Spinal Reflexes

Reciprocal Innervation

A disynaptic excitatory pathway that excites motoneurons of the antagonist muscles.

Muscle Tone - Normal

At varying muscle lengths, the muscle spindle fires tonically and provides input to maintain activation of the motoneuron pool resulting in normal muscle tone.

Jendrassik's Maneuver

Clinically, weak reflexes can often be facilitated by increasing the overall level of central excitation among motoneurons. This is referred to as the Jendrassik's maneuver and is accomplished by having the patient hook his hands together and try to pull them apart isometrically. This results in an increase in central facilitation of extensors and an exaggerated knee jerk reflex is observed.

Divergence

Each of the Ia afferents that originate from a given muscle spindle project to practically all the motoneurons that innervate that muscle (the entire homonymous motoneuron pool). This single to many projection is an example of divergence.

Flexor Reflex Afferents

Flexion or withdrawal reflexes can be initiated by action of group II, III or group IV fibers. These are primarily somatosensory or pain afferents and are collectively referred to as the Flexor Reflex Afferents (FRA). The function of the flexor reflex is primarily to withdraw a stimulated body part from a potentially dangerous stimulus.

Flexor-Withdrawal Reflex

Flexor reflexes act to produce excitation of all flexor muscles in the limbs from which the afferent fibers come, and inhibition of extensor muscles in that limb. In addition, flexor reflex afferents can produce excitation of contralateral extensors and inhibition of contralateral flexors (crossed extension reflex) which is important in maintaining balance and postural support.

Polysynaptic Reflexes

Flexor reflexes are more complex than the Ia and Ib spinal reflexes because they often involve coordination of an entire limb or even the entire body. Ia and Ib reflexes are limited to a simple joint.All flexor reflexes involve several interneurons (are polysynaptic reflexes) and have widespread connections within the spinal cord.

Hypertonia

Hypertonia is an increase in resistance to passive manipulation of the limbs. Hypertonia or spasticity occurs when there is an increased sensitivity of muscle spindles. This takes place when there is an increase in gamma motoneuron discharge rate.

Hypotonia

Hypotonia is a decrease in resistance to passive manipulation of the limbs

Homonymous Motoneurons

Ia afferents enter the spinal cord and relay information about the dynamic changes in muscle length directly (monosynaptically) to alpha motoneurons innervating the muscle that was stretched (homonymous motoneurons). In most cases, Ia fibers from a muscle spindle terminate on all cells in the homonymous motoneuron pool, although the level of excitation varies depending on the proximity of the fiber near the spindle to the muscle spindle initiating the response.

Atonia

If the dorsal roots are cut, input from the spindles is removed and motoneuron activity decreases, muscles become flaccid, and a condition known as atonia results. Under these conditions the muscle feels floppy and limp. A decrease in central excitatory influences on the motoneurons can also cause hypotonia. Clinically, under these conditions, voluntary movements would require greater effort and patients would report fatigue and weakness. In the case of spinal cord transections, a decrease in gamma motoneuron activity limits the dynamic range of muscles spindles and thus, decreases muscle tone.

Discharge Zone

In a motoneuron pool, small motoneurons will be discharged by weaker stimuli (low thresholds) whereas larger ones require stronger stimuli (higher thresholds). The inputs from Ia afferents are distributed among larger and smaller motoneurons within the motoneuron pool. The population of small motoneurons that is very excited is referred to as the discharge zone.

Synergistic Excitation

In addition to the direct monosynaptic excitation of homonymous motoneurons, Ia fibers also terminate, although to a lesser extent on motoneurons innervating synergistic muscles. This is referred to as synergistic excitation.

Antagonist Inhibition

In contrast to monosynaptic excitation of homonymous and synergistic muscles, Ia afferents synapse on inhibitory interneurons that inhibit motoneurons of antagonist muscles. This disynaptic pathway illustrates the principle of reciprocal innervation where there is a coordination of muscles. There is excitation and contraction of agonist muscles but at the same time relaxation or inhibition of antagonist muscles. This is very important if you want to make movements around a given joint.

Specific Spinal Reflex

In the example of a specific spinal reflex (the stretch reflex) the stimulus (muscle stretch) is sensed by a receptor organ in the muscle (muscle spindle) and a signal is transmitted (via Ia afferent fibers) to spinal cord (motoneurons) in the central nervous system. Motoneurons send efferent signals via its axon (alpha motoneuron) to the effector organ (muscle) which results in a response (muscle contraction).

General Mechanism

In the nervous system, specialized receptor organs receive stimulus information, convert this into electrical signals (usually nerve impulses) and transmit this information via afferent nerve fibers to the central nervous system. The central nervous system processes the information by sending a command signal via an efferent nerve fiber to an effector organ and a response is elicited.

