LMNs Flashcards
Types of motor coordination (from MOST to LEAST automatic)
- Reflex
- Rhythmic/Repetitive (walking)
- Skilled Voluntary (writing, typing)
Cell bodies of LMNs are located where?
Ventral horn
OR
Cranial Nerve motor nuclei
LMNs receive input from
UMNs
Sensory N
Interneurons
Motor N Pools are located where
ventral horn
Medial pools - which muscles?
Axial
Lateral pools - which muscles?
Distal
___ interneurons are long-distance circuits
Medial
___ interneurons are short-distance circuits
Lateral
This type of interneuron crosses the midline (symmetrical movements)
Medial
This type of interneuron terminates unilaterally (fine control, manipulative tasks)
Lateral
2 types of LMNs found in pools
Alpha
Gamma
Alpha motor neurons invx ___ fibers
extrafusal
Gamma motor neurons invx ___ fibers
intrafusal
This type of Alpha Motor N is easily excitable, but low velocity
Small
This type of Alpha Motor N is hard to excite, but fast velocity
Large
Small Alpha N invx ____ muscle fibers, forming a ____ MU
Red (Type 1) muscle fibers
Slow (S) MU
Large Alpha N invx ____ muscle fibers, forming a ____ MU
Pale (Type IIb) muscle fibers Fast Fatigueable (FF) MU
Intermediate Alpha N invx ____ muscle fibers, forming a ____ MU
Type IIa muscle fibers
Fast Fatigue Resistant (FR) MU
Muscle force is regulated by these 2 mechanisms
- Size Principle: S recruited first, then FR, then FF.
2. Frequency of APs: greater firing rate = increased force of contraction.
Types of contractions with low to high AP frequencies
Single twitch (lowest)
Temporal summation
Unfused tetanus
Fused tetanus (highest)
2 types of afferent axons in muscle spindles
- Group 1a (rate of stretch)
2. Group 2 (degree of stretch)
Gamma motor neuron function
When activated, intrafusal fiber contracts
Why is co-activation of Alpha & Gamma important?
Allows spindle to function at all lengths.
If only alpha was stimulated, 1a afferent response decreases as muscle contracts.
When both are stimulated, 1a afferent activity continues sending info to CNS about muscle length.
Gamma Bias/Gain
Gamma N activity is constantly adjusted to meet functional requirements.
Higher activity during difficult, precise, rapid movements.
Lower activity during rest.
GTO invx by ____ afferent
Group 1b
Purpose of 1b Afferents
Project to Inhibitory Interneurons, which decrease Alpha Motor N activity to the same muscle.
Prevents muscle from contracting too much (prevents tearing).
Flexion Reflex
Withdraw limb from painful stimulus.
A-Delta nociceptive N stimulated –> excites ipsi flexor & inhibits ipsi extensor
Crossed Extension Pathway
Opposite reaction of Flexion Reflex to contralateral limb to maintain balance.
Excites contra ext & inhibits contra flex.
LMN Syndrome can result from damage to…
LMN cell body
OR
Their peripheral axons
Symptoms of LMN Syndrome
Paralysis or Paresis
Areflexia or Hyporeflexia
Atrophy or loss of muscle tone
Fibrillations/Fasciculations
Examples of LMN diseases
Polio - viral destruction of motor Ns in ventral horn.
Trauma/laceration of N (e.g. Bells Palsy).
ALS (Lou Gehrigs) - neurodegenerative disease of ventral horn.
Characteristics of ALS
Initially affects LMNs, eventually affects UMNs.
CN motor nuclei affected too.
Sensory & cognition remain intact.
