Flashcards in Motor System I, II, and III Deck (31):
Interestingly, there is a tunicate (a small, waterborne organism) that ________ its brain after implanting into a stationary location.
The local circuit neurons do what?
They coordinate lower motor neurons (so that similar groups function together)
The motor unit is defined as _______________.
the motor neuron and the muscle fibers that it innervates
Another name for lower motor neuron is ____________.
alpha motor neuron
Describe the three size categories of motor units.
Slow: recruited first; small number of muscle fibers; generate small forces; fatigue slowly; high input resistance
Fast, fatigue-resistant: recruited second; medium number of muscle fibers; generate large forces; fatigue slowly
Fast, fatigable: recruited last; large number of muscle fibers; generate large forces; fatigue fast
True or false: small motor neurons have higher resistance.
True! (Fewer channels)
What is the consequence of increased resistance in smaller neurons?
Smaller neurons will have a higher voltage EPP with the same current input compared to large neurons.
Weirdly, the muscle fiber type (fast-twitch or slow-twitch) is determined by ____________.
the type of motor neuron that stimulates it; switching neuron types will induce the muscle fiber to switch
Just anterior to the lateral corticospinal tract in the spine is the _____________. Where does this come from?
Rubrospinal tract; midbrain
The stretch reflex is also called the __________.
A alpha sensory fibers connect to what subtype of muscle fibers?
Intrafusal – a type of muscle spindle that senses stretching; it is embedded within extrafusal muscle (the standard kind of muscle that responds to lower motor neuron stimulation
How many lower motor neurons do A alpha neurons innervate?
Many; they innervate multiple neurons that work together as well as inhibitory neurons that synapse on opposite muscles to relax opposite to direction of the reflex
What is the basis of the stretch reflex (that is, what is the reason we have it)?
It is a feedback mechanism for when things are heavier or lighter than we expect. For instance, if you expect something to be heavy, your extrafusal muscle fibers will contract a lot in anticipation; if it is lighter, then you will suddenly over contract and the intrafusal fiber will become slackened, producing a decrease in basal firing tone and causing the extrafusal muscles to stop contracting as much. The opposite also happens.
Golgi tendon organs lie between _____________.
muscles and tendons in a cartilaginous capsule
Overstretching muscles leads to inhibition via the _____________.
A beta neurons leading off the Golgi tendon organ
True or false: Golgi tendon neurons mediate the passive stretch reflex.
False! Golgi tendon organs balance active stretching, while the A alpha neurons from the intrafusal fibers mediate the passive reflex
Golgi tendon organs are not ___________.
contractile (they just sense stretching and inhibit excessive motor neuron stimulation)
Pain fibers synapse on ___________ in the spinal cord to mediate the pain reflex. The coupling is opposite, so if you step on a nail, then the injured leg will flex while the opposite leg will extend.
excitatory interneurons (a network of neurons that coordinates a retracting movement)
What is central pattern generation?
A process in which rhythmic motor actions occur in the absence of sensory or cerebral involvement
What is the evidence for central pattern generation being generated in the spinal cord?
Cats with thoracic spinal cord transections can still generate left-right walking with the assistance of a treadmill; further, severing the dorsal roots eliminates this ability, but adding L-DOPA regenerates the CPG!
What part of the brain controls the axial muscles?
What is channelrhodopsin?
An experimentally created sodium channel that opens in response to blue light; other channels exist for inhibitory channels.
Central pattern generation seems to begin in the _____________.
What tract mediates the unexpected fall reflex?
The vestibulospinal tract
The cortex is not set up to innervate just one muscle. How does this happen?
Upper motor neurons synapse on many lower motor neurons.
What research indicates that the motor cortex operates at a higher level (that is, not simple muscle activation)?
Stimulating the brains of macaque monkeys elicits certain complex actions that seem purpose-driven; for instance, stimulating one area caused the monkeys to bring their hands to their mouths regardless of where the hand started out.
What is the big problem in neural bypass technology?
The electrodes "die." A few months after implanting them, the electrodes stop working.
What is the basic purpose of gamma motor neurons?
They stimulate the intrafusal muscle spindles to contract when the extrafusal muscles contract, thus making sure that muscle spindles remain sensitive to stretch across a range of lengths.
Intrafusal spindles rely on ___________ neurons, and Golgi tendon organs rely on ___________.
I-a neurons also synapse on _______________.
inhibitory interneurons for the muscles that go counter to direction of the reflex (I-b neurons do the opposite)