Page 495 - Fundamentals of anatomy physiology

482 Unit 3 Control and Regulation

patterns based in the spinal cord. Descend- Figure 13–17 The Babinski Reflexes.
ing tracts originating in the brain synapse on
interneurons and motor neurons throughout
the spinal cord. These synapses are continu-
ously active, producing EPSPs or IPSPs at the
postsynaptic membrane.

Voluntary Movements
and Reflex Motor Patterns

Spinal reflexes produce consistent, stereo-

typed motor patterns that are triggered by

specific external stimuli. However, the same a The plantar reflex b The Babinski sign (positive Babinski
motor patterns can also be activated as needed (negative Babinski reflex), reflex) occurs in the absence of
by centers in the brain. By making use of these a curling of the toes, is descending inhibition. It is normal
preexisting patterns, relatively few descending seen in healthy adults. in infants, but pathological in adults.

fibers can control complex motor functions.

For example, neuronal pools in the spinal cord direct the The person may be consciously suppressing the response, the

motor patterns for walking, running, and jumping. The de- nerves involved may be damaged, or there may be underly-

13 scending pathways from the brain provide appropriate facilita- ing problems inside the CNS. The clinician may then ask the

tion, inhibition, or “fine-tuning” of the established patterns. p­ atient to perform an action designed to provide reinforce-

This is a very efficient system that is similar to an order given in ment, such as the Jendrassik maneuver. Reinforced reflexes are

a military drill: A single command triggers a complex, predeter- usually too strong to suppress consciously. If the reflex still fails

mined sequence of events. to appear, the likelihood of nerve or CNS damage is increased.

Motor control involves a series of interacting levels. At the The clinician may then order more sophisticated tests, such as

lowest level are monosynaptic reflexes that are rapid, but ste- nerve conduction studies or scans.

reotyped and relatively inflexible. At the highest level, centers &T i p s T r i c k s
in the brain can modulate or build on reflexive motor patterns.

Reinforcement and Inhibition Facilitation and inhibition are similar to what happens when
a symphony conductor raises or lowers one hand to control

A single EPSP may not depolarize the postsynaptic neuron suffi- the music’s volume while keeping the rhythm going with the
ciently to generate an action potential, but it does make that neu- baton hand: The basic pattern of beats doesn’t change, but
ron more sensitive to other excitatory stimuli. We introduced this the loudness does.

process of facilitation in Chapter 12. Alternatively, an IPSP makes Other descending fibers have an inhibitory effect on spinal
the neuron less responsive to excitatory stimulation, through the reflexes. In adults, stroking the lateral sole of the foot produces
process of inhibition. p. 447 By stimulating excitatory or inhib- a curling of the toes, called a plantar reflex, or negative Babinski
itory interneurons within the brain stem or spinal cord, higher reflex, after about a 1-second delay (Figure 13–17a). In contrast,
centers can adjust the sensitivity of reflexes by creating EPSPs or stroking an infant’s foot on the lateral sole produces a fanning
IPSPs at the motor neurons involved in reflex responses. of the toes known as the Babinski sign, or positive Babinski
reflex (Figure 13–17b). This response disappears as descending
When many of the excitatory synapses are chronically motor pathways develop. If either the higher centers or the
active, the postsynaptic neuron can enter a state of gener- descending tracts are damaged, the Babinski sign reappears in
alized facilitation. This facilitation of reflexes can result in an adult. As a result, clinicians often test this reflex when CNS
reinforcement, an enhancement of spinal reflexes. For ex- injury is suspected.
ample, a method used to overemphasize the patellar reflex is
called the Jendrassik maneuver. To do this, the subject hooks

the hands together by interlocking the fingers and then tries to Checkpoint
pull the hands apart while a light tap is applied to the patella.
This reinforcement produces a big kick rather than a twitch. 22. Define reinforcement as it pertains to spinal reflexes.
This distractive technique still produces a larger reflex response
even if the individual realizes it is just a distraction. 2 3. After injuring her back, Tina exhibits a positive
Babinski reflex. What does this imply about Tina’s
If a stimulus fails to elicit a particular reflex response dur- injury?

ing a clinical exam, there can be many reasons for the failure. See the blue Answers tab at the back of the book.