Chapter 4: Reflexes & the Reflex Examination

Tap a tendon and a muscle jumps; touch something hot and your hand is gone before you feel pain. These are reflexes, the simplest circuits in the nervous system and some of the most useful signs in clinical medicine. This chapter covers the second half of the "Fenomena Kontraksi & Refleks Otot" session (definition, mechanism and types of reflex) and the "Refleks Tubuh" part of the practicum, where you examine six reflexes on a partner.

What a reflex is

A reflex is any response that happens automatically, without conscious effort. More precisely, it is a rapid, spontaneous and predictable motor reaction to a stimulus acting on a receptor, which happens without passing through conscious control.

Three everyday examples from the session:

• stepping on a drawing pin and pulling the foot away (withdrawal reflex);
• a toddler touching a hot pot and snatching the hand back;
• the knee jerk when the patellar tendon is tapped (stretch reflex).

Reflexes are fast (few synapses, fast-conducting fibers), stereotyped (the same stimulus gives the same response each time) and involuntary (you may become aware of the response, but only after it has started). Awareness comes separately, through collateral branches of the sensory neuron that ascend to the brain.

An adequate stimulus

A reflex appears only if the stimulus is adequate:

1. It matches the receptor. A tendon reflex needs a sharp tap that stretches the muscle spindle; stroking the tendon will not trigger it.
2. It reaches threshold. Too weak a tap gives nothing.
3. It is sudden. A slow, gradual stretch lets the spindle adapt.
4. It is not anticipated. A subject who watches the hammer and tenses up can suppress the response. This is why the examiner distracts the subject in the practicum.

The reflex arc

Every reflex runs through a reflex arc with five components:

1. Sensory receptor: detects the stimulus (a muscle spindle, a pain ending in the skin, a photoreceptor).
2. Sensory (afferent) neuron: carries the impulse to the central nervous system. Its cell body is in the dorsal root ganglion (or a cranial nerve ganglion), and it enters the spinal cord through the dorsal root.
3. Integration center: one or more synapses in the gray matter of the spinal cord or brainstem. In the simplest reflex it is a single synapse between the sensory and motor neurons; in others it includes one or more interneurons.
4. Motor (efferent) neuron: carries the command out of the CNS, leaving the spinal cord through the ventral root.
5. Effector: the muscle or gland that responds.

Reflex time is the delay between stimulus and response. It depends mostly on the length of the pathway and on the number of synapses, because every synapse adds a delay of about 0.5 ms. A monosynaptic stretch reflex is therefore faster than a polysynaptic withdrawal reflex, and both are much faster than a voluntary reaction, which must pass through the cerebral cortex.

Types of reflex

The session classified reflexes in four ways:

Cranial vs spinal

Cranial reflexes are integrated in the brainstem and use cranial nerves: the pupillary light reflex, the corneal (blink) reflex, the gag reflex and the jaw jerk. Spinal reflexes are integrated in the spinal cord and can work even after the cord is cut off from the brain, like the patellar and withdrawal reflexes. In the frog wet lab (Chapter 5), the brain and spinal cord are destroyed precisely so that neither kind of reflex can interfere with the recording.

Somatic vs autonomic

A somatic reflex ends on skeletal muscle through a single motor neuron running from the spinal cord, like withdrawing the hand from a candle flame. An autonomic reflex ends on viscera through a two-neuron efferent chain: a preganglionic neuron from the spinal cord synapses in an autonomic ganglion with a postganglionic neuron that supplies the organ. Examples include the baroreceptor reflex that adjusts heart rate, emptying of the bladder, salivation and the pupillary light reflex (whose effector, the sphincter pupillae, is smooth muscle).

Monosynaptic vs polysynaptic

• A monosynaptic reflex has one synapse, directly between the afferent and efferent neurons. The only true monosynaptic reflexes in humans are the stretch (tendon) reflexes, such as the knee jerk.
• A polysynaptic reflex has two or more synapses, with one or more interneurons in between. Most reflexes are polysynaptic, including the withdrawal reflex and the superficial (skin) reflexes such as the abdominal reflex. Interneurons let one stimulus produce a coordinated response in several muscles, on both sides of the body.

