Carlos Manuel Rodrigues Machado Autonomic Ubiquitous Computing

3 – Interneuron
Transverse section
through spinal cord
2 – Sensor Neuron
4 – Motor Neurons
Figure 5 – Human Reflex Arc (knee-jerk reflex) [Purves, 2004]
1 - Stimulus
5 - Response
In Figure 5 it is presented the Human Reflex Arc for the knee-jerk reflex. This is the simplest
reflex arc which only uses two neurons in the spinal cord. The muscles in the leg have stretch
receptors which react to a change in length of the muscle. When the hammer hits the tendon at the
knee, it makes a muscle in the front of the thigh longer. That stimulates the stretch receptors in the
muscle resulting in muscles in the front of the thigh contract and, at same time, the back muscle of
the thigh relaxes. The muscle reactions make the foot jerks. The main function of this reflex is
related with maintaining an upright posture, which requires fast and immediate responses to
changes in the environment.
The stimulus travels through the sensor neuron to the spinal cord where it is processed by
the interneuron and the result signals are transmitted to the muscles by the motor neurons that
finally produce the muscle response. The information that arrives to the spinal cord is also
transmitted to the brain, allowing the acknowledgement of the changes in the environment that may
- 70 -