Chapter 11 fundamental of nervous system and nervous tissue word doc notes PDF

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CHAPTER 11: FUNDAMENTALS OF NERVOUS SYSTEM AND NERVOUS TISSUE The Nervous System: Functions 1. Sensory input: monitors changes that occur both inside and outside the body 2. Integration: processing and interpretation of input information  nervous system decides what response to make 3. Motor output (motor response): nervous system activates effector organs to cause a response Cell types in the nervous system: 1. Neuroglia (glial cells) – provide support and maintenance to neurons 2. Neurons – nerve cells that can respond to stimuli and transmit electrical signals Components of the Nervous system: 1. The central nervous system (CNS) a. Composed of the brain and spinal cord b. Function: responsible for interpreting sensory input and deciding motor output 2. The peripheral nervous system (PNS) a. Composed of bundles of nerves coming from the brain/spinal cord b. Function: spinal nerves and cranial nerves link the rest of the body to the CNS c. Two subdivisions: i. Afferent division: carries impulses from the body to the central nervous system  impulse allow CNS to interpret info and send out a response ii. Efferent division: carries impulses from CNS to the effector organs  impulses activate muscle or glands to carry out motor response Microanatomy of Nervous System Tissue: Neuroglia Types of neuroglia: 1. Astrocytes (CNS): most abundant and versatile of the neuroglia (aka glial cells) a. star shaped, with projections connecting to and wrapping around neurons, synaptic nerve endings, and surrounding blood capillaries b. main functions: i. provide nutrient supply to neuron cells ii. allow migration of young neurons

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iii. clean up outside neurons cells 1. leaked K+ ions, neurotransmitter Microglial cells (CNS): a. Functions: i. Contact nearby neurons to monitor health ii. Migrate toward injured neurons, where they transform into a macrophage and phagocytize the neuron  important bc the immune system has limited access to the CNS Ependymal cells (CNS): a. most cells have cilia b. function: lines central cavities of CNS to circulate cerebrospinal fluid (CFS) within cavities Oligodendrocytes (CNS): a. Associated with thicker nerve fibers in CNS, wrapping around each fiber to produce a myelin sheath - a wrapping of myelin around certain nerve axons, serving as an electrical insulator that speeds nerve impulses to muscles and other effectors. b. Function: create an insulting covering for individual neurons of the CNS  important bc it allows for fast and efficient transmission of electrical impulses Satellite cells (PNS): support and protect neurons in the PNS, functionally similar to astrocytes Schwann Cells (PNS): a. Surround nerve fibers of PNS to form myelin sheaths b. Functionally similar to oligodendrocytes (CNS)

Microanatomy of Nervous System Tissue: Neurons Neurons: cells of the nervous system specialized to generate or transmit electrical signals (nerve impulses), longevity, amitotic, metabolism -

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Structure: o Cell body of neuron: portion of cell containing the nucleus  Function: plasma membrane can receive information from surrounding neurons  Most are found in the CNS protected by bone  Clusters of cell bodies in CNS are called nuclei, those in PNS are called ganglia Process of neuron: arm-like extensions from the cell body of all neurons o Two types:  Dendrites: main receptive region of neuron  A single neuron can have dozens of dendrites  Function: provide increased surface area for incoming signals, convey incoming messages toward the cell body

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Axons: single nerve fiber coming form a cell body that serves as the conduction region  the axon is the conducting region of the neuron  function: generates and transmits nerve impulses away from the cell body  bundles of axons in the CNS are called tracts, those in the PNS are called nerves  axons branch at the end to form terminal branches and axon terminals o function: neurotransmitter released at axon terminal to pass along the impulse to the next neuron Myelin Sheath: o Function: protects and electrically insulates long and/or larger nerve fibers, increases speed at which these impulses are transmitted o Found only on axon portion of the neuron o Not all axons are myelinated o Myelination in the PNS  Accomplished by schwann cells, which wrap themselves around the axon multiple times to create layers of insulation  Multiple schwann cells on the axon, but do not make contact with one another  Myelin sheath gaps o Myelination in the CNS  Accomplished by oligodendrocytes  A single oligodendrocyte can cover over 60+ axons with branching process