Innate vs learned

Innate reflexes are present from birth and need no learning: blinking, sucking, withdrawing from pain, the tendon reflexes. Learned (acquired) reflexes are built up by practice until they run without conscious attention, like a guitarist's fingers finding a chord or a driver braking at a red light. Because learned responses involve higher brain centers, their reaction time is generally longer than that of innate spinal reflexes. Comparing the two is one of the aims of the reflex practicum.

Two important spinal reflexes

The stretch reflex

Stretch reflexes are what you test with the reflex hammer.

1. A sharp tap on a tendon suddenly stretches the muscle and the muscle spindles inside it. Spindles are stretch receptors made of small intrafusal fibers lying parallel to the ordinary fibers.
2. The spindle's type Ia afferent fibers fire and enter the spinal cord through the dorsal root.
3. In the ventral horn, they synapse directly on alpha motor neurons of the same muscle (one synapse, so monosynaptic).
4. The muscle contracts, shortening and "jerking" the limb.
5. At the same time, Ia collaterals excite inhibitory interneurons that relax the antagonist muscle (reciprocal inhibition), so the joint can move freely.

Stretch reflexes normally help maintain posture and muscle tone: if the knee begins to buckle while you stand, the quadriceps is stretched and contracts reflexly to straighten it.

The Golgi tendon organ does the opposite. It lies in the tendon, senses tension, and through an inhibitory interneuron relaxes the muscle when tension becomes very high. This protects the tendon, and it is why stretching a cramping muscle helps (Chapter 2).

The withdrawal and crossed extensor reflexes

A painful stimulus to the foot triggers a withdrawal (flexor) reflex: through several interneurons, flexors of the whole limb contract and the leg is pulled away. At the same time the crossed extensor reflex stiffens the extensors of the opposite leg so it can take the body's weight. Both are polysynaptic, which is why they spread across several spinal segments and both sides of the cord.

Examining the reflexes

The practicum tests six reflexes. You need a reflex hammer, a penlight and a timer (to record reaction times).

Reflex hammers come in two common shapes: the triangular Taylor hammer and the round-headed Queen Square or Tromner type. Whichever you use, hold it loosely at the end of the handle and let it swing like a pendulum from the wrist, so it strikes briskly and bounces off.

General rules for tendon reflexes:

• The subject must be relaxed, with the muscle in a slightly stretched position.
• Distract the subject (talk to them, ask them to look away) so they do not tense the muscle.
• Strike the tendon, not the muscle belly, with a brisk tap.
• Always compare left and right: asymmetry is more important than absolute briskness.
• If a reflex seems absent, try reinforcement before recording it as absent. In the Jendrassik maneuver, the subject hooks the fingers of both hands together and pulls hard just as you tap the knee or ankle; for arm reflexes, they clench the teeth or make a fist with the other hand.

Tendon reflexes are graded on a standard scale:

The six reflexes of the practicum, at a glance:

A useful mnemonic counts up the segments: "S1–2 buckle my shoe" (ankle), "L3–4 kick the door" (knee), "C5–6 pick up sticks" (biceps), "C7–8 lay them straight" (triceps).

Pupillary light and consensual reflex

Procedure:

1. In a dim room, ask the subject to look at a distant point.
2. Shine the penlight into the right eye from the side.
3. Watch both pupils.
4. Repeat for the left eye and again watch both.

Normal result: the illuminated pupil constricts (direct light reflex) and the other pupil constricts equally (consensual reflex).

Pathway: light → retina → optic nerve (CN II) → optic tract → pretectal nucleus in the midbrain → both Edinger–Westphal nuclei (each pretectal nucleus projects to both sides) → parasympathetic fibers in the oculomotor nerve (CN III) → ciliary ganglion → short ciliary nerves → sphincter pupillae contracts.

