Kenyon1-Motor Neurons Flashcards
What is a lower motor neuron? Where are their cell bodies found?
these are neurons that innervate muscle directly. axons end in neuromuscular jcns w/ skeletal muscle fibers.
cell bodies are found in brainstem or spinal cord
**they just tell the muscle what to do & it has no choice.
What NTs & receptors are involved in the neuromuscular jcns of lower motor neurons?
Ach is the NT
Nicotinic Receptors
Where are the cell bodies of upper motor neurons found?
brainstem or cerebral cortex
What do upper motor neurons synapse with?
upper motor neurons OR
lower motor neurons OR
local circuit neurons
OR a combination
Is it excitatory or inhibitory?
Lower motor neurons:
Upper motor neurons:
Lower motor neurons: excitatory
Upper motor neurons: excitatory or inhibitory or both
If you see a neuron in the brainstem synapsing with a lower motor neuron–>it could be 1 of 2 things. What are your options?
Upper Motor Neuron
OR Local Circuit Neuron
Upper motor neurons descend from the motor cortex or the brainstem. What is the motor cortex involved in? Brainstem?
Motor cortex: planning, initiating, & directing voluntary movements.
Brainstem: basic movement & postural control
The UMN descend & communicate with which 2 things? which is a more major pathway? Is this excitatory or inhibitory?
Local Circuit Neurons (major pathway)
Lower Motor Neurons (minor pathway)
Can be excitatory or inhibitory
What do the local circuit neurons do? Is this excitatory or inhibitory?
they synapse with the LMN
can be excitatory or inhibitory
What do the LMN do? Is this excitatory or inhibitory?
they synapse @ the neuromuscular jcn w/ the skeletal muscle
this is excitatory always.
If the LMN are inhibited, no excitation of the skeletal muscle.
Sensory inputs go to which things in the general pathway?
to the local circuit neurons & LMN
so essentially to both types of motor neurons…
also go to the cerebellum
T/F The local circuit neurons & LMN are located far apart.
False. they are close together.
What do the basal ganglia contribute to the descending systems?
they gate proper initiation of movement
What does the cerebellum contribute to the descending systems?
sensory motor coordination of ongoing movements
So…we know that the LMN control the movements of the skeletal muscles. But where are they located really?
When they are found in the spinal cord: they are in the ventral horn!
When they are found in the brainstem: found in somatic or branchial motor nuclei
The LMN that control the skeletal muscles of the body are found where?
in the ventral horn of the spinal cord
The LMN that control the skeletal muscles of the head are found where?
in the somatic & branchial motor nuclei of the brainstem
Which lower motor neurons are found in the somatic motor nucleus in the brainstem?
Oculomotor
Trochlear
Abducens
Hypoglossal
Which lower motor neurons are found in the branchial motor nucleus in the brainstem?
Trigeminal
Facial
Ambiguus
Spinal Accessory Nucleus
Once again, aside from the brainstem…where do you find the upper motor neurons?
In the cortex
Primary Motor Cortex & Premotor Cortex
Cingulate Gyrus
What is the function of the primary motor cortex & premotor cortex in terms of UMN?
planning, initiating, and directing series of movements in limbs & eyes
Where would you find the UMN for a lower limb in the premotor cortex or primary motor cortex?
you find this in the medial portion of the cortex.
Where would you find the UMN for an upper limb in the premotor cortex or primary motor cortex?
you would find this in the lateral portion of the cortex.
The fcn of the cingulate gyrus with respect to UMN is what?
expression of emotions especially with facial muscles
Where exactly are upper motor neurons found in the brainstem? What is their corporate function?
Vestibular Nuclei Reticular Formation Superior Colliculus **regulate muscle tone **orient eyes, head, body with respect to incoming info.
From the brainstem, what direction do the UMN go?
they project medially & inferiorly to the cervical spinal cord
Which is located more laterally in the brainstem…the vestibular nuclei or the reticular formation?
the vestibular nuclei
Cerebellar neurons influence the activity of which types of neurons? What is the function of the cerebellum?
they influence the activity of UMN
**they compare actual movement via proprioceptive input to intended movement & command corrections.
They basically modulate ongoing movements
The basal ganglia are responsible for what situation in Parkinson’s? In Huntington’s?
