Somatic Reflexes

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Objectives

When you have completed this section, you should be able to

• define reflex and explain how reflexes differ from other motor actions;

• describe the general components of a typical reflex arc; and

• explain how the basic types of somatic reflexes function.

Most of us have had our reflexes tested with a little rubber hammer; a tap near the knee produces an uncontrollable jerk of the leg, for example. In this section, we discuss what reflexes are and how they are produced by an assembly of receptors, neurons, and effectors. We also survey the different types of neuromuscular reflexes and how they are important to motor coordination.

The Nature of Reflexes

Reflexes are quick, involuntary, stereotyped reactions of glands or muscles to stimulation. This definition sums up four important properties of a reflex:

1. Reflexes require stimulation—they are not spontaneous actions but responses to sensory input.

2. Reflexes are quick—they generally involve few if any interneurons and minimum synaptic delay.

3. Reflexes are involuntary—they occur without intent, often without our awareness, and they are difficult to suppress. Given an adequate stimulus, the response is essentially automatic. You may become conscious of the stimulus that evoked a reflex, and this awareness may enable you to correct or avoid a potentially dangerous situation, but awareness is not a part of the reflex itself. It may come after the reflex action has been completed, and somatic reflexes can occur even if the spinal cord has been severed so that no stimuli reach the brain. Reflexes are stereotyped—they occur in essentially the same way every time; the response is very predictable.

Brain Reflex Zones

-T12

Figure 13.19 A Dermatome Map of the Anterior Aspect of the Body. Each zone of the skin is innervated by sensory branches of the spinal nerves indicated by the labels. Nerve C1 does not innervate the skin.

Figure 13.19 A Dermatome Map of the Anterior Aspect of the Body. Each zone of the skin is innervated by sensory branches of the spinal nerves indicated by the labels. Nerve C1 does not innervate the skin.

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Reflexes include glandular secretion and contractions of all three types of muscle. They also include some learned responses, such as the salivation of dogs in response to a sound they have come to associate with feeding time, first studied by Ivan Pavlov and named conditioned reflexes. In this section, however, we are concerned with unlearned skeletal muscle reflexes that are mediated by the brainstem and spinal cord. They result in the involuntary contraction of a muscle—for example, the quick withdrawal of your hand from a hot stove or the lifting of your foot when you step on something sharp. These are somatic reflexes, since they involve the somatic nervous system. Chapter 15 concerns visceral reflexes. The somatic reflexes have traditionally been called spinal reflexes, although some visceral reflexes also involve the spinal cord, and some somatic reflexes are mediated more by the brain than by the spinal cord.

A somatic reflex employs a reflex arc, in which signals travel along the following pathway:

1. somatic receptors in the skin, a muscle, or a tendon;

2. afferent nerve fibers, which carry information from these receptors into the dorsal horn of the spinal cord;

3. interneurons, which integrate information; these are lacking from some reflex arcs;

4. efferent nerve fibers, which carry motor impulses to the skeletal muscles; and

5. skeletal muscles, the somatic effectors that carry out the response.

The Muscle Spindle

Many somatic reflexes involve stretch receptors in the muscles called muscle spindles. These are among the body's proprioceptors—sense organs that monitor the position and movements of body parts. Muscle spindles are especially abundant in muscles that require fine control. The hand and foot have 100 or more spindles per gram of muscle, whereas there are relatively few in large muscles with coarse movements and none at all in the middle-ear muscles. Muscle spindles provide the cerebellum with the feedback it needs to regulate the tension in the skeletal muscles.

Muscle spindles are about 4 to 10 mm long, tapered at the ends, and scattered throughout the fleshy part of a muscle (fig. 13.20). A spindle contains 3 to 12 modified muscle fibers and a few nerve fibers, all wrapped in a fibrous capsule. The muscle fibers within a spindle are called intra-fusal23 fibers, while those of the rest of the muscle are called extrafusal fibers. Only the two ends of an intrafusal fiber have sarcomeres and are able to contract. The middle portion acts as the stretch receptor. There are two classes of intrafusal fibers: nuclear chain fibers, which have a single file of nuclei in the noncontractile region, and nuclear bag fibers, which are about twice as long and have nuclei clustered in a thick midregion.

