Motor Functions of the Spinal Cord pdf - The Nervous System

roots. After entering the cord, every sensory signal travels to two separate

The cord gray matter is the integrative area for the cord reflexes. Figure 54–1

Organization of the Spinal Cord

sic excitatory motor functions of the cord itself.

sensory input signals to the cord. Using this preparation, one can study the intrin-

reflexes become very excitable and, therefore, easy to activate by even the slightest

the spinal cord motor control circuits. The result is that the spinal cord motor

these nuclei to become tonically active, transmitting facilitatory signals to most of

centers of the brain to the pontile and vestibular muscle control nuclei. This allows

blocks normal inhibitory signals

encephalic level, which

study.

a few days to weeks in monkeys, most of the intrinsic spinal cord functions return

the level of transection is severely depressed. After a few hours in rats and cats or

still remains functional, and (2) the

which the spinal cord is transected, frequently in the neck so that most of the cord

been especially useful in studying spinal cord function: (1) the

Two types of experimental preparations have

circuits provide all but a small fraction of the direct control of the muscles.

ronal circuits of the spinal cord that are the objects of the commands. These

“command” signals generated in the brain. But it also requires the many neu-

tinuously and control equilibrium. All this is done through “analytical” and

change the movements from walking to jumping as needed, and to monitor con-

ments when they are required, to lean the body forward during acceleration, to

Let us not belittle the role of the brain, however, because the brain gives

signals to the spinal cord to set into motion the walking process.

Instead, the circuits for these movements are in the cord, and the brain simply

causes the specific to-and-fro movement of the legs that is required in walking.

To give an example, there is no neuronal circuit anywhere in the brain that

Without the special neuronal circuits of the cord, even the most complex motor

In this chapter, we discuss the control of muscle function by the spinal cord.

brum, where the most complicated muscle skills are

plicated responses, and finally extend to the cere-

reflexes, extend into the brain stem with more com-

We have already noted that sensory information is

Cord; the Cord Reflexes

673

Motor Functions of the Spinal

integrated at all levels of the nervous system and
causes appropriate motor responses that begin in
the spinal cord with relatively simple muscle

controlled.

control systems in the brain could not cause any purposeful muscle movement.

sends command

directions that control the sequential cord activities—to promote turning move-

Spinal Animal and the Decerebrate Animal.

spinal animal, in

decerebrate animal, in which the brain stem is

transected in the middle to lower part of the mesencephalon.

Immediately after preparation of a spinal animal, most spinal cord function below

to nearly normal and provide a suitable experimental preparation for research

In the decerebrate animal, the brain stem is transected at the middle to lower mes-

from the higher control

for Motor Functions

shows the typical organization of the cord gray matter in a single cord segment.
Sensory signals enter the cord almost entirely through the sensory (posterior)

where they are appropriately processed. Thus, in

signals are transmitted first through interneurons,

the anterior motor neurons. Instead, almost all these

types of circuits. In this chapter, we examine many

diverging, converging, repetitive-discharge,

interneuron pool of cells of the spinal cord, including

chapter.

in Figure 54–1. The interconnections among the

directly with the anterior motor neurons, as shown

with one another, and many of them also synapse

times per second. They have many interconnections

neous activity and capable of firing as rapidly as 1500

small and highly excitable, often exhibiting sponta-

as numerous as the anterior motor neurons. They are

shown in Figure 54–1. These cells are about 30 times

horns, and the intermediate areas between them, as

the cord gray matter—in the dorsal horns, the anterior

later in this chapter.

which helps control basic muscle “tone,” as discussed

muscle spindle,

shown in Figure 54–2. These

intrafusal fibers,

eter, which go to small, special skeletal muscle fibers

motor nerve fibers, averaging 5 micrometers in diam-

impulses through much smaller type A gamma (A

anterior horns. These gamma motor neurons transmit

muscle fibers, about one half as many much smaller

neurons, which excite contraction of the skeletal

Gamma Motor Neurons.

Transmission of

three to several hundred skeletal muscle fibers, which

innervate the large skeletal muscle fibers. Stimulation

averaging 14 micrometers in diameter; these fibers

) motor nerve fibers,

rise to large type A alpha (A

The alpha motor neurons give

Alpha Motor Neurons.

neurons.

of two types,

innervate the skeletal muscle fibers. The neurons are

They give rise to the nerve fibers that leave

neurons.

interneurons.

are of two types: (1)

discussed in Chapters 47 and 48, the other neurons

gray matter. Aside from the sensory relay neurons

bral cortex, as described in earlier chapters.

in the cord itself, to the brain stem, or even to the cere-

local effects. (2) Another branch transmits signals to

destinations: (1) One branch of the sensory nerve ter-

The Nervous System: C. Motor and Integrative Neurophysiology

674

Unit XI

minates almost immediately in the gray matter of the
cord and elicits local segmental cord reflexes and other

higher levels of the nervous system—to higher levels

Each segment of the spinal cord (at the level of

each spinal nerve) has several million neurons in its

anterior motor neurons and (2)

Anterior Motor Neurons.

Located in each segment of the

anterior horns of the cord gray matter are several
thousand neurons that are 50 to 100 per cent larger
than most of the others and are called anterior motor

the cord by way of the anterior roots and directly

alpha motor neurons and gamma motor

branch many times after they enter the muscle and

of a single alpha nerve fiber excites anywhere from

are collectively called the motor unit.
nerve impulses into skeletal muscles and their stimu-
lation of the muscle motor units are discussed in
Chapters 6 and 7.

Along with the alpha motor

gamma motor neurons are located in the spinal cord

called
fibers constitute the middle of the

Interneurons.

Interneurons are present in all areas of

interneurons and anterior motor neurons are respon-
sible for most of the integrative functions of the spinal
cord that are discussed in the remainder of this

Essentially all the different types of neuronal cir-

cuits described in Chapter 46 are found in the

and other

applications of these different circuits in the perform-
ance of specific reflex acts by the spinal cord.

