MODULE 8- Lower Motor Neurons Circuits + Motor Control Flashcards
4 neural centers responsible for movement
-lower motor neurons
-upper motor neurons
-cerebellum
-basal ganglia
what are considered the “final common path” for initiating movement
lower motor neurons
basal ganglia
initiation of intended movement + suppression of unwanted movement
cerebellum
coordination of ongoing movement
motor cortex
planning, initiating, + directing voluntary movements
brainstem centers
stereotyped movements, postural control, + gain adjustments
local circuit neurons
sensorimotor integration + central pattern generation
motor neuron pools
all the motor neurons innervating a single muscle
lower motor neurons in the spinal cord + brainstem map what
the body’s musculature
each lower motor neuron innervates muscle fibers within a single/multiple muscles
a single muscle
somatotopic organization of motor neuron pools in ventral horn of spinal cord
-proximal muscles towards center of ventral horn
-distal muscles towards periphery of ventral horn
describe axons of medical ventral horn
extend several spinal cord segments + terminate bilaterally
axons of medial ventral horn are concerned with what
control of posture + locomotion
describe axons of lateral ventral horn
extend over fewer spinal cord segments + terminate only ipsilaterally
axons of lateral ventral horn are concerned with what
fine control of distal extremities
2 types of lower motor neurons found in the ventral horn
-alpha (α) motor neurons
-gamma (γ) motor neurons
alpha motor neurons
-innervate striated muscles
-make up part of the motor unit
gamma motor neurons
sensory receptors arranged in parallel with muscle spindles
the motor unit
-1 α-motor neuron +all the muscle fibers it innervates.
-1 α-motor neuron contacts a relatively wide area of muscle
-smallest unit of force that can be activated by the muscle
individual motor axons branch within muscles to do what
to contact fibers distributed over a wide area
-allows spread of activation for smoother force
-protects the muscle from becoming dysfunctional when 1 motor neuron dies out
where are motor neurons controlling muscles in the head, eyes + neck located
in the brainstem
innervation number
the number of muscle fibers innervated by 1 alpha motor neuron
-aka the number of fibers innervated by a motor unit
innervation differs across ____
muscle groups
gastroc force innervation number
1:2000
extraocular eye muscles innervation number
1:5
poliovirus
invades the motor neurons, leading to muscle weakness + impaired motor control due to fewer nerve signals reaching muscle fibers
polio- what happens to motor neurons that survive
develop new terminal axon sprouts that re-innervate orphaned muscle fibers
polio- total number of motor units increase/decrease
decrease
polio- muscle fibers that belong to a single motor unit increase/decrease
increase
what does polio result in
enlarged, less efficient motor units, which are easier to fatigue, further affecting fine motor control
how do alpha motor neurons vary
in size
alpha motor neuron size is proportional to what
the amount of muscle force that can be generated
slow (S) motor units
sustained muscular contraction
-such as for maintaining posture
fast fatigable (FF) motor units
generating large forces for short periods of time
-such as for jumping
-few mitochondria
fast fatigue resistant (FR) motor units
generate 2X force as slow ones
-resistant to fatigue
what motor units have few mitochondria
fast fatigable (FF) motor units
twitch contraction
muscle tension in response to a single AP
see slides 13-15
motor neuron innervation to striated muscle changes in response to ____
exercise
chronic electrical nerve stimulation transforms the metabolic + contractile properties of muscle from FF to ____ type
S type
size principle
the orderly systematic recruitment of motor neurons from S to FR to FF muscle fibers as the physical demands of the task increases
motor units are recruited from ____ to ____
weakest to strongest
as the rate of stimulation on muscle tension increases…
AP firing frequency increases ->
temporal summation ->
unfused tetanus ->
fused tetanus
increase in force occurs with increased ____
firing rate
as the amount of voluntary force increases, what increases
the number + firing rate of active motor units
the lowest firing rates of motor units during a voluntary movement are on the order of ____/second
8/second
firing rate rises to about ____ per second
20-25 per second
normal firing rates are much less than those required for ____
fused tetanus
local circuitry mediates reflexes that rapidly adjust muscle tension in response to sensory input
slide 23?
