Exam #6: Spinal Mechanisms of Motor Control Flashcards
What is the muscle spindle?
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
Where is the muscle spindle located?
In skeletal muscle, in PARALLEL to extrafusal fibers
What innervates the muscle spindle?
Gamma motor axons
What is the golgi tendon organ?
This is a stretch receptor found in tendons, which senses contraction of muscle and activates group Ib afferent nerves
Where is the golgi tendon organ located?
Tendons
What innervates the golgi tendon organ?
Single group Ib fiber
What is the difference between alpha motor neurons and gamma motor neurons? Which innervates intrafusal muscle fibers? Which type is larger?
alpha motor neuron= innervated extrafusal fibers of skeletal muscle
- Much LARGER than intrafusal
gamma-motor neurons= innervate intrafusal fibers of skeletal muscle
What are β-motor neurons or skeletofusimotor fibers? What is the function of β-motor neurons or skeletofusimotor fibers?
These are motor neurons that innervate BOTH intrafusal and extrafusal fibers
What is the function of coactivation of alpha and gamma motoneuron?
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
What is the difference between the way static and dynamic sensory information is relayed from muscle spindles?
Static= length only
Dynamic= rate or change in muscle length
What kind(s) of information is/are carried by group Ia fiber?
- Length of the muscle (static)
- How fast the muscle is changing (dynamic)
**Innervates nuclear chain & nuclear bag fibers
What kind(s) of information is/are carried by group II fiber?
Length of the muscle ONLY
*****Innervates nuclear chain fibers only
Draw the neuronal circuit for the stretch reflex (or myotatic reflex).
p. 100 Costanzo
1) Ia afferent from intrafusal fibers
2) Synapse on homonymous muscle (same)
3) alpha-motoneuron innervation to SAME muscle
What are the clinical applications of the stretch reflexes?
Evaluation for signs of LMN vs. UMN lesion
- Hypoactive= LMN
- Normal= normal
- Hyperactive= UMN
What is a DTR or tendon jerk?
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
What’s the difference between phasic and tonic stretch reflexes?
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
How are phasic and tonic stretch reflexes they tested?
Phasic= reflex hammer Tonic= stretching muscle at varying speeds
What is meant by, reciprocal innervation in the spinal cord?
Innervation of the homonymous muscle agonists & antagonists
- Ia fibers synapse with alpha-motoneuron of homonymous muscle
- Ia collateral synapse on interneurons of antagonist muscle
How does the reciprocal innervation of the spinal cord relate to the concept to the stretch reflex?
Reciprocal innervation yields reciprocal inhibition in the stretch reflex i.e.
- Contraction of homonymous muscle
- Relaxation of antagonist muscle
How do clinicians test muscle tone?
Stretching muscle at varying speeds i.e. testing the tonic stretch reflex
What is co-contraction?
Simultaneous activation of agonist and antagonist muscles
In what situations is co-contraction used?
- 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)
Draw the neuronal circuits that include the Ib interneuron and describe the “normal” group Ib reflex.
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
What is the flexor withdrawal reflex?
Reflex withdrawal from noxious stimuli
- Pain afferents cause flexion & withdrawal of affected body part from stimulus
- Extension in contralateral side
What is the crossed extension reflex?
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
Compare and contrast the output of a muscle spindle and a Golgi tendon organ during muscle contraction and passive muscle stretch.
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
What is the clasp-knife response? When does it occur?
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”
Why do UMN lesions result in hypertonia?
Loss of presynaptic inhibition of muscle spindle afferents allows the muscle stretch reflex to elicit continual contraction
