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Physiology II Exam 2 > Spinal Cord, Brainstem, Cortex Control Of Motor Function > Flashcards

Spinal Cord, Brainstem, Cortex Control Of Motor Function Flashcards

1
Q

What are some characteristics of the upper motor neurons?

A
  • originate in motor cortices
  • 75-85% decussate in pyramids and form the lateral corticospinal tracts. remainder decussate near synapse with lower motor neurons to form anterior corticospinal tracts
  • most synapse with association neurons in spinal cord central gray
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2
Q

How are the upper motor neurons of the pyramidal system classified?

A

-classified according to where they synapse in the ventral horn

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3
Q

What are the classes of upper motor neurons of the pyramidal system?

A
  • 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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4
Q

What is the difference between the layer and anterior corticospinal tract?

A

Lateral:

  • made up of corticospinal fibers that have crosses in medulla
  • supply all levels of spinal cord

Anterior:

  • made up of uncrossed corticospinal fibers that cross near level of synapse with LMNs
  • supply neck and upper limbs
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5
Q

What is the corticospinal tract? Where does it originate?

A
  • aka pyramidal tract
  • origin of tract:
  1. Primary motor cortex
  2. Premotor cortex
  3. Somatosensory area
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6
Q

What is the pathway of the corticospinal tract?

A
  • site of origin -> internal capsule -> medullary pyramids -> cross in lower medulla(most fibers) -> lateral columns of spinal cord (lateral corticospinal tract)
  • some fibers do not cross but continue down ipsilateral lay in ventral corticospinal tract
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7
Q

What are giant pyramidal (Betz) cells?

A
  • located in motor cortex
  • large cells
  • large fibers
  • transmit at 70/sec
  • make up about 3% of fibers in the tract
  • send collaterals back to cortex
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8
Q

What happens to the other fibers from the cortex?

A
  • pass into caudate nucleus and putamen
  • pass to red nucleus
  • pass to reticular substance and vestibular nuclei
  • large numbers of fibers pass to pontine nuclei
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9
Q

What are the functions of the corticospinal tract?

A
  • adds speed and agility to conscious movement -> especially the hand
  • provides a high degree of motor control -> ex. Movement of individual fingers
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10
Q

What are symptoms of corticospinal tract lesions?

A
  • reduced muscle tone
  • clumsiness
  • weakness
  • not complete paralysis (only complete if both pyramidal and extrapyramidal systems are involved)
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11
Q

What is the corticobulbar tract?

A

-innervates the head

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12
Q

Where does the corticobulbar tract terminate?

A

-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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13
Q

Describe the red nucleus.

A
  • fibers from primary motor cortex (corticorubral pathway) and branches from corticospinal tract synapse in magnocellular portion of red nucleus
  • large neurons from magnocellular region of red nucleus give rise to rumors pineal tract which decussate said in lower brain stem
  • magnocellular region has somatotropin representation of all the muscles of the body
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14
Q

What does stimulation of the the red nucleus result in?

A
  • stimulation of flexors

- inhibition of extensors (antigravity muscles)

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15
Q

What is the extrapyramidal system? What does it include?

A
  • pathways that contribute to motor control but that are not part of the corticospinal system
  • includes descending motor tracts that do not pass through medullary pyramids or corticobulbar tracts

Includes:

  • rubrospinal tracts
  • vestibulspinal tracts
  • reticulospinal tracts
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16
Q

Describe the rubrospinal tract.

A
  • originates in red nucleus
  • decussates in midbrain
  • descends in lateral funiculus
  • function is closely related to cerebellar function
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17
Q

What do lesions of the rubrospinal tract result in?

A
  • impairment of distal arm and hand movement

- intention tremors (similar to cerebellar lesions)

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18
Q

Describe the vestibulospinal tract.

A
  • originates in vestibular nuclei -> receives major input from vestibular nerve
  • descends in anterior funiculus
  • synapses with LMNs to extensor muscles -> primarily involved in maintenance of upright posture
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19
Q

Describe the reticulospinal tract.

A
  • originates in various regions of reticular formation
  • descends in anterior portion of lateral funiculus
  • through to mediate larger movements of trunk and limbs that do not require balance or fine movements of upper limbs
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20
Q

Describe the pontine reticular nuclei.

A
  • fibers that make up pontine reticulospinal tract (anterior column)
  • stimulators effects on both extensors and flexors, especially flexors
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21
Q

Describe the medullary reticular nuclei.

A
  • fibers make up the medullary reticulospinal tract (lateral column)
  • inhibitory effect on both extensors and flexors, especially extensors
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22
Q

What is the vestibular apparatus?

A

-membranous labyrinth consisting of three semicircular canals and vestibule (includes cochlea)

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23
Q

What is the vestibule composed of?

A

-utricle and saccule

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24
Q

Describe the functions of the vestibular apparatus.

