Control of movement Flashcards
4 divisions in the control of motor movement:
Spinal cord and brainstem circuits (lower motor neurons)
Descending Systems (upper motor neurons)
Basal Ganglia
Cerebellum
Lower Motor Neurons
-location
- Lower Motor Neurons live in spinal cord or brainstem that transmit the motor message directly to the skeletal muscle (synapse is known as the neuromuscular junction)
- The lower motor neuron serves as the final common pathway for transmitting to and controlling the muscles
α motor neurons
Lower Motor Neurons
Local circuit neurons
-receive info from
-receive sensory information from sensory neurons and information from Upper motor neurons (cortex, brainstem)
Organization of Lower Motor Neurons
- topographically arranged i think.. actually i dont think so?
- Cell bodies arranged in an orderly manner in the ventral horn of the spinal cord
*Muscles closer to your midline are controlled by motor neurons that have cell bodies closer to your midline
More distal muscles- more laterally placed cell bodies
Medial: posture/balance, Distal: doing things
*Also arranged in a top-bottom fashion (muscles near the top of the spinal cord are controlled by motor neurons closer to the top of the spinal cord…)
Each lower motor neuron innervates muscle fibers of the same muscle
All motor neurons that innervate a single muscle (motor neuron pool) are grouped together in the spinal cord
Form a rod-shaped cluster that runs parallel to the long axis of the spinal cord for 1 or more segments
*Areas of the spinal cord that are responsible for controlling more or more complex musculature have more neurons in them (CERVICAL ENLARGEMENT and
LUMBAR ENLARGEMENT)
*Medial motor neurons vs Lateral motor neurons
Each lower motor neuron innervates _______
Each lower motor neuron innervates muscle fibers of the same muscle
Areas of the spinal cord that are responsible for controlling more or more complex musculature have ____ neurons in them (2 examples)
Areas of the spinal cord that are responsible for controlling more or more complex musculature have more neurons in them
Cervical enlargement- arm and hand control
Lumbar enlargement- leg and foot control
Cervical enlargement-
arm and hand control
Lumbar enlargement-
leg and foot control
Medial motor neurons vs lateral motor neurons
-where receive their input:
*Medial motor neurons receive their input from upper motor neurons in the brainstem vestibular nuclei and reticular formation
Pathways run medial down the ventral spinal cord
-Posture and balance (The ones in the middle are concerned about posture and balance)
*Lateral motor neurons receive their input from upper motor neurons in the cerebral cortex
Pathways run lateral down the ventral spinal cord
Controlling further away muscles: more deliberate actions, moving fingers I think
Local Circuits
- local circuit neurons
- Mostly interneurons
- Lie primarily in the intermediate zone of the spinal cord
- Supply much of the direct input to the lower motor neurons
- motor neurons have different patterns of connection with their local circuits (since motor neurons in the medial and lateral portions of the ventral horn care about different functions)
- Medial local circuit neurons vs Lateral local circuit neurons
distance of local circuit units
medial vs lateral
- both running in the ventral horn (the gray matter of the spinal cord)
- local circuits in the lateral portion of the ventral horn: short distance local circuit neurons
- local circuits in the medial portion of the ventral horn: long-distance local circuit unit
Muscle Spindles
- specialized muscle fibers that do not contribute to the work
- Also known as intrafusal fibers
- Embedded within connective tissue capsules in the muscle
- Innervated by sensory axons to relay information about the length of the muscle
stretch receptors that detect changes in the length of the muscle (how much it is stretched)
Involved in the sensation of position and movement of the body (proprioception)
Stretch and Speed of Stretch
Motor Unit
*The relationship between the α motor neuron and the muscle fibers it innervates
Motor units and α motor neurons vary in size
Small α motor neurons innervate fewer muscle fibers and make small motor units
- In the adult, each α motor neuron innervates multiple muscle fibers, but each muscle fiber is only innervated by a single α motor neuron
- Why?
