08. Nervous Systemcontents.kocw.net/KOCW/document/2014/gacheon/shimwonsik/5.pdf · Numerous, short, highly branched Receives impulses from other neurons and sensory receptors Carry

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The nervous systemIs the primary coordinating and controlling system of the bodyUses electrochemical impulses to fulfill its functions

General functions includeDetect internal and external changesAnalysis of informationOrganization of informationInitiation of appropriate action

3 8.1 Divisions of the Nervous System

Anatomical divisionsCentral nervous system (CNS)Brain and spinal cordBody’s neural control center

Peripheral nervous system (PNS)Located outside of the CNSConsists of cranial and spinal nerves and sensory receptorsPNS carries signals formed by receptors to the CNS, and delivers impulses from the CNS to effectors

4Functional divisionsSensory divisionCarries impulses from sensory receptors to the CNS

Motor divisionCarry impulses from CNS to effectorsMotor division has two sub-divisionsSomatic nervous systemVoluntary control of skeletal muscle

Autonomic nervous systemInvoluntary control of cardiac muscle, smooth muscle, and glands

56 8.2 Nerve Tissue

NeuronsSpecialized to transmit neural impulsesStructural and functional units of nervous systemThough they vary in size and shape, neurons have many common features

7Structure of a neuronCell bodyPossesses the nucleus and organellesTwo types of neuron processes extend from the cell body: dendrites (many) and axon (only one)

DendritesNumerous, short, highly branchedReceives impulses from other neurons and sensory receptorsCarry impulses toward cell body and axons

8Axon (or nerve fiber)A long, thin process that has one or more side branchesEnds in axon terminals with synaptic knobsCarry impulses away from cell body or dendritesSome axons have an insulating myelin sheath formed by special neuroglial cellsuch nerve fiber is called myelinatedIncreases speed of impulse transmissionNodes of RanvierThe tiny spaces between adjacent myelin-forming cells, where the axon is exposed

9Structural classification of neurons

101) Multipolar neurons several dendrites single axon

several dendrites single axon e.g. brain and spinal cord neurons

113) Unipolar neurons a single process extending from the cell body. process divides into two branches extending in opposite directionsbranches function as a single axon.

Unipolar neurons carry impulses from sensory receptors to the brain and spinal cordGanglia - Clusters of cell bodies of unipolar neurons

121) Sensory neuronsCarry impulses from peripheral body parts to the CNSDetect homeostatic changes directly or via sensory receptorsGanglia in the PNS house sensory neuron cell bodiesMost sensory neurons are unipolar neurons2) InterneuronsLocated entirely in CNS and associate with other neuronsProcess and interpret impulses from CNSActivate motor neuronsInterneurons are multipolar neurons3) Motor neuronsCarry impulses from CNS to effectors to produce an actionCell bodies and dendrites are in CNSAxon is located in nerves of PNSMotor neurons are multipolar neurons

13Neuroglia (or neuroglial cells)Support and protect neurons1 in the PNS, 4 in the CNS

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14They wrap tightly around an axon of PNS many times → nucleus and the most of the cytoplasm are squeezed into the outermost layerThe outermost layer: neurolemma (or neurilemma) → essential for axon regeneration

The inner layers: form myeline sheath

15OligodendrocytesForm myelin sheath around CNS axonsDo not form neurolemmaAxon regeneration is not possible

16The primary supporting cells for neurons in the brainStimulate neuronal growthInfluence synaptic transmissionJoin with the epithelium of blood vessels to form the blood-brain barrier (BBB), which regulates exchange of materials between blood and neurons

17Microglial cellsScattered throughout the CNSKeep the tissue clean by engulfing and digesting cellular debris and bacteria

Ependymal cellsForm an epithelial-like lining of cavities in brain and spinal cord

1819 8.3 Neuron Physiology

Neurons have two unique functional characteristicsIrritabilityAbility to respond to a stimulus by forming an impulse

