Crossed-extensor Diagram

Roots

Brachial Plexus Mcgraw Hill

Anterior root Posterior root

Supraclavicular nerve

Branch to brachial plexus Phrenic nerve

Great Occipital Nerve
Segmental branch Hypoglossal nerve (XII) Lesser occipital nerve Great auricular nerve Transverse cervical nerve

Anterior root Posterior root

-Ansa cervicalis

Supraclavicular nerve

Branch to brachial plexus Phrenic nerve

Figure 13.14 The Cervical Plexus.

Aphren = diaphragm

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Table 13.4 The Brachial Plexus

The brachial plexus (figs. 13.15 and 13.16) is formed by the ventral rami of nerves C4 to T2. It passes over the first rib into the axilla and innervates the upper limb and some muscles of the neck and shoulder. It gives rise to nerves for cutaneous sensation, muscle contraction, and proprioception from the joints and muscles.

The subdivisions of this plexus are called roots, trunks, divisions, and cords (color-coded in figure 13.15). The five roots are the ventral rami of nerves C5 to T1, which provide most of the fibers to this plexus (C4 and T2 contribute partially). The five roots unite to form the upper, middle, and lower trunks. Each trunk divides into an anterior and posterior division, and finally the six divisions merge to form three large fiber bundles—the posterior, medial, and lateral cords.

Axillary Nerve

Composition: Motor and somatosensory

Origin: Posterior cord of brachial plexus

Sensory innervation: Skin of lateral shoulder and arm; shoulder joint

Motor innervation: Deltoid and teres minor

Radial Nerve

Composition: Motor and somatosensory Origin: Posterior cord of brachial plexus

Sensory innervation: Skin of posterior aspect of arm, forearm, and wrist; joints of elbow, wrist, and hand Motor innervation: Muscles of posterior arm and forearm: triceps brachii, supinator, anconeus, brachioradialis, extensor carpi radialis brevis, extensor carpi radialis longus, and extensor carpi ulnaris

Trunks

  • Anterior divisions
  • Posterior divisions

C5 flv I

T1 S

C5 flv I

T1 S

Sensory Innervation Arm

Medial cord

Figure 13.15 The Brachial Plexus.

Medial cord

Dorsal scapular nerve Long thoracic nerve Suprascapular nerve Subclavian nerve Posterior cord Axillary nerve Subscapular nerve Thoracodorsal nerve Radial nerve Lateral cord

Musculocutaneous nerve Medial and lateral pectoral nerves Median nerve Ulnar nerve

Medial cutaneous antebrachial nerve Medial brachial cutaneous nerve v.-i^—__

Thoracodorsal Nerve Innervation

Clavicle Lateral cord Posterior cord Medial cord Axillary nerve Scapula

Musculocutaneous nerve

Median nerve Humerus Radial nerve Ulna

Ulnar nerve

Median nerve

Radial nerve Radius

Superficial branch of ulnar nerve

Digital branch of median nerve

Digital branch of ulnar nerve

Figure 13.15 The Brachial Plexus.

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Musculocutaneous Nerve

Composition: Motor and somatosensory

Origin: Lateral cord of brachial plexus

Sensory innervation: Skin of lateral aspect of forearm

Motor innervation: Muscles of anterior arm: coracobrachial, biceps brachii, and brachialis Median Nerve

Composition: Motor and somatosensory Origin: Medial cord of brachial plexus

Sensory innervation: Skin of lateral two-thirds of hand, joints of hand

Motor innervation: Flexors of anterior forearm; thenar muscles; first and second lumbricals

Ulnar Nerve

Composition: Motor and somatosensory

Origin: Medial cord of brachial plexus

Sensory innervation: Skin of medial part of hand; joints of hand

Motor innervation: Flexor carpi ulnaris, flexor digitorum profundus, adductor pollicis, hypothenar muscles, interosseous muscles, and third and fourth lumbricals

Figure 13.16 Photograph of the Brachial Plexus. Anterior view of the right shoulder, also showing three of the cranial nerves, the sympathetic trunk, and the phrenic nerve (a branch of the cervical plexus). Most of the other structures resembling nerves in this photograph are blood vessels. (a. = artery; m. = muscle; n. = nerve.)

Anterior View Right Shoulder Image

Figure 13.16 Photograph of the Brachial Plexus. Anterior view of the right shoulder, also showing three of the cranial nerves, the sympathetic trunk, and the phrenic nerve (a branch of the cervical plexus). Most of the other structures resembling nerves in this photograph are blood vessels. (a. = artery; m. = muscle; n. = nerve.)

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Table 13.5 The Lumbar Plexus

The lumbar plexus (fig. 13.17) is formed from the ventral rami of nerves L1 to L4 and some fibers from T12. With only five roots and two divisions, it is less complex than the brachial plexus.