Muscle spindles have three types of nerve fibers:

1. Primary afferent fibers, which end in annulospiral endings that coil around the middle of nuclear chain and nuclear bag fibers. These respond mainly to the onset of muscle stretch.

2. Secondary afferent fibers, which have flower-spray endings, somewhat resembling the dried head of a wildflower, wrapped primarily around the ends of the nuclear chain fibers. These respond mainly to prolonged stretch.

3. Gamma (7) motor neurons, which originate in the ventral horn of the spinal cord and lead to the contractile ends of the intrafusal fibers. The name distinguishes them from the alpha (a) motor neurons, which innervate the extrafusal fibers. Gamma motor neurons adjust the tension in a muscle spindle to variations in the length of the muscle. When a muscle shortens, the 7 motor neurons stimulate the ends of the intrafusal fibers to contract slightly. This keeps the intrafusal fibers taut and responsive at all times. Without this feedback, the spindles would become flabby when a skeletal muscle shortened. This feedback is clearly very important, because 7 motor neurons constitute about one-third of all the motor fibers in a spinal nerve.

The Stretch Reflex

When a muscle is stretched, it "fights back"—it contracts, maintains increased tonus, and feels stiffer than an unstretched muscle. This response, called the stretch (myotatic24) reflex, helps to maintain equilibrium and posture. For example, if your head starts to tip forward, it stretches muscles such as the semispinalis and splenius capitis of the nuchal region (back of your neck). This stimulates their muscle spindles, which send afferent signals to the cerebellum by way of the brainstem. The cerebellum integrates this information and relays it to the cerebral cortex, and the cortex sends signals back to the nuchal muscles. The muscles contract and raise your head.

Stretch reflexes often feed back not to a single muscle but to a set of synergists and antagonists. Since the contraction of a muscle on one side of a joint stretches the antagonistic muscle on the other side, the flexion of a joint triggers a stretch reflex in the extensors, and extension

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Connective tissue sheath

Extrafusal fibers

Chapter 13 The Spinal Cord, Spinal Nerves, and Somatic Reflexes 505

Connective tissue sheath

Extrafusal fibers

Stretch Reflex Newborn

Figure 13.20 A Muscle Spindle and Its Innervation.

Figure 13.20 A Muscle Spindle and Its Innervation.

stimulates a stretch reflex in the flexors. Consequently, stretch reflexes are valuable in stabilizing joints by balancing the tension of the extensors and flexors. They also dampen (smooth) muscle action. Without stretch reflexes, a person's movements tend to be jerky. Stretch reflexes are especially important in coordinating vigorous and precise movements such as dance.

A stretch reflex is mediated primarily by the brain and is not, therefore, strictly a spinal reflex, but a weak component of it is spinal and occurs even if the spinal cord is severed from the brain. The spinal component can be more pronounced if a muscle is stretched very suddenly. This occurs in a tendon reflex—the reflexive contraction of a muscle when its tendon is tapped, as in the familiar knee-jerk (patellar) reflex. Tapping the patellar ligament with a reflex hammer suddenly stretches the quadriceps femoris muscle of the thigh (fig. 13.21). This stimulates numerous muscle spindles in the quadriceps and sends an intense volley of signals to the spinal cord, mainly by way of primary afferent fibers.

In the spinal cord, the primary afferent fibers synapse directly with the a motor neurons that return to the muscle, thus forming monosynaptic reflex arcs. That is, there is only one synapse between the afferent and efferent neuron, therefore little synaptic delay and a very

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Form and Function, Third Reflexes Edition

506 Part Three Integration and Control

@ Primary afferent neuron stimulates a motor neuron to extensor muscle

3) Primary afferent neuron excited

@ Primary afferent neuron stimulates inhibitory interneuron

(2 Muscle spindle stimulated

@ Primary afferent neuron stimulates inhibitory interneuron

@ Primary afferent neuron stimulates a motor neuron to extensor muscle

3) Primary afferent neuron excited

(2 Muscle spindle stimulated

Neuron Muscle Spindle

(T) Extensor muscle stretched

Figure 13.21 The Patellar Tendon Reflex Arc and Reciprocal Inhibition of the Antagonistic Muscle. Plus signs indicate excitation of a postsynaptic cell (EPSPs) and minus signs indicate inhibition (IPSPs). The tendon reflex is occurring in the quadriceps femoris muscle (red arrow), while the hamstring muscles are exhibiting reciprocal inhibition (blue arrow) so they do not contract and oppose the quadriceps. Why is no IPSP shown at point 8 if the contraction of this muscle is being inhibited?

