DESCENDING TRACTS

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DESCENDING TRACTS
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Fiber Types

• A Fibers
• Somatic, myelinated.
• Alpha (a)
• Largest, also referred to as Type I.
• Beta (ß)
• Also referred to as Type II.
• Gamma (?)
• Delta (d)
• Smallest, referred to as Type IV.

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Fiber Types

• B Fibers
• Lightly myelinated.
• Preganglionic fibers of ANS.
• C Fibers
• Unmyelinated.
• Found in somatic and autonomic systems.
• Also referred to as Type IV fibers.

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Fiber Types

• Sensory fibers are either
• A-a or A-ß fibers
• Conduction rate 30-120 m/sec.
• A-d fibers
• Conduction rate 4-30 m/sec.
• C fibers
• Conduction rate is less than 2.5m/sec.

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Fiber Types

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

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Generalizations Motor Paths

• Typical descending pathway consists of a series
  of two motor neurons
• Upper motor neurons (UMNs)
• Lower motor neurons (LMNs)
• Does not take into consideration the
  association neurons between UMNs and LMNs

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Upper Motor Neurons

• Are entirely within the CNS.
• Originate in
• Cerebral cortex
• Cerebellum
• Brainstem
• Form descending tracts

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Lower Motor Neurons

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

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UMN Classification

• Classified according to where they synapse in the
  ventral horn
• Medial activation system
• Innervate postural and girdle muscles
• Lateral activation system
• Associated with distally located muscles used
  for fine movements
• Nonspecific activating system
• Facilitate local reflex arcs

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Pyramidal System

• Characteristics
• Upper motor neurons
• 75 85 Decussate in pyramids.
• Remainder decussate near synapse with lower
  motor neurons.
• Most synapse with association neurons in
  spinal cord central gray.

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Pyramidal System

• Components
• Corticospinal Tract
• Corticobulbar Tract

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Corticospinal Tract Divisions

• Lateral corticospinal tract
• Made up of corticospinal fibers that have
  crossed in medulla.
• Supply all levels of spinal cord.
• Anterior corticospinal tract
• Made up of uncrossed corticospinal fibers that
  cross near level of synapse with LMNs.
• Supply neck and upper limbs.

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Corticospinal Tract Functions

• Add speed and agility to conscious movements
• Especially movements of hand.
• Provide a high degree of motor control
• (i.e., movement of individual fingers)

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Corticospinal Tract Lesions

• Reduced muscle tone
• Clumsiness
• Weakness
• Not complete paralysis
• Note complete paralysis results if both
  pyramidal and extrapyramidal systems are involved
  (as is often the case).

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Corticobulbar Tract

• Innervates the head
• Most fibers terminate in reticular formation near
  cranial nerve nuclei.
• Association neurons
• Leave reticular formation and synapse in
  cranial nerve nuclei.
• Synapse with lower motor neurons.

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Extrapyramidal System

• Includes descending motor tracts that do not pass
  through medullary pyramids or corticobulbar
  tracts.
• Includes
• Rubrospinal tracts
• Vestibulospinal tracts
• Reticulospinal tracts

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Rubrospinal Tract

• Begins in red nucleus.
• Decussates in midbrain.
• Descends in lateral funiculus (column).
• Function closely related to cerebellar function.
• Lesions
• Impairment of distal arm and hand movement.
• Intention tremors (similar to cerebellar
  lesions)

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Vestibulospinal Tract

• Originates in vestibular nuclei
• Receives major input from vestibular nerve
• (CN VIII)
• Descends in anterior funiculus (column).
• Synapses with LMNs to extensor muscles
• Primarily involved in maintenance of upright
  posture.

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Reticulospinal Tract

• Originates in various regions of reticular
  formation.
• Descends in anterior portion of lateral funiculus
  (column).
• Thought to mediate larger movements of trunk and
  limbs that do not require balance or fine
  movements of upper limbs.

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BASAL NUCLEI
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Basal Ganglia Functions

• Compare proprioceptive information and movement
  commands.
• Sequence movements.
• Regulate muscle tone and muscle force.
• May be involved in selecting and inhibiting
  specific motor synergies.

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Basal Ganglia Functions

• Basal ganglia are vital for normal movement but
  they have no direct connections with lower motor
  neurons.
• Influence LMNs
• Through planning areas of cerebral cortex.
• Pedunculopontine nucleus of midbrain.

