Motor Control: Reflexes

Question 1

motor control reflexes in the cortex

Answer 1

• placing reaction

- hopping reaction

Question 2

motor control reflexes in the brainstem/midbrain

Answer 2

• vestibular
• righting reflex
• suckle*
• yawn*
• eye/head movements*

*these occur even with encephalic defects, so even after traumatic brain injuries, these can often be mistaken for a thing that is volitional, but that is not true

Question 3

motor control reflexes in the SC

Answer 3

• stretch/myotatic
• golgi tendon reflex
• crossed extensor–withdrawal from something but then move the rest of the body to make up for the movement

Question 4

difference b/w reflex and volitional motion

Answer 4

reflex

• precise motions in response to afferent stimuli
• mediated at all levels of CNS
• rapid initiation
• many elicited even during unconsciousness

volitional activity

• originates in cortical areas associated with judgment, initiative, and motor control
• longer onset latency due to processing
• require conscious awareness

Question 5

myotatic reflex

Answer 5

• stretch reflex
• appearance: contraction (shortening) of the stretched muscle
• purpose: protect muscle from tearing from stretch
• characteristics:
  
  • initiated by muscle spindle
  • monosynaptic, segmental reflex–afferent info comes into the same segment of the SC that the motor will leave

Question 6

muscle spindle

Answer 6

• myotatic reflex
• extrafusal fibers–fibers you see when looking at a M and they are the fibers that do the work
• intrafusal fibers
  
  • parallel to extrafusal fibers
  • have an afferent sensory component in the center and an efferent motor component at the periphery/ends

Question 7

sensory portion of the intrafusal M fibers of the muscle spindle

Answer 7

• sensory portion:
  
  • not contractile
  • portion sensitive to length
  • actually 2 different afferents
    
    1. nuclear bag fiber–nuclei are in the center in a swelling
    2. nuclear chain fiber–nuclei are lined up in a row

Question 8

primary afferent in the sensory portion of the intrafusal M fibers of M spindle

Answer 8

type 1 A fiber

• innervates both the nuclear bag and nuclear chain fiber
• large, myelinated 1a fiber
• sensitive to both:
  
  • length of M
  • how fast the length is changing
• fires a few APs per sec at rest, but fires inc APs when the muscle spindle is stretched

Question 9

secondary afferent in the sensory portion of the intrafusal M fibers of M spindle

Answer 9

type 2 fiber

• slightly smaller than type 1A fiber
• innervates only the nuclear chain fiber
• sensitive to only the length of the M–tells us what the length of the M is RIGHT NOW, and doesn’t give any info o if the M is changing
• less myelin

Question 10

motor portion of the intrafusal M fibers of the muscle spindle

Answer 10

• same as skeletal M
• innervated by a gamma motoneuron–this neuron goes thru NMJ and releases ACh
• control the length of the sensory portion

Question 11

how does the contraction of the intrafusal sensory and motor fibers

Answer 11

• by contracting the intrafusal Ms, we stretch the sensory portion, this renders the sensory portion more sensitive to a superimposed stretch
  
  • the motor portion on each end is attached to ligaments/tendons that the M is attached to and as we contract the motor part the sarcomeres move closer together, and we stretch the sensory portion nuclear bag/chain
• gamma motoneurons control the sensitivity

Question 12

alpha vs. gamma motoneurons

Answer 12

alpha motoneurons

• large, heavily myelinated fiber
• innervates (via NMJ) the skeletal M
• responsible for activating M
• activity directly leads to motion

gamma motoneurons

• slightly smaller, slower than alpha–still fast overall
• innervates the contractile component of the M spindle via the NMJ
• activity causes contraction
• controls sensitivity of M spindle
• activity does NOT lead to motion

Question 13

how does the myotatic reflex work for the agonistic M

Answer 13

• 1a afferent neuron comes into the SC and releases EAA onto the alpha motor neuron
  
  • motoneuron is excited by the activation of the 1a afferent–>contraction relieves the stretch, returning the 1a discharge rate back to normal

Question 14

how does the myotatic reflex work for the antagonistic M

Answer 14

• when elicit a stretch reflex on the agonistic M and contract the M, the antagonistic M is also stretched but don’t get another stretch reflex
• when 1a afferent comes in, it sends a branch to an interneuron that inhibits the alpha motor neuron to the antagonist M which allow the antagonist M to relax and allow the original M to experience the reflex–RECIPROCAL INHIBITION
  
  • anytime you contract one M, their antagonist M must relax
  • there is a decrease in the activity of the alpha motoneuron innervating the antagonist M allowing it to RELAX and LENGTHEN

Question 15

golgi tendon reflex

Answer 15

• also called inverse myotatic reflex and autogenic inhibition
• appearance: sudden (abrupt) relaxation of a contracted M
• purpose: protect M from damage due to excessive force
• chracteristics:
  
  • initiated by the golgi tendon organ
  • polysynaptic, segmental reflex–more than one synapse, but afferent and efferent come out the same segment

