The Reflex Arc
Reflexes travel along specific circuits known as reflex arcs. This arc carries the initial stimulus in the form of an action potential from the sensory receptor to the spinal cord, where it triggers some type of motor output. A reflex arc contains 5 components.
1. A receptor
2. Sensory neuron
3. Integration Center
4. Motor Neuron
Reflex Arc illustration below (this particular reflex arc is known as the flexor withdrawal reflex).
The Stretch Reflex
An incredibly simple reflex, this particular arc is activated by a quick stretch of skeletal muscle tissue. The most familiar example of this reflex would be the patellar reflex in which a small reflex hammer strikes the patellar tendon resulting in a quick stretch of the quadriceps muscle. This quick stretch provides the stimuli that activates this reflex. Let’s examine this reflex in detail below:
The muscle spindle serves as the receptor for the stretch reflex. In response to a quick stretch it sends nerve impulses along a 1st order sensory neuron through the dorsal root into the spinal cord. Here, the sensory neuron branches and synapses with a motor neuron, a sensory neuron, while also relaying awareness of the stretch to the brain as shown above. The sensory neuron directly excites the motor neuron innervating the quadriceps muscles resulting in a quick knee extension jerk. The sensory neuron simultaneously stimulates an inhibitory interneuron which then inhibits a motor neuron that innervates the hamstrings. This results in further amplification of the reflex.
The Tendon Reflex
The golgi tendon organ serves as the receptor for this arc, and it is stimulated by tension extremes. In this case, a tendon organ located in the patellar tendon is stimulated by excessive tension in this tendon.
When stimulated the golgi tendon organ sends action potentials along a 1st order sensory neuron through the dorsal horn into the spinal cord. Once in the spinal cord the sensory neuron stimulates one excitatory interneuron and one inhibitory interneuron. The excitatory interneuron synapses with the motor neuron innervation the hamstrings, while the inhibitory interneuron synapses with the motor neuron innervating the quadriceps. This results in the production of a knee flexion force.