Motor System: Neurons, Reflexes, and Motor Control

• The lecture differentiates between types of neurons, basic circuitry for motor control, subdivisions of the motor system, motor neurons, primitive reflexes, hypertonicity, and hypotonicity.

Nervous System Subdivisions

• The nervous system divides into the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).
• The nervous system also divides into the motor and sensory systems.

Neuron Types

• Neurons classify by structure and function.
• Sensory (afferent) neurons originate in the PNS and send information from sensory receptors to the CNS.
• Motor (efferent) neurons originate in the CNS and send information from the CNS to an effector (muscle) in the PNS.
• Interneurons reside within the CNS and link sensory and motor neurons.

Motor Neurons

• The motor system involves CNS Upper Motor Neurons (UMN) and Lower Motor Neurons (LMN).
• UMN cell bodies originate in the cerebrum, while LMN cell bodies originate in the brainstem or spinal cord.
• LMN axons exit the CNS to enter the PNS and activate muscles.
• LMNs originating from the brainstem innervate face muscles.
• LMNs originating from the spinal cord innervate trunk and limb muscles.

Descending Motor Tracts

• Corticospinal tracts run from the cortex (brain) to the spinal cord.
  • They govern voluntary movements of the limbs and trunk.
  • UMNs in the cortex connect to LMNs in the anterior horn of the spinal cord, leading to the effector muscle.
• Corticobulbar tracts run from the cortex to the brainstem.
  • They govern voluntary movements of the face.
  • UMNs in the cortex connect to LMNs in the brainstem, leading to the effector muscle.
• Reticulospinal tracts contribute to postural motor control
  • They run from the brainstem (reticular formation) to the spinal cord.
• Vestibulospinal tracts also contribute to postural motor control.
  • They run from the brainstem (vestibular nuclei) to the spinal cord.

Voluntary Movement

• Voluntary movement involves motor output from the cortex to the spinal cord via UMNs.
• LMNs then carry the signal to the muscles.

Muscle Spindle

• A muscle spindle is a receptor within muscles.

Stretch Reflex

• The stretch reflex involves sensory and motor neurons.

Cerebral Cortex

• The cerebral cortex allows conscious awareness, sensory perception, voluntary motor initiation, communication, memory storage, and understanding.
• The cerebral cortex has motor areas for voluntary movement control, sensory areas for conscious sensation awareness, and association areas which integrate diverse information.
• Conscious behavior involves the entire cerebral cortex.

Newborn Reflexes

• Newborns should exhibit reflexes such as rooting, sucking, Moro (startle), tonic neck, grasping, and stepping.

Muscle Tone

• Muscle tone is the involuntary resistance of a muscle to passive stretch.
• Normal muscle tone resists gravity while allowing free movement.
• Hypertonia refers to resistance to passive movement, independent of velocity, with or without spasticity.
• Spasticity involves an increase in resistance to sudden passive movement and is velocity-dependent; the faster the movement, the stronger the resistance.
• It is considered a sustained efferent muscular hyperactivity dependent on continuous supraspinal drive to the alpha motor neuron.

Hypotonia

• Hypotonia describes muscles that appear floppy attributed to problems with the nerves or muscles.
• Signs and symptoms of low muscle tone include:
  • Limpness when lifted.
  • Increased joint flexibility.
  • Poor posture.
  • Easy fatigue due to increased effort to activate muscles/maintain posture.
  • Reduced strength.

Conclusion

• Muscle contraction can be voluntary, postural, or reflexive.
• The corticobulbospinal, reticulospinal, and vestibulospinal tracts serve specific functions.
• The stretch reflex and primitive reflexes are critical neurological functions.