Motor System I-III Flashcards
What is a motor unit?
Motor neuron and the muscle fibers it innervates
Each muscle fiber innervated by ONE motor neuron
One motor neuron innervates multiple muscle fibers
Explain the concept of the “size principle”
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)
How does exercise influence the motor unit
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
Where within the motor systems are alpha motor neurons organized somatotopically?
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.
Muscle spindle innervation
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.
Golgi tendon organ innervation
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
Describe the basic stretch reflex circuit
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
Slow motor units
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
Fast-fatigue-resistant motor units
Intermediate sized alpha motor neurons Innverate an intermediate number of fast oxidative glycolytic muscle fibers Generate large forces Fatigue slowly Recruited second
Fast-fatigable motor units
Large alpha motor neurons Innervate a large number of fast glycolytic muscle fibers Generate large forces Fatigue quickly Recruited last
What sensory info do muscle muscle spindles encode
Detects muscle stretch
Activated in unexpected stretch
Where are muscle spindles located?
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)
What sensory information do golgi tendon organs encode
Proprioceptor
Preferentially sensitive to muscle tension, not passive stretch (like a Chinese finger trap)
Used to regulate force
Where are golgi tendon organs located?
Reside at the junction of a muscle and a tendon
Situated in series with the muscle and tendon
What happens if a muscle shortens unexpectedly?
Spindle longer than length of extrafusal muscle fibers (under contracted)
1a afferent drops firing rate
Alpha motor neuron experiences reduced drive
Lengthens muscle
What serves as a mechanism for rapid error-correction in muscle contraction?
Coactivation of α and ɣ motor neurons
If something is lighter than you expect when you pick it up, what happens?
Ia rate drops and muscle relaxes
If something is heavier than you expect when you pick it up, what happens?
Muscle spindles experience stretch, so Ia afferents rate increases and muscle contracts
Extensor-flexor coupling circuits
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.
Crossed-extensor reflex
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
Central pattern generator definition, describe a behavior that uses one, and where it resides
Neural networks that can produce patterned, rhythmic outputs in the absence of sensory or central input
Example behavior: Locomotion, swimming
- Part of the locomotor CPG is the rhythm generator, or ‘clock’.
- The ‘clock’ component innervates and drives interneuronal networks that amplify the clock signal and distribute it appropriately to coordinate muscle contraction and relaxation.
- CPGs for both limbs interact via commissural fibers to coordinate between-limb use.
- CPGs are modulated by descending pathways that can affect clock rate and motor patterns.
- Some of the same circuits involved in simple reflexes discussed earlier are flexibly engaged in locomotion.
Where it resides: In spinal cord we think
What is meant by the ‘hierarchical’ organization of the motor system
Increasingly flexible, voluntary control as neuron populations shift from the periphery to the forebrain.
Descending pathways that control finger movement (where is spinal cord?)
Motor: Axons that course laterally
All decussate
Lateral corticospinal tract
Descending pathways that control axial musculature (where in spinal cord?)
Vestibulospinal and reticulospinal pathways participate in organizing axial musculature and maintaining balance
Axons that course medially
Some decussate, some don’t
Ventral corticospinal tract