ConductivityAbility to transmit an impulse along a neuron to another cell

20Resting potential

20Resting potentialA resting (inactive) neuron actively pumps sodium (Na+) out of the cells, whereas potassium (K+) into the cells.Outside: Na+Inside: K+, PO4-3, and SO4-2

This results in a net excess of positively- charged ions outside and a net excess of negatively-charged ions inside.The membrane is polarized due to this unequal distribution of electrical charges on each side of the plasma membraneThis condition is known as the resting potentialPotential is the difference in electrical charge between the outside and inside of the neuron’s plasma membrane

21Impulse FormationWhen stimulated, neurons exhibit an all-or-none responseThe weakest stimulus that will activate a neuron is called threshold stimulusAll impulses are alike: There are no strong or weak impulses.When activated by a stimulus, membrane instantly becomes permeable to Na+ → Na+ diffuse into the neuron (due to Na+ channel opening) → causes inside of the cell become positive

This switch in polarity is called depolarizationThe depolarization “is” the nerve impulse or action potential

22RepolarizationAfter depolarization → K+ channel opens and Na+ channel closes → K+ diffuses outward to reestablish the resting potential

Then, Na+ is pumped out and K+ is pumped in to reestablished resting-state ion distributionIn this way, the neuron is repolarizedWhen repolarization occurs, the neuron is ready to respond to another stimulusDepolarization and repolarization are accomplished in about 1/10,000 of a second

23Impulse ConductionDepolarization at one point triggers depolarization in adjacent portions.Forms a wave of depolarization sweeping along the neuronConduction of nerve impulses is more rapid in myelinated neurons than unmyelinated neurons

Conduction of nerve impulses is more rapid in myelinated neurons than unmyelinated neuronsImpulses jumps from Ranvier node to node and does not have to depolarize the intervening segments of the neuron process

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Synaptic TransmissionSynapseJunction of an axon with another neuron or an effector cell

Synapse structureSynaptic knob (axon tip) of presynaptic neuronPostsynaptic neuron’s dendrite or cell bodySynaptic cleft, a tiny space between the neurons → so they are not in physical contact

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Steps of synaptic transmissionArrival of an impulse causes synaptic knob to release a neurotransmitter into the synaptic cleftNeurotransmitters bind to receptors on postsynaptic neuron’s plasma membraneThus, a response is triggered in the postsynaptic neuron

Some neurotransmitters stimulate impulse formation, whereas others may inhibit impulse formationSignal transmission across a synapse is only in one directionBecause only the synaptic knob can release neurotransmitters, impulses can only pass in one direction across a synapse (presynaptic → postsynaptic)

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Some neurotransmitters are reabsorbed into the synaptic knob for reuseOthers diffuse out of the synaptic cleft or decomposed by enzymes in synaptic cleft (the decomposed products are then reabsorbed for reuse)This quick removal of a neurotransmitter prevents continuous stimulation of postsynaptic neuron or effectorIt only takes a fraction of a second from start to finish

30Enable neurons to communicate with each other

30Enable neurons to communicate with each otherMore than 40 neurotransmitters has been identified

But, they can be categorized as two types: excitatory and inhibitoryExcitatory neurotransmitters: Acetylcholine (Ach), Norepinephrine (NE)…Inhibitory neurotransmitters: Dopamine, Endorphins

Each neuron releases only one or two neurotransmittersA postsynaptic neurons receives input from hundreds of presynaptic synaptic knobsSome knobs secrete excitatory and other inhibitoryImpulse formation depends on type of neurotransmitters predominant at any given period of time

3132 8.4 Protection for the

Central Nervous System

BonesCranial bonesVertebrae

331) Pia materThin, innermost layerTightly envelops the CNS and penetrates into each groove and depressionContains many blood vessels to nourish brain and spinal cord

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2) Arachnoid materThe middle layer of the meningesA thin, web-like membrane without blood vesselsDoes not penetrate smaller depressions as does the pia materSubarachnoid spaceFilled with cerebrospinal fluid (CSF), which serves as a shock absorber

