Reflex arc. Stretch reflex. Golgi tendon reflex. Withdraw reflex. What is a reflex? Reflexes are rapid, involuntary responses to stimuli which are mediated over simple nerve pathways called reflex arcs. Involuntary reflexes are very fast, traveling in milliseconds.
The fastest impulses can reach miles per hour. Definition of a reflex. Reflex arcs have five essential components:. The receptor at the end of a sensory neuron reacts to a stimulus. The sensory neuron conducts nerve impulses along an afferent pathway towards the CNS. The integration center consists of one or more synapses in the CNS. A motor neuron conducts a nerve impulse along an efferent pathway from the integration center to an effector.
An effector responds to the efferent impulses by contracting if the effector is a muscle fiber or secreting a product if the. Reflexes can be categorized as either autonomic or somatic. Autonomic reflexes are not subject to conscious control, are mediated by the autonomic division of the nervous system, and usually involve the activation of smooth muscle, cardiac muscle, and glands.
Somatic reflexes involve stimulation of skeletal muscles by the somatic division of the nervous system. Most reflexes are polysynaptic involving more than two neurons and involve the activity of interneurons or association neurons in the integration center. Some reflexes; however, are monosynaptic "one synapse" and only involve two neurons, one sensory and one motor.
Since there is some delay in neural transmission at the synapses, the more synapses that are encountered in a reflex pathway, the more time that is required to effect the reflex. The knee jerk reflex is called a monosynaptic reflex. This means that there is only 1 synapse in the neural circuit needed to complete the reflex. It only takes about 50 milliseconds of time between the tap and the start of the leg kick The tap below the knee causes the thigh muscle to stretch.
Information is sent to the spinal cord. After one synapse in the ventral horn of the spinal cord, the information is sent back out to the muscle Tonicity of Skeletal Muscle. Therefore, to understand the control of tone it is imperative to understand function of the muscle spindle. The functional value of reflexes. The Stretch Reflex. As briefly described above the muscle spindle plays an integral role in the stretch reflex. In brief:.
As a muscle lengthens the MS is stretched. Impulses are conducted towards the CNS spinal cord where the afferent fiber divides into several colateral fibers. Spinal reflexes include the stretch reflex, the Golgi tendon reflex, the crossed extensor reflex, and the withdrawal reflex.
The stretch reflex myotatic reflex is a muscle contraction in response to stretching within the muscle. This reflex has the shortest latency of all spinal reflexes. It is a monosynaptic reflex that provides automatic regulation of skeletal muscle length.
When a muscle lengthens, the muscle spindle is stretched and its nerve activity increases. This increases alpha motor neuron activity, causing the muscle fibers to contract and thus resist the stretching. A secondary set of neurons also causes the opposing muscle to relax. The reflex functions to maintain the muscle at a constant length. The Golgi tendon reflex is a normal component of the reflex arc of the peripheral nervous system.
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 Golgi tendon receptors, and 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 a passive stretch or muscular contraction. Jendrassik maneuver : The Jendrassik maneuver is a medical maneuver wherein the patient flexes both sets of fingers into a hook-like form and interlocks those sets of fingers together note the hands of the patient in the chair.
This maneuver is used often when testing the patellar reflex, as it forces the patient to concentrate on the interlocking of the fingers and prevents conscious inhibition or influence of the reflex.
The crossed extensor reflex is a withdrawal reflex. The reflex occurs when the flexors 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 nail, the leg that is stepping on the nail pulls away, while the other leg takes the weight of the whole body. The crossed extensor reflex is contralateral, meaning the reflex occurs on the opposite side of the body from the stimulus.
This interneuron excites the alpha motor neurons that excite the hip flexor muscle, allowing the coordinated activity of two muscle groups to withdraw the whole leg away from the painful stimulus. Thus, spinal reflexes work not only at a single joint; they can also coordinate the activity of multiple joints simultaneously. Nolte, When the knee joints and the hip joints are flexed, the antagonist extensor muscles must be inhibited just as in the stretch reflex. Thus, the Group III afferents innervate inhibitory interneurons that in turn innervate the alpha motor neurons controlling the antagonist muscle.
Further circuitry is needed to make the flexor reflex adaptive. Because the weight of the body is supported by both legs, the flexor reflex must coordinate the activity not only of the leg being withdrawn but also of the opposite leg Figure 2. Imagine stepping on a tack, and having the flexor reflex withdraw your right leg immediately. The left leg must simultaneously extend in order to support the body weight that would have been supported by the right leg.
Without this coordination of the two legs, the shift in body mass would cause a loss of balance. Thus, the flexor reflex incorporates a crossed extension reflex. A branch of the Group III afferent innervates an excitatory interneuron that sends its axon across the midline into the contralateral spinal cord.
There it excites the alpha motor neurons that innervate the extensor muscles of the opposite leg, allowing balance and body posture to be maintained. Axons of alpha motor neurons bifurcate in the spinal cord and innervate a special inhibitory interneuron called the Renshaw cell Figure 2.
