DESCENDING TRACTSDESCENDING TRACTS

Fiber TypesFiber Types

A Fibers:A Fibers:Somatic, myelinated.Somatic, myelinated.Alpha (Alpha (αα):):

Largest, also referred to as Type I.Largest, also referred to as Type I.Beta (Beta (ββ):):

Also referred to as Type II.Also referred to as Type II.Gamma (Gamma (γγ):):Delta (Delta (δδ):):

Smallest, referred to as Type IV.Smallest, referred to as Type IV.

Fiber TypesFiber Types

B Fibers:B Fibers:Lightly myelinated.Lightly myelinated.

Preganglionic fibers of ANS.Preganglionic fibers of ANS. C Fibers:C Fibers:

Unmyelinated.Unmyelinated.

Found in somatic and autonomic systems.Found in somatic and autonomic systems.

Also referred to as Type IV fibers.Also referred to as Type IV fibers.

Fiber TypesFiber Types

Sensory fibers are either:Sensory fibers are either:A-A-αα or A- or A-ββ fibers: fibers:

Conduction rate = 30-120 m/sec.Conduction rate = 30-120 m/sec.

A-A-δδ fibers: fibers:

Conduction rate = 4-30 m/sec.Conduction rate = 4-30 m/sec.

C fibers:C fibers:

Conduction rate is less than Conduction rate is less than 2.5m/sec.2.5m/sec.

Fiber TypesFiber Types

Nociceptors and thermoreceptors are Nociceptors and thermoreceptors are related to C fibers or A-related to C fibers or A-δδ fibers. fibers.

Generalizations: Motor PathsGeneralizations: Motor Paths

Typical descending pathway consists of a Typical descending pathway consists of a series of two motor neurons:series of two motor neurons:

Upper motor neurons (UMNs)Upper motor neurons (UMNs)

Lower motor neurons (LMNs)Lower motor neurons (LMNs)

Does not take into consideration the Does not take into consideration the association neurons between UMNs and association neurons between UMNs and LMNsLMNs

Upper Motor NeuronsUpper Motor Neurons

Are entirely within the CNS.Are entirely within the CNS.Originate in:Originate in:

Cerebral cortexCerebral cortexCerebellum Cerebellum Brainstem Brainstem

Form descending tractsForm descending tracts

Lower Motor NeuronsLower Motor Neurons

Begin in CNS.Begin in CNS.From anterior horns of spinal cord.From anterior horns of spinal cord.From brainstem cranial nerve nuclei.From brainstem cranial nerve nuclei.Made up of alpha motor neurons Made up of alpha motor neurons (A-(A-αα))..Make up spinal and cranial nerves.Make up spinal and cranial nerves.

UMN ClassificationUMN Classification

Classified according to where they Classified according to where they synapse in the ventral horn:synapse in the ventral horn:

Medial activation system:Medial activation system:

Innervate postural and girdle musclesInnervate postural and girdle muscles

Lateral activation system:Lateral activation system:Associated with distally located muscles Associated with distally located muscles

used used for fine movementsfor fine movements

Nonspecific activating system:Nonspecific activating system:Facilitate local reflex arcsFacilitate local reflex arcs

Pyramidal SystemPyramidal System

Characteristics:Characteristics:Upper motor neurons:Upper motor neurons:

75 – 85% Decussate in pyramids.75 – 85% Decussate in pyramids.

Remainder decussate near synapse with Remainder decussate near synapse with lower motor neurons.lower motor neurons.

Most synapse with association neurons in Most synapse with association neurons in spinal cord central gray.spinal cord central gray.

Pyramidal SystemPyramidal System

Components:Components:

Corticospinal TractCorticospinal Tract

Corticobulbar TractCorticobulbar Tract

Corticospinal Tract DivisionsCorticospinal Tract Divisions

Lateral corticospinal tract:Lateral corticospinal tract:Made up of corticospinal fibers that have Made up of corticospinal fibers that have crossed in medulla.crossed in medulla.

Supply all levels of spinal cord.Supply all levels of spinal cord.Anterior corticospinal tract:Anterior corticospinal tract:

Made up of uncrossed corticospinal fibers Made up of uncrossed corticospinal fibers that cross near level of synapse with that cross near level of synapse with

LMNs.LMNs.

