Ch. 10: Motor System: Motor Neurons Flashcards
Ongoing Voluntary Movement
sensory info (from spindles, tendon organs, skin rec etc) may modify movement at level of motor planning area and/or central circuit and/or spinal cord
LMN
- Lower motor neurons
- “peripheral” motor neurons
- Start in ventral horn and go to Mm
UMN
- “central”/upper motor neurons
- from brainstem/cortex to spinal cord
- 2 types: medial and lateral
Lateral UMN
- Fine motor tracts
- control peripheral Mm
- connect to LMN that go to peripheral Mm
:Active Stiffness:
UMN
UMN–>activation of LMG
-conscious & intentional
or
-unconscious & unintentional
Intrinsic Stiffness
- weak actin-myosin attachment in resting state
- “velcro”
- happens after a muscle is immobile for a while
Sarcomere Remodeling
- Mm immobilized in shortened position results in decreased number of sarcomeres=contracture (bad)
- Mm immobilized in lengthened position results in an increased number of sarcomeres
M Line
anchors myosin
Z Line
anchors actin
Titin
- Structural protein that connects M line to Z line
- increases structural integrity of sarcomere contributes to stiffness of Mm at end range
:Active Stiffness:
Reflex Activity
- branches of sensory neurons can activate LMN reflexively
- (Patellar tendon reflex)
Passive Stiffness
- Elastic resistance of titin
- particularly troublesome with contractures
- muscle shorter than it should be
Medial UMN
- postural/gross motor
- control midline/proximal Mm
- connect to LMN that go to midline Mm
4 factors of Mm Stiffness
- Active Stiffness: UMN activating LMN
- Active Stiffness: reflex activity
- Intrinsic Stiffness
- Passive Stiffness
Cocontraction
- “joint stiffness”
- partial/full contraction of Mm on both sides of a joint or around a body segment
- Static/Dynamic
Static Cocontraction
Stiffly locking a joint
Dynamic Cocontraction
control movement of a joint
2 Types of LMN
- Alpha motor neurons (to extrafusal Mm)
- Gamma motor neurons (to intrafusal Mm)
Henneman’s Size Principle
- order of recruitment
- Slow twitch first, then fast twitch
- EXCEPTION: when you need explosive/high level force right away
alpha-gamma coactivation
- UMN send parallel signals to alpha and gamma neurons
- contracts Mm (alpha) and keeps spindle sensitive (gamma) to stretch during contraction
Motor Neuron Pools
- Clusters of LMN somas in grey matter of spinal cord arranged in vertical fusal shapes (fusiform)
- axons project to a single muscle per pool
Motor Unit
- one alpha motor neuron & all the Mm fibers it’s connected to
- slow OR fast (not mixed)
Size of Motor Units
- Gross Motor Control has many Mm fibers for each motor axon
- Fine Motor Control has few Mm fibers for each motor axon
Convergence of info to alpha motor neuron
- each LMN gets input from a variety of sources
- Sources include: UMN, sensory receptors
Vertical Orginization of Neuron Pools
- single pool may include several spinal levels and myotomes
- every Mm gets innervation from 2+ spinal levels
Horizontal Organization of Neuron Pools
- Medial Pools: axial and proximal Mm
- Lateral Pools: distal Mm
- Anterior: Extensor Mm
- Posterior: Flexor Mm
Muscle Synergies
-Many Mm working/contracting together
-activation of multiple UMN
OR
-UMN branching to several UMN
OR
-Interneuron Networks
Phasic Stretch reflex
- Spinal region reflex from Mm spindles to LMN (alpha)
- Quick stretch to a spindle–>Ia fiber–>monosynaptic connection with alpha motor neuron of same Mm–>brief activation of alpha motor neurons of same Mm
UMN Director Tracts
- Start in cerebral cortex
- end in ventral horn spinal cord
- 2 divisions: medial/lateral
- highest amount of voluntary control of movement
UMN Director Tract:
Medial Division
-to LMN of proximal arm/leg Mm
UMN Director Tract:
Lateral Division
-to LMN of distal arm/leg Mm
Reciprocal Inhibition
- when 1 Mm contracts the antagonist relaxes
- connections from UMN to alpha motor neurons
- connectiosn from Mm spindle afferents to alpha motor neurons
- uses an interneuron
Normal Mm Synergies vs Abnormal
-group of Mm that work together under control for a functional outcome
VS
-group of Mm working together w/o control and don’t produce functional outcome
Withdrawal Reflex
- spinal region reflex from cutaneous receptors to LMN (alpha)
