Motor Control Reflexes Karius T#2

Question 1

What reflexes are the spinal cord mediated reflexes?

Answer 1

• Stretch (Myotatic)
• Golgi Tendon:
  
  • a lot of force when lifting osmething heavy and could potentiatlly cause damage, so this reflex causes you to drop whatever it is
• Crossed extensor:
  
  • When you step on a tack and move your leg up this reflex kicks in to keep you from falling, you brace your extended leg

Question 2

What are the brainstem/midbrain reflexes??

Answer 2

• Vestibular
• Righting reflex
• Suckle*
• Yawn*
• Eye/head movements*

* occurs in anencephalic babies, so they occur in the absence of the cortex. It is confusing for families who see these movements while being told their family member’s brain is not functioning.

Question 3

What are cortical reflexes?

Answer 3

• Placing reaction
  
  • Top of baby foot in contact with table the baby will move foot into a weight bearing position
• Hopping reaction:
  
  • When you get shoved one direction you will hop away to try and keep your balance

Question 4

What is the purpsoe of a reflex?

Answer 4

• Protection
• Quick action without consious thought
  
  • when you touch a hot pan you don’t want to have to think about moving your hand away
• In infants it’s things they need to do but can’t voluntarily do

Question 5

Characteristics of reflexes?

Answer 5

• Involuntary
• Fast
• Short acting
• Precise

Question 6

How can you tell the differnece between a precise reflex and volitional movement?

Answer 6

The speed at which the motion occurs. The volitional movement is slower as it takes thought from cortex and subcortex. The reflex just occurs.

Question 7

What is the initaition in reflexes vs volitional motion?

Answer 7

Sensory input is the only initiator for reflexes, but higher cognition along with sensory input allow for volitional movment

Question 8

Describe the cirtcuitry in reflexes vs volitional movement.

Answer 8

Reflexes have fixed circuitry and volitional movements are variable depending on motion.

Question 9

Describe the myototatic stretch reflex appearance, purpose and characteristics.

Answer 9

• Contraction of a stretched muscle
• Protect muscle from tearing due to stretch, most active during passive stretch
• Initiated by muscle spindles, it is a monosynapitc segmental reflex
  
  • there is only one synapse- the muscle spindle sensory axon synapses directly on the alpha motor neuron

Question 10

Describe the structure of the muscle spindle?

Answer 10

• found within skeletal mm embedded din fusifoem capsule
• Parallel to muscle fibers
• Contains afferent and efferent fibers
  *

Question 11

Describe sensory portion of a muscle spindle.

Answer 11

• Not contractile
• Sensitive to length
• TWO sensorys with different afferents:
  
  • Nuclear bag fiber
  • Nuclear chain fiber

Question 12

What innervates the muscle spindle afferent part?

Answer 12

• large myelinated 1a fiber, it has a high conduction velocity and low threshold
• It is sensitive to both length of mm and how fast the length changes
  
  • aka dynamic fiber

Question 13

What rate will the 1a fiber fire when the muscle length changes rapidly?

Answer 13

• It will fire quickly vs if the length changes slowly it will fire slower.
• Seen most with passive stretch, as the brain didn’t plan on the action occuring.

Question 14

Describe the Group II fibers.

Answer 14

• Smaller than 1a still myelinated it has a slwoer conductioni velocity and decreased sensitivity and slightly higher threshold
• Only innervates nuclear chain fiber
• Ssensitive only to Length of muscle

Question 15

Compare & contrast functions of primary and secondary afferent fibers associted with the muscle spindle.

Answer 15

• The 1a fiber is heavily myelinated has fast velocity and low threshold while the group II fiber has a smaller diameter, less myelin, and is still very fast
• The Primary innervates nuclear bag and chain while the secondary innervates only nuclear chian
• The primary detects length and speed of that change and the secondary only detects length

Question 16

What inenrvates the motor portion of the myotatic reflex?

Answer 16

Gamma motor neurons they control length of the sensory portion and therefore its sensitivity to length changes

Question 17

What happens when the intrafusal muslce are contracted and the snensory portion stretches?

Answer 17

• The sensory portion will be more sensitive to additional stretches
• The muscle contracts and shortens and this causes the nuclear bag to elonngate and causes increase of 1a firing but upon the next stretch the reflex is even stronger

Question 18

Compare contrast functions of alpha and gamma motor neurons.

Answer 18

• Alpha:
  
  • large highly myelinated
  • Innervates extrafusal fibers (skeletal mm via NMJ)
  • Responsbile for activating muscle
  • Leads directly to motion
• Gamma:
  
  • Slightly smaller and slower than alpha, still fast
  • Innervates intrafusal fibers, the contractile component, of mm spindle via NMJ
  • Activity causes contraction but does not direcly lead to motion
  • Controls sensitivity of muscle spindles

Question 19

Describe the pathway for myotatic reflex.

Answer 19

• In the spinal cord the 1a affernet fiber from the muscle spindle synapses directly on the alpha motor neuron releasing EAA causing depolarization EPSP’s in the alpha motor neuron leading to AP’s. This causes contraction in the stretched muscle

Question 20

What else does the 1a afferent fiber do in regards to the antagonist muscle?

Answer 20

It releases IAA such as glycine or GABA causing IPSP’s to occur in the antagonist muscle relaxing it so it can move with the reflex

Question 21

Descsribe the appearance purpose and characteristics of the golgi tendon reflex

Answer 21

• Sudden abrupt relaxation of a contracted muscle
• Protects a muscle from damage due to excessive force
• Initiated by golgi tendon organ and it is polysynaptic segmental reflex
  
  • More than one synapse

Question 22

What does the Golgi tendon organ do

Answer 22

• Innervates the tendon
• It is a bare nerve ending with lots of branches
• Action potentials increase with tension
• 1b fiber goes to spinal cord
  
  • slightly smaller diameter than 1a, slight decrease in myelin, slightly slower conduction velocity and slightly higher threshold

Question 23

Describe the Golgi tendon reflex pathway

Answer 23

The 1b afferent from the golgi tendon releases EAA on the inhibitory interneuron which then releases GABA to the alpha motor neuron of the contracting muscle. This causes IPSP’s to occur causing relaxation of the muscle.

• the 1b firing on the inhibitory neuron has a high threshold, you don’t want this reflex to occur when you are lifting something slightly heavy

Question 24

Compare contrast muscle spindle and golgoi tendon reflex

Answer 24

• Myotitaic:
  
  • Initiated by muscle spindle
  • Passive stretch of the muscle
  • Monosynaptic!
  • Contraction of a stretched muscle back to normal length
• Golgi Tendon:
  
  • Initiated by golgi tendon organ
  • requires active contraction of a muscle
  • Polysynapic inhibitory reflex!
  • Abrupt relaxation of a muscle