In this article we will discuss about the definition and properties of reflex.
Definition of Reflex:
A reflex is defined as automatic/involuntary, stereotyped/repetitive, purpose serving or goal oriented response for afferent stimulation which can be internal or external.
For any reflex action to be brought about, basic reflex arc should be intact.
The components of basic reflex arc are (Fig. 9.11):
b. Afferent limb
d. Efferent limb
e. Effector organ.
Center can be present either in brain or spinal cord. Damage to any part of basic reflex arc results in loss of reflex activity in that part of body.
Classification of reflexes may be done based on different criteria:
a. Unconditioned or conditioned reflex classification is based on whether the reflexes are present at birth. Unconditioned reflex is present by birth.
b. Monosynaptic or polysynaptic reflex is based on number of synapses involved in reflex arc. Stretch reflex is the only example for monosynaptic reflex in body.
c. Clinical classification is based on location of receptors which are involved in reflex action:
1. Superficial reflexes—plantar, abdominal, corneal, etc.
2. Deep reflexes—knee jerk, biceps jerk, ankle jerk, jaw jerk, etc.
3. Visceral reflexes—Marey’s reflex, micturition reflex, vomiting reflex, defecation reflex, etc.
Properties of Reflexes:
Many of the properties of synapses are also applicable for reflexes because in any reflex arc, there will be at least one synapse. Certain other specific properties, which are specific to reflexes, are after discharge is, even after stimulus is over, reflex response continues for prolonged duration. This is possible because of parallel and reverberating circuits in CNS. This is seen in flexion withdrawal reflex.
Habituation occurs when stimulus is benign (not harmful). For any benign stimulus, initially there will be response, but as stimulus is repeated, intensity of reflex response decreases and will be absent after sometime.
Example for habituation is, ticking of a new clock; initially person may not get sleep due to this sound. But as person continues to stay in same environment, his sleep would not get affected in the course of time.
Sensitization is just opposite of habituation. When stimulus is dangerous or harmful, for subsequent exposure to same stimulus, intensity of reflex response will be much more.
Some of the other properties of reflexes are:
iii. Rebound phenomenon
Importance of knowledge of reflexes in clinical practice:
1. To differentiate between upper motor and lower motor lesions. In lower motor neuron lesions, all reflexes (superficial and deep) are lost, whereas in upper motor neuron lesions, all superficial reflexes are lost except plantar reflex which is Babinski +ve and deep reflexes are exaggerated. In cerebellar lesion, knee jerk (deep reflex) becomes pendular.
2. To assess level of lesion in CNS, e.g. if reflexes are normal in upper half of body but altered or absent in lower half of body, it can be concluded that lower half of spinal cord function is not normal. Lower motor neuron is anterior horn cell (AHC) or corresponding cranial nerve motor nuclei with its axon. It forms the final common efferent pathway along which motor impulses reach any part of body.
There are two types of lower motor neurons, namely alpha and gamma motor neurons:
i. The alpha motor neuron supplies extrafusal muscle fibers.
ii. Gamma motor neuron supplies contractile part (polar ends) of intrafusal muscle fibers.
Upper motor neuron (UMN) is any motor neuron which takes origin from cerebral cortex or subcortical regions and influence activity of the lower motor neuron. All descending tracts in general constitute extensions of upper motor neuron in spinal cord. They ultimately act on lower motor neuron.
Effect of Dorsal/Posterior/Afferent Nerve Sectioning:
Dorsal nerve carries all sensory input from peripheral parts of body.
It forms afferent part of basic reflex arc. Features are (in affected part of body):
a. Loss of all modalities of sensations in particular dermatome (if 3 consecutive nerve roots of spinal cord are involved) due to damage in afferent nerve fibers (Fig. 9.12).
b. Loss of all reflexes in the affected part of the body due to lesion in afferent part of basic reflex arc (Fig. 9.13).
c. Tone of muscle is lost or decreased due to damage to the alpha-gamma linkage pathway.
d. Voluntary movements are still present but not normal. This is because even though both lower and upper motor neurons are intact, there are no feedback signals from concerned part of body as afferent nerve has got damaged. These afferent signals are essential for coordination of movements.
Due to loss of afferent feedback signals, there will ataxia (in-coordination of movements). This type of ataxia is termed as sensory ataxia.
e. Tissue damage and tropic ulcers due to loss of protective pain sensations.