- pain–>A delta neurons–>multisynaptic connection with alpha motor neurons–>automatic w/d of painful limb
(one pain fiber splits to many spinal levels)
UMN Helper Tracts
- Start: in brain stem
- End: in ventral horn
- medal/lateral divisions
- lowest level of voluntary control (hold me up against gravity/pick up against gravity)
UMN Helper Tracts:
Medial Division
-to LMN that control prox arm/leg Mm
UMN Helper Tracts:
Lateral division
-to LMN that control distal leg/arm Mm
director UMN help control_______
activation of helper UMN in brainstem
Paralysis
inability to move
Atrophy
- shrinking of Mm bulk
- disuse versus denervation
Paresis
- weakness
- difficulty moving
Myoclonus
-rhythmic & repetitive involuntary contraction of the whole muscle
3 Descending Motor Tracts
- Lateral activation system
- medial activation system
- Nonspecific activation system
Motor planning areas send _____
movement signals to director and helper UMN
7 Involuntary Mm Contractions
- Mm spasms
- Mm Cramps
- Fasciculation
- Mhyoclonus
- Tremor
- Firillations
- Abnormal Movement from dysfunctional basal ganglia
Lateral Activation System
- Axons in Lateral columns connect to LMN in ventral horn
- 2 pathways (lateral corticospinal tract, rubrospinal tract)
2 Pathways of Lateral Activation System
- lateral corticospinal tract
- rubrospinal tract
Mm Spasms/Cramps
- involuntary Mm contraction
- problem at Mm level
Hypotonia
- flaccidity
- decreased muscle tone
Fasciculations
- contraction of entire motor unit
- sign of NS irritation
Tremor
- shaking back and forth of limb
- at rest (Parkinson’s sign) vs Intention (cerebellar damage)
Fibrillations
- spontaneous depol of 1 denervted Mm fiber
- can’t see
- evidence of severe nerve damage
Hypertonia
- increased tone
- spasticity (Velocity dependent)
- Rigidity (nonvelocity dependent)
5 S/Sx of LMN Disorder
- loss of reflexes (stretch and cutaneous)
- atrophy of denervation
- disorders of Mm tone
- paralysis (flaccid)
- fibrillations
5 S/Sx UMN Disorders
- Paresis/paralysis
- loss fractionated movement
- abnormal reflexes
- disorders of Mm tone
- disorders of Mm contraction
Mm Stretch hyperreflexia
- exaggerated Mm response to a quick stretch
- LMN not getting inhibitory signal from damaged UMN, so post synaptic membrane upregulates more receptors to get a signal and causes increased strength of reflexes
Clasp Knife Response
- catch and release
- muscle stretch hyperreflexia (catch) that relaxes (release)
Rigid Hypertonia
- UMN damage–>send too many signals all the time
- nonvelocity dependent
- ex. Parkinson’s
Clonus
-repeated stretch hyperreflexia
Spastic Hypertonia
- Mm stretch hyperreflexia
- velocity depenent
Decerebrate Rigidity
- whole head cut off from brainstem
- unconscious tonic rigidity in arm and leg extensors
- Ex. TBI
Decorticate Rigidity
- cut off from cortex
- unconscious tonic flexion of arms and extension of legs
- ex. TBI
6 Disorders of Mm contraction
- delayed initiation
- slow force production
- prolonged contraction time
- disordered corrdination of agonists and antagonists
- decreased fractionation
- abnormal (stereotyped) Mm synergies
Myoplastic Hyperstiffness
- due to paresis & habitual positioning
- increased stiffness from increased intrinsic stiffness and connective tightness
- not due to increased tone
- decreased sarcomere number (contractures)
- increased weak binding actin/myosin from decreased movement
- selective loss fast twitch fibers
Mm overactivity
- unconscious
- excess tone is activity dependent
- ex. stroke
- at rest no hypertonia
- moving increases hypertonia
4 types of Mm hyperstiffness
- myoplastic hyperstiffness
- spasticity
- rigidity & Mm overactivity
- Mm overactivity
Abnormal Tone
- hypotonia
- hypertonia
- shock
Shock
- period of time after trauma when motor neurons are unexcitable
- cerebral–post stroke
- Spinal–post SCI
- may wake up again
Example of LMN Disorders
- peripheral N injury
- ALS
ALS
- Amyotrophic Lateral Sclerosis
- LMN in ventral horn attacked and die all the way out to the periphery
2 Types of Rigid Hypertonia
- decerebrate
- decorticate
(too many signals all the time)
3 disorders of Mm tone
(hypertonia)
- spastic hypertonia
- rigid hypertonia
- Mm overactivity