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3) Dura materThe tough, outermost layerAttached to the inner surface of cranial bones. In the vertebral canal, it forms a protective tube that extends to the sacrumDoes not attached to the bones of vertebrae, but separated by epidural space

3940 8.5 Brain

Contains about 100 billion neurons and innumerable neuron processes and synapsesFour major components: cerebrum, cerebellum, diencephalon, brain stem

41 Cerebrum

Largest portion of the brainPerforms higher brain functionsSensationsVoluntary actionsReasoningPlanningProblem solving

42Structure Left and right cerebral hemispheresSeparated by longitudinal fissureConnected by corpus callosum, which is a mass of myelinated neuron processes

Surface shows numerous gyri (folds) with sulci (shallow grooves) between them

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Outer surface is composed of gray matter (neuron cell bodies and unmyelinated fibers), which is called cerebral cortex

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Outer surface is composed of gray matter (neuron cell bodies and unmyelinated fibers), which is called cerebral cortex

46Cerebral hemisphere is divided into 4 lobesFrontal lobeParietal lobeTemporal lobeOccipital lobe

47Three major type of functional areasSensory areasMotor areasAssociation areas

481) Sensory areasReceive impulses from sensory receptors and interprets impulses as sensationsVision: occipital lobeHearing: temporal lobesSomatosensory (Cutaneous sensation): postcentral gyri of the parietal lobeSmell (Olfactory sensation): inferior part of frontal lobe

Sensory areas receive information from opposite sides of the bodyDue to cross over of ascending sensory fibers in the spinal cord or brain stem

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2) Motor areasLocated in frontal lobePrimary motor areacontrols skeletal muscleslie along the precentral gyri of the frontal lobe

Premotor areacomplex learned activities e.g. writing, problem solving, planning…

Premotor areacomplex learned activities e.g. writing, problem solving, planning…anterior to the primary motor area

Broca’s areacontrols ability to speaknear the inferior end of the primary motor areaonly found in one hemisphere (90% people has it in their left hemisphere)

Left side of the cerebrum controls right side skeletal muscles due to the crossover of descending motor fibers

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3) Association areasInterrelate sensory inputs and motor outputsinterrelationships of sensations, memory, will, and coordination of motor responsesGeneral interpretative areaa major association area that is located at the junction of the temporal, parietal, and occipital lobesinvolved with the interpretation of complex sensory experiences and thought processes

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Hemisphere SpecializationEach hemisphere commonly performs basic functions such as receiving sensory input and initiating voluntary motor output

However, in most people (90%), two hemispheres have different rolesLeft hemisphereAnalytical and verbal skillse.g. mathematics, reading, writing, speech…

Right hemisphereEmotionale.g. musical, artistic, spatial awareness, imagination, and insight

5657 Diencephalon

Lies between the brain stem and midbrain

57 Diencephalon

Lies between the brain stem and midbrainConsists of two major areasThalamusHypothalamus

58 Thalamus = 시상 (視床)?

Thalamus: Thalamo Gk. storeroom, bedroomThomas Willis, an English anatomist in 17C, coined the word thalamus opticus, believing that thalamus is related to visionEven though opticus was later removed since thalamus is a region not specific to vision, the translation from thalamus opticus has survived.

