Unit 3_Motor Systems and Motor Units Flashcards
How does a single neuron work and how do neurons talk to eachother?
Sensory systems
Sensorimotor integration
How does information get into the nervous system and what do we perceive based on this information?
Sensory systems
How do we respond to sensory inputs, use sensory inputs for planning movements and how do we execute movements?
Sensorimotor integration
What does purposeful reaching for an object require?
The nervous system to interact with and/or control the physical machine- our musculoskeletal system - and the musculoskeletal system’s interaction with a varying environment
What is it called when the nervous system can solve problems in many ways and this flexibility is normal?
Motor Equivalence
Loss of flexibility is what?
Dysfunctional
What mechanisms help us prepare for movement and respond to perturbations of movement, respectively?
Feed-forward and feedback control
What axons of alpha motor neurons innervate skeletal muscle (extrafusal muscle fibers) and form part of motor units? These axons form the motor part of peripheral nerves (from spinal cord and cranial nerves).
Alpha Motor Neurons (lower motor neuron; LMN’s)
What are located in the ventral horn of every spinal cord segment and motor nuclei for cranial nerves in the brainstem?
Cell bodies
(note: Gamma motor neurons for the spindle also have cell bodies in ventral horn)
What axons travel in spinal and cranial nerves that have a motor component and innervate skeletal muscle?
Alpha motor neuron (LMN)
What are known as each spinal cord segment that has its own set of alpha motor neurons (LMN) whose axons form the ventral root of the spinal nerve at that segment only?
Myotomes
What type of organization is found in the spinal cord where alpha motor neuron cell bodies are located?
Topographic organization
What are organized in a way to most easily access the motor neurons they impact?
Descending pathways
What is a motor neuron (motoneuron) [including its cell body (soma), dendrites and axon] and all of the (extrafusal) muscle fibers it innervates?
Motor unit
There are usually _____ motor units within a named muscle.
many
There are _________ types of motor units that allow us to do things for a long time and to generate large forces for short times.
different
What are the three types of motor units that allow us to do things for a long time and to generate large forces for short times?
Slow, fast fatigue-resistant, fast fatigable
Most muscles are a mixture of what?
slow and fast twitch muscle fibers
Motor units from muscles that need to generate a lot of force tend to have ______ muscle fibers in the motor unit (e.g., MG) while those used for fine control have _______ muscle fibers in the motor unit (e.g., lateral rectus and tensor tympani).
more
fewer
Are there distinct types of motor units?
It is more of a continuum. Not all motor units fall into a specific force to twitch ratio.
What governs recruitment of motor units into action? Small motor units with small alpha motor neurons are recruited before larger motor units with larger motor neurons.
The size principle
What motor units are recruited first?
Small motor units with small alpha motor neurons are recruited before larger motor units with larger motor neurons
Force can be increased by what two mechanisms that are like sensory mechanisms?
Population coding (recruiting more motor units)
Frequency coding (rate coding; each motor unit firing faster)
What mechanism increases force by recruiting more motor units?
Population coding
What mechanism increases force by rate coding; each motor unit firing faster?
Frequency coding
What are Common Signs and Symptoms of Alpha Motor Neuron and Motor Unit Neuropathology?
(“Lower Motor Neuron”) Disorders
Weakness (paresis)
Hypotonia
Hyporeflexia
Muscle atrophy
Fasciculations and fibrillations
What are the following disorders?
Weakness (paresis)
Hypotonia (floppy/lax)
Hyporeflexia (decreased/absent reflex responses)
Muscle atrophy
Fasciculations (small muscle twitches that you can see) and fibrillations (small muscle twitches that you necessarily can’t see)
“Lower Motor Neuron” Disorder
The distribution of problems, when one has a lesion causing “LMN” signs and symptoms, is always what (i.e. only in that segment or nerve)? For example, a lesion in the ventral horn of C6 will give you “lower motor neuron” signs in those muscles supplied by C6 only. NOT in all muscles below the lesion like when a pathway is cut.
Segmental and ipsilateral
Diseases of the motor unit do what? They may also affect the smoothness of contraction and demonstrate plasticity within the neuromuscular system.
change the force output
What is caused by a viral infection that affects the alpha motor neurons in the ventral horn of the spinal cord and cranial nerve motor nuclei in the brainstem?
It is an alpha motor neuron (lower motor neuron) disorder that affects muscles in the body and well as muscled supplied by cranial nerves.
The affected muscles exhibit lower motor neuron signs and symptoms (weakness (paresis), hypotonia, hyporeflexia, muscle atrophy, fasciculations and fibrillations).
The viral infection is self-limiting and the destruction of cells occurs within the first few weeks of infection.
Polio
What syndrome do patients usually recover up to a certain extent and is left with residual deficits?
This is a syndrome that occurs many years after a person first experiences polio.
Individuals who have been stable and functioning with their residual deficits for years begin to experience new weakness and muscle fatigue.
These new symptoms are likely due to a combination of overuse and aging.
This condition is usually not life threatening but it often requires individuals to make significant lifestyle changes to reduce fatigue and conserve remaining muscle strength.
Post-polio syndrome
What disease destroys alpha motor neurons in the spinal cord and brainstem and is also classified as a lower motor neuron disorder?
However, unlike polio this disease is progressive and usually fatal within an average of 5 years.
It is also multisegmental with destruction of the ventral horn alpha motor neurons in many spinal segments as well as cranial nerve motor nuclei.
Unlike polio, in the later stages of this disease it may also affect the lateral white matter columns of the spinal cord and the patient may present with UMN signs such as spasticity.
ALS (Amyotrophic Lateral Sclerosis; Lou Gehrig’s)
What disease may be caused by spinal cord injuries, spinal cord tumors, and damage caused by inflammation in and around the spinal cord? In some cases, the cause is unknown (idiopathic).