Only a few incoming sensory signals from the spinal

nerves or signals from the brain terminate directly on

Sensory root

Solitary cell

External basal cells

Corticospinal tract

Interneurons

Anterior motor
neurons

Motor root

cord.

with the interneurons and anterior motor neurons of the spinal

Connections of peripheral sensory fibers and corticospinal fibers

Figure 54–1

Motor

Sensory

Motor

Sheath

Primary

Alpha motor

ending

Gamma motor

ending

Fluid

cavity

ending

1 cm

Extrafusal

fibers

Secondary

ending

Intrafusal

fibers

tal muscle fibers. Note also both motor and sensory innervation of

Muscle spindle, showing its relation to the large extrafusal skele-

Figure 54–2

the muscle spindle.

type Ia fiber averaging 17 micrometers in diameter,

This nerve fiber is a

annulospiral ending.

of each intrafusal fiber, forming the so-called

large sensory nerve fiber encircles the central portion

In the center of the receptor area, a

secondary ending.

central receptor area of the muscle spindle. They are

Two types of sensory endings are found in this

of the spindle and therefore excites the receptor.

spindle’s intrafusal fibers stretches the midportion

change, contraction of the end portions of the

2. Even if the length of the entire muscle does not

the receptor.

midportion of the spindle and, therefore, excites

1. Lengthening the whole muscle stretches the

stretching of this midportion of the spindle. One can

fibers originate in this area. They are stimulated by

Figure 54–2 and in more detail in Figure 54–3, sensory

myosin and actin contractile elements. As shown in

this area, the intrafusal muscle fibers do not have

portion of the muscle spindle is its central portion. In

The receptor

Sensory Innervation of the Muscle Spindle.

muscle.

(type A alpha

gamma efferent fibers,

as described earlier. These gamma motor nerve fibers

that originate from small type A gamma motor

receptor, as described later. The end portions that

when the ends do. Instead, it functions as a sensory

ments.Therefore, this central portion does not contract

of these fibers—that is, the area midway between

muscle fiber. However, the central region of each

fibers.

54–2. Each spindle is 3 to 10 millimeters long. It is built

organization of the muscle spindle is shown in Figure

The

Structure and Motor Innervation of the Muscle Spindle.

Receptor Function of the

subconscious level. Even so, they transmit tremendous

muscle control. They operate almost completely at a

The signals from these two receptors are either

(see Figure 54–7), which are located in

length or rate of change of length, and (2)

(see Figure 54–2), which are distrib-

dantly with two special types of sensory receptors: (1)

tion, the muscles and their tendons are supplied abun-

length or tension changing? To provide this informa-

is its instantaneous tension, and how rapidly is its

instant. That is, what is the length of the muscle, what

information from each muscle to the spinal cord, indi-

Roles in Muscle Control

Tendon Organs—And Their

Muscle Sensory Receptors—

limbs.

later in this chapter, including reflexes that coordinate

transmit signals to many segments. These ascending and

transmit signals to only a segment or two, while others

both up and down the spinal cord; some of the branches

from the posterior cord roots, they bifurcate and branch

another. In addition, as the sensory fibers enter the cord

These fibers run from one segment of the cord to

propriospinal fibers.

Multisegmental Connections from One Spinal Cord Level

signals to spread laterally.

desired direction while suppressing the tendency for

sensory system uses the same principle—that is, to allow

focus, or sharpen, its signals in the same way that the

reason: The motor system uses this lateral inhibition to

This effect is important for the following major

adjacent motor neurons, an effect called

Thus, stimulation of each motor neuron tends to inhibit

mit inhibitory signals to the surrounding motor neurons.

cent Renshaw cells. These are

neuron, collateral branches from the axon pass to adja-

Renshaw cells.

motor neurons, are a large number of small neurons

horns of the spinal cord, in close association with the

Renshaw Cell Inhibitory System.

rons, where the signals from this tract are combined

Figure 54–1, the corticospinal tract from the brain is

Motor Functions of the Spinal Cord; the Cord Reflexes

Chapter 54

675

shown to terminate almost entirely on spinal interneu-

with signals from other spinal tracts or spinal nerves
before finally converging on the anterior motor
neurons to control muscle function.

Also located in the anterior

called

Almost immediately after the

anterior motor neuron axon leaves the body of the

inhibitory cells that trans-

lateral inhibi-

tion.

unabated transmission of the primary signal in the

to Other Levels—Propriospinal Fibers

More than half of all the nerve fibers that ascend and
descend in the spinal cord are

descending propriospinal fibers of the cord provide
pathways for the multisegmental reflexes described

simultaneous movements in the forelimbs and hind

Muscle Spindles and Golgi

Proper control of muscle function requires not only
excitation of the muscle by spinal cord anterior motor
neurons but also continuous feedback of sensory

cating the functional status of each muscle at each

muscle spindles
uted throughout the belly of the muscle and send
information to the nervous system about muscle

Golgi

tendon organs
the muscle tendons and transmit information about
tendon tension or rate of change of tension.

entirely or almost entirely for the purpose of intrinsic

amounts of information not only to the spinal cord but
also to the cerebellum and even to the cerebral cortex,
helping each of these portions of the nervous system
function to control muscle contraction.

Muscle Spindle

around 3 to 12 very small intrafusal muscle fibers that
are pointed at their ends and attached to the glycoca-
lyx of the surrounding large extrafusal skeletal muscle

Each intrafusal muscle fiber is a very small skeletal

its two ends—has few or no actin and myosin fila-

do contract are excited by small gamma motor nerve
fibers
neurons in the anterior horns of the spinal cord,

are also called

in contradistinc-

tion to the large alpha efferent fibers
nerve fibers) that innervate the extrafusal skeletal

readily see that the muscle spindle receptor can be
excited in two ways:

the primary ending and the

Primary Ending.

primary

ending or

and it transmits sensory signals to the spinal cord at a

stretched suddenly, excitation of the spindles causes

Whenever a muscle is

muscle stretch reflex.