*There are more muscle fibers than α motor neurons
So, activation of one α motor neuron leads to contraction of all of the muscle fibers it innervates
Motor Units Differ in the types of muscle fibers that are innervated: name them
Differ in the types of muscle fibers that are innervated
Slow (S) motor units
Fast fatigable (FF) motor units
Fast fatigue-resistant (FR) motor units
Slow (S) motor units (size, control___ muscle fibers, force, fatigue, importance)
smaller motor units controlling “red” muscle fibers that contract slowly, generate small forces, and resistant to fatigue
Important for activities that require sustained contraction
(FF) motor units (size, control, force, fatigue, importance/when used)
Fast fatigable
- use larger α motor neurons, control larger “pale” muscles, generate more force, and are easily fatigued
- Important for brief exertions that require a lot of force (running, jumping)
(FR) motor units- (size, control, force, fatigue, importance)
- Fast fatigue-resistant
- somewhere between the Fast fatigable (FF) and Slow (S) motor units; intermediate size, intermediate speed, generate 2x the force of slow motor units, are resistant to fatigue
What About Fast and Slow Twitch?
*Fast and Slow Twitch are basically the same thing, but the term mostly is referring to the muscle
Slow twitch (Type I) basically same as slow motor unit Fast twitch (Type IIa) basically same as fast fatigue-resistant motor unit Fast twitch (Type IIb) basically same as fast fatigable motor unit
Motor Units and Contribution to Activity (standing vs walking vs jumpting)
(slide 25)
Standing- minimal force required, but must be sustained; mostly S motor units
Walking- more force required; recruit FR motor units
Jumping- requires full force of muscle; recruit FF motor units
How Do Rabbits Jump a Lot Then?
The occurrence of muscle fibers depends on exertion, innervation, and the type of innervation.
Inside a muscle there may appear different type of muscle fibers—for instance, closer to the bones, muscles are more reddish than close to the surface.
Muscle that is exposed to constant activity (such as the muscle of a wild rabbit or thighs of chickens) is reddish, and composed of slow-twitch muscle fibers
*Muscles which are not exposed to constant exertion (muscle of a domesticated rabbit, and chicken breast) are lighter in color and are composed of fast-twitch muscle fibers.
how is our discussion of muscles really an Oversimplification
Muscles posses a broader spectrum of fiber phenotypes
Vary in speed of contraction, tension generation, oxidative capacity, and endurance
These variations have corresponding variations in α motor neurons
Motor Unit Plasticity
- def
- examples
*Repeated use or stimulation can change the physiological properties of the motor unit
*Example: chronic electrical stimulation of the of the nerve can change FF fibers to perform more like S fibers
α motor neuron had increased excitability and longer after-hyperpolarizations
*Exercise, and the coincident tensions exerted on the fibers and the duration of the activity, can also lead to adaptations in the motor units
–Changes in physiological properties of the α motor neurons and changes to the muscle fibers
Frequency of Action Potentials
- generated by :
- contributes to:
*Frequency of APs generated by α motor neurons contributes to regulation of muscle tension
*Increased force occurs following increased frequency
Muscle fibers are activated by the next AP before they can relax after the first one –> sustained contraction
Asynchronous firing of different α motor neurons provides steady level of input to muscle
Muscle Stretch Reflexes (the players and their roles)
*Group Ia- respond phasically to small stretches
Activity dominated by signals from dynamic nuclear bag fibers (velocity of stretch)
*Group II- signal level of sustained fiber stretch; fire tonically with frequency proportional to degree of stretch
Activity dominated by static nuclear bag fibers and nuclear chain fibers
*Sensory axons form excitatory monosynaptic connections on the α motor neurons
Also synapse on inhibitory local circuit interneurons that innervate antagonistic muscle groups
Reciprocal innervation!!!
No brain input required for the muscle stretch reflex!!!!!