@ Interneuron inhibits a motor neuron to flexor muscle

(5) a motor neuron stimulates extensor muscle to contract

@ Interneuron inhibits a motor neuron to flexor muscle

(5) a motor neuron stimulates extensor muscle to contract

Delayed Return Tendon Reflex

@ Flexor muscle

(antagonist) relaxes

(T) Extensor muscle stretched

@ Flexor muscle

(antagonist) relaxes

Figure 13.21 The Patellar Tendon Reflex Arc and Reciprocal Inhibition of the Antagonistic Muscle. Plus signs indicate excitation of a postsynaptic cell (EPSPs) and minus signs indicate inhibition (IPSPs). The tendon reflex is occurring in the quadriceps femoris muscle (red arrow), while the hamstring muscles are exhibiting reciprocal inhibition (blue arrow) so they do not contract and oppose the quadriceps. Why is no IPSP shown at point 8 if the contraction of this muscle is being inhibited?

prompt response. The a motor neurons excite the quadriceps muscle, making it contract and creating the knee jerk.

There are many other tendon reflexes. A tap on the calcaneal tendon causes plantar flexion of the foot, a tap on the triceps brachii tendon causes extension of the elbow, and a tap on the cheek causes clenching of the jaw. Testing somatic reflexes is valuable in diagnosing many diseases that cause exaggeration, inhibition, or absence of reflexes, such as neurosyphilis, diabetes mellitus, multiple sclerosis, alcoholism, electrolyte imbalances, and lesions of the nervous system.

Stretch reflexes and other muscle contractions often depend on reciprocal inhibition, a reflex phenomenon that prevents muscles from working against each other by inhibiting antagonists. In the knee jerk, for example, the quadriceps femoris would not produce much joint movement if its antagonists, the hamstring muscles, contracted at the same time. But reciprocal inhibition prevents that from happening. Some branches of the sensory fibers from the muscle spindles in the quadriceps stimulate spinal cord interneurons which, in turn, inhibit the a motor neurons of the hamstring muscles (fig. 13.21). The hamstring muscles therefore remain relaxed and allow the quadriceps to extend the knee.

The Flexor (Withdrawal) Reflex

A flexor reflex is the quick contraction of flexor muscles resulting in the withdrawal of a limb from an injurious stimulus. For example, suppose you are wading in a lake and step on a broken bottle with your right foot (fig. 13.22). Even before you are consciously aware of the pain, you quickly pull your foot away before the glass penetrates any deeper. This action involves contraction of the flexors and relaxation of the extensors in that limb; the latter is another case of reciprocal inhibition.

The protective function of this reflex requires more than a quick jerk like a tendon reflex, so it involves more complex neural pathways. Sustained contraction of the flexors is produced by a parallel after-discharge circuit in the spinal cord (see fig. 12.27, p. 473). This circuit is part of a polysynaptic reflex arc—a pathway in which signals travel over many synapses on their way back to the muscle. Some signals follow routes with only a few synapses and return to the flexor muscles quickly. Others follow routes with more synapses, and therefore more delay, so they reach the flexor muscles a little later. Consequently, the flexor muscles receive prolonged output from the spinal cord and not just one sudden stimu

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lus as in a stretch reflex. By the time these efferent signals begin to die out, you will probably be consciously aware of the pain and begin taking voluntary action to prevent further harm.

The Crossed Extensor Reflex

In the preceding situation, if all you did was to quickly lift the injured leg from the lake bottom, you would fall over. To prevent this and maintain your balance, other reflexes shift your center of gravity over the leg that is still on the ground. The crossed extensor reflex is the contraction of extensor muscles in the limb opposite from the one that is withdrawn (fig. 13.22). It extends that limb and enables you to keep your balance. To produce this reflex, branches of the afferent nerve fibers cross from the stimulated side of the body to the contralateral side of the spinal cord. There, they synapse with interneurons, which, in turn, excite or inhibit a motor neurons to the muscles of the contralateral limb.

