week 1 motor units

Upper Motor Neurons (UMNs)

• Cell bodies are localized in the cerebral cortex or brainstem.
• UMNs in the cortex initiate voluntary movements and manage complex skilled sequences.
• Axons of UMNs synapse with local circuit neurons and sometimes directly with lower motor neurons for distal muscles.
• UMNs in the brainstem regulate muscle tone and control posture and balance influenced by sensory inputs (vestibular, auditory, visual, somatic).

Local Circuit Neurons

• Interneurons responsible for activating α-motor neurons.
• Located near corresponding α-motor neurons in the spinal cord or cranial nerve motor nuclei.
• Receive descending projections from higher centers and mediate sensory-motor reflexes.
• Maintain interconnections for rhythmic and stereotyped behaviors.
• Can coordinate involuntary movements like walking independently from brain inputs.

Cerebellum and Basal Ganglia

• Complex circuits that regulate upper motor neurons without containing motor neurons themselves.
• The cerebellum detects and corrects motor errors and is essential for motor learning.
• The basal ganglia suppress unwanted movements and prepare UMNs for movement initiation, with malfunctions linked to Parkinson’s and Huntington’s diseases.

Motor Neuron – Muscle Relationship

• Each lower motor neuron innervates muscle fibers within a single muscle and can branch to synapse on multiple fibers.
• Each muscle fiber is innervated by one α-motor neuron, which activates all fibers it innervates upon reaching an action potential threshold.
• A motor unit consists of a motor neuron and the muscle fibers it innervates, ensuring even distribution of contractile force across a muscle.

Motor Unit Types

• Motor units vary in size, with size correlated to the number of muscle fibers innervated and the force generated.
• Small α-motor neurons innervate fewer muscle fibers for low force, while larger neurons innervate more fibers for greater force.
• Three major motor unit types include:
  • Slow (S) Motor Units: Small size, low activation threshold, high endurance, important for sustained contractions.
  • Fast Fatigue-Resistant (FR) Motor Units: Intermediate size, generates higher force than S units, fatigue-resistant.
  • Fast Fatigable (FF) Motor Units: Largest size, highest force output, easily fatigable.

Muscle Fiber Typing

• Muscle fiber types can be classified via ATPase histochemistry or SDS-PAGE electrophoresis.
• ATPase histochemistry identifies fiber types (Type I, IIa, IIb) based on staining.
• SDS-PAGE separates proteins by mass to determine MyHC protein types, including additional type IIx.
• Potential overlap in fiber types due to MyHC isoform variations (e.g., IIb may correspond to MyHC-IIb or IIx).

Muscle Unit Distribution

• Different muscles have varying innervation ratios; for example, the soleus has 180 fibers per motor neuron, while gastrocnemius has 1000-2000.
• Extraocular muscles have a low innervation ratio of 3 fibers per unit, allowing for fine control.
• Use-dependent motor unit plasticity allows for fiber phenotype shifts after chronic electrical stimulation, which has implications for therapies in paralyzed patients.

Regulation of Muscle Force

• Recruitment follows the size principle; smaller motor neurons are activated first with lower thresholds.
• Describes the relationship defined by Ohm’s Law (I = V / R), with regard to current, voltage, and resistance.
• Smaller neurons have higher resistance and lower voltage change for action potential induction, maintaining the same synaptic input efficacy across neuron sizes.

Explore the functions and roles of upper motor neurons (UMNs) located in the cerebral cortex and brainstem. This quiz covers their significance in the initiation of voluntary movements and the regulation of muscle tone. Understand the connections UMNs make with local circuit neurons and lower motor neurons for skilled movements.