Motor Control: Reflexes

Question 1

What Reflexes are monitored by the Cortex?

Answer 1

Placing reaction

Hopping reaction

Question 2

What reflexes is monitored by the Brainstem/Midbrain?

Answer 2

Vestibular 
Righting reflex
Suckle
Yawn
Eye/head movement

Question 3

What reflexes is monitored by the Spinal Reflexes?

Answer 3

Stretch (myotactic)
Golfing tendon reflex
Crossed extensor

Question 4

Describe the myotatic reflex.

Answer 4

Appearance: contraction of a stretched muscles
Purpose: Protect muscle from tearing due to stretch
Characteristics: initiated by muscle spindle; monosynaptic, segmental reflex

Question 5

What fiber are involved in the muscle spindle?

Answer 5

Extrafusal Fibers: all muscle fiber that make you move

Infrafusal Fibers: encapsulated; have both motor and sensory components

Question 6

What are the characteristics of the sensory portion of a muscle spindle?

Answer 6

Is not contractile 
Is the portion sensitive to length
Is actually two sensory: w/ different afferent 
  Nuclear bag fiber 
  Nuclear chain fiber

Question 7

Describe the characteristics of the primary afferent neuron in the intrafusal fibers?

Answer 7

Innervates both nuclear bag and nuclear chain
Large,myelinated la fiber (fast) 
Sensitive to both:
  Length of muscle
  How fast the length is changing

Question 8

Describe the Action potentials of the intrafusal fibers

Answer 8

At rest the primary afferent send a specific number of APs

With stretch: increase the number of APs

With Fast Stretch: hugely increase the number of APs

Question 9

What are the characteristics of the secondary afferent?

Answer 9

Smaller, myelinated Group II fiber (slight slower, and higher threshold than 1a fiber)
Innervates only the nuclear chain fiber
Sensitive only to the length of the muscle

Question 10

What are the characteristics of the motor portion of the intrafusal contractile elements?

Answer 10

Same as skeletal muscles
Innervates by gamma motor neuron
Control the length of the sensory portion

Question 11

Describe the relationship between the intrafusal muscles and sensory portion.

Answer 11

By contracting the intrafusal muscles( via gamma motor neuron), we stretch the sensory portion

Sensory portion more sensitive to a superimposed stretched.

Question 12

Describe the myotatic reflex in the spinal cord

Answer 12

1. 1a afferent enters dorsal horn
2. Synapses on an alpha-neuron in the ventral horn (releases EAAs)
3. Increases APs of alpha-neuron; contracting muscles
4. Relieves stretch
5. 1a fiber returns to normal firing rate

Question 13

Describe what happens to the antagonist muscles during a myotatic reflex.

Answer 13

1. 1a fiber sends collateral fiber to synapse of an interneurons
2. The interneurons (releases glycine) synapse on the alpha motor neuron of the antagonist muscle
3. Inhibits the alpha-motor neuron; causes relaxation and lengthening

Question 14

What is the purpose, appearance, and characteristics of the golgi tendon reflex?

Answer 14

Golgi tendon Reflex: inverse myotatic, auto genie inhibition

Appearance: sudden relaxation of a contracted muscle
Purpose: Protect muscle from damage due to excessive force
Characteristics: initiated by Golgi tendon organ, polysynaptic

Question 15

What is Golgi Tendon Organs innervate?

Answer 15

Innervates tendon with bare nerve ending w/ lots of branches

APs increases w/ tension via 1b fibers to the spinal cord

Question 16

Describe the synapses within the spinal cord for the Golgi tendon Reflex.

Answer 16

1. 1b afferent from Golgi tendon enters spinal cord via dorsal horn
2. 1b afferent (releasing EAAs) synapses on a interneuron
3. The interneuron releases glycine onto an alpha-motor neuron (of contracted muscle)
4. Decreasing APs; less tension in muscles; abrupt relaxation
5. The Golgi tendon organ discharge back to normal

Question 17

How does one recover from spinal shock?

Answer 17

Axonal sprouting below the levels of the transaction

Expression of receptor phenotypes that are self-activating

Question 18

How does decerebrate posturing occur?

Answer 18

Results from loss of all structures rostral to the pons

Question 19

How does rigidity occur?

Answer 19

Rigidity: resists motion in all direction

Continual activation of alpha-motor neurons

*** not due to Reflexes, brainstem is activating alpha- motor neurons

Question 20

How does Spasticity Occur?

Answer 20

Spasticity: myotatic reflex hyperactive, continual activation of gamma-motor neurons

Contracts intrafusal muscle, lengthens nuclear bag/chain fibers, brainstem

Question 21

What is the Brainstem facilitatory region in spasticity?

Answer 21

Brainstem facilitatory region: by activating the gamma-motor neuron, makes muscles spindle more sensitive

Brainstem inhibitory region: inhibits gamma motor neurons, making muscle spindle less sensitive; requires activation from cortical regions

With the loss of the cortex, no inhibitory signals thus the gamma- motor neurons continue to signal

Question 22

What is the presentation of decorticating posturing?

Answer 22

Flexion of the upper limbs
Extension of the lower limbs
Dependent on the head position

Question 23

What leads to decorticate posturing?

Answer 23

Stroke in the vicinity of the internal capsule