Reflexes are integrated at various levels of the nervous system.
A reflex movement is a specific pattern of response that occurs
without volition and without the need of direction from the cerebrum.
The anatomical basis for a reflex act is the reflex arc (Figure
4.10). This consists of an afferent neuron that comes from a receptor
organ, enters the spinal cord, and there makes a synaptic connection
either directly with the dendrites and the cell body of an efferent
neuron or indirectly through one or more connector neurons. The
axon of the efferent neuron extends from the cord to the muscle,
where its distal branches terminate in muscle fibers. The point
of contact between an axon and a muscle fiber is known as a myoneural
junction, or motor endplate. The number of reflex arcs and the number
of motor units involved depend both on the nature of the reflex
and on the extent of muscular activity needed. Automatic reflex
motions accompany all normal voluntary motion. Indeed, very few
muscles in most movement patterns are under conscious control.
Reflex action, by its very nature, is complex and highly integrated
with other control mechanisms. The complicated relationship between
propioceptive afferents, exteroceptive afferents, and subconscious
levels of control is even now not well understood. What is presented
here is a highly simplified discussion of reflex activity and is
intended as a basic introduction to the concepts of reflex activity.
A simplistic but useful list of the reflexes covered is presented
in Table 4.2.
Although some overlap occurs, as could be inferred from the discussion
of receptors, two main classes of reflexes are related to skeletal
movements—namely, exteroceptive and proprioceptive. Many
of the exteroceptor reflexes exhibited by animals and human beings
are familiar to us. A horse will twitch its skin when flies alight
on it; a dog will scratch when its skin is irritated by a flea,
or perhaps tickled by a person. A human jumps upon hearing a sudden
loud noise. A person also blinks when a foreign body strikes the
eyeball, or even threatens to strike it. Three exteroceptive reflexes
that may be of special interest are the extensor thrust, flexor,
and crossed extensor reflexes.
Pressure against the sole of the foot stimulates the pacinian
corpuscles in the subcutaneous tissue and elicits the reflex contraction
of the extensor muscles of the lower extremity. When the weight is
supported by the feet, the pressure of the floor is sufficient to
bring about this reaction. As the weight is shifted to the balls
of the foot in preparation for a jump, or to the palms of the hand
in preparation for a handspring, the pressure results in the extensor
thrust reflex facilitating contractions of the extensor muscles
of the legs or arms, respectively, thus assisting the push-off from
the floor (Figure 4.11). Similarly in archery, the pacinian corpuscles
in the bow hand are stimulated, and facilitation of the extensor
muscles of the bow arm occurs. However, in this instance, the archer
must counteract this reflex action and prevent full extension at
the elbow or suffer a painful lesson of being whipped by the released
strings. The extensor thrust reflex is often included as a proprioceptive
reflex in some classifications.
Extensor thrust reflex. As the weight is transferred
to the ball of the foot in preparation for the hurdle, the concentrated
pressure on the ball of the foot may enable the extensor thrust
reflex to facilitate the push-off from the ground.
The flexor reflex most frequently operates in response to pain
and is a device for self-protection. Because of the flexor reflex,
we quickly withdraw a part of the body the instant it is hurt. If
a finger is pricked by a pin or if it inadvertently touches a hot
pan, we do not have to decide to remove our hand from the source
of pain; we jerk it back even before we realize what happened to
it. Furthermore, all the necessary muscles for withdrawing it are
activated promptly, not just those in the immediate vicinity of
the injury. Although we are all too aware of the pain, this awareness
occurs after the withdrawal and plays no part in the reflex action.
The value of the awareness is rather in teaching us to avoid repeating
the act that caused the pain. This reflex is also called the nociceptive
This reflex functions cooperatively with the flexor reflex in
response to pain in a weight-bearing limb. For instance, when an
animal injures its paw, the flexor reflex causes it to withdraw
the paw. Simultaneously, owing to the crossed extensor reflex, the
extensor muscles of the opposite limb contract to support the additional
weight thrust upon it. Similarly, if a person who is barefoot happens
to step on a tack with the right foot, the body weight quickly shifts
to the left foot and the right foot is withdrawn from the floor.
As the flexors of the right limb contract to enable the lifting of
the right foot, the extensors of the left limb contract more strongly
to support the weight of the entire body.
