In a reflex arc that comprises a pain receptor cell in the skin, three neurones and a muscle (effector), the number of synapses found in the central nervous system (CNS) is:
A. 3
B. 5
C. 4
D. 1
E. 2
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The reflex arc is the neuron pathway that controls a reflex. They are usually composed of sensory neurons, spinal cord neurons, and motor neurons. A basic reflex arc pathway is mentioned in the question, and the data mentioned should be sufficient to solve this question even if you don’t remember the basics of Reflex arcs. A sensory neuron (Peripheral neuron) enters the spinal cord, and you might assume that it will immediately synapse with the motor neuron (as in some cases of reflex arc).
However, we have another neuron mentioned in our case, an interneuron which can be found in the CNS, the central nervous system.
The interneuron will synapse with the motor neuron (effector) and synapse with the muscle fibers.
Do not forget to find the question itself in the paragraph.
“the number of synapses found in the central nervous system (CNS) is” - this question is explicitly referring to the spinal cord (CNS) only, and not the rest of the synapses we can find in this reflex arc. As this question is short, under exam conditions, the student can easily miss this necessary detail. We suggest following a specific sequence of steps while reading the exam questions. There are two synapses in the CNS: the sensory neuron and the interneuron, the other with the interneuron and the motor neuron.
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