A
  • detects angular (semicircular canals) and linear (utricle and saccule) acceleration of head
  • involved in reflex adjustment of head, eyes, and postural muscles
  • provides a stable visual image and steady posture
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25
Q

Describe the structure of the vestibular apparatus.

A
  • filled with endolymph and surrounded by perilymph
  • utricle and saccule each contain a macula, which contains hair cells
  • each semicircular canal has an enlargement at one end called the ampulla -> contains hair cells
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26
Q

What structures have a macula?

A

-utricle and saccule

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27
Q

What role does the utricle play in orientation?

A
  • located on horizontal plane

- plays role in determining orientation of head when head is upright

28
Q

What does the saccule do?

A
  • located in a vertical plane

- signals head orientation when person is lying down

29
Q

What is a macula?

A
  • covered in gelatinous layer
  • contains lathe number of small CaCO3 crystals (statoconia)
  • contains thousands of hair cells which project cilia into the gelatinous layer
  • the weight of the statoconia bends cilia in the direction of gravitational pull
30
Q

What are the functions of the hair cells in the macula?

A
  • bending of stereocilia toward kinocilium opens hundreds of cation channels causing receptor membrane depolarization
  • bending of cilia in opposite direction closes channels and hyperpolarizes membrane
  • hair cells are oriented such that bending the head in different directions causes different groups of hair cells to depolarize
31
Q

How do semicircular canals function?

A
  • when head begins to rotate in any direction, inertia of the fluid in 1+ of the canals reminds stationary while semicircular canal rotates with the head
  • fluid flows from the duct and through the ampulla and causes the cupula to bend to one side
  • hundreds of hair cells within each cupula detect this bending and send signals via the vestibular nerve
32
Q

What happens when the head is bent forward 30 degrees?

A
  • lateral ducts are horizontal
  • anterior ducts are in vertical planes projecting forward and 45 degrees outward
  • posterior ducts are in vertical planes projecting backward and 45 degrees outward
33
Q

What is the ampulla? What structures can be found in it?

A
  • enlargement at one end of each semicircular canal
  • filled with endolymph
  • crista ampullaris: small crest within each ampulla
  • cupula: loose mass of gelatinous tissue on top of the crista
34
Q

What is the pyramidal system? What are the components?

A
  • tracts that pass through the medullary pyramidals -> other components are extrapyramidal
  • consists of the corticospinal tract and the corticobulbar tract
35
Q

What the different groups of neurons in the spinal cord?

A

-sensory (afferent)
-anterior motor neurons (efferent)
+alpha motor neurons give rise to A alpha fibers
-interneurons

36
Q

What are some characteristics of interneurons?

A
  • 30x as numerous as anterior motor neurons
  • small and highly excitable
  • capable of spontaneous activity
  • responsible for most of spinal cord integrative function
37
Q

What is a motor unit?

A
  • composed of a single motor neuron and the muscle fibers it innervates
  • composed of extrafusal fibers
38
Q

What is a motor neuron pool?

A

-group of motor neurons that innervate fibers within the same muscle

39
Q

What is recruitment?

A

-refers to the increase in tension of muscle contractions by the activation of additional motor units (size principle)

40
Q

What is the difference between small and large motor neurons?

A

Small:

  • innervate a few muscle fibers
  • lowest thresholds
  • fire first
  • generate smallest force

Large:

  • innervate many muscle fibers
  • highest thresholds
  • fire last
  • generate largest force
41
Q

What are Renshaw cells?

A
  • inhibitory cells in anterior horns of spinal cord
  • receive collateral branches from alpha motor neurons

-transmit inhibitory signals to surrounding motor neurons
+results in lateral inhibition
+enhance fluidity of limb movement

-transmit inhibitory signals to same motor neuron
+results in recurrent inhibition

42
Q

What do muscle spindle sensors (group Ia and II afferents) detect? How is it arranged?

A
  • detect both dynamic and static changes in muscle length

- arranged in parallel with extrafusal fibers

43
Q

What do group Ib afferents/Golgi tendon organ sensors detect? How is in arranged?

A
  • arranged in series with extrafusal fibers

- detects muscle tension

44
Q

What do group II afferents (Pacinian corpuscles) detect?

A

-detect vibration

45
Q

What do group III and IV afferents (free nerve endings) detect?

A

-detect noxious stimuli

46
Q

What groups of nerve fibers are myelinated? Which ones are not?

A
  • groups I, II, and III are myelinated

- group IV is not

47
Q

Describe muscle spindle fibers/ group Ia and II afferents.

A

-3-10mm long

-consists of 3-12 intrafusal fibers
+innervated by small gamma motor neurons (group II)
+encapsulated within a sheath to form muscle spindle
+run parallel to extrafusal fibers

-central region of spindle has no contractile fibers, functions as a sensory receptor
+sensory fibers originate from central region
+stretching of central region of intrafusal fiber stimulates sensory fibers

  • detects change in muscle length
  • with finer movements, the number of muscle spindles required increase (gamma motor neurons)
48
Q

What are the two types of intrafusal fibers? What are the differences between them?