This reflex is an example for monosynaptic reflex. Passive stretch does not involve y motor neuron. Tapping on tendon of the muscle, brings about stimulation of stretch (muscle spindle) receptors as intrafusal fibers (IFF) are arranged parallel to extrafusal fibers.
Impulses from stretch receptors of IFF reach spinal cord along la fibers. They stimulate alpha motor neurons present in spinal cord and this in turn brings about contraction of EFF. While eliciting this type of reflex passive stretch of IFF occurs.
Functional Significance of α-ϒ Linkage:
Partial state of contraction that is tone has to be maintained even at rest.
This tone is important:
1. For maintenance of postural balance even at rest
2. To maintain/facilitate load effect acting on body against gravity that is while threading a needle, tone of muscle should be maintained during various skilled activities.
3. To exert damping effect—after intended movement is over, contractions of muscle should not continue. It should be stopped immediately. If contractions do not stop instantaneously, it will bring about oscillations in that part of body. It is because of this reason, in UMN lesions, knee jerk gets exaggerated. In cerebellar lesions, knee jerk becomes pendular.
If dorsal nerve root is sectioned, afferent impulses are not able to reach CNS. Therefore, a-y linkage is broken. This will also lead to oscillation movements or physiological tremors. The a-y linkage provides a feedback mechanism for smooth movement in any part of body.
Inverse Stretch Reflex:
i. This reflex is an example for polysynaptic reflex.
ii. The receptors involved are Golgi tendon organs (tendon end organ).
iii. Golgi tendon end organ is arranged serially with extrafusal fibers.
iv. In an inverse stretch reflex, concerned muscle relaxes.
v. Reflex operates when muscle is in isometric contractile state, that is length of muscle remains same but tension (due to contraction) in muscle is increased.
vi. When muscle contracts isometrically, there is no change in length of muscle but tendon of muscle gets pulled at either ends like what is seen in arm wrestling or tug of war, etc.
vii. If this pulling is beyond certain limit, tendon might snap and get detached from bone/muscle.
viii. In order to avoid snapping of tendon or rupture of muscle fibers, muscle is released by a reflex mechanism.
Reflex pathway involved:
Due to sudden excessive stretching, Golgi tendon end organ gets stimulated, afferent impulses are carried to spinal cord Ib fibers. When impulses reach spinal cord, they stimulate activity of internuncial neuron present between afferent fiber and alpha motor neuron.
Stimulation of internuncial neuron in turn brings about inhibition of alpha motor neuron. Hence there will be no more excitatory impulses to EFF of muscle. This leads to relaxation of muscle. The tension in the muscle decreases. This prevents muscle from getting damaged.
Hence this reflex is a protective reflex. The inverse stretch reflex is an example for autogenic inhibition.
i. When a noxious stimulus is applied to any part of body, there will be a reflex withdrawal of that part of body away from stimulus.
ii. This is a protective reflex as it prevents further damage by removing that affected part of body from source of stimulus.
iii. When noxious stimulus is applied, nociceptors are stimulated.
iv. This brings about production of action potentials in afferent nerve fibers.
v. Fast pain is carried by A 8 fibers.
vi. The afferent impulses on reaching spinal cord stimulates motor neuron supplying flexor group of muscles and inhibit motor neurons supplying extensor group of muscles.
vii. This brings about withdrawal of part of body away from source of noxious stimulus.
viii. This reflex is also an example for polysynaptic reflex.
Crossed Extensor Reflex:
i. In the case of humans, it is more pronounced in lower limbs since human beings are biped unlike most of animals which are quadrupeds.
ii. In the case of human beings whenever noxious stimulus is applied to one of the lower limbs, crossed extensor reflex can be elicited.
iii. Due to noxious stimulus, limb which is getting injured flexes reflexly thereby bringing about withdrawal flexion reflex and at same time contralateral limb becomes extended.
iv. The purpose of flexion of affected limb is to prevent further damage and opposite limb has to be extended in order to maintain posture and equilibrium of body.
v. Receptors involved are nociceptors (pain receptors).
vi. Afferent impulses reach spinal cord through small diameter (A delta) fibers.
