Chapter 7

Chapter 7

The Nervous SystemNeurons and Synapses

Cell Body• Enlarged part of the cell that containsnucleus• Nutritional center of the neuron• Cell bodies clustered in groups• CNS – nuclei• PNS - ganglia

Dendrites• Thin, branched processes that extendfrom the cytoplasm of the cell body• Pick up electrical impulses and transmitto the cell body

Axon• Conducts impulses away from the cell body• Longer than dendrites• Vary in length• 1mm to over 1 m• Axon hillock• Beginning of axon near cell body• Where nerve impulses originate• Axon collaterals may extend from axon

Classification of neuronsand nerves• Classified according to structure or function1. Sensory (afferent) neurons• Conduct impulses from sensory receptors to CNS2. Motor (efferent) neurons• Conduct impulses out of CNS to effector organs(muscles or glands)3. Association neurons (interneurons)• Located in CNS – integrative function of nervoussystem

Nerve• A bundle of axons located outside theCNS• Mixed nerves – motor and sensory fibers• Cranial nerves – sensory fibers only

Supporting Cells - PNS• Schwann Cell• Form myelin sheaths around peripheralaxons• Satellite Cells (Ganglionic gliocytes)• Support neuron cell bodies in ganglia ofPNS

Supporting Cells - CNS• Oligodendrocytes• Form myelin sheaths around axons of CNS• Microglia• Migrate through CNS and phagocytose foreign anddegenerated material• Astrocytes• Help regulate the external environment of neuronsin the CNS• Ependymal Cells• Line the ventricles of the brain and the central canalof the spinal cord

Astrocytes• Most abundant glial cells in CNS• Have processes that have “end-feet”• Surrounding capillaries of CNS• Other extensions by connections (synapses)between axon terminal of one neuron anddendrite or cell of another neuron

Functions of Astrocytes1. Take up K + from extracellular fluid• Maintain ionic balance2. Take up some neurotransmitters releasedfrom axon terminals• Remove neurotransmitters from synapse3. Take up glucose from the blood• Glucose converted to lactic acid and released intoneurons4. Needed for formation of synapses in CNS5. Induce formation of blood-brain barrier

Blood-Brain Barrier• Capillaries in brain do not have pores betweenendothelial cells• Brain cannot get nutrients from blood by filtering• Molecules moved through endothelial cells by diffusion, activetransport, phagocytoses and endocytosis• Causes a blood-brain barrier• Very specific• Could be due to effects of astrocytes on brain capillaries• Many drugs cannot enter the brain that can enter otherorgans• Parkinson’s disease – dopamine• Give predursor called levodopa• L-dopa can cross and dopamine cannot

Myelin Sheath• All axons in PNS surrounded byNeurilemma (sheath of Schwann)• Some axons in PNS and CNS havemyelin sheath• Wrapping of the cell membrane of Schwanncells• In CNS formed by Oligodendrocytes• Axons smaller than 2 μm – unmyelinated• Larger axons are myelinated

Myelin Sheath in PNS• Schwann cells wrap around axon liketape• Cytoplasm forced to outer layer• Each Schwann cells wraps 1 mm of axon• Provide insulation around axon• Node of Ranvier• Gaps between Schwann cells• Only exposed area of axon• Allows nerve impulses

Myelin Sheath in CNS• Formed by oligodendrocytes• Have extensions that form myelinsheaths around several axons• Give the tissue a white color (white matter)

Regeneration of a cutaxon• When axon of peripheral nerve is cut• Severed part degenerates and is phagocytosed by Schwanncells• Schwann cells from a regeneration tube• Remaining axon grows and is directed by regeneration tube• Nerves can be surgically reconnected and functionreestablished• If before tissue death• Injury to CNS nerve stimulates growth of axonterminals• Limited ability to regenerate• No continuous neurilemma – no regeneration tube• Inhibitory molecules produced by oligodendrocytes andastrocytes• New research – injury to spinal cord can cause cell suicide inneurons not directly damaged by injury

Resting MembranePotential• The inside of cell is (-) charged comparedto outside• -70mV• Unequal distribution of ions acrossmembranes• (-) charged organic molecules in cell• Limited diffusion of (+) inorganic ions

Excitability or Irritability• Ability of neurons and muscle cells tocause changes in the membranepotential• Only certain cells can alter membranepotential is response to stimulation

Depolarization• Stimulation causes (+) charges to flow inthe cell• Excitatory

Repolarization• Return to the resting membrane potential

Hyperpolarization• Stimulation causes inside of cell tobecome more negative than restingmembrane potential• Inhibitory

Resting MembranePotential• Resting membrane• Outside (+)• Inside (-)• Separation of charges = potential energy• Measured in volts• -70 mV (inside negative compared to outside)• Resting membrane potential – unequaldistribution of ions in cytoplasm and interstitialfluid• Insterstitial fluid – Na - and Cl -• Inside of cell – K + , P of ATP (-charge), AA (-charge)

Resting MembranePotential• Gated Channels• K + - 2 gates• Na + - always closed in resting cell• Resting cell more permeable to K + than Na +• K + leaks out of cell• Large (-) molecules cannot leave cell• Very little Na + leading• Na + - K + pump helps maintain potential

