Chapter. 55-56

• Sensory information is integrated at all levels of the nervous system and causes appropriate motor responses beginning in the spinal cord into the brain stem and finally extend to the cerebrum, where the most complicated muscle skills are controlled
• Organization of the Spinal Cord for Motor Function:

  • Gray Matter
  • White Matter
  • Posterior Horn
  • Anterior Horn
  • The cord gray matter is the integrative area of the cord reflexes
  • The sensory signal enters through the posterior root
  • Neurons in the spinal cord gray matter:
  • Anterior Motor Neuron located in the anterior horn
  • α Motor Neuron Type A α Motor Nerve Fiber (14µm in diameter) innervates large skeletal muscle fibers
  • γ Motor Neuron Type A γ Motor Nerve Fiber (5 µm in diameter) innervates intrafusal fibers and helps control basic muscle "tone"
• • Interneurons present in all areas of the gray matter, 30 times as numerous as the anterior motor neuron, responsible for most of the integrative functions of the spinal cord
  • Renshaw Cells in the anterior horn transmit inhibitory signals to the surrounding motor neuron
  • Propriospinal Fibers ascend or descend in the spinal cord, providing pathways for multisegmental reflexes
• • Role in voluntary motor activity: γ-Efferent System
  • Brain areas for control of the γ-Motor System
  • Manifestation of Muscle Spindle Function – Muscle Stretch Reflex:
  • Neuronal Circuit of the Stretch Reflex
  • 2 Component Divisions of the Stretch Reflex: Dynamic Stretch Reflex
• Muscle Sensory Receptors & their Roles in Muscle Control:

  • Muscle Spindle
  • Golgi Tendon Organs
  • Muscle Sensory Receptors:
  • 2 special types of sensory receptors: Golgi Tendon Organ and Muscle Spindle
  • Muscle Spindle:
  • Intrafusal muscle fibers built around Intrafusal Muscle Fiber, function as sensory receptors
  • 2 types of Muscle Spindle Intrafusal Fiber: Nuclear Bag Muscle Fiber and Nuclear Chain Fiber
  • Sensory and motor innervation details of Muscle Spindle
  • Response of nerve endings to change of receptor length: Static Response and Dynamic Response
• Muscle sensory receptors play a crucial role in muscle control
• Stretch reflex function: opposes sudden changes in muscle length
• Elicited by continuous static receptor signal transmitted by both primary & secondary endings
• Causes the degree of muscle contraction to remain reasonably constant
• Ability to prevent oscillation or jerkiness of body movement ("Damping" function)
• Stabilizes body position during tense action
• Clinical application: Knee Jerk Reflex stretches quadriceps muscles, lower leg jerks forward
• Two special types of sensory receptors: Muscle Spindle and Golgi Tendon Organ
• Muscle Spindle detects muscle length & changes, while the tendon organ detects muscle tension
• Golgi Tendon Organ: Encapsulated sensory receptor through which muscle tendon fibers pass
• Transmission of impulses from the Tendon Organ to the CNS inhibits individual muscles without affecting adjacent muscles
• Motor Cortex: Primary Motor Area, Premotor Area, Supplementary Motor Area
• Primary Motor Cortex: Topographical representation of different muscle areas of the body
• Premotor Cortex: Generates complex patterns of movement, contains Mirror Neurons
• Supplementary Motor Cortex: Elicits bilateral movements, e.g., bilateral grasping
• Transmission of signals from the Motor Cortex to the muscles through the Corticospinal (Pyramidal) Tract
• Corticospinal Tract originates from different areas of the cortex and passes through the brain stem
• Pyramids in the medulla: Majority cross to the opposite side (Lateral Corticospinal Tract)
• Terminates in interneurons in the cord gray matter, sensory relay neurons in the dorsal horn, and anterior motor horn
• Concerned with control of bilateral postural movements by the supplementary motor cortex
• Transmission of signals from the motor cortex to the muscles occurs through the corticospinal (pyramidal) tract
• Betz cell:

