Motor Control - Reflexes

Question 1

List the various Spinal Reflexes.

Answer 1

1. Stretch (myotactic)
2. Golgi Tendon reflex
3. Crossed extensor (withdraw reflex)

Question 2

List the various reflexes associated with the Brainstem/Midbrain.

Answer 2

1. Vestibular
2. Righting reflex
3. Suckle*
4. Yawn*
5. Eye/Head Movements*

*** Often confused for VOLITIONAL activity!!! These are REFLEXES!!!

Question 3

List the various reflexes that are associated with the Cortex.

Answer 3

1. Placing Reaction (most in animals)
2. Hopping Reaction –> when you are pushed to the side, you are going to perform a little hop

Question 4

Describe the rules of reflexes.

Answer 4

In order to be effective, reflexes must be PRECISE

Reflex movements occur FASTER than the FASTEST voluntary motion

Question 5

Differentiate between Reflex Activity and Volitional Movement.

Answer 5

• *Reflex Activiy**
• Precise motions in response to afferent stimuli
• Mediated at all levels of the CNS
• Rapid initiation
• Many elicited even during unconsciousness
• *Volitional Activity**
• Originates in corical areas associated with judgement, initiative and motor control
• Longer onset latenct due to processing
• Require conscious awareness

Question 6

Describe the Myotactic (Stretch) Reflex.

Answer 6

“Patellar Reflex”

Looks like: Shortening of a stretched muscle

Function: Protect muscle from tearing due to stretch

Characteristics: Initiated by the muscle spindle; Monosynaptic, Segmental Reflex (occurs @ the same level of the spinal cord)

Ia Afferent comes into the spinal cord and SYNAPSES in the SAME LEVEL of the SPINAL cord on an ALPHA-MOTOR NEURON (EAA is used)

Reciprocol Inhibition takes place here!

Question 7

Describe the intrafusal fibers of the muscle spindle.

Answer 7

Have BOTH Afferents and Efferents!
Sensory Portion - Not contractile, Sensitive to LENGTH, has two sensory fibers: 1. Nuclear Bag (Ia afferent) 2. Nuclear Chain (Ia afferent, Group II fiber)

• *Ia fiber** –> Large diameter and Myelinated
• Sensitive to –> Length of Muscle AND How fast the length is changing (GET MORE action potentials if you stretch the muscle really FAST!)
• *II Fiber** –> Not as fast
• Sensitive to –> ONLY CHANGE in LENGTH of the fiber!

Motor Portion - Innervated by a Gamma Neuron
- Function: Control the Length of the Sensory Portion!
**** DOES NOT directly produce a MOTION!

Question 8

Describe the concept of Reciprocal Inhibition.

Answer 8

Ia Afferent is coming into the spinal cord and is going to synapse on an Alpha Motor Neuron AND an Inhibitory Neuron (Spinal Interneuron)(Glycine) which is going to INHIBIT the ANTAGONIZING muscle

*** You will have a DECREASE in activity in the antagonistic Muscle which allows it to relax and lengthen

Question 9

Describe the Golgi Tendon Reflex.

Answer 9

Looks Like: Sudden (abrupt) RELAXATION of a contracted muscle

Function: Protect muscle from damage due to excessive force (Active Contraction about to do damage to the muscle!)

Characteristics: Initiated by Golgi Tendon Organ; Polysynaptic, Segmental Reflex

*** Synapses on an INHIBITORY Interneuron and will Inhibit the Alpha Motor Neuron! Uses Glycine

Question 10

Describe the Golgi Tendon Organ.

Answer 10

Golgi Tendon Organ

• Innervates TENDON
• Action Potentials INCREASE with Tension
• Ib fiber to spinal cord (Ib is slower than Ia but faster than II)

Question 11

Describe Spinal Shock.

Answer 11

Period of Areflexia that follows transection of the Spinal Cord.

**** Even though the neurons producing the reflex are below the level of the transection (and are therefore intact), the reflex FAILS to occur
- Fails to occur because the Afferent neurons are not able to bring the Alpha Motor neuron to Thresdhold!

• *Recovery** –> Axonal sprouting below the level of the Transection!
• Expression of receptor phenotypes that are self-activating (5HTC receptor)

Question 12

Describe Decerebrate Posturing.

Answer 12

Results from loss of ALL structures ABOVE the Pons!

Rigidity –> Resists motion in ALL directions; Results from maintained muscle contraction (everything is Extended); Continual activation of ALPHA-motor neurons (Brainstem is activating the alph-motor neurons)

• Spasticity** –> Resisting a passive motion in a given direction; *Myotatic reflex is HYPERACTIVE; Results from CONTINUAL activation of Gamma-Motor Neurons (contracts intrafusal muscles, lengthens nuclear bag/chain fibers)
• LOSS of Brainstem Inhibitory Region and overactive Brainstem Facilitatory Region

Question 13

Differentiate between the Brainstem Facilitory and Inhibitory Regions.

Answer 13

• *BRAINSTEM facilitatory region**
• Spontaneously Active
• Going to activate gamma-motor neurons (making the muscle spindles more active)
• *BRAINSTEM inhibitory region**
• Requires activation from CORTICAL REGIONS
• Inhibits gamma motor neurons, making the muscle spindle LESS sensitive

*** With the Spasticity that is involved in Decerebrate Posturing, you are going to lose the Brainstem inhibitory region and that is why you present with SPASTICITY!

Question 14

Describe Decorticate posturing.

In which structure might you see a stroke?

Answer 14

Disruption of structures in the cortex (Corticospinal Tract)

Presentation: “Mummy Position”; FLEXION of the Upper limbs, EXTENSION of the Lower Limbs; DEPENDENT on HEAD Positioning!

Might see this with patients that have a STROKE in the Internal Capsule

Question 15

Answer 15

C. The Hopping Reaction

“Shoving you to the side”

Question 16

Answer 16

D. Gamma Motor Neuron

Question 17

Answer 17

B. A Ib Fiber

“Golgi Tendon Reflex”

Question 18

Answer 18

B. 2

Primary Afferents –> Interneuron –> Alpha Motor Neuron

Question 19

Answer 19

E. Glycine