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LMNs
Lower Motor Neuron lecture
208
Anatomy
Graduate
04/03/2012

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Term
Are lower motor neurons GVE, GVA, GSE, or GVA?
Definition
GSE
Term
what do motor systems consist of
Definition
all the muscles and neurons that control movements
Term
what kinds of neurons control movements
Definition
UMNs, LMNs, interneurons, parts of the brain that deal with motor
Term
is the brain required for all complex motor patterns
Definition
no
Term
what are central pattern generators
Definition
circuitry within the spinal cord and brainstem for coordinated control of rhythmic movements
Term
what are some examples of rhythmic movements that central pattern generators control
Definition
walking, running, swimming
Term
what controls motor programs
Definition
brain, but they're contained in the spinal cord
Term
what are motor programs
Definition
circuitry that gives us the ability to do alternating/rhythmic movements
Term
how are motor programs accessed, executed, and modulated?
Definition
by descending commands from the brain
Term
what are the 2 basic parts of motor control
Definition
1. spinal cord's command and control of coordinated rhythmic muscle contractions; 2. the brain's command and control of motor programs in the spinal cord
Term
where do UMNs originate and where do they go?
Definition
UMNs always originate AND remain in the CNS
Term
where do LMNs originate and where do they go
Definition
LMNs originate in the CNS and project OUT to skeletal muscles
Term
what is a LMN
Definition
somatic motor nerve that leaves the spinal cord/brainstem and goes out to innervate skeletal muscles
Term
where are the cell bodies of most LMNs
Definition
ventral horn of the spinal cord
Term
what is the path of an LMN axon?
Definition
axon goes out ventral root into spinal nerve, then through dorsal or ventral ramus depending on which muscle it will innervate
Term
what do LMNs innervate when they go through the ventral rami
Definition
anterior/lateral trunk, arms, legs
Term
what do LMNs innervate when they go through the dorsal rami
Definition
intrinsic back muscles
Term
what kind of nerve enters the spinal cord through the dorsal root?
Definition
sensory
Term
what synapses on LMNs in the spinal cord
Definition
UMNs
Term
other than ventral horn of the spinal cord, what is another location where cell bodies of LMNs can be located
Definition
cranial nerve nuclei within the brainstem
Term
what initiates skeletal muscle contraction?
Definition
LMNs
Term
what modulates the activity of LMNs
Definition
UMN descending pathways
Term
aside from UMNs, what else can LMNs receive input from
Definition
sensory neurons, interneurons
Term
what is the final common pathway for transmitting neural information to the muscles
Definition
LMNs
Term
what 3 things can synapse on an LMN
Definition
UMN, interneuron, sensory neuron
Term
what is the 3 neuron pathway for motor control
Definition
UMN - interneuron - LMN
Term
what are the 4 neural centers that are responsible for movement
Definition
1. spinal cord and brainstem circuits; 2. descending modulatory systems or pathways; 3. cerebellum; 4. basal ganglia
Term
to be able to contract skeletal muscles, which of the 4 neural centers have to be working properly?
Definition
all 4
Term
do the 4 neural centers responsible for movement interact with each other?
Definition
yes, they are highly interactive systems providing essential and unique contributions to motor control
Term
what does the neural center of the local circuitry within spinal cord/brainstem involve (which 2 types of neurons)?
Definition
LMNs, interneurons
Term
what are local circuit neurons?
Definition
interneurons
Term
what is the major source of synaptic input to the LMNs
Definition
local circuit neurons/interneurons
Term
from what sources do interneurons receive information/
Definition
local circuit neurons receive SENSORY INPUTS and DESCENDING PROJECTIONS (UMNs) from higher centers
Term
what conveys all commandes for movement, both voluntary and involuntary, to muscles?
Definition
LMNs
Term
what does a retrograde tracer do if we inject it into a muscle.
Definition
Retrograde tracers will follow the axon backwards. It will go up the motor nerve into the spinal cord. You can see where the cell bodies are of all the motor nerves that innervate that muscle.
