Spinal Mechanisms of motor control Flashcards
What is the route of innervation of the patellar tendon reflex?
Tapping stretches muscle spindles, causing reflex arc via Ia fibers to A motor neurons of extensors, and inhibitors of flexor muscles
What is a phasic stretch reflex?
very brief stretch usually elicited by tapping on a tendon
What is a tonic stretch reflex?
longer lasting stretch caused by stretching a muscle and holding it at its new longer length
What are the two fibers involved in a monosynaptic reflex?
Ia from muscle spindle
Alpha motor neuron
What are the two phases of the tonic stretch? What are each of these phases?
- Dynamic phase-period during which length is changing
2. Static/steady phase– when muscle has stabilized at its new length
Type Ia fibers carry static or dynamic information about stretch? How about type II?
Ia = both II = static
Which type of muscle fibers are sensitive to dynamic changes in stretch: nuclear bags or chains?
Bags
Muscle spindles lie parallel or perpendicular to muscle fibers?
Parallel
What are the three types of intrafusal muscle fibers?
a. Dynamic nuclear bag
b. Static nuclear bag
c. Nuclear chain
What type of fibers innervate muscle spindles?
Type Ia
Firing pattern of Group Ia afferent is related to what two factors?
both how fast the muscle length is changing and the current length of the muscle.
Group II fibers relay what information?
Firing rate gives information about muscle length but does not emphasize changes in muscle length
True or false: patellar tendon reflexes do not stimulate group II fibers
True –too fast for them to respond
What do gamma efferent fibers innervate?
Stretches the ends of the nuclear bag/chain fibers.
What is the role of coactivation of alpha and gamma neurons?
• To prevent unloading of the muscle spindle gamma motor neurons are activated and they make intrafusal fibers contract. Now a small lengthening of the muscle can be sensed immediately.
The firing rate of muscle spindle afferents depends on what two factors?
both
muscle length and the level of gamma activation of the
intrafusal muscle fibers
What are the three functions of muscle spindles?
a. sensory input for stretch reflexes
b. sensory input to cerebellum
c. Muscle spindle output relayed to cerebral cortex for
conscious proprioception.
What are the two ways in which muscle spindle fibers can be stretched?
gamma motor neuron activation
Alpha motor neuron activation of muscle
What is an “adequate stimulus” for a muscle spindle?
Stretch of the central portion of the intrafusal muscle fiber
True or false: there are usually more than one type Ia fiber per muscle spindle
False–usually 1-1
True or false: Ia fibers usually innervate all the intrafusal fibers, so innervates both nuclear bags and chain
true
Where are type II fibers found?
at the ends of the nuclear bags/chains
Review alpha-gamma coactivation.
Both are needed to maintain the muscle spindle sensitivity when a muscle contracts
Why is DTR a misnomer?
uses muscle spindles, not GTOs
What is the tonic stretch reflex used to measure?
measure muscle tone= the resistance of a relaxed muscle to passive stretch.
What is the route of innervation of the tonic stretch reflex?
type Ia fibers from bags/chains synapse directly of alpha motor neurons.
Group II fibers from muscle spindles also synapse on alpha motor neurons of homonymous muscles
Spastic limbs show a blank-dependent increase in tonic stretch reflexes
velocity
What type of reflexes are seen with parkinsons?
Rigidity
What is the MOA of spasticity of UMN lesions?
Loss of inhibitory signals from corticospinal tract
What is reciprocal inhibition?
Inhibit antagonist muscle in a reflex arc
What is co-contraction?
simultaneous activation of agonist and antagonists at a joint
True or false: co-contraction is normal in neurologically intact adults
true
When is co-contraction important?
When stabilization is important
Inappropriate co-activation is implicated in what disease?
Thought that inappropriate coactivation contributes to the motor control problems found in children with cerebral palsy.
What is the MOA of the stretch reflex?
