In our discussion we will study four major reflexes that are combined within the spinal cord: the large reflex, the Golgi tendon reflex, the withdrawal reflex and the crossed extensor reflex. Although every of these reflexes is integrated within the spinal cord, they have the right to be affected or modification by higher brain centers come either exaggerate or suppress the response. Somatic reflexes involve specialized sensory receptors called proprioceptors the monitor the place of our limbs in space, body movement, and the quantity of stress, overload on our musculoskeletal system. The effectors associated in these reflexes are situated within skeletal muscle.

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Stretch Reflex

Think back to the critical time you had actually a sports physical or a regimen physical examination. Why did the doctor tap your leg just below the knee? What information deserve to he probably gather indigenous this simple procedure? The magic of analyzing reflexes originates from the phenomenon that, under regular circumstances, a certain stimulus will certainly elicit a predictable response. In the situation of the knee-jerk reflex the expected response is extension of the leg at the knee. If the reflex is higher than expected (hyperactive), much less than expected (hypoactive) or totally absent, that argues potential pathology. Currently let"s watch at just how the big reflex works.

Muscle spindles are dedicated proprioceptors the monitor muscle length. They room bundles of modified bones muscle yarn with substantial sensory and also motor innervation. This fibers, called intrafusal fibers, operation parallel come the contractile bones muscle fibers referred to as extrafusal fibers that consist of the mass of skeletal muscle. Muscle spindles are scattered transparent skeletal muscle, however they happen in the highest thickness near tendinous insertions and in muscles affiliated in good motor regulate (i.e. The small muscles that the hand etc). Intrafusal fibers are only qualified of contraction at your tapered ends whereby they space innervated by gamma engine neurons. (The convulsion is too weak to add to pistol movement but is important in maintaining the sensitivity that the muscle spindle while the muscle is either shortened or lengthened.) Sensory neurons innervate the noncontractile central region of the intrafusal fibers. If stretched, the sensory fiber linked with the muscle spindle will certainly be activated and result in stimulation of an alpha engine neuron (a form of reduced motor neuron) in the anterior horn the the spinal cord. The alpha engine neurons directly innervate the bones muscle whereby the muscle spindle is located. This is an example of a monosynaptic reflex because the sensory neuron synapses directly with the motor neuron and occurs without any type of input from the top motor neuron.

Imagine stepping out of the driver"s chair of your car onto a spot of ice in the parking lot. Together your load transfers to your left foot and starts to slide the end from under you, what happens? The muscle spindles in your left within thigh (adductors) are conveniently stretched and send a blog post to your alpha engine neurons in the spinal cord begging for help. The alpha engine neurons then cause contraction the the very same inner thigh muscles (adductors) that were stretched, and you directly avoid the pain of a groin injury. All of this happens so quick (signals are sent out at speeds roughly 350 miles per hour) the you have already recovered by the time you are mindful that you were in trouble. When a muscle is stretched, the muscle spindles room stimulated and thus rise the frequency of activity potentials sent out to the lower motor neurons in the CNS. The increased activity potential frequency causes alpha engine neurons to promptly fire, leading to muscle shortening. This reflex contraction, in the direction directly opposite come the early stage stretch, protects bones muscle from damage due come overstretching.


Image drawn by BYU-I college student Jared Cardinet Winter 2015

The same process that we described above also relates come other an extremely common situations. For example, together you are analysis this you may be experiencing some drowsiness. We will assume that is due to the fact that you have actually stayed up method too late! together you get exhausted you may have actually experienced the feeling of nodding off, where your head beginning to fall forward complied with by an virtually violent jerking motion as you lug your head upright again. Your muscle spindles are vital in keeping posture, even if it is we are talking about nodding off in course or even if it is we space talking about staying upright together you walk under the street.

So, currently that the muscle the was being stretched is shortened, what wake up to the muscle spindle? go it come to be insensitive to further changes in the muscle"s length? Remember, we said that gamma engine neurons innervate the contractile end of the muscle spindle. As the alpha motor neurons activate extrafusal fibers, causing shortening that the muscle, gamma motor neurons activate the muscle spindle. We describe this together alpha-gamma co-activation. This reasons the tapered end to contract, hence maintaining a baseline tension on the central region of the muscle spindle the is sensitive to stretch. The is in this manner the the muscle spindle is may be to maintain its sensitivity v a wide range of muscle length.

In fact, also when a muscle is at rest the muscle spindle sends out a relatively steady present of action potentials which help to keep a short level of muscle activity. This constant tension that the muscle is what we describe as muscle tone.

Up to this suggest we have only handle activation that the muscle team that is gift stretched. This is important but body motion is managed by opposing muscle groups, the agonist and also antagonist muscles. The agonist muscle is the muscle that contracts to reason a specific movement come happen and also the antagonist is the muscle team that would execute the the contrary action. In the instance of the knee jerk reflex the quadriceps would certainly be the agonist and the hamstring would be the antagonist. In bespeak to prolong the leg in ~ the knee we have to contract the quadriceps, i beg your pardon we do via activation the the alpha engine neurons, however we must likewise relax, or inhibit, the hamstring. We accomplish this with a phenomenon dubbed reciprocal inhibition. The sensory neuron that synapses with and excites alpha engine neurons giving the quadriceps additionally synapses with an inhibitory interneuron. The inhibitory interneuron efficiently shuts under the alpha engine neurons to the hamstring. This allows the leg to extend at the knee.