Term
Where are the cell bodies of most LMNs, according to information from retrograde tracers injected into muscle?
Definition
ventral horn of the spinal cord
Term
what is the motor neuron pool
Definition
grouping together of all the motor neurons innervating a single muscle into a rod-shaped cluster that runs parallel to the long axis of the spinal cord
Term
where are the cell bodies of LMN that innervate quads
Definition
L2-4
Term
where are the cell bodies for LMNs that innervate thenaor muscles?
Definition
C8 T1
Term
where are the cell bodies for LMNs that innervate biceps?
Definition
C56
Term
does every muscle have its own motor neuron pool?
Definition
yes
Term
where is the motor neuron pool for the quads?
Definition
L2-4
Term
where is the motor neuron pool for thenar muscles?
Definition
C8 T1
Term
where is the motor neuron pool for biceps
Definition
C56
Term
in what dimension of the spinal cord are motor neuron pools arranged?
Definition
mediolateral
Term
what is the distribution of the location of muscle group motor neuron pools in the spinal cord?
Definition
proximal muscle are medial/distal muscles are lateral; extensors are anterior/flexors are posterior
Term
what is the function of axial muscles?
Definition
trunk movement (erector spinae, obliques are axial muscles)
Term
where in the spinal cord are axial muscle motor neuron pools located?
Definition
the most medially
Term
where in the spinal cord are proximal or girdle muscle motor neuron pools located?
Definition
medially in the ventral horn
Term
where in the spinal cord are distal muscle motor neuron pools located?
Definition
laterally in the ventral horn
Term
describe the relationship between motor neurons and muscles
Definition
orderly
Term
how is the orderliness of the relationship between motor neurons and muscles evident
Definition
evident along the length of the cord as well as the mediolateral dimension: it provides a spatial map of the body's musculature
Term
what are the 2 types of LMNs found in the motor neuron pools
Definition
small gamma motor neurons, alpha motor neurons
Term
what do gamma motor neurons innervate
Definition
intrafusal fibers of muscle spindles
Term
besides gamma motor neurons, what else innervates intrafusal fibers of muscle spindles
Definition
sensory neurons
Term
what do gamma motor neurons do to intrafusal fibers of muscle spindles
Definition
function to regulate the length and tension of the intrafusal fibers
Term
what do alpha motor neurons innervate
Definition
extrafusal fibers
Term
what are extrafusal fibers
Definition
the striated skeletal muscle fibers that contract and generate forces needed for movement
Term
what cranial nerves have LMNs?
Definition
the ones that innervate skeletal muscles
Term
define "motor unit"
Definition
the collection of skeletal muscle fibers innervated by a single motor neuron
Term
what does a single motor neuron synapse with
Definition
multiple muscle fibers
Term
are motor units all the same size?
Definition
no
Term
are alpha motor neurons all the same size?
Definition
no
Term
describe small motor neurons/motor units
Definition
small alpha motor neurons innervate a small number of muscle fibers and form small motor units that generate small forces
Term
describe large motor neurons/motor units
Definition
large alpha motor neurons innervate a large number of muscle fibers and form large motor units that generate large, powerful forces
Term
what do small motor units innervate?
Definition
small red fibers that contract slowly and generate small forces
Term
why are the small fibers innervated by small motor units red
Definition
rich vascular supply and a lot of mitochondria
Term
what makes the small red fibers innervated by small motor units resistant to fatigue
Definition
they have lots of mitochondria
Term
what do we call the small motor units?
Definition
slow motor units (S)
Term
for what kind of activities are slow motor units important
Definition
sustained activities that require us to not fatigue, such as maintenance of upright posture
Term
what are some muscles with lots of slow motor unitis
Definition
obliques, hamstrings
Term
what do larger motor units innervate
Definition
larger, pale fibers that generate a lot of force
Term
why are the large fibers innervated by large motor units pale?