The simplest manifestation of muscle spindle function

—that is, increased numbers of

firing. Thus, the spindles can send to the spinal cord

the muscle spindles increases the rate of firing,

emit sensory nerve impulses continuously. Stretching

degree of gamma nerve excitation, the muscle spindles

Normally, particularly when there is some

little influence on the dynamic response. Subsequent

chain fibers, enhances the static response while having

tion of the gamma-s fibers, which excite the nuclear

static response is hardly affected. Conversely, stimula-

spindle becomes tremendously enhanced, whereas the

nuclear bag fibers, the dynamic response of the muscle

intrafusal fibers. When the gamma-d fibers excite the

fibers, and the second excites mainly the nuclear chain

dynamic (gamma-d) and gamma-static (gamma-s).The

muscle spindle can be divided into two types: gamma-

The gamma motor nerves to the

Gamma Motor Nerves.

any change in length of the spindle receptor.

or negative, signals to the spinal cord to apprise it of

primary ending sends extremely strong, either positive

exactly opposite sensory signals occur. Thus, the

Conversely, when the spindle receptor shortens,

length stops increasing, this extra rate of impulse dis-

while the length is actually increasing.

large 17-micrometer sensory nerve fiber,

in a fraction of a second, the primary receptor trans-

only a fraction of a micrometer, if this increase occurs

rate of change

dynamic response,

especially powerfully. This excess stimulus of the

the spindle receptor increases suddenly, the primary

When the length of

ondary Ending) to Rate of Change of Receptor

of the spindle receptor,

mit these impulses for several minutes. This effect is

degree of stretching, and the endings continue to trans-

slowly,

When the receptor portion of the muscle

Endings to the Length of the Receptor—“Static”

tions are shown in Figure 54–3.

usually excited only by nuclear chain fibers. These rela-

the nuclear chain fibers. Conversely, the sec-

as shown by the bottom fiber in the figure. The primary

(three to nine), which are about half as large in diam-

nuclear chain fibers

top fiber in Figure 54–3, and (2)

central portion of the receptor area, as shown by the

nuclei are congregated in expanded “bags” in the

three in each spindle), in which several muscle fiber

intrafusal fibers: (1)

There are also two types of muscle spindle

that the type Ia fiber does, but often it spreads like

ending, as shown in Figures 54–2 and 54–3. This

nerve fiber in the entire body.

velocity of 70 to 120 m/sec, as rapidly as any type of

The Nervous System: C. Motor and Integrative Neurophysiology

676

Unit XI

Secondary Ending.

Usually one but sometimes two

smaller sensory nerve fibers—type II fibers with an
average diameter of 8 micrometers—innervate the
receptor region on one or both sides of the primary

sensory ending is called the secondary ending; some-
times it encircles the intrafusal fibers in the same way

branches on a bush.

Division of the Intrafusal Fibers into Nuclear Bag and Nuclear
Chain Fibers—Dynamic and Static Responses of the Muscle
Spindle.

nuclear bag muscle fibers (one to

eter and half as long as the nuclear bag fibers and have
nuclei aligned in a chain throughout the receptor area,

sensory nerve ending (the 17-micrometer sensory
fiber) is excited by both the nuclear bag intrafusal
fibers and
ondary ending (the 8-micrometer sensory fiber) is

Response of Both the Primary and the Secondary

Response.

spindle is stretched

the number of impulses

transmitted from both the primary and the secondary
endings increases almost directly in proportion to the

called the static response
meaning simply that both the primary and secondary
endings continue to transmit their signals for at least

several minutes if the muscle spindle itself remains
stretched.

Response of the Primary Ending (but Not the Sec-

Length—“Dynamic” Response.

ending (but not the secondary ending) is stimulated

primary ending is called the

which

means that the primary ending responds extremely
actively to a rapid

in spindle length.

Even when the length of a spindle receptor increases

mits tremendous numbers of excess impulses to the

but only

As soon as the

charge returns to the level of the much smaller static
response that is still present in the signal.

Control of Intensity of the Static and Dynamic Responses by the

first of these excites mainly the nuclear bag intrafusal

paragraphs illustrate that these two types of muscle
spindle responses are important in different types of
muscle control.

Continuous Discharge of the Muscle Spindles Under Normal
Conditions.

whereas shortening the spindle decreases the rate of

either positive signals
impulses to indicate stretch of a muscle—or negative
signals—below-normal numbers of impulses to indi-
cate that the muscle is unstretched.

Muscle Stretch Reflex

is the

Group II fiber
(secondary
efferent)

Nuclear bag fiber
(intrafusal muscle)

Nuclear chain fiber
(intrafusal muscle)

Plate

ending

Dynamic

fiber

(efferent)

Static

fiber

(efferent)

Trail ending

Group Ia fiber
(primary afferent)

control of movement. Physiol Rev 54:225, 1974.)

chain muscle spindle fibers. (Modified from Stein RB: Peripheral

Details of nerve connections from the nuclear bag and nuclear

Figure 54–3

selves be jerky. This effect can also be called a

smooth muscle contractions, even though the primary

cally demonstrates the damping mechanism’s ability to

unsmooth muscle contraction. Thus, curve A graphi-

nerves had been sectioned 3 months earlier. Note the

signals per second. Curve B illustrates the same exper-

the muscle is excited at a slow frequency of only 8

is relatively smooth, even though the motor nerve to

the excited muscle is intact. Note that the contraction

Figure 54–5. In curve A, the muscle spindle reflex of

course of such a signal. This effect is demonstrated in

factorily, the muscle contraction is jerky during the

changing to another intensity level, and so forth. When

a few milliseconds, then decreasing in intensity, then

muscle in an unsmooth form, increasing in intensity for

, or smoothing, function,

movements. This is a

Stretch Reflexes

person’s nervous system specifically wills otherwise.

to remain reasonably constant, except when the

secondary endings. The importance of the static stretch

after. This reflex is elicited by the continuous static

stretch reflex

unstretched) to its new length, but then a weaker

The dynamic stretch reflex is over within a fraction

the reflex functions to oppose sudden changes in muscle

same muscle from which the signal originated. Thus,

to the spinal cord; this causes an instantaneous strong

stretched or unstretched, a strong signal is transmitted

or unstretch. That is, when a muscle is suddenly

endings of the muscle spindles, caused by rapid stretch

dynamic stretch reflex

The

stretch reflex can be divided into two components: the

The

serve other functions.

interneurons in the cord gray matter, and these trans-

muscle after excitation of the spindle. Most type II

monosynaptic pathway

which the muscle spindle fiber originated. Thus, this is

dorsal root of the spinal cord. A branch of this fiber

stretch reflex, showing a type Ia proprioceptor nerve

Figure 54–4

Neuronal Circuitry of the Stretch Reflex.

gistic muscles.

