Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 9

Reflexes 159

CLINICAL BOX 9–1

Knee Jerk Reflex

Tapping the patellar tendon elicits the

knee jerk,

a stretch

reflex of the quadriceps femoris muscle, because the tap on

the tendon stretches the muscle. A similar contraction is ob-

served if the quadriceps is stretched manually. Stretch re-

flexes can also be elicited from most of the large muscles of

the body. Tapping on the tendon of the triceps brachii, for

example, causes an extensor response at the elbow as a re-

sult of reflex contraction of the triceps; tapping on the Achil-

les tendon causes an ankle jerk due to reflex contraction of

the gastrocnemius; and tapping on the side of the face

causes a stretch reflex in the masseter. The knee jerk reflex is

an example of a

deep tendon reflex (DTR)

in a neurological

exam and is graded on the following scale: 0 (absent), 1+ (hy-

poactive), 2+ (brisk, normal), 3+ (hyperactive without clonus),

4+ (hyperactive with mild clonus), and 5+ (hyperactive with

sustained clonus). Absence of the knee jerk can signify an ab-

normality anywhere within the reflex arc, including the mus-

cle spindle, the Ia afferent nerve fibers, or the motor neurons

to the quadriceps muscle. The most common cause is a pe-

ripheral neuropathy from such things as diabetes, alcoholism,

and toxins. A hyperactive reflex can signify an interruption of

corticospinal and other descending pathways that influence

the reflex arc.

FIGURE 9–2
Mammalian muscle spindle. A)
Diagrammatic representation of the main components of mammalian muscle spindle includ-
ing intrafusal muscle fibers, afferent sensory fiber endings, and efferent motor fibers (
γ
-motor neurons).
B)
Three types of intrafusal muscle fibers:
dynamic nuclear bag, static nuclear bag, and nuclear chain fibers. A single Ia afferent fiber innervates all three types of fibers to form a primary
sensory ending. A group II sensory fiber innervates nuclear chain and static bag fibers to form a secondary sensory ending. Dynamic
γ
-motor neu-
rons innervate dynamic bag fibers; static
γ
-motor neurons innervate combinations of chain and static bag fibers.
C)
Comparison of discharge pat-
tern of Ia afferent activity during stretch alone and during stimulation of static or dynamic
γ
-motor neurons. Without
γ
-stimulation, Ia fibers show
a small dynamic response to muscle stretch and a modest increase in steady-state firing. When static
γ
-motor neurons are activated, the steady-
state response increases and the dynamic response decreases. When dynamic
γ
-motor neurons are activated, the dynamic response is markedly
increased but the steady-state response gradually returns to its original level.
(From Kandel ER, Schwartz JH, Jessell TM [editors]:
Principles of Neural Science,
4th
ed. McGraw-Hill, 2000.)

Afferent

axons

Efferent

axons

Gamma

motor

endings

Sensory

endings

Capsule

Intrafusal

muscle

fibers

Ia

II

Static

Dynamic

A Muscle spindle

Dynamic nuclear

bag fiber

Static nuclear

bag fiber

Nuclear

chain fiber

B Intrafusal fibers of the muscle spindle C Response of Ia sensory fiber to selective

activation of motor neurons

Stretch alone

Dynamic response

Steady-state response

0

Stimulate static gamma fiber

200

0

Stimulate dynamic gamma fiber

Stretch

Imp/s

200

0

6

0.2 s

Imp/s

0

200

Imp/s