59ThalamusTwo lateral masses joined by the intermediate massReceives all incoming sensory impulses (except smell) before sending them to the cerebral cortexProvides a general, nonspecific awareness of sensationspain, pressure, touch, and temperature…Interpretation is carried out by the cerebral cortex

Relays motor impulses from cerebral cortex to lower brain regions

60HypothalamusInferior to thalamus and anterior to midbrainCommunicates with thalamus, cerebrum, and other brain regionsMajor control center for autonomic nervous systemIt controls virtually all internal organs

Also, it connects between brain and endocrine systemHypothalamus controls pituitary gland

61Thus, primary function of hypothalamus is to maintain homeostasis by regulatingBody temperatureMineral and water balanceAppetite and digestive processes

Mineral and water balanceAppetite and digestive processesHeart rate and blood pressureSleep and wakefulnessEmotions of fear and rageSecretion of hormones by pituitary gland

62 Limbic System

A complex of thalamus, hypothalamus and parts of the cerebral cortex and deep nuclei of cerebrumnuclei (neuroanatomy): a brain structure consisting of a relatively compact cluster of neurons, often in gray matter.

Involved in:MemoryEmotions e.g. sadness, happiness, anger, fear…Emotional behaviors

Malfunctions can result in mood disorderse.g. depression

63 Limbic System

64 Brain Stem

Stalk-like portion connecting higher brain centers with the spinal cordContains nuclei (localized gray matter) surrounded by white matterAscending and descending fibers pass through the brain stemConsists ofMidbrainPonsMedulla oblongata

65MidbrainThe topmost portion of the brain stemPosterior to hypothalamus and superior to ponsContains Reflex center for visual and auditory stimulie.g. reflexively turing head to enable better vision or hearing

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66Medulla oblongataMost inferior portion of the brain that connects to the spinal cordThe crossover of ascending/descending fiber happens in the medulla oblongataThree control centers1) Respiratory control centerRegulate depth and rate of breathingReflexes like coughing and breathing

2) Cardiac control centerRegulate rate of heart contractions

3) Vasomotor centerRegulates blood pressure and blood flow

67Reticular FormationA network of fibers and nuclei in brain stem and spinal cordGenerates and transmits impulses that arouse the cerebrum to wakefulness → Decrease in activity causes sleep

Damage can cause unconsciousness or coma

68 Cerebellum

Second largest brain regionInferior to occipital and temporal lobes; posterior to pons and medulla oblongataTwo hemispheres connected by the vermisGray matter forms a thin outer layer covering the underlying white matter, which forms most of the cerebellumA reflex center that controls and coordinates the interaction of skeletal musclesposture, balance, and muscle coordination

Damage results in loss of equilibrium, muscle coordination, and muscle tone

69 Ventricles and Cerebrospinal Fluid

Brain contains four interconnecting ventriclesEach ventricle is lined by ependymal cells and filled with cerebrospinal fluid (CSF)Two lateral ventriclesWithin cerebral hemispheres

Third ventricle

Within cerebral hemispheresThird ventricleMidline of diencephalon

Fourth ventricleMidline of brain stemContinuous with central canal of spinal cord

70 Ventricles and Cerebrospinal Fluid

Each ventricle contains a chroid plexus, a mass of special capillaries that secrete the CSFMost of CSF is produced in the lateral ventricles

71Flow of cerebrospinal fluidLateral ventricles (1) → third ventricle (2) → fourth ventricle (3) → subarachnoid space (4, most) or central canal of the spinal canal (5, some)

72Within the subarachnoid space, CSF flows in two directionsUpwards around the brain (1) orDown posterior side of spinal cord (2), then up anterior side (3)

73Cerebrospinal fluid is reabsorbed into the dural sinus (1)Secretion and absorption of CSF is at equal rates to keep a constant hydrostatic pressure

7475 8.6 Spinal Cord

Descends from medulla oblongata through foramen magnumPasses through vertebral canal and extends to the second lumbar vertebra (L2)Below L2, only spinal nerves occupy lower levels of vertebral canal

76Cylindrical shapeTwo grooves divide left and right of the spinal cordAnterior median fissure (wider)Posterior median sulcus (narrower)

31 pairs of spinal nerves branch from the spinal cord

Posterior median sulcus (narrower)31 pairs of spinal nerves branch from the spinal cord

77Gray matterButterfly shapeCentral canal: CSFHornsAnterior horns (1): motor neuronPosterior horns (2): sensory interneuron