Functional classification of neurons: groups neurons according to direction in which nerve impulses travel relative to the CNS 1. Sensory (afferent) neuron: afferent neurons transmit signals from the body to the CNS a. Cell bodies found outside the CNS b. Receptive endings can as actual sensory structure, or are associated with larger sensory receptors (other cell types) 2. Motor (efferent) neuron: efferent neurons transmit motor response from CNS to the body a. Cell bodies found inside the CNS b. Impulses travel to effector organs (muscle + glands) 3. Interneuron: lie between sensory and motor neurons a. Function: pass signals through CNS pathways where integration occurs b. Mostly confined to only CNS, make up 99% of all neurons in the body

Membrane Potentials ** all cells have a resting membrane potential (approximately -70mV) o Inside of cell is more negatively charged than the outside of the cell - Neurons are able to change their resting membrane potential faster than other cell types o Without this quality the nervous system loses its function - Neuron communication occurs when the membrane potential changes - Important defintions: o Voltage: the measure of the potential energy by separate electrical charges (measured in V or mV)  The greater the difference in charge between two points, the higher the voltage  In the human body  difference in charge on either side of the plasma membrane creates voltage (also called potential) o Current: flow of electrical charge from one point to antoher  Can be used to do work  In the human body  currents reflect movenet of ions across a membrane along the axon o Resistance: hindrance of charge flow from substances through which charge must travel  In the human body  plasma membranes provide the resistance  Substances with high resistance  insulator  Substance with low resistance  conductor -

Changing resting membrane potential and production of communication signals - Changing the resting membrane potential allows a neuron to receive, integrate, and send information - Cause of change in resting membrane potential: o Alteration of ion concentrations on either side of membrane o Change in membrane permeability to 1+ ion** - Type of signal that can be produced by change: o Graded potential: have variable (graded) strength  Usually incoming signals over short distances o Action potential: have a constant strength  Long distance signals of axons - Ion channels and membrane potentials o Selective proteins in plasma membrane allow passage of ions into/out of cell (ion channels) o Types of proteins:  Leakage (non-gated) channels: always open, allow free flow of ions  Gated proteins: part of the protein forms a “gate” that must be opened before ions can move

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Chemically gated: only open when a certain chemical (neurotransmitter) binds to protein  Voltage gated: open and close in response to changing membrane potentials  Mechanically gated: open in response to physical deformation of receptor When gate channels open  ions move across membrane o What determines this direction of movement??  electrochemical gradients  Components of electrochemical gradients:  Concentration gradient: ions move from higher to lower concentration  Electrical gradient: ions move to area opposite of charge  **these components barely work together Change in membrane potential voltage due to opening of ion channels can result in: o Depolarization: decrease in membmrane potential  The inside of the membrane becomes less negative than resting potential  Potential (mV) becomes more positive  Function: depolarizing the membrane increases probability of producing a nerve impulse  Excitation of neuron o Hyperpolarization: increase in membrane potential  the inside of the membrane becomes more negative than resting potential  potential (mV) becomes more negative  fuction: hyperpolarizing a membrane decreases the probability of producing a nerve impulse  inhibits a neuron graded potentials o graded  magnitude varies directly with stimulus strength  strong stimulus = strong graded potential; therefore direct relationship o can be depolarizing or hyperpolarizing o types of graded potentials:  receptor potential: produced when sensory receptor is excited by its specific stimulus  postsynaptic potential: produced when stimulus is released by another neurotransmitter o graded potentials only occur over short distances  charge is lost quickly due to leaky channels  current dies off quickly as well  important function: graded potentials are necessary to initiate an action potential

Action potentials (nerve impulses) Action potentials (AP): a very brief reversal of membrane potential (from -70mV to around +30mV) - only produced by neurons and muscle cells - difference in graded potentials: o long distance, do not decay with distance, only occur at axons - action potentials are generated from graded potentials, originate at the beginning of axon coming from cell body (“trigger point”) o change in membrane potential from graded potential caused voltage-gated channels to open generating action potentials: - generation of an AP involves opening of voltage-gated ion channels in membrane in response to changing membrane potential o Na+ channel has two gates:  Activation gate: voltage-sensitive, opens at depolarization  Inactivation gate: blocks channel once it is open o K+ only has one gate that opens slowly at depolarization - The process: o All voltage gated channels are closed at the resting state of (-70mV)  Leakage channels still open here o Depolarization: voltage gated Na+ channels open  Na+ rushes into the cell, causing more surrounding gates to open  Inside of the cell becomes less negative  At the threshold voltage (-55mV), depolarization becomes self generating  Positive feedback mechanism  More Na+ channels open to make inside of cell significantly less negative (30 mV at its peak) o Repolarization: Na+ channels inactivated, K+ channels open  Na+ permeability drops rapidly  Net influx of Na+ into cell stops completely o This causes the AP to stop rising  Voltage gated K+ ions open at this point o K+ leaves the cell  restores (-) internal charge of cell o Hyperpolarization: excess K+ leaves cells  Result  inside of the cell becomes more negative than resting membrane potential  While this happens  Na+ activation channels have closed, inactivation gates reopen  Na+ - K+ pump works to re-establish normal Na+ and K+ concentrations outside and inside the cell Action potentials: Reaching threshold

