Motor Neurons Flashcards
The anticipatory use of sensory information to prepare for movement is referred to as what?
feedforward
Give an example of feedforward
In patients who have ACL ruptures they increase hamstring activity before joint loafing in order to prevent anterior tibial translation during acceptance of loading
The use of sensory information during or after movement to make corrections to the ongoing movement or to future movements is referred to as what?
feedback
What types of things continually give feedback?
proprioceptors, skin, vision, hearing, and vestibular receptors
Describe symptoms of patients with complete loss of somatosensation below the neck
Joint movements are not well synchronized and fast movements are decomposed such that only one joint is moved at a time, in an attempt to simplify control by eliminating interaction torques
What does the act of reaching depend on in the absence of vision?
Somatosensation and proprioception to locate objects which requires more time and is less accurate
What does loss of somatosensation in individuals with complete deafferentation disrupt?
positioning of limbs
Smooth, accurate movement requires information from what 3 systems?
- visual
- somatosensory
- gravitational
Describe the pathway through the motor system from the time a decision is made to the point in which the muscle contracts
1) a decision is made in the anterior part of the frontal lobe
2) motor planning areas are activated
3) control circuits are activated to regulate the activity in upper motor neuron tracts
4) UMN tracts deliver signals to spinal interneurons and LMNs
5) LMNs transmit signals directly to skeletal muscles which elicit the contraction of muscle fibers that move the upper limbs and fingers
Where are the control circuits of the motor system located?
In the cerebellum and the basal ganglia
What is the basal ganglia involved in?
gating proper initiation of movement
What is the cerebellum involved in?
sensory motor coordination of ongoing movement
What are the 3 classifications of UMN tracts?
- postural/gross movement tracts
- fine movement tracts
- nonspecific tracts
What do postural/gross movement tracts control?
autonomic skeletal muscle activity
What do fine movement tracts control?
fractioned movements (independent contractions of single muscles) of the limbs and face
What do nonspecific tracts control?
all motor neurons
Control circuits adjust the activity of the ___ motor neurons
upper
In summary _____ motor neurons have their cell bodies in the spinal cord or brainstem and synapse with skeletal muscle fibers. Whereas _____ motor neurons arise in the cerebral cortex or brainstem and synapse with LMNs or interneurons in the brainstem or spinal cord.
lower
upper
What are the 4 properties of skeletal muscle?
- excitable
- contractile
- extensible
- elastic
Describe the structure of an individual muscle fiber
Myofibrils are arranged in parallel to the long axis of the muscle fiber. These myofibrils consist of proteins arranged in sarcomeres.
What is the functional unit of muscle?
sarcomere
What are the proteins that provide structure to the sarcomere?
- Z line
- M line
- titin
What is the Z line?
The fibrous structure at each end of the sarcomere
What is the M line?
It is the structure that anchors the fibers in the center of the sarcomere
What are the 3 functions of titin?
- connects the Z line with the M line
- maintains the position of myosin relative to actin
- prevents the sarcomere from being pulled apart
When is muscle contraction produced?
when actin slides relative to myosin
What initiates muscle contraction?
When calcium binds to troponin, which causes a conformational change in the troponin which induces movement of the tropomyosin to uncover active sites on actin. This allows myosin cross-bridges to bind on the actin binding sites
What does the resistance to muscle stretch depend on?
Muscle length, for example a stretched spring generates more resistance to stretch than the same spring when it is shortened
What 3 things determine the total resistance to muscle stretch?
- active contraction from descending motor commands and proprioceptive information
- titin
- weak actin-myosin bonds
What is muscle tone?
the resistance to stretch in a resting muscle
Clinically how can you assess muscle tone?
Passively rotate the limb through it’s range of motion
When muscle tone is normal, resistance to passive stretch is _____.
minimal
Normal resting muscle tone is provided by what 2 things?
- titin
- weak actin-myosin bonds
When are weak actin-myosin bonds formed?
When myosin attaches to actin but the myosin heads do not swivel, so there is no power stroke. Thus no muscle contraction occurs.
Are fast or slow stretches more resistant to stretch? Explain why…
Fast stretches because cross-bridges do not have the opportunity to detach, making the muscle more resistant to stretch.
This is apparent in a muscle that has been immobilized for a period of time in which many weak actin-myosin bonds have formed.
What happens to sarcomeres when a healthy innervated muscle is continuously immobilized in a shortened position for a prolonged period?
They begin to disappear from the ends of myofibrils (contracture)
Why do sarcomeres disappear when a healthy muscle is immbolized in a shortened position?
It is a structural adaptation that allows the muscle to generate optimal force at the new (shorter) resting length
What happens to sarcomeres when a muscle is immobilized in a lengthened position for a prolonged period?
The muscle will add new sarcomeres
How can a joint’s resistance to movement be increased?
cocontraction
What does concontraction refer to?
The simultaneous contraction of antagonist muscles
essentially what does cocontraction do?
stabilizes joints
What does cocontraction in the UEs do?
enables precise movements
What does cocontraction in the LEs do?
allows an individual to stand on unstable surfaces
___ motor neurons are the only neurons that convey signals to extrafusal and intrafusal skeletal muscle fibers.
lower
What are the 2 types of LMNs?
- alpha
- gamma
Where are the cell bodies of all LMNs found?
in the ventral horn of the spinal cord
Describe the path LMN axons take once they leave the spinal cord
They leave the spinal cord via the ventral root, travel through the spinal nerve, and then travel through the peripheral nerve to reach skeletal muscle
Alpha motor neurons have ____-sized myelinated axons
large
Alpha motor neurons project to ___fusal skeletal mucle
extrafusal
What type of NT do LMNs release?