Parkinson’s hypokinetic–not enough wanted movements.
Huntington’s hyperkinetic–too many unwanted movements.
What is the function of the basal ganglia?
initiation of movements
suppression of unwanted movements
Which structures make up the basal ganglia? Where is it found?
found in the forebrain made of forebrain nuclei including: caudate putamen globus pallidus substantia nigra subthalamic nucleus
What do local circuit neurons basically do?
they basically receive input from UMN & sensory neurons & excite or inhibit lower motor neurons.
Describe the relative position in the spinal cord of LMN that are controlling distal muscles v. proximal muscles.
Distal Muscles–Located Laterally in Spinal Cord
Proximal Muscles–Located Medially in Spinal Cord
Compare & contrast local circuit neurons that control posture v. those that control limbs.
Posture: They are located medially in the spinal cord. They have long projections & commissural axons. They control a larger set of muscles.
Limbs: They are located laterally in the spinal cord. They have short projections. They control a limited set of muscles.
Where are the axons of UMN that control limbs found in the spinal cord? Do they cross over pyramids on their journey?
they are found laterally near the local circuit neurons they project to…
they are found in the lateral white matter–>here there is the lateral corticospinal tract
**they do cross over pyramids on their journey
Where are the axons of UMN that control postured located in the spinal cord? Do they cross over pyramids on their journey?
they are found in medial white matter (closer to the local circuits that they communicate with)
Here in the medial white matter you can find: Reticulospinal, Vestibulospinal, & a little Ventral Corticospinal Tracts
**don’t cross over the pyramids on their journey.
What is a motor unit?
the motor neuron & its muscle fibers
LMN innervates 3-3000 muscle fibers
Where are the LMN for a given muscle found?
clustered in the ventral horns in one or more spinal segments
T/F LMN are found in organized arrangements in the muscles they control.
False. They are found scattered throughout the muscles they control.
Which of the following is a ratio of LMN to muscle fibers for an eye muscle & which is for the gastrocnemius?
1: 2000
1: 3
1: 3 eye muscle–fine control
1: 2000–gastrocnemius
T/F The motor neuron type is matched perfectly with the type of muscle fibers it innervates.
True. Different motor neurons used for fast fibers v. slow fibers.
What are the 3 flavors of motor units?
Slow motor units
Fast Fatigable Motor units
Fast Fatigue-Resistant Motor units
When you see red muscle–think which type of motor unit? When you see pale muscle?
Red Muscle–think slow motor units
Pale Muscle–think fast fatigable motor units
So…if you were making a human body & deciding which motor units to put in the posture muscles…which would be your first choice? 2nd? 3rd?
1st Choice: Slow fibers (can be continuously active w/o tiring)
2nd Choice: Fast Fatigue Resistant Fibers
3rd Choice: Fast Fatigable Fibers (best for large forces of short bursts)
Describe how the various motor units perform in terms of force generated & time at 100% of maximum force.
Slow Fibers: Not much force, best at staying at 100% for a long time!
Fast Fatigue-Resistant Fibers: Medium force, medium capacity to stay at 100% for a small period
Fast Fatigable Fibers: Greatest force, lowest capacity to stay at 100%
As the synaptic activity driving muscle contraction increases…say going from standing to walking to running to jumping…what happens to fiber recruitment?
Start at slow, move next to fast fatigue-resistant & finally fast fatigable.
What are the 3 feedback systems in the spinal cord that help control muscle length & tension?