Supply neck and upper limbs.Supply neck and upper limbs.

Corticospinal Tract FunctionsCorticospinal Tract Functions

Add speed and agility to conscious Add speed and agility to conscious movements:movements:

Especially movements of hand.Especially movements of hand.Provide a high degree of motor control:Provide a high degree of motor control:

(i.e., movement of individual fingers)(i.e., movement of individual fingers)

Corticospinal Tract LesionsCorticospinal Tract Lesions

Reduced muscle toneReduced muscle toneClumsinessClumsinessWeaknessWeaknessNot complete paralysisNot complete paralysisNote: complete paralysis results if both Note: complete paralysis results if both

pyramidal and extrapyramidal systems are pyramidal and extrapyramidal systems are involved (as is often the case).involved (as is often the case).

Corticobulbar TractCorticobulbar Tract

Innervates the headInnervates the headMost fibers terminate in reticular formation Most fibers terminate in reticular formation

near cranial nerve nuclei.near cranial nerve nuclei.Association neurons:Association neurons:

Leave reticular formation and synapse in Leave reticular formation and synapse in cranial nerve nuclei.cranial nerve nuclei.

Synapse with lower motor neurons.Synapse with lower motor neurons.

Extrapyramidal SystemExtrapyramidal System

Includes descending motor tracts that do Includes descending motor tracts that do not pass through medullary pyramids or not pass through medullary pyramids or corticobulbar tracts.corticobulbar tracts.

Includes:Includes:

Rubrospinal tractsRubrospinal tracts

Vestibulospinal tractsVestibulospinal tracts

Reticulospinal tractsReticulospinal tracts

Rubrospinal TractRubrospinal Tract

Begins in red nucleus.Begins in red nucleus.Decussates in midbrain.Decussates in midbrain.Descends in lateral funiculus (column).Descends in lateral funiculus (column).Function closely related to cerebellar Function closely related to cerebellar

function.function.Lesions:Lesions:

Impairment of distal arm and hand movement.Impairment of distal arm and hand movement.

Intention tremors (similar to cerebellar lesions)Intention tremors (similar to cerebellar lesions)

Vestibulospinal TractVestibulospinal Tract

Originates in vestibular nuclei:Originates in vestibular nuclei:

Receives major input from vestibular Receives major input from vestibular nerve: nerve:

(CN VIII)(CN VIII)

Descends in anterior funiculus (column).Descends in anterior funiculus (column).Synapses with LMNs to extensor muscles:Synapses with LMNs to extensor muscles:

Primarily involved in maintenance of Primarily involved in maintenance of upright upright posture.posture.

Reticulospinal TractReticulospinal Tract

Originates in various regions of reticular Originates in various regions of reticular formation.formation.

Descends in anterior portion of lateral Descends in anterior portion of lateral funiculus (column).funiculus (column).

Thought to mediate larger movements of Thought to mediate larger movements of trunk and limbs that do not require balance trunk and limbs that do not require balance or fine movements of upper limbs.or fine movements of upper limbs.

BASAL NUCLEIBASAL NUCLEI

Basal Ganglia FunctionsBasal Ganglia Functions

Compare proprioceptive information and Compare proprioceptive information and movement commands.movement commands.

Sequence movements.Sequence movements.Regulate muscle tone and muscle force.Regulate muscle tone and muscle force.May be involved in selecting and inhibiting May be involved in selecting and inhibiting

specific motor synergies.specific motor synergies.

Basal Ganglia FunctionsBasal Ganglia Functions

Basal ganglia are vital for normal Basal ganglia are vital for normal movement but they have no direct movement but they have no direct connections with lower motor neurons.connections with lower motor neurons.

Influence LMNs:Influence LMNs:

Through planning areas of cerebral cortex.Through planning areas of cerebral cortex.

Pedunculopontine nucleus of midbrain.Pedunculopontine nucleus of midbrain.

Basal Ganglia FunctionsBasal Ganglia Functions

Basal nuclei set organism’s level of Basal nuclei set organism’s level of responsiveness to stimuli.responsiveness to stimuli.