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**not all depolarizing events will result in AP Depolarization must reach a “threshold point” to generate an AP o Threshold value = -55mV Aps are an “all or none” event: if threshold is not reached, an AP does not occur

Propagation of action potentials - AP beings at trigger point, but must travel the entire length of the axon  how is this accomplished? o Na+ influx causes local current  This change causes depolarization of adjacent areas of membrane  Na+ channels open in these new areas  Na+ channels nearer the AP origin are inactivated  no new AP can be generated here  ** this causes an AP to always propagate away from the origin  Action potentials are unidirectional Action potentials: stimulus strength - Aps are independent of stimulus strength, are all alike - If this is the case  how does the nervous system discriminate between a strong stimulus and weak stimulus? o How frequent nerve impulses are generated!  Strong stimuli: impulses are sent more frequently in a given time period  Weak stimuli: impulses sent less frequently in a given time period Action Potentials: Refractory Periods Refractory period: a period of time in which a second AP cannot be generated at an axon - Occurs when Na+ voltage gated channels are open - Types: o Absolute refractory period:  Begins when Na+ gated channels open, continues until Na+ channels reset to their original state  During this time, another AP cannot be generated in the area, no matter how strong the stimulus is  Importance:  Ensures each AP is a separate all or none event  Enforces one-way transmission of the AP o Relative refractory period:  Follows the absolute refractory period  Occurs after depolarization  Stimuli that are relatively weak cannot stimulate an AP, but an exceptionally strong stimulus can  Why? Hyperpolarization causes mV to be more negative  need stronger stimulus to reach threshold Action Potentials: Conducton speed

Impulses can be conducted quickly (ex: postural changes) or more slowly (ex: Gi tract, etc) - Speed is dependent on two main factors: o Axon diameter: larger axon = faster conduction o Degree of myelination: more myelination = faster conduction - Myelin sheaths influence conduction speed in the following ways: o Continuous conduction: propagation in unmyelinated fibers  Voltage gated ion channels are adjacent  Occurs slowly o Saltatory conduction: propagation in myelinated fibers  Voltage gated ion channels found in myelin sheath gaps  AP can only be generated in these gaps  Electrical signals “jump” from gap to gap Action Potentials: Transmission of signals - Signals are transmitted between neurons at synapses o Synapse: junction between two neurons that sends information from one neuron to the next - Presynaptic neurons conduct impulses toward the synapse - Postsynaptic neurons conduct signal away from the synapse - Neurons are separated by synaptic clef  fluid filled space - Two types of synapses: o Electrical syanpses: transmit signals very quickly  Have gap juntions that connect cytoplasm of one neuron to another, allowing ions and small molecules to travel directly between them  Found in regions of the brain with stereotyped movements  Are more common in embryos than in adults o Chemical sysapses: allow release and reception of neurotransmitters  Most common synapse type in the body  Presynaptic neuron releases neurotransmitter at the axon terminal via synaptic vesicles  Postsynaptic neuron has receptor region (dendrite/body cell) that sense neurotransmitter Transmission of action potential from one neuron to antoher The process: 1. Action potential arrives at axon terminal of presynaptic neuron 2. Voltage gated Ca2+ channels open in response to AP a. Ca2+ flows into the axon terminal of presynaptic neuron 3. Synaptic vesicles fuse with membrane in response to Ca2+ influx a. Neurotransmitter enters the synaptic cleft 4. Neurotransmitter crosses cleft, binds to proteins on postsynaptic cleft -

Postsynaptic Potentials - Neurotransmitter binding cause graded potentials that vary in strength according to:

