Motor system: lower motor neuron circuits Flashcards
What is a synonym for motor neuron pools?
Final Common path
What is a motor neuron pool?
All the motor neurons innervating the same muscle.
Where are located lower motor neuron pools in the spinal cord?
Ventral horn.
How is the specific organization of lower motor neuron pools called?
Somatotopic organization.
List three differences between long distance local neuron circuits and short distance local neuron circuits.
Long distance local neuron circuits:
- Medial ventral horn
- Axons cross the midline = bilateral innervation
- Control posture and locomotion (SYNCHRONOUS movements)
Short distance local neuron circuits:
- Lateral ventral horn
- Axons do not cross the midline = unilateral
- Control of distant limb muscles (DEXTEROUS movements)
What is a motor unit?
Smallest unit capable of controlled movement → all the muscle fibers innervated by the projections of a single alpha motor neuron.
Does size matter………… for alpha motor neurons?
Yes.
Low innervation ratio is an indication of what?
Muscle controlling fine movement.
What are the motor unit types?
Slow, fast fatigue-resistant, fast fatigable.
What motor unit type is particularly important for forceful movements?
Fast fatigable.
Slower motor units have ____ activation thresholds.
Lower.
List three differences between gamma motor neurons and alpha motor neurons.
Gamma motor neurons:
- Small diameter
- Innervate muscle spindles
- Control excitability of sensory afferents in muscle spindles
Alpha motor neurons:
- Large diameter
- Innervate extrafusal muscle fibres
- Generate force for posture and movement
Does motor unit plasticity exist?
Yes, motor units can change accordingly to the demands of the body.
True or false: when muscle tension is increasing, the largest to smallest motor neurons are recruited.
False: smallest to largest.
What is a simple solution to graded muscle contraction?
The muscle size principle.
Associate the Hz to the stimulation: “twitch”, “unfused tetanus” and “fused tetanus”.
Respectively 5 Hz, 50 Hz and 100 Hz.
What are the three factors controlling muscle force?
- Type of motor units activated
- Number of motor units activated
- Rate of firing of each motor unit
What are the two forms that an intrafusal fibre can have in a muscle spindle?
Chain or bag.
** SEEN IN SOMATOSENSORY SYSTEM **
What are the two types of sensory afferents in muscle spindles? List three differences between the two.
Group Ia sensory afferents:
- Dynamic sensibility
- Respond phasically
- Spiral up on bags and chain fibres
Group II sensory afferents:
- Static sensibility
- Respond tonically
- Spiral up mainly on chain fibres
What motor neurons innervate the muscle spindles? What are its types?
Gamma motor neurons:
- Static
- Dynamic
Muscle spindles monitor and maintain ____ ____ and are sensible to ____ force.
muscle length… passive
Are group Ia and II afferents synapsing on alpha motor neurons? If yes, for what purpose?
Yes, for the stretch reflex (only carried in spinal cord).
Explain the reciprocal innervation mechanism and its consequences.
Mechanism: group Ia sensory afferents EXCITE alpha motor neurons that innervate synergistic muscles AND indirectly INHIBIT alpha motor neurons innnervating antagonist muscles via interneurons (intervening reciprocal-sensory Ia-inhibitory interneurons).
Consequences: contraction of stretched muscle and relaxation of antagonist muscle.
What is muscle tone? How is it important?
Muscle tone is the steady level of muscle tension. Important for adapting to demands of environment and self.
What is responsible for regulating the muscle gain? How?
Gamma motor neurons by adjusting level of tension in intrafusal muscle fibers.
True or false: often gamma and alpha motor neurons are co-activated.
True.
How do gamma motor neurons prevent Ia sensory afferents to become loose when extrafusal muscles contract?
By innervating the contractile regions of intrafusal fibers and being often co-activated with alpha motor neurons (the gamma motor neurons).
What is the sensory afferent in golgi tendon organs?
Group Ib sensory axons.
What is the negative feedback loop mechanism of GTO?
- Ib sensory afferents of GTOs synapse to inhibitory interneurons in spinal cord that synapse alpha motor neurons of the same muscle = decreasing muscle tension
- Ib sensory afferents of GTOs also synapse on excitatory interneurons that synapse alpha motor neurons of antagonist muscles
What is the role of Golgi tendon organs?
Help to maintain a steady level of force to counteract fatigue or muscle damage.
Golgi tendon organs monitor and maintain ____ ____ and are sensible to ____ force.
muscle force… active.
Where is controlled locomotion?
In central pattern generators in spinal cord.
What are the two states composing locomotion?
Swing and stance.
How is the pace of locomotion increased?
Stance time reduced and swings kept constant.
