2: Intro to the Motor Systems Flashcards
Define the terms motor unit and motor pool and discuss their location in the spinal cord.
Motor unit: all of the muscle fibers innervated by a single motor neuron
Motor pool: all of the motor neurons innervating a single muscle
–Grouped together in clusters in the ventral horn, spread over several spinal segments -> most muscles receive innervation from several spinal segments and nerve roots
Discuss the types of extrafusal muscle fibers in relation to their size, speed of contraction, tension/fiber, metabolism, and susceptibility to fatigue. (3)
Type I (red): for endurance
- -Small fibers
- -Small neurons
- -Moderate speed of contraction
- -Low tension/fiber
- -High aerobic metabolism
- -Resistant to fatigue
Type II (white): for powerful movements
- -Large fibers
- -Large neurons
- -Very fast speed of contraction
- -High tension/fiber
- -Low aerobic metabolism
- -Fatigues easily
Type III: intermediate
–Fast twitch, powerful, but more fatigue resistant
Describe the structure and function of muscle spindle receptors.
Muscle spindle receptor: sets LENGTH of muscle, RESISTS STRETCH from this position
- -Intrafusal fibers, much smaller than extrafusal
- -Innervated by A-gamma neuron
- -Center portion wrapped in nerve ending (ANNULOSPIRAL ENDING) of 1A afferent nerve fiber
- -Contraction at ends of intrafusal muscle stretches center portion of muscle fiber -> 1A afferent nerve fiber activated
- -Activated either by stretch of muscle or activation of gamma motor neuron
- -Burst of activity during active stretch, followed by baseline firing pattern
List the three classic spinal cord reflexes.
Muscle stretch (myotatic, "deep tendon") reflex: stretching a muscle results in contraction --Tested with reflex hammer
Inverse myotatic/Golgi tendon organ reflex: stretch (to the point of muscle damage) results in relaxation of muscle
Flexion reflex (withdrawal): painful stimulus of a limb results in a withdrawal
Explain the general principles of alpha-gamma coactivation in the regulation of movement.
Aligns muscle spindle tension with new muscle length so that muscle spindle can respond to any deviations from new set length
–If muscle is shortened, intrafusal fiber would be unloaded and unable to detect stretch IF they didn’t contract along with extrafusal fibers
–Since contraction of extrafusal fibers = activation of gamma motor neurons -> contraction of ends of intrafusal fibers -> alignment of muscle spindle with new position of muscle -> restored ability to detect stretch from new position
***Describe the gamma loop and its participation in normal movements. (4 steps)
Gamma loop: the mechanism by which many smooth, routine movements take place
–Allows you to shorten biceps without thinking about lengthening triceps
- Activating gamma motor neurons will contract intrafusal fibers
- Annulospiral endings are activated as a result of stretch of center of intrafusal fibers
- 1a afferent neuron activated
- Alpha motor neuron -> agonist motor neurons activated, antagonist motor neurons inhibited
- Reflex contraction of extrafusal fibers takes tension off intrafusal fibers
Explain the clinical findings in spasticity in terms of abnormalities of normal function.
Spasticity: overactivity of gamma motor neurons -> excessively sensitive stretch reflex
- -Resistance to passive movement that is greater with faster movement
- -Due to activation of muscle stretch reflex
- -Also have “clasp knife” phenomena: muscle gives way when it is passively stretched (from overactive GTO)
- -Typically results from damage to descending projections from brainstem that inhibit gamma motor runs
Give a BRIEF overview of motor systems in the spinal cord, including the type of motor neurons and how they are controlled.
Spinal cord:
- Alpha (“lower”) motor neurons
- -Project to skeletal muscles - Controlled by INTERNEURONS that pattern particular types of movements
- —Interneurons also respond to sensory input: muscle stretch, muscle tension, pain
Alpha motor neurons can also be activated by higher centers of the NS for rapid, skilled movements
What types of motor neurons are present in the spinal cord? Where are they located?
Located in the ventral horn
Alpha motor neurons: large, rapidly conducting (80-120 m/s)
–EXTRAFUSAL
Gamma motor neurons: axons in the A-gamma range of size/speed (4-24 m/s)
–INTRAFUSAL (spindle)
Where are interneurons of the motor systems located?
In the center of the gray matter of the spinal cord, next to the motor neurons they control
Does the amount of white matter increase or decrease as you progress down the spinal cord?
Decrease. Ascending sensory tracts add as you ascend and motor tracts terminate as you progress down
Briefly describe the somatotopic organization of lower motor neurons in the ventral horn, using the cervical level as an example.
Motor neurons to arm are lateral
Axial muscles are medial
Extensor muscles are ventral
Flexor muscles are dorsal
What happens when you gradually try to increase the force of contraction of a muscle?
- Single motor unit fires slowly
- Single motor unit fires quickly
- Second motor unit becomes active, fires slowly
- Second motor unit fires quickly
….
n. All motor units fire quickly
Additionally, more white fibers are recruited as percent of maximum force is increased
What determines the numbers and types of muscle fibers contained within a muscle?
The nerve innervating the muscle
Explain the organization and functions of the muscle stretch reflex. (6)
Muscle stretch (myotatic, “deep tendon”) reflex: stretching a muscle results in contraction
- -Tested with reflex hammer
- -Requires receptors for stretch and tension
- Strike tendon -> sudden/rapid stretch of muscle
- Annulospiral ending activated
- Signal of muscle stretch sent along 1A afferent nerve fiber
- 1A afferent fiber synapses directly on motor neurons to muscle, activating them
- Collateral branches of afferent fiber synapse on interneurons to inhibit antagonist muscles and facilitate agonist muscles
- Muscle contracts -> limb is stabilized, unintended movement prevented
