Initiating Stretch Reflexes

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Reflex testing is an important diagnostic tool for assessing the condition of the nervous system. This video will introduce inborn and learned reflexes; distinguish between somatic and autonomic reflexes; describe the components of a reflex arc; examine how the stretch reflex works; describe how to perform a test of the patellar reflex; and predict how mental distraction might affect the vigor of the patellar reflex response. A reflex is the ability to respond to a stimulus in a very rapid, predictable, and involuntary manner. Some reflexes are acquired, such as riding a bike or driving a car. On the other hand, intrinsic reflexes are inborn and serve to protect the body. For example, we rapidly withdraw our hand when we touch a hot surface to avoid permanent tissue damage. Intrinsic reflexes are divided into two functional categories: somatic and autonomic. Somatic reflexes activate skeletal muscle, while autonomic reflexes activate smooth muscle, cardiac muscle, or a gland. Let's check your understanding. Which of the following describes the type of reflex when you pull your hand away from a hot stove: intrinsic and somatic; learned and autonomic; intrinsic and autonomic; or learned and somatic? >> Pulling your hand away from a hot stove is intrinsic because it does not need to be learned, and somatic because it requires skeletal muscle. Reflexes are predictable because the pathway or signal for the reflex is built into our anatomy. The pathway for this "hardwired" signal is called a reflex arc, and it has five essential components. The first component is the receptor, which is responsible for detecting the stimulus. In its simplest form, the receptor is a nerve ending that detects a physical or chemical change in your environment. The receptor transmits the stimulus to a sensory neuron, which carries the afferent impulses to the central nervous system. Next in the pathway is the integration center. The integration center consists of two or more neurons synapsing with each other. In this example, an interneuron is involved, so the integration would be polysynaptic since there are two synapses. For spinal reflexes, the integration center is located in a particular segment of the spinal cord. The fourth component of the reflex arc is the motor neuron. The motor neuron conducts efferent impulses from the integration center to the effector organ. The fifth and final component is the effector, a muscle fiber or a gland cell that responds to the efferent impulse by contracting or secreting, respectively. For somatic reflexes, the effector is skeletal muscle, as shown in the figure. For some reflexes, the integration center is as simple as a single synapse between a sensory neuron and a motor neuron. This reflex would be classified as monosynaptic, and it would cause a faster response than a polysynaptic reflex arc because it involves fewer neurons. Notice that this reflex arc does not have an interneuron between the sensory and motor neurons. Let's check your understanding. The minimum number of neurons present in a reflex arc is one, two, three, or four? >> The minimum number of neurons is two because a monosynaptic reflex can occur between two neurons, a sensory neuron and a motor neuron. Stretch reflexes are monosynaptic somatic reflexes important to maintaining muscle tone. We will explore a specific stretch reflex, the patellar or "knee-jerk" reflex. The receptor in the stretch reflex is the muscle spindle. The muscle spindle is a receptor that is located within the muscle and detects a change in the length of the muscle, the stretch. Testing this reflex checks the functional integrity of spinal cord sections L2 to L4. To test the patellar reflex, the patellar ligament is tapped with the broad side of the reflex hammer. Tapping the ligament with a reflex hammer stretches the quadriceps femoris muscle, which activates the muscle spindles within the muscle fibers. When the muscle is stretched, the stretch reflex is initiated and the muscle contracts, extending the leg at the knee. The contraction of the muscle counteracts the stretching caused by the tapping of the hammer. Let's check your understanding. Which of the following is the effector in the patellar reflex: the muscle spindle, sensory neuron, motor neuron, or quadriceps femoris muscle? >> The correct answer is the quadriceps femoris muscle. Recall that the effector in a reflex is a muscle or a gland. In the patellar reflex, the effector is skeletal muscle. Even though inborn reflexes do not require conscious thought, they can be modified by the cerebral cortex. In the lab, you will test the effect of mental distraction on the patellar reflex response. You will ask the subject to add a column of three-digit numbers while you test the reflex a second time. As you head into the lab, think about the effect you expect mental distraction to have on the strength of the patellar reflex response.