Interneurons form most of the gray matter

78Functions of the spinal cord

Transmit impulses to and from brainAscending (sensory) tracts: carry impulses to the brainDescending (motor) tracts: carry impulses from the brain

Serves as a reflex center for spinal reflexes

79 8.7 Peripheral Nervous System

Consists of cranial and spinal nerves that connect to parts of the bodyNerves are neuronal processes bundled together by connective tissueThere are three types of nerves

Motor nervesSensory nervesMixed nerves (most): motor + sensory

Nerves can contain fibers of both somatic (voluntary) and autonomic (involuntary) nervous systems

80Cranial NervesTwelve pairs of nervesArise from brain and connect with organs near head and neckCranial nerves are identified by both roman numerals and names

Cranial nerves are identified by both roman numerals and namesThe numerals indicates the order of nerves from the inferior surface of the brainI is most anterior: XII is most posterior

Most cranial nerves are motor nerves: a few are sensory and some are mixed

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Spinal Nerves31 pairsThe first pair branches between atlas and occipital boneThe remaining pairs emerge through intervertebral foramina

84Spinal nerves are identified by the spinal region from which they branch and are numbered in sequence within each region8 pairs of cervical nerves (C1-C8)12 pairs of thoracic nerves (T1-T12)5 pairs of lumbar nerves (L1-L5)5 pairs of sacral nerves (S1-S5)1 pair of coccygeal nerves

85Spinal cord terminates at L2Descending from this point are proximal portions of spinal nervesCauda equina

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Spinal nerves branch from two roots from the spinal cordVentral rootMotor axons

Dorsal (posterior) rootSensory axonsDorsal root ganglion

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Dorsal root ganglion

88Spinal PlexusesSpinal nerves divide into parts after branching from the vertebral columnAnterior branchPosterior branchVisceral branch (In thoracic, lumbar spinal nerves only) → sympathetic chain ganglia

The anterior branches of many spinal nerves merge to form a spinal plexusa network of nerves, before continuing to the innervated structures

89In a plexus, the nerve fibers in the anterior branches are sorted and recombined so that fibers going to a specific body part are carried in the same spinal nerve, although they may originate in several different spinal nerves.

There are four pairs of plexuses: cervical, brachial, lumbar, and sacral.

90Cervical plexusFormed by first four cervical nerves (C1~C4)Nerves for muscles and skin of neck, head, and shouldersphrenic nerves: stimulate diaphragm

Brachial plexusFormed by last four cervical nerves and first thoracic nerve (C5~T1)Nerves for skin and muscles of arms and shoulders

91Sacral plexusLast two lumbar nerves and first four sacral nerves (L5-S4)Nerves from it service skin and muscles of buttocks and legs

92ReflexesRapid, involuntary, and predictable responses to internal and external stimuliReflexes require few neurons in their pathways and therefore produce very rapid responses to stimuli.

Rapid, involuntary, and predictable responses to internal and external stimuliReflexes require few neurons in their pathways and therefore produce very rapid responses to stimuli.Reflex pathways are called reflex arcsTwo types of reflexesAutonomic reflexesAct on smooth muscle, cardiac muscle, and glands

e.g. heart rate, blood pressure, digestion…Maintain homeostasis and body functions at unconscious level

Somatic reflexesAct on skeletal muscle

e.g. withdrawal reflexConscious awareness of these reflexes exists

9394 8.8 Autonomic Nervous System (ANS)

ANS consists of portions of central and peripheral nervous systemsANS functions involuntarily without conscious controlmostly by involuntary reflexes

Purpose is to maintain homeostasis in response to changes in internal conditionsaffects cardiac muscle, smooth muscle, and glands

95OrganizationAutonomic nerve fibers are motor neuronsUnlike somatic nervous system, ANS uses two motor neurons to relay impulses to effectorsPreganglionic neuron extends from CNS to autonomic ganglionPostganglionic neuron extends from ganglion to visceral effector