ACh
Gamma motor neurons have ____-sized myelinated axons
medium
Gamma motor neurons project to ___fusal skeletal mucle
intrafusal
What is the function of a gamma motor neuron?
To control and adjust the sensitivity of spindle afferents
In other words to “gain” control
Define “gain”
the amount of extrafusal contraction elicited by a given stretch (output/input)
When is gain adjustment necessary?
When muscle is shortened to prevent the muscle spindle from being unloaded
During most movements, the alpha and gamma motor neuron systems function simultaneously. This pattern is called what?
alpha-gamma coactivation
What does alpha-gamma coactivation maintain?
The stretch on the central region of the muscle spindle intrafusal fibers when the muscle actively contracts
Alpha-gamma coactivation occurs because of what 2 things?
- most sources of input to alpha motor neurons have collaterals that project to gamma motor neurons
- gamma motor neurons, with their smaller cell bodies, require less excitation to reach threshold than alpha motor neurons do
What does a motor unit consist of?
An alpha motor neuron and all of the muscle fibers it innervates
What are the 2 classifications of motor units?
- Slow-twitch fibers
- Fast-twitch fibers
What type of motor neurons innervate fast-twitch muscle fibers?
larger-diameter, faster-conducting motor neuron
Fast-twitch muscle fibers are recruited in what types of movements?
walking, running, etc.
explosive movements
What type of motor neurons innervate slow-twitch muscle fibers?
Smaller-diameter, slower-conducting alpha motor neuron
Slow-twitch muscle fibers are found in what type of muscle?
Postural and slowly contracting muscles
Which muscle fibers are activated first and why?
Slow twitch fibers are activated first due to smaller cell bodies
The order of recruitment from small to larger motor neurons is called what?
The size principle
When are movements generated?
When somatosensory information is integrated with descending motor commands in the spinal cord
What are LMN pools?
Groups of cell bodies in the spinal cord whose axons project to a single muscle
Where are LMN pools located?
In the ventral horn of the spinal cord
The actions of LMN pools are correlated with what?
their anatomic position
What do medially located LMN pools innervate?
axial and proximal muscles
What do laterally located LMN pools innervate?
distal muscles
What do anteriorly located LMN pools innervate?
extensors
What do posteriorly located LMN pools innervate?
flexors
The inhibition of antagonist muscles during agonist contraction is called what?
Reciprocal Inhibition
When is reciprocal inhibition used extensively and why?
During voluntary motion to prevent antagonist opposition to the movement
What are central pattern generators (CPGs)?
They are basic patterns of coordinated rhythmical activity that are “wired” into the spinal cord circuits
What do CPGs produce?
locomotory behaviors including walking, running or swimming
What are the 3 types of spinal reflexes?
- stretch reflex
- GTO reflex
- flexor-withdrawal reflex
What are the 2 types of stretch reflexes?
phasic and tonic
Describe the steps involved in a phasic stretch reflex
1) the muscle is stretched
2) group Ia afferent fibers in the muscle spindle are activated and their firing rate increases
3) group Ia afferents enter the spinal cord and synapse directly on and activate α motoneurons which innervate the homonymous muscle
4) Activation of the α motoneurons causes contraction of the muscle that was originally stretched
5) this contraction decreases the stretch on the muscle spindle and the firing rate of the group Ia afferents returns to normal
When is the only time in which tonic stretch reflexes are present?
Only following UMN lesions
What is the difference between phasic and tonic receptors
The tonic stretch reflex continues as long as the stretch is maintained
Describe the steps involved in the Golgi tendon reflex
1) the muscle contracts and extrafusal fibers shorten, activating the Golgi tendon organs attached to them
2) group Ib afferent fibers are activated and synapse on inhibitory interneurons in the spinal cord
3) the inhibitory interneurons synapse on α motoneurons inhibiting them, producing relaxation of the homonymous muscle
4) as the homonymous muscle relaxes, the reflex also causes synergistic muscles to relax and antagonistic muscles to contract
What is the function of the GTO reflex?
To protect against potential damage and prevent muscle fatigue
Describe the steps involved in the flexor-withdrawal reflex
1) painful stimulus occurs
2) groups II, III, and IV afferents are activated
3) these flexor reflex afferents synapse on multiple interneurons in the spinal cord
4) on the same side of the pain stimulus flexor muscles contract and extensor muscle relax causing a withdrawal from the noxious stimulus
5) on the opposite side of the pain stimulus extensor muscles contract and flexor muscles relax producing extension which helps to maintain balance
6) the contracted muscles remain contracted for a period of time after the reflex is activated because of a persistent neural discharge called an afterdischarge
What is the Hoffmann’s Reflex or H-Reflex?
A monosynaptic reflex elicited by electrically stimulating a nerve
What is the purpose of the H-reflex?
To quantify the level of alpha motor neuron facilitation or inhibition
Describe the process of eliciting a H-reflex
1) electrodes are placed over the tibial nerve in the popliteal fossa to and on the inferomedial gastrocnemius
2) a weak current, adequate to stimulate only the largest axons is administered to the popliteal fossa electrode
3) APs spread travel both toward the muscle and toward the spinal cord each with different latencies that can be recorded over the skin
What are the 2 latencies that are produced?
The M-wave and the H-reflex
The shorter-latency M-wave is produced by what?
Impluses traveling along motor fibers, casuing an almost immediate muscle contraction
What elicits the H-reflex?
Afferent fiber activity which transmits across synapses to facilitate alpha motor neurons. If activation of this alpha motor neuron is near threshold it will cause depolarization of the calf muscle membranes, and the H-reflex is recorded
