Spinal reflexes

Reflexes may be classified in several ways. They may be named according to the effector tissues that carry out the reflex response:

• Skeletal muscle reflexes control skeletal muscles.

• Autonomic reflexes control cardiac muscle, smooth muscle and glands.

They may be named according to the region of the CNS that integrates incoming sensory information and elicits the reflex response:

• Cranial reflexes are processed within the brain.

• Spinal reflexes are processed at the level of the spinal cord.

Finally, reflexes may be innate or learned:

• Simple, or basic, reflexes are preprogrammed (built-in), unlearned responses.

• Acquired, or conditioned, reflexes are learned responses that require experience or training.

This section will examine the mechanism of simple or basic spinal reflexes that control skeletal muscles.

A reflex occurs when a particular stimulus always elicits a particular response. This response is automatic and involuntary; in other words, it occurs without conscious effort. Therefore, reflexes are specific, predictable, and, furthermore, often purposeful. For example, the withdrawal reflex causes a body part to be pulled away from a painful stimulus so that tissue injury is avoided. Spinal reflexes require no input from the brain because they are elicited entirely at the level of the spinal cord. However, while the reflex is underway, nervous impulses are also transmitted to the brain for further processing. In fact, input from the brain may modulate a reflex or alter the response to a stimulus through conscious effort.

A reflex response requires an intact neural pathway between the stimulated area and the responding muscle. This pathway is referred to as a reflex arc and includes the following components (see Figure 7.3):

• Sensory receptor

• Afferent or first-order sensory neuron

• Integrating center in the spinal cord (synapses)

Sensory Receptor

MONOSYNAPTIC REFLEX Afferent Neuron

POLYSYNAPTIC REFLEX

Efl Ti

Sensory Receptor

MONOSYNAPTIC REFLEX Afferent Neuron

POLYSYNAPTIC REFLEX

Efl Ti

Integrating Center (synapse)

Figure 7.3 Components of a reflex arc. As illustrated by the components of the reflex arc, reflexes may be processed entirely at the level of the spinal cord with no need for input from the brain. A monosynaptic reflex has a single synapse between afferent and efferent neurons; a polysynaptic reflex has two or more synapses between these neurons. In this case, interneurons lie between the sensory and motor neurons. The more interneurons involved, the more complex the response is.

Integrating Center (synapse)

Figure 7.3 Components of a reflex arc. As illustrated by the components of the reflex arc, reflexes may be processed entirely at the level of the spinal cord with no need for input from the brain. A monosynaptic reflex has a single synapse between afferent and efferent neurons; a polysynaptic reflex has two or more synapses between these neurons. In this case, interneurons lie between the sensory and motor neurons. The more interneurons involved, the more complex the response is.

• Efferent or motor neuron

• Effector tissue (skeletal muscle)

A reflex is initiated by stimulation of a sensory receptor located at the peripheral ending of an afferent or first-order sensory neuron. This afferent neuron transmits impulses to the spinal cord. Within the gray matter of the spinal cord, the afferent neuron synapses with other neurons. As such, the spinal cord serves as an integrating center for the sensory input. The afferent neuron must ultimately synapse with an efferent or motor neuron. When the afferent neuron synapses directly with the motor neuron, it forms a mono-synaptic reflex. An example of this type of reflex is the stretch reflex. When the afferent neuron synapses with an interneuron that then synapses with the motor neuron, it forms a polysynaptic reflex, e.g., the withdrawal reflex. Most reflexes are polysynaptic. The motor neuron then exits the spinal cord to innervate an effector tissue, which carries out the reflex response.

Withdrawal reflex. The withdrawal reflex is elicited by a painful or tissue-damaging stimulus. The response is to move the body part away from the source of the stimulus quickly, usually by flexing a limb. Any of the major joints, and therefore muscle groups, may be involved in a reflex, depending upon the point of stimulation. For example, all of the joints of a limb are involved when a digit, such as a finger, is stimulated (e.g., finger, wrist, elbow, shoulder). Furthermore, the withdrawal reflex is a very powerful reflex and may override other nervous impulses, such as those regarding locomotion, or walking.

Toward brain

Direction of movement (Flexion)

Direction of movement (Extension)

Figure 7.4 The withdrawal reflex coupled with the crossed-extensor reflex. A painful stimulus will elicit the withdrawal reflex, which causes flexor muscles to contract and move the affected body part away from the stimulus. At the same time, the crossed-extensor reflex causes extensor muscles in the opposite limb to contract. The straightening of the opposite limb provides support for the body.

Toward brain

Pain V& receptor -j.

Direction of movement (Flexion)

Direction of movement (Extension)

Figure 7.4 The withdrawal reflex coupled with the crossed-extensor reflex. A painful stimulus will elicit the withdrawal reflex, which causes flexor muscles to contract and move the affected body part away from the stimulus. At the same time, the crossed-extensor reflex causes extensor muscles in the opposite limb to contract. The straightening of the opposite limb provides support for the body.

An example of the mechanism of the withdrawal reflex is illustrated in Figure 7.4. When a painful stimulus activates a sensory receptor on the right foot, action potentials are transmitted along the afferent neuron to the spinal cord. By way of divergence, this neuron synapses with several other neurons within the gray matter of the spinal cord:

• Excitatory interneuron

• Inhibitory interneuron

• Second-order sensory neuron

The excitatory interneuron then synapses with the alpha motor neuron that innervates the flexor muscles of the right leg. Consequently, stimulation of the excitatory interneuron leads to stimulation of the alpha motor neuron, which then stimulates the flexor muscles to contract and pick up or withdraw the foot from the painful stimulus. The inhibitory interneuron synapses with the alpha motor neuron that innervates the extensor muscles of the right leg. Therefore, stimulation of the inhibitory interneuron leads to inhibition of the alpha motor neuron. As a result, the extensor muscles relax.

The flexor muscles and the extensor muscles are antagonistic — they cause opposite effects. Therefore, when one of these groups of muscles is activated, the other group must be inhibited. This is referred to as reciprocal inhibition. In this way, activation of the withdrawal reflex leads to unimpeded flexion.

The second-order sensory neuron transmits impulses ultimately to the left side of the brain. This permits the awareness of pain, identification of its source, and, if necessary, postural adjustment. As discussed, impulses in this pathway do not play a role in the reflex per se.

Crossed-extensor reflex. Where appropriate, the withdrawal reflex may be accompanied by the crossed-extensor reflex. In the example discussed, when the right leg is flexed or lifted, the left leg must be extended or straightened in order to support the body. In addition to stimulating interneurons on the right side of the spinal cord to influence skeletal muscle activity on the right side of the body, the afferent neuron may also stimulate interneurons on the left side of the spinal cord to influence skeletal muscle activity on the left side of the body. Once again, excitatory and inhibitory interneurons are involved; however, in this case these interneurons influence the activity of the opposite muscle groups. Stimulation of the excitatory interneuron on the left side of the spinal cord leads to stimulation of the alpha motor neuron that innervates the extensor muscles, causing the left leg to straighten. Stimulation of the inhibitory interneuron on the left side of the spinal cord leads to inhibition of the alpha motor neuron that innervates the flexor muscles. This results in unimpeded extension of the left leg and support of the body during withdrawal of the right leg.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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