Motor Control & Learning (Lower Motor Neurons)

Motor Control

ability of the nervous system to control posture and movement for a given task

Motor learning

process of acquiring and refining movements through practice and experience

skill acquisition

long term motor learning

Lower motor neurons

alpha and gamma motor neurons, local circuit neurons

Motor cortex

contains M1, pre motor area, and supplemental motor area

Descending system function

create a motor plan

AKA upper motor neurons

Lower motor neuron functions

carry out the motor plan

Postural control

process of maintaining balance by keeping the center of gravity within the base of support

Static postural control

ability to quietly sit or stand upright, not 100% possible

Active postural control

ability to balance in voluntary movements

Reactive Postural control

ability to respond to a perturbation/push/force

Lower Motor Neuron synapse

directly onto the muscle

Local circuit neurons

sensory hub for spinal cord, modulate activity of motor neurons, main source of synaptic input to lower motor neurons

Alpha motor neurons

innervate extrafusal fibers, contract and generate tension

Gamma Motor Neurons

innervate intrafusal fibers (spindles), contract but no NOT generate tension, provides stretch info

Muscle Anatomy

muscle → fasicle → fiber → myofibril

Each muscle fiber is innervated…

by only one alpha motor neuron

One motor neuron innervates…

multiple muscle fibers

Motor unit

smallest level of activation, one motor neuron & the fibers it innervates

Motor Neuron Pool

all motor neurons together than innervate a single muscle

Motor neuron organization in the spinal cord

medial - axial/trunk movement (posture)

lateral - appendicular/limb movement

Motor neurons for head and neck muscles

located in the brainstem

Slow (S) Motor Units

smallest motor units, less tension, slow contraction, resistant to fatigue, lowest threshold

Fast Fatigue Resistant (FR) Motor Units

medium motor units, medium resistance to fatigue, medium amount of tension

Fast Fatigable (FF) Motor Units

largest motor units, highest tension, fastest contraction, fastest fatigue, highest threshold

Slow motor unit example

postural muscles

Fast Fatigue Resistant motor unit example

steady state jogging

Fast fatigue motor unit example

Sprinting or jogging, short burst/high intensity

Modulating force

1) force regulation by Henneman’s size principal or 2) action potential frequency

Henneman’s size principle

orderly recruitment of motor units based on task requirements, more F needed → higher threshold reached

Changing AP frequency

not letting a motor unit fully relax before firing again

gamma motor neuron function

keep spindles taut to remain sensitive to changes even when muscle is fully contracted

Baseline muscle state

always under some version of stretch

Monosynaptic stretch reflex

muscle contraction in response to muscle stretch (tendon strike, dropping a book in hands, etc.)

Muscle tone

steady level of tension, regulated by monosynaptic stretch reflex

Gain

level of gamma motor neuron activity in muscle

low gain

greater stretch required to activate stretch reflex

high gain

less stretch required to activate stretch reflex, often when we know something is coming

Golgi tendon organ sensitivity

sensitive to muscle tension but not passive stretch

GTO Innervation

1b afferents

Muscle spindle innervation

1a afferents

Central Pattern Generator

Oscillatory spinal cord local circuits responsible for alternating flexion/extension

Limb specificity in central pattern generators

explains why walking pattern is different than running

Central pattern generators in humans

less effective since bipedal, postural demands, and increased reliance on local circuits

Amyotrophic Lateral Sclerosis

slow degeneration of alpha motor neurons in ventral horn and brainstem

Amyotrophic Lateral Sclerosis Symptoms

progressive weakness, skeletal muscle wasting, death within 5 years

Fibrillation

spontaneous twitch of denervated muscle fibers

Fasciculation

spontaneous twitch of entire motor unit