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Nervous TissueNervous Tissue• Controls and integrates all body activities

within limits that maintain life

• Three basic functions

• sensing changes with sensory receptors

• fullness of stomach or sun on your face

• interpreting and remembering those changes

• reacting to those changes with effectors

• muscular contractions

• glandular secretions

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Major Structures of the Nervous Major Structures of the Nervous SystemSystem

• Brain, cranial nerves, spinal cord, spinal Brain, cranial nerves, spinal cord, spinal nerves, ganglia, enteric plexuses and nerves, ganglia, enteric plexuses and sensory receptorssensory receptors

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Organization of the Organization of the Nervous SystemNervous System

• CNS is brain and spinal CNS is brain and spinal cordcord

• PNS is everything elsePNS is everything else

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Nervous System Nervous System DivisionsDivisions

• Central nervous system (CNS) Central nervous system (CNS)

• consists of the brain and spinal cordconsists of the brain and spinal cord

• Peripheral nervous system (PNS)Peripheral nervous system (PNS)

• consists of cranial and spinal nerves that consists of cranial and spinal nerves that contain both sensory and motor fiberscontain both sensory and motor fibers

• connects CNS to muscles, glands & all connects CNS to muscles, glands & all sensory receptorssensory receptors

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Subdivisions of the Subdivisions of the PNSPNS• Somatic (voluntary) nervous system (SNS)Somatic (voluntary) nervous system (SNS)

• neurons from cutaneous and special sensory neurons from cutaneous and special sensory receptors to the CNSreceptors to the CNS

• motor neurons to skeletal muscle tissuemotor neurons to skeletal muscle tissue• Autonomic (involuntary) nervous systemsAutonomic (involuntary) nervous systems• sensory neurons from visceral organs to CNSsensory neurons from visceral organs to CNS• motor neurons to smooth & cardiacmotor neurons to smooth & cardiac muscle muscle

and glandsand glands• sympathetic division (speeds up heart rate)sympathetic division (speeds up heart rate)• parasympathetic division (slow down heart parasympathetic division (slow down heart

rate)rate)• Enteric nervous system (ENS)Enteric nervous system (ENS)• involuntary sensory & motor neurons control involuntary sensory & motor neurons control

GI tractGI tract

• neurons function independently of ANS & CNSneurons function independently of ANS & CNS

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NeuronsNeurons• Functional unit of nervous systemFunctional unit of nervous system• Have capacity to produce action potentialsHave capacity to produce action potentials• electrical excitabilityelectrical excitability

• Cell bodyCell body• single nucleus with prominent nucleolussingle nucleus with prominent nucleolus• Nissl bodies (chromatophilic substance) Nissl bodies (chromatophilic substance) • rough ER & free ribosomes for protein rough ER & free ribosomes for protein

synthesissynthesis• neurofilaments give cell shape and neurofilaments give cell shape and

supportsupport• microtubules move material inside cellmicrotubules move material inside cell• lipofuscin pigment clumps (harmless lipofuscin pigment clumps (harmless

aging)aging)

• Cell processes = dendrites & axonsCell processes = dendrites & axons

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Nucleus with

Nucleolus

Parts of a NeuronParts of a Neuron

Axons or Dendrites

Cell body

Neuroglial cells

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DendritesDendrites

• Conducts impulses Conducts impulses towards the cell bodytowards the cell body

• Typically short, highly Typically short, highly branched & branched & unmyelinatedunmyelinated

• Surfaces specialized Surfaces specialized for contact with other for contact with other neuronsneurons

• Contains neurofibrils Contains neurofibrils & Nissl bodies& Nissl bodies

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AxonsAxons• Conduct impulses Conduct impulses

away from cell bodyaway from cell body

• Long, thin cylindrical Long, thin cylindrical process of cell process of cell

• Arises at axon hillock Arises at axon hillock

• Impulses arise from Impulses arise from initial segment (trigger initial segment (trigger zone)zone)

• Swollen tips called Swollen tips called synaptic end bulbs synaptic end bulbs contain vesicles filled contain vesicles filled with neurotransmitterswith neurotransmitters Synaptic

boutons

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Axonal TransportAxonal Transport• Cell body is location for most protein Cell body is location for most protein

synthesissynthesis

• neurotransmitters & repair proteins neurotransmitters & repair proteins

• Axonal transport system moves substances Axonal transport system moves substances

