REFLEXES AND REFLEX ARCS: Meaning, features, components, stretch reflex (meaning, components, mechanism), tendon reflex (meaning, components, mechanism), flexor & crossed extensor reflexes (meaning, components, mechanis,).

The second way the spinal cord promotes homeostasis is by serving as an integrating center for some reﬂexes. A reﬂex is a fast, automatic, unplanned sequence of actions that occurs in response to a particular stimulus. Some reﬂexes are inborn, such as pulling your hand away from a hot surface before you even feel that it is hot. Other reﬂexes are learned or acquired. For instance, you learn many reﬂexes while acquiring driving expertise. Slamming on the brakes in an emergency is one example. When integration takes place in the spinal cord gray matter, the reﬂex is a spinal reﬂex. An example is the familiar patellar reﬂex (knee jerk). If integration occurs in the brain stem rather than the spinal cord, the reﬂex is called a cranial reﬂex. An example is the tracking movements of your eyes as you read this sentence. You are probably most aware of somatic reﬂexes, which involve contraction of skeletal muscles. Equally important, however, are the autonomic (visceral) reﬂexes, which generally are not consciously perceived. They involve responses of smooth muscle, cardiac muscle, and glands. The body functions such as heart rate, digestion, urination, and defecation are controlled by the autonomic nervous system through autonomic reﬂexes.

Nerve impulses propagating into, through, and out of the CNS follow speciﬁc pathways, depending on the kind of information, its origin, and its destination. The pathway followed by nerve impulses that produce a reﬂex is a reﬂex arc (reﬂex circuit). A reﬂex arc includes the following ﬁve functional components.

Sensory receptor. The distal end of a sensory neuron (dendrite) or an associated sensory structure serves as a sensory receptor. It responds to a speciﬁc stimulus a change in the internal or external environment by producing a graded potential called a generator (or receptor) potential. If a generator potential reaches the threshold level of depolarization, it will trigger one or more nerve impulses in the sensory neuron.
Sensory neuron. The nerve impulses propagate from the sensory receptor along the axon of the sensory neuron to the axon terminals, which are located in the gray matter of the spinal cord or brain stem.
Integrating center. One or more regions of gray matter within the CNS act as an integrating center. In the simplest type of reﬂex, the integrating center is a single synapse between a sensory neuron and a motor neuron. A reﬂex pathway having only one synapse in the CNS is termed a monosynaptic reﬂex arc (mono- one). More often, the integrating center consists of one or more interneurons, which may relay impulses to other interneurons as well as to a motor neuron. A polysynaptic reﬂex arc (poly- many) involves more than two types of neurons and more than one CNS synapse.

COMPONENTS OF REFLEX CIRCUIT

Motor neuron. Impulses triggered by the integrating center propagate out of the CNS along a motor neuron to the part of the body that will respond.
Effector. The part of the body that responds to the motor nerve impulse, such as a muscle or gland, is the effector. Its action is called a reﬂex. If the effector is skeletal muscle, the reﬂex is a somatic reﬂex. If the effector is smooth muscle, cardiac muscle, or a gland, the reﬂex is an autonomic (visceral) reﬂex.

Because reﬂexes are normally so predictable, they provide useful information about the health of the nervous system and can greatly aid diagnosis of disease. Damage or disease anywhere along its reﬂex arc can cause a reﬂex to be absent or abnormal. For example, tapping the patellar ligament normally causes reﬂex extension of the knee joint. Absence of the patellar reﬂex could indicate damage of the sensory or motor neurons, or a spinal cord injury in the lumbar region. Somatic reﬂexes generally can be tested simply by tapping or stroking the body surface. Next, we examine four important somatic spinal reﬂexes: the stretch reﬂex, the tendon reﬂex, the ﬂexor (withdrawal) reﬂex, and the crossed extensor reﬂex.

The Stretch Reﬂex

A stretch reﬂex causes contraction of a skeletal muscle (the effector) in response to stretching of the muscle. This type of reﬂex occurs via a monosynaptic reﬂex arc. The reﬂex can occur by activation of a single sensory neuron that forms one synapse in the CNS with a single motor neuron. Stretch reﬂexes can be elicited by tapping on tendons attached to muscles at the elbow, wrist, knee, and ankle joints. An example of a stretch reﬂex is the patellar reﬂex (knee jerk); a stretch reﬂex operates as follows:

1. Slight stretching of a muscle stimulates sensory receptors in the muscle called muscle spindles. The spindles monitor changes in the length of the muscle.

2. In response to being stretched, a muscle spindle generates one or more nerve impulses that propagate along a somatic sensory neuron through the posterior root of the spinal nerve and into the spinal cord.