Applications of the crossed extensor reflex have also been made
to a non-weight-bearing limb. When a pain stimulus is applied to
one hand, at the same moment that the hand is withdrawn, the opposite
arm will extend as though to push the body away. Some individuals
have also suggested that walking may reflect an application of the
crossed extensor reflex. Release of pressure from the sole of the
back foot results in facilitation of the extensor muscles of the
front leg to accommodate the shift in weight over that foot.
Earlier in this chapter receptors were classified as exteroceptors
and interoceptors, the latter being subdivided into visceral receptors
and proprioceptors. Proprioceptive reflexes are generally described
as those reflexes that occur in response to stimulation of receptors
located in the skeletal muscles, tendons, joints, and labyrinths
of the inner ear. According to this interpretation, the proprioceptive
reflexes are those interoceptors related to motions and positions
of the body. The stretch, or myotatic, reflex is always included
among these. Some classifications also include the extensor thrust,
the labyrinth and neck, and the tendon organ reflex.
The stretch reflex is so named because stretch on the muscle
stimulates the muscle spindle, resulting in the reflex contraction
of the stretched muscle and its synergists and relaxation of its
antagonists. The muscle spindle picks up the stretch stimulus and
transmits it by way of the afferent neuron to the spinal cord. There
the central terminal branches of the sensory neuron synapse directly
with the dendrites of the motor neuron, which activates the same
muscle fibers that were stretched. These fibers then contract. This
two-neuron reflex arc is the simplest type of reflex arc, consisting
of a single sensory neuron and single motor neuron, which may involve
one or more synapses. The important characteristic of this type
of reflex arc is that it does not make use of connector neurons
in the spinal cord.
There are two responses to the stretch reflex—phasic
or tonic—depending on the velocity at which the stretch
occurs. The frequency of discharge from the primary endings of the
muscle spindle is directly related to the velocity at which the
muscle fibers are being stretched. The greater the frequency of
these impulses, the greater will be the resulting muscle fiber contraction.
The phasic type includes familiar clinical tests, such as the
knee jerk. Reflexes of this type are extremely rapid and the contraction
is of brief duration. The word “jerk” gives an
accurate picture. Although it is true that the cause of the stimulus
in this instance is exteroceptive in nature (a rubber-headed hammer
or the edge of the hand), it is nevertheless classed as a proprioceptive reflex.
If the stretch is sudden and sufficiently strong, the stretched
muscle contracts quickly and forcefully because of the response
of the primary endings of the muscle spindles.
A slow stretch, such as is elicited in postural sway, will result
in a tonic response (Figure 4.12). Another demonstration of this
reflex may be observed when a weight is placed in the hand, with the
forearm in 90 degrees of flexion at the elbow joint. The result
is likely to be a subtle depression of the hand and forearm (slight
extension at the elbow) immediately followed by a return movement
Electromyograms: Stretch reflex. Subject is maintaining
the elbow joint at 90 degrees of flexion while holding a bucket.
Arrow (↓) indicates loading the bucket. Tracings
1 and 2 show the biceps and triceps muscle activity when the load
is dropped from decreasing heights (phasic response). Tracing 3
shows muscle activity when the load is placed into the bucket (tonic
Movements that put muscles on a stretch in the backswing or preparatory
phase can take advantage of the stretch reflex. If the desired outcome
is a strong application of force, the preparatory movement should
be rapid, to benefit from the phasic increase in spindle discharge,
and long, to increase the tonic response. The backswing in forceful
striking and throwing patterns, the crouch before a vertical jump,
and the stretch before a tuck dive are all examples (Figure 4.13a).
If, however, the desired outcome is accuracy, as in a badminton
low serve or a golf putt, the backswing should be short and slow,
with a pause before the force application (Figure 4.13b). This approach allows
the phasic frequencies of the primary endings to slow down to tonic
(a) The phasic type of stretch reflex will facilitate
force development if the backswing is long and rapid with little
pause between the backswing and force phase as demonstrated in a
forceful overarm throw. (b) To take advantage of the tonic type
of stretch reflex when accuracy is desired, the backswing should
be short and slow, with a pause before the force phase. An example
is the golf putt.
In the static type of stretch reflex, the muscle is stretched
slowly. This causes primary and secondary endings of several spindles
to be stimulated and results in a more sustained muscular contraction.
