a) Explain why the knee-jerk reflex still functions in a person with a severed spinal cord.
b) Why is there no sensation of the stimulus?
a) The knee-jerk reflex, also known as the patellar reflex, is a reaction that occurs when the knee is tapped and the leg involuntarily extends. The reflex arc for the knee-jerk reflex involves a sensory neuron, an interneuron in the spinal cord, and a motor neuron.
Even though the spinal cord is severed, the knee-jerk reflex can still function because it is a monosynaptic reflex. This means that the sensory neuron from the quadriceps muscle in the thigh directly synapses with the motor neuron in the spinal cord, bypassing the need for higher brain input.
When the knee is tapped, the sensory neuron detects the stimulus and sends an electrical signal to the spinal cord. The interneuron in the spinal cord receives the signal and immediately sends a signal to the motor neuron, instructing it to contract the quadriceps muscle and extend the leg. This quick and automatic response does not require input from the brain, and can still occur even when the communication between the brain and the spinal cord is disrupted.
b) The lack of sensation associated with the knee-jerk reflex in a person with a severed spinal cord is due to the interruption of the sensory pathway to the brain. Normally, when the knee is tapped, sensory neurons detect the stimulus and send the information to the brain, where it is perceived as a sensation. However, when the spinal cord is severed, the signals from the sensory neurons cannot reach the brain.
Without a functional pathway to transmit sensory information, the person with a severed spinal cord cannot perceive the stimulus or any associated sensations. This lack of sensation does not mean that the knee-jerk reflex itself is absent; it simply means that the person cannot consciously experience or feel the reflex occurring.