Motor Functions of the Spinal Cord; the Cord Reflexes

Chapter 54

677

reflex contraction of the large skeletal muscle fibers of
the stretched muscle and also of closely allied syner-

demonstrates the basic circuit of the muscle spindle

fiber originating in a muscle spindle and entering a

then goes directly to the anterior horn of the cord gray
matter and synapses with anterior motor neurons that
send motor nerve fibers back to the same muscle from

that allows a reflex signal to

return with the shortest possible time delay back to the

fibers from the muscle spindle terminate on multiple

mit delayed signals to the anterior motor neurons or

Dynamic Stretch Reflex and Static Stretch Reflexes.

dynamic stretch reflex and the static stretch reflex.

is elicited by the potent

dynamic signal transmitted from the primary sensory

reflex contraction (or decrease in contraction) of the

length.

of a second after the muscle has been stretched (or

static

continues for a prolonged period there-

receptor signals transmitted by both primary and

reflex is that it causes the degree of muscle contraction

“Damping” Function of the Dynamic and Static

An especially important function of the stretch reflex
is its ability to prevent oscillation or jerkiness of body

damping

as explained in the following paragraph.

Damping Mechanism in Smoothing Muscle Contraction.

Signals from the spinal cord are often transmitted to a

the muscle spindle apparatus is not functioning satis-

iment in an animal whose muscle spindle sensory

input signals to the muscle motor system may them-

signal

averaging function of the muscle spindle reflex.

Motor nerve

Stretch reflex

Muscle spindle

Sensory nerve

Neuronal circuit of the stretch reflex.

Figure 54–4

Stimulus

(8 per second)

Seconds

Force of contraction

sity Press, 1932.)

et al: Reflex Activity of the Spinal Cord. New York: Oxford Univer-

of the muscle spindle reflex in curve A. (Modified from Creed RS,

rior roots of the cord 82 days previously. Note the smoothing effect

whose muscle spindles were denervated by section of the poste-

ditions: curve A, in a normal muscle, and curve B, in a muscle

Muscle contraction caused by a spinal cord signal under two con-

Figure 54–5

the degree of facilitation of spinal cord centers. When

The muscle jerks are used by neurologists to assess

other words, sudden stretch of muscle spindles is all that

muscle or by striking the belly of the muscle itself. In

part of Figure 54–6 shows a myogram from the quadri-

that causes the lower leg to “jerk” forward. The upper

dynamic stretch reflex

with a reflex hammer; this instantaneously stretches the

elicit the knee jerk and other muscle jerks.The knee jerk

Clinically, a method used

spinal cord. This reflex is elicited as follows.

ground excitation, or “tone,” the brain is sending to the

reflexes. The purpose is to determine how much back-

ination on a patient, he or she elicits multiple stretch

Clinical Applications of the

procedures.

joints. This aids tremendously in performing the

tioning, excitation of the appropriate muscle spindles

opposing each other at the joint. The net effect is that

joint also increases, producing tight, tense muscles

sides of each joint are activated at the same time, reflex

their signal output. However, if the spindles on both

stretches the central receptor regions, thus increasing

spindles. This shortens the ends of the spindles and

tense motor action. To do this, the bulboreticular facil-

Position During Tense Action

during walking and running.

high density of muscle spindles, emphasis is given to

ticularly concerned with antigravity contractions, and

control of the gamma efferent system. However,

basal ganglia,

the brain stem and, secondarily, by impulses transmit-

The gamma efferent system is excited specifically by

Brain Areas for Control of the Gamma

be flail and sometimes be overstretched, in neither

tract and relax along with the large muscle fibers,

length. For instance, if the muscle spindle did not con-

the muscle spindle, regardless of any change in muscle

Second, it maintains the proper damping function of

contraction. Therefore, coactivation keeps the muscle

muscle fibers contract is twofold: First, it keeps the

The purpose of contracting the muscle spindle intra-

muscle fibers to contract at the same time.

motor neurons. This causes both the extrafusal skele-

gamma motor neurons are stimulated simultaneously,

brain to the alpha motor neurons, in most instances the

alpha motor fibers. Whenever signals are transmitted

A gamma efferent fibers rather than large type A

system, one needs to recognize that 31 per cent of all

To emphasize the importance of the gamma efferent

in Voluntary Motor Activity

Role of the Muscle Spindle

The Nervous System: C. Motor and Integrative Neurophysiology

678

Unit XI

the motor nerve fibers to the muscle are the small type

from the motor cortex or from any other area of the

an effect called coactivation of the alpha and gamma

tal muscle fibers and the muscle spindle intrafusal

fusal fibers at the same time that the large skeletal

length of the receptor portion of the muscle spindle
from changing during the course of the whole muscle

spindle reflex from opposing the muscle contraction.

the receptor portion of the spindle would sometimes

instance operating under optimal conditions for
spindle function.

Motor System

signals from the bulboreticular facilitatory region of

ted into the bulboreticular area from (1) the cerebel-
lum, (2) the

and (3) the cerebral cortex.

Little is known about the precise mechanisms of

because the bulboreticular facilitatory area is par-

because the antigravity muscles have an especially

the importance of the gamma efferent mechanism for
damping the movements of the different body parts

Muscle Spindle System Stabilizes Body

One of the most important functions of the muscle
spindle system is to stabilize body position during

itatory region and its allied areas of the brain stem
transmit excitatory signals through the gamma nerve
fibers to the intrafusal muscle fibers of the muscle

excitation of the skeletal muscles on both sides of the

the position of the joint becomes strongly stabilized,
and any force that tends to move the joint from its
current position is opposed by highly sensitized stretch
reflexes operating on both sides of the joint.

Any time a person must perform a muscle function

that requires a high degree of delicate and exact posi-

by signals from the bulboreticular facilitatory region
of the brain stem stabilizes the positions of the major

additional detailed voluntary movements (of fingers
or other body parts) required for intricate motor

Stretch Reflex

Almost every time a clinician performs a physical exam-

Knee Jerk and Other Muscle Jerks.

to determine the sensitivity of the stretch reflexes is to

can be elicited by simply striking the patellar tendon

quadriceps muscle and excites a

ceps muscle recorded during a knee jerk.

Similar reflexes can be obtained from almost any

muscle of the body either by striking the tendon of the

is required to elicit a dynamic stretch reflex.