Definition
not very much mitochondria
Term
what easily happens to large pale fibers innervated by large motor units
Definition
easily fatigued
Term
what do we call the large motor units
Definition
fast fatigable motor units (FF)
Term
what are fast fatigable motor units important for
Definition
brief exertions like running, jumping
Term
how big are fast fatigue-resistant motor units (small, intermediate, or large)
Definition
intermediate
Term
how much force do fast fatigue-resistant motor units produce compared to force produced by slow motor units
Definition
twice as much as slow motor units
Term
how resistant to fatigue are fast fatigue-resistant motor units?
Definition
somewhat
Term
what are the 3 different types of motor units?
Definition
Slow, Fast Fatigue Resistant, Fast Fatigable
Term
which type of muscle unit can produce the most force
Definition
fast fatigable motor units (FF)
Term
which type of muscle unit produces the least force
Definition
Slow
Term
why does an alpha motor neuron to a small motor unit generate only a small amount of force?
Definition
it doesn't innervate a lot of muscle fibers
Term
how may muscle fibers are innervated when an alpha motor neuron innervates a fast fatigable motor unit
Definition
1000-3000 muscle fibers
Term
which motor units are able to sustain the same force for the longest time
Definition
slow
Term
why do we need different types of motor units
Definition
allows the nervous system to produce movements appropriate for different circumstances
Term
what is types of movements is the soleus muscle important for?
Definition
posture and "strolling"
Term
what type of motor units are there a majority of in the soleus muscle?
Definition
slow
Term
on average, about how many muscle fibers/motor neuron exist in the soleus
Definition
180 muscle fibers per motor neuron
Term
on average, about how many muscle fibers/motor neuron exist in the gastroc
Definition
1000-2000 muscle fibers per motor neuron
Term
what is the result of having so many muscle fibres per motor neuron in the gastrocnemius
Definition
the gastroc can produce sudden bursts of force in jumpng
Term
what is it that changes/regulates the force produced by a muscle
Definition
increasing or decreasing the number of motor units that are active at any one time
Term
what is the order of recruitment of motor units to increase muscle tension
Definition
S units are activated 1st, then FR units, then FF units
Term
what is the term for the orderly recruitment of motor units based on their size (small to large) in order to increase muscle tension?
Definition
size principle
Term
what motor units are activated for standing
Definition
slow motor units (S)
Term
what motor units are activated for walking/running
Definition
fast fatigue resistant motor units (FR)
Term
what motor units are required for strenuous activities such as jumping?
Definition
fast fatigable motor units (FF)
Term
besides recruitment of motor units in a fixed order according to their size, what else contributes to the regulatoin of muscle tension
Definition
frequency of action potentials
Term
what is the result of an increase in firing rate of a motor neuron?
Definition
increase in force of contraction
Term
what is the result of low frequency stimulation/action potentials
Definition
a single twitch
Term
what happens when you have higher frequencies of action potentials?
Definition
the twitches "sum" to produce a force greater than that of single twitches
Term
what do we call the summing of twitches to produce a greater force
Definition
temporal summation
Term
if frequncies of action potentials are even higher than that required for temporal summation, what is the result?
Definition
the force produced is greater than before, but twitches are still evident
Term
what do we call it when there is a greater force than with temporal summation but twitches are still evident?
Definition
unfused tetanus
Term
what happens at the highest rates of motor neuron activation from high frequency action potentials?
Definition
individual twitches are no longer present
Term
what do we call it when the individual muscle twitches are no longer present?
Definition
fused tetanus
Term
define temporal summation
Definition
the increase in force that occurs with increased firing rate that reflects the summation of successive muscle contractions
Term
what is happening to the muscle fibers in temporal summation
Definition
the muscle fibers are activated by the next action potential before they have had time to relax from the previous action potential; forces generated by the overlapping contractions are summed
Term
what is size principle
Definition
orderly recruitment of larger motor units increases muscle tension
Term
what is temporal summation
Definition
increase in motor neuron frequency of action potentials increases muscle tension
Term
go to slide 47 and watch the video
Definition
ok
Term
what are muscle spindles
Definition
sensory receptors found in skeletal muscles
Term
what is the structure of muscle spindles
Definition
4-8 specialized intrafusal fibers surrounded by a capsule
Term
what is the nuclear bag fiber
Definition
in the muscle spindle, largest intrafusal fiber.