Same as the patellar tendon reflex plus descending inhibition/facilitation
What is the gain of stretch reflex?
how large a response you get for a stretch of a certain amount (e.g. large gain if small stretch caused strong contraction, small gain if large stretch caused weak muscle contraction)
What is the stretch reflex dependent on? (3)
- level of activity in gamma motor neurons
- background inhibition/excitation of alpha motor neurons
- modified by presynaptic inhibition of Ia afferents onto motor neurons
What is resting tone?
slight tension that can be felt in a relaxed muscle
Clinically, how would you test resting tone?
measure muscle tone by testing the tonic stretch reflex.
What causes muscle tone?
Passive (viscoelastic) properties of muscles and joints.
What is atonia? When is it seen?
No muscle tone–spinal shock
When is hypotonia seen? (2)
With pure pyramidal tract lesions and cerebellar lesions
What is the most common type of hypertonia?
Spasticity
Rigidity is another form of what tone? What disease is this classically found in?
Patients with Parkinson’s disease can have rigidity, another kind of hypertonia.
Spasticity is usually found with what type of motor neuron lesion?
UMN
What are the three major causes of spasticity?
Brain trauma
Spinal cord injury
MS
Spasticity is what type of disorder? What is the tone like? What is the major variable that this depends on?
Motor disorder
Hypertonia
Velocity dependent (resistance to tonic stretch increases as speed of stretch increases)
Spasticity is found particularly in which muscles?
Antigravity muscles
The more severe the patient’s spasticity, the (faster or slower) you can test the tonic stretch reflex and still feel increased muscle tone.
Slower
If a pt has spasticity in the LE, what other sign would you be likely to see?
Babinski sign
What is clonus? How is it tested? What type of motor lesion is it seen in?
oscillation in muscle stretch reflexes
Pushing up on the foot.
UMN lesion
What is the clasp-knife reflex? What type of motor lesion is it associated with?
The sudden decrease in resistance when flexing a knee
UMN lesion
What causes the clasp-knife reflex?
Hyperactive stretch reflexes of the spastic patient
Ia fibers innervate what? Ib?
Ia = muscle spindle Ib = GTO
Do GTOs have a high or low threshold of activation with muscle stretch?
High
What measures muscle length, and what measures muscle tension?
Length - muscle spindles
Tension - GTOs
Where are GTOs found?
Between muscles and tendons
Are GTO’s stimulated by stretch at all?
Yes, but high threshold
What happens to muscle spindle fiber firing if you only stimulate alpha motor neurons?
muscle contraction and lower muscle spindle firing
What are the three places the GTOs send their signals?
a. spinal cord (for reflex)
b. cerebellum
c. cerebral cortex
What is the Ib circuit reflex?
Very similar to the stretch reflex, but GTOs send Ib fibers to INHIBIT homonymous muscle, while STIMULATING antagonist
What is the result of the Ib reflex (hint, has to do with tension in a muscle since it’s GTOs involved)?
increased tension in muscle causes muscle to be inhibited. Decreased tension in muscle causes inhibition by Ib interneuron to decrease
True or false: Ib reflexes are only innervated via Ib fibers.
False–Ib inhibitory neurons also receive input from cutaneous receptors, joint receptors, muscle spindles and descending pathways. The GTO reflex can thus be modified by higher centers and /or additional sensory receptors.
True or false: the Ib reflex can protect its muscle from injury via excessive tension by reflexively inhibiting it when tension got too high
False-ish–the reflex is not strong enough to protect muscle by itself.
The Ib reflex is also called what (like it was called in neuroanatomy)?
Autogenic inhibition
What are the fibers that are involved in the flexion reflex afferents?
II, III, and IV
What is the crossed extension reflex?
The withdrawal reflex on one side of the body is counteracted by increased extensor muscle activation on the other side (e.g. stepping on a tack)
What is a renshaw cell?
A cell that receives innervation from the Alpha motor neuron that is stimulating a muscle, and has an inhibitor effect on that same alpha motor neuron (thus inhibits overactive neurons)
What is the effect of the renshaw cells?
Recurrent inhibition to alpha motor neurons
What is the most common inhibitory neurotransmitter in the spinal cord?
Glycine
Inhibiting an inhibitor produces what?
Excitation
What is the effect of Renshaw cells in the descending pathways?
Descending pathways can excite or inhibit Renshaw cells and thus adjust the excitability of all the motor neurons innervating muscles around a joint.
