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Motor Function
Reflexes
Reflexes are involuntary and relatively stereotyped responses to specific stimuli. Afferent
nerve fibers conduct the impulses generated by
activated receptors to neurons in the central
nervous system, which fire impulses that are
then transmitted through efferent nerve fibers
to the cells, muscles, or organs that carry out the
reflex response. The pathway as a whole is
known as the reflex arc. Receptors are found at
the origin of all sensory pathways—in the skin,
mucous membranes, muscles, tendons, and periosteum, as well as in the retina, inner ear, olfactory mucosa, and taste buds. A reflex response may involve the somatic musculature or
the internal organs. Most reflexes are relatively
independent of the state of consciousness. An
interruption of the reflex arc at any point
weakens or abolishes the reflex. Intrinsic reflexes
are those whose receptors and effectors are located in the same organ (e. g., the quadriceps reflex), while the receptors and effectors of extrinsic reflexes are in different organs (e. g., the
oculovestibular reflex). Reflexes are important
for normal function (e. g., for postural control
and goal-directed movement), and an impaired
reflex is an important objective finding in clinical neurological examination.
Intrinsic Muscle Reflexes (Phasic Stretch
Reflexes, Tendon Reflexes)
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Intrinsic muscle reflexes are triggered by stretch
receptors within the muscle (annulospiral nerve
endings of muscle spindles). The impulses
generated at the receptors are conveyed via afferent fast-twitch Ia fibers to spinal alpha-motor
neurons, whose efferent α1 processes excite the
agonistic muscle of an opposing muscle pair.
The antagonistic muscle is simultaneously inhibited by spinal interneurons. The resulting
muscle contraction relaxes the muscle spindles,
thereby stopping impulse generation at the
stretch receptors. The spinal reflex arc is also
under the influence of higher motor centers.
Abnormal reflex responses imply an abnormality of the musculature, the reflex arc, or higher
motor centers. The most important reflexes in
clinical diagnosis are the biceps (C5–C6), brachioradialis (C5–C6), triceps (C7–C8), adductor
(L2–L4), quadriceps (L2/3–L4), posterior tibial
(L5), and Achilles (S1–S2) reflexes.
Extrinsic Reflexes
Intrinsic muscle reflexes, discussed above, are
monosynaptic, but extrinsic reflexes are polysynaptic: between their afferent and efferent arms
lies a chain of spinal interneurons. They may be
activated by stimuli of various types, e. g.,
muscle stretch, touch on the skin (abdominal reflex) or cornea (corneal reflex), mucosal irritation (sneezing), light (eye closure in response to
a bright flash), or sound (acoustic reflex). The intensity of the response diminishes if the
stimulus is repeated (habituation). Because they
are polysynaptic, extrinsic reflexes have a longer
latency (stimulus-to-response interval) than intrinsic reflexes. Some important extrinsic reflexes for normal function are the postural and
righting reflexes, feeding reflexes (sucking,
swallowing, licking), and autonomic reflexes
(p. 110).
The flexor reflex is triggered by noxious stimulation, e. g., from stepping on a tack. Excitatory interneurons activate spinal cord alpha-motor
neurons, which, in turn, excite ipsilateral flexor
muscles and simultaneously inhibit ipsilateral
extensor muscles via inhibitory interneurons.
Meanwhile, the contralateral extensors contract, and the contralateral flexors relax. The response does not depend on pain, which is felt
only when sensory areas in the brain have been
activated, by which time the motor response has
already occurred. This spinal reflex arc, like that
of the intrinsic muscle reflexes, is under the influence of higher motor centers.
Abnormalities of the extrinsic reflexes imply an
interruption of the reflex arc or of the corticospinal tracts (which convey impulses from
higher motor centers). Some clinically important extrinsic reflexes are the abdominal (T6–
T12), cremasteric (L1–L2), bulbocavernosus (S3–
S4), and anal wink (S3–S5) reflexes.
Reflexes that can be elicited only in the diseased
state are called pathological reflexes. Pathological reflexes indicating dysfunction of the pyramidal (corticospinal) tract include the Babinski sign (tonic dorsiflexion of the great toe on
stimulation of the lateral sole of the foot), the
Gordon reflex (same response to squeezing of
the calf muscles), and the Oppenheim reflex
(same response to a downward stroke of the examiner’s thumb on the patient’s shin).
Rohkamm, Color Atlas of Neurology © 2004 Thieme
All rights reserved. Usage subject to terms and conditions of license.
Reflexes
Symbol
0
Can only be elicited by maneuvers
(e.g., Jendrassik maneuver)
+
_
Diminished
Normal intensity
Heightened
Persistent clonus
1
2
3
4
Extensor muscle
Reflex response
(Proprioceptive muscle reflex)
Afferent (Ia) fiber
Efferent fiber (excitatory)
Efferent fiber (inhibitory)
Flexor muscle
Pseudounipolar nerve cells in
spinal ganglion
Supraspinal control
(inhibitory)
Afferent fiber
Proprioceptive
(intrinsic) muscle
reflex
Motor Function
Reflex response
Absent, cannot be elicited
by maneuvers
Efferent to
extensors
Annulospiral
ending of
muscle spindle
Extensor
Excitatory synapse
Efferent fibers to
contralateral extensors
and flexors
Interneuron
Flexor
Inhibitory synapse
Efferent fibers to
ipsilateral flexors
and extensors
Efferent to flexors
Afferent (Ia) fiber
Free ending of
afferent fiber (pain,
temperature)
Supraspinal
control
(inhibitory)
Interneurons
Pressure receptor
(Vater-Pacini corpuscle)
Afferent fiber
Fibers to contralateral side of
commissural cell
Inhibitory synapse
Excitatory synapse
Extrinsic muscle reflex
Extensor muscle
Rohkamm, Color Atlas of Neurology © 2004 Thieme
All rights reserved. Usage subject to terms and conditions of license.
Flexor muscle
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