A

NUCLEAR BAG FIBERS
-detect rate of change in muscle length

  • innervated by group Ia afferents and dynamic gamma efferents
  • multiple nuclei located in a central bag-like configuration

NUCLEAR CHAIN FIBERS
-detect static change in muscle length

  • innervated by group II afferents and static gamma efferents
  • more numerous than nuclear bag fibers
  • multiple nuclei arranged in a single row
49
Q

What do muscle spindle sensory fibers correct for? How does this happen?

A
  • corrects for increase in muscle length (stretch)
  • sensory fibers from central region of intrafusal fiber

-stimulation results from:
+lengthening of entire muscle and contraction of ends of intrafusal fibers

-stimulation of group Ia and II results in stimulation of alpha motor neurons, resulting in contraction and shortening of muscle

50
Q

What are muscle spindle gamma motor neurons?

A
  • innervate intrafusal fibers
  • adjust sensitivity of muscle spindle
  • coactivated with alpha motor neurons (results from lengthening of entire muscle)
51
Q

What are the different types of gamma motor neurons? What do they do?

A

A(gamma):

  • 5 micrometer diameter
  • supply small intrafusal fibers in middle of muscle spindle

gamma-dynamic:
-excite nuclear bag intrafusal fibers

Gamma-static:
-X cite nuclear chain intrafusal fibers

52
Q

What brain areas control the gamma motor neurons?

A
  • bulboreticular region of brain stem
  • cerebellum
  • basal nuclei
  • cerebral cortex
53
Q

What is the dynamic stretch reflex?

A
  • signals transmitted from primary nerve endings
  • elicited by rapid stretch or unstretch
  • opposes sudden changes to muscle length
54
Q

What is the static reflex?

A
  • transmitted by both primary and secondary endings

- causes degree of muscle contraction to remain relatively constant

55
Q

What do the dynamic and static reflexes result in?

A

Prevention of jerkiness of body movements (dampening)

56
Q

How does the stretch reflex work?

A
  • stretching of muscle stretches group I1 afferent fibers
  • group Ia afferents synapse directly on alpha motor neurons of same muscle
  • the muscle contracts and decreases tension on muscle spindle
  • synergistic muscles are activated and antagonistic muscles are inhibited
57
Q

What is the Golgi tendon organ? How does it function?

A
  • encapsulated sensory receptor through which muscle tendon fibers pass
  • arranged in series with extrafusal fibers
  • 10-15 muscle fibers attached to each Golgi organ
  • Golgi organ is stimulated by contracting or stretching of muscle
  • detects muscle tension
  • Golgi tendon reflex is opposite of the stretch reflex
58
Q

What is the circuitry if the Golgi tendon?

A

Type Ib afferent (+) -> inhibitory interneuron (-) -> anterior motor neuron

59
Q

What are the functions of pre motor and supplementary motor areas?

A
  • generate a plan for movement -> transfer to primary motor cortex
  • signals generated cause more complex patterns of movement than the more discrete pattern generated by the primary motor cortex
  • anterior part of the promoter cortex develops a “motor image” of the total muscle movement that is to be performed
  • supplementary motor cortex programs complex motor sequences and is responsible for mental rehearsal for a movement
  • image in posterior motor cortex excites each successive pattern of muscle activity required to achieve the image
  • posterior motor cortex sends signals to primary motor cortex and basal nuclei and thalamus (primary motor cortex)
60
Q

What are mirror neurons?

A

-these have been suggested to fire when another person does an action and the neurons of the brain fire, “mimicking” the behavior of the observed person

61
Q

How are the cells in the motor cortex organized?

A
  • organized in vertical columns
  • each column stimulates a group of synergistic muscles or even a single muscle
  • 6 distinct layers
62
Q

What layer are pyramidal cells found?

A

5th layer

63
Q

Where do input signals enter (what layers of the motor cortex)?

A

-2-4

64
Q

What do neurons of the 6th layer of the motor cortex do?

A

-communicate with other regions of the cerebral cortex

65
Q

What are the two descending pathways that consist of a series of motor neurons?

A
  • upper motor neurons (UMN)

- lower motor neurons (LMN)

66
Q

Where are motor neurons found?

A

-entirely within the CNS

-originate in:
+cerebral cortex
+cerebellum
+brainstem

-form descending tracts

67
Q

Where are lower motor neurons found?

A

-begin in CNS
+from anterior horns of spinal cord
+from brainstem cranial nerve nuclei

  • made up of alpha motor neurons
  • make up spinal and cranial nerves

Physiology II Exam 2 (6 decks)

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