A simple reflex arc: stretch in muscle spindle
A simple reflex arc:
Sensory response to stretch in muscle spindle –> excitatory feedback to the muscle that is being stretched
**Group Ia afferents- large diameter axons; coiled around middle of intrafusal fibers
**Group II afferents- also large; mostly contact nuclear chain fibers
Mediate fast reflex adjustments in response to stretch
Muscle Stretch Reflex (add weight–>)
-also seen in…
Add weight –> increase stretch –> sensory neuron lets motor neuron know –> proportionally increase contraction in the muscle doing the work and increase relaxation in counter muscle group
Also seen in knee-jerk reflex
-Tap on tendon creates the sensory signal and it causes the motor response
Contribution of γ motor neurons
Control the functional characteristics of the muscle spindles by modulating their level of excitability
- Muscle stretched –> spindle is stretched –> increase in axons firing
- Muscle is relaxed –> less tension –> muscle spindle not stretched –> SENSORY AXONS DO NOT GO SILENT
*The γ motor neurons synapse on the contractile poles of the intrafusal fibers –> can cause intrafusal fiber contraction
Contraction picked up by sensory afferents –> maintain some activity
*Co-activation of α and γ motor neurons allows for muscle spindles to function at all muscle lengths
Spinal Cord Circuitry and Regulation of Force
*Group Ia and II: collectively provide information about stretch
*Golgi tendon organs-: provide information about tension
-Muscle contraction –> increases tension on tendon
If it reaches threshold, it send signals through Group Ib axons (!!!!)
Axons synapse on inhibitory interneurons –> inhibit α motor neuron controlling same muscle
Woah, that’s too much tension. Stop it.
Axons also synapse on excitatory interneurons that pass the message to antagonistic muscles
Step on a tack
*Team Work
*Sensory input –> sensory axon directly firing on α motor neurons of flexors and inhibitory interneurons of extensors to mediate withdrawal of the leg from the tack
*On the other side of the spinal cord, local circuit neurons excite the α motor neurons of extensors and inhibitory interneurons of the flexors make those relax, allowing the other leg to compensate
And behind the scenes, everything else coming together to make sure you do not over-extend or over-contract or tip over
more complex motor
movements (examples)
-example of complex motor movemnts: walking, swimming
*Central pattern generators are contained in local circuits of the spinal cord
Control the timing and coordination of more complex, yet regular, patterns of movement (walking, swimming)
Limbs need to have opposing actions and work in time with each other
Flex one limb to bring it off the ground while you extend the other limb towards the ground
-Timing really matters
Central pattern generators
general: location, control
- are contained in local circuits of the spinal cord
- Control the timing and coordination of more complex, yet regular, patterns of movement (walking, swimming)
Central Pattern Generators
- region
- circuit (dont have to know details)
Initiation of locomotion requires the contribution of some higher brain centers, such as the mesencephalic motor region
However, the act of coordination between the limbs occurs in the spinal cord
Transection of the spinal cord at the thoracic level isolates the hindlimb segments of the cord.
Hindlimbs are still able to walk on a treadmill, and reciprocal bursts of electrical activity can be recorded from flexors during the swing phase and from extensors during the stance phase of walking.
Circuit for central pattern generation of locomotion.
Neuronal modules for flexion and extension antagonism (dashed boxes) comprise excitatory neurons and reciprocally connected Ia inhibitory interneurons (rIa-INs).
These modules receive input from excitatory rhythm-generating interneurons (E/R), which are reciprocally inhibited by different classes of interneurons
Damage to the lower motor neurons => (2)
- Lower Motor Neuron Syndrome
* Amyotrophic Lateral Sclerosis (ALS)
Amyotrophic Lateral Sclerosis (ALS)
- type of disease, def
- how starts
- treatment
- why does it happen?
as result of damage to the lower motor neurons
- Neurogenerative disease
- Slow, but progressive, loss of α motor neurons in the spinal cord and brainstem, as well as upper motor neurons in the cortex
- Starts with progressive weakness
- Muscles waste
- Currently no effective treatment
- Why? Genetics, reactive oxygen species, pro-inflammatory interactions between neurons and microglia, mitrochondrial dysfunction…
- Leading current hypothesis is hyperexcitability in cortical networks which may lead to glutamate excitotoxicity
- Lower Motor Neuron Syndrome
- paralysis?
- muscle tone?
- twitch?
- atrophy?
