Anatomy Exam 2 Lecture 18A: Spinal Cord & Pathway

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ANATOMY EXAM 2 LECTURE 18a: SPINAL CORD & PATHWAY

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.

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Ascending pathway

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Carry info to more rostral areas of CNS

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Descending pathway

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Carry info to more caudal regions of CNS.

Deliver MOTOR instructions from brain to spinal cord

2 groups:

1. Pryramidal (corticospinal) tracts: somatic motor (voluntary)

2. Extrapyramidals

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Five features of ascending and descending paths:

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1. Contralateral
2. Chain of neurons
3. Connected by white matter tracts.
4. Somatotopy
5. Bilateral

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Lateral corticospinal tract

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Pyramidal path
Somatic motor

A two neuron path
1. Upper motor neuron in motor cortex in precentral gyrus.
2. Lower motor neuron in Ventral horn of spinal cord.

Upper neuron sends axon via:
Internal capsule-->ipsilateral brainstem-->pyramidal decussation-->contralateral spinal cord-->lateral column--> SYNAPSE with lower motor neuron of ventral horn--> ventral root--> spinal nerve--> target muscle

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Homunculus

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A drawing of body regions on a brain region representing those body regions.

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Flaccid paralysis

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Damage to lower motor neurons

Muscle atrophy

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Spastic paralysis

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Damage to upper motor neurons

Hyper reflexive
Tense musculature

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For ascending or descending paths, ask yourself:

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1. How many neurons involved?
2. Where are cell bodies found?
3. Where does the path decussate?

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Four main ascending pathways

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1. Dorsal column
2. Spinothalamic
3. Posterior spinocerebellar
4. Anterior spinocerebellar

Conduct general somatic sensory impulses.

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Somatosensory systems

1. where is the first neuron found?
2. what kind of neuron is it?
3. how does the axon enter the spinal cord?
4. where will the info go in the cortex?
5.contra or ipsilateral cortex?
6. where does almost all sensory info synapse on its way to cerebral cortex.

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1. DRG
2. unipolar
3. dorsal root
4. postcentral gyrus
5. contra
6. thalamus

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Somatosensory paths

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Nuclei connected by white matter tracts.

Synapse in thalamus

Contralateral representation

Somatotopy mapping

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Dorsal columns

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2 pairs of columns

1. cuneate fasciculus (upper body)

2. gracile fasciculus (lower body)

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Dorsal column/Medial lemniscus (DCML)

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Ascends in ipsilateral spinal cord

Discriminate touch

1st Neuron: DRG dorsal columns (cuneate fasciculus and gracile fasciculus)

2nd dorsal column nuclei (cuneate nucleus-lateral upper body and gracile nucleus-medial lower body)

Decussate after synapse

3rd VPN: ventral posterior nucleus--> thalamus

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Dorsal column

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Cuneate fasciculus + gracile fasciculus

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Dorsal column nuclei

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Cuneate nucleus + gracile nucleus

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Spinothalamic (STP)/ Anterolateral pathway (ALS)

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Pain and temperature
Crude touch

1st neuron: DRG
Decussate at level of spinal cord in VWC (ventral white commissure)

2nd neuron: in dorsal horn of spinal cord (first synapse)

Axon of second neuron ascends in contralateral spinal cord.

3rd neuron: VPN in thalamus.

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Rules that apply to BOTH sensory paths:

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First order neurons
Afferent fibers in spinal nerve
Ipsilateral DRG/unipolar
Axon terminal synapses in ipsilateral CNS.

Second order neurons
Soma ipsilateral in
Spinal cord (for spinothalamic)
Medulla (for dorsal column)
Decussates first, then ascends rostrally to synapse in the contralateral thalamus.

Third order
Soma in VPN of thalamus
Sends axon to postcentral gyrus of cortex via internal capsule.

NOTE: the second order neuron is the only one with a different location btw the two pathways.

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Differences btw the two paths:

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Spinothalamic: ascends on the contralateral side. Decussates in the spinal cord.

Dorsal: ascends on the ipsilateral side. Decussates in the brainstem?

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Where does the 2nd neuron of the spinothalamic pathway cross?

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In the VWC.

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Ascending pathways (sensory)

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1. chain of 3 neurons (primary, secondary, and tertiary).
a. 1st in DRG
b. 2nd neuron decussates
c. 3rd neuron in the thalamus.

2. pathways and nuclei display somatotopy.

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Descending pathways (motor)

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2 neurons
a. upper in motor cortex
b. lower in ventral horn

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Where is the upper neuron found?

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Sacral spinal cord
Cervical spinal cord
Medulla
Thalamus
Cortex

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Where is the lower neuron found?

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Same as upper neurons.

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Where does the pyramidal tract decussate?

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sacral spinal cord
cervical spinal cord
medulla
midbrain
thalamus
cortex

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Where is the lower motor neuron found in the spinal cord?

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dorsal horn
intermediate
ventral horn

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Where is the second cell body in the dorsal column path?

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thalamus
cortex
medulla
spinal cord
DRG

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Where is the second cell body in the spinothalamic path?

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thalamus
cortex
medulla
spinal cord
DRG

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Which axon decussates in the Dorsal column path?

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1, 2, or 3?

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Which axon decussates in the spinothalamic path?

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1, 2, or 3?