The importance of the static stretch is that it causes muscle contraction
as long as a muscle is put on excessive stretch. When such a reflex
is elicited by the stretch caused by the tendency of weight-bearing
joints to flex, the response of the extensor muscles is commonly
referred to as the antigravity reflex. Some
also use this term to include the response of lower extremity and
trunk muscles to the involuntary forward-backward swaying that usually
occurs when a person stands in one position for a long time.
The gamma efferent system functions to adjust the muscle spindle
length by causing the intrafusal fibers to contract (see Figure 4.7).
This mechanism receives signals whenever the alpha motor system stimulates
the extrafusal (skeletal muscle) fibers. Thus, a balance between
the length of the skeletal and spindle fibers is maintained. In
addition, the gamma efferent system is under voluntary control (gamma
bias). For example, this mechanism is at work when a subject anticipates
receiving a weight. Impulses are sent along the gamma system, adjusting
the threshold level of the muscle spindle so that as soon as any
change occurs, a reflex contraction results. Remember what happens
when lifting an empty box after thinking it would be weighted.
The flower-spray endings, or secondary afferents, are less sensitive
to stretch and only result in the tonic response. Therefore, a greater
or prolonged stretch is necessary to stimulate the secondary endings.
Whereas the primary endings always facilitate the contraction of
the muscle in which they are located, the influence of the secondary
afferents depends on the type of muscle in which it is located.
Secondary afferents facilitate flexor muscles and inhibit extensor
muscle contraction. Therefore, the secondary afferent response would
reinforce the primary afferent response if the stimulated spindle
was located in a flexor muscle but would result in a cocontraction
if the stimulated spindle were in an extensor muscle. This would
be due to the reflex contraction facilitated by the primary endings
in the extensor muscle and the facilitation of the flexor muscle
by the secondary ending. Two-joint muscles respond to secondary
afferent stimulation as though they are flexor muscles.
Spindles may also be stimulated by pressure and vibration. This
fact is important whenever pressure must be placed on another individual.
In gymnastics, if pressure is placed on flexor muscles, the gymnast
may fall rather than be helped to hold a position. Similarly, pressure
on the extensor muscles when working in rehabilitation may inhibit
elbow flexion. Use of this technique may provide additional variations
to the rehabilitation setting.
In some instances it would be desirable to minimize the effect
of the stretch reflex, as in flexibility exercises. This can be
done if the stretch is done slowly and is held, as in static stretching.
A quick stretch or bounce will evoke a stronger, phasic, reflex
The second muscle proprioceptor, the Golgi
tendon organ (GTO), is located in series at the musculotendinous
junction. It too is stimulated by tension, but because of its location,
the stretch may be induced on the tendon when the muscle contracts
as well as when the muscle is stretched. The threshold of the GTO
to muscle stretch is higher than that of the muscle spindle. That
is, if the stimulus is strong enough to stimulate the GTO, the muscle
spindle response is overridden. On the other hand, the GTO is very
sensitive to tension when it is produced by tendon stretch resulting
from muscle contraction. In either case, the reflex effect is to inhibit
impulses from the motor nerve to the muscle and its synergists,
thus causing the muscle to relax. Simultaneously, the antagonist
contracts from being facilitated.
When tension on the muscle is extreme, the Golgi tendon organs’ inhibiting
action results in sudden relaxation of the muscle. This effect is
called the lengthening reaction and
undoubtedly serves as protection for muscles or tendons that could
be torn or ruptured by the strong contractile force. Guyton (2000)
has suggested that the tendon reflex serves as a feedback mechanism
to control the tension in the muscle. If the tension in the muscle
became too great, the inhibiting action of the receptors would cause
relaxation. If the muscle tension became too little, the receptors
would stop firing, and the muscle tension would be able to increase.
The behavior of the tendon reflex in the performance of some
skills by beginners is also a matter of interest. It may be that
the reason beginners do not follow through has to do with the fact
that the tendon reflex causes relaxation of vigorously contracting
muscle. It is suggested that until an increase in the Golgi tendon
organ threshold develops with learning, beginners may need voluntary
effort to counteract the inhibitory effect of the tendon reflex.
Newborn infants start out with very simple reflexes, which in
the process of normal motor development are suppressed or modified.
