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

REFLEXES

• A reflex is a specific, stereotyped, involuntary response to peripheral stimulation.
• The spinal cord is a reflex neural system and its activity is modified by influences transmitted from the brain via descending fiber system
• The regulating system of the brain are informed of the reflex activity initiated within the spinal cord and is able to initiate descending influences to modify this reflex activity so that it is of maximum economy to the animal
• Reflexes involve several components of CNS and PNS connected in series
• Skeletal motor system and visceral motor system ( ANS) are closely related to spinal cord reflexes.

• The components of a reflex arc are,
  • Receptor organ
  • Afferent neuron
  • Efferent neuron
  • Effector organ
• Sometimes one or more internuncial neurons
• Only the reflex arcs utilized by the muscle spindle stretch receptor afferents do not contain internuncial neurons Eg : The myotatic reflex arc in which fibers terminate directly on the efferent neurons . Such an reflex arc is known as an monosynaptic reflex arc because only one synapse is involved
• All other reflexes are referred to as polysynaptic reflex arcs indicating that more than one synapse is involved
• Reflex arcs that do not involve a synapse either in the PNS or CNS involve in reflexes referred to as axonal reflexes
• Many autonomic reflexes take place within peripheral ganglia without spinal cord component and are known as ganglionic reflexes which are mostly monosynaptic
• All autonomous reflex arcs that pass through the spinal cord are polysynaptic or atlest bisynaptic in nature.
• Alpha motor neurons are the final common pathway to skeletal muscle.
• Alpha motor neurons possess large cell bodies which are filled with well-developed, granular endoplasmic reticulum called Nissel substance.
• Before emerging out of the spinal cord , the axon gives rise to form one to six branches which terminate within the confines of the spinal cord called the recurrent collaterals of Alpha motor neuron axons.
• They terminate upon neurons that exert an inhibitory effect on Alpha motor neurons called the Renshaw cells.
• According to Dale’s principle, a given neuron can produce only one type of transmitter agent at all its terminal synapses. The Alpha motor neurons produce acetylcholine at all its neuro effector junction.
• Prolonged discharges of action potentials upto 50 msec at high frequencies of 500 to 1000 per sec are possible at these synapses.