200

400

Milliseconds

Muscle length

Patellar tendon struck

Knee jerk

Ankle clonus

600

800

and from the gastrocnemius muscle

Myograms recorded from the quadriceps muscle during elicita-

Figure 54–6

tion of the knee jerk (above)
during ankle clonus (below).

such destructive effects.

opposed by this negative reflex, can occasionally cause

ulation of muscles in the laboratory, which cannot be

bone. We know, for instance, that direct electrical stim-

taneous relaxation of the entire muscle. This effect is

tendon becomes extreme, the inhibitory effect from

When tension on the muscle and, therefore, on the

ment of too much tension on the muscle.

tive feedback

Thus, this reflex provides a

inhibitory.

cause reflex effects in the respective muscle. This reflex

muscle, signals are transmitted to the spinal cord to

When the Golgi tendon organs of a muscle tendon are

Its Importance

Inhibitory Nature of the Tendon Reflex and

without affecting adjacent muscles. The relation

This local circuit directly inhibits the individual muscle

cortex. The local cord signal excites a single

in a dorsal horn of the cord, through long fiber path-

both into local areas of the cord and, after synapsing

from the primary spindle endings, transmit signals

endings of the muscle spindle. These fibers, like those

ter, only slightly smaller than those from the primary

Ib nerve fibers that average 16 micrometers in diame-

are transmitted through large, rapidly conducting type

Central Nervous System.

Transmission of Impulses from the Tendon Organ into the

muscle.

response). Thus, Golgi tendon organs provide the

static response,

muscle spindle, has both a

The tendon organ, like the primary receptor of the

reflected by the tension in itself.

detects muscle length and changes in muscle length,

“tensed” by contracting or stretching the muscle. Thus,

nected to each Golgi tendon organ, and the organ is

pass. About 10 to 15 muscle fibers are usually con-

tendon organ, shown in Figure 54–7, is an encapsulated

The Golgi

Golgi Tendon Organ Helps Control Muscle Tension.

Golgi Tendon Re

ing force to it. If clonus occurs, the degree of facilitation

spinal cord, neurologists test patients for clonus by sud-

readily. To determine the degree of facilitation of the

stretch reflexes are highly facilitated, clonus develops

brain. For instance, in a decerebrate animal, in which the

clonus.

continues to oscillate, often for long periods; this is

this way, the stretch reflex of the gastrocnemius muscle

and the body falls once more to begin a new cycle. In

body, but this too dies out after a fraction of a second,

second time. Again, a dynamic stretch reflex lifts the

and the body falls again, thus stretching the spindles a

of a second, the reflex contraction of the muscle dies out

muscle, which lifts the body up again. After a fraction

cord. These impulses reflexively excite the stretched

the gastrocnemius muscles, stretch reflex impulses are

ankle clonus, as follows.

(see lower myogram, Figure 54–6). Oscil-

tions, the muscle jerks can oscillate, a phenomenon

of the body.

facilitatory area of the brain stem. Ordinarily, large

ably weakened or absent. These reflexes are used most

depressed or abrogated, the muscle jerks are consider-

gerated. Conversely, if the facilitatory impulses are

system into the cord, the muscle jerks are greatly exag-

Motor Functions of the Spinal Cord; the Cord Reflexes

Chapter 54

679

large numbers of facilitatory impulses are being trans-
mitted from the upper regions of the central nervous

frequently in determining the presence or absence of
muscle spasticity caused by lesions in the motor areas
of the brain or diseases that excite the bulboreticular

lesions in the motor areas of the cerebral cortex but not
in the lower motor control areas (especially lesions
caused by strokes or brain tumors) cause greatly exag-
gerated muscle jerks in the muscles on the opposite side

Clonus—Oscillation of Muscle Jerks.

Under some condi-

called clonus
lation can be explained particularly well in relation to

If a person standing on the tip ends of the feet sud-

denly drops his or her body downward and stretches

transmitted from the muscle spindles into the spinal

Clonus ordinarily occurs only when the stretch reflex

is highly sensitized by facilitatory impulses from the

denly stretching a muscle and applying a steady stretch-

is certain to be high.

flex

sensory receptor through which muscle tendon fibers

stimulated when this small bundle of muscle fibers is

the major difference in excitation of the Golgi tendon
organ versus the muscle spindle is that the spindle

whereas the tendon organ detects muscle tension as

dynamic response and a

responding intensely when the muscle

tension suddenly increases (the dynamic response) but
settling down within a fraction of a second to a lower
level of steady-state firing that is almost directly

proportional to the muscle tension (the static

nervous system with instantaneous information on the
degree of tension in each small segment of each

Signals from the tendon organ

ways such as the spinocerebellar tracts into the cere-
bellum and through still other tracts to the cerebral

inhibitory

interneuron that inhibits the anterior motor neuron.

between signals to the brain and function of the cere-
bellum and other parts of the brain for muscle control
is discussed in Chapter 56.

stimulated by increased tension in the connecting

is entirely

nega-

mechanism that prevents the develop-

the tendon organ can be so great that it leads to a
sudden reaction in the spinal cord that causes instan-

called the lengthening reaction; it is probably a pro-
tective mechanism to prevent tearing of the muscle or
avulsion of the tendon from its attachments to the

Tendon

Muscle

Nerve fiber (16

Figure 54–7

Golgi tendon organ.

discharge may last for a second or more.

discharge, but after a strong pain stimulus, the after-

reflex; a weak tactile stimulus causes almost no after-

to occur. The duration of afterdischarge depends on

of afterdischarge, it takes many milliseconds for this

of the muscle returns toward the baseline, but because

cord. Finally, after the stimulus is over, the contraction

fatigue,

response appears. Then, in the next few seconds, the

after a pain nerve begins to be stimulated, the flexor

muscle during a flexor reflex.Within a few milliseconds

Figure 54–9 shows a typical myogram from a flexor

stimulus is over.

discharge

circuits to inhibit the antagonist muscles, called

the reflex to the necessary muscles for withdrawal; (2)

basic types of circuits: (1) diverging circuits to spread

pathway; however, most of the signals of the reflex tra-

The shortest possible circuit is a three- or four-neuron

neurons and only secondarily to the motor neurons.

The pathways for eliciting the flexor reflex do not

the hand from the painful stimulus.

of the upper arm become excited, thus withdrawing

lus is applied to the hand; as a result, the flexor muscles

for the flexor reflex. In this instance, a painful stimu-

portion of Figure 54–8 shows the neuronal pathways

The left-hand

withdrawal reflexes.

same type of reflex. Therefore, the many patterns of

fined to flexor muscles, even though it is basically the

drawn from the stimulus,

is painfully stimulated, that part will similarly be

a pinprick, heat, or a wound, for which reason it is also

powerfully by stimulation of pain endings, such as by

In its classic form, the flexor reflex is elicited most

object. This is called the

In the spinal or decerebrate animal, almost any type

that originate in all these areas.