Generation of ActionPotential• Reverse membrane potential and restore tooriginal charge• Electrical excitability• Ability of neurons and muscle fibers to convertstimuli into action potential• Stimulus• Anything in cell’s environment that can changeresting membrane potential• Threshold• Stimulation causes membrane to depolarize• Causes action potential

Generation of ActionPotential• 2 phases of action potential1. Depolarizing phase – sequence of eventsthat decreases and reverses thepolarization of the membrane, making theinside more (+)2. Repolarizing phase – membrane potentialrestore to resting state

Generation of ActionPotential• Depolarization – opens voltage gatedchannels in the plasma membrane of theaxon and axon terminals1. Threshold depolarization opens voltage-gated Na +channels• 20,000 Na + rush into cell• Charge reaches +30 mV2. Opens voltage gated K + channels• Open more slowly, open same time Na + close• K + out – repolarizing• After – hyperpolarizing – membrane potential becomesmore (-) than rest• K+ close – membrane potential returns to rest (-70 mV)

All-or-None Principle• If stimulus is strong enough to causedepolarization the action potential occurs• A stronger stimulus does not cause alarger action potential

If stronger stimulus isrequired . . .• To increase the strength of a stimulus thefrequency of the stimulus is increased and/orthe number of neurons recruited is increased• When a greater stimulus is needed AP areproduced more frequently• Recruitment – can also cause increasestimulus strength• A weak stimulus will activate a few axons with lowthresholds• A strong stimulus activates axons with higherthesholds

Refractory Period• The period of time when an axonmembrane cannot respond to furtherstimuli

Conduction of NerveImpulses• Impulse travels from where signaloriginates (axon hillock) to axon terminal• Inflow of Na + depolarizes adjacentmembrane and opens more sodiumgates

Types of Conduction1. Continuous conduction• Unmyelinated axons• Each adjacent membrane depoarizes tothreshold and generates another actionpotential (slow)2. Saltatory conduction• Myelinated axons• Voltage-gated channels in nodes of ranvier• Current flows through the interstitial fluidaround myelin sheath and throughcytoplasm to next cell

What influences speed ofimpulse?• Large diameter (myelinated) – faster• Higher speeds when warm, lower whencool• Pain from injury reduced by ice• Cooling slows conduction of nerve impulsesalong axons of pain sensitive neurons

Local Anesthetics• Block opening of voltage gated Na +channels• Nerve impulses obstructed• Pain signals do not reach CNS

Synapse Transmission• Synapse• Where neurons communicate• Presynaptic neuron• Neuron sending signal• Post synaptic neuron• Neuron receiving signal• Separated by synaptic cleft• Filled with interstitial fluid

What Happens1. Nerve impulse at synaptic end bulb of presynapticaxon2. Depolarization opens voltage-gated Ca 2+ channels inmembrane of synaptic end bulbs3. Increase in Ca 2+ inside synaptic end bulb causesexocytosis of synaptic vesicles which release theneurotransmitter into the synaptic cleft4. Neurotransmitter molecules diffuse across synapticcleft and bind to postsynaptic neuron’s plasmamembrane• Opens ion channels in postsynaptic membrane5. As ions flow through open channels, voltage acrossmembrane changes• Depolariztion – excitatory• Hyperpolarization - inhibitory

How are neurotransmittersremoved from the synapticcleft?1. Some of the released neurotransmittersdiffuse away from the synaptic cleft2. Some neurotransmitters are destroyedby enzymes3. Most of the neurotransmitters areactively transported back to the neuronthat released or into the neighboringneuroglia

Acetylcholine• Excitatory neurotransmitter• Some neurons in CNS• Somatic motor neurons at neuromuscularjunction• Can be excitatory or inhibitory,depending on organ involved

Other Neurotransmitters• Serotonin• Sensory perception, temperature regulation, controlof mood and appetite, onset of sleep• Derived from tryptophan• Amount can be affected by diet• Tryptophan rich foods are turkey and milk• Drugs – Prozac, Paxil, Zoloft, and Luvox• Serotonin specific reuptake inhibitors• Increase effectiveness of serotonin transmission atsynapses• Many different types of serotonin receptors• Some drugs that promote serotonin action are usedto treat appetite of obese people• Some drugs treat anxiety and others treat migraineheadaches

• Dopamine• Emotional responses, addictive behavior,pleasure experiences, muscle tone• Parkinson's disease• Degeneration of the dopaminergic neurons in thesubstantia nigra• Alcohol, amphetamines, cocaine, marijuana,morphine and nicotine• Promote activity of dopaminjergic neurons• Over activity of dopamine pathwayscontributes to schizophrenia

• Norepinephrine• Awakening from sleep, dreaming, regulatingmood, muscles and glands• Amphetamines stimulate norepinephrinepathways• General behavioral arousal

Amino Acids asNeurotransmitters• Glutamic acid and aspartic acid• Excitatory neurotransmitters in CNS• Glycine• Inhibitory• Opens Cl - channels – produces hyperpolarization• Involved with muscle relaxation• Gamma-aminobutyric acid (GABA)• Derived from glutamic acid• Most prevalent neurotranmitter in the brain• Inhibitory – opens Cl - channels• Involved in motor control• Functions as neurotransmitter involved in mood and emotion• Valium – increases ability of GABA to activate receptors inbrain and spinal cord – used to treat anxiety and depression

Polypeptides asNeurotransmitters• Neuropeptides• Endorphins – body’s natural painkillers

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