3. In the spinal cord (integrating center), the sensory neuron makes an excitatory synapse with and thereby activates a motor neuron in the anterior gray horn.

4. If the excitation is strong enough, one or more nerve impulses arise in the motor neuron and propagate along its axon, which extends from the spinal cord into the anterior root and through peripheral nerves to the stimulated muscle. The axon terminals of the motor neuron form neuromuscular junctions (NMJs) with skeletal muscle ﬁbers of the stretched muscle.

5. Acetylcholine released by nerve impulses at the NMJs triggers one or more muscle action potentials in the stretched muscle (effector), and the muscle contracts. Thus, muscle stretch is followed by muscle contraction, which relieves the stretching.

COMPONENTS OD STRETCH REFLEX

In the reﬂex arc just described, sensory nerve impulses enter the spinal cord on the same side from which motor nerve impulses leave it. This arrangement is called an ipsilateral reﬂex ( same side). All monosynaptic reﬂexes are ipsilateral. In addition to the large-diameter motor neurons that innervate typical skeletal muscle ﬁbers, smaller-diameter motor neurons innervate smaller, specialized muscle ﬁbers within the muscle spindles themselves. The brain regulates muscle spindle sensitivity through pathways to these smaller motor neurons. This regulation ensures proper muscle spindle signaling over a wide range of muscle lengths during voluntary and reﬂex contractions. By adjusting how vigorously a muscle spindle responds to stretching, the brain sets an overall level of muscle tone, which is the small degree of contraction present while the muscle is at rest. Because the stimulus for the stretch reﬂex is stretching of muscle, this reﬂex helps avert injury by preventing overstretching of muscles. Although the stretch reﬂex pathway itself is monosynaptic (just two neurons and one synapse), a polysynaptic reﬂex arc to the antagonistic muscles operates at the same time. This arc involves three neurons and two synapses. Axon collateral (branch) from the muscle spindle sensory neuron also synapses with an inhibitory interneuron in the integrating center. In turn, the interneuron synapses with and inhibits a motor neuron that normally excites the antagonistic muscles. Thus, when the stretched muscle contracts during a stretch reﬂex, antagonistic muscles that oppose the contraction relax. This type of arrangement, in which the components of a neural circuit simultaneously cause contraction of one muscle and relaxation of its antagonists, is termed reciprocal innervation. Reciprocal innervation prevents conﬂict between opposing muscles and is vital in coordinating body movements. Axon collaterals of the muscle spindle sensory neuron also relay nerve impulses to the brain over speciﬁc ascending pathways. In this way, the brain receives input about the state of stretch or contraction of skeletal muscles, enabling it to coordinate muscular movements. The nerve impulses that pass to the brain also allow conscious awareness that the reﬂex has occurred.

The stretch reﬂex can also help maintain posture. For example, if a standing person begins to lean forward, the gastrocnemius and other calf muscles are stretched. Consequently, stretch reﬂexes are initiated in these muscles, which cause them to contract and reestablish the body’s upright posture. Similar types of stretch reﬂexes occur in the muscles of the shin when a standing person begins to lean backward.

The Tendon Reﬂex

The stretch reﬂex operates as a feedback mechanism to control muscle length by causing muscle contraction. In contrast, the tendon reﬂex operates as a feedback mechanism to control muscle tension by causing muscle relaxation before muscle force becomes so great that tendons might be torn. Although the tendon reﬂex is less sensitive than the stretch reﬂex, it can override the stretch reﬂex when tension is great, making you drop a very heavy weight, for example. Like the stretch reﬂex, the tendon reﬂex is ipsilateral. The sensory receptors for this reﬂex are called tendon (Golgi tendon) organs, which lie within a tendon near its junction with a muscle. In contrast to muscle spindles, which are sensitive to changes in muscle length, tendon organs detect and respond to changes in muscle tension that are caused by passive stretch or muscular contraction.

A tendon reﬂex operates as follows:

1. As the tension applied to a tendon increases, the tendon organ (sensory receptor) is stimulated (depolarized to threshold).

2. Nerve impulses arise and propagate into the spinal cord along a sensory neuron.

3. Within the spinal cord (integrating center),the sensory neuron activates an inhibitory interneuron that synapses with a motor neuron.

4. The inhibitory neurotransmitter inhibits (hyperpolarizes) the motor neuron, which then generates fewer nerve impulses.

