Answer to Question 1
Sherrington found that synapses have the property of spatial summationthat is, summation over space. Synaptic inputs from separate locations combine their effects on a neuron. Sherrington again began with a pinch too weak to elicit a reflex. This time, instead of pinching one point twice,
he pinched two points at once. Although neither pinch alone produced a reflex, together they did. Sherrington concluded that pinching two points activated separate sensory neurons, whose axons converged onto one neuron in the spinal cord. Excitation from either sensory axon excited that
spinal neuron, but not enough to reach the threshold. A combination of excitations exceeded the threshold and produced an action potential. Again, Eccles confirmed Sherrington's inference, demonstrating that EPSPs from several axons summate their effects on a postsynaptic cell.
Spatial summation is critical to brain functioning. Sensory input to the brain arrives at synapses that individually produce weak effects. However, each neuron receives many incoming axons that might produce synchronized responses. Spatial summation assures that those synchronized inputs
excite a neuron enough to activate it.
Answer to Question 2
d
Summation of Heart Sounds, Aortic Ejection Click
Summation of Heart Sounds, Normal Physiological Split of S2
Spatial and Response Strategies
Cradle the head briefly in both hands to end this section. Hold for a few seconds until you can feel ...
Graded Potentials: Summation
Summation of Postsynaptic Potentials