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DESCENDING TRACTS

DESCENDING TRACTS. Fiber Types. A Fibers: Somatic, myelinated. Alpha ( α ): Largest, also referred to as Type I. Beta ( β ): Also referred to as Type II. Gamma ( γ ): Delta ( δ ): Smallest, referred to as Type IV. Fiber Types. B Fibers: Lightly myelinated.

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DESCENDING TRACTS

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  1. DESCENDING TRACTS

  2. Fiber Types • A Fibers: Somatic, myelinated. Alpha (α): Largest, also referred to as Type I. Beta (β): Also referred to as Type II. Gamma (γ): Delta (δ): Smallest, referred to as Type IV.

  3. 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.

  4. Fiber Types • Sensory fibers are either: A-α or A-β fibers: Conduction rate = 30-120 m/sec. A-δ fibers: Conduction rate = 4-30 m/sec. C fibers: Conduction rate is less than 2.5m/sec.

  5. Fiber Types • Nociceptors and thermoreceptors are related to C fibers or A-δ fibers.

  6. 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

  7. Upper Motor Neurons • Are entirely within the CNS. • Originate in: Cerebral cortex Cerebellum Brainstem • Form descending tracts

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

  9. 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

  10. 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.

  11. Pyramidal System • Components: Corticospinal Tract Corticobulbar Tract

  12. 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.

  13. 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)

  14. 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).

  15. 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.

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

  17. 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)

  18. 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.

  19. 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.

  20. BASAL NUCLEI

  21. 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.

  22. 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.

  23. Basal Ganglia Functions • Basal nuclei set organism’s 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. Huntington’s chorea

  24. Basal Nuclei Components • Corpus striatum • Substantia nigra (within the midbrain) • Subthalamic nuclei (diencephalon) • Red nucleus (?) • Claustrum (?) • Nucleus accumbens (?)

  25. 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.

  26. Corpus Striatum

  27. 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).

  28. Substantial Nigra

  29. Input Nuclei • Striatum Caudate nucleus Putamen Nucleus accumbens • Receive widespread input from: Neocortex Intralaminar nuclei Substantia nigra Dorsal raphe nucleus

  30. 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.

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

  32. General Core Circuit • Cerebral cortex to: • Striatum to: • Globus pallidus to: • Thalamus to: • Portions of motor cortex to: • Upper motor neurons

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

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

  35. Dopamine Neuronal System • Consists of nigrostriatal fibers • From pars compacta of substantia nigra • To striatum • Dopaminergic

  36. 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)

  37. 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

  38. 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

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

  40. 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)

  41. Indirect Basal Ganglia Circuit • Subthalamic nuclei to output nuclei (substantia nigra reticularis) Excitatory (glutamate) • Output nuclei to VA-VL complex (motor thalamus) Inhibitory (GABA)

  42. Indirect Basal Ganglia Circuit • VA-VL complex to motor cortex: Excitatory • Therefore: decrease in corticofugal pathways.

  43. 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

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