How does reflex arc work

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. In a monosynaptic reflex, the message travels from the sensory neuron to the motor neuron with only one synapse. If you think about a relay race with a baton, this is similar to the way a reflex works. The runner with the baton is the action potential, and it is carried down the neuron, which is part of the track.

When the runner and baton get to a specific area on the track, the baton is transferred to the next runner to move along the track. A reflex is just like a relay race, but in a reflex, it is the action potential that moves between neurons rather than a baton and runner along parts of the track Figure 3. In the above example of a monosynaptic reflex, the interneuron is not used. It is easy to add an interneuron; just add another runner and baton, but this runner travels a very short distance.

In our body, interneurons are in the spinal cord. So, when an interneuron participates in a reflex, there is more than one synapse, and these reflexes are then called complex reflexes. There are many different reflexes in the body. Some of them are complicated and involve multiple interneurons and many synapses. These reflexes, just like the simple monosynaptic reflex, exist in living beings, especially humans, to keep us safe! Sometimes reflexes create more than one action.

Imagine removing your foot from something sharp—like a Lego piece that was left on the floor. Crucially this does not involve the conscious part of the brain, which makes it much quicker.

This in turn reduces damage to the body. These are the steps in the reflex arc in more detail:. The reflex arc There are different types of neurones that work together in a reflex action. Monosynaptic refers to the presence of a single chemical synapse. In the case of peripheral muscle reflexes patellar reflex, achilles reflex , brief stimulation to the muscle spindle results in the contraction of the agonist or effector muscle.

By contrast, in polysynaptic reflex arcs, one or more interneurons connect afferent sensory and efferent motor signals. For example, the withdrawal reflex nociceptive or flexor withdrawal reflex is a spinal reflex intended to protect the body from damaging stimuli. It causes the stimulation of sensory, association, and motor neurons. Spinal reflexes include the stretch reflex, the Golgi tendon reflex, the crossed extensor reflex, and the withdrawal reflex.

The stretch reflex myotatic reflex is a muscle contraction in response to stretching within the muscle. This reflex has the shortest latency of all spinal reflexes. 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.