8-10 intrafusal fibers arranged in parallel
-dynamic nuclear bag fibers
-static nuclear bag fibers
-nuclear chain fibers
_____ coil around fibers
large-diameter afferents
Ia afferent innervates what
all 3 types of fibers
what does Ia afferent signal
velocity
II afferent innervates what
nuclear chain fibers
what do II afferent signal
length
activation of ____ pulls central region of intrafusal fiber
gamma motor neuron
activiation of gamma motor neuron contracts intrafusal fibers, making which afferents more sensitive
Ia
stretching muscle leads to what
increased discharge (APs) from Ia afferents
alpha motor neurons of agonist/antagonist increase discharge (APs) causing extrafusal muscle fibers to contract
agonist
-monosynaptic
reciprocal innervation
-excite agonist muscle that is stretched
-inhibits antagonist muscle
-monosynaptic
disynaptic pathway is mediated by what
reciprocal Ia inhibitory interneurons
what happens during extrafusal contraction
the spindle afferents are quiet
during extrafusal contraction, why are spindle afferents quiet
because the muscle spindle is not stretched – it is not loaded
during alpha + gamma coactivation, what happens
when extra + intrafusal fibers contract, there is no change in spindle afferent firing, as they are being stretched or loaded
stretch reflex operates as a feedback loop to regulate what
muscle length
gain of stretch reflex depends on the level of what
gamma motor neuron activity
high gain of stretch reflex
small stretch ->
large increase in alpha motor neuron activation
low gain of stretch reflex
larger stretch is required to produce same amount of tension in extrafusal fibers
gain of stretch reflex is high during what tasks
difficult tasks or uncertain conditions
-ex: walking across a narrow beam or standing on a moving bus
golgi tendon organ
encapsulated nerve endings at junction of tendon + muscle
what axons innervate GTOs
Ib axons
how are GTOs arranged with extrafusal fibers
in series
GTOs sense changes in ____ of the muscle
tension
GTOs are sensitive/insensitive to passive stretch
insensitive
GTOs work by positive/negative feedback system
negative
decreased/increased GTO leads to APs of the Ib neuron
increasd
which horn do GTOs enter
dorsal horn
GTOs excite/inhibit alpha motor neuron of agonist muscle
inhibit
GTOs excite/inhibit alpha motor neuron of antagonist muscle
excite
GTOs prevent what
excessive tension
GTOs maintain what during muscle fatigue
steady tone
2 functions of GTOs
-prevent excessive tension
-maintain steady tone during muscle fatigue
under passive stretch or tension, what happens to muscle spindle activity
increases + is maintained
under passive stretch or tension, what happens to GTO activity
relatively unaffected
when load is increased…
the increased force is transmitted to the GTO ->
increase in firing rate of its Ib afferents
flexion reflex
-cutaneous (nociceptor) stimulated
-excites interneuron circuits in the spinal cord
flexion reflex causes inhibition of flexor/extensor of stimulated foot
extensor
flexion reflex causes excitation of flexor/extensor of stimulated foot
flexor
flexion reflex is ___ in the other limb
opposite
-crossed extension reflex
what does flexion reflex lead to
withdrawal of the limb from painful stimuli
central pattern generator
local circuits in the spinal cord for generating rhythmic stereotypical patterns of movement, such as in walking, running, etc.
central pattern generator extra info
-alternating between stance phase and swing phase
-flexors + extensors activated reciprocally
-increased speed means less time between each cycle
-module receives input from excitatory rhythm-generating interneurons, which are reciprocally inhibited by interneurons
what alternates during stepping
contractions of flexor + extensor muscles
after recovering from surgery (transection of spinal cord), are the cats hindlimbs able to walk on treadmill
yes
-reciprocal bursts of electrical activity can be recorded from flexors during the swing phase and from extensors during the stance phase of walking
the basic rhythmicity of stepping is produced by what
neuronal circuits contained entirely within the spinal cord
-separate CPG for each leg
are supraspinal commands necessary for producing the basic motor pattern for stepping
NO
what initiates + modulates CPG activity
mesencephalic locomotor region
do CPG networks require sensory input
NO
what are CPG networks strongly regulated by
input from limb proprioceptors (e.g., speed changes)
lower motor neuron syndrome
caused by damage to lower motor neurons
-depending on the extent of damage, can lead to paralysis, weakness, or loss of tone
-muscles may exhibit fasciculations and fibrillations – spontaneous twitches characteristic of degenerating motor units + fibers, respectively
how is lower motor neuron syndrome clinically diagnosed
EMG
amyotrophic lateral sclerosis (ALS)/Lou Gehrig’s disease
degeneration of lower motor neurons +/or upper motor neurons
