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Neural Reflexes
∗
Steven Telleen
This work is produced by OpenStax-CNX and licensed under the
Creative Commons Attribution License 4.0†
Abstract
This module describes neural reexes as a fundamental component in body activity. By the end of
this section the student should understand what reexes are and why they are important; be able to
apply the input-integration-output model when describing reexes; should know the dierence between
monosynaptic and polysynaptic reexes and examples of each; and should be able to identify specic
types of somatic and cranial reexes.
Reexes form the neural core of most body activity, including complex voluntary movement.
Most
higher level commands coming from the brain do not activate and coordinate the actions of every body cell
or organ cell involved in the response. The brain activates and adjusts the performance patterns of smaller
precongured modules called reexes.
You can think of the process as similar to driving a car. Many complex, precongured components are
involved in making the car move, such as the engine, the transmission, the anti-locking brakes, and the
power steering. Most of these components, for example the engine, are made by combining and coordinating
multiple simpler modules that control dierent aspects of engine performance. Many of these simple modules
are created by combining even simpler modules. It would be very slow and inecient for you as the driver
(the brain) to micromanage each of the repetitive lower level functions when driving the car. Instead, when
you press on the gas pedal this activates the appropriate pattern of lower level modules that "speed up" the
engine and engage the transmission.
Just like a car, simple reexes (functional modules) can be precongured into more complex reexes,
which can be precongured into higher level reexes. When a higher level reex is activated, the activation
cascades down these reex arcs nally activating or inhibiting the individual eector cells at the bottom of
the reex cascade in a coordinated pattern that results in the desired body response.
1 Reex Arcs
The most basic unit in any body activity under nervous system control is the
Reex Arc.
All reex arcs
consists of three specic functions that were discussed way back in the introductory chapter of this book:
Input -> Integration -> Output.
Note that we encountered this three part process again when the
functions controlling a single neuron was discussed. A reex follows this same process extending it to more
than one neuron working together as a group to change the state of a body eector.
Input occurs when
a peripheral neuron responds to a specic external (outside the central nervous system) state. When that
state is encountered, the neuron generates an action potential that is transmitted to one or more neurons in
the central nervous system that make up the
Integration Center (IC). In its simplest form, the signal is
enough to generate an action potential in a single IC neuron. This IC action potential is transmitted to a
specic body eector cell causing it to change its state in response (see: Figure 1).
∗ Version
1.1: Sep 4, 2016 1:06 pm -0500
† http://creativecommons.org/licenses/by/4.0/
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Figure 1
Most reex arcs are more complicated than the simple reex arc described above. However, they all have
two things in common: the Input->Integration->Output process and a response (output) that is consistent
and automatic.
Reexes provide two major advantages:
speed of response and freeing limited working
memory space to attend to other inputs that require more complicated integration.
2 Classication of Reexes
In our simple example above there were only two neurons (the sensory neuron and the motor neuron) with
one synapse between them.
There is not much
this single input. This kind of reex (
integration
going on as the motor neuron is activated by
Monosynaptic) is analogous to a unipolar neuron at the individual
cell level. Monosynaptic reex arcs play an important role when very simple, rapid, responses are required.
However, many reex arcs are
Polysynapic meaning there is more than one synapse and neuron providing
input, analogous to multipolar neurons at the individual cell level.
Having inputs from multiple sensory
and integrating neurons allows for more nuanced responses to changing conditions. The neurons providing
input to the motor neuron in the arc generally are
Internuerons, neurons that reside and terminate inside
the central nervous system (see: Figure 2). Their inputs may come either from sensory neurons or other
interneurons and their outputs may go either to motor neurons or other interneurons.
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Figure 2
In addition to the number of synapses, other features used to describe and classify reexes are: whether
the reex is
innate or learned, whether the integration neurons reside in the brain or spinal cord, and
somatic or autonomic eectors (see: Figure 3). You will notice that any
whether the reex activates
combination of these feature states can be found in a given reex, with one exception: learned reexes are
always polysynaptic. Monosynaptic reexes are less common and innate.