*as result of damage to the lower motor neurons
*Results in paralysis of the affected muscles
Loss of reflexes because sensorimotor reflex arcs are disrupted
Loss of muscle tone (muscle spindles no longer linked to a lower motor neuron)
Muscles may spontaneously twitch due to changes in excitability or due to pathological activity of the motor fiber
Can lead to muscular atrophy from disuse
Upper Motor Neurons
Cell bodies are in “higher” centers (like the cerebral cortex and brainstem)
Axons descent to influence lower motor neurons and their circuits in the brainstem and spinal cord
- cerebral cortex upper motor neurons influencing the lower motor neurons of the lateral ventral horn
- brainstem upper motor neurons influencing the lower motor neurons of the medial ventral horn
Medial ventral horn**
*contains lower motor neurons that govern posture, balance, locomotion, and orienting movements of the head and neck during shifts of visual gaze.
Receive descending input from pathways that originate mainly in the BRAINSTEM, course through the anterior-medial white matter of the spinal cord, and then terminate bilaterally.
-brainstem upper motor neurons influencing the lower motor neurons of the medial ventral horn
Lateral ventral horn**
- controls:
- receives info from:
- contains lower motor neurons that mediate the expression of skilled voluntary movements of the distal extremities.
- Receive a major descending projection from the contralateral motor cortex via the main (lateral) division of the corticospinal tract, which runs in the lateral white matter of the spinal cord.
-cerebral cortex upper motor neurons influencing the lower motor neurons of the lateral ventral horn
Upper motor neurons of the cortex
- reside where?
- receive info from?
- Upper motor neurons reside in several interconnected areas of the posterior frontal lobe
- Receive regulatory input from the basal ganglia and cerebellum, by way of the ventrolateral thalamus, as well as the somatosensory cortex
-upper motor neurons are found in layer 5 of the primary motor cortex
Other adjacent areas of cortex are PREMOTOR AREAS
Primary motor cortex: general
- large and direct pathway to the lower motor neurons and their circuits
Low intensity of stimulation needed to evoke movement (low threshold)
Other adjacent areas of cortex are PREMOTOR AREAS
Primary Motor Cortex: upper motor neurons
- layer?
- cell type(s)?
- Upper motor neurons reside in LAYER 5
* Betz cells and SMALLER, non-Betz pyramidal neurons
The motor tracts, in general, can be functionally divided into two major groups:
Pyramidal tracts:
Extrapyramidal tracts:
Pyramidal tracts:
- originate in :
- pass through:
one of the two major motor groups (divided by function)
These tracts originate in the cerebral cortex, carrying motor fibers to the spinal cord and brain stem.
- They are responsible for the VOLUNTARY control of the musculature of the body and face.
- Named after themedullary pyramids of the medulla oblongata, which they pass through.
Extrapyramidal tracts:
one of the two major motor groups (divided by function)
These tracts originate in the brain stem, carrying motor fibers to the spinal cord.
They are responsible for the involuntary and automatic control of all musculature, such as muscle tone, balance, posture and locomotion
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Pyramidal Tracts: Functionally, these tracts can be subdivided into two
Corticospinal tracts– supplies the musculature of the body.
Corticobulbar tracts– supplies the musculature of the head and neck.
Descending Tracts (2)
*the corticobulbar (to brainstem) or corticospinal tracts
Corticobulbar Tract
-explain the projections
Many of the descending fibers that want to govern the actions of the cranial nerve motor nuclei send bilateral projections onto local circuit neurons in the reticular formation and the cells of the reticular formation send their axons out to make contact with the lower motor neurons in these nuclei
Coordinate action
Why bilateral projections? Built in back-up system. Damage to fibers on one side will not result in major functional deficits.
*Note: Your textbook says most of the descending fibers synapse in the reticular formation and not the motor nuclei of the cranial nerves. I cannot fully support this statement
*Not all innervation to the cranial nerve motor nuclei or local circuit neurons in the reticular formation is symmetrical and bilateral
In these exceptions, there is a bias to favor inputs from the CONTRALATERAL MOTOR CORTEX