Motor System I-III

Question 1

What is a motor unit?

Answer 1

Motor neuron and the muscle fibers it innervates

Each muscle fiber innervated by ONE motor neuron

One motor neuron innervates multiple muscle fibers

Question 2

Explain the concept of the “size principle”

Answer 2

Systematic recruitment of smaller-to-larger motor units

Motor units vary in size and properties

3 types: Slow motor units, Fast-fatigue-resistant motor units, Fast-fatigable motor units

Small neurons recruited first because of V=IR and because large neurons have more channels and are leakier

If enough action potentials, get recruitment of the larger motor units (to run for example)

Question 3

How does exercise influence the motor unit

Answer 3

Motor unit properties are use-dependent

Endurance exercise tends to slow the contractile properties of motor units and increase endurance and strength. High intensity strength training increases amount of contractile protein

Effects not entirely on motor unit phenotype; also central changes that may alter recruitment

Question 4

Where within the motor systems are alpha motor neurons organized somatotopically?

Answer 4

Alpha motor neuron cell bodies reside in the ventral horn of the spinal cord and they are organized somatotopically

Lateral musculature innervated by laterally situated motor neurons

Medial musculature innervated by medially situated motor neurons

The same arrangement holds along the rostro-caudal extent of the spinal cord: Cervical and lumbar enlargements can be seen within the spinal cord, representing the enlarged motor neuron populations that innervate the upper and lower limb musculature respectively.

Question 5

Muscle spindle innervation

Answer 5

Proprioceptor (sensory receptor)

They send info out (stretch) but they also receive info from gamma motor neurons

Stretch of the muscle spindle is communicated to the spinal cord through group Ia and group II sensory afferents, which are large, fast axons that fire action potentials in response to mechanical stretch of the intrafusal muscle fibers.

Ia sensory afferents contact α motor neurons in the spinal cord which trigger muscle contraction of the homonymous muscle fiber in response to stretch (reflex)

Also, innervated by gamma neurons

During voluntary contraction, α and ɣ motor neurons fire together, shortening both the extrafusal (normal skeletal muscle) and intrafusal (spindle) muscle fibers together. By shortening the spindle (i.e. intrafusal muscle fiber), the system maintains sensitivity to stretch, thus it can detect stretch of a contracted or relaxed muscle.

Question 6

Golgi tendon organ innervation

Answer 6

Innervated by and signal via type Ib sensory afferents that wind around and within the collagen strands

During muscle contraction, force increases the tension on collagen strands and pinches the intertwined afferent fibers, causing them to fire.

Activated in active contractions with a load

Question 7

Describe the basic stretch reflex circuit

Answer 7

Monosynaptic reflex

The hammer tap stretches the muscle, stimulating activity in the Ia sensory axons (fast!), reporting stretch of muscle spindles. This sensory information is relayed to and activates α motor neurons in the spinal cord which in turn contract the stretched muscle

Question 8

Slow motor units

Answer 8

Small alpha motor neurons
Innervate a small number of slow oxidative muscle fibers
Generate small forces
Fatigue slowly
Recruited first due to high resistance V=IR

Question 9

Fast-fatigue-resistant motor units

Answer 9

Intermediate sized alpha motor neurons
Innverate an intermediate number of fast oxidative glycolytic muscle fibers
Generate large forces
Fatigue slowly
Recruited second

Question 10

Fast-fatigable motor units

Answer 10

Large alpha motor neurons 
Innervate a large number of fast glycolytic muscle fibers
Generate large forces
Fatigue quickly
Recruited last

Question 11

What sensory info do muscle muscle spindles encode

Answer 11

Detects muscle stretch

Activated in unexpected stretch

Question 12

Where are muscle spindles located?

Answer 12

Embedded within a muscle (parallel orientation)

Specialized muscle fiber (intrafusal muscle fibers) run in parallel with the main “extrafusal” muscle fiber (the force-generating muscles we are all familiar with)

Question 13

What sensory information do golgi tendon organs encode

Answer 13

Proprioceptor

Preferentially sensitive to muscle tension, not passive stretch (like a Chinese finger trap)

Used to regulate force

Question 14

Where are golgi tendon organs located?

Answer 14

Reside at the junction of a muscle and a tendon

Situated in series with the muscle and tendon

Question 15

What happens if a muscle shortens unexpectedly?

Answer 15

Spindle longer than length of extrafusal muscle fibers (under contracted)
1a afferent drops firing rate
Alpha motor neuron experiences reduced drive
Lengthens muscle

Question 16

What serves as a mechanism for rapid error-correction in muscle contraction?

Answer 16

Coactivation of α and ɣ motor neurons

Question 17

If something is lighter than you expect when you pick it up, what happens?