  • Myelinated fiber (16 µm in diameter) found only in the primary motor cortex
  • Originates from the Giant Pyramidal Cells
  • Transmits nerve impulses to the spinal cord at about 70 m/sec (most rapid rate of transmission of any signals from the brain to the cord)
• Short Collateral Axons from the Giant Betz Cells back to the Cortex:

  • Inhibit adjacent regions of the cortex when the Betz cell discharges
  • "Sharpening" the boundaries of the excitatory signals
• Large Numbers of Fibers from the Motor Cortex into the Caudate Nucleus & Putamen:

  • Additional pathway extends from there to the brain stem & spinal cord
  • Mainly to control body postural muscle contractions
• Moderate number of motor fibers pass to Red Nucleus of Midbrain to the Rubrospinal Tract:

  • Others go to the cerebellum via Reticulocerebellar & Vestibulocerebellar Tracts
  • Deviation of motor fibers into the reticular substance & vestibular nuclei of the brain stem then to the spinal cord via Reticulospinal & Vestibulospinal Tracts
• Motor fiber that synapse in the pontine nuclei (pontocerebellar fibers) carrying signals into the cerebellar hemisphere
• Collaterals terminating in the inferior olivary nuclei, then secondary olivocerebellar fibers transmit signals to multiple areas of the cerebellum
• Corticorubrospinal System:

  • Fibers synapse in the lower portion of the Red Nucleus (Magnocellular Portion)
  • Descends in the Rubrospinal Tract
  • Crosses to the opposite side in the lower brain stem, follows the corticospinal tract into the lateral column of the spinal cord
  • Terminates mostly on the interneurons of the immediate area of the gray matter
  • Some directly on anterior motor neuron
  • Corticospinal Tract + Rubrospinal Tract = Lateral Motor System of the Cord
• Functions of the Corticorubrospinal System:

  • The magnocellular portion of the Red Nucleus has a somatographic representation of all the muscles of the body
  • Accessory route for transmission of relatively discrete signals from the motor cortex to the spinal cord
  • Stimulation causes contraction of either a single muscle or a small group of muscles
  • Discrete movements can still occur if the corticospinal fibers are destroyed but the corticorubrospinal pathway is intact, except that movements for fine control of the fingers & hands are considerably impaired
• Extrapyramidal System:

  • Portions of the brain & brain stem that contribute to motor control but are not part of the direct corticospinal-pyramidal system
  • Include pathways through the Basal ganglia, Reticular Formation of the brain stem, Vestibular nuclei, and Red Nucleus
• Brain Stem consists of the medulla, pons & the midbrain (mesencephalon) and provides special functions in controlling respiration, cardiovascular system, gastrointestinal function, stereotyped movements of the body, equilibrium, eye movements, and serves as a way station for "command signals" from higher neuronal centers
• Support of the body against gravity – Roles of Reticular & Vestibular Nuclei:

  • 2 Divisions of the Reticular Nuclei: Pontine Reticular Nuclei and Medullary Reticular Nuclei
  • Vestibular Nuclei function in association with the pontine reticular nuclei to control antigravity muscles and maintain equilibrium in response to signals from the vestibular apparatus
• Vestibular Apparatus:

  • Sensory organ for detecting sensations of equilibrium
  • Encased in bony tubes & chambers in the petrous portion of the temporal bone (Bony Labyrinth)
  • Major sensory organ for hearing
  • Integral parts of the Equilibrium Mechanism
• Maculae:

  • Located on the inside surface of each utricle & saccule
  • Utricular Maculae lies in the horizontal plane on the inferior surface of the utricle and plays an important role in determining the orientation of the head when the head is upright
  • Saccular Maculae lies in the vertical plane & signals head orientation when the head is lying down
  • Detects linear acceleration
• Semicircular Ducts:

  • The 3 semicircular ducts (anterior, posterior & lateral) are arranged in right angles to one another to represent all 3 planes in space
  • Each semicircular duct has an enlargement at its end (Ampulla) with a small crest (Crista Ampullaris)
  • Rotation of the head causes the cupula to bend and stimulate hair cells, informing the brain of the position of the head with respect to the pull of gravity