Why do reflexes occur quickly

The patellar reflex is illustrated in Figure 2. The patellar tendon attaches the quadriceps muscle to the tibia bone of the lower leg. The quadriceps is an extensor muscle because when it contracts it extends the angle of the knee joint by raising the lower leg. Tapping the patellar tendon stretches the quadriceps muscle and causes the sensory receptor of the muscle, called a spindle fiber, to send a signal along the afferent neuron to the spinal cord.

This causes the efferent neuron to return a signal to the quadriceps muscle to contract and lift the lower leg. This action resists the initial stretch and is a classic example of negative feedback. To consider more carefully the events which results of in the "sensation" of muscle stetch, we have to define a few more muscle fiber types.

Muscle spindles, called intrafusal fibers, are innervated by sensory neurons and are arranged in parallel to normal muscle fibers extrafusal. Intrafusal fibers respond to tension by depolarizing a sensory neuron. The sensory neuron synapses with a motor neuron in the spinal cord that innervates extrafusal fibers.

The contraction of the extrafusal fibers releases tension on the intrafusal fibers, decreasing stimulation to neuron. The extensor muscle of the knee quadriceps femoris is attached just below the knee to the anterior part of the tibia by the patellar tendon. Hitting the patellar tendon with a rubber mallet stretches the spindles in the quadriceps muscle, causing contraction Sherwood, Neural activity at other sites in the body may influence the patellar reflex response.

Some evidence suggests that the magnitude of the patellar reflex can be increased when the contractile tone of the quadriceps muscle is increased. Mental activity, which increases muscle tone, may increase the magnitude of the response. Decreased mental activity sleep, restfulness can decrease the magnitude of the patellar reflex while physical activity and mental stress may make the reflex less sensitive. The muscle spindle, afferent sensory and efferent motor neurons, neuromuscular junctions, and the muscle are working appropriately.

Tests for simple muscle reflexes, such as the patellar reflex, are basic to any physical exam when motor nerve or spinal damage is suspected.

These tests can help locate the damage, because motor nerves above the damage aren't affected, but nerves that originate at or below the injury will produce abnormal reflexes. Your doctor may also test similar stretch rflexes that exist in the triceps muscle and also the Achilles tendon. The hardware has already been connected for you.

The transducer a goniometer , which converts a biological event to an electrical signal, is constructed as two levers set an an angle to each other. Changes in the angle can be read by the computer. A hammer is connected to the goniometer and acts as a signal to the computer to begin gathering data when it impacts the patellar tendon. Have the subject sit on the edge of the table with right leg dangling freely. With the straps provided, attach the transducer on the outer side of the right knee with the box facing outwards; make sure the transducer hinge coincides with the knee joint, and that the levers are parallel to the leg bones.

The goniometer should be as close to 90 degrees with the legs dangling freely. You should first practice obtaining the patellar reflex using the rubber reflex mallet provided at your lab station: hit the patellar ligament just below the knee-cap with the pointed end of the mallet. You may mark the area on the knee as a guide for subsequent data collection. Practice getting the hand of a good involuntary knee jerk using the white hammer.

Your leg kicks forward, seemingly on its own. And in a sense, your leg has a mind of its own—in your spinal cord. When the mallet hits your knee, it stretches the tendon just below the kneecap. That causes a signal to travel along a nerve to your spinal cord.

There, a waiting motor neuron sends out an automatic command to contract the muscle attached to the tendon. When the muscle contracts, your leg kicks. Most are located in the spinal cord. But some are in the motor centers of your brain. They work to protect your body from injury. And they also form the basis for more complicated physical activities, such as standing, walking, or riding a bike.

Your brain gets involved by modifying and fine-tuning reflex actions. For example, when you trip and fall, reflexes automatically command your hands and arms to reach out and break your fall.

Muscles will contract throughout your body to minimize injury. But what if you were carrying a priceless object, say, a Ming vase?

It is a monosynaptic reflex that provides automatic regulation of skeletal muscle length. When a muscle lengthens, the muscle spindle is stretched and its nerve activity increases. This increases alpha motor neuron activity, causing the muscle fibers to contract and thus resist the stretching. A secondary set of neurons also causes the opposing muscle to relax.

The reflex functions to maintain the muscle at a constant length. The Golgi tendon reflex is a normal component of the reflex arc of the peripheral nervous system. The tendon reflex operates as a feedback mechanism to control muscle tension by causing muscle relaxation before muscle force becomes so great that tendons might be torn.

Although the tendon reflex is less sensitive than the stretch reflex, it can override the stretch reflex when tension is great, making you drop a very heavy weight, for example. Like the stretch reflex, the tendon reflex is ipsilateral.

The sensory receptors for this reflex are called Golgi tendon receptors, and lie within a tendon near its junction with a muscle. In contrast to muscle spindles, which are sensitive to changes in muscle length, tendon organs detect and respond to changes in muscle tension that are caused by a passive stretch or muscular contraction. Jendrassik maneuver : The Jendrassik maneuver is a medical maneuver wherein the patient flexes both sets of fingers into a hook-like form and interlocks those sets of fingers together note the hands of the patient in the chair.

This maneuver is used often when testing the patellar reflex, as it forces the patient to concentrate on the interlocking of the fingers and prevents conscious inhibition or influence of the reflex.

The crossed extensor reflex is a withdrawal reflex. The reflex occurs when the flexors in the withdrawing limb contract and the extensors relax, while in the other limb, the opposite occurs.

An example of this is when a person steps on a nail, the leg that is stepping on the nail pulls away, while the other leg takes the weight of the whole body. The crossed extensor reflex is contralateral, meaning the reflex occurs on the opposite side of the body from the stimulus. To produce this reflex, branches of the afferent nerve fibers cross from the stimulated side of the body to the contralateral side of the spinal cord.

There, they synapse with interneurons, which in turn, excite or inhibit alpha motor neurons to the muscles of the contralateral limb. The withdrawal reflex nociceptive or flexor withdrawal reflex is a spinal reflex intended to protect the body from damaging stimuli. It is polysynaptic, and causes the stimulation of sensory, association, and motor neurons. When a person touches a hot object and withdraws his hand from it without thinking about it, the heat stimulates temperature and danger receptors in the skin, triggering a sensory impulse that travels to the central nervous system.

The sensory neuron then synapses with interneurons that connect to motor neurons. Some of these send motor impulses to the flexors to allow withdrawal.

Some motor neurons send inhibitory impulses to the extensors so flexion is not inhibited—this is referred to as reciprocal innervation.

Although this is a reflex, there are two interesting aspects to it:. Golgi tendon organ : The Golgi tendon organ, responsible for the Golgi tendon reflex, is diagrammed with its typical position in a muscle left , neuronal connections in spinal cord middle , and expanded schematic right.