Motor Unit

Examples of Motor Unit in the following topics:

• Motor Units

  • The motor unit is the functional unit of muscle contraction and includes the motor nerve fiber and the muscle fibers it innervates.
  • A motor unit consists of the motor neuron and the grouping of muscle fibers innervated by the neuron.
  • Precision is inversely proportional to the size of the motor unit.
  • Thus, small motor units can exercise greater precision of movement compared to larger motor units.
  • Groups of motor units are innervated to coordinate contraction of a whole muscle and generate appropriate movement; all of the motor units within a muscle are considered a motor pool.

• Overview of Motor Integration

  • A motor unit is comprised of a single alpha-motor neuron and all the muscle fibers it innervates.
  • When a motor unit is activated, all of its fibers contract.
  • All of the motor units that subserve a single muscle are considered a motor unit pool.
  • Motor unit recruitment is a measure of how many motor neurons are activated in a particular muscle.
  • These small motor units may contain only 10 fibers per motor unit.

• Interactions of Skeletal Muscles

  • Within a muscle summation can occur across motor units to recruit more muscle fibers, and also within motor units by increasing the frequency of contraction.
  • When a weak signal is sent by the central nervous system to contract a muscle, the smaller motor units, being more excitable than the larger ones, are stimulated first.
  • As the strength of the signal increases, more (and larger) motor units are excited.
  • The largest motor units have as much as 50 times the contractile strength as the smaller ones; thus, as more and larger motor units are activated, the force of muscle contraction becomes progressively stronger.

• Motor Areas

  • The motor areas of the brain are located in both hemispheres of the cortex.
  • The right half of the motor area controls the left side of the body, and the left half of the motor area controls the right side of the body.
  • Premotor cortex: Located anterior to the primary motor cortex, it is responsible for some aspects of motor control.
  • It is located on the midline surface of the hemisphere anterior to the primary motor cortex.
  • Various experiments examining the motor cortex map showed that each point in motor cortex influences a range of muscles and joints, indicating significant overlapping in the map.

• Organization of Motor Neuron Pathways

  • The motor pathway, also called the pyramidal tract or the corticospinal tract, serves as the motor pathway for upper motor neuronal signals coming from the cerebral cortex and from primitive brainstem motor nuclei.
  • The motor impulses originate in the giant pyramidal cells (Betz cells) of the motor area, i.e., precentral gyrus of the cerebral cortex.
  • These are the upper motor neurons of the corticospinal tract.
  • Peripheral motor nerves carry the motor impulses from the anterior horn to the voluntary muscles.
  • The midbrain nuclei include four motor tracts that send upper motor neuronal axons down the spinal cord to lower motor neurons.

• Basal Ganglia

  • The basal ganglia (or basal nuclei, ) are a group of nuclei of varied origin in the brains of vertebrates that act as a cohesive functional unit.
  • The basal ganglia are associated with a variety of functions, including voluntary motor control, procedural learning relating to routine behaviors or "habits" such as bruxism, eye movements, and cognitive, emotional functions.
  • Experimental studies show that the basal ganglia exert an inhibitory influence on a number of motor systems, and that a release of this inhibition permits a motor system to become active.
  • The pallidum receives its most important input from the striatum (either directly or indirectly), and sends inhibitory output to a number of motor-related areas, including the part of the thalamus that projects to the motor-related areas of the cortex.

• The Role of the Basal Ganglia in Movement

  • The basal ganglia are responsible for voluntary motor control, procedural learning, eye movement, and cognitive, emotional functions.
  • The basal ganglia (or basal nuclei) are a group of nuclei of varied origin in the brains of vertebrates that act as a cohesive functional unit.
  • The basal ganglia are associated with a variety of functions, including voluntary motor control, procedural learning relating to routine behaviors or "habits" such as bruxism, eye movements, and cognitive, emotional functions.
  • Experimental studies show that the basal ganglia exert an inhibitory influence on a number of motor systems, and that a release of this inhibition permits a motor system to become active.
  • Although the role of the basal ganglia in motor control is clear, there are also many indications that it is involved in the control of behavior in a more fundamental way, at the level of motivation.

• Sacral and Coccygeal Plexuses

  • The sacral plexus is a nerve plexus which provides motor and sensory nerves for the posterior thigh, most of the lower leg, the entire foot, and part of the pelvis .
  • The largest and longest nerve of the human body, the sciatic nerve, is the main branch and gives rami to the motor innervation of the muscles of the foot, the leg, and the thigh.
  • The nerves forming the sacral plexus converge toward the lower part of the greater sciatic foramen and unite to form a flattened band from the anterior and posterior surfaces of which several branches arise.

• Microscopic Anatomy

  • Cardiac muscle appears striated due to the presence of sarcomeres, the highly organized basic functional unit of muscle tissue.
  • A sarcomere is the basic unit of muscle tissue in both cardiac and skeletal muscle.
  • Binding of Acetylcholine at the motor end plate leads to intracellular calcium release and interactions between myofibrils, eliciting contraction.

• Structural Diversity of Neurons

  • Neurons are electrically excitable cells that are the structural unit of the nervous system.
  • Motor neurons receive signals from the brain and spinal cord initiating muscle contractions and affecting glands.