Term
how are the nuclei in nuclear bag fibres
Definition
in the center
Term
what are nuclear chain fibers
Definition
intrafusal fibers of muscle spindles
Term
how are the nuclei arranged in nuclear chain fibers
Definition
lined up in a single file
Term
what are group Ia axons
Definition
sensory axons that encircle and innervate both types of intrafusal fibers to encode the rate of stretch
Term
are Ia axons large
Definition
yes
Term
are Ia axons myelinated
Definition
yes
Term
do Ia axons have fast conduction velocities
Definition
yes
Term
what are group II axon
Definition
axons that innervate nuclear chain fibers and give off minor branches to nuclear bag fibers to encode the degree of stretch
Term
what motor neurons innervate and contract intrafusal fibers
Definition
specialized gamma motor neurons
Term
what is the function of muscle spindles
Definition
provide information to the CNS about muscle length
Term
where does the sensory signal for the stretch reflex originate
Definition
muscle spindles
Term
what happens to the intrafusal muscle fibers when the muscle is stretched?
Definition
they are lengthened/stretched too
Term
what is the result of stretching intrafusal muscle fibers
Definition
the stretching initiates an action potential in the 1a afferent axons coiled around the spindle.
Term
what do centrally projecting branches of 1a afferent axons form with homonymous alpha motor neurons?
Definition
monosynaptic excitatory connections
Term
what does homonymous mean
Definition
the 2 nerves innervate the same muscle
Term
describe the excitatory stretch-reflex circuitry that occurs when beer is poured into a glass you are holding
Definition
As beer is poured into the mug, we get passive stretch of the bicep. 1a afferent axons send action potentials to the spinal cord saying that the bicep has been stretched. The 1a afferent axon makes monosynaptic excitatory connections with the alpha motor neuron to the bicep. THe alpha motor neuron causes contraction of the bicep so that you don't spill your beer.
Term
besides the monosynaptic excitatory connection with the homonymous alpha motor neuron, to what else does the 1a afferent neuron innervate centrally?
Definition
through an interneuron, the central projection of the 1a sensory neuron has inhibitory connections with alpha motor neurons of heteronymous muscles
Term
what is a heteronymous muscle?
Definition
antagonistic muscle
Term
what is reciprocal innervation?
Definition
the 1a afferent neuron's inhibitory connections with alpha motor neurons of heteronymous muscles
Term
what is the result of reciprical innervation
Definition
rapid and efficient responses in muscle length and tension
Term
what gives us muscle tone
Definition
the continual occurrence of reciprocal innervation activity
Term
describe the inhibitory stretch-reflex circuitry that occurs as beer is poured into a glass that you are holding
Definition
as beer is poured into the mug, we get passive stretch of the bicep. 1a afferent axons send action potentials to the spinal cord saying that the bicep has been stretched. The 1a afferent axon makes inhibitory connections via an interneuron with alpha motor neurons of the triceps
Term
the stretch reflex arc is an example of what?
Definition
feedback loop
Term
what is the purpose of the stretch reflex arc
Definition
maintain the muscle length at a desired value (ie muscle tone)
Term
what do gamma motor neurons terminate on?
Definition
contractile poles of intrafusal fibers
Term
activation of what neurons cause intrafusal fiber contraction
Definition
gamma motor neurons
Term
what is the purpose of contracting the intrafusal fiber when the muscle contracts?
Definition
maintain tension on the equatorial region where sensory axons terminate
Term
what would happen to the muscle spindle if you contract your muscle but you don't have a gamma motor neuron?