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Figure 3
•
Reex Development: Innate reexes
are those present when you are born.
The input/output
wiring is already present at birth. For example, dilating your pupils or rapidly withdrawing your hand
from a hot stove. In contrast,
Learned reexes
(or conditioned reexes) develop through conscious or
unconscious repetition. For example, learning to ride a bicycle, play a musical instrument, or eciently
execute a sports move are all examples of consciously conditioning learned reexes. The repetition in
these cases is called practice and the objective is to move the activity from slower conscious initiation
to a faster unconscious reex. However, any repetitive behavior, even when you are not consciously
trying to learn the response, can develop into reexive behavior. Many of our less eective (bad) habits
became reexive in this manner.
•
Synapses in Reex Arc: Monosynaptic reexes
Some elds also nd it useful to distinguish
and polysynaptic reexes were introduced above.
disynaptic reexes within the traditional polysynaptic
class. This is where there is only one interneuron between the sensory and motor neurons creating two
synapses. Disynaptic reexes are common in inhibitory circuits that keep antagonist muscle groups
from becoming active during a muscle contraction. The polysynaptic reex shown in Figure 2 is more
specically a disynaptic reex.
•
Integration Level: Spinal reexes
cord.
reex
are those where the neuron synapses are located in the spinal
Cranial reexes are those where the neuron synapses are located in the brain.
An
Intersegmental
refers to a spinal reex that has synapses (and therefore integration activities) in more than one
segment of the spinal cord.
•
Type of Eectors Controlled: Somatic reexes
control skeletal muscle tension, contraction, and
inhibition, generally aecting organismal movement and balance.
Autonomic reexes
control smooth
muscle, cardiac muscle, and glands. Autonomic reexes are generally involved with maintaining internal
homeostasis and controlling internal motility of substances through body tubes.
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3 Motor Reex Patterns
In the introduction to this section it was noted that reexes are modular response patterns. Simple reexes
can be combined into more complex reexes. Skeletal muscle reexes are an example of combining more than
one simple reex into a more complex reex pattern of coordinated action. This is because the neuromuscular
junction is strictly excitatory, the muscle will contract when the motor nerve is active. Skeletal muscles do
not actively relax. Instead the motor neuron needs to quiet down, or be inhibited. This means when a
reex activates a skeletal muscle its antagonist needs to be inhibited in order for the eective change in
muscle tone to occur.
This occurs through an interneuron that inhibits the antagonist muscle when the
reex is activated.
Stretch Reex:
In this reex, when a skeletal muscle is stretched, a muscle spindle receptor is activated.
The axon from this receptor structure will cause direct contraction of the muscle (a monosynaptic reex). A
collateral of the muscle spindle ber will also inhibit the motor neuron of the antagonist muscles (a bisynaptic
reex). The reex helps to maintain muscles at a constant length. A common example of this reex is the
knee jerk that is elicited when a rubber hammer struck against the patellar ligament in a physical exam.
The blow "articially" stretches the tendon of the quadriceps femoris muscles. Normally this stretch would
result from an increased contraction of the hamstring muscles, and unless movement is desired, an increased
contraction by the quadriceps would hold the leg in place. However, since the hamstring muscles did not
cause the stretch, the compensating quadriceps contraction causes the leg to jerk upward.
Withdrawal Reex:
As you withdraw your hand from a hot stove or other painful stimulus, you do
not want to slow that reex down. As the biceps brachii contracts, the antagonistic triceps brachii needs to
relax. The interneuron's cell body is located in the dorsal horn of the spinal cord. The interneuron receives
a synapse from the axon of the sensory neuron that detects that the hand is being burned. In response to
this stimulation from the sensory neuron, the interneuron then inhibits the motor neuron that controls the
triceps brachii. This is done by releasing a neurotransmitter or other signal that hyperpolarizes the motor
neuron connected to the triceps brachii, making it less likely to initiate an action potential. With this motor
neuron being inhibited, the triceps brachii relaxes. Without the antagonistic contraction, withdrawal from
the hot stove is faster and keeps further tissue damage from occurring.