• slow axonal flowslow axonal flow

• movement at 1-5 mm per day movement at 1-5 mm per day

• movement in one direction only -- away movement in one direction only -- away from cell bodyfrom cell body

• fast axonal flowfast axonal flow

• moves organelles & materials along moves organelles & materials along surface of microtubulessurface of microtubules

• at 200-400 mm per dayat 200-400 mm per day

• transports in either directiontransports in either direction

• for use or for recycling in cell bodyfor use or for recycling in cell body

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Axonal Transport & Axonal Transport & DiseaseDisease

• Fast axonal transport route by which Fast axonal transport route by which toxins or pathogens reach neuron cell toxins or pathogens reach neuron cell bodiesbodies

• tetanus (Clostridium tetani bacteria)tetanus (Clostridium tetani bacteria)

• disrupts motor neurons causing disrupts motor neurons causing painful muscle spasmspainful muscle spasms

• Bacteria enter the body through a Bacteria enter the body through a laceration or puncture injurylaceration or puncture injury

• more serious if wound is in head or more serious if wound is in head or neck because of shorter transit timeneck because of shorter transit time

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Functional Classification Functional Classification of Neuronsof Neurons

• Sensory (afferent) neuronsSensory (afferent) neurons

• transport sensory information from transport sensory information from skin, muscles, joints, sense organs & skin, muscles, joints, sense organs & viscera to CNSviscera to CNS

• Motor (efferent) neuronsMotor (efferent) neurons

• send motor nerve impulses to send motor nerve impulses to muscles & glandsmuscles & glands

• Interneurons (association) neuronsInterneurons (association) neurons

• connect sensory to motor neuronsconnect sensory to motor neurons

• 90% of neurons in the body90% of neurons in the body

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Structural Classification Structural Classification of Neuronsof Neurons

• Based on number of processes found on cell bodyBased on number of processes found on cell body

• multipolar = several dendrites & one axonmultipolar = several dendrites & one axon

• most common cell typemost common cell type

• bipolar neurons = one main dendrite & one axonbipolar neurons = one main dendrite & one axon

• found in retina, inner ear & olfactory found in retina, inner ear & olfactory

• unipolar neurons = one process only(develops from a bipolar)unipolar neurons = one process only(develops from a bipolar)

• are always sensory neuronsare always sensory neurons

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Association or Association or InterneuronsInterneurons• Named for histologist that first Named for histologist that first

described them or their appearancedescribed them or their appearance

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• Half of the volume of the CNS Half of the volume of the CNS

• Smaller cells than neuronsSmaller cells than neurons

• 50X more numerous50X more numerous

• Cells can divide Cells can divide

• rapid mitosis in tumor formation (gliomas)rapid mitosis in tumor formation (gliomas)

• 4 cell types in CNS4 cell types in CNS

• astrocytes, oligodendrocytes, microglia & astrocytes, oligodendrocytes, microglia & ependymalependymal

• 2 cell types in PNS2 cell types in PNS

• schwann and satellite cellsschwann and satellite cells

Neuroglial CellsNeuroglial Cells

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AstrocytesAstrocytes• Star-shaped cellsStar-shaped cells

• Form blood-brain Form blood-brain barrier by covering barrier by covering blood capillariesblood capillaries

• Metabolize Metabolize neurotransmittersneurotransmitters

• Regulate K+ Regulate K+ balancebalance

• Provide structural Provide structural supportsupport

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OligodendrocytesOligodendrocytes

• Most common Most common glial cell typeglial cell type

• Each forms Each forms myelin sheath myelin sheath around more around more than one axons than one axons in CNSin CNS

• Analogous to Analogous to Schwann cells of Schwann cells of PNSPNS

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MicrogliaMicroglia

• Small cells found near blood vesselsSmall cells found near blood vessels

• Phagocytic role -- clear away dead cellsPhagocytic role -- clear away dead cells

• Derived from cells that also gave rise to Derived from cells that also gave rise to macrophages & monocytesmacrophages & monocytes

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Ependymal cellsEpendymal cells

• Form epithelial membrane lining cerebral Form epithelial membrane lining cerebral cavities & cavities & central canalcentral canal

• Produce cerebrospinal fluid (CSF)Produce cerebrospinal fluid (CSF)

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Satellite CellsSatellite Cells

• Flat cells surrounding neuronal cell Flat cells surrounding neuronal cell bodies in peripheral gangliabodies in peripheral ganglia

