Page 494 - Fundamentals of anatomy physiology
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Chapter 13 The Spinal Cord, Spinal Nerves, and Spinal Reflexes 481
unexpectedly hot pan on the stove, a dramatic flexor reflex will They Involve Pools of Interneurons. Processing takes place in
occur. When the pain receptors in your hand are stimulated,
the sensory neurons activate interneurons in the spinal cord pools of interneurons before motor neurons are activated.
that stimulate motor neurons in the anterior gray horns. The
result is a contraction of flexor muscles that yanks your hand The result may be excitation or inhibition. The tendon
away from the stove.
reflex inhibits motor neurons, and the flexor and crossed
When a specific muscle contracts, opposing muscles must
relax to permit the movement. For example, flexor muscles that extensor reflexes direct specific muscle contractions.
bend the elbow (such as the biceps brachii muscle) are opposed
by extensor muscles (such as the triceps brachii muscle) that They Are Intersegmental in Distribution. The interneuron
straighten it out. A potential conflict exists: In theory, the con-
traction of a flexor muscle should trigger a stretch reflex in the pools extend across spinal segments and may activate
extensors that would cause them to contract, opposing the move-
ment. Interneurons in the spinal cord prevent such competition muscle groups in many parts of the body.
through reciprocal inhibition. When one set of motor neurons
is stimulated, those neurons that control antagonistic muscles They Involve Reciprocal Inhibition. Reciprocal inhibition co-
are inhibited. The term reciprocal refers to the fact that the system
works both ways. When the flexors contract, the extensors relax, ordinates muscular contractions and reduces resistance to
and when the extensors contract, the flexors relax.
movement. In the flexor and crossed extensor reflexes, the
Withdrawal reflexes are much more complex than any
monosynaptic reflex. They also show tremendous versatility, contraction of one muscle group is associated with the in-
because the sensory neurons activate many pools of interneu-
rons. If the stimuli are strong, interneurons will carry excitatory hibition of opposing muscles.
and inhibitory impulses up and down the spinal cord, affecting
motor neurons in many segments. The end result is always the They Have Reverberating Circuits, Which Prolong the Reflexive
same: a coordinated movement away from the stimulus. But
the distribution of the effects and the strength and character Motor Response. Positive feedback between interneurons
of the motor responses depend on the intensity and location
of the stimulus. that innervate motor neurons and the processing pool
Mild discomfort might provoke a brief contraction in mus- maintains the stimulation even after the initial stimulus
cles of your hand and wrist. More powerful stimuli would
produce coordinated muscular contractions affecting the posi- has faded.
tions of your hand, wrist, forearm, and arm. Severe pain would
also stimulate contractions of your shoulder, trunk, and arm Several Reflexes May Cooperate to Produce a Coordinated, 13
muscles. These contractions could last for several seconds, due
to the activation of reverberating circuits. In contrast, monosyn- Controlled Response. As a reflex movement gets under way,
aptic reflexes are invariable and brief. For example, the patellar
reflex is completed in about 20–40 msec. antagonistic reflexes are inhibited. For example, during the
Crossed Extensor Reflexes stretch reflex, antagonistic muscles are inhibited. In con-
The stretch, tendon, and withdrawal reflexes involve ipsilateral trast, in the tendon reflex, antagonistic muscles are stimu-
(ipsi, same + lateral, side) reflex arcs: The sensory stimulus and
the motor response occur on the same side of the body. The lated. In complex polysynaptic reflexes, commands may be
crossed extensor reflex involves a contralateral reflex arc (contra,
opposite), because the motor response occurs on the side op- distributed along the length of the spinal cord, producing a
posite the stimulus (Spotlight Figure 13–14).
well-coordinated response.
General Characteristics of Polysynaptic Reflexes
Checkpoint
Polysynaptic reflexes range in complexity from a simple ten-
don reflex to the complex and variable reflexes associated with 1 8. Identify the basic characteristics of polysynaptic
standing, walking, and running. Yet all polysynaptic reflexes reflexes.
share the following basic characteristics:
19. For the patellar (knee-jerk) reflex, how would the
stimulation of the muscle spindle by gamma motor
neurons affect the speed of the reflex?
2 0. A weight lifter is straining to lift a 200-kg barbell
above his head. Shortly after he lifts it to chest height,
his muscles appear to relax and he drops the barbell.
Which reflex has occurred?
21. During a withdrawal reflex of the foot, what happens
to the limb on the side opposite the stimulus? What is
this response called?
See the blue Answers tab at the back of the book.
13-8 The brain can affect
spinal cord–based reflexes
Learning Outcome Explain how higher centers control and modify
reflex responses.
Reflex motor behaviors happen automatically, without instruc-
tions from higher centers. However, higher centers can have
a profound effect on the performance of a reflex. Process-
ing centers in the brain can facilitate or inhibit reflex motor
