VPB 121: Veterinary Physiology - II (2+1)

Internuncial Neuron function

INTERNUNCIAL NEURONS

• Classified into 2 types based on their synaptic capacity :
  • Excitatory Internuncial Neurons – exert a depolarizing influence on the postsynaptic with which they have contact
  • Inhibitory Internuncial Neurons – exert hyper polarizing influence on the neurons with which they have contact
• With the production of presynaptic inhibition, Internuncial Neuron produces inhibition by depolarizing the presynaptic terminals of other neurons.
• The presence of two types of Internuncial neurons ensure that activity and signaling inside the spinal cord remain flexible
• Reciprocal innervation
  • A phenomenon exhibited by afferent nerves to the spinal cord on somatic efferent nerve activities
  • When motor neurons supplying a skeletal muscle are depolarized by excitatory interneurons or by dorsal root afferents, the motor neurons supplying muscles that are antagonistic to this muscle are hyperpolarized by inhibitory interneurons.
  • All voluntary motor activity is made meaningful because of this integration by inhibitory and excitatory interneurons
• Interneurons are classified based on their pattern of convergence into 5 basic groups
• They can have one or two or three muscle groups projecting muscle spindle fibers to them
• One synergistic muscle group :
• Some of these are excited by exteroceptive afferent sources especially from the foot pads or hair in between the toes
• Two different muscle groups : Usually antagonists acting on the same joint
• Or they may be synergists  acting at neighboring joints
• Or antagonists acting at neighboring joints
• Few are influenced by sensory information from the skin (cutaneous afferents )
• Three muscle groups are also seen
• Interneurons form the system that is within the spinal cord
• Interneurons are capable of prolonged rapid discharge of action potentials following afferent stimulation
• Interneurons play an important role as convergence systems for sensory (afferent ) fibers of sensory modalities
• The convergence systems are not necessarily anatomical but rather are ones that are active at that time
• Spinal reflexes usually involve interneurons except some autonomic reflex arcs through the spinal cord and the myotatic reflex
• The corticospinal, rubrospinal, vestibulospinal, tectospinal, interstitiospinal and reticulospinal motor pathways for somatic and autonomous controls exert their influence through these interneurons
• Many interneurons are organized to perform specific functions
• Alpha motor neurons give rise to large fibers which innervate skeletal muscle
• These fibers also give rise to collateral branches which reenter spinal gray matter to synapse upon inhibitory interneurons
• These Internuncial neurons are called Renshaw cells
• They control the duration, intensity and distribution for motor neuron discharge
• Thus they function in localization of reflex motor activity
• Disinhibition : The above neurons in certain cases increase the firing of motor neurons inhibitory to the alpha motor neuron
• Discharge of Renshaw cells is inhibited by stimulation of the cutaneous and muscle afferents
• A group of neurons within the nucleus intermedialis and nucleus proprius give rise to the ascending fiber system
• These neurons give rise to ventral and rostral spino cerebellar tracts , the spino cervical tracts, the spino olivary tract and spino reticular tracts