Question 16

golgi tendon organ

Answer 16

• innervate tendon
• bare N ending with lots of branches
• APs increase with tension
• 1b fiber goes to spinal cord

Question 17

how does the golgi tendon reflex work

Answer 17

• 1b afferent come from the golgi tendon organ and into the SC and is excitatory to so releases EAA to the interneuron in the SC
  
  • activated by the EAA and then this interneuron releases an inhibitory NT (glycine) which inhibits the alpha motor neuron of the contraction M
    
    • so this alpha motor neuron gets input from brain to contract and inhibitory input form the SC to relax–so when inhibition finally comes thru, it stops contraction of the alpha motor neuron
• these reflexes allow us to avoid going to the brain to get the contraction to stop

-motoneuron is inhibited by the activation of the spinal interneuron–>abrupt relaxation of the M occurs, returning the golgi tendon organ discharge rate to normal

Question 18

myotatic reflex vs. golgi tendon reflex

Answer 18

myotatic reflex:

• initiated by M spindle
• passive stretch of the M
• monosynaptic reflex
• contraction of stretched M back to normal length

golgi tendon reflex:

• initiated by golgi tendon organ
• active contraction of M
• polysynaptic reflex
• abrupt relaxation of the contracted M to prevent damage

Question 19

modification of reflexes

Answer 19

• occurs all the time–healthy

- a lot of the time, the influence for modification is inhibitory

Question 20

spinal shock

Answer 20

• occurs from spinal transection–from hyper polarization of spinal neurons due to loss of excitatory input form cortex
• reflexes that are below the level of the transection, the reflex fails to occur even though the wiring is intact
  
  • occurs right after the spinal cord is transected but usually goes away and reflexes can return, except in humans, the Ms often atrophy w/in the year and the reflex return is never noticed
• reversible
  
  • recovery results from axonal sprouting below the level of the transection–occurs due to neurons need for input b/c without input they will die
  • recovery also occurs due to expression of R phenotypes that are self-activating

Question 21

decerebrate posturing

Answer 21

results from loss of all structures rostral to the pons

• involves the whole body
• spinal reflexes are intact and hyperactive
  
  • experience extension in all Ms, so rigid–happens instantly
• usually bilateral

Question 22

rigidity vs. spasticity

Answer 22

rigidity:

• resists motion in all directions
• results from maintained M contraction; loss of cortical influence that inhibits a medullary input to the alpha motoneurons
• continual activation of alpha motoneurons
  
  • not due to reflexes
  • brainstem is activating alpha motoneurons

spasticity:

• resists motion in a given direction–so when move arm, the arm will spring back
• myotatic reflex is hyperactive due to inc gamma motoneuron firing
• continual activation of gamma motoneurons
  
  • contracts intrafusal M
  • lengthens nuclear bag/chain fibers
  • brainstem
• cause: damage to cortex that abolishes activation of the brainstem inhibitory region

Question 23

spasticity from alteration to what locations?

Answer 23

• brainstem facilitatory region
  
  • by activating the gamma motoneurons, makes M spindle more sensitive
  • region is spontaneously active
  • dominates with decerebrate posturing
• brainstem inhibitory region
  
  • inhibits gamma motoneurons, making M spindle less sensitive
  • region requires activation from cortical regions
  • this doesn’t work in decerebrate posturing

Question 24

decorticate posturing

Answer 24

• rigidity
  
  • flexion of the upper limb joints
  • extension of the lower limbs
  • dependent on head position
    
    • head motion in one direction will flex the contralateral arm and extend the ipsilateral arm
• changes as the head is moved passively
• happens often b/c it happens in pts that experience strokes in the vicinity of the internal capsule
  
  • cause: loss of cortical inputs due to lesion of internal capsule

Question 25

``` which of the following reflex requires cortical input? withdrawing hand from hot stove turning heads towards sound hopping run turning eyes towards motion righting response ```

Answer 25

hopping rxn

Question 26

what innervates the intrafusal M fibers?

Answer 26

gamma motoneuron

Question 27

in order to save time, Dr. K tries to lift 47 cases of Pepsi into car in one lift. She is not up the challenge and a reflex is elicited, resulting in complete relaxation of her Ms. Activation of what produces this?

Answer 27

1b fiber b/c this is involved in the golgi tendon reflex which protects M from damage due to excessive force and causes abrupt relaxation of a contracted M

Question 28

in order to save time, Dr. K tries to lift 47 cases of Pepsi into car in one lift. She is not up the challenge and a reflex is elicited, resulting in complete relaxation of her Ms. How many synapses in the SC are required for reflex?

Answer 28

2 | 1b afferent from golgi tendon--interneuron--alpha motorneuron

Question 29

in order to save time, Dr. K tries to lift 47 cases of Pepsi into car in one lift. She is not up the challenge and a reflex is elicited, resulting in complete relaxation of her Ms. what R is most likely on the alpha motoneuron in this reflex?

Answer 29

glycine