Extrapyramidal disorders are associated Extrapyramidal disorders are associated with basal nuclei pathology:with basal nuclei pathology:

Negative symptoms of Negative symptoms of underresponsiveness:underresponsiveness:

AkinesiasAkinesiasi.e. Parkinson diseasei.e. Parkinson disease

Positive symptoms of over-responsiveness:Positive symptoms of over-responsiveness:Choreas, athetoses, ballismsChoreas, athetoses, ballismsi.e. Huntington’s choreai.e. Huntington’s chorea

Basal Nuclei ComponentsBasal Nuclei Components

Corpus striatumCorpus striatumSubstantia nigra (within the midbrain)Substantia nigra (within the midbrain)Subthalamic nuclei (diencephalon)Subthalamic nuclei (diencephalon)Red nucleus (?)Red nucleus (?)Claustrum (?)Claustrum (?)Nucleus accumbens (?)Nucleus accumbens (?)

Corpus StriatumCorpus Striatum

Composed of caudate nucleus + lentiform Composed of caudate nucleus + lentiform nucleus:nucleus:

Striatum = caudate nucleus + putamen.Striatum = caudate nucleus + putamen.

Pallidum = globus pallidus.Pallidum = globus pallidus.Putamen + globus pallidus = lentiform Putamen + globus pallidus = lentiform

nucleus.nucleus.Controls large subconscious movements Controls large subconscious movements

of the skeletal muscles.of the skeletal muscles.The globus pallidus regulates muscle tone.The globus pallidus regulates muscle tone.

Corpus StriatumCorpus Striatum

Corpus Striatum(Telencephalon)

Striatum Pallidum

Caudate Nucleus Putamen Globus Pallidus

Substantia Nigra SubdivisionsSubstantia Nigra Subdivisions

Dorsal pars compacta:Dorsal pars compacta:

Has melanin containing neurons and Has melanin containing neurons and dopaminergic neurons.dopaminergic neurons.

Ventral pars reticularis:Ventral pars reticularis:

Has iron-containing glial cells.Has iron-containing glial cells.

Has serotonin and GABA (no Has serotonin and GABA (no melanin).melanin).

Substantial NigraSubstantial Nigra

Substantia NigraMesencephalon

DorsalPars compacta

VentralPars reticularis

Melanin containing neurons Dopaminergic neurons

Iron-containing glial cellsSerotonin and GABA

Input NucleiInput Nuclei

Striatum Striatum Caudate nucleusCaudate nucleusPutamen Putamen Nucleus accumbensNucleus accumbens

Receive widespread input from:Receive widespread input from:NeocortexNeocortexIntralaminar nucleiIntralaminar nucleiSubstantia nigraSubstantia nigraDorsal raphe nucleusDorsal raphe nucleus

Input NucleiInput Nuclei

Striatum projects to:Striatum projects to:Globus pallidusGlobus pallidusSubstantia nigraSubstantia nigra

Pars reticularisPars reticularis

Via gabaminergic fibersVia gabaminergic fibersMotor and sensory cortices project to Motor and sensory cortices project to

putamen.putamen.Association areas of all lobes project to Association areas of all lobes project to

caudate nucleus.caudate nucleus.

Output NucleiOutput Nuclei

Globus pallidus (medial part)Globus pallidus (medial part)Substantia nigra:Substantia nigra:

Pars reticularisPars reticularisVentral pallidumVentral pallidumFibers project to:Fibers project to:

VA/VL nucleiVA/VL nuclei

Mostly inhibitoryMostly inhibitory

General Core CircuitGeneral Core Circuit

Cerebral cortex to:Cerebral cortex to:Striatum to:Striatum to:Globus pallidus to:Globus pallidus to:Thalamus to:Thalamus to:Portions of motor cortex to:Portions of motor cortex to:Upper motor neuronsUpper motor neurons

Thalamic FasciculiThalamic Fasciculi

Ansa lenticularis:Ansa lenticularis:

Consists of fibers from dorsal portion Consists of fibers from dorsal portion of of globus pallidus.globus pallidus.

Loops under internal capsule.Loops under internal capsule.

To VA/VL complex.To VA/VL complex.