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Basal Ganglia Functions

• Basal nuclei set organisms level of
  responsiveness to stimuli.
• Extrapyramidal disorders are associated with
  basal nuclei pathology
• Negative symptoms of underresponsiveness
• Akinesias
• i.e. Parkinson disease
• Positive symptoms of over-responsiveness
• Choreas, athetoses, ballisms
• i.e. Huntingtons chorea

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Basal Nuclei Components

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

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Corpus Striatum

• Composed of caudate nucleus lentiform nucleus
• Striatum caudate nucleus putamen.
• Pallidum globus pallidus.
• Putamen globus pallidus lentiform nucleus.
• Controls large subconscious movements of the
  skeletal muscles.
• The globus pallidus regulates muscle tone.

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Corpus Striatum
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Substantia Nigra Subdivisions

• Dorsal pars compacta
• Has melanin containing neurons and
  dopaminergic neurons.
• Ventral pars reticularis
• Has iron-containing glial cells.
• Has serotonin and GABA (no melanin).

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Substantial Nigra
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Input Nuclei

• Striatum
• Caudate nucleus
• Putamen
• Nucleus accumbens
• Receive widespread input from
• Neocortex
• Intralaminar nuclei
• Substantia nigra
• Dorsal raphe nucleus

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Input Nuclei

• Striatum projects to
• Globus pallidus
• Substantia nigra
• Pars reticularis
• Via gabaminergic fibers
• Motor and sensory cortices project to putamen.
• Association areas of all lobes project to caudate
  nucleus.

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Output Nuclei

• Globus pallidus (medial part)
• Substantia nigra
• Pars reticularis
• Ventral pallidum
• Fibers project to
• VA/VL nuclei
• Mostly inhibitory

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General Core Circuit

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

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Thalamic Fasciculi

• Ansa lenticularis
• Consists of fibers from dorsal portion of
  globus pallidus.
• Loops under internal capsule.
• To VA/VL complex.

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Thalamic Fasciculi

• Lenticular fasciculus
• Consists of fibers from ventral portion of
  globus pallidus.
• Passes across the internal capsule.
• To VA/VL complex.

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Dopamine Neuronal System

• Consists of nigrostriatal fibers
• From pars compacta of substantia nigra
• To striatum
• Dopaminergic

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Direct Basal Ganglia Circuit

• Motor cortex projects to putamen
• Excitatory (glutamate)
• Putamen projects to output nuclei (globus
  pallidus internus and substantia nigra
  reticularis)
• Inhibitory (GABA and substance P)

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Basal Ganglia ConnectionsRed excitatory Black
Inhibitory
Motor areas of cerebral cortex
Ventrolateral thalamus
Putamen
Globus pallidus externus
Output nuclei
Subthalamic nuclei
Pedunculo- Pontine nuclei
Lateral Activation pathways
Reticulospinal and Vestibulospinal pathways
Substantia nigra compacta
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Direct Basal Ganglia Circuit

• Output nuclei project to motor thalamus (VA-VL)
  and pedunculopontine nuclei
• Inhibitory (GABA)
• Ventrolateral (VA-VL) thalamus projects to motor
  cortex
• Excitatory
• Therefore
• Increasing input to putamen increases activity
  in corticofugal fibers

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Direct Basal Ganglia Circuit

• Pedunculopontine nuclei project to reticulospinal
  and vestibulospinal pathways.
• Stimulation of pedunculopontine nuclei elicit
  rhythmical behaviors such as locomotor patterns.

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Indirect Basal Ganglia Circuit

• Motor cortex to putamen
• Excitatory (glutamate)
• Putamen to globus pallidus externus
• Inhibitory (GABA and enkephalins)
• Globus pallidus externus to subthalamic nuclei
• Inhibition (GABA)

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Indirect Basal Ganglia Circuit

• Subthalamic nuclei to output nuclei (substantia
  nigra reticularis)
• Excitatory (glutamate)
• Output nuclei to VA-VL complex (motor thalamus)
• Inhibitory (GABA)

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Indirect Basal Ganglia Circuit

• VA-VL complex to motor cortex
• Excitatory
• Therefore decrease in corticofugal pathways.

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Input from Substantia Nigra Compacta

• Projects to putamen
• Excitatory (dopamine)
• Two kinds of receptors in basal ganglia
  circuit
• D1 facilitates activity in direct pathway
• D2 inhibits activity in indirect pathway