Golgi Tendon body organ (GTO)

Whereas muscle spindles respond come stretch another form of sensory system responds come tension. You could think the stretch and also tension room pretty much the same thing however they room not. Have you ever tried tying your shoes really tight and as you room pulling on the laces, which rises tension, among the laces snaps? that is quite inconvenient as soon as you have to replace a shoelace however think if that was your muscle! At times our muscles are qualified of generating enough power to damages tendons or even break bones. Castle can cause avulsion, whereby the tendon tears off a item of the bone in ~ its attachments site. In stimulate to prevent this we have a safety mechanism in place referred to as the Golgi tendon organ. Where we could consider the stretch reflex to it is in excitatory and also cause convulsion of the extended muscle team the Golgi tendon reflex would be taken into consideration inhibitory and causes relaxation of the affected muscle. As such the an outcome of activation that a GTO would be opposing of the activation of a muscle spindle. The main purpose the GTOs is come prevent too much tension ~ above tendons and thus avoids injury.

Golgi tendon organs room composed the encapsulated nerve endings the are discovered interwoven through collagen fibers close to the shift from muscle come tendon. This nerve end monitor stress on the tendon quite than muscle size as muscle spindles do. As a muscle contracts it develops tension ~ above the tendon i beg your pardon is detect by the GTO. The GTO then sends activity potentials, via afferent neurons, to the dorsal horn the the spinal cord whereby they synapse with inhibitory interneurons. The interneuron then synapse with and inhibits the alpha engine neurons in the anterior horn of the spinal cord. Inhibition the alpha engine neurons will effectively shut turn off the "power" to the muscle causing it come relax. You can think of this phenomenon nearly like a circuit breaker. If over there is a spike in strength coming into your residence that could potentially damage electrical gadgets the circuit breaker is tripped, in the interim shutting off electricity to those electrical devices.


Image drawn by BYU-I college student Jared Cardinet Winter 2015

You could ask yourself, "If this avoids excessive anxiety on muscles, what around those stories I have heard around mothers lifting cars off of babies and also such?" Well, remember that this is a reflex and is generally managed from the bottom up without too much oversight native the top motor neurons. In part circumstances, such as the super human feats of toughness you have actually heard about, the CNS has the capacity to override the reflex that the GTO. This wake up as upper motor neurons change the reflex at the level of the spinal cord. This enables extreme amounts of force and tension to it is in achieved, however the downside is that it usually causes pretty severe damages to the musculoskeletal system.

Withdrawal Reflex and Reciprocal Inhibition


Image attracted by BYU-I student Nate Shoemaker spring 2016

Have you ever before stepped on something sharp with your ceiling feet or touched something warm with her hand? If the answer is yes climate you have experienced the grace of the withdrawl reflex. If the answer is no, you must live a little! The withdrawl reflex is yet another means that we are tough wired to stop pain and tissue damage. Us have free nerve endings, dubbed nociceptors, scattered throughout our human body that are sensitive come pain. When stimulated these sensory neurons activate lower motor neurons in the spinal cord. The lower motor neurons then stimulate convulsion of bones muscle to eliminate or retract ourselves from the pain generator. In general, this will certainly take location as flexor muscle are engendered to contract, such as the hamstrings and hip flexors if you step on a tack or the biceps once you touch a hot stove. For this reason the withdrawl reflex is sometimes called the flexor reflex.

In order because that this to occur efficiently, we need to stimulate the flexor muscles and also at the same time inhibit the extensor muscles. This phenomenon, referred to as reciprocal inhibition, that was debated in terms of the knee-jerk reflex is also at play here. The ache neuron, together it enters the dorsal horn that the spinal cord, will branch to stimulate an excitatory interneuron and an inhibitory interneuron. The excitatory interneuron then stimulates muscle convulsion of the flexor muscle if the inhibitory interneuron reasons the antagonist muscle, or the extensors come relax.

Crossed Extensor Reflex

The overcome extensor reflex is however another method that your body protects itself. As soon as you action on the tack and also reflexively pull your foot away you quickly discover yourself supporting every one of your load on one leg. There is no the crossed extensor reflex, instead of stand on one foot after stepping top top a tack friend would most likely wind up on your backside.

Again, when you step on a tack and stimulate the pain yarn in your foot castle send signals to the spinal cord through the dorsal horn. In enhancement to sending out branches come excitatory and inhibitory interneurons top top the exact same side that the human body the ache neuron likewise sends a branch to an excitatory interneuron that crosses over to the opposite next of the spinal cord and also stimulates a reduced motor neuron. This reduced motor neuron stimulates the extensor muscles on the opposite next of the human body in ready for the increased load as you transition your weight to the side.

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Now the you understand what reflexes are and also how lock work, let"s revisit the question, "How can the doctor tell that your girlfriend was ok just by looking in her eyes?" The price is the he was checking the integrity of a different reflex, the pupillary light reflex. Remember, the clinical usefulness of check reflexes is that details stimuli must elicit predictable responses. Therefore, you would certainly anticipate the shining a bright light in a person"s eyes would reason the pupils to constrict, and that is precisely what have to happen, yet how? There room special receptor in the eye that room sensitive come light. When stimulated, like when the medical professional shined the bright irradiate in her roommate"s eye, castle transmit signals through the optic nerve come the midbrain. In the midbrain these neurons stimulate the occulomotor nerves, which supply the muscle that cause constriction of the pupil. Hence by checking the pupillary light reflex the medical professional was maybe to quickly evaluate the seriousness that the injury. In the case of severe mind injury this reflex have the right to be jeopardized so the the bright light would certainly not reason the anticipated pupil constriction.

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