Figure 1. Block diagrams showing two main classes of control. (a) In open-loop control, the system accepts input and produces output. The output is not fed back to the input. (b) In closed-loop control, certain aspects of the output are measured and compared with a desired value. This ability requires a reference copy of the ideal performance within the nervous system.

Figure 2. The knee-jerk reflex. When the skin overlying the patellar tendon is ‘hit’ with a hammer the extensor muscles, including their muscle spindles, stretch. The stretched muscle spindles, which sense muscle strain and strain rate, activate afferent sensory neurons which in turn stimulate motor neurons (and interneurons) in the grey matter of the spinal cord. The fast axons of these neurons subsequently stimulate the extensor muscle fibres, resulting in the knee-jerk.

Figure 3. Diagram of the main types of neurons in a segment of the spinal cord of the lamprey. C, E, L, interneurons; SR, stretch receptor; M, motor neurons. Redrawn after Ekeberg and Grillner (1999).