Neuro 13 descending tracts student
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- 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:
- 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
- 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
- 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
- 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
- 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.
- 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.
- 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:
- 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
- Caudate nucleus
- Nucleus accumbens
- Receive widespread input from:
- 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