Imagine removing your foot from something sharp—like a Lego piece that was left on the floor. So, the withdrawal reflex to remove your foot works with a reflex on the other side of the body telling you to put your other foot down. That reflex is called the crossed extensor reflex. These actions are all done without you thinking or planning, but your brain helps to assess the situation as an afterthought. Who left the Lego there? Did you notice how the brain was not on the five-part list of what makes up a reflex?
Well, reflexes are functions of the nervous system, which coordinates our actions. The nervous system is the network of neurons that transmits the action potentials. The sensor and neurons are outside the spinal cord, in what is called the peripheral nervous system PNS.
The other part of the system is called the central nervous system CNS , and it consists of the brain and spinal cord. The CNS is used to think, plan, and learn.
Remember the earlier examples when you ducked down in response to a loud noise or lifted your foot after stepping on the Lego? In those examples, the CNS is what helped you understand what the noise was or why you moved automatically. It did not create the initial movement, but the CNS allowed you to understand why you ducked down or that you stepped on a Lego.
The CNS also plans the voluntary movements that come after the initial involuntary reflex. Perhaps you will bend down and pick up the Lego—that is a voluntary movement. Do you remember the example of the doctor tapping your knee? That is a test to see how well your body is prepared to react. There are many different types of reflexes that protect your body. The doctor tapping your tendon is a test of the simple monosynaptic reflex, which consists of a sensor, a sensory neuron, a motor neuron, and a muscle.
Once the sensory neuron is activated, an action potential travels along the sensory neuron to the spinal cord. The action potential then moves across the synapse to a motor neuron and leaves the spinal cord. More advanced reflex responses will use an interneuron, and these are called polysynaptic reflexes.
Reflexes do not involve the CNS at first, but after the reflex has occurred in the body to keep it safe, the brain assists in understanding what happened. Reflexes protect your body from things that can harm it.
For example, if you put your hand on a hot stove, a reflex causes you to immediately remove your hand before a "Hey, this is hot! Other protective reflexes are blinking when something flies toward your eyes or raising your arm if a ball is thrown your way. Even coughing and sneezing are reflexes.
They clear the airways of irritating things. The reflex that the doctor checks by tapping your knee is called the patellar, or knee-jerk, reflex. It is also known as a deep tendon reflex DTR because the doctor is actually tapping on a tendon called the patellar say: puh-TEL-ur tendon. Within 10 to 30 milliseconds after tripping, the conscious motor centers of the brain would take control of the fall. They weigh the chances of breaking the vase versus breaking your neck.
If the object in your hands is important enough to you, you can modify the reflex action and keep a grip on the vase. Reflexes do play a role in reaction time. Some people are born with faster reflexes. Electrical impulses actually travel more quickly through their nerves. But you can also speed up nerve conduction through practice. A soccer player, for example, can improve his running or kicking. In doing so, his knee jerk might get faster. But those kinds of improvements are specific to the activity.
A soccer player's feet and legs might develop faster nerve conduction than average. This travels to the spinal cord where it passes, by means of a synapse, to a connecting neuron called the relay neuron situated in the spinal cord. The relay neuron in turn makes a synapse with one or more motor neurons that transmit the impulse to the muscles of the limb causing them to contract and pull away from the sharp object.
Reflexes do not require involvement of the brain, although in some cases the brain can prevent reflex action. Reflex arc : The path taken by the nerve impulses in a reflex is called a reflex arc. This is shown here in response to a pin in the paw of an animal, but it is equally adaptable to any situation and animal including humans.
There are two types of reflex arcs:the autonomic reflex arc, affecting inner organs, and the somatic reflex arc, affecting muscles. When a reflex arc consists of only two neurons, one sensory neuron, and one motor neuron, it is defined as monosynaptic. 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.
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