Motor Pool

Examples of Motor Pool in the following topics:

• Motor Units

  • A motor unit consists of the motor neuron and the grouping of muscle fibers innervated by the neuron.
  • 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.
  • There are often multiple sizes of motor unit within a motor pool as a means of modulating the precision and force produced by a single muscle.
  • These multiple motor units of different sizes within a motor pool allow for very fine control of force either spatially or temporally.

• 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.

• 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 and responsible for some aspects of motor control.
  • 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.
  • $$Topography of the human motor cortex, including the premotor cortex, SMA, primary motor cortex, primary somatosensory cortex, and posterior parietal cortex.

• 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., the 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.

• Peripheral Motor Endings

  • A neuromuscular junction exists between the axon terminal and the motor end plate of a muscle fiber where neurotransmitters are released.
  • A neuromuscular junction is the synapse or junction of the axon terminal of a motor neuron with the motor end plate, as shown in Figures 1 and 2.
  • Acetylcholine diffuses into the synaptic cleft and binds to the nicotinic acetylcholine receptors located on the motor end plate.
  • As intracellular calcium levels rise, the motor proteins responsible for the contractile response are able to interact, as shown in Figure 3, to form cross-bridges and undergo shortening.
  • The binding of acetylcholine at the motor end plate leads to intracellular calcium release and interactions between myofibrils to elicit contraction.

• Disorders of the Basal Ganglia

  • Basal ganglia disease refers to a group of physical dysfunctions that occur when the group of nuclei in the brain, known as the basal ganglia, fail to properly suppress unwanted movements or to properly prime upper motor neuron circuits to initiate motor function.
  • Along with other structures, the basal ganglia are part of a circuit that is integral to voluntary motor function.
  • Hypokinetic disorders are movement disorders that are described as having reduced motor function.
  • Hyperkinetic disorders are movement disorders characterized by increased uncontrollable motor function.
  • Distinguish between hypokinetic, hyperkinetic, and non-motor disorders of the basal ganglia

• Neural Mechanisms (Cortex)

  • The primary motor cortex is the neural center for voluntary respiratory control.
  • More broadly, the motor cortex is responsible for initiating any voluntary muscular movement.
  • The inferior portion of the primary motor cortex may be involved in controlled exhalation.
  • Topography of the primary motor cortex, on an outline drawing of the human brain.
  • Each part of the primary motor cortex controls a different part of the body.

• Branches of Spinal Nerves

  • The dorsal ramus: Contains nerves that serve the dorsal portions of the trunk carrying visceral motor, somatic motor, and sensory information to and from the skin and muscles of the back.
  • The ventral ramus: Contains nerves that serve the remaining ventral parts of the trunk and the upper and lower limbs carrying visceral motor, somatic motor, and sensory information to and from the ventrolateral body surface, structures in the body wall, and the limbs.
  • The rami communicantes: Contain autonomic nerves that carry visceral motor and sensory information to and from the visceral organs.

• Trigeminal (V) Nerve

  • The trigeminal nerve (cranial nerve V), and it contains both sensory and motor fibers.
  • It is responsible for sensation in the face and certain motor functions such as biting, chewing, and swallowing.
  • The motor function activates the muscles of the jaw, mouth, and inner ear.
  • The mandibular nerve has both sensory and motor functions.
  • Immediately adjacent to the sensory root, a smaller motor root emerges from the pons at the same level.

• Spinal Cord Trauma

  • SCI can have a number of causes; examples include motor vehicle accidents, falls, sports injuries, and violence.
  • A indicates a "complete" spinal cord injury where no motor or sensory function is preserved in the sacral segments S4-S5.
  • This is typically a transient phase and if the person recovers any motor function below the neurological level, that person essentially becomes a motor incomplete, i.e.
  • E indicates "normal" where motor and sensory scores are normal.
  • Note that it is possible to have spinal cord injury and neurological deficits with completely normal motor and sensory scores.