Syringomyelia
What can sensory input evoke?
reflex activation of muscle
What helps coordinate postural control?
Long propriospinal pathway
What helps coordinate limb movement?
Short propriospinal pathway
What is responsible for much of movement?
The spinal cord
Much of what you see as movement is the result of patterns of motor unit activity generated by what?
Reflexes give us a hint of the patterns that can be generated.
Reflexes are more flexible than previously thought and they are not rigidly fixed. That said many neurologic diseases/injuries result in loss of flexibility.
The spinal cord
What are the following primary functions for?
1. To adjust for unexpected perturbations
2. To organize patterns for coordinated movement (examples are flexor withdrawal and crossed extension; reciprocal inhibition)
3. To allow for rapid protection from painful or damaging stimuli (not a major function)
Reflexive motor control at the level of the spinal cord
You do not need what to obtain a reflex contraction?
Any lesions in the sensory cortex or ascending sensory white matter tracts that affect our ability to detect a sensation will not abolish a reflex, but it will not have its normal flexibility.
perception of sensation
What can influence or regulate reflexes by increasing or decreasing the size of the reflex response but you do not need descending motor pathways to have a reflex present?
You don’t need descending motor or ascending sensory white matter pathways for a reflex to be present.
Descending control from the cerebral cortex and brainstem
What may be used to obtain a motor response in a patient?
Stimulation of a spinal reflex pathway
What occurs over a very short time frame and usually does not last much longer after the removal of the stimulus? The only way to lose or abolish a reflex is to remove the sensation that stimulates it or denervate the muscles that contract.
The motor response
What will not abolish reflexes? They may make the spinal reflex presentation abnormal and less flexible.
Descending pathway (UMN) lesions
What may be used to initiate movements for rehabilitation? They may be of particular relevance in the acute phase of rehabilitation to stimulate muscle activity and give the patient a proprioceptive sense of movement.
Reflexes
What may be used to inhibit spastic muscles?
Certain reflexes
What help us prepare for movement and respond to perturbations of movement, respectively?
Feedback and feed-forward control (mechanisms)
What are 3 ways reflexes can help you adjust?
- Can adjust for unexpected perturbations
- To organize patterns for coordinated movement (i.e. reciprocal inhibition)
- To allow for rapid protection from painful or damaging stimuli. The stretch reflex acts to protect muscles from over stretching.
What do the following relate to in regards to reflexes?
If an object is placed in your hand is unexpectedly heavy, you will not exert enough force to hold the weight and your biceps may be stretched. This triggers a reflexive contraction of biceps to support the weight of the object correctly.
If you are standing and you are displaced posteriorly, the muscles on the anterior aspect of your lower leg (anterior tibialis) will be stretched. They will contract in response to your stretch and dorsiflex the ankle to assist in bringing you back to midline. However, longer latency responses are more involved in controlling large perturbations in humans.
Can adjust for unexpected perturbations
What do the following relate to in regards to reflexes?
Many movement patterns require coordination of agonist and antagonist so that as the agonist is excited the antagonist is inhibited. Descending pathways use the components of the stretch reflex loop to organize these activation patterns for reciprocal inhibition.
To organize patterns for coordinated movement (i.e. reciprocal inhibition)
What does the following relate to in regards to reflexes?
The GTO regulates the force of a contraction and is regulated by descending pathways.
- For example, if you are holding a videotape in your hand and it starts to slip, the reflex can be inhibited to allow you to develop more force to hold the tape. That said cutaneous input has been shown to be the key to prevent slip.
To organize patterns for coordinated movement
What type of reflex is a painful stimulus stimulates nociceptors, the cutaneous afferent nerve fibers (flexor reflex afferents or FRA’s) transmit the signal to the spinal cord and synapse on alpha motor neurons.
The reflex is an example of an intersegmental reflex as many muscles are coordinated to activate at once to withdraw the limb.
Clinically, this reflex may be used to stimulate muscle contraction in the flexors. For example, it may be used in comatose patients for arousal and to initiate a motor response.
In normal life these reflexes may utilize similar circuitry as locomotion (walking).
Flexor Withdrawal Reflex and Crossed Extension
What reflex do the following relate to?
1. Can adjust for unexpected perturbations
2. To organize patterns for coordinated movement
3. To allow for rapid protection from painful or damaging stimuli
Flexor withdraw and Crossed Extension
What flexor withdraw/crossed extension reflex support the following: Response to pain or nociceptive stimulus?
Can adjust for unexpected perturbations
What flexor withdraw/crossed extension reflex support the following:
- The flexor withdrawal reflex is an example of an intersegmental reflex as many muscles are coordinated to activate at once to withdraw the limb.
- Clinically, this reflex may be used to stimulate muscle contraction in the flexors. For example, it may be used in comatose patients for arousal and to initiate a motor response.
- In normal life the flexor withdrawal and crossed extension may utilize similar circuitry as locomotion (walking)
To organize patterns for coordinated movement
What flexor withdraw/crossed extension reflex support the following: Flex away from the stimulus but also maintain body support and positioning.
To allow for rapid protection from painful or damaging stimuli
What type of reflex involves fast cutaneous sensory stimulation such as fast “icing”, brushing, or stroking to the skin will cause the muscle underneath the stimulated area to contract?
In this case, the cutaneous afferent excites the homonymous muscle. Therefore, one would want to avoid using these types of stimuli over a spastic muscle.
However, these techniques would be appropriate for a hypotonic muscle.
Other forms of cutaneous stimulation, such as maintained heat, cause the muscle under the skin that is heated to relax or be inhibited. This might be a good technique to apply to the skin over a spastic muscle.
Cutaneous Surface Stimulation