Muscle Spindles
Golgi Tendon Organs
Flexion Reflex–helps you to move away from danger in periphery
What do muscle spindles help you to control? Which reflex are they involved in?
help you to control muscle length
deep tendon reflex aka stretch reflex aka myotatic reflex
**muscle tendon receives a blow & the muscle stretches.
What do golgi tendon organs help you to control?
help you to control muscle tension
You start off with sensory info from the muscle spindle…how do you eventually get a muscle twitch?
muscle spindle sensory–>spinal cord–>muscle twitch
Which type of motor neuron innervates the muscle spindles?
gamma motor neurons (as opposed to alpha motor neurons)
they innervate 8-10 specialized intrafusal muscle fibers in proprioception
Which type of motor neuron innervates the body of the muscle itself?
the alpha motor neurons
what is the muscle spindle wrapped in?
fast Ia primary afferent neurons
So…the Ia afferent sensory neurons from the muscle spindle give proprioceptive info…they come back towards the spinal cord & can go 2 ways according to the TVP pathway. Explain this.
Get back to the spinal cord & will go up to the cuneate & gracilis nuclei so that we have knowledge of where our muscle is…
Also involved in reflex @ the spinal cord level.
Activates an alpha neuron that acts on that muscle.
Activates a motor neuron to a synergist muscle.
Activates local circuit neuron to inhibit antagonist muscles.
What will stretching a muscle do to 1a afferent activity?
It will increase it.
What would happen if you activate the alpha motor neuron to a muscle w/o activating the gamma motor neuron to its muscle spindle?
this would be bad.
1a afferent input would shut down as the muscle contracted & shortened
bad b/c cerebellum needs to receive proprioception!
What happens when the alpha motor neuron & the gamma motor neurons are activated together?
this causes the muscle to contract/shorten & the spindle to shorten. Never lose 1a afferent info.
good b/c control center needs to know what’s going on!
What determines the activity of gamma motor neurons? Hint: same thing that determines activity of lower motor neurons.
Upper Motor Neurons
When would you have low & when would you have high muscle spindle sensitivity & gamma activity?
Low Muscle Spindle Sensitivity/Low Gamma Activity:
low activity levels, sitting etc.
High Muscle Spindle Sensitivity/High Gamma Activity:
standing on a moving bus or skiing (need to make sure that the muscles stay the appropriate length)
T/F YOu should keep a pretty constant output of info from the 1a afferents. And if you don’t–you know there is a problem.
True.
then you would know there is too much stretch of the muscle & i must adjust something–let’s synapse with some alpha neurons.
What is a Golgi Tendon Organ?
this is found in b/w a tendon & a muscle
it is encapsulated afferent nerve endings of 1b fibers.
they are activated by tension from muscle contraction
T/F GTO are equally activated by muscle tension created by muscle contraction & by passive stretch.
False. Weak response to passive stretch. STrong to muscle contraction.
What will muscle contraction due to afferent activity of muscle spindles & GTO?
Muscle Spindles: afferent activity will be decreased
GTO: afferent activity will be increased
Describe the afferent pathway from the GTO.
afferent 1b fibers from GTO travel to the dorsal horn of the spinal cord & DON’T synapse.
Parts of it ascend via the TVP pathway to the cortex.
Other parts participate in a protective reflex.
It synapses with a local circuit interneuron to inhibit the homonymous muscle & activate the antagonist muscle. This will cause a person to drop whatever they are holding if they weight becomes so heavy that it affects the GTO.
**there are also descending pathways from UMN that affect this reflex-not closed loop.
What is the flexion reflex pathway? What is another name for this?
aka somatosomatic reflex pathway
**step on a nail and you hop off right away as a reflex!
Pain–>DRG–>Dorsal horn–>synapses–>crosses to go to the cortex as the anterolateral tract.
Interneurons in the spinal cord also activate the flexor on the nail side & the extensor on the other side. Interneurons inhibit the extensor on the nail side & the flexor on the other side.
Is the flexion reflex pathway the same as the tomato-somatic reflex?
NO
What’s the deal with the cat on the treadmill?
the cat’s spinal cord was sectioned, and yet it is still able to walk on the treadmill as a reflex & its extensors & flexors alternate properly.
don’t know how it works–lesion to dorsal horn & it still works.
shows how complex reflexes can be.
What’s the deal with lower motor neuron syndromes?
you can lose LMNs at the spinal cord & periphery (probably in the brainstem too)
get paralysis of the muscle it innervated
paresis (weakness)
areflexia
loss of tone
atrophy
single muscle fiber twitches (fibrillations, fasciculations)