5. The muscle relaxes and relieves excess tension.

COMPONENTS OF TENDON REFLEX

Thus, as tension on the tendon organ increases, the frequency of inhibitory impulses increases; inhibition of the motor neurons to the muscle developing excess tension (effector) causes relaxation of the muscle. In this way, the tendon reﬂex protects the tendon and muscle from damage due to excessive tension. Note that the sensory neuron from the tendon organ also synapses with an excitatory interneuron in the spinal cord. The excitatory interneuron in turn synapses with motor neurons controlling antagonistic muscles. Thus, while the tendon reﬂex brings about relaxation of the muscle attached to the tendon organ, it also triggers contraction of antagonists. Here we have another example of reciprocal innervation. The sensory neuron also relays nerve impulses to the brain by way of sensory tracts, thus informing the brain about the state of muscle tension throughout the body.

The Flexor and Crossed Extensor Reﬂexes

Another reﬂex involving a polysynaptic reﬂex arc results when, for instance, you step on a tack. In response to such a painful stimulus, you immediately withdraw your leg. This reﬂex, called the ﬂexor or withdrawal reﬂex, operates as follow:

Stepping on a tack stimulates the dendrites (sensory receptor) of a pain-sensitive neuron.

This sensory neuron then generates nerve impulses, which propagate into the spinal cord.

Within the spinal cord (integrating center),the sensory neuron activates interneurons that extend to several spinal cord segments.

The interneurons activate motor neurons in several spinal cord segments. As a result, the motor neurons generate nerve impulses, which propagate toward the axon terminals.

Acetylcholine released by the motor neurons causes the ﬂexor muscles in the thigh (effectors) to contract, producing withdrawal of the leg. This reﬂex is protective because contraction of ﬂexor muscles moves a limb away from the source of a possibly damaging stimulus.

FLEXOR (WITHDRAWAL) REFLEX

The ﬂexor reﬂex, like the stretch reﬂex, is ipsilateral—the incoming and outgoing impulses propagate into and out of the same side of the spinal cord. The ﬂexor reﬂex also illustrates another feature of polysynaptic reﬂex arcs. Moving your entire lower or upper limb away from a painful stimulus involves contraction of more than one muscle group. Hence, several motor neurons must simultaneously convey impulses to several limb muscles. Because nerve impulses from one sensory neuron ascend and descend in the spinal cord and activate interneurons in several segments of the spinal cord, this type of reﬂex is called an intersegmental reﬂex arc (inter- between). Through intersegmental reﬂex arcs, a single sensory neuron can activate several motor neurons, thereby stimulating more than one effector. The monosynaptic stretch reﬂex, in contrast, involves muscles receiving nerve impulses from one spinal cord segment only.

COMPONENTS OF FLEXOR REFLEX

Something else may happen when you step on a tack: You may start to lose your balance as your body weight shifts to the other foot. Besides initiating the ﬂexor reﬂex that causes you to withdraw the limb, the pain impulses from stepping on the tack also initiate a crossed extensor reﬂex to help you maintain your balance; it operates as follows:

Stepping on a tack stimulates the sensory receptor of a painsensitive neuron in the right foot.

This sensory neuron then generates nerve impulses, which propagate into the spinal cord.
Within the spinal cord (integrating center),the sensory neuron activates several interneurons that synapse with motor neurons on the left side of the spinal cord in several spinal cord segments. Thus, incoming pain signals cross to the opposite side through interneurons at that level, and at several levels above and below the point of entry into the spinal cord.
The interneurons excite motor neurons in several spinal cord segments that innervate extensor muscles. The motor neurons in turn generate more nerve impulses, which propagate toward the axon terminals.
Acetylcholine released by the motor neurons causes extensor muscles in the thigh (effectors) of the unstimulated left limb to contract, producing extension of the left leg. In this way, weight can be placed on the foot that must now support the entire body. A comparable reﬂex occurs with painful stimulation of the left lower limb or either upper limb.

Unlike the ﬂexor reﬂex, which is an ipsilateral reﬂex, the crossed extensor reﬂex involves a contralateral reﬂex arc Sensory impulses enter one side of the spinal cord and motor impulses exit on the opposite side. Thus, a crossed extensor reﬂex synchronizes the extension of the contralateral limb with the withdrawal (ﬂexion) of the stimulated limb. Reciprocal innervation also occurs in both the ﬂexor reﬂex and the crossed extensor reﬂex. In the ﬂexor reﬂex, when the ﬂexor muscles of a painfully stimulated lower limb are contracting, the extensor muscles of the same limb are relaxing to some degree. If both sets of muscles contracted at the same time, the two sets of muscles would pull on the bones in opposite directions, which might immobilize the limb. Because of reciprocal innervation, one set of muscles contracts while the other relaxes.

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Friday, 6 March 2015