This interneuron innervates and inhibits the very same motor neuron that caused it to fire. Thus, a motor neuron regulates its own activity by inhibiting itself when it fires. This negative feedback loop is thought to stabilize the firing rate of motor neurons. The reflex circuits demonstrate that sophisticated neural processing occurs at the lowest level of the motor hierarchy.
These automatic reflexes can be modulated, however, by higher levels of the hierarchy. For example, when touching an iron to see if it is hot, your flexor reflex may be hypersensitive. As a result, you pull your hand away repeatedly before even touching the iron, anticipating that it may be hot. Conversely, if you remove a hot dish from the oven and the heat starts to go through the oven mitt, you will suppress the flexor response so that you do not drop your dinner as you rush to put it down on a table.
These modulations both facilitatory and inhibitory of the spinal reflexes arise from the descending pathways from the brainstem and cortex. Voluntary movement and some sensory-driven reflex actions are also controlled by the descending pathways. The corticospinal system controls motor neurons and interneurons in the spinal cord.
The corticobulbar system controls brainstem nuclei that innervate cranial muscles. Although the motor system is organized hierarchically, the hierarchy is not a simple chain of processing from higher to lower areas. Many pathways enable the different levels of the hierarchy to influence each other. Thus, the flow of information through the motor system has both a serial organization communication between levels and a parallel organization multiple pathways between each level. This parallel organization is critically important in understanding the various dysfunctions that can result from damage to the motor system.
If the motor hierarchy had a strictly serial organization, like a series of links on a chain, then damage to any part of the system would produce severe deficits or paralysis in almost all types of movements. However, because of the parallel nature of processing, paralysis is actually a relatively rare outcome, produced by damage to the lowest level of the hierarchy. Damage to higher levels results in deficits in motor planning, initiation, coordination, and so forth, but movement is still possible.
The parallel nature of organization is also important for the ability of undamaged parts of the motor system to compensate at least partially for injuries to other parts of the system. Descending motor pathways arise from multiple regions of the brain and send axons down the spinal cord that innervate alpha motor neurons, gamma motor neurons, and interneurons. The motor neurons are topographically organized in the anterior horn of the spinal cord according to two rules: the flexor-extensor rule and the proximal-distal rule Figure 2.
Flexor-extensor rule : motor neurons that innervate flexor muscles are located posteriorly to motor neurons that innervate extensor muscles. Proximal-distal rule : motor neurons that innervate distal muscles e. C lick on the labels to see the highlighted area. Corticospinal tracts. The corticospinal tract originates in the motor cortex Figure 2. The axons of motor projection neurons collect in the internal capsule, and then course through the crus cerebri cerebral peduncle in the midbrain.
At the level of the medulla, these axons form the medullary pyramids on the ventral surface of the brainstem hence, this tract is also called the pyramidal tract. At the level of the caudal medulla, the corticospinal tract splits into two tracts.
These axons continue to course through the lateral funiculus of the spinal cord, before synapsing either directly onto alpha motor neurons or onto interneurons in the ventral horn. When they reach the spinal segment at which they terminate, they cross over to the contralateral side through the anterior white commissure and innervate alpha motor neurons or interneurons in the anterior horn.
Thus, both the lateral and anterior corticospinal tracts are crossed pathways; they cross the midline at different locations, however. The corticospinal tract along with the corticobulbar tract is the primary pathway that carries the motor commands that underlie voluntary movement.
Reflexes allow your body to react in ways that help you to be safe, to stand upright, and to be active. Imagine a typical day. You might be thinking of practicing your sport or musical instrument, walking to school, or making a snack. In all of these actions, you are thinking, but at the same time, there are also reflexes that you are unaware of happening inside your body. These reflexes are built naturally into the body, and they exist at birth and change as we grow older.
Reflexes are kind of like safety features for survival that allow us to move in response to something in the environment. Reflexes can act to protect you in many ways, including removing your hand from a hot or sharp object, or ducking when a loud and sudden sound occurs. These fast actions are reflex responses!
The fact that these responses are automatic shows that reflexes occur at a rate that is far too fast for the brain to be involved with the response. Actions that occur without the involvement of the brain are called involuntary actions, while planned actions from the brain, like throwing a ball or strumming a guitar, are called voluntary actions. After the reflex action has happened, the brain does become aware and tells you what happened.
At this point, the brain might even add to the action. For example, you might have ducked as an involuntary response to a very loud noise, but when the brain becomes involved you learn why you ducked down and the brain sends the voluntary action to respond—maybe to stand back up.
In order for reflexes to work, messages need to move around the body. These messages are action potentials , and they travel along the neurons and send messages, special parts of the neurons are involved.
The neuron has three different parts that allow signals to be sensed, to travel, and then move to another neuron or muscle. These three parts are called the dendrites, the axon, and the nerve ending Figure 1. The dendrites receive information from the sensor or other neurons. This information then moves to the axon, which travels to or from the spinal cord.
The action potential travels from the nerve endings at one end of the neuron to the next neuron. Many reflexes start at the muscle or skin and go to the spinal cord.
When the action potential reaches the nerve ending, the signal is transferred to another neuron, such as an interneuron or motor neuron. The action potential then travels outside the spinal cord to a muscle. But the neurons do not touch each other in the spinal cord and do not touch at the muscle.
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