Spinal Cord, Brainstem, Cortex Control Of Motor Function

Question 1

What are some characteristics of the upper motor neurons?

Answer 1

• 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

Question 2

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

Answer 2

-classified according to where they synapse in the ventral horn

Question 3

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

Answer 3

• 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

Question 4

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

Answer 4

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

Question 5

What is the corticospinal tract? Where does it originate?

Answer 5

• aka pyramidal tract
• origin of tract:

1. Primary motor cortex
2. Premotor cortex
3. Somatosensory area

Question 6

What is the pathway of the corticospinal tract?

Answer 6

• 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

Question 7

What are giant pyramidal (Betz) cells?

Answer 7

• 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

Question 8

What happens to the other fibers from the cortex?

Answer 8

• 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

Question 9

What are the functions of the corticospinal tract?

Answer 9

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

Question 10

What are symptoms of corticospinal tract lesions?

Answer 10

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

Question 11

What is the corticobulbar tract?

Answer 11

-innervates the head

Question 12

Where does the corticobulbar tract terminate?

Answer 12

-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

Question 13

Describe the red nucleus.

Answer 13

• 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

Question 14

What does stimulation of the the red nucleus result in?

Answer 14

• stimulation of flexors

- inhibition of extensors (antigravity muscles)

Question 15

What is the extrapyramidal system? What does it include?

Answer 15

• 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

Question 16

Describe the rubrospinal tract.

Answer 16

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

Question 17

What do lesions of the rubrospinal tract result in?

Answer 17

• impairment of distal arm and hand movement

- intention tremors (similar to cerebellar lesions)

Question 18

Describe the vestibulospinal tract.

Answer 18

• 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

Question 19

Describe the reticulospinal tract.

Answer 19

• 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

Question 20

Describe the pontine reticular nuclei.

Answer 20

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

Question 21

Describe the medullary reticular nuclei.

Answer 21

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

Question 22

What is the vestibular apparatus?

Answer 22

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

Question 23

What is the vestibule composed of?

Answer 23

-utricle and saccule

Question 24

Describe the functions of the vestibular apparatus.

Answer 24

• 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

Question 25

Describe the structure of the vestibular apparatus.

Answer 25

- 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

Question 26

What structures have a macula?

Answer 26

-utricle and saccule

Question 27

What role does the utricle play in orientation?

Answer 27

- located on horizontal plane | - plays role in determining orientation of head when head is upright

Question 28

What does the saccule do?

Answer 28

- located in a vertical plane | - signals head orientation when person is lying down

Question 29

What is a macula?

Answer 29

- 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

Question 30

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

Answer 30

- 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

Question 31

How do semicircular canals function?

Answer 31

- 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

Question 32

What happens when the head is bent forward 30 degrees?

Answer 32

- 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

Question 33

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

Answer 33

- 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

Question 34

What is the pyramidal system? What are the components?

Answer 34

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

Question 35

What the different groups of neurons in the spinal cord?

Answer 35

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

Question 36

What are some characteristics of interneurons?

Answer 36

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

Question 37

What is a motor unit?

Answer 37

- composed of a single motor neuron and the muscle fibers it innervates - composed of extrafusal fibers

Question 38

What is a motor neuron pool?

Answer 38

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

Question 39

What is recruitment?

Answer 39

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

Question 40

What is the difference between small and large motor neurons?

Answer 40

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

Question 41

What are Renshaw cells?

Answer 41

- 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

Question 42

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

Answer 42

- detect both dynamic and static changes in muscle length | - arranged in parallel with extrafusal fibers

Question 43

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

Answer 43

- arranged in series with extrafusal fibers | - detects muscle tension

Question 44

What do group II afferents (Pacinian corpuscles) detect?

Answer 44

-detect vibration

Question 45

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

Answer 45

-detect noxious stimuli

Question 46

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

Answer 46

- groups I, II, and III are myelinated | - group IV is not

Question 47

Describe muscle spindle fibers/ group Ia and II afferents.

Answer 47

-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)

Question 48

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

Answer 48

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

Question 49

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

Answer 49

- 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

Question 50

What are muscle spindle gamma motor neurons?

Answer 50

- innervate intrafusal fibers - adjust sensitivity of muscle spindle - coactivated with alpha motor neurons (results from lengthening of entire muscle)

Question 51

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

Answer 51

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

Question 52

What brain areas control the gamma motor neurons?

Answer 52

- bulboreticular region of brain stem - cerebellum - basal nuclei - cerebral cortex

Question 53

What is the dynamic stretch reflex?

Answer 53

- signals transmitted from primary nerve endings - elicited by rapid stretch or unstretch - opposes sudden changes to muscle length

Question 54

What is the static reflex?

Answer 54

- transmitted by both primary and secondary endings | - causes degree of muscle contraction to remain relatively constant

Question 55

What do the dynamic and static reflexes result in?

Answer 55

Prevention of jerkiness of body movements (dampening)

Question 56

How does the stretch reflex work?

Answer 56

- 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

Question 57

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

Answer 57

- 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

Question 58

What is the circuitry if the Golgi tendon?

Answer 58

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

Question 59

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

Answer 59

- 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)

Question 60

What are mirror neurons?

Answer 60

-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

Question 61

How are the cells in the motor cortex organized?

Answer 61

- organized in vertical columns - each column stimulates a group of synergistic muscles or even a single muscle - 6 distinct layers

Question 62

What layer are pyramidal cells found?

Answer 62

5th layer

Question 63

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

Answer 63

-2-4

Question 64

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

Answer 64

-communicate with other regions of the cerebral cortex

Question 65

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

Answer 65

- upper motor neurons (UMN) | - lower motor neurons (LMN)

Question 66

Where are motor neurons found?

Answer 66

-entirely within the CNS -originate in: +cerebral cortex +cerebellum +brainstem -form descending tracts

Question 67

Where are lower motor neurons found?

Answer 67

-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