Answer 17

Ia rate drops and muscle relaxes

Question 18

If something is heavier than you expect when you pick it up, what happens?

Answer 18

Muscle spindles experience stretch, so Ia afferents rate increases and muscle contracts

Question 19

Extensor-flexor coupling circuits

Answer 19

Ib afferents innervating the golgi tendon organs (GTOs) directly contact inhibitory and excitatory interneurons in the spinal cord. In contrast to the jerk that is produced during the stretch reflex, this reflex protects the musculature from over exertion by relaxing the synergist (homonymous) muscle and contracting the antagonist.

The key take home message is that activity of synergist and antagonist musculature is coordinated in part by spinal cord interneuronal circuitry.

Question 20

Crossed-extensor reflex

Answer 20

Stepping on tack

Body reflexively shifts weight to the alternate leg and lifts the pricked foot

Cutaneous (nociceptors in this case) sensory receptors 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 21

Central pattern generator definition, describe a behavior that uses one, and where it resides

Answer 21

Neural networks that can produce patterned, rhythmic outputs in the absence of sensory or central input

Example behavior: Locomotion, swimming

1. Part of the locomotor CPG is the rhythm generator, or ‘clock’.
2. The ‘clock’ component innervates and drives interneuronal networks that amplify the clock signal and distribute 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.

Where it resides: In spinal cord we think

Question 22

What is meant by the ‘hierarchical’ organization of the motor system

Answer 22

Increasingly flexible, voluntary control as neuron populations shift from the periphery to the forebrain.

Question 23

Descending pathways that control finger movement (where is spinal cord?)

Answer 23

Motor: Axons that course laterally
All decussate
Lateral corticospinal tract

Question 24

Descending pathways that control axial musculature (where in spinal cord?)

Answer 24

Vestibulospinal and reticulospinal pathways participate in organizing axial musculature and maintaining balance

Axons that course medially
Some decussate, some don’t
Ventral corticospinal tract

Question 25

Two ways in which motor cortical plasticity is advantageous for recovery and/or treatment of diseases or damage to the motor system?

Answer 25

1. Sprouting of new connections from adjacent areas subserve motor control over the affected body part
2. Practice- subtle synaptic changes likely occur during practice and repeated performance of an action leads to expansion of that region of the cortex

Question 26

Reciprocal innervation

Answer 26

Example:
In the stretch reflex, Type Ia afferents within the reflex arc directly contact alpha motor neurons that contract the homonymous muscle (the muscle that has been stretched).

Ia afferents also contact inhibitory interneurons that reside within the spinal cord that then inhibit motor neurons controlling the opposing “antagonist” muscle

Synergist and antagonist muscles are coordinated to contract and relax nearly simultaneously.

Question 27

Lower motor neuron syndrome symptoms and causes

Answer 27

Refers to the loss or degeneration of motor neurons in the spinal cord and results in : Paralysis; Weakness; Loss of deep reflexes; Decreased muscle tone; Muscle atrophy; Spontaneous twitches due to changes in muscle excitability after denervation

Syphilis : dorsal root ganglia
Herniated disc: sensory nerve root
Polio or ALS: motor neuron
Guillan-Barre: demyelination of peripheral motor nerve
Lambert-Eaton syndrome: peripheral Ca channel attack hits NMJ
Muscular dystrophy: can affect the muscle directly.

Question 28

Upper motor neuron syndrome

Answer 28

Refers to the lesion of premotor neurons (corticospinal; brainstem)

Results in: Contralateral muscle flaccidity (body and lower face). Weakness; Spasticity (including increased muscle tone, hyperactive deep reflexes, clonus, or large involuntary rhythmic movements because of loss of inhibitory tone mediated by descending projections); Babinski sign; Loss of voluntary movements

Question 29

Colliculospinal tract

Answer 29

“tectospinal”

orientation of gaze and stance to a stimulus in space

Question 30

Vestibulospinal tract

Answer 30

Important for posture, balance, and orienting movements (if you fall, brings arms out)

Question 31

Reticulospinal tract

Answer 31

Posture, balance, and anticipatory movements

Question 32

Lateral Premotor Cortex

Answer 32

Responsible for learned movements in response to external stimuli. This area also plans movements that are to be executed with delay (i.e. conforming your hand to the shape you are going to grasp). Planning here begins up to a second before the activity begins. The supplementary motor cortex is involved in rehearsing movements in your head. Activity from the premotor cortex and the supplementary motor cortex can result in bilateral movements. Most projections from these areas go to the primary motor cortex but 25% go directly to the corticospinal tract.

Question 33

Supplementary motor cortex

Answer 33

involved in the mental rehearsal of planned movements. Both the supplementary and premotor cortices are located ANTERIOR to the primary motor cortex.