• Support neurons in the PNS gangliaSupport neurons in the PNS ganglia

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Schwann CellSchwann Cell

• Cells encircling PNS axonsCells encircling PNS axons

• Each cell produces part of the myelin Each cell produces part of the myelin sheath surrounding an axon in the PNSsheath surrounding an axon in the PNS

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Axon Coverings in Axon Coverings in PNSPNS• All axons surrounded by a lipid & protein covering All axons surrounded by a lipid & protein covering

(myelin sheath) produced by Schwann cells(myelin sheath) produced by Schwann cells

• Neurilemma is cytoplasm & nucleusNeurilemma is cytoplasm & nucleusof Schwann cellof Schwann cell

• gaps called nodes of Ranviergaps called nodes of Ranvier

• Myelinated fibers appear whiteMyelinated fibers appear white

• jelly-roll like wrappings made of jelly-roll like wrappings made of lipoprotein = myelinlipoprotein = myelin

• acts as electrical insulatoracts as electrical insulator

• speeds conduction of nerve impulsesspeeds conduction of nerve impulses

• Unmyelinated fibersUnmyelinated fibers

• slow, small diameter fibersslow, small diameter fibers

• only surrounded by neurilemma but no myelin sheath only surrounded by neurilemma but no myelin sheath wrappingwrapping

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Myelinated vs. Unmyelinated

Axons In a myelinated axon, the nerve impulse

“jumps” from neurofibril node to neurofibril node and is known as saltatory conduction.

In an unmyelinated axon, the nerve impulse must travel the entire length of the axon, a process called continuous conduction.

A myelinated axon produces a faster nerve impulse.

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Gray and White Gray and White MatterMatter

• White matter = myelinated processes (white in color)White matter = myelinated processes (white in color)

• Gray matter = nerve cell bodies, dendrites, axon Gray matter = nerve cell bodies, dendrites, axon terminals, bundles of unmyelinated axons and terminals, bundles of unmyelinated axons and neuroglia (gray color)neuroglia (gray color)

• In the spinal cord = gray matter forms an H-shaped inner In the spinal cord = gray matter forms an H-shaped inner core surrounded by white mattercore surrounded by white matter

• In the brain = a thin outer shell of gray matter covers the In the brain = a thin outer shell of gray matter covers the surface & is found in clusters called nuclei inside the CNSsurface & is found in clusters called nuclei inside the CNS

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Electrical Signals in NeuronsElectrical Signals in Neurons

• Neurons are electrically excitable due to Neurons are electrically excitable due to the voltage difference across their the voltage difference across their membrane membrane

• Communicate with 2 types of electric Communicate with 2 types of electric signalssignals

• action potentials that can travel long distancesaction potentials that can travel long distances

• graded potentials that are local membrane graded potentials that are local membrane changes onlychanges only

• In living cells, a flow of ions occurs In living cells, a flow of ions occurs through ion channels in the cell through ion channels in the cell membranemembrane

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Two Types of Ion Two Types of Ion ChannelsChannels

• Leakage (nongated) channels are always Leakage (nongated) channels are always openopen

• Gated channels open and close in response Gated channels open and close in response to a stimulus results in neuron excitabilityto a stimulus results in neuron excitability

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Gated Ion ChannelsGated Ion Channels

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Local AnestheticsLocal Anesthetics

• Prevent opening of voltage-gated Na+ Prevent opening of voltage-gated Na+ channelschannels

• Nerve impulses cannot pass the Nerve impulses cannot pass the anesthetized regionanesthetized region

• Novocaine and lidocaineNovocaine and lidocaine

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Encoding of Stimulus Encoding of Stimulus IntensityIntensity

• How do we differentiate a light touch How do we differentiate a light touch from a firmer touch?from a firmer touch?

• frequency of impulsesfrequency of impulses

• firm pressure generates impulses at firm pressure generates impulses at a higher frequencya higher frequency

• number of sensory neurons activatednumber of sensory neurons activated

• firm pressure stimulates more firm pressure stimulates more neurons than does a light touchneurons than does a light touch

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Signal Transmission Signal Transmission at Synapsesat Synapses

• 2 Types of synapses2 Types of synapses

• electrical electrical

• ionic current spreads to next cell through ionic current spreads to next cell through gap junctionsgap junctions

• faster, two-way transmission & capable faster, two-way transmission & capable of synchronizing groups of neuronsof synchronizing groups of neurons

