthesizes proteins, membranes, chemicals o Rough ER- chromatophilic substance or Nissl bodies  Contains spherical nucleus with nucleolus  Some contain pigments o Helps to protect cells  Plasma membrane is part of receptive region o Receive input info from other neurons  Most neuron cells bodies are all located in CNS o Nuclei: cluster of neuron cell bodies in CNS o Ganglia: cluster of neuron cells bodies in PNS Neuron Processes  Arm-like extension from cell body o CNS contains both neuron cell bodies and their processes o PNS contains mainly neuron processes  Tracts: bundles of neuron processes in CNS  Nerves: bundles of neuron processes in PNS  Two types of processes o Dendrite- more important in CNS o Axon- PNS  Dendrites o Motor neurons can contain 100s of short, tapering, diffusely branched processes  Contain same organelles as cell body o Receptive region- input

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o Convey incoming messages toward cell body as graded potential (short distance signals) o In brain, finer dendrites are highly specialized  Contain dendrite spines, appendages with bulbous or spiky ends Axon o Structure Each neuron has one axon that starts at cone-shaped area called axon hillock  Some neurons- axons are short or absent  Others- axon comprises almost entire length  Have occasional branches called axon collaterals  Branch profusely at their end (terminus)  Can number as many as 10,000 terminal branches  Distal endings are called axon terminals o Functional characteristics  Conducting region of neuron  Generates nerve impulses and transmits them along axolemma (membrane) to axon terminal  Terminal: secretes neurotransmitters into extracellular space  Can excite or inhibit neurons it contacts  Carries on many conversations with different neurons at the same time  Rely on cell bodies to renew proteins and membranes  Quickly delay if damaged o Axonal transport  Axons have efficient internal transport mechanisms  Molecules and organelles are moved along axons by motor proteins and cytoskeletal elements  Movement occurs in both directions  Anterograde: away from cell body o Ex- mitochondria, enzymes,  Retrograde: toward cell body o Ex- organelles to be degraded, signal molecules, viruses, toxins o Myelin Sheath (CNS)  Formed by oligodendrocytes  Can coil around 60 axons at once  Gap is present- important for check point  Insultation and protection of axon  Thin fibers are unmyelinated  Do not have myelin sheath  White matter: dense collection of myelinated fibers in brain/spinal cord  Gray matter: contains neuron cell bodies and nonmyelinated fibers o Myelin Sheath (PNS)  Whitish, fatty segmented sheath surrounding most long or larger-diameter axons  Functions:

 Protect and electrically insulate axon  Increases speed of nerve impulse transmission  Nonmyelinated fibers do not contain sheath  Conduct impulses more slowly  Formed by Schwann cells- PNS only  One cell forms one segment of sheath  Plasma membranes have less protein o Makes good electrical insulator  Myelin sheath gaps- nodes of Ranvier o Gaps between adjacent Schwann cells  Nonmyelinated fibers o One Schwann cell may surround 15 thin fibers Neuron- membrane potential  Difference in electrical potential inside (ICF) and outside (ECF) of cell  Neurons can rapidly change membrane potential o Highly excitable o Unlike most other cells  Opposite charges are attracted to each other o Energy is required to keep opposite charges separated across membrane o Energy is released when opposite charges come together  Basic principles of electricity o Voltage (V): measure of potential energy o Current (I): flow of electrical charge between two points o Resistance (R): limit to charge flow o Ohm’s law: relationship of V, I, R  I = V/R  Current is directly proportional to voltage  Current is inversely proportional to resistance  Roles of membrane ion channels o Large proteins serve as selective membrane ion channels  Sodium and potassium  K+ ion channel only allows K+ to pass o Two types of ion channels  Leakage- always open, ions move down gradient  Ex- high conc of Na+ outside cell, tries to move inside  Gated- protein changes shape to open/close channel  Membrane potential for a neuron o ~ -70mV o Inside neuron is negatively charged relative to outside o Polarized membrane  Resting o Potential generated by:  Differences in ionic composition of ICF and ECF  High Na+ conc in extracellular fluid o Wants to move into the cell

 High K+ conc in intracellular fluid Differences in plasma membrane permeability  Large proteins cannot pass  Slightly permeable to Na+  25x more permeable to K+ than Na+ o Why inside is more negative o Easier for K+ to leave  Sodium-potassium pump (ATPase) stabilizes resting mem potential  Maintains conc gradients for Na+ and K+  Three Na+ pumped out for two K+ pumped in  ATPase used for substance transport (active) o Need to go against conc gradient Changes to resting o Used as signals to receive, integrate, and send info o Changes occur when  Conc of ions across membrane change  Na+ and K+  Membrane permeability to ions change o Produce two types of signal  Graded potentials  Incoming signals operating over short distances  Localized  Happens from dendrite  Action potentials  Long-distance signals of axons o Depolarization: decrease in membrane potential  Moves toward zero and above  Inside membrane becomes less negative than resting mem potential  -70  -60  Probability of producing impulse increases o Hyperpolarization: increase in membrane potential  Away from zero  Inside of membrane becomes more negative than resting mem potential  -70  -75  Probability of producing impulse decreases o Graded potential  Short-lived, localized chains  Receptive region of dendrites  Results in depolarization  Sometimes hyper  Magnitude varies directly with strength of stimulus  Stronger stimulus- more voltage changes  Farther current flows  Triggered by stimulus that opens gated ion channels  Named according to location and function  Receptor potential: in receptors of sensory neurons 