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ANS is subdivided intoSympathetic divisionParasympathetic Division

Parasympathetic Division

1) Sympathetic divisionPreganglionic fibers arise from thoracic and lumbar regions (T1~L2)Preganglionic fibers either synapse with postganglionic neurons in sympathetic chain ganglia (paravertebral) or a collateral ganglion

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2) Parasympathetic divisionPreganglionic fibers arise from brain stem and sacral region of spinal cordSynapse with postganglionic neurons in ganglia close to visceral organs

Most visceral organs are innervated by both divisions of the ANSBut a few, such as sweat glands and blood vessels, receive only sympathetic fibers.

100Autonomic Neurotransmitters

All preganglionic neurons release acetylcholine (Ach)Parasympathetic postganglionic use acetylcholine (Ach)Sympathetic postganglionic use norepinephrine (NE)

101Functions of the ANS: The two divisions have opposing effects on organsSympathetic divisionprepare the body for physical action to meet emergencies“Fight or Flight”

Parasympathetic divisiondominates during normal, non-stressful conditionsusually opposite of sympathetic division

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103104 8.9 Disorders of the Nervous System

Disorders are characterized asInflammatory disorders

Non-inflammatory disorders

105 Inflammatory Disorders

MeningitisBacterial, fungal, or viral infection of the meninges

If the brain is involved, it is called encephalitis

106NeuritisInflammation of nerve or nerves

Caused by infection, compression, or trauma

Severe to moderate pain

107SciaticaNeuritis involving the sciatic nervePain can be severe with radiating pain down thigh and leg

ShinglesCaused by reactivation of the chickenpox virus that has been dormant in the nerve rootPainful blisters form at sensory nerve endings

108 Non-inflammatory Disorders

Alzheimer’s diseaseCharacterized by progressive loss of memory, disorientation, and mood swingsExhibit loss of cholinergic neurons in brainAlso have reduced ability to secrete acetylcholine

Exhibit loss of cholinergic neurons in brainAlso have reduced ability to secrete acetylcholine

109Cerebral palsyPartial paralysis and possible mental retardationCaused by prenatal brain damage, German measles, delivery brain trauma

Cerebrovascular accidentsDisorders of brain blood vesselsCaused by blood clots, aneurysms, or hemorrhageCause severe brain damage

110ComasPatient is unconscious and cannot be arousedAlteration of reticular formation function may be a cause

ConcussionCaused by severe jarring of brain due to blow to the headCan be accompanied by unconsciousness, amnesia, and confusion

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DyslexiaPerson reverses letters or syllables in words or words within sentencesDue to malfunctioning language center

113EpilepsyCan be hereditary or triggered by injuries, infections, or tumorsTwo types of epilepsyGrand mal epilepsyConvulsive seizures

Convulsive seizures

Petit mal epilepsyMomentary loss of reality without convulsion or unconsciousness

114FaintingLoss of consciousness due to sudden reduction in brain blood supplyPhysical or psychological causes

115HeadachesVarious causesMost due to dilation of blood vessels within the meningesSome are caused by tension in head and neck muscles

Migraine headachesVisual or digestive side effectsCauses include stress, allergies, or fatigue

Sinus headachesCause may be inflammation that increases pressure within the sinuses

116Mental illnessesNeurosesMild maladjustments to life situationsProduce anxiety and interfere with normal behavior

PsychosesSevere mental disordersCause delusions, hallucinations, or withdrawal from reality

117Multiple sclerosisProgressive degeneration of myelin sheath around CNS processesAlso have development of scleroses

Also have development of sclerosesEffect is a short-circuiting of neural pathways, which impairs motor function

NeuralgiaPain arising from a nerve regardless of cause of pain

118ParalysisPermanent loss of motor control over body partsCommon cause is injury to CNS

Parkinson’s diseaseInsufficient dopamine production by basal nuclei in cerebrumCauses tremors and impairs skeletal muscle contraction