Because the signal is shared between both sides in the midbrain, light in either eye constricts both pupils. This lets you localize a lesion:

• Afferent defect (optic nerve): light in the affected eye produces little constriction of either pupil, but light in the healthy eye constricts both. In the swinging flashlight test, moving the light quickly from the healthy eye to the affected one makes both pupils dilate: a relative afferent pupillary defect (RAPD, Marcus Gunn pupil).
• Efferent defect (oculomotor nerve): the affected pupil stays large whichever eye is lit, while the other pupil reacts normally.

Biceps reflex

Procedure:

1. The subject's arm is relaxed, with the elbow partly flexed and the forearm resting on the examiner's forearm or on the subject's lap.
2. Distract the subject.
3. Place your thumb (or index finger) firmly on the biceps brachii tendon in the elbow crease.
4. Strike your own thumb with the reflex hammer.
5. Observe the response.

Normal result: contraction of the biceps and flexion of the elbow. Segment C5–C6, musculocutaneous nerve.

Triceps reflex

Procedure:

1. Flex the subject's elbow to about 90°. Either let the forearm hang while you support the arm at the upper arm, or rest the forearm across the subject's abdomen.
2. Distract the subject.
3. Strike the triceps tendon just above the olecranon.
4. Observe the response.

Normal result: contraction of the triceps and extension of the elbow. Segment C7 (C6–C8), radial nerve.

Abdominal reflex

The abdominal reflex is a superficial (cutaneous) reflex. Unlike tendon reflexes, it is polysynaptic and its circuit loops up through the brain.

Procedure:

1. The subject lies supine and relaxed, abdomen exposed.
2. With a blunt pointed object (the pointed end of a reflex hammer, a wooden stick or a key; never a sharp needle), stroke the skin briskly from the side toward the umbilicus.
3. Do this in each region: epigastric, supraumbilical, umbilical and infraumbilical, on both sides.
4. Watch for the abdominal skin wrinkling as the muscle contracts.

Normal result: the abdominal muscles under the stroke contract and the umbilicus moves toward the stimulated side. Upper abdomen T8–T9, around the umbilicus T10, lower abdomen T11–T12.

The reflex may be absent in healthy people with obese or lax abdominal walls, after abdominal surgery or after several pregnancies. Its loss on one side together with brisk tendon reflexes suggests an upper motor neuron lesion.

Patellar reflex (knee jerk)

Procedure:

1. The subject sits on the edge of the table with the legs hanging freely, feet off the floor.
2. The examiner's free hand rests on the subject's knee or thigh to feel the quadriceps contract.
3. Strike the patellar ligament just below the kneecap.
4. Observe the response.

Normal result: contraction of the quadriceps femoris and extension of the knee (the leg kicks forward). Segment L2–L4, femoral nerve. Use the Jendrassik maneuver if the reflex seems absent.

Achilles reflex (ankle jerk)

Procedure:

1. The subject sits with the legs hanging (or kneels on a chair with the feet over the edge).
2. Hold the foot in slight dorsiflexion to put the calf muscles on a little stretch.
3. Strike the Achilles tendon just above the heel.
4. Observe the response.

Normal result: contraction of the gastrocnemius and soleus and plantar flexion of the foot. Segment S1 (S1–S2), tibial nerve. A slowly relaxing ankle jerk is a classic sign of hypothyroidism.

Interpreting abnormal reflexes

Key points

• A reflex is a rapid, predictable, involuntary motor response to an adequate stimulus: appropriate to the receptor, above threshold, sudden and unanticipated.
• Reflex arc: receptor → sensory neuron → integration center → motor neuron → effector.
• Reflexes are classified as cranial or spinal, somatic or autonomic, monosynaptic or polysynaptic, and innate or learned. Stretch reflexes are the only monosynaptic reflexes.
• Tendon taps stretch muscle spindles; Ia afferents excite the alpha motor neurons of the same muscle and inhibit its antagonist.
• Practicum reflexes: pupillary light (CN II → CN III), biceps (C5–6), triceps (C7), abdominal (T8–12), patellar (L3–4), Achilles (S1).
• Distract the subject, compare sides, grade 0–4+, and reinforce before calling a reflex absent. Brisk reflexes point to an upper motor neuron lesion, absent ones to a lower motor neuron lesion.