Iliohypogastric Nerve

Femoral Nerve

Composition: Motor and somatosensory Sensory innervation: Skin of anterior abdominal wall Motor innervation: Internal and external obliques and transversus abdominis

Ilioinguinal Nerve

Composition: Motor and somatosensory

Sensory innervation: Skin of anterior and lateral thigh; medial leg and foot Motor innervation: Anterior muscles of thigh and extensors of leg; iliacus, psoas major, pectineus, quadriceps femoris, and sartorius

Saphenous (sah-FEE-nus) Nerve

Composition: Motor and somatosensory

Sensory innervation: Skin of upper medial thigh; male scrotum and root of penis; female labia majora Motor innervation: Joins iliohypogastric nerve and innervates the same muscles

Genitofemoral Nerve

Composition: Somatosensory

Sensory innervation: Skin of medial aspect of leg and foot; knee joint

Obturator Nerve

Composition: Somatosensory

Sensory innervation: Skin of middle anterior thigh; male scrotum and cremaster muscle; female labia majora

Lateral Femoral Cutaneous Nerve

Composition: Motor and somatosensory

Sensory innervation: Skin of superior medial thigh; hip and knee joints Motor innervation: Adductor muscles of leg: external obturator, pectineus, adductor longus, adductor brevis, adductor magnus, and gracilis

Composition: Somatosensory

Sensory innervation: Skin of lateral aspect of thigh

U Roots

U Anterior divisions U Posterior divisions

U Anterior divisions U Posterior divisions

Figure 13.17 The Lumbar Plexus.

Anterior view

Iliohypogastric nerve Ilioinguinal nerve Genitofemoral nerve Lateral femoral cutaneous nerve Femoral nerve

Saphenous nerve Obturator nerve Lumbosacral trunk

I I From lumbar plexus From sacral plexus

Os coxae Sacrum Femoral nerve - Pudendal nerve Sciatic nerve Femur

Tibial nerve Common fibular nerve

Superficial fibular nerve

Deep fibular nerve

Fibula

Tibia

Tibial nerve

Medial plantar nerve Lateral plantar nerve

Posterior view

Figure 13.17 The Lumbar Plexus.

I I From lumbar plexus From sacral plexus

Os coxae Sacrum Femoral nerve - Pudendal nerve Sciatic nerve Femur

Posterior view

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Table 13.6 The Sacral and Coccygeal Plexuses

The sacral plexus is formed from the ventral rami of nerves L4, L5, and S1 to S4. It has six roots and anterior and posterior divisions. Since it is connected to the lumbar plexus by fibers that run through the lumbosacral trunk, the two plexuses are sometimes referred to collectively as the lumbosacral plexus. The coccygeal plexus is a tiny plexus formed from the ventral rami of S4, S5, and Co (fig. 13.18).

The tibial and common fibular nerves listed in this table travel together through a connective tissue sheath; they are referred to collectively as the sciatic (sy-AT-ic) nerve. The sciatic nerve passes through the greater sciatic notch of the pelvis, extends for the length of the thigh, and ends at the popliteal fossa. Here, the tibial and common fibular nerves diverge and follow their separate paths into the leg. The sciatic nerve is a common focus of injury and pain.

Superior Gluteal Nerve

Composition: Motor

Motor innervation: Gluteus minimus, gluteus medius, and tensor fasciae latae

Inferior Gluteal Nerve

Composition: Motor

Motor innervation: Gluteus maximus

Nerve to Piriformis

Composition: Motor Motor innervation: Piriformis

Nerve to Quadratus Femoris

Composition: Motor and somatosensory Sensory innervation: Hip joint

Motor innervation: Quadratus femoris and gemellus inferior

Nerve to Internal Obturator

Composition: Motor

Motor innervation: Internal obturator and gemellus superior

Perforating Cutaneous Nerve

Composition: Somatosensory

Sensory innervation: Skin of posterior aspect of buttock

Posterior Cutaneous Nerve

Composition: Somatosensory

Sensory innervation: Skin of lower lateral buttock, anal region, upper posterior thigh, upper calf, scrotum, and labia majora Tibial Nerve

Composition: Motor and somatosensory

Sensory innervation: Skin of posterior leg and sole of foot; knee and foot joints

Motor innervation: Semitendinosus, semimembranosus, long head of biceps femoris, gastrocnemius, soleus, flexor digitorum longus, flexor hallucis longus, tibialis posterior, popliteus, and intrinsic muscles of foot

(continued)

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Table 13.6 The Sacral and Coccygeal Plexuses (continued)

Common Fibular (peroneal) Nerve

Composition: Motor and somatosensory

Sensory innervation: Skin of anterior distal one-third of leg, dorsum of foