<?xml version="1.0" encoding="us-ascii"?>
<!-- DFIN ActiveDisclosure(SM) XBRL Schema Document - http://www.dfinsolutions.com/ -->
<!-- Creation Date      : 2021-01-09T20:27:51.0919573+00:00 -->
<!-- Version            : 5.0.1.571 -->
<!-- Package ID         : 68df5d2304b64c5bb75fac3539217e03 -->
<!-- Copyright (c) 2021 Donnelley Financial Solutions, Inc. All Rights Reserved. -->
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        <link:definition>100360 - Disclosure - Other Income (Expense), Net (Tables)</link:definition>
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  <xsd:element id="atha_StockIssuedDuringPeriodSharesWarrantsExercised" name="StockIssuedDuringPeriodSharesWarrantsExercised" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="atha_TemporaryEquityStockAndWarrantsIssuedDuringPeriodSharesPreferredStockAndWarrants" name="TemporaryEquityStockAndWarrantsIssuedDuringPeriodSharesPreferredStockAndWarrants" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="atha_TemporaryEquityStockAndWarrantsIssuedDuringPeriodValuePreferredStockAndWarrants" name="TemporaryEquityStockAndWarrantsIssuedDuringPeriodValuePreferredStockAndWarrants" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="atha_TemporaryEquityStockIssuedDuringPeriodSharesConversionOfConvertibleSecurities" name="TemporaryEquityStockIssuedDuringPeriodSharesConversionOfConvertibleSecurities" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="atha_TemporaryEquityStockIssuedDuringPeriodValueConversionOfConvertibleSecurities" name="TemporaryEquityStockIssuedDuringPeriodValueConversionOfConvertibleSecurities" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="atha_AdjustmentsToAdditionalPaidInCapitalStockOfInitialPublicOfferingCosts" name="AdjustmentsToAdditionalPaidInCapitalStockOfInitialPublicOfferingCosts" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="atha_DeferredOfferingCostsIncludedInAccountsPayableAndAccruedLiabilities" name="DeferredOfferingCostsIncludedInAccountsPayableAndAccruedLiabilities" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="atha_IssuanceOfCommonStockWarrantsInConnectionWithSeriesBConvertiblePreferredStockFinancing" name="IssuanceOfCommonStockWarrantsInConnectionWithSeriesBConvertiblePreferredStockFinancing" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="atha_RecognitionOfWarrantLiabilityInConnectionWithConvertiblePreferredStockFinancing" name="RecognitionOfWarrantLiabilityInConnectionWithConvertiblePreferredStockFinancing" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="atha_DerivativeLiabilityRecordedUponIssuanceOfConvertibleNotes" name="DerivativeLiabilityRecordedUponIssuanceOfConvertibleNotes" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="atha_ConvertibleNotesDisclosureTextBlock" name="ConvertibleNotesDisclosureTextBlock" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="atha_EmployeeBenefitPlansAbstract" name="EmployeeBenefitPlansAbstract" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="atha_GrantLiabilityPolicyTextBlock" name="GrantLiabilityPolicyTextBlock" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="atha_DerivativeLiabilityConvertibleNotesDiscountAndAmortizationPolicyTextBlock" name="DerivativeLiabilityConvertibleNotesDiscountAndAmortizationPolicyTextBlock" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="atha_GrantIncomePolicyTextBlock" name="GrantIncomePolicyTextBlock" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="atha_UnauditedProFormaNetLossPerShareAttributableToCommonStockholders" name="UnauditedProFormaNetLossPerShareAttributableToCommonStockholders" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="atha_RelatedPartyTransactionsPolicyTextBlock" name="RelatedPartyTransactionsPolicyTextBlock" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="atha_EmergingGrowthCompanyStatusPolicyTextBlock" name="EmergingGrowthCompanyStatusPolicyTextBlock" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="atha_UnauditedInterimCondensedFinancialStatementsPolicyTextBlock" name="UnauditedInterimCondensedFinancialStatementsPolicyTextBlock" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="atha_ConvertiblePreferredStockWarrantsLiability" name="ConvertiblePreferredStockWarrantsLiability" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="atha_GrantLiabilityMember" name="GrantLiabilityMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="atha_ConvertiblePreferredStockWarrantLiabilityMember" name="ConvertiblePreferredStockWarrantLiabilityMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="atha_ScheduleOfReservedSharesOfCommonStockForFutureIssuanceTableTextBlock" name="ScheduleOfReservedSharesOfCommonStockForFutureIssuanceTableTextBlock" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="atha_DescriptionOfBusinessLineItems" name="DescriptionOfBusinessLineItems" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="atha_DescriptionOfBusinessTable" name="DescriptionOfBusinessTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="atha_CommonStockWarrantsMember" name="CommonStockWarrantsMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="atha_SeriesBConvertiblePreferredStockWarrantMember" name="SeriesBConvertiblePreferredStockWarrantMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="atha_ConvertiblePreferredStockCommonStockWarrantsOrSeriesBConvertiblePreferredStockWarrantsMember" name="ConvertiblePreferredStockCommonStockWarrantsOrSeriesBConvertiblePreferredStockWarrantsMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="atha_BasisOfPresentationAndSignificantAccountingPoliciesLineItems" name="BasisOfPresentationAndSignificantAccountingPoliciesLineItems" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="atha_BasisOfPresentationAndSignificantAccountingPoliciesTable" name="BasisOfPresentationAndSignificantAccountingPoliciesTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="atha_AlzheimersAssociationMember" name="AlzheimersAssociationMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="atha_AccruedExpensesMember" name="AccruedExpensesMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="atha_UnsecuredRelatedPartyNoteReceivableMember" name="UnsecuredRelatedPartyNoteReceivableMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="atha_GrantsAwarded" name="GrantsAwarded" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="atha_GrantsReceived" name="GrantsReceived" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="atha_RemainingGrantAmount" name="RemainingGrantAmount" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="atha_GrantsAmountReceivedInExcessOfQualifyingExpense" name="GrantsAmountReceivedInExcessOfQualifyingExpense" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="atha_DebtInstrumentMaturityMonthAndYear" name="DebtInstrumentMaturityMonthAndYear" type="xbrli:gYearMonthItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="atha_ShortTermInvestmentsMaturityPeriod" name="ShortTermInvestmentsMaturityPeriod" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="atha_MilestonePaymentsAmountPayableUponTrialInitiation" name="MilestonePaymentsAmountPayableUponTrialInitiation" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="atha_MilestonePaymentsAmountPayableUponMarketingApproval" name="MilestonePaymentsAmountPayableUponMarketingApproval" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="atha_WashingtonStateUniversityLicenseAgreementMember" name="WashingtonStateUniversityLicenseAgreementMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="atha_PhaseTwoClinicalTrailInitiationOfLicenseAgreementMember" name="PhaseTwoClinicalTrailInitiationOfLicenseAgreementMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="atha_PhaseThreeClinicalTrailInitiationOfLicenseAgreementMember" name="PhaseThreeClinicalTrailInitiationOfLicenseAgreementMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="atha_WashingtonLifeServicesDiscoveryFundGrantAgreementMember" name="WashingtonLifeServicesDiscoveryFundGrantAgreementMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="atha_AnnualMinimumRoyaltyPaymentsAfterFirstCommercialSale" name="AnnualMinimumRoyaltyPaymentsAfterFirstCommercialSale" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="atha_NetSalesExemptFromRoyaltyPayment" name="NetSalesExemptFromRoyaltyPayment" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="atha_RoyaltyObligation" name="RoyaltyObligation" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="atha_GrantLiabilityMaximumAmountPayableUnderAgreement" name="GrantLiabilityMaximumAmountPayableUnderAgreement" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="atha_EstimatedFairValueOfGrantLiability" name="EstimatedFairValueOfGrantLiability" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="atha_UnsecuredConvertibleNotesMember" name="UnsecuredConvertibleNotesMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="atha_SeriesB1ConvertiblePreferredStockMember" name="SeriesB1ConvertiblePreferredStockMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="atha_PaycheckProtectionProgramCARESActMember" name="PaycheckProtectionProgramCARESActMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="atha_ConvertibleNotesUnamortizedDiscountRelatedToDerivativeLiability" name="ConvertibleNotesUnamortizedDiscountRelatedToDerivativeLiability" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="atha_LaboratoryAndOfficeFacilitiesMember" name="LaboratoryAndOfficeFacilitiesMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="atha_SeriesAConvertiblePreferredStockMember" name="SeriesAConvertiblePreferredStockMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="atha_SeriesA1ConvertiblePreferredStockMember" name="SeriesA1ConvertiblePreferredStockMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="atha_SeriesBConvertiblePreferredStockMember" name="SeriesBConvertiblePreferredStockMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="atha_ClassOfWarrantOrRightPurchasePriceOneMember" name="ClassOfWarrantOrRightPurchasePriceOneMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="atha_StockOptionsOutstandingMember" name="StockOptionsOutstandingMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="atha_SharesAvailableForFutureGrantsMember" name="SharesAvailableForFutureGrantsMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="atha_SharesAvailableForFutureGrantUnderTwoThousandTwentyEquityIncentivePlanMember" name="SharesAvailableForFutureGrantUnderTwoThousandTwentyEquityIncentivePlanMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="atha_SharesAvailableForFutureGrantUnderEmployeeStockPurchasePlanMember" name="SharesAvailableForFutureGrantUnderEmployeeStockPurchasePlanMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="atha_WarrantsExpirationDate" name="WarrantsExpirationDate" type="xbrli:dateItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="atha_EquityIncentivePlanMember" name="EquityIncentivePlanMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="atha_EmployeesAndDirectorsMember" name="EmployeesAndDirectorsMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="atha_AdvisorsMember" name="AdvisorsMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="atha_NonEmployeesMember" name="NonEmployeesMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="atha_OptionAgreementOptionTerm" name="OptionAgreementOptionTerm" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="atha_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsWeightedAverageGrantDateFairValueOfOptionsGranted" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsWeightedAverageGrantDateFairValueOfOptionsGranted" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="atha_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAvailableForGrantAuthorized" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAvailableForGrantAuthorized" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="atha_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAvailableForGrantGranted" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAvailableForGrantGranted" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="atha_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAvailableForGrantForfeitedAndExpired" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAvailableForGrantForfeitedAndExpired" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="atha_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAvailableForGrantExercised" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAvailableForGrantExercised" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="atha_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePrice" name="ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePrice" type="num:perShareItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="atha_ExercisePriceOneMember" name="ExercisePriceOneMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="atha_ExercisePriceTwoMember" name="ExercisePriceTwoMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="atha_ExercisePriceThreeMember" name="ExercisePriceThreeMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="atha_ExercisePriceFourMember" name="ExercisePriceFourMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="atha_ExercisePriceFiveMember" name="ExercisePriceFiveMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="atha_ExercisePriceSixMember" name="ExercisePriceSixMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="atha_ExercisePriceSevenMember" name="ExercisePriceSevenMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="atha_ExercisePriceEightMember" name="ExercisePriceEightMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="atha_ExercisePriceNineMember" name="ExercisePriceNineMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="atha_ShareBasedCompensationArrangementByShareBasedPaymentAwardSubjectToRepurchaseUponDiscontinuedService" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardSubjectToRepurchaseUponDiscontinuedService" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="atha_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseRelatedToConvertibleNotesAndDerivativeLiability" name="EffectiveIncomeTaxRateReconciliationNondeductibleExpenseRelatedToConvertibleNotesAndDerivativeLiability" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="atha_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseRelatedToGrantLiability" name="EffectiveIncomeTaxRateReconciliationNondeductibleExpenseRelatedToGrantLiability" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="atha_NetOperatingLossAndTaxCreditCarryForwardsExpirationPeriod" name="NetOperatingLossAndTaxCreditCarryForwardsExpirationPeriod" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="atha_ConvertiblePreferredStockOnConvertedBasisMember" name="ConvertiblePreferredStockOnConvertedBasisMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="atha_UnvestedRestrictedStockAwardsMember" name="UnvestedRestrictedStockAwardsMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="atha_StockOptionsToPurchaseCommonStockMember" name="StockOptionsToPurchaseCommonStockMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="atha_LossOnRemeasurementOfGrantLiability" name="LossOnRemeasurementOfGrantLiability" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="atha_NetIncomeLossUsedInComputingProFormaNetLossPerShareAttributableToCommonStockholdersBasicAndDiluted" name="NetIncomeLossUsedInComputingProFormaNetLossPerShareAttributableToCommonStockholdersBasicAndDiluted" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="atha_ProformaAdjustmentToReflectNetExerciseOfCommonStockWarrants" name="ProformaAdjustmentToReflectNetExerciseOfCommonStockWarrants" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="atha_ChiefOperatingOfficerAndAffiliatesMember" name="ChiefOperatingOfficerAndAffiliatesMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="atha_SeriesBConvertiblePreferredStockWarrantsMember" name="SeriesBConvertiblePreferredStockWarrantsMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="atha_ClassOfWarrantOrRightExercised" name="ClassOfWarrantOrRightExercised" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="atha_ConvertiblePreferredStockConversionPrice" name="ConvertiblePreferredStockConversionPrice" type="num:perShareItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="atha_ConvertiblePreferredStockConversionPricePercentage" name="ConvertiblePreferredStockConversionPricePercentage" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="atha_CommonSharesIssuedUponExerciseOfWarrants" name="CommonSharesIssuedUponExerciseOfWarrants" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="atha_CashCashEquivalentsAndShortTermAndLongTermInvestments" name="CashCashEquivalentsAndShortTermAndLongTermInvestments" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="atha_ProceedsFromIssuanceOfConvertiblePreferredStockAndCommonStockWarrants" name="ProceedsFromIssuanceOfConvertiblePreferredStockAndCommonStockWarrants" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="atha_CashAndCashEquivalentsAccumulatedGrossUnrealizedGainBeforeTax" name="CashAndCashEquivalentsAccumulatedGrossUnrealizedGainBeforeTax" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="atha_CashAndCashEquivalentsAccumulatedGrossUnrealizedLossBeforeTax" name="CashAndCashEquivalentsAccumulatedGrossUnrealizedLossBeforeTax" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="atha_LongTermInvestmentsMember" name="LongTermInvestmentsMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="atha_PaymentOnAchievementOfDevelopmentMilestone" name="PaymentOnAchievementOfDevelopmentMilestone" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="atha_OperatingLeaseExpirationMonthAndYear" name="OperatingLeaseExpirationMonthAndYear" type="xbrli:gYearMonthItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="atha_BothellWashingtonMember" name="BothellWashingtonMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="atha_UniversityOfWashingtonSeattleMember" name="UniversityOfWashingtonSeattleMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
</xsd:schema>