Chapters 55 and 56, the information from these recep-

motor areas of the cerebral cortex. As discussed in

brain stem and, to a lesser extent, all the way to the

brain or spinal cord. Additional pathways transmit

120 m/sec, the most rapid conduction anywhere in the

For instance, the dorsal spinocerebellar tracts carry

of instantaneous changes taking place in the muscles.

cord control of motor function, these two sensory

with Motor Control from Higher Levels

Golgi Tendon Organs in Conjunction

Function of the Muscle Spindles and

inhibition. This spreads the muscle load over all the

reflex, whereas those that exert too little tension

forces of the separate muscle fibers.That is, those fibers

Possible Role of the Tendon Reflex to Equalize Contractile

The Nervous System: C. Motor and Integrative Neurophysiology

680

Unit XI

Force Among the Muscle Fibers.

Another likely function

of the Golgi tendon reflex is to equalize contractile

that exert excess tension become inhibited by the

become more excited because of absence of reflex

fibers and prevents damage in isolated areas of
a muscle where small numbers of fibers might be
overloaded.

of the Brain

Although we have emphasized the function of the
muscle spindles and Golgi tendon organs in spinal

organs also apprise the higher motor control centers

instantaneous information from both the muscle
spindles and the Golgi tendon organs directly to the
cerebellum at conduction velocities approaching

similar information into the reticular regions of the

tors is crucial for feedback control of motor signals

Flexor Reflex and the
Withdrawal Reflexes

of cutaneous sensory stimulus from a limb is likely to
cause the flexor muscles of the limb to contract,
thereby withdrawing the limb from the stimulating

flexor reflex.

called a nociceptive reflex, or simply a pain reflex. Stim-
ulation of touch receptors can also elicit a weaker and
less prolonged flexor reflex.

If some part of the body other than one of the limbs

with-

but the reflex may not be con-

these reflexes in the different areas of the body are
called

Neuronal Mechanism of the Flexor Reflex.

pass directly to the anterior motor neurons but instead
pass first into the spinal cord interneuron pool of

verse many more neurons and involve the following

recip-

rocal inhibition circuits; and (3) circuits to cause after-

lasting many fractions of a second after the

reflex begins to

which is characteristic of essen-

tially all complex integrative reflexes of the spinal

the intensity of the sensory stimulus that elicited the

FLEXOR

REFLEX

CROSSED EXTENSOR

REFLEX

Painful
stimulus
from hand

Polysynaptic

circuit

Excited

RECIPROCAL INHIBITION

Inhibited

Excited

Inhibited

Flexor reflex, crossed extensor reflex, and reciprocal inhibition.

Figure 54–8

This is the phenomenon of

often simultaneously inhibits the antagonist muscles.

instance, when a stretch reflex excites one muscle, it

associated with inhibition of another group. For

In the previous paragraphs, we pointed out several

Reciprocal Innervation

Reciprocal Inhibition and

cause the entire body to move away.

of the stimulus. The prolonged afterdischarge is of

This demonstrates the relatively long latency before

Figure 54–10 shows a typical myogram recorded

the interneuronal cells.

reflex. Again, it is presumed that this prolonged after-

removed, the crossed extensor reflex has an even

crossed extension. After the painful stimulus is

of the initial pain stimulus, it is certain that many

muscles. Because the crossed extensor reflex usually

right-hand portion of Figure 54–8 shows the neuronal

The

drawn limb.

This is called the

reflex in one limb, the opposite limb begins to extend.

Crossed Extensor Reflex

limbs because of their highly developed flexor reflexes.

any part of the body, it is especially applicable to the

principle, called the principle of “local sign,” applies to

away from the object causing the pain. Although this

outward. In other words, the integrative centers of

which sensory nerve is stimulated. Thus, a pain stimu-

The pattern of withdrawal that

from the painful stimulus.

over. During this time, other reflexes and actions of the

charge, the reflex can hold the irritated part away from

body from a stimulus. Further, because of afterdis-

Thus, the flexor reflex is appropriately organized to

sensory signal is over.

transmit impulses to the anterior motor neurons,

reverberating interneuron circuits. These, in turn,

after strong pain stimuli, almost certainly resulting

themselves. Also, prolonged afterdischarge occurs

charge, lasting for about 6 to 8 milliseconds, results

discharge circuits discussed in Chapter 46. Electro-

The afterdischarge that occurs in the flexor reflex

Motor Functions of the Spinal Cord; the Cord Reflexes

Chapter 54

681

almost certainly results from both types of repetitive

physiologic studies indicate that immediate afterdis-

from repetitive firing of the excited interneurons

from recurrent pathways that initiate oscillation in

sometimes for several seconds after the incoming

withdraw a pained or otherwise irritated part of the

the stimulus for 0.1 to 3 seconds after the irritation is

central nervous system can move the entire body away

Pattern of Withdrawal.

results when the flexor reflex is elicited depends on

lus on the inward side of the arm elicits not only
contraction of the flexor muscles of the arm but also
contraction of abductor muscles to pull the arm

the cord cause those muscles to contract that can
most effectively remove the pained part of the body

About 0.2 to 0.5 second after a stimulus elicits a flexor

crossed extensor reflex. Extension of

the opposite limb can push the entire body away from
the object causing the painful stimulus in the with-

Neuronal Mechanism of the Crossed Extensor Reflex.

circuit responsible for the crossed extensor reflex,
demonstrating that signals from sensory nerves cross
to the opposite side of the cord to excite extensor

does not begin until 200 to 500 milliseconds after onset

interneurons are involved in the circuit between the
incoming sensory neuron and the motor neurons of
the opposite side of the cord responsible for the

longer period of afterdischarge than does the flexor

discharge results from reverberating circuits among

from a muscle involved in a crossed extensor reflex.

the reflex begins and the long afterdischarge at the end

benefit in holding the pained area of the body away
from the painful object until other nervous reactions

times that excitation of one group of muscles is often

reciprocal inhibition, and

the neuronal circuit that causes this reciprocal relation

Seconds

Flexor contraction

Duration of stimulus

Fatigue

Afterdischarge

stimulus is over.

an interval of fatigue, and, finally, afterdischarge after the input

Myogram of the flexor reflex showing rapid onset of the reflex,

Figure 54–9

Extensor contraction

Seconds

Duration of

stimulus

Afterdischarge

longed afterdischarge.