Crossed-Extensor Reex:
is a withdrawal reex. The exors in the withdrawing limb contract and
the extensors relax, while in the other limb, the opposite occurs. An example of this is when a person steps
on a sharp object.
The leg that is stepping on the sharp object exes to pull away, while the other leg
extends to support the weight of the whole body. To produce this reex, branches of the aerent nerve bers
cross from the stimulated side of the body to the opposite side of the spinal cord. There, they synapse with
interneurons, which, in turn, excite or inhibit alpha motor neurons to the muscles of the opposite limb. If
this did not happen, the person would fall down. The crossed-extensor reex is also seen in the upper limbs.
When a person withdraws their arm from the hot stove, as in the example above, the opposite arm will
reexively extend.
Corneal Reex:
or the eye blink reex is a specialized reex to protect the surface of the eye. When
the cornea is stimulated by a tactile stimulus, or even by bright light in a related reex, blinking is initiated.
This is an autonomic cranial, as opposed to spinal, reex as the integration takes place in the brain via the
cranial nerves. The sensory component travels through the trigeminal nerve, which carries somatosensory
information from the face (or through the optic nerve, if the stimulus is bright light). The motor response
travels through the facial nerve and innervates the orbicularis oculi on the same side. This reex is commonly
tested during a physical exam using an air pu or a gentle touch of a cotton-tipped applicator.
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Watch this video
to learn more about the reex arc of the corneal reex. When the right cornea
senses a tactile stimulus, what happens to the left eye? Explain your answer.
1 http://openstaxcollege.org/l/reexarc
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Watch this video
to learn more about newborn reexes. Newborns have a set of reexes that are
expected to have been crucial to survival before the modern age. These reexes disappear as the
baby grows, as some of them may be unnecessary as they age. The video demonstrates a reex
called the Babinski reex, in which the foot exes dorsally and the toes splay out when the sole of
the foot is lightly scratched. This is normal for newborns, but it is a sign of reduced myelination
of the spinal tract in adults. Why would this reex be a problem for an adult?
4 Section Summary
Reexes form the neural core of most body activity, including complex voluntary movement. The most basic
unit in any body activity under nervous system control is the
specic functions:
Input -> Integration -> Output.
2 http://openstaxcollege.org/l/newreex
http://cnx.org/content/m62681/1.1/
Reex Arc.
All reex arcs consists of three
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Reexes can be classied by: When they develop:Innate reexes are those present when you are
born, while
Learned reexes
spinal cord.
Cranial reex
(or conditioned reexes) develop through conscious or unconscious repetition.
Number of synapses:Monosynaptic reexes and polysynaptic reexes with disynaptic reexes sometimes
distinguished from the full polysynaptic class. Integration level:Spinal reex synapses are located in the
synapses are located in the brain.
than one segment of the spinal cord.
while
Autonomic reexes
Several
An
Intersegmental reex
synapses in more
Type of eector controlled:Somatic reexes control skeletal muscles
control smooth muscle, cardiac muscle, and glands.
Motor Reex Patterns were discussed. Stretch Reex:
when a skeletal muscle is stretched,
a muscle spindle receptor is activated. The axon from this receptor structure will cause direct contraction of
the muscle (a monosynaptic reex). A collateral of the muscle spindle ber will also inhibit the motor neuron
of the antagonist muscles (a bisynaptic reex).
Withdrawal Reex:
is similar to the stretch reex, but
the stimulus is the axon of a sensory neuron that detects a painful stimulus. By inhibiting the antagonistic
contraction while stimulating the agonist contraction, withdrawal from the painful stimulus is faster and
keeps further tissue damage from occurring.
Crossed-Extensor Reex:
is a withdrawal reex where the
exors in the withdrawing limb contract and the extensors relax, while in the other limb, the opposite occurs.
This helps maintain body balance during the reexive action.
Corneal Reex:
is an autonomic cranial
reex that protects the surface of the eye. When the cornea is stimulated by a tactile stimulus, or even by
bright light in a related reex, blinking is initiated.
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