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Reflex Arcs
The neural pathway for reflexes, starting from a sensory stimulus to the motor response, is called a reflex arc. In general, all reflexes follow the same steps. First, a sensory neuron detects a stimulus and sends a signal to the spinal cord. Within the spinal cord, the signal is integrated by an interneuron, which then sends another signal to a motor neuron. Lastly, the motor neuron sends a signal to the muscle which will then produce a response.
Afferent & Efferent Neurons
Sensory neurons are also known as afferent neurons. The word "afferent" means "towards". Sensory neurons are called afferent neurons because they send signals towards the spinal cord. Motor neurons, on the other hand, are also known as efferent neurons. The word "efferent" means "away". Motor neurons are called efferent neurons because they send signals away from the spinal cord. A useful mnemonic to use to remember the difference between afferent and efferent neurons is SAME: Sensory, Afferent/Motor, Efferent.
Monosynaptic Reflexes
Not all reflexes involve all of the above steps. In particular, the second step involving an interneuron is not present in some reflexes. One example is the patellar reflex, also known as the “knee jerk” reflex, which is shown in Figure 1. When the patellar tendon, located below the knee is tapped, the quadriceps muscle fires and the knee extends. The steps involved in this reflex are as follows: first, an action potential travels along the sensory neuron to the spinal cord. At the spinal cord, the sensory neuron synapses directly onto the motor neuron. The motor neuron then sends a signal that elicits the contraction of the quadriceps muscle. Since this reflex involves only one synapse, it is known as a monosynaptic reflex.
Polysynaptic Reflexes
Most reflexes, however, are not monosynaptic. Instead, they are polysynaptic. Figure 2 shows that in a polysynaptic reflex, a sensory neuron detects a stimulus and sends a signal to the spinal cord. In the spinal cord, the sensory neuron synapses onto an interneuron, which integrates the signal and sends a new signal to a motor neuron, which in turn elicits a motor response. So polysynaptic reflexes involve one or more interneurons connecting the afferent and efferent signals. Most reflexes are complicated polysynaptic reflexes that involve multiple interneurons.