In the ipsilateral leg (the side that was hurt), you would contract your flexors and relax your extensors to lift the leg from the ground. On the contralateral side, you would relax your flexors and contract the extensors to stiffen that leg, since it must suddenly support your entire body. At the same time, signals travel up the spinal cord and cause contraction of contralateral muscles of the hip and abdomen to shift your center of gravity over the

(?) Sensory neuron activates multiple interneurons

(?) Sensory neuron activates multiple interneurons

Spine Primary Afferent Neuron

Contralateral motor neurons to extensor excited

6) Contralateral extensor contracts

(T) Stepping on glass stimulates pain receptors in right foot

Contralateral motor neurons to extensor excited

6) Contralateral extensor contracts

(T) Stepping on glass stimulates pain receptors in right foot

Withdrawal of right leg (flexor reflex)

Extension of left leg (crossed extensor reflex)

Figure 13.22 The Flexor and Crossed Extensor Reflexes. The pain stimulus triggers a withdrawal reflex, which results in contraction of flexor muscles of the injured limb. At the same time, a crossed extensor reflex results in contraction of extensor muscles of the opposite limb. The latter reflex aids in balance when the injured limb is raised. Note that for each limb, while the agonist contracts, the a motor neuron to its antagonist is inhibited, as indicated by the red minus signs in the spinal cord.

Would you expect this reflex arc to show more synaptic delay, or less, than the ones in figure 13.15? Why?

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508 Part Three Integration and Control extended leg. To a large extent, the coordination of all these muscles and maintenance of equilibrium is mediated by the cerebellum and cerebral cortex.

The flexor reflex employs an ipsilateral reflex arc— one in which the sensory input and motor output are on the same sides of the spinal cord. The crossed extensor reflex employs a contralateral reflex arc, in which the input and output are on opposite sides. An intersegmental reflex arc is one in which the input and output occur at different levels (segments) of the spinal cord—for example, when pain to the foot causes contractions of abdominal and hip muscles higher up the body. Note that all of these reflex arcs can function simultaneously to produce a coordinated protective response to pain.

The Golgi Tendon Reflex

Golgi tendon organs are proprioceptors located in a tendon near its junction with a muscle (fig. 13.23). A tendon organ is about 1 mm long and consists of an encapsulated tangle of knobby nerve endings entwined in the collagen fibers of the tendon. As long as the tendon is slack, its collagen fibers are slightly spread and they put little pressure on the nerve endings woven among them. When muscle contraction pulls on the tendon, the collagen fibers come together like the two sides of a stretched rubber band and squeeze the nerve endings between them. The nerve fiber sends signals to the spinal cord that provide the CNS with feedback on the degree of muscle tension at the joint.

The Golgi tendon reflex is a response to excessive tension on the tendon. It inhibits a motor neurons to the muscle so the muscle does not contract as strongly. This serves to moderate muscle contraction before it tears a tendon or pulls it loose from the muscle or bone. Nevertheless, strong muscles and quick movements sometimes damage a tendon before the reflex can occur, causing such athletic injuries as a ruptured calcaneal tendon.

The Golgi tendon reflex also functions when some parts of a muscle contract more than others. It inhibits the fibers connected with overstimulated tendon organs so that their contraction is more comparable to the contraction of the rest of the muscle. This reflex spreads the workload more evenly over the entire muscle, which is beneficial in such actions as maintaining a steady grip on a tool.

Table 13.7 and insight 13.5 describe some injuries and other disorders of the spinal cord and spinal nerves.

Insight 13.5 Clinical Application

Spinal Cord Trauma

Each year in the United States, 10,000 to 12,000 people become paralyzed by spinal cord trauma, usually as a result of vertebral fractures. The group at greatest risk is males from 16 to 30 years old, because of

Golgi Tendon Organ
Figure 13.23 A Golgi Tendon Organ.

their high-risk behaviors. Fifty-five percent of their injuries are from automobile and motorcycle accidents, 18% from sports, and 15% from gunshot and stab wounds. Elderly people are also at above-average risk because of falls, and in times of war, battlefield injuries account for many cases.

