Reflexes
In this lesson, we’ll examine reflexes, rapid and automatic responses that help the nervous system react quickly to certain stimuli. We'll cover a couple of examples of reflexes here, and a couple more when we discuss the cranial nerves in the next chapter.
1) Start with this video about the knee-jerk reflex:
2) Then, watch this video about a slightly more complicated reflex, the withdrawal reflex:
3) Finally, answer these questions to test your understanding:
1. The knee-jerk (patellar) reflex involves contraction of the __________ muscle and inhibition of the _________ muscle.
- A. quadriceps; hamstring
- B. hamstring; quadriceps
- C. gastrocnemius; quadriceps
- D. quadriceps; gastrocnemius
Show Answer
Correct answer: A
The knee-jerk reflex causes immediate contraction of the quadriceps muscle to produce movement of the leg. Additionally, the sensory neurons traveling from muscle spindles stimulate interneurons that inhibit the activity of motor neurons that supply opposing muscles like the hamstring muscle. The stimulation of these inhibitory interneurons causes the action of the quadriceps muscle to be unopposed.
2. During the knee-jerk (patellar) reflex, tapping the patellar tendon briefly stretches the quadriceps muscle. Which receptor detects this stretch and initiates the reflex arc?
- A. golgi tendon organ
- B. Pacinian corpuscle
- C. muscle spindle
- D. nociceptor
Show Answer
Correct answer: C
When the patellar tendon of the quadriceps muscle is stretched, the stretch is detected by stretch receptors known as muscle spindles that are found in the quadriceps muscle. The muscle spindles stimulate sensory neurons that travel to the spinal cord, where they synapse with motor neurons that control the contraction of the quadriceps muscle.
3. True or false: The main mechanism of the knee-jerk (patellar) reflex does NOT require input from the brain.
- A. True
- B. False
Show Answer
Correct answer: A
The knee-jerk reflex is a monosynaptic spinal reflex; the sensory neuron from the muscle spindle synapses directly onto the motor neuron in the spinal cord, causing contraction of the quadriceps. The brain is not required for the reflex itself, although it can modulate the strength of the response.
4. You step on a piece of sharp glass and reflexively pull your foot away from the painful stimulus. This reflex is known as the:
- A. knee-jerk reflex
- B. withdrawal reflex
- C. plantar reflex
- D. golgi tendon reflex
Show Answer
Correct answer: B
The withdrawal reflex is an automatic response enacted to withdraw a limb from a painful stimulus. If, for example, you were to touch a hot stove or step on a tack, the withdrawal reflex would cause you to pull your arm or leg away from the painful stimulus without having to consciously plan to do so. The reflex is also known as the flexor-withdrawal reflex.
5. The withdrawal reflex begins with these types of receptors:
- A. photoreceptors
- B. nociceptors
- C. baroreceptors
- D. chemoreceptors
Show Answer
Correct answer: B
Nociceptors are sensory receptors that detect painful or noxious stimuli. This is the critical first step in the withdrawal reflex, where a signal is sent from the nociceptors to the spinal cord, triggering a rapid, involuntary withdrawal of the body part from the source of the pain.