vii. These afferent impulses on reaching spinal cord stimulate motor neurons supplying flexor muscles on same side. In addition to this, they also inhibit through an internuncial neuron the motor neurons supplying extensor group of muscles on same side. This brings about flexion of affected limb.
viii. These afferent impulses also affect motor neurons supplying muscles of opposite limb. Afferent impulses stimulate activity of motor neurons supplying extensor muscles and through internuncial neuron.
ix. This leads to extension of opposite limb. This is how posture and equilibrium can be maintained even on one limb.
x. Stimulation of agonist muscle and inhibition of antagonist muscle is an example for reciprocal inhibition.
i. This is one of the examples for superficial reflex.
ii. In UMN lesions, all superficial reflexes are absent except plantar reflex which will show Babinski +ve sign.
iii. In LMN or afferent nerve lesions, plantar reflex will be absent since there is damage to basic reflex arc.
iv. When sole of foot is firmly stroked from lateral side and continued up to area below toes, normally there will be plantar flexion of all five toes. This forms a normal plantar response (Fig. 9.35).
v. When plantar response is Babinski +ve type, there will be dorsiflexion of great toe and fanning out of other toes (Fig. 9.35). This type of response is normally seen in UMN lesion particularly in pyramidal tract lesion in which all superficial reflexes are absent except plantar reflex.
Babinski +ve sign can be seen in normal infants also till the age of one to one and half years. In infancy, it occurs due to incomplete myelination of pyramidal tract. Babinski +ve sign is also seen in normal adults especially when they are dreaming (during REM sleep), and in person who is in state of coma.
Stage of Recovery of Reflex Activity:
i. During this stage, voluntary motor activity will not be regained because in CNS regeneration of nerve fibers is not possible. Therefore, descending tracts nerve fibers cannot get regenerated.
ii. There is no recovery of sensations as ascending tract nerve fibers also cannot regenerate.
iii. Lateral horn cells function is recovered to a certain extent. Regain of sympathetic activity leads to increase of vasomotor tone and rise of blood pressure.
iv. There will be functional recovery of lower motor neurons. Because of this, reflex activity is possible, but not normal. Only spinal reflexes recover.
v. Visceral reflex also get recovered. Micturition reflex becomes completely automatic even in adults but complete evacuation of bladder would not take place. This will be due to loss of reinforcing influences from pons over sacral segments of spinal cord. So there will be retention of some urine in bladder.
vi. Defecation reflex also recovers but it too becomes completely an automatic reflex even in adults.
Somatic reflexes observed will be:
a. Flexor withdrawal reflex:
Motor neurons supplying flexor group of muscles recover earlier. So muscle tone is regained in flexor compartment first. Because of this, withdrawal reflex can be elicited. Most of the times limbs are held in flexion. Hence this is known as paraplegia in flexion.
b. Later on, extensor group of muscles start recovering (these muscles have a delayed recovery as compared to flexor muscles). So reflexes can also be elicited in extensor compartment. Now limbs will be in extended position. This is known as paraplegia in extension.
c. Coitus reflex:
After complete transaction, impulses from higher centers, like cerebral cortex, are unable to reach nerves supplying reproductive organs. Hence any erogenic thought will not bring about erecting of penis. But such people can still have erection of penis and coitus reflex responses when there is physical stimulation of glans penis.
During coitus reflex, there will be curling up of skin around scrotal sac, stiffening of penis, erection, engorgement of penis with blood due to dilation of vessels, and at times ejaculation of seminal fluid.
Mass reflex (pathological reflex):
Whenever skin on medial upper part of thigh is scratched, there will be flexion of limb, contraction of abdominal muscles, rise in arterial blood pressure, sweating, reflex micturition and defecation. Mass reflex will be due to irradiation of afferent impulses into many segments of spinal cord and heightened excitability of spinal motor neurons.
Stage of Reflex Failure:
Sometimes, even when a person has gone into stage of reflex recovery of reflex activity, it can still suddenly get into a stage of reflex failure. Some volume of residual urine will always be present in urinary bladder, which can lead to infection of bladder either due to constant presence of urine or bedsores, etc.
These infections may be more generalized, and later may affect blood pressure. Fall in blood pressure affects normal functioning of kidney and leads to renal failure. Thus internal environment (homeostasis is impaired). All these may lead to failure of heart and nervous system function and can lead to death.