Motor Systems

Question 1

Where do alpa motor neurons live?

Answer 1

• Cell bodies reside in the ventral horn of the spinal cord and brainstem cranial motor nuclei

Question 2

What are the higher motor centers in the cortex and brainstem?

Answer 2

• Vestibular nucleus
		○ Anticipatory motor control
	• Reticular formation
		○ Rhythmic motor output (locomotion)
	• Superior colliculus
		○ Eye movements
	• Motor cortex
		○ voluntary motor control

Question 3

What does the cerebellum act as (general)?

Answer 3

• Feedback error correction circuit

* Computes short and long-term (learned) corrections to the errors in the circuits from the other motor systems

Question 4

What role (general) does the basal ganglia play?

Answer 4

• Roles in movement initiation and action selection as well as roles in motor learning, reinforcement, motivated behavior

Question 5

Alpha motor neurons (LMNs) in the ventral horn of the spinal cord are organized somatotopically. What does that mean?

Answer 5

• Lateral musculature is innervated by laterally situated motor neurons and medial musculature is innervated by medially situated motor neurons
  
  • That allows for symptoms of ascending or descending motor loss

Question 6

What is the definition of a motor unit?

Answer 6

• Alpha motor neuron and the muscle fibers it innervates

• more muscle fibers than nerves so one nerve will often innervate a ton of muscle fibers
• leads to different characteristics of fatigueability and force between the different motor units

Question 7

How does the gradual recruitment of force help you hold an egg?

Answer 7

• The fact that smaller alpha motor neurons innervate smaller motor units allows the nervous system to recruit less movement for a given task
• Gradients from small to large motor units exist, generating graded forces
• They can be recruited systematically by higher motor centers, resulting in a gradual increase in force
*to grip a fragile object you recruit the smaller motor units

Question 8

Describe the physiology of graded motor unit recruitment

Answer 8

• Small neurons have high input resistances because of fewer channels in the membrane
  
  • V=IR
  • A smaller current is needed to cause a voltage increase sufficient to stimulate an EPSP
  • Thus, less NT is needed to fire the smaller motor neurons than the larger ones
  • “size principle”

Question 9

What are the three basic types of muscle fibers that differ in fatigability?

Answer 9

• Tonic muscle fibers
○ Super low fatigability, mostly in spindles and in extraocular muscles
○ Generate isometric tension, shorten very slowly
• Slow twitch muscle fibers
○ Low fatigability with lots of myoglobin and mitochondria
• Fast twitch oxidative fibers
○ Activate quickly and have lots of mitochondria so fatigue moderately slowly
• Fast twitch glycolytic fibers
○ Activate quickly
○ Fatigue rapidly as they rely on anaerobic glycolysis ATP generation
○ Less mitochondria

Question 10

The size principle of motor neurons matches up how with the type of muscle fibers?

Answer 10

• One motor unit is all the same type of muscle fiber
  
  • Small neurons innervate slow motor units (posture)
  • Large neurons innervate the fast-twitch glycolytic fibers
  • Medium neurons innervate the fast twitch oxidative fibers

Question 11

What is a muscle spindle?

Answer 11

• Technically, a muscle spindle is a speciaal type of muscle fiber
○ Intrafusal muscle fiber
• Run in parallel with the main extrafusal muscle fiber
• Stretch of the muscle spindle is communicated to the spinal cord through group Ia and II sensory afferents
○ Large, fast axons that have the modality of stretch
• Ia sensory afferents contact alpha motor neurons in spinal cord and form the DTRs or stretch reflexes
• Remember that muscle spindles are contractile

Question 12

What motor neurons innervate muscle spindle cells?

Answer 12

• Gamma motor neurons
  
  • Alpha and gamma neurons fire together during voluntary contraction
  • This allows a maintenance of stretch receptivity (even a shorter muscle will have the same(ish) stretch receptivity)

Question 13

What is the GTO?

Answer 13

• Golgi tendon organ
  
  • Collagen structures at the junction of a muscle and a tendon and are innervated by and signal bia Ib sensory afferents that wind around and within the collagen strands
  • Situated in series with the muscle and tendon (not parallel like muscle spindles)
  • Are preferentially sensitive to muscle tension since passive stretch lengthens the muscle before straning the tendon
  • During muscle contraction force increases the tension on collagen strands and pinches the intertwined afferent fibers causing them to fire
  • GTOs preferentially detect muscle tension rather than passive stretch
  • REGULATE muscle force

Question 14

Describe the neuronal process behind the patellar tendon DTR

Answer 14

• Hammer tap stretches the muscle
• Stimulates activity in the Ia sensory axons (fast and fat)
• This reports the stretch of muscle spindles to both cortex and the relay to alpha motor neurons in the spinal cord that contract the stretched muscle
○ Synergist muscle
• Considered monosynaptic but the Ia spindle afferents branch and diverge in the spinal cord to contact large populations of motor neurons which innervate populations of muscle fibers to create the reflex motion
• Thus it produces large contraction and maintains faitfulness by not depending on one or a few neurons

Question 15

Besides contraction of the synergist muscle, what muscles are coordinated to act by the type Ia afferents in the reflex arc?