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o Amount of neurotransmitter released o How long neurotransmitter stays in synaptic cleft Chemical synapses can be either excitatory or inhibitory o Depends mostly on how membrane potential is affected Excitatory synapses and EPSPs o If enough neurotransmitter is bound to postsynaptic neuron, an excitatory postsynaptic potential (EPSP) occurs at postsynaptic membrane  Depolarization of postsynaptic membrane occurs o EPSP triggers AP at the beginning of the axon  If current reaching this portion of the axon is at the threshold, an AP is generated Inhibitory synapses and IPSPs o Reduces ability to generate an AP by hyperpolarizing the postsynaptic membrane  K+ channels or Cl- channels are opened, making inside of cell more negative  Pushes membrane potential farther from the threshold value o This results in inhibitory postsynaptic potentials (IPSP)

Integration and Modification of Synaptic Events - A single EPSP cannot induce AP alone o Several EPSPs can summate to influence postsynaptic neuron to generate AP - Most neurons will receive both excitatory and inhibitory input from 1000+ other neurons o So what would be the response?  Whichever signal (EPSP or IPSP) is stronger will have greater influence on postsynaptic neuron  If EPSP is predominate  AP is generated Integration & Modification of Synaptic Events: Summation - Two types of summation: o Temporal summation: One (or more) presynaptic neurons transmit impules in rapid fire order o Spatial summation: postsynaptic neuron stimulated by a large number of terminals on multiple dendrites at the same time Neurotransmitters: Effects - Neurotransmitters: chemical signals produced in the cell body, moves into axon via anterograde movement - Most neurons produce at least two types o Neurons can release one or more neurotransmitters simultaneously - Effects o Function effects: can be excitatory, inhibitory, or can do either depending on the receptor type they bind o Action effects: can be direct or indirect

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Direct neurotransmitters cause opening of ion channels directly Indirect neurotransmitters have longer lasting effects, act through second messenger molecules  Can increase likelihood direct neurtransmitters can bind

Neurotransmitters: Types - Acetylcholine: released at neuromuscular junctions o Function: mostly stimule skeletal muscle and neurons in autonomic nervous system (ANS) o Formed from acetic acid and choline o Degraded quickly by enzyme acetylcholinesterase (AChE) - Biogenic Amines o Includes dopamine, norepinephrine, ephinphrine, serotonin, histamin  All synthesized from various amino acids o Functions: mostly involved in emotional behavior and regulates biological clock o Imbalances in some are often associated with mental illness  Ex: schizophrenia, hallucinations (with drug use) - Amino acids o Glutamate (excitatory) o Asparate (excitatory) o Glycine (inhibitory) o Gamma-aminobutyric acid (GABA) (inhibitory) - Peptides o Strings of amino acids that take on a broad spectrum of molecules, all of which have various effects  Substance P: mediator for pain signaling  Endorphins: reduce pain perception (opiates) - Purines: one type of nitrogen-containing base that make up DNA and RNA o ATP  More primitive neurotransmitter found in CNS and PNS  Can produce fast and slow responses o Adenosine  Inhibitor in the brain  Caffeine blocks adenosine receptors  prevents inhibition o Gases and Lipids  Nitric oxide, carbon monoxide, hydrogen sulfide  Functions: NO participates in formation of new memories by increasing synapse strength in the brain  Not stored in vesicles, but are produced by cells on demand  Do not attach to receptors, but pass right through membrane of nearby cells, bind to intracellular receptors - Endocannabinoids

o Act at same receptors as THC (active ingredient in cannabis plant) o Synthesized on demand, not stored in vesicles o Fuctions: involved in neuronal development, controls appetite, suppresses nausea, play role in learning/memory Neurotransmitter Receptors - Two types: o Channel linked receptors: mediate fast synaptic transmission  Ligand-gated ion channels  When ligand binds  protein changes shape and channels open o Na+ influx  depolarization o Cl- influx  hyperpolarization  Action is immediate, but brief o G – protein coupled receptors  Contains transmembrane protein complexes  Response is indirect, complex, prolonged  Effects tend to include widespread metabolic changes in postsynaptic cell  General process  Neurotransmitter binds to receptor  G-protein activated  G-protein activated adenylate cyclase  second messengers are produced o Second messengers can activate certain genes to produce proteins, can open/close ion channels, etc...