Definition
the muscle spindle wouldn't contract. It would be long and limp inside the muscle. there would also be a void of afferent signals going back to the CNS because the muscle spindle wasn't changing and reporting change to the CNS
Term
when both alpha and gamma motor neurons are activated to shorten both the muscle and the muscle spindle, what is the effect on 1a afferents?
Definition
they send action potentials to the CNS to say that the muscle/muscle spindle has shortened
Term
what is the function/purpose of gamma motor neurons
Definition
gamma motor neurons regulate the tension of muscle spindles so they can operate efficiently at any length of the muscle
Term
what are 2 other ways to refer to gamma activity
Definition
gamma bias, gain
Term
what sets the tension/gain in the muscle spindle?
Definition
gamma moto neurons
Term
what controls the gamma activity?
Definition
upper motor neurons
Term
how does gamma activity relate to the gain
Definition
they are the exact same thing
Term
how does gain/gamma activity from a stretch reflex relate to change in muscle force
Definition
the larger the gain, the greater the change in muscle force
Term
what is the result of a stretch to the muscle when the gain of a reflex is high?
Definition
a small stretch applied to the intrafusal fibers produces a large increase in alpha motor neurons recruited and a large increase in tension produced by extrafusal fibers
Term
what is the result of a stretch to the muscle when the gain of a reflex is low?
Definition
a greater stretch is required to generate the same amount of tension in extrafusal fibers
Term
co-activation of what 2 neurons allows muscle spindles to function properly at all lengths and movements?
Definition
alpha and gamma motor neurons
Term
what must happen to the gain of the stretch reflex in order to meet different functional requirements?
Definition
must be continuously adjusted
Term
when does the CNS set the gain high?
Definition
when you're worried about balance or watchign a scary movie
Term
what is the golgi tendon organ
Definition
another sensory receptor that is important in the reflex regulation of motor activity
Term
where are golgi tendon organs
Definition
distributed among collagen fibers in tendons
Term
what innervates golgi tendon organs
Definition
Group 1b afferents
Term
what information does group 1b convey to the CNS
Definition
info about muscle tension
Term
what detects change in muscle length?
Definition
muscle spindles
Term
when does muscle action act on tendons?
Definition
when muscles actively contract
Term
what happens to the GTO when muscles actively contract?
Definition
the contraction force acts on the tendon, leading to tension on the collagen fibril in the GTO and compression of the intertwined sensory axon
Term
how does firing rate from the GTO change when the muscle contracts?
Definition
increases
Term
why is it important to monitor the tension of a muscle?
Definition
to protect the muscle's integrity (keep us from generating exceptionally large forces)
Term
what would happen if we didn't have GTO's
Definition
we would tear muscle fibers
Term
what do 1b afferent axons from GTO's connect to centrally?
Definition
1b inhibitory interneurons in the spinal cord
Term
what do 1b inhibitory interneurons do?
Definition
decrease the activity of alpha motor neurons that innervate the same muscle
Term
what does the arrangement of 1b afferent axons innervating 1b interneurons to decrease alpha motor neurons to the same muscle do?
Definition
prevents muscles from generating excessive force and regulates muscle tension within an optimal range
Term
do 1b inhibitory interneurons receive any synapses from descending pathways?
Definition
yes
Term
what is the flexion reflex pathway
Definition
an important reflex that causes withdrawal of limb from a painful stimulus
Term
what are the steps of the flexion reflex pathway
Definition
A delta fibers bring pain info into the dorsal horn, where they excite ipsilateral flexor muscles via interneurons and they also inhibit ipsilateral extensor muscles via interneurons
Term
when you have the flexion reflex pathway, what else happens?
Definition
there is an opposite reaction of flexion reflex that occurs on contralateral limb causing contralateral extensor muscles to be excited and contralateral flexor muscles to be inhibited
Term
what is the purpose of the crossed extension reflex?