Thalamic FasciculiThalamic Fasciculi

Lenticular fasciculus:Lenticular fasciculus:

Consists of fibers from ventral portion Consists of fibers from ventral portion of globus pallidus.of globus pallidus.

Passes across the internal capsule.Passes across the internal capsule.

To VA/VL complex.To VA/VL complex.

Dopamine Neuronal SystemDopamine Neuronal System

Consists of nigrostriatal fibersConsists of nigrostriatal fibersFrom pars compacta of substantia nigraFrom pars compacta of substantia nigraTo striatumTo striatumDopaminergicDopaminergic

Direct Basal Ganglia CircuitDirect Basal Ganglia Circuit

Motor cortex projects to putamen:Motor cortex projects to putamen:Excitatory (glutamate)Excitatory (glutamate)

Putamen projects to output nuclei (globus Putamen projects to output nuclei (globus pallidus internus and substantia nigra pallidus internus and substantia nigra reticularis):reticularis):

Inhibitory (GABA and substance P)Inhibitory (GABA and substance P)

Basal Ganglia ConnectionsRed = excitatory; Black = Inhibitory

Motor areas of cerebral cortex

Ventrolateralthalamus Putamen

Globus pallidusexternus

Outputnuclei Subthalamic

nuclei

Substantia nigracompacta

LateralActivationpathways

Pedunculo-Pontinenuclei

Reticulospinal and Vestibulospinal

pathways

Direct Basal Ganglia CircuitDirect Basal Ganglia Circuit

Output nuclei project to motor thalamus Output nuclei project to motor thalamus (VA-VL) and pedunculopontine nuclei:(VA-VL) and pedunculopontine nuclei:

Inhibitory (GABA)Inhibitory (GABA)

Ventrolateral (VA-VL) thalamus projects to Ventrolateral (VA-VL) thalamus projects to motor cortex:motor cortex:

ExcitatoryExcitatory

Therefore:Therefore:Increasing input to putamen increases activity in Increasing input to putamen increases activity in corticofugal fiberscorticofugal fibers

Direct Basal Ganglia CircuitDirect Basal Ganglia Circuit

Pedunculopontine nuclei project to Pedunculopontine nuclei project to reticulospinal and vestibulospinal reticulospinal and vestibulospinal pathways.pathways.

Stimulation of pedunculopontine nuclei Stimulation of pedunculopontine nuclei elicit rhythmical behaviors such as elicit rhythmical behaviors such as locomotor patterns.locomotor patterns.

Indirect Basal Ganglia CircuitIndirect Basal Ganglia Circuit

Motor cortex to putamen:Motor cortex to putamen:Excitatory (glutamate)Excitatory (glutamate)

Putamen to globus pallidus externus:Putamen to globus pallidus externus:Inhibitory (GABA and enkephalins)Inhibitory (GABA and enkephalins)

Globus pallidus externus to subthalamic Globus pallidus externus to subthalamic nuclei:nuclei:

Inhibition (GABA)Inhibition (GABA)

Indirect Basal Ganglia CircuitIndirect Basal Ganglia Circuit

Subthalamic nuclei to output nuclei Subthalamic nuclei to output nuclei (substantia nigra reticularis)(substantia nigra reticularis)

Excitatory (glutamate)Excitatory (glutamate)Output nuclei to VA-VL complex (motor Output nuclei to VA-VL complex (motor

thalamus)thalamus)Inhibitory (GABA)Inhibitory (GABA)

Indirect Basal Ganglia CircuitIndirect Basal Ganglia Circuit

VA-VL complex to motor cortex:VA-VL complex to motor cortex:ExcitatoryExcitatory

Therefore: decrease in corticofugal Therefore: decrease in corticofugal pathways.pathways.

Input from Substantia Nigra Input from Substantia Nigra CompactaCompacta

Projects to putamen:Projects to putamen:

Excitatory (dopamine)Excitatory (dopamine)

Two kinds of receptors in basal ganglia Two kinds of receptors in basal ganglia circuit:circuit:

D1: facilitates activity in direct pathwayD1: facilitates activity in direct pathway

D2: inhibits activity in indirect pathwayD2: inhibits activity in indirect pathway