Effects of Injury

Complete transection (severance) of the spinal cord causes immediate loss of motor control at and below the level of the injury. Transection superior to segment C4 presents a threat of respiratory failure. Victims also lose all sensation from the level of injury and below, although some patients temporarily feel burning pain within one or two dermatomes of the level of the lesion.

In the early stage, victims exhibit a syndrome (a suite of signs and symptoms) called spinal shock. The muscles below the level of injury exhibit flaccid paralysis and an absence of reflexes because of the lack of stimulation from higher levels of the CNS. For 8 days to 8 weeks after the accident, the patient typically lacks bladder and bowel reflexes and thus retains urine and feces. Lacking sympathetic stimulation to the blood vessels, a patient may exhibit neurogenic shock in which the vessels dilate and blood pressure drops dangerously low. Fever may occur because the hypothalamus cannot induce sweating to

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Table 13.7 Some Disorders of the Spinal Cord and Spinal Nerves

Guillain-Barre syndrome

An acute demyelinating nerve disorder often triggered by viral infection, resulting in muscle weakness, elevated heart rate, unstable blood pressure, shortness of breath, and sometimes death from respiratory paralysis

Neuralgia

General term for nerve pain, often caused by pressure on spinal nerves from herniated intervertebral discs or other causes

Paresthesia

Abnormal sensations of prickling, burning, numbness, or tingling; a symptom of nerve trauma or other peripheral nerve disorders

Peripheral neuropathy

Any loss of sensory or motor function due to nerve injury; also called nerve palsy

Rabies (hydrophobia)

A disease usually contracted from animal bites, involving viral infection that spreads via somatic motor nerve fibers to the CNS and then autonomic nerve fibers, leading to seizures, coma, and death; invariably fatal if not treated before CNS symptoms appear

Spinal meningitis

Inflammation of the spinal meninges due to viral, bacterial, or other infection

Disorders described elsewhere

Amyotrophic lateral sclerosis p. 490 Leprosy p. 589 Sciatica p. 494

Carpal tunnel syndrome p. 365

Multiple sclerosis p. 453 Shingles p. 493

Crutch paralysis p. 494

Poliomyelitis p. 490 Spina bifida p. 484

Diabetic neuropathy p. 670

Paraplegia p. 509 Spinal cord trauma p. 508

Hemiplegia p. 509

Quadriplegia p. 509

cool the body. Spinal shock can last from a few days to 3 months, but typically lasts 7 to 20 days.

As spinal shock subsides, somatic reflexes begin to reappear, at first in the toes and progressing to the feet and legs. Autonomic reflexes also reappear. Contrary to the earlier urinary and fecal retention, a patient now has the opposite problem, incontinence, as the rectum and bladder empty reflexively in response to stretch. Both the somatic and autonomic nervous systems typically exhibit exaggerated reflexes, a state called hyperreflexia or the mass reflex reaction. Stimuli such as a full bladder or cutaneous touch can trigger an extreme cardiovascular reaction. The systolic blood pressure, normally about 120 mmHg, jumps to as high as 300 mmHg. This causes intense headaches and sometimes a stroke. Pressure receptors in the major arteries sense this rise in blood pressure and activate a reflex that slows the heart, sometimes to a rate as low as 30 or 40 beats/minute (bradycardia), compared to a normal rate of 70 to 80. The patient may also experience profuse sweating and blurred vision. Men at first lose the capacity for erection and ejaculation. They may recover these functions later and become capable of climaxing and fathering children, but without sexual sensation. In females, menstruation may become irregular or cease.

The most serious permanent effect of spinal cord trauma is paralysis. The flaccid paralysis of spinal shock later changes to spastic paralysis as spinal reflexes are regained, but lack inhibitory control from the brain. Spastic paralysis typically starts with chronic flexion of the hips and knees (flexor spasms) and progresses to a state in which the limbs become straight and rigid (extensor spasms). Three forms of muscle paralysis are paraplegia, a paralysis of both lower limbs resulting from spinal cord lesions at levels T1 to L1; quadriple-gia, the paralysis of all four limbs resulting from lesions above level C5; and hemiplegia, paralysis of one side of the body, resulting not from spinal cord injuries but usually from a stroke or other brain lesion. Spinal cord lesions from C5 to C7 can produce a state of partial quadriplegia—total paralysis of the lower limbs and partial paralysis (paresis, or weakness) of the upper limbs.