• chemicalchemical

• one-way information transfer from a one-way information transfer from a presynaptic neuron to a postsynaptic presynaptic neuron to a postsynaptic neuronneuron

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Chemical SynapsesChemical Synapses• Action potential reaches end Action potential reaches end

bulb and voltage-gated Ca+ 2 bulb and voltage-gated Ca+ 2 channels openchannels open

• Ca+2 flows inward triggering Ca+2 flows inward triggering release of neurotransmitterrelease of neurotransmitter

• Neurotransmitter crosses Neurotransmitter crosses synaptic cleft & binding to synaptic cleft & binding to ligand-gated receptors ligand-gated receptors

• the more neurotransmitter the more neurotransmitter released the greater the change released the greater the change in potential of the postsynaptic in potential of the postsynaptic cellcell

• Synaptic delay is 0.5 msecSynaptic delay is 0.5 msec

• One-way information transferOne-way information transfer

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Removal of Removal of NeurotransmitterNeurotransmitter• DiffusionDiffusion

• move down concentration move down concentration gradientgradient

• Enzymatic degradationEnzymatic degradation

• acetylcholinesteraseacetylcholinesterase

• Uptake by neurons or glia Uptake by neurons or glia cellscells

• neurotransmitter transportersneurotransmitter transporters

• Prozac = serotonin reuptake Prozac = serotonin reuptake inhibitorinhibitor

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Strychnine PoisoningStrychnine Poisoning

• In spinal cord, Renshaw cells normally In spinal cord, Renshaw cells normally release an inhibitory neurotransmitter release an inhibitory neurotransmitter (glycine) onto motor neurons preventing (glycine) onto motor neurons preventing excessive muscle contractionexcessive muscle contraction

• Strychnine binds to and blocks glycine Strychnine binds to and blocks glycine receptors in the spinal cordreceptors in the spinal cord

• Massive tetanic contractions of all skeletal Massive tetanic contractions of all skeletal muscles are producedmuscles are produced

• when the diaphragm contracts & remains when the diaphragm contracts & remains contracted, breathing can not occurcontracted, breathing can not occur

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Neurotransmitter EffectsNeurotransmitter Effects

• Neurotransmitter effects can be modifiedNeurotransmitter effects can be modified

• synthesis can be stimulated or inhibitedsynthesis can be stimulated or inhibited

• release can be blocked or enhancedrelease can be blocked or enhanced

• removal can be stimulated or blocked removal can be stimulated or blocked

• receptor site can be blocked or activatedreceptor site can be blocked or activated

• AgonistAgonist

• anything that enhances a transmitters effects anything that enhances a transmitters effects

• AntagonistAntagonist

• anything that blocks the action of a anything that blocks the action of a neurotranmitterneurotranmitter

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Small-Molecule Small-Molecule NeurotransmittersNeurotransmitters• Acetylcholine (ACh)Acetylcholine (ACh)

• released by many PNS neurons & some CNSreleased by many PNS neurons & some CNS

• excitatory on NMJ but inhibitory at othersexcitatory on NMJ but inhibitory at others

• inactivated by acetylcholinesteraseinactivated by acetylcholinesterase

• Amino AcidsAmino Acids

• glutamate released by nearly all excitatory glutamate released by nearly all excitatory neurons in the brain ---- inactivated by neurons in the brain ---- inactivated by glutamate specific transportersglutamate specific transporters

• GABA is inhibitory neurotransmitter for 1/3 GABA is inhibitory neurotransmitter for 1/3 of all brain synapses (Valium is a GABA of all brain synapses (Valium is a GABA agonist -- enhancing its inhibitory effect)agonist -- enhancing its inhibitory effect)

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• Biogenic AminesBiogenic Amines

• modified amino acids (tyrosine)modified amino acids (tyrosine)

• norepinephrine -- regulates mood, norepinephrine -- regulates mood, dreaming, awakening from deep sleepdreaming, awakening from deep sleep

• dopamine -- regulating skeletal muscle dopamine -- regulating skeletal muscle tonetone

• serotonin -- control of mood, serotonin -- control of mood, temperature regulation, & induction of temperature regulation, & induction of sleepsleep

• removed from synapse & recycled or removed from synapse & recycled or destroyed by enzymes (monoamine destroyed by enzymes (monoamine oxidase or catechol-0-methyltransferase)oxidase or catechol-0-methyltransferase)

Small-Molecule Small-Molecule Neurotransmitters (2)Neurotransmitters (2)