Answer 15

• Type Ia afferents directly contact alpha motor neurons in the spinal cord of the homogenous/syergist muscle leading to contraction
  
  • IN ADDITION the type Ia afferents stimulate inhibitory neurons in the spinal cord to RELAX the opposing muscle
  • Reciprocal innervation, happens simultaneously

Question 16

What is flexor-extensor coupling?

Answer 16

• Type 1b afferents innervating the GTOs directly contact inhibitory and excitatory interneurons in the spinal cord
  
  • Not a jerk that is produced, but a protection of the syngergist muscle by contracting the antagonist

Question 17

How does Guillan-barre mess with muscle action?

Answer 17

• Peripheral motor nerve acute demyelination

Question 18

What is the normal function of the neuronal circuit tested in DTRs?

Answer 18

• Maintaining muscle tone, or the resistance of muscle to stretch
  
  • Important for standing, walking, running
  • Can be controlled top-down (think gamma-neuron innervation of spindles)
  • Hypotonia can result from damage to either the Ia sensory afferents innervating the spindles or the alpha motor neurons innervating the muscle
  • Hypertonia can result from damage to descending motor pathways that influence the spinal cord premotor circuits (spasticity is included here)

Question 19

How does Lambert-eaton syndrome mess with muscle action?

Answer 19

• NMJ destruction by immunological attack of peripheral nerve Ca channels

Question 20

Describe the native firing rate of the type Ia afferents

Answer 20

• These innervate the spindle muscle fibers (intrafusal)
• Maintain a low, but non-zero firing rate at baseline
• Passive stretch and shortening are discerned as increased or decreased frequency of the type Ia afferent AP
○ Increase = stretch

Question 21

Using a heavy box as an example, how do the gamma and alpha motor neurons result in a coordinated error correction reflex?

Answer 21

• You know the box is heavy so both alpha and gamma neurons are highly active shortening the spindle and the muscle to the same amount to give a large force
  
  • If the box were lightened without you knowing it, the alpha motor neuron activity would result in a shortening out of synch with the gamma motor neurons’ work
  • Essentially, the muscle shortens faster than the spindle and the type 1a afferent would tell the spinal cord to correct the discrepancy
  • The result in this scenario is a drop in type1a firing rate and reduciton of alpha motor neuron drive and reduction of muscle contraction
  • The opposite is a heavier than intended box and now there is a percieved stretch, increase of Ia firing rate and resultant increase in reflexive tone

Question 22

Type Ia sensory afferents direclty contact which motor neurons?

Answer 22

• Alpha, not gamma
  
  • Gamma is controlled by the descending voluntary control pathways
  • You want gamma to be the “expected stretch response” to mediate rapid error-correction reflexes

Question 23

How does the crossed extension reflex work?

Answer 23

• Classic example - stepping on tack and not falling from being off-balance
  
  • Cutaneous nocieceptors innervate spinal interneuronal motor networks
  • These coordinate extensor relaxation and flexor contraction on the same side as the stimulus and a converse extensor contraction and flexor relaxation on the contralateral side

Question 24

What are CPGs?

Answer 24

• CPG - central pattern Generators
  
  • Neuronal networks in spinal cord that, without downward control, can produce coordinated action like locomotion and swimming

Question 25

What are the 5 general main points of the CPG system?

Answer 25

• CPG - central pattern generator

1) Part of the locomotor CPG is the rhythm generator/clock
2) The clock component innervates and drives interneuronal networks that amplify the clock signal and distrubte it appropriately to coordinate muscle contraction and relaxation
3) CPGs for both limbs interact via commissural fibers to coordinate between limb use
4) CPGs are modulated by descending pathways that can affect clock rate and motor patterns
5) Some of the same circuits involved in simple reflexes discussed earlier are flexibly engaged in locomotion

Question 26

What three higher order structures that coordinate movement did we discuss in class?

Answer 26

• Vestibular complex
  
  • Reticular formation
  • Supperior colliculus (tectum)

Question 27

Where do the vestibular nuclei send axons to?

