Neuro 13 descending tracts student

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lecture 10-19-11

Transcript of Neuro 13 descending tracts student

  • 1. DESCENDING TRACTS

2. Fiber Types

  • A Fibers:
  • Somatic, myelinated.
  • Alpha ( ):
  • Largest, also referred to as Type I.
  • Beta ( ):
  • Also referred to as Type II.
  • Gamma ( ):
  • Delta ( ):
  • 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- or A- fibers:
  • Conduction rate = 30-120 m/sec.
  • A- 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- 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 theassociation neurons between UMNs andLMNs

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- ) .
  • 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 usedfor 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 withlower motor neurons.
  • Most synapse with association neurons inspinal cord central gray.

11. Pyramidal System

  • Components:
  • Corticospinal Tract
  • Corticobulbar Tract

12. Corticospinal Tract Divisions

  • Lateral corticospinal tract:
  • Made up of corticospinal fibers that havecrossed in medulla.
  • Supply all levels of spinal cord.
  • Anterior corticospinal tract:
  • Made up of uncrossed corticospinal fibersthat 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 incranial 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 uprightposture.

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 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

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 = lentiformnucleus.
  • Controls large subconscious movements of the skeletal muscles.
  • The globus pallidus regulates muscle tone.

26. Corpus Striatum Corpus Striatum (Telencephalon) Striatum Pallidum Caudate Nucleus Putamen Globus Pallidus 27. Substantia Nigra Subdivisions

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

28. Substantial Nigra Substantia Nigra Mesencephalon Dorsal Pars compacta Ventral Pars reticularis Melanin containing neurons Dopaminergic neurons Iron-containing glial cells Serotonin and GABA 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 ofglobus pallidus.
  • Loops under internal capsule.
  • To VA/VL complex.

34. Thalamic Fasciculi

  • Lenticular fasciculus:
  • Consists of fibers from ventral portionof globus pal