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NeuropeptidesNeuropeptides• 3-40 amino acids linked by peptide 3-40 amino acids linked by peptide

bondsbonds

• Substance P -- enhances our perception Substance P -- enhances our perception of painof pain

• Pain reliefPain relief

• enkephalins -- pain-relieving effect by enkephalins -- pain-relieving effect by blocking the release of substance P blocking the release of substance P

• acupuncture may produce loss of pain acupuncture may produce loss of pain sensation because of release of sensation because of release of opioids-like substances such as opioids-like substances such as endorphins or dynorphinsendorphins or dynorphins

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Regeneration & Regeneration & RepairRepair

• Plasticity maintained throughout lifePlasticity maintained throughout life

• sprouting of new dendritessprouting of new dendrites

• synthesis of new proteinssynthesis of new proteins

• changes in synaptic contacts with changes in synaptic contacts with other neuronsother neurons

• Limited ability for regeneration Limited ability for regeneration (repair)(repair)

• PNS can repair damaged dendrites PNS can repair damaged dendrites or axonsor axons

• CNS no repairs are possibleCNS no repairs are possible

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Neurogenesis in the Neurogenesis in the CNSCNS• Formation of new neurons from stem cells Formation of new neurons from stem cells

was not thought to occur in humanswas not thought to occur in humans

• 1992 a growth factor was found that 1992 a growth factor was found that stimulates adult mice brain cells to stimulates adult mice brain cells to multiplymultiply

• 1998 new neurons found to form within 1998 new neurons found to form within adult human hippocampus (area adult human hippocampus (area important for learning)important for learning)

• Factors preventing neurogenesis in CNSFactors preventing neurogenesis in CNS

• inhibition by neuroglial cells, absence of inhibition by neuroglial cells, absence of growth stimulating factors, lack of growth stimulating factors, lack of neurolemmas, and rapid formation of scar neurolemmas, and rapid formation of scar tissuetissue

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• Axons & dendrites may be repaired ifAxons & dendrites may be repaired if

• neuron cell body remains intactneuron cell body remains intact

• schwann cells remain active and form a schwann cells remain active and form a tube tube

• scar tissue does not form too rapidlyscar tissue does not form too rapidly

• ChromatolysisChromatolysis

• 24-48 hours after injury, Nissl bodies break 24-48 hours after injury, Nissl bodies break up into fine granular massesup into fine granular masses

Repair within the Repair within the PNSPNS

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Multiple Sclerosis Multiple Sclerosis (MS)(MS)

• Autoimmune disorder causing Autoimmune disorder causing destruction of myelin sheaths in CNSdestruction of myelin sheaths in CNS

• sheaths becomes scars or plaquessheaths becomes scars or plaques

• 1/2 million people in the United States1/2 million people in the United States

• appears between ages 20 and 40 appears between ages 20 and 40

• females twice as often as malesfemales twice as often as males

• Symptoms include muscular weakness, Symptoms include muscular weakness, abnormal sensations or double visionabnormal sensations or double vision

• Remissions & relapses result in Remissions & relapses result in progressive, cumulative loss of functionprogressive, cumulative loss of function

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• The second most common neurological The second most common neurological disorder disorder

• affects 1% of populationaffects 1% of population

• Characterized by short, recurrent attacks Characterized by short, recurrent attacks initiated by electrical discharges in the braininitiated by electrical discharges in the brain

• lights, noise, or smells may be sensedlights, noise, or smells may be sensed

• skeletal muscles may contract involuntarilyskeletal muscles may contract involuntarily

• loss of consciousnessloss of consciousness

• Epilepsy has many causes, including;Epilepsy has many causes, including;

• brain damage at birth, metabolic brain damage at birth, metabolic disturbances, infections, toxins, vascular disturbances, infections, toxins, vascular disturbances, head injuries, and tumors disturbances, head injuries, and tumors

EpilepsyEpilepsy

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Neuronal Structure & Neuronal Structure & FunctionFunction

NeurotransmittersNeurotransmitters Chemical that reacts with specific receptors Chemical that reacts with specific receptors

to create a nerve impulseto create a nerve impulse Acetylcholine – controls skeletal muscle actionsAcetylcholine – controls skeletal muscle actions

Norepinephrine – “good” feeling, low levels may Norepinephrine – “good” feeling, low levels may lead to depressionlead to depression