Pathogenesis

Spinal cord trauma produces two stages of tissue destruction. The first is instantaneous—the destruction of cells by the traumatic event itself. The second wave of destruction, involving tissue death by necrosis and apoptosis, begins in minutes and lasts for days. It is far more destructive than the initial injury, typically converting a lesion in one spinal cord segment to a lesion that spans four or five segments, two above and two below the original site.

Microscopic hemorrhages appear in the gray matter and pia mater within minutes and grow larger over the next 2 hours. The white matter becomes edematous (swollen). This hemorrhaging and edema spread to adjacent segments of the cord, and can fatally affect respiration or brainstem function when it occurs in the cervical region. Ischemia (iss-KEE-me-uh), the lack of blood, quickly leads to tissue necrosis. The white matter regains circulation in about 24 hours, but the gray matter remains ischemic. Inflammatory cells (leukocytes and macrophages) infiltrate the lesion as the circulation recovers, and while they clean up necrotic tissue, they also contribute to the damage by releasing destructive free radicals and other toxic chemicals. The necrosis worsens, and is accompanied by another form of cell death, apoptosis (see chapter 5). Apoptosis of the spinal oligodendrocytes, the myelinating glial cells of the CNS, results in demyelination of spinal nerve fibers, followed by death of the neurons.

In as little as 4 hours, this second wave of destruction, called posttraumatic infarction, consumes about 40% of the cross-sectional area of the spinal cord; within 24 hours, it destroys 70%. As many as five segments of the cord become transformed into a fluid-filled cavity, which is replaced with collagenous scar tissue over the next 3 to 4 weeks. This scar is one of the obstacles to the regeneration of lost nerve fibers.

Treatment

The first priority in treating a spinal injury patient is to immobilize the spine to prevent further injury to the cord. Respiratory or other life support may also be required. Methylprednisolone, a steroid, dramatically

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510 Part Three Integration and Control improves recovery. Given within 3 hours of the trauma, it reduces injury to cell membranes and inhibits inflammation and apoptosis.

After these immediate requirements are met, reduction (repair) of the fracture is important. If a CT or MRI scan indicates spinal cord compression by the vertebral canal, a decompression laminectomy may be performed, in which the vertebral arch is removed from the affected region. CT and MRI have helped a great deal in recent decades for assessing vertebral and spinal cord damage, guiding surgical treatment, and improving recovery. Physical therapy is important for maintaining muscle and joint function as well as promoting the patient's psychological recovery.

Treatment of spinal cord injuries is a lively area of medical research today. Some current interests are the use of antioxidants to reduce free radical damage, and the implantation of embryonic stem cells, which has produced significant (but not perfect) recovery from spinal cord lesions in rats.

Before You Go On

Answer the following questions to test your understanding of the preceding section:

9. Name five structural components of a typical somatic reflex arc. Which of these is absent from a monosynaptic arc?

10. State the function of each of the following in a muscle spindle: intrafusal fiber, annulospiral ending, and 7 motor neuron.

11. Explain how nerve fibers in a tendon sense the degree of tension in a muscle.

12. Why must the withdrawal reflex, but not the stretch reflex, involve a polysynaptic reflex arc?

13. Explain why the crossed extensor reflex must accompany a withdrawal reflex of the leg.

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Responses

  • Leon Krueger
    What is a reflex arc that lacks an interneuron?
    8 years ago
  • Federica
    When does a somatic reflex occur?
    8 years ago
  • Biniam
    How reflexes differ from motor actions?
    6 years ago
  • lillo
    What are the examples of three classes of infant reflexes and discuss their functions?
    5 years ago
  • Pia Palermo
    What reflex phenomenon prevents muscles from working against each other by inhibiting antagonists?
    3 years ago
  • Paula
    Why must the crossed extension reflex accompany a withdrawal reflex of the leg?
    1 year ago

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