Lower Motor Neurons Objectives Understand the motor pathway

Lower Motor Neurons

Objectives • Understand the motor pathway from spinal cord to skeletal muscle • Articulate the influence of afferent sources on muscle tone and reflex response

1. Lower motor neurons A. Alpha motor neurons • Innervate muscle • Big axons B. Gamma motor neurons • Innervate intrafusal fibers in the muscle spindles • Smaller axons

A. Alpha motor neuron • Multi-polar • Large diameter axon with myelin sheath • Cell body is in the ventral horn of the grey matter of the spinal cord • Innervates skeletal muscle fibers ipsilaterally • Alpha motor neuron + fibers it innervations = motor unit • Neurotransmitter: acetylcholine

Axial Extensor Distal Flexor Axial, proximal mm S R O X E FL EN T X E S R SO Distal mm

B. Gamma Motor Neuron cell body in the ventral horn smaller axon slower speed innervates muscle spindles (intrafusal fibers) • Neurotransmitter: acetylcholine • • ag

2. Local circuit neurons • Interneurons that run up and down the spinal cord • Inhibitory or excitatory synapse on alpha motor neurons • Some cross the midline, some don’t • Receive input from upper motor neurons

Motor Units Small • Fine motor control • Smaller diameter axon, not as fast • Lower activation threshold (more excitable) Large • Gross motor control • Larger diameter axon, faster • Higher activation threshold (not as excitable)

Muscle Tone

Myotatic Reflex

Regulation of muscle force

Motor Neuron Pathology

Definitions • Muscle tone • Resistance offered by a muscle to passive stretch • The level of contraction seen in normally innervated, resting skeletal muscle • Reflects the number of motor units in a muscle that are currently active • Hypertonia- more tone than normal • Hypotonia/atonia- less tone than normal

Abnormal tone Lower motor neuron lesion • Efferent and afferent contributions • Loss of tone, flaccid • Fibrillations/ fasciculation • Impaired or absent reflexes • Weakness/paralysis Upper motor neuron lesion • Corticospinal tract on up • Initial loss (spinal shock) • Spasticity, clasp knife resistance and clonus • Hyper reflexes • Weakness/ paralysis

Cutaneous Reflex Example: Plantar Reflex • Similar to the Myotatic Reflex but interneurons in between • Stroke the bottom of the foot and should get toe flexion • Babinski Sign is abnormal: big toe extension and fanning of the other toes

Amyotrophic Lateral Sclerosis (ALS) • Degeneration of upper and lower motor neurons • Leaves cognition intact

References • Purves, D. , Augustine, G. , Fitzpatrick, D. , Hall, W. , La. Mantia, A. , Mooney, R. , Platt, M. and White, L. (2018). Neuroscience (6 th ed. ). Sunderland, MA: Sinauer Press.