Exam #6: Spinal Mechanisms of Motor Control

Question 1

What is the muscle spindle?

Answer 1

Sometimes referred to as the “fusiform organ,” this is the spindle-shaped stretch receptor associated with intrafusal fibers that senses muscle length
- Oriented in PARALLEL with extrafusal fibers

**Function is to correct for changes in extrafusal fiber length

Question 2

Where is the muscle spindle located?

Answer 2

In skeletal muscle, in PARALLEL to extrafusal fibers

Question 3

What innervates the muscle spindle?

Answer 3

Gamma motor axons

Question 4

What is the golgi tendon organ?

Answer 4

This is a stretch receptor found in tendons, which senses contraction of muscle and activates group Ib afferent nerves

Question 5

Where is the golgi tendon organ located?

Answer 5

Tendons

Question 6

What innervates the golgi tendon organ?

Answer 6

Single group Ib fiber

Question 7

What is the difference between alpha motor neurons and gamma motor neurons? Which innervates intrafusal muscle fibers? Which type is larger?

Answer 7

alpha motor neuron= innervated extrafusal fibers of skeletal muscle
- Much LARGER than intrafusal

gamma-motor neurons= innervate intrafusal fibers of skeletal muscle

Question 8

What are β-motor neurons or skeletofusimotor fibers? What is the function of β-motor neurons or skeletofusimotor fibers?

Answer 8

These are motor neurons that innervate BOTH intrafusal and extrafusal fibers

Question 9

What is the function of coactivation of alpha and gamma motoneuron?

Answer 9

Coactivation is to keep spindles sensitive to changes in muscle length even as the muscle contracts & shortens

• Extrafusal contraction alone would cause UNLOADING of intrafusal fibers & subsequent changes in length would not be immediately detected
• Gamma motor neuron activation keeps intrafusal fibers the same length as the extrafusal fibers

Question 10

What is the difference between the way static and dynamic sensory information is relayed from muscle spindles?

Answer 10

Static= length only

Dynamic= rate or change in muscle length

Question 11

What kind(s) of information is/are carried by group Ia fiber?

Answer 11

• Length of the muscle (static)
• How fast the muscle is changing (dynamic)

**Innervates nuclear chain & nuclear bag fibers

Question 12

What kind(s) of information is/are carried by group II fiber?

Answer 12

Length of the muscle ONLY

*****Innervates nuclear chain fibers only

Question 13

Draw the neuronal circuit for the stretch reflex (or myotatic reflex).

Answer 13

p. 100 Costanzo

1) Ia afferent from intrafusal fibers
2) Synapse on homonymous muscle (same)
3) alpha-motoneuron innervation to SAME muscle

Question 14

What are the clinical applications of the stretch reflexes?

Answer 14

Evaluation for signs of LMN vs. UMN lesion

• Hypoactive= LMN
• Normal= normal
• Hyperactive= UMN

Question 15

What is a DTR or tendon jerk?

Answer 15

This is the same thing as the stretch reflex or myotatic reflex

** “Deep Tendon Reflex” is a misnomer, the sensory receptor is IN THE MUSCLE spindle

Question 16

What’s the difference between phasic and tonic stretch reflexes?

Answer 16

Phasic= v. brief stretch elicited by tapping a tendon

Tonic= longer lasting stretch caused by stretching a muscle and holding it in its new position
- This is how spasticity is elicited

Question 17

How are phasic and tonic stretch reflexes they tested?

Answer 17

Phasic= reflex hammer 
Tonic= stretching muscle at varying speeds

Question 18

What is meant by, reciprocal innervation in the spinal cord?

Answer 18

Innervation of the homonymous muscle agonists & antagonists

• Ia fibers synapse with alpha-motoneuron of homonymous muscle
• Ia collateral synapse on interneurons of antagonist muscle

Question 19

How does the reciprocal innervation of the spinal cord relate to the concept to the stretch reflex?

Answer 19

Reciprocal innervation yields reciprocal inhibition in the stretch reflex i.e.

• Contraction of homonymous muscle
• Relaxation of antagonist muscle

Question 20

How do clinicians test muscle tone?

Answer 20

Stretching muscle at varying speeds i.e. testing the tonic stretch reflex

Question 21

What is co-contraction?

Answer 21

Simultaneous activation of agonist and antagonist muscles

Question 22

In what situations is co-contraction used?

Answer 22

• Neurologically intact adults & kids when learning new skills
• Infants & children during postural development

*****Note that this is thought to be the physiologic basis of the phenotype of Cerebral Palsy (CP)

Question 23

Draw the neuronal circuits that include the Ib interneuron and describe the “normal” group Ib reflex.

Answer 23

Costanzo p. 101

1) Muscle contraction
2) Afferent signal via Ib fivers to inhibitory interneurons
3) Inhibitory interneurons synapse on alpha-motoneurons
4) Inhibition of alpha motoneurons causes relaxation of homonymous muscle

Question 24

What is the flexor withdrawal reflex?

Answer 24

Reflex withdrawal from noxious stimuli

• Pain afferents cause flexion & withdrawal of affected body part from stimulus
• Extension in contralateral side

Question 25

What is the crossed extension reflex?

Answer 25

This terminology refers to the extension part of the “flexor withdrawal” reflex
- Extensor muscle contraction & flexor relaxation on the CONTRALATERAL side of the body to the painful stimulus

Question 26

Compare and contrast the output of a muscle spindle and a Golgi tendon organ during muscle contraction and passive muscle stretch.

Answer 26

Golgi tendon= “inverse myotaxic reflex”

• GT is in SERIES w/ extrafusal fibers vs. parallel spindle
• Ib innervation in GT vs. Ia in muscle spindle
• Spindle= stretch, Contraction= GT
• GT produces inhibition/ relaxation of homonymous muscles

Question 27

What is the clasp-knife response? When does it occur?

Answer 27

This is considered to be a sign of a UMN lesion/ occurs when there has been a UMN lesion

• Passive flexion of joint is initially opposed by opposing muscles
• Flexion continues & resistance mounts
• Suddenly the Gogli Tendon Reflex kicks in causing a sudden relaxation & closing of the joint

*****This resembles a pocket-knife closing i.e. “Clasp Knife”

Question 28

Why do UMN lesions result in hypertonia?

Answer 28

Loss of presynaptic inhibition of muscle spindle afferents allows the muscle stretch reflex to elicit continual contraction