Motor Neurons Flashcards
Describe UMN
-
- Motor tracts
- cell bodies in CNS
- target/synapse in CNS
- connect with LMN
Describe LMN
- cell bodies in CNS/Spinal cord
- target periphery
Describe what occurs when we decide to move
- Frontal lobe makes decision to move
- motor planning areas activated
- control circuits (basal ganglia/cerebellum) regulate activity of UMN
- UMN tracts deliver signals to interneurons and LMNs
- LMNs signal skeletal muscle
Where are the cell bodies of the LMN located
- ventral horn
- their axons leave the spinal cord via the ventral horn
Classification of UMNs (motor tracts)
- postural/gross motor tracts: control automatic skeletal muscle activity
- fine movement tracts: fractionated movements of limbs and face
- nonspecific tracts: facilitate all other motor neurons
Multipolar neuron
describe look
- cell bod with many dendrites
- axon with multiple collaterals
Types of LMN
- alpha motor neurons
- gamma motor neurons
Alpha motor neurons
- synpase with extrafusal muscle
- innervate skeletal muscle
- when we test when we do MMT
Gamma Motor neuron
- medium myelination
- synpase with intrafusal muscle fibers in the mucsle spindle
- responsble for volume control of a muscle (reflex)
- excited by the brainstem
muscle tone imbalance
is caused by?
is caused by an imbalance between alpha and gamma activation
What is the path of a lower motor neuron
- Axons leave spinal cord via ventral root
- Travel through the spinal nerve
- Travel through peripheral nerve
- Skeletal muscle
Coactivation
alpha-gamma
- stretch on central region of the muscle is maintained while the extrafusal fibers contract
- Alpha-gamma coactivation
what does alpha motor neuron synapse with
- synpases with extrafusal muscle
- extrafusal muscle is responsible for muscle action
What does gamma motor neurons synpase with
- muscle spindle/intrafusal muscle fibers
- montiors and controls muscle length
What is a motor unit
- alpha motor neuron and the muscle fibers it innervates
- when an alpha motor neuron ia activated ACh is released at all the neuromuscular junctions and all muscle fibers that are innervated by the neuron contract
Slow twitch muscle fibers
- twitch-innervated by smaller diameter,
- slower conducting alpha motor neurons
- postural muscles/slower contracting muscles (Ex: soleus tonically active in standing, physically active with walking)
Fast Twitch muscle fibers
- innervated by larger diameter, faster conducting alpha motor neurons
- Ex: gastroc-sprinting
Henneman’s principle
- Slow twitch fibers continue to contribute during faster actions, as fast twitch units are recruited.
- Order of recruitment known as Henneman’s. (LE)
- Order of recruitment is modified depending on the task and phase during walking and running
What gives sensory input to the LMN
- Golgi tendon organs (GTO)
- muscle spindle
Muscle spindle
- Provides information about and responds to muscle length
- 3 components
- The more delicate the movement, the higher the spindle count/More spindles in muscles that produce fine motor movement vs. larger muscles responsible for posture and gross movements
what are the three components of a muscle spindle
- Intrafusal muscle fibers
- sensory endings (primary 1a and secondary type 2 afferents)
- motor endings
Describe the intrafusal muscle fibers of the muscle spindle
- muscle fiber
- found on the inside
Describe the sensory endings fibers of the muscle spindle
- Sensory endings respond to stretch
- change in muscle length and velocity of length change
- Stretch of the central region of intrafusal fibers is sensed by these primary and secondary endings
- sensory info conveyes to the CNS by type 1a and type 2 afferents
Primary 1a afferents in a muscle spindle
- annulospiral
- discharge is phasic and tonic
- phasic = velocity of length change
- tonic = stretch over time
- responds partly to muscle length but more powerfully to changes in length
Seconary type 2 afferent in a muscle spindle
- discharge is tonic only
- better at registering length alone (ie static stretch)
Motor endings component of muscle spindle
- intrafusal fibers provided by gamma motor neurons
Gamma motor neuron activity
- maintain sensitivity of spindle throughout normal range of muscle lengths
- Gamma motor neurons fire, causing end of intrafusal fibers to contract and maintain sensory activity from spindle
- 2 types of intrafusal fibers: Nuclear bag fibers and Nuclear chain fibers
Nuclear bag fibers
- clump of nuclei
- contain info about length and velocity of length
- 1a acts on both but mostly on bag fibers
Nuclear Chain fibers
- nuclei arranged single file
- contains info about static length only
What is the gamma loop
- gamma motor neurons fire to maintain the sensitivity of the spindle throughout the normal rnage of the muscle lengths
- no activation of gamma = flaccid and extrafusal cannot contract
GTO
- sensory organ of a tendon near musclotendinous junction
- respond to tension in the tendons
- contraction of a muscle pulls on the collagen strands which compressess the GTO and leads to action potential generation
- info is transmitted from GTOs into the SC by type 1 b afferents
- depending on the task GTO inputs either inhibits or facilitates LMN
Stretch reflex
- DTR
- monosynpatic
- stimulus: muscle stretch
- stretches the spindle
- primary afferents 1a activated
- alpha motor neuron activated
- response: muscle contracts
Reciprocal inhibition
- used to coordinate movement
- during agonist contraction antagonist is inhibited
- allows agonist to act unopposed/prevents unwanted activity
- achieved by inhibitory interneurons in the spinal cord
Tendon Organ reflex
- Autogenic Inhibition?
- muscle contracts; GTO detects tension and afferent activiated stimualtes interneuron
- THEORY NO SUPPORTED BY RESEARCH
Recurrent inhibition
- opposite of reciprocal inhibition
- inhibits agonist + disinhibition of antagonist
- helps to isolate fine motor from gross motor
Withdrawal/cross extension reflex
- polysynpatic
- cutaneous reflex
- activation of muscles innervated at numerous spinal cord segments
- noxious afferent stimulation => withdrawl of limb on ipsilateral side
- activation of extensor muscles on contralateral side = crossed extension
Spinal cord and movement coordination
- muscle syergies: coordinated muscular action; type 2 affernet and interneurons to and from the spinal cord
- proprioceptive body schema: used to plan and adapt movements,
- reciprocal inhibition
- GTO input
- Stepping pattern generators
Joint capsule and ligament receptors, muscle spindle receptors, and Golgi tendon organs (GTOs) provide the proprioceptive input required to generate the body
GTO and coordination
- GTO contributes proprioception by registering tendon tension
- information is conveyed by type Ib afferents to spinal cord and stimulate interneurons that either excite or inhibit motor neurons
- Overally play a role in adjusting muscle contraciton
Stepping Pattern Generators/Central Pattern Generators
- adaptable networks of spinal interneurons (activate LMN)
- each LE has its own SPG
- SPGs recieve proprioceptive input
- Descending signals from the brain required to activate and coordinate SPG
- body weight support and SPGs
UMN lesion symptoms
Location: brain, brainstem, corticospinal tracts, SC
Diagnosis: CVA, TBI, SCI, MS, CP
tone: hypertonia
reflexes: hyperreflexia
involuntary movemens: muscle spasms
strenght: weakness/paralysis
Muscle bulk: disuse atrophy
Voluntary movements: impaired or absent
LMN lesion symptoms
- Location: CN nuclei/nerves, ventral horn cells, spinal roots, peripheral nerve
- Diagnosis: polio, guillain-barre, peripheral Neuropathy, radiculopathy
- tone: hypotonia
- reflexes: hyporeflexia
- involuntary movemens: with denervation: fasciculations
- strenght: ipsilateral weakness/paralysis
- Muscle bulk: neurogenic atrophy
- Voluntary movements: weak or absent
