Lower Motor Neurons (α motor neurons)
Located in the ventral horn of the spinal cord and initiate the contraction of muscles
Known as the final common pathway for transmitting information from a variety of sources to the skeletal muscles
Distribution of lower motor neurons
Each neuron acts on muscle fibers in a single muscle
Neurons are grouped together in a motor neuron pool for a specific muscle (like a column)
These neurons are in the ipsilateral ventral horn
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Lower Motor Neurons (α motor neurons)
Located in the ventral horn of the spinal cord and initiate the contraction of muscles
Known as the final common pathway for transmitting information from a variety of sources to the skeletal muscles
Distribution of lower motor neurons
Each neuron acts on muscle fibers in a single muscle
Neurons are grouped together in a motor neuron pool for a specific muscle (like a column)
These neurons are in the ipsilateral ventral horn
Topographical Organization of Ventral Horn
Medial to lateral topographical map of lower motor neuron pools
Medial → act on proximal muscles
Lateral → act on more distal muscles (ex. fingers and toes)
Medial Ventral Horn
This part controls posture and locomotion
Cover several segments vertically (different layers of spinal cord)
Bilateral → can contact the opposite side of the ventral horn
Lateral Ventral Horn
This part controls fine movement in distal extremities (ex. moving fingers)
Interneurons only cover their own local layer
Ipsilateral → only same side of the ventral horn
α motor neurons
A type of lower motor neuron that acts on the extrafusal and force-producing muscle fibers
Needed for posture and movement
γ motor neurons
A type of lower motor neuron that acts on the intrafusal muscle fibers (muscle spindles)
Sets muscle fibers to an appropriate length
Motor Unit
The α motor neuron and the muscle fibers it innervates
A single α motor neuron acts on multiple extrafusal muscle fibers (in the same muscle)
Spreads force evenly across the muscle and the motor neuron generates contraction
Fast Fatigable (FF)
α Motor Neurons / Motor Units → that are large, have a high threshold, and conduct very fast.
Generate a lot of force and fatigues quickly → used for brief force (ex. running or jumping)
Fast Fatigue-Resistant (FR)
α Motor Neurons / Motor Units → that are medium size, have a medium threshold, and have medium conduction speed
Generate medium force and have intermediate fatigue → used for intermediate movements (ex. walking)
Slow (S)
α Motor Neurons / Motor Units → that are small size, have a low threshold, and have slow conduction speed
Generate small force and have low fatigue → used for sustained movements (ex. standing)
Group Ia afferents
Respond phasically (in bursts) to small stretches
Sensitive to the velocity of a fiber stretch
Group II
Signal the level of sustained fiber stretch
Fire tonically (steadily) with little dynamic sensitivity
Stretch Reflex (Negative Feedback Loop)
Ia afferents are activated which excites α motor neurons which indirectly inhibit the antagonist muscle
Ex. Adding drink to a glass that you’re holding
myotatic reflex (knee-jerk response)
Ex. Hammer tap on the knee
Hammer tap stretches tendon which stretches sensory receptors → sensory neurons excite motor neurons (activators) and interneurons (inhibitors) → leg extends
This reflex helps keep posture and balance without having to think about every step
γ-motor neurons
Adjust the excitability of muscle spindles
→ Leads to change in afferent activity
→ Allows for reliable sensory information
Golgi tendon organs
Located in between the muscles and the tendon
Circuit is a negative feedback system that regulates muscle tension
→ decreases muscle activation when there is too much force and helps protect the muscle
Group Ib sensory axons
Acts on golgi tendon orgons
Contact GABAergic inhibitory local circuit neurons that contact the α motor neurons that act on the same muscle
Passive stretch
Both muscle spindles and golgi tendons are activated → but muscle spindles are activated to a larger extent
Active muscle contraction
Muscle spindles are unloaded and activity decreases
Golgi tendon organs are (more and more) activated
Flexion-crossed extension reflex
Painful sensory stimulation leads to this flexion reflex
→ The extensor is inhibited
→ The flexor is activated on the affected side
crossed extension reflex
Accompanies the flexion reflex
→ Activation of the opposite side (Ex. support leg)
→ Helps to maintain posture
Central Pattern Generators
Local circuits in the spinal cord that control timing and coordination
Paralysis
The loss of movement
Paresis
Weakness of the affected muscles
areflexia
The loss of reflexes
Amyotrophic Lateral Sclerosis (ALS)
Characterized by the slow degeneration of α motor neurons in the ventral horn of the spinal cord and brainstem & of neurons in the motor cortex
Progressive weakness and wasting of skeletal muscles