Disynaptic Reflex

In this reflex the adequate stimulus is muscle tension. When the muscle contracts, it stretches receptors in the golgi tendon organ and transmits impulses to the spinal cord via type Ib fibers. Unlike the myotatic reflex, this is a disynaptic reflex. Ib fibers from the golgi tendon synapse first on an inhibitory interneuron, and not directly on the homonymous motoneuron pool. These interneurons function to decrease activity in the homonymous motoneuron pool and cause relaxation rather than contraction of the muscle.

Reflex Pathways in the Spinal Cord - Purpose

Reflex pathways in the spinal cord play an important physiological role in the processing of information leading to motor output. Reflexes are perhaps the simplest form of behavior, involving a response to a stimulus.

Renshaw Cells

Located within the ventral horn of the spinal cord are intrinsic inhibitory neurons called Renshaw cells.

Muscle Tone

Muscle tone is defined as the resistance of a muscle to active or passive stretch. The stretch reflex plays an important role in maintaining muscle tone. Under normal conditions, there is a balance of excitatory and inhibitory influences on motoneurons controlling muscle tone.

Recurrent Inhibition

Renshaw cells receive input from collaterals of motoneuron axons and when stimulated project back onto the same motoneuron that stimulated it. This negative feedback is referred to as recurrent inhibition and serves to limit the discharge of that motoneuron.

Facilitation

Spatial and temporal summation are examples of facilitation. They act to increase the excitability level in a cell and thereby facilitate its discharge. When two motoneuron pools have overlapping subliminal fringe regions, there is a summation of the stimulus in the region of overlap and a discharge or response may occur. This is an example of facilitation.

Spatial Summation

Spatial summation occurs when you have convergence of inputs where a single input alone is not capable of eliciting a response (subthreshold stimuli) but several inputs together can sum to produce a response (suprathreshold).

Inverse Myotatic Reflex

Stimulation of the Golgi Tendon organs lead to a spinal reflex that is just the opposite of the stretch reflex and is sometime referred to as the inverse myotatic reflex.

Temporal Summation

Temporal summation can also occur if multiple subthreshold stimuli from the same input arrive in rapid succession and sum to give a suprathreshold response.

Convergence

The connections of Ia fibers onto homonymous motoneurons illustrate two important patterns of connection in the spinal cord. A single motoneuron receives input from practically all muscle spindle afferents (Ia fibers) that originate from that same muscle. This projection of many fibers onto a single cell is referred to as convergence.

Clasp-Knife Reflex

The golgi tendon organ reflex can be dramatically demonstrated in the hypertonic or spastic limb. If one flexes the limb, there would be a strong resistance to stretching the muscle mediated by the highly sensitive muscle spindles. This resistance would increase up to a point at which the muscle tension resisting the stretch was sufficient to trigger the Golgi tendon organ. This would set off the inverse myotatic reflex and the muscle would suddenly relax and the joint would rapidly flex. This initial resistance to flexion followed by a rapid flexion is similar to the way a pocket knife closes (flexes) and is referred to as the claspknife reflex.

Knee Jerk Reflex

The knee jerk reflex is a classic example of a simple stretch reflex. A tap of the patellar tendon produces a stretch of the quadriceps muscles and an extension of the leg. The stimulus is primarily a dynamic or rapid stretch which excites the nuclear bag Ia afferents. The knee jerk response is often used clinically to test for sensitivity of the stretch reflex. With high levels of fusimotor activity (hyperactive gamma motoneurons) a brief tap on the patella tendon could give an abnormally brisk reflex possibly followed by several additional contractions. With high fusimotor discharge, the spindle is hypersensitive and instead of the muscle relaxing back to its resting position after contraction, only a partial muscle relaxation (spindle stretch) would be sufficient to set off another reflex.

Post-stimulus Discharge

The onset of the flexor response is very rapid (a few milliseconds) and will begin to decrease (fatigue) after a few seconds of sustained stimulation. However after the stimulus is over, the muscle begins to relax but does not fully relax for some time. This is the result of after discharge. After discharge is a consequence of persistent neural activity among polysynaptic circuits and neuronal pools within the spinal cord. This is the result of parallel, reverberating or recurrent circuits.

Subliminal Fringe

The population of larger motoneurons requiring stronger stimuli to excite them make up the subliminal fringe.

Stretch Reflex

The stretch reflex is the simplest of all central pathways. It is sometimes referred to as the myotatic, tendon-jerk, or knee jerk reflex. The stretch reflex is an important postural reflex. The stretch reflex begins with stimulation of the muscle spindle following muscle stretch. Ia afferents from the spindle provide information on dynamic changes in muscle length (velocity of stretch). The stretch reflex plays an important role in maintaining muscle tone.

Occlusion

When there is overlap of discharge zones of two afferents, the number of motoneurons excited when both afferents fire simultaneously may be less than the sum of motoneurons excited when the two afferents fire independently. This is referred to as occlusion.