Why is knee jerk reflex essential for walking

To test your knee jerk reflex, a doctor or nurse practitioner raps a small hammer on the tendon below your kneecap. A split second later, when you kick the person with the hammer, it's because the lightning-quick knee jerk reflex occurred within the spinal cord. The kick simply indicates that a section of your spinal cord, and the nerves extending from it, are working properly. Testing a series of different reflexes gives an indication of the health of your whole nervous system. Now let's follow the path of the knee jerk reflex to see how it works.

The tendon below your kneecap connects to the quadriceps muscle on top of your thigh. The hammer's rap on the tendon slightly stretches the muscle. The sensory neuron synapses with a motor neuron in the spinal cord that innervates contractile extrafusal fibers. The contraction of the extrafusal fibers, that is, contraction of the belly of the muscle, releases tension on the intrafusal fibers, decreasing stimulation to neuron.

In the case of the monosynaptic knee-jerk reflex, hitting the patellar tendon with a mallet stretches the intrafusal fibers of the spindles in the quadriceps muscle, leading to contraction. In parallel, an inhibitory impulse is sent from the spinal cord to cause relaxation of the hamstring muscles, via a polysynaptic pathway. It is important to note that, even with the simplest of reflexes, there are multiple inhibitory and stimulatory influences that can affect the excitability of the motor neuron.

These can amplify or suppress a reflex response and may somewhat vary from subject to subject. Lesions that damage the sensory or motor fibers, or damage to the spinal cord, generally diminish a reflex unless the spinal cord has been completely transected. Additionally, neural activity at other sites in the body may influence a reflex arc. The Jendrassik maneuver JM is a special method for reinforcement that is applied in the clinical setting when it is difficult to initiate a reflex in a patient.

Sometimes, though, outside of the clinical setting, experimental outcomes with the JM maneuver can result in no effect or inhibition of a reflex. Different results when implementing the JM may be due to differences in the physiology of healthy experimental subjects versus patients in the clinical setting who are being assessed for nerve problems. Reflex testing is of clinical value. Testing of the patellar response indicates:. Tests for simple muscle reflexes, such as the patellar reflex, are basic to any physical exam when motor nerve or spinal damage is suspected.

To determine this, doctors may also test stretch reflexes in the triceps muscle and the Achilles tendon ankle-jerk reflex and compare results. The Achilles reflex is a monosynaptic stretch reflex similar to the patellar reflex. In the Achilles reflex, the hammer taps the Achilles tendon while the foot is dorsiflexed, and the foot, in response, should jerk toward the plantar surface. The Achilles reflex originates in the S1 and S2 nerve roots. When comparing reflexes from different sites of the body, the locations of the corresponding nerve roots along the spinal cord should be considered in order to determine possible sites of injuries and differences in reflex path lengths.

In this lab, students will record electromyograms EMGs for the fibers in a muscle, and use them to determine the time between the stretch of a tendon and the arrival of a motor impulse at the muscle. Surface electromyography EMG measures the electrical activity in a muscle by placing recording electrodes on the skin over the muscle. To be more precise, an EMG measures fluctuations in the electrical activity of muscles due to muscle cell action potentials.

The EMG produces an electromyogram, which records both the relative amplitude relative strength and timing of muscle contractions. In all of these actions, you are thinking, but at the same time, there are also reflexes that you are unaware of happening inside your body.

These reflexes are built naturally into the body, and they exist at birth and change as we grow older. Reflexes are kind of like safety features for survival that allow us to move in response to something in the environment.

Reflexes can act to protect you in many ways, including removing your hand from a hot or sharp object, or ducking when a loud and sudden sound occurs. These fast actions are reflex responses!

The fact that these responses are automatic shows that reflexes occur at a rate that is far too fast for the brain to be involved with the response. Actions that occur without the involvement of the brain are called involuntary actions, while planned actions from the brain, like throwing a ball or strumming a guitar, are called voluntary actions. After the reflex action has happened, the brain does become aware and tells you what happened.

At this point, the brain might even add to the action. For example, you might have ducked as an involuntary response to a very loud noise, but when the brain becomes involved you learn why you ducked down and the brain sends the voluntary action to respond—maybe to stand back up. In order for reflexes to work, messages need to move around the body. These messages are action potentials , and they travel along the neurons and send messages, special parts of the neurons are involved.

The neuron has three different parts that allow signals to be sensed, to travel, and then move to another neuron or muscle. These three parts are called the dendrites, the axon, and the nerve ending Figure 1. The dendrites receive information from the sensor or other neurons. This information then moves to the axon, which travels to or from the spinal cord.

The action potential travels from the nerve endings at one end of the neuron to the next neuron. Many reflexes start at the muscle or skin and go to the spinal cord. When the action potential reaches the nerve ending, the signal is transferred to another neuron, such as an interneuron or motor neuron. The action potential then travels outside the spinal cord to a muscle. But the neurons do not touch each other in the spinal cord and do not touch at the muscle.

There are tiny spaces called synapses that the action potential must jump across. Doctors will perform a test to make sure reflexes are working properly because reflexes can change if you are sick and as you grow. Imagine you are sitting up on the exam table and the doctor taps you just below the knee with a rubber hammer.

Hopefully, the doctor moved out of the way! The response to the tap of the rubber hammer is called a knee-jerk reflex, but scientists and doctors call it a monosynaptic reflex —the simplest reflex that occurs inside your body [ 2 ]. Monosynaptic is an important word because it describes how the reflex works. When broken into two parts, the word is easier to remember. That means, in the knee-jerk reflex, there is only one point where the message transfers between neurons, so it is monosynaptic.

This monosynaptic reflex is called simple because it works through only four separate parts, whereas most reflexes work through five parts. The five parts of most reflexes are:. A sensor, which is a receptor that senses or detects a change. The monosynaptic knee-jerk reflex skips the interneuron, so it involves the sensor, sensory neuron, motor neuron, and muscle fiber Figure 2.