Myogram of a crossed extensor reflex showing slow onset but pro-

Figure 54–10

another manifestation of reciprocal innervation, this

the forelimbs and hindlimbs. This diagonal response is

limbs. In general, stepping occurs diagonally between

shown in Figure 54–12, the stretch on the limbs occa-

up from the floor and its legs are allowed to dangle, as

Diagonal Stepping of All Four Limbs—“Mark Time” Reflex.

from reciprocal innervation between the two limbs.

site limb ordinarily moves backward.This effect results

occurs in the forward direction in one limb, the oppo-

cord is not split down its center, every time stepping

controller.

Thus, the cord is an intelligent walking

forward to be placed over the obstruction. This is the

sequence, the foot will be lifted higher and proceed

the forward thrust will stop temporarily; then, in rapid

be even more complex. For instance, if the top of the

fact, the cord mechanism for control of stepping can

when the foot is allowed to walk along a surface. In

controlling foot pressure and frequency of stepping

The sensory signals from the footpads and from the

neurons controlling agonist and antagonist muscles.

matrix of the cord itself, oscillating between the

nerves have been cut, and it seems to result mainly

This oscillation back and forth between flexor and

repeated over and over.

extension. Then flexion occurs again, and the cycle is

individual stepping functions. Forward flexion of the

between the two limbs, each hindlimb can still perform

limbs of spinal animals. Indeed, even when the lumbar

Stepping and Walking Movements

forelimbs.

of the thoracic cord, the walking movements of

walk using its hindlimbs in addition to its forelimbs.

posture are integrated in the spinal cord. Indeed, an

cord righting reflex.

raise itself to the standing position. This is called

its side, it will make incoordinate movements trying to

When a spinal animal is laid on

animal from falling to that side.

This helps keep an

sure on one side causes extension in that direction, an

mines the direction in which the limb will extend; pres-

The locus of the pressure on the pad of the foot deter-

responsible for the flexor and cross extensor reflexes.

The positive supportive reaction involves a complex

support the weight of the body. This reflex is called the

feet, the reflex often stiffens the limbs sufficiently to

been transected for several months—that is, after the

the pressure applied to the foot. Indeed, this reflex

Reflexes of Posture and

the original reflex to reassume its previous intensity.

flexion. Finally, removal of the stronger reflex allows

body. This stronger reflex sends reciprocal inhibitory

while this reflex is still being elicited, a stronger flexor

inhibition. In this instance, a moderate but prolonged

Figure 54–11 shows a typical example of reciprocal

extensor muscle reflexes described earlier.

sides of the body, as exemplified by the flexor and

Likewise, reciprocal

The Nervous System: C. Motor and Integrative Neurophysiology

682

Unit XI

is called reciprocal innervation.
relations often exist between the muscles on the two

flexor reflex is elicited from one limb of the body;

reflex is elicited in the limb on the opposite side of the

signals to the first limb and depresses its degree of

Locomotion

Postural and Locomotive Reflexes
of the Cord

Positive Supportive Reaction.

Pressure on the footpad of

a decerebrate animal causes the limb to extend against

is so strong that if an animal whose spinal cord has

reflexes have become exaggerated—is placed on its

positive supportive reaction.

circuit in the interneurons similar to the circuits

effect called the magnet reaction.

Cord “Righting” Reflexes.

the

Such a reflex demonstrates

that some relatively complex reflexes associated with

animal with a well-healed transected thoracic cord
between the levels for forelimb and hindlimb innerva-
tion can right itself from the lying position and even

In the case of an opossum with a similar transection

the hindlimbs are hardly different from those in a
normal opossum—except that the hindlimb walking
movements are not synchronized with those of the

Rhythmical Stepping Movements of a Single Limb.

Rhythmi-

cal stepping movements are frequently observed in the

portion of the spinal cord is separated from the
remainder of the cord and a longitudinal section is
made down the center of the cord to block neuronal
connections between the two sides of the cord and

limb is followed a second or so later by backward

extensor muscles can occur even after the sensory

from mutually reciprocal inhibition circuits within the

position sensors around the joints play a strong role in

foot encounters an obstruction during forward thrust,

stumble reflex.

Reciprocal Stepping of Opposite Limbs.

If the lumbar spinal

a well-healed spinal animal (with spinal transection in
the neck above the forelimb area of the cord) is held

sionally elicits stepping reflexes that involve all four

Flexor contraction

Duration of inhibitory stimulus

Duration of flexor reflex stimulus

Seconds

site side of the body.

by an inhibitory stimulus from a stronger flexor reflex on the oppo-

Myogram of a flexor reflex showing reciprocal inhibition caused

Figure 54–11

tion, all the segmental reflexes can at times be elicited

(see Chapter 31) or the colon (see Chapter 63). In addi-

response to peritoneal irritation (see Chapter 66); and

functions of the gut (see Chapter 62); (4) peritoneoin-

heat on the surface of the body (see Chapter 73); (3)

Chapter 73); (2) sweating, which results from localized

other chapters. Briefly, these include (1) changes in vas-

grated in the spinal cord, most of which are discussed in

Many types of segmental autonomic reflexes are inte-

Spinal Cord

blown muscle cramp ensues.

itive feedback develops, so that a small amount of initial

cord to increase the intensity of contraction. Thus, pos-

sensory receptors even more, which causes the spinal

The contraction is believed to stimulate the same

flow, or overexercise, can elicit pain or other sensory

mality of a muscle, such as severe cold, lack of blood

follows: Any local irritating factor or metabolic abnor-

typical muscle cramp. Electromyographic studies indi-

abdominal operations.

this reason, deep anesthesia is usually required for intra-

extruding the intestines through the surgical wound. For

abdominal muscles to contract extensively, sometimes

for instance, during abdominal operations, pain im-

peritonitis allows the spastic muscle to relax. The same

peritonitis. Here again, relief of the pain caused by the

priate positions.

as ether anesthesia, also relieves the spasm. One of

spasm; a deep general anesthetic of the entire body, such

tonically. Pain relief obtained by injecting a local anes-

initiated from the broken edges of the bone, which

round a broken bone. This results from pain impulses

In many, if not most, instances, localized pain is the

In human beings, local muscle spasm is often observed.

