We accomplish this through a phenomenon called reciprocal inhibition. The sensory neuron that synapses with and excites alpha motor neurons supplying the quadriceps also synapses with an inhibitory interneuron. The inhibitory interneuron effectively shuts down the alpha motor neurons to the hamstring. This allows the leg to extend at the knee. Whereas muscle spindles respond to stretch another type of sensory system responds to tension.
You might think that stretch and tension are pretty much the same thing but they are not. Have you ever tried tying your shoes really tight and as you are pulling on the laces, which increases tension, one of the laces snaps? It is pretty inconvenient when you have to replace a shoelace but think if that was your muscle!
At times our muscles are capable of generating sufficient power to damage tendons or even break bones. They can cause avulsion, where the tendon tears off a piece of the bone at its attachment site. In order to prevent this we have a safety mechanism in place called the Golgi tendon organ. Where we could consider the stretch reflex to be excitatory and cause contraction of the stretched muscle group the Golgi tendon reflex would be considered inhibitory and causes relaxation of the affected muscle.
Therefore the result of activation of a GTO would be the opposite of the activation of a muscle spindle. The main purpose of GTOs is to prevent excessive tension on tendons and thus prevents injury. Golgi tendon organs are composed of encapsulated nerve endings that are found interwoven with collagen fibers near the transition from muscle to tendon. These nerve endings monitor tension on the tendon rather than muscle length as muscle spindles do. As a muscle contracts it develops tension on the tendon which is detected by the GTO.
The GTO then sends action potentials, via afferent neurons, to the dorsal horn of the spinal cord where they synapse with inhibitory interneurons. The interneuron then synapses with and inhibits the alpha motor neurons in the anterior horn of the spinal cord. Inhibition of alpha motor neurons will effectively shut off the "power" to the muscle causing it to relax.
You can think of this phenomenon almost like a circuit breaker. If there is a spike in power coming into your home that could potentially damage electrical devices the circuit breaker is tripped, temporarily shutting off electricity to those electrical devices. You might ask yourself, "If this prevents excessive tension on muscles, what about those stories I have heard about mothers lifting cars off of babies and such?
In some circumstances, such as the super human feats of strength you have heard about, the CNS has the ability to override the reflex of the GTO.
This happens as upper motor neurons modify the reflex at the level of the spinal cord. This allows extreme amounts of force and tension to be achieved, but the downside is that it usually causes pretty severe damage to the musculoskeletal system. Have you ever stepped on something sharp with your bare feet or touched something hot with your hand? If the answer is yes then you have experienced the grace of the withdrawl reflex.
If the answer is no, you need to live a little! The withdrawl reflex is yet another way that we are hard wired to avoid pain and tissue damage. We have free nerve endings, called nociceptors , scattered throughout our body that are sensitive to pain. When stimulated these sensory neurons activate lower motor neurons in the spinal cord. The lower motor neurons then stimulate contraction of skeletal muscle to remove or withdraw ourselves from the pain generator. In general, this will take place as flexor muscles are stimulated to contract, such as the hamstrings and hip flexors if you step on a tack or the biceps when you touch a hot stove.
For this reason the withdrawl reflex is sometimes called the flexor reflex. These may include more selective electrodes, inactivation of nerves by specific stimulus parameters, greater use of sensors, and networking of implanted components connected to the central and peripheral nervous system.
Abstract The principles of using electrical stimulation of peripheral nerves or nerve roots for restoring useful bladder, bowel, and sexual function after damage or disease of the central nervous system are described. Publication types Review. The autonomic system, however, targets cardiac and smooth muscle, as well as glandular tissue. Whereas the basic circuit is a reflex arc, there are differences in the structure of those reflexes for the somatic and autonomic systems. The peripheral nervous system can be divided into two parts: 1 the somatic system that controls the activity of voluntary or striated muscle, and 2 the autonomic nervous system that controls the activity of smooth muscle of the blood vessels and viscera, and other internal organs such as the heart, glands, etc.
In order, the five parts of a reflex are sensor, sensory neuron, control center, motor neuron, and muscle. These five parts work as a relay team to take information from the sensor to the spinal cord or brain and back to the muscles.
Somatic Reflex Examples A touch on the roof of the mouth by the sucking tongue causes swallowing to occur allowing baby to obtain nourishment. A stroke along the side of the sole of the foot causes the foot to grasp, moving the toes toward the heel.
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