This phenomenon is especially evident in the labyrinth and neck
reflexes. The tonic labyrinth reflexes emanate from utricle and
semicircular canal receptors, which are sensitive to changes in
position of the head with respect to gravity. Because of the primitive
tonic labyrinth reflex present in the newborn child, a supine position
of the head facilitates extension of the extremities, whereas a
prone position inhibits extension and facilitates flexion. At a
few months of age or more, this primitive reflex is suppressed,
and the labyrinth righting reflex is evident. In cooperation with
other righting reflexes of the neck and eyes, it evokes muscular responses
to restore the body to a normal upright position.
Righting reflexes can be demonstrated in adults. They respond
to any body-tilting action by attempting to restore balance through
facilitating limb actions, such as an arm or leg being thrust out.
In spinning, the limbs facilitate restoration of the normal head
position by actions that inhibit the rotation. The arm on the same
side as the direction of the spin is thrust out during the spin,
and the opposite arm is thrust out at the termination of the spin.
This latter action is probably a response to the imbalance caused
by the dizziness experienced at the conclusion of the spin because
of the continuation of movement (inertia) of the fluid and resulting
stimulation of the hair cells.
Tonic neck reflexes are evident when the joint receptors in the
neck are stimulated by any movement in the neck. They are also present
at birth, and even though they are masked by further development,
they remain in adults. Like the tonic labyrinth reflex, they can
also be demonstrated in adults. Actions due to tonic neck reflexes
in the primitive infant form are predictable. If the head is flexed,
flexion will occur in the upper extremities and extension in the
lower ones. With head extension, the opposite occurs—extension
in the upper extremities and flexion in the lower ones. Rotation
of the head to the right is accompanied by extension and abduction
of the limbs on the chin side, and flexion and adduction on the
Tonic neck and tonic labyrinth reflexes become most apparent
in adults in stressful situations. They are hard to distinguish
from each other because movements of the head and neck occur together
and labyrinth and neck receptors are stimulated simultaneously.
In some instances they reinforce each other in their effect on joint
actions of the extremities, and in some instances they oppose each
other. There are times when we should take advantage of them to
facilitate actions and other times when it would be beneficial to
The neck reflexes are more effective in modifying upper extremity
actions and the labyrinthine in modifying those of the lower extremities,
and these results are still accepted. There is no question that
head position influences the actions of the arms. When a strong
pulling action is required, flexion of the head will facilitate
it to some extent. On the other hand, the neck reflex facilitation suggests
that the head should be extended for strong pushing actions. For
facilitating a one-armed pull, the head should be turned away in
addition to being flexed and, in a one-armed push, the head should
be turned toward the pushing arm. Other examples of reflex facilitation
include extension of the head in a handstand to reinforce extension
in the arms, flexion of the head to reinforce trunk, arms, and leg
flexion in forward or backward rolls, and head rotation in archery
to facilitate bow arm pushing on the chin side and bowstring arm
pulling by the opposite side (Deutsch et al. 1987; Gowitzke and
Milner 1988; Le Pellec and Maton 1993).
Undoubtedly one of the difficulties in learning new motor skills
is the failure to suppress reflex responses. In falling backward,
our natural reaction is to extend the arms and throw the head forward.
If a beginner did this in attempting a back dive, the entry into
the water would be uncomfortable, to say the least. The labyrinth
righting reflex must be suppressed consciously so that the head
may be extended back and the body follow. Belly flops in front dives
can also be attributed to the labyrinth righting reflex.
Whether because of reflex action or to some other mechanism,
it seems apparent that the human body has certain provisions for
remaining more or less erect and for engaging in locomotion and that
these follow the general pattern of reflex behavior. The coordinated
efforts of the body to resist the downward pull of gravity include
the extensor thrust reflex, the static type of stretch reflex in
response to gravitational pull, the muscular action evoked by forward-backward
swaying, and the various mechanisms for preserving equilibrium,
including visual orientation and labyrinthine reflexes.
In regard to locomotion, the action of the legs of a four-footed
animal has been attributed to reflex action. Some classify this
reflex as a division of the crossed extensor reflex. Because research
in this area has been done primarily on dogs and cats, it has been
suggested that this reflex exists only in quadrupeds. Nevertheless,
it is logical to assume that it exists also in humans inasmuch as the
early forms of locomotion—creeping and crawling—resemble
the locomotion of quadrupeds. Even after the erect position is assumed,
the swing of the arms in opposition to the lower extremities reflects
the earlier four-footed gait.