Spinal Cord Reflexes That

ments of locomotion, involves reciprocal innervation

to-and-fro movement,

The

crosses the midline, the first paw stops scratching and

make the reflex even more complicated, when the flea

to bring the paw to the position of the crawling flea. To

find its position, even though 19 muscles in the limb

as the shoulder of a spinal animal, the hind paw can still

developed function. If a flea is crawling as far forward

The

movement.

to-and-fro scratching

the surface of the body, and (2) a

It involves two functions: (1) a

tickle sen-

itch

the scratch reflex, which is initiated by

poses the animal to keep galloping and, therefore, con-

hindlimbs are stimulated about equally; this predis-

ground during galloping, both forelimbs and both

walking. Conversely, when the animal strikes the

ulated, which would predispose the animal to continue

terns of walking and galloping, because in walking,

walking reflex. This is in keeping with the normal pat-

same time; unequal stimulation elicits the diagonal

while both hindlimbs move forward. This often occurs

cord between the forelimbs and hindlimbs. Such a

Motor Functions of the Spinal Cord; the Cord Reflexes

Chapter 54

683

time occurring the entire distance up and down the

walking pattern is called a mark time reflex.

Galloping Reflex.

Another type of reflex that occasion-

ally develops in a spinal animal is the galloping reflex,
in which both forelimbs move backward in unison

when almost equal stretch or pressure stimuli are
applied to the limbs on both sides of the body at the

only one forelimb and one hindlimb at a time are stim-

tinues this pattern of motion.

Scratch Reflex

An especially important cord reflex in some animals is

sation.

position sense that

allows the paw to find the exact point of irritation on

position sense of the scratch reflex is a highly

must be contracted simultaneously in a precise pattern

the opposite paw begins the to-and-fro motion and
eventually finds the flea.

like the stepping move-

circuits that cause oscillation.

Cause Muscle Spasm

cause of the local spasm.

Muscle Spasm Resulting from a Broken Bone.

One type of

clinically important spasm occurs in muscles that sur-

cause the muscles that surround the area to contract

thetic at the broken edges of the bone relieves the

these two anesthetic procedures is often necessary
before the spasm can be overcome sufficiently for the
two ends of the bone to be set back into their appro-

Abdominal Muscle Spasm in Peritonitis.

Another type of

local spasm caused by cord reflexes is abdominal spasm
resulting from irritation of the parietal peritoneum by

type of spasm often occurs during surgical operations;

pulses from the parietal peritoneum often cause the

Muscle Cramps.

Still another type of local spasm is the

cate that the cause of at least some muscle cramps is as

signals transmitted from the muscle to the spinal cord,
which in turn cause reflex feedback muscle contraction.

irritation causes more and more contraction until a full-

Autonomic Reflexes in the

cular tone resulting from changes in local skin heat (see

intestinointestinal reflexes that control some motor

testinal reflexes that inhibit gastrointestinal motility in

(5) evacuation reflexes for emptying the full bladder

simultaneously in the form of the so-called mass reflex,
described next.

Figure 54–12

Diagonal stepping movements exhibited by a spinal animal.

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cases they eventually return. These effects are

first few weeks after cord transection, but in most

3. The sacral reflexes for control of bladder and colon

stepping reflexes.

postural antigravity reflexes, and remnants of

progressively more complex reflexes: flexor reflexes,

stretch reflexes, followed in order by the

transected. The first reflexes to return are the

hyperexcitable, particularly if a few facilitatory

humans, some reflexes may eventually become

sometimes required. In both animals and

in human beings, 2 weeks to several months are

In lower animals, a few hours to a few days are

2. All skeletal muscle reflexes integrated in the spinal

in human beings.

ordinarily returns to normal within a few days, even

blocked almost to extinction. The pressure

1. At onset of spinal shock, the arterial blood

functions.

complete; conversely, sometimes recovery is excessive,

to a day or so, but in human beings, the return is often

the loss. In most nonprimates, excitability of the cord

facilitatory impulses, they increase their own natural

nervous system—that is, after they lose their source of

gradually regain their excitability. This seems to be a

After a few hours to a few weeks, the spinal neurons

tracts, and corticospinal tracts.

mitted through the reticulospinal tracts, vestibulospinal

cord from higher centers, particularly discharge trans-

The reason for this is that normal activity of the cord

spinal shock.

the point of total silence, a reaction called

ing the cord reflexes, immediately become depressed to

upper neck, at first, essentially all cord functions, includ-

When the spinal cord is suddenly transected in the

Spinal Cord Transection and

seizures, which involve reverberating circuits that occur

at once. This is similar to the mechanism of epileptic

Because the mass reflex can last for minutes, it pre-

areas of the body break out into profuse sweating.

to a systolic pressure well over 200 mm Hg; and (4) large

rial pressure often rises to maximal values, sometimes

colon and bladder are likely to evacuate; (3) the arte-

skeletal muscles goes into strong flexor spasm; (2) the

cord. The effects are (1) a major portion of the body’s

of the type of stimulus, the resulting reflex, called the

as overdistention of the bladder or the gut. Regardless

stimulus to the skin or excessive filling of a viscus, such

cord. The usual stimulus that causes this is a strong pain

active, causing massive discharge in large portions of the

In a spinal animal or human being, some-

The Nervous System: C. Motor and Integrative Neurophysiology

684

Unit XI

Mass Reflex.

times the spinal cord suddenly becomes excessively

mass reflex, involves large portions or even all of the

sumably results from activation of great numbers of
reverberating circuits that excite large areas of the cord

in the brain instead of in the cord.

Spinal Shock

neurons depends to a great extent on continual tonic
excitation by the discharge of nerve fibers entering the

natural characteristic of neurons everywhere in the

degree of excitability to make up at least partially for

centers returns essentially to normal within a few hours

delayed for several weeks and occasionally is never

with resultant hyperexcitability of some or all cord

Some of the spinal functions specifically affected

during or after spinal shock are the following:

pressure falls instantly and drastically—sometimes
to as low as 40 mm Hg—thus demonstrating that
sympathetic nervous system activity becomes

cord are blocked during the initial stages of shock.

required for these reflexes to return to normal;

pathways remain intact between the brain and the
cord while the remainder of the spinal cord is

evacuation are suppressed in human beings for the

discussed in Chapters 31 and 66.

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