Lecture 14 - Descending Pathways Part 2 Flashcards
Name 2 types of lower motor neurons
- Alpha motor neuron
- Gamma motor neuron
Alpha motor neuron function
Innervate muscle fibers (extrafusal fibers) to trigger generation of force by muscle
Gamma motor neuron function
Innervate intrafusal fibers in the muscle spindle
Describe the properties of alpha and gamma motor neurons (in terms of size, conduction speed and function)
Motor unit
One alpha motor neuron and all the muscle fibers it innervates
Motor neuron pool
Collection of alpha motor neurons for a single muscle
Axial muscles are
postural muscles
Distal muscles are
extremities
How do motor neuron pools travel in the spinal together and why
As a cluster in rods along the spinal cord for redundancy (muscles are innervated at many areas of the spinal cord 1/2 segments)
How did we visualize the motor neuron pools
Used a retrograde tracer to visualize distribution
T or F: A motor unit is a 1:1 relationship
F
T or F: An increased rate of innervation of a motor unit means that there is a higher level of precise control
T (eg. lateral rectus is 1:5 vs medial gastrocnemius is 1:1800)
Name the 3 types of motor units
1) type I (Slow twitch fibers)
2) Type IIA (Fast twitch, slow fatigue)
3) Type IIB (Fast twitch, fast fatigue)
4 characteristics of Type 1 (Slow twitch fibers)
- Slow to contract
- Sustain contraction
3) Resistant to fatigue - Small motor neurons
4 characteristics of type II A: Fast twitch, slow fatigue
- Rapid contraction/powerful
- Moderately strong and fast
- Relatively resistant to fatigue
- Intermediate-sized motor neurons
3 characteristics of Type II B: Fast twitch, fast fatigue
- Strongest and fastest contractions
- quickly fatigue
- Largest motor neurons
Neuromuscular Matchmaking
Match between neuron and the properties of the muscle fiber
Who came up with neuromuscular matching, how, and the result?
Who: John Eccles
How: Removed normal innervation of fast muscle fiber and replaced with a nerve that innervated slow muscle fiber
Result: Muscle fiber acquired slow properties (type of contraction and biochemistry of muscle fiber)
What did Terje Lomo and colleagues do
Altered the pattern of activity in the motor neuron changed the muscle phenotype, which results in synaptic plasticity of the periphery (input is important for how motor unit behaves)
T or F: Varying activity/exercise will not change muscles
F, it will (eg. inactivity causes atrophy and isometric exercise results in hypertrophy)
ALS
Degeneration of large alpha motor neurons that causes significant muscle weakness and atrophy (Excitotoxicity)
How does excitotoxicity relate to ALS
Release of too much glutamate triggers degeneration of large alpha motor neurons
Graded control of contraction results in
Precision of the muscle (eg. how much force were using to pick up a can)
What are 2 mechanisms to control the force of muscle contraction?
- Rate coding
- Recruitment
-Size Principle
Rate Coding
Rate of firing of motor units (impacts force production)
Recruitment
Recruits motor unit based on size
T or F: In recruitment largest motor units are recruited first
F, smaller motor units are recruited first and largest last
Size principle
Smaller motor units will be recruited first so specific and appropriate units are matched to the task at hand (only using fatigable motor units when needed)
eg. Stand -> Run -> Jump (muscles involved)
Order of motor unit recruitment based on size principle
- Type 1 - Slow Twitch Fibers
- Type IIA - Fast twitch, slow fatigue
- Type II B, Fast twitch, fast fatigue
What are the 3 main sources of input to the alpha motor neuron
- Input from UMN in motor cortex and brainstem
- Sensory input from muscle spindles
- Input from interneurons (largest input**)
Muscle spindles
Stretch receptors that are specialized for detecting change in muscle length (proprioceptor)
Ia sensory axons wrap around
muscle fibers of the spindle
Ia sensory axons
Provide information of muscle length (thickest and myelinated so very fast) and is necessary for sensory info input to LMN
Another name for the stretch reflex
Myotatic Reflex
Stretch/Myotatic reflex
When a muscle is pulled on it contracts in response
Monosynaptic stretch reflex arc is composed of: (2)
- Ia axon
- Alpha motor neuron its synapses on
T or F: Motor neurons receive continual input from muscles
T
Discharge of Ia sensory axons is closely related to
the length of the muscle
Reciprocal Inhibition of stretch reflex
Contraction of agonist requires relaxation of antagonist
What is an example of a stretch reflex
Knee-jerk reflex
Muscle spindles contain
intrafusal fibers
Muscle spindles are innervated by
Gamma motor neurons at its two poles and has sensory info medially
Gamma and alpha motor neurons are a [positive/negative] feedback loop
Positive
Extrafusal fibers are innervated by
Alpha motor neurons
When muscle is stretched what is firing
The muscle spindle
When a muscle is contracting/shortening
the muscle spindle relaxes and it cannot provide us with information
How do we get information during contraction from muscle if muscle spindle is turned off
The alpha and gamma motor neurons cause contraction in intrafusal fibers to activate muscle spindle and provide proprioception
Without the gamma motor neuron what would happen to sensory information of muscle spindle?
We would lose the sensory information (proprioception) but the muscle would still contract
Golgi tendon organs is located
at the junction of the muscle and tendon
Function of the golgi tendon organs
Monitors muscle tension/force contraction (protective mechanism against damage)
The golgi tendon organs is innervated by
Ib sensory axons
Golgi tendon organs are a [negative/positive] feedback
Negative
Autogenic inhibition of the golgi tendon organs
Ib axons synapse on ib inhibitory neurons in ventral horn to form inhibitory connection with the alpha motor neuron on the same muscle (decreases contraction for protection by monitoring force)
Name 5 differences/characteristics between the muscle spindle and golgi tendon organ
Do we only receive proprioception information from muscle spindle and golgi tendon organ?
No also joint capsules and ligaments
What are 3 strategies the body uses to control the motor system?
- Feedback Systems
- Feedforward Systems
- Adaptive Systems
Feedback system function, its advantage, disadvantages and an example
Function: Closed loop of continuous control
Advantages: Simple and compensates for disturbances
Disadvantages: Not perfect and slow
Example: Stretch reflex
Feedforward system function, its advantage, disadvantages and an example
Function: Moment-to-moment control via an open loop
Advantages: Increases speed of response and is anticipatory
Disadvantages: None
Example: Exercise, thermoreceptors of skin and pelvic floor activation
Adaptive Systems Functions, advantages, disadvantages
Function: Elements of the system are modified rather than the output (work in adjunct to feedforward control)
Advantages: Useful and effective
Disadvantages: Slow and complex
Causes of LMN disease
Infections (polio), injury (prolapsed disc), and disease (spinal muscular atrophy)
What are 5 characteristics of LMN disease
- Interruption of final common pathway
- Hypotonia
- Flaccid paralysis
- Muscle wasting
- Segmental loss
Causes of UMN disease
Stroke, infections, trauma, or demyelinating diseases
3 effects of UMN disease
- Hyperreflexia
- Hypertonia (spasticity)
- Loss of regulatory influence from descending inputs
How to eliminate spasticity of UMN disease
Cut dorsal roots (sensory input)
Compare LMN and UMN disease in terms of tone, reflexes, babinski, muscle bulk and description
