The Patellar Reflex as a Model for Neural Function

To set the stage for discussing the generation and transmission of signals in the nervous system, it will be useful to describe the characteristics of those signals using a simple example: the patellar reflex, also known as the knee-jerk reflex. Figure 1-1 shows the neural circuitry underlying the patellar reflex. Tapping the patellar tendon, which connects the knee cap (patella) to the bones of the lower leg, pulls the knee cap down and stretches the quadriceps muscle at the front of the thigh. Specialized nerve cells (sensory neurons) sense the stretch of the muscle and send a signal that travels along the thin fibers of the sensory

Figure 1-1 A schematic representation of the patellar reflex. The sensory neuron is activated by stretching the thigh muscle. The incoming (afferent) signal is carried to the spinal cord along the nerve fiber of the sensory neuron. In the spinal cord, the sensory neuron activates motor neurons, which in turn send outgoing (efferent) signals along the nerve back to the thigh muscle, causing it to contract.

Figure 1-1 A schematic representation of the patellar reflex. The sensory neuron is activated by stretching the thigh muscle. The incoming (afferent) signal is carried to the spinal cord along the nerve fiber of the sensory neuron. In the spinal cord, the sensory neuron activates motor neurons, which in turn send outgoing (efferent) signals along the nerve back to the thigh muscle, causing it to contract.

neurons from the muscle to the spinal cord. In the spinal cord, the sensory signal is received by other neurons, called motor neurons. The motor neurons send nerve fibers back to the quadriceps muscle and command the muscle to contract, which causes the knee joint to extend.

The reflex loop exemplified by the patellar reflex embodies in a particularly simple way all of the general features that characterize the operation of the nervous system. A sensory stimulus (muscle stretch) is detected, the signal is transmitted rapidly over long distance (to and from the spinal cord), and the information is focally and specifically directed to appropriate targets (the quadriceps motor neurons, in the case of the sensory neurons, and the quadriceps muscle cells, in the case of the motor neurons). The sensory pathway, which carries information into the nervous system, is called the afferent pathway, and the motor output constitutes the efferent pathway. Much of the nervous system is devoted to processing afferent sensory information and then making the proper connections with efferent pathways to ensure that an appropriate response occurs. In the case of the patellar reflex, the reflex loop ensures that passive stretch of the muscle will be automatically opposed by an active contraction, so that muscle length remains constant.

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