Dopamine – “good” feelingDopamine – “good” feeling

Serotonin – sleepSerotonin – sleep

Histamine – alertnessHistamine – alertness

Endorphins – reduce painEndorphins – reduce pain

Nitric oxide – vasodilation, memoryNitric oxide – vasodilation, memory

pg. 218 chart of neurotransmitterspg. 218 chart of neurotransmitters

Types of NervesTypes of Nerves

Nerves are cordlike bundles of nerve Nerves are cordlike bundles of nerve fibers held together by connective tissue fibers held together by connective tissue layers that conduct impulses layers that conduct impulses

Sensory nerves – impulses into the Sensory nerves – impulses into the brain or spinal cordbrain or spinal cord

Motor nerves – carry impulses to Motor nerves – carry impulses to muscles or glandsmuscles or glands

Mixed nerves – include both of the Mixed nerves – include both of the aboveabove

Nerve PathwaysNerve Pathways Routes that nerve impulses follow in the nervous Routes that nerve impulses follow in the nervous

systemsystem

Reflex arcs are the simplest pathways that Reflex arcs are the simplest pathways that constitute reflexes.constitute reflexes.

Reflexes are subconscious responses to stimuli Reflexes are subconscious responses to stimuli within or outside the body. Help maintain within or outside the body. Help maintain involuntary actions such as heart rate, breathing involuntary actions such as heart rate, breathing rate, blood pressure and digestionrate, blood pressure and digestion

Knee-jerk reflex – simple, only using two Knee-jerk reflex – simple, only using two neurons; helps maintain upright postureneurons; helps maintain upright posture

Withdrawal reflex – unexpected touch to Withdrawal reflex – unexpected touch to something painful; protective by limiting something painful; protective by limiting tissue damagetissue damage

MeningesMeninges Membranes beneath the bony coverings of the skull Membranes beneath the bony coverings of the skull

and vertebral column for protectionand vertebral column for protection

Three layersThree layers

Dura mater – outermost layer of connective Dura mater – outermost layer of connective tissue, blood vessels, and nerves; interior tissue, blood vessels, and nerves; interior periosteum of skull bones and between lobes of periosteum of skull bones and between lobes of brain; surround spinal cord for protection.brain; surround spinal cord for protection.

Arachnoid mater – thin membrane lacking blood Arachnoid mater – thin membrane lacking blood vessels in the middlevessels in the middle

Pia Mater – thin layer of nerves and blood Pia Mater – thin layer of nerves and blood vessels that nourish the cells of the brain and vessels that nourish the cells of the brain and spinal cord; hugs surfaces of organsspinal cord; hugs surfaces of organs

Cerebrospinal fluid (CSF) – in between Cerebrospinal fluid (CSF) – in between arachnoid and pia maters.arachnoid and pia maters.

Spinal CordSpinal Cord

Nerve column passing from brain into Nerve column passing from brain into vertebral canalvertebral canal

Begins at the foramen magnum and Begins at the foramen magnum and terminates at the first and second terminates at the first and second lumbar vertebrae.lumbar vertebrae.

Structure of Spinal Structure of Spinal CordCord

Consists of 31 segments that give rise to Consists of 31 segments that give rise to spinal nerves which branch to various body spinal nerves which branch to various body partsparts

Cervical enlargement is the thickening of Cervical enlargement is the thickening of the spinal cord in the neck region that the spinal cord in the neck region that supplies nerves to upper limbssupplies nerves to upper limbs

The lumbar enlargement is in the lower The lumbar enlargement is in the lower back giving nerves to lower limbs.back giving nerves to lower limbs.

Divided into right and left halves by the Divided into right and left halves by the anterior median fissure and posterior anterior median fissure and posterior median sulcus grooves.median sulcus grooves.

Functions of Spinal Functions of Spinal CordCord

Two major functions: conducting nerve Two major functions: conducting nerve impulses and center for spinal reflexesimpulses and center for spinal reflexes

Axons of the spinal cord provide two way Axons of the spinal cord provide two way communication between the brain and the communication between the brain and the body partsbody parts

Ascending tracts carry sensory information to Ascending tracts carry sensory information to the brainthe brain

Descending tracts conduct motor impulses Descending tracts conduct motor impulses from the brain to the musclesfrom the brain to the muscles

Knee-jerk and withdrawal reflexes are spinal Knee-jerk and withdrawal reflexes are spinal reflexes because they pass through the spinal reflexes because they pass through the spinal cordcord