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

PROPERTIES OF REFLEX ARC

One way conduction (Bell and Magendie law)

    

• The dorsal root fibres of the spinal cord are afferent (sensory), while the ventral root fibres are efferent (motor).
• The sensory impulses are centripetal (towards the CNS) and motor impulses are centrifugal (away from the CNS).

Slow speed of conduction

• Slow build up of EPSP to threshold stimulus causes the synaptic delay, thus slows the speed of conduction.
• Fatigue: Due to exhaustion of the neurotransmitter substance at the synapse.
• Refractory period: Show both absolute and relative refractory periods.
• Reinforcement: (Successive induction): By discharging signals from many neurons. It is of two types,
  • Positive successive induction: Irritation at a point of the skin by a tick leads to scratch reflex
  • Negative successive induction: Reciprocal stimulation and inhibition; e.g., biting reflex, reflex flexion and extension of the limbs.
• Rebound: Increased reflex response following the withdrawal of inhibitory stimulus.
• After discharge: Continuous impulse discharge from the centre, even after the cessation of the original stimulation.
• Summation: Subminimal stimuli at rapid succession develop effective EPSP, the temporal summation.
• Recruitment: Prolonged and altered intensity of stimulus progressively activates more number of motor neurons.
• Irradiation: Gradual increase in the intensity of stimulus excites the additional neurons, and the effectors.