Definition
serves to enhance postural support during withdrawal of affected limb from stimulus
Term
what do central pattern generators allow us to do
Definition
rhythmic movements
Term
what are interneurons capable of besides reflex responses to sensory inputs?
Definition
interneurons are capable of controlling the timing and coordination of rhythmic movememnts
Term
what are the 2 phases of walking?
Definition
stance phase, swing phase
Term
what happens when you increase walking speed?
Definition
you reduce the time it takes to complete a cycle by shortening the stance phase
Term
in dogs and cats, what accompanies change in locomotin speed?
Definition
changes in the sequence of limb movements
Term
what is the progression of leg movements in dogs at low speeds
Definition
LH, LF, RH, RF: back to front, first one side then the other
Term
at high speeds, what is the leg movements in dogs?
Definition
2 front legs are synchronized, 2 rear legs are synchronized
Term
is locomotion in animals fully controlled by higher centers?
Definition
no
Term
if you transect a cat's spinal cord in the thoracic region, what happens if you put the cat on a treadmill?
Definition
hindlimbs still make coordinated locomotor movements.
Term
what determines the speed of the transected cat's locomotor movements
Definition
speed of the treadmill
Term
is the locomotion of transected cats on a treadmill a reflex response to the stretching of the limb muscles?
Definition
no. if you destroy the hindlimb's DRG in these animals, appropriate locomotor movements are still evident
Term
what is it that tells the transected cat how to move?
Definition
some kind of Central Pattern Generator inside the spinal cord that tells it how to do the rhythmic movements
Term
are basic rhythmic patterns in locomotion dependent on sensory input?
Definition
no
Term
are basic rhythmic patterns in locomotion dependent on descending input from higher centers?
Definition
no
Term
what is a central pattern generator
Definition
an oscillatory spinal cord local circuit responsible for alternating flexion and extension in locomotion. Each limb has its own CPG
Term
can humans depend on CPG alone to control rhythmic movements?
Definition
no. we require UMNs to control rhythmic movements
Term
define lower motor neuron syndromes
Definition
collection of symptoms that arise following damage to LMN cell bodies or their peripheral axons
Term
how does LMN syndrome affect muscles
Definition
paralysis, paresis, atrophy, fibrillations, or fasciculations of affected muscles
Term
what is paresis
Definition
weakness
Term
what is areflexia
Definition
loss of reflexes
Term
how does LMN syndrome affect reflexes and why
Definition
causes loss of reflexes due to loss of the efferent limb of the reflex arc
Term
how does LMN syndrome affect muscle tone and why
Definition
loss of muscle tone because tone is dependent on the reflex arc
Term
what are fibrillations and fasciculations
Definition
spontaneous muscle twitches which arise from altered excitability of muscle fibers and damaged alpha neurons
Term
what determines whether you get paralysis or paresis in LMN syndrome?
Definition
if you completely cut the nerve, you get paralysis. If you just damage it, you'll get paresis
Term
what do fasciculations look like?
Definition
a little wave in the muscle that you can see through the skin
Term
name 3 LMN syndromes?
Definition
polio, trauma, ALS
Term
what is polio
Definition
viral destruction of neurons in the ventral horn of the spinal cord
Term
what type of trauma causes LMN syndrome?
Definition
damage/laceration of a peripheral nerve
Term
what does ALS stand for
Definition
amyotrophic lateral sclerosis
Term
what's another name for ALS
Definition
Lou Gehrigs Disease
Term
what is ALS
Definition
neurodegenerative disease
Term
how common is ALS
Definition
affects .05% of the population
Term
what happens in ALS
Definition
slow degeneration of the alpha motor neurons in ventral horn and brainstem and eventually neurons in the cortex
Term
does ALS start as a UMN or LMN disease
Definition
LMN
Term
can ALS affect UMNs
Definition
yes, some UMNs can be destroyed too
Term
what is the progression of ALS
Definition
weakness and wasting of skeletal muscles with sensory and cognitive functions intact. Death within 5 years
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