Lecture 10: Lower Motor Neurons

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