L41 Muscle Innervation and Motor Unit Flashcards
where are the cell bodies of all muscle afferents?
DRG
what are the two major sensory organs?
muscle spindle
golgi tendon organ
(also mechanoreceptors)
what does the muscle spindle measure and what type of fiber does it use?
measures length and rate of change of length
Ia fiber
what does the golgi tendon organ measure and what type of fiber does it use?
measure tension
Ib fiber
what make up the additional mechanoreceptors?
thermo and nociceptors (temperature and pain)
what is a muscle spindle
encapsulated structures found in most skeletal muscles used to monitor its length
the number of spindles in the muscle is directly related to what?
the muscles function
muscles involved in fine movement (hand) have ___ spindles than coarse movement (back)
more
what are the components of the muscle spindle?
- small group of 2-12 intrafusal gamma fibers
- central regions are non contractile
- large diameter myelinated sensory axon (Ia axon) with sensory ending in the intrafusal belly
- small diameter myelinated motor axon that innervate the distal contractile regions (gamma motor ending)
where do the intrafusal fibers lie?
within the spindle which is between/parallel to the extrafusal fibers and are attached to the muscles tendons
when the muscle contracts, it shortens and ____ I a firing rate
decreases
when the muscle stretches, Ia firing rate _____
increases
what are the 3 types of intrafusal fibers?
dynamic nuclear bag fibers
static nuclear bag fibers
nuclear chain fibers
what are dynamic nuclear bag fibers sensitive to?
length change and rate of change
what are static nuclear bag fibers sensitive to?
length change
what are nuclear chain fibers sensitive to?
length change
what are the primary sensory ending and what does it wrap around
group Ia
each intrafusal nuclear bag and chain fiber
static and dynamic changes
what does Group II sensory ending wrap around?
all nuclear chain and static nuclear bag fiber
only static changes
describe A-Gamma fibers responsible for
small myelinated and low velocity axons carry motor innervation to the intrafusal fibers
what are the A-Alpha fibers responsible for?
motor innervation of the extrafusal fibers
during a sustained stretch reflex, as the spindle is continuously stretched, there is a _____ AP firing in the muscle spindle afferent
sustained
what happens to the muscle spindle during alpha motor stimulation?
the muscle shortens and briefly alleviates the stretch on the spindle, at this time, there is no stimulation of the Ia axon = NO afferent discharge!
what happens to the muscle spindle during simultaneous alpha and gamma stimulation?
the intrafusal and extrafusal fibers will contract together
because the intrafusal spindle consensually contracts, it continues to relay afferent information thus able to continually measure muscle length
where are golgi tendon organs located?
in series between tendon and extrafusal fibers which are sensitive to tension
what are each golgi tendon organ innervated by?
single Ib axon which is large myelinated and has high velocity
once the Ib axon enters the tendon organ and intertwine in the collagen chains what happens?
the Ib axon will lose its myelin
what happens to the Ib axon firing rate when the contraction of the muscle compresses the free, non-myelinated endings?
increase firing rate
what happens to the alpha motor neuron when you increase Ib axon firing rates?
increase inhibition of alpha motor neurons - ensures even distribution of tension and contraction as well as avoiding excessive contraction to the point of damage
what is the reverse myotactic reflex?
increasing Ib axon firing increases inhibition of alpha motor neurons - ensures even distribution of tension and contraction as well as avoiding excessive contraction to the point of damage
what are the 2 types of motor neurons?
A-alpha
A-gamma
describe A-alpha motor neurons?
large, myelinated, high velocity passing via ventral horn to limb muscle extrafusal fibers
describe A-gamma motor neurons
small, myelinated, lower velocity passing via ventral horns to limb muscle intrafusal fibers
where are the alpha and gamma motor neurons located in the spinal cord?
ventral horn
what are renshaw cells?
inhibitory interneurons of the anterior horn that receive collaterals from the alpha motor neuron and work to avoid damage in the event of excessive contraction (like golgi)
what makes up a motor unit?
motor neuron and the muscle it activates
what is a motor nuclei?
100 motor neurons (average) to control a typical muscle
they congreate to create a nuclei in the anterior horn of the spinal cord as well as in the brainstem for motor CNs
a motor axon branches at its muscle and each fiber receives ____ ending of the branching
one
where is the NMJ located
in the center of the muscle fiber where APs are generated
the number of muscle fibers in a motor unit ___ with increasing fine motor control
decreases
what are the 3 types of motor units?
Type I
Type IIA
Type IIB
describe Type IA motor units
slow twitch muscle fibers, low tension, fatigue resistant, aerobic, innervated by small motor neuron and axon
describe Type IIA motor units
fast fatigue resistant, large tension, some aerobic capacity, innervated by large motor neuron and axon
describe Type IIB motor units
fast fatigable, large tension, anaerobic, innverated by large motor neuron and axon
the force developed by the muscle fibers increase with _____ firing rate of the motor neuron
increasing
force will also increase as ____ motor units are recruited to the excited state
more
describe hypotonia?
reduced muscle tone (atrophy) due to damage to either Ia afferent or the alpha motor neuron
describe LMN syndrome
causes by destruction of the motor neuron in the anterior horn, the axons in the ventral roots or the peripheral nerves
can cause
- atrophy
- loss of voluntary and reflex responses
- hyporeflexia
- fasciculations and fibrillations
what does conduction velocity depend on
- myelination
2. axon diameter - the larger the axon diameter, the faster the conduction velocity
what factors control contraction
- firing rates of motor neurons
2. number of motor units recruited