Answer 27

• Variety of targets, there is lots of coordination here
  
  • Medial vestibulospinal tract to the medial spinal cord where they regulate head orientation and neck muscle activation
  • Via the lateral vestibulospinal tract to the lateral motor pools controlling proximal limb musculature
  • Vestibular information is used to adjust posture of the head and torso in the vestibulocervical or vestibulospinal reflexes
  • Example is falling down forward and throuwing arms out while pulling head up

Question 28

What is the VOR?

Answer 28

• Vestibular ocular reflex
• Produces eye movements that counter head movements to keep gaze fixed
*example of the most simple of vestibular-coordinated movements (or at least most mapped out)

Question 29

Describe the neuronal mechanism of the VOR

Answer 29

• The vestibular nuclei project bilaterally to the abducens nuclei
• The abducens neurons cross again
○ On the side contralateral to the activated vestibular neurons, its axons excite motor neurons that contract the lateral rectus of the contralateral eye and the medial rectus of the ipsilateral eye
○ On the side ipsilateral to the activated vestibular nucleus, its axons inhibit motor neurons that control the medial rectus muscle of the contralateral eye
○ Also relax the lateral rectus muscle of the ipsilateral eye
○ This results in the eyes moving exactly opposite the movement of the head with no temporal delay

Question 30

What is the clinical efficacy in testing the VOR?

Answer 30

• It’s a reflex. It will happen in a comatose patient
• If the reflex is absent this suggests brainstem damage
• Damage to the circuitry can also result in reports of bouncing vision
○ Oscillopsia
• Patient is unable to maintain stable gaze fixed to movement making his visual scene seem to bounce

Question 31

What is oscillopsia?

Answer 31

• Damage to the circuitry can also result in reports of bouncing vision
○ Oscillopsia
• Patient is unable to maintain stable gaze fixed to movement making his visual scene seem to bounce

Question 32

What is the reticular formation?

Answer 32

• Collection of loosely connected areas that lie within the midbrain tegmentum
○ Ventral covering of midbrain
• Formation has many functions that nevertheless fall into two broad categories:
○ Modulatory functions
§ Cardiovascular control
§ Respiratory control
§ Some sensorimortor reflexes
§ Eye movement coordination
§ Sleep/wake regulation
§ Coordination of limb and trunk movements
○ Premotor functions

Question 33

What is the (general) role that the reticular formation plays in motor control?

Answer 33

• Anticipatory reflex action
  
  • Example of lifting a dumbell
  • Legs will move to anticipate the change in center of mass as arm is working
  • RF is responsible for many anticipatory responses to a voluntary movement that help maintatin postural control and balance

Question 34

What is the superior colliculus important for?

Answer 34

• Critical for orienting gaze
• (general) directing body position
• Descending projections of the colliculospinal (tectospinal) tract target motor neurons that control the axial musculature of the neck
○ Generates coordinated orienting responses
• Siren goes off you look towards the siren

Question 35

Describe the route cells take from the primary motor cortex

Answer 35

• Precentral gyrus of cerebral cortex
• Pyramidal neurons of laryer 5 project to spinal cord via the corticospinal tract
○ Internal capsule
○ Cerebral peduncle
○ Ventral midbrain
○ Through pons where they collateralize
○ Medulla - now called pyramids
○ Cross midline at caudal medulla and form lateral corticospinal tract
• Still some fibers that don’t cross over and continue medially to innervate alpha motor neurons for posture
• Most fibers make synapses on interneurons (circuits) in the ventral horns
• Fewer neurons carry on the classic function of direct innervation of alpha motor neurons

Question 36

What brodmann’s area is the primary motor cortex?

Answer 36

• Brodmann’s area 4 = 4 primary motor cortex

Question 37

What is just anterior to the primary motor cortex?

Answer 37

• Premotor cortex
  
  • Fewer direct descending projections as M1 but still 25% of corticospinal tract
  • Involved when movement is initiated by an external cue
  • Moving hand in response to a light stimulus
  • Stimulation of premotor or supplementary motor cortices are complex, multijoint movements, suggesting a more complex motor map than M1

Question 38

What does the supplementary motor cortex do?

Answer 38

• Involved primarily in self-cue movements
  
  • Contrast this with premotor cortex
  • Highly active during mental rehearsal of movement
  • Stimulation of premotor or supplementary motor cortices are complex, multijoint movements, suggesting a more complex motor map than M1

Question 39

In what area of the brain are mirror neurons?

Answer 39

• Mirror neurons - active during visualization or observation of an action. They are also identically active while performing the action.
  
  • You watch someone throw a ball, and these neurons are active then and when you throw the ball too.

Question 40

How can plasticity in the motor cortices be generated?

Answer 40

• Practice
○ Repeated performance of a given action leads to expansion of that region of cortex
○ Contraint induced movement therapy helping in stroke recovery
• Stroke/injury (new connections take over certain functions)