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 Postsynaptic potential: neuron graded potential o Action potentials  Principle way neurons send signals  Long distance (in axon)  More important in PNS  Occur only in muscle cells and axons  In neurons- referred to as nerve impulse  Do not decay over distance like graded potential  Finishes signal at axon terminal  Involves opening of specific voltage-gated channels  Brief reversal of mem potential with change in voltage of 100mV  -70  30  Generation  Resting state- gated channels are closed  Depolarization- Na+ channels open o Ion change o Happens everywhere in neuron  Repolarization- Na+ channels inactivate, K+ channels open o Back to normal o Resets electrical conditions o Goes back to -70mV  Hyperpolarization- some K+ channels remain open o Restores ionic conditions  Propagation  Allows AP to be transmitted entire length of neuron o Origin (dendrite)  terminals  Happens in axon  Only uses electrical signal  Na+ influx through voltage gates in one membrane area cause local currents that cause opening of Na+ voltage gates in adjacent membrane areas o Leads to depolarization in one area  depolarization in next  Once initiated, AP is self-propagated o In nonmyelinated axons, successive segments on membrane depolarize, then repolarize  AP occurs only in a forward direction Neuron- synapse  Nervous system works b/c info flows from neuron to neuron  Neurons are connected by synapses- junctions that mediate info transfer o Neuron to neuron o Neuron to effector cell (skeletal muscle)  Presynaptic: conducting impulses toward synapse o Sends info  Postsynaptic: transmits electrical signal away from synapse

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o Receives info o In PNS, may be nerve cell, gland Chemical synapse o Most common type o Specialized for release and reception of chemical neurotransmitters o Typically composed of two parts  Axon terminal of presynaptic neuron  Contains synaptic vesicles filled with neurotransmitter o Exocytosis  Receptor region on postsynaptic neuron  Receives neurotransmitter o Usually on dendrite or cell body  Two parts separated by fluid filled synaptic cleft o Electrical impulse changed to chemical across synapse, then back to electrical  Energy transfer o Synaptic cleft transmission (neuron to neuron)  Synaptic cleft prevents nerve impulses from directly passing from one neuron to next  Depends on release, diffusion, receptor binding of neurotransmitters  Ensures unidirectional communication  Stages (6)  AP arrives at axon terminal of presynaptic neuron  Voltage gated calcium channels open, Ca2+ enter axon terminal  Ca2+ entry causes synaptic vesicles to release neurotransmitter  Neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on postsynaptic membrane  Binding of neurotransmitters opens ion channels o Creates graded potentials  Neurotransmitter effects are terminated o Neurotransmitters  Language of nervous system (more than 50 types of neurotransmitters)  Most neurons make two or more neurotransmitters  Neurons can exert several influences  Usually released at different stimulation frequencies  Action potential can be strong or weak  Classified by  Structure o Acetylcholine  First identified and best understood  Most well known  For skeletal muscle contraction  Released at neuromuscular junctions  Also used by many ANS neurons and some CNS  Synthesized from acetic acid and choline  Enzyme = choline acetyltransferase

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 Degraded by enzyme = acetylcholinesterase o Biogenic amines  Catecholamines  Dopamine, epinephrine, norepinephrine (NE)  Important for brain function  Increase heart rate and blood pressure  Indolamines  Serotonin, histamine  All widely used in brain: play roles in emotional behaviors and biological clock (wake-up)  Used by some ANS motor neurons  Especially NE  Imbalances are associated with mental illness o Peptides (neuropeptides)  Strings of amino acids that have diverse functions  Can easily pass through membrane with fluid (pinocytosis)  Substance P  Mediator of pain signals  Endorphins  Act as natural opiates  reduce pain perception  Endogenous  Gut-brain peptides  Play a role in regulating digestion  Feeling of satiety and hunger Function o Two groups  Effects: excitatory vs inhibitory  Effect determined by receptor to which it binds o GABA and glycine- inhibit o Glutamine- excite o Acetylcholine and NE bind to two receptor types with opposite effects  Actions: direct vs indirect  Direct- rapid response o Ach and amino acids  Indirect- long lasting o Act through intracellular second messengers (G-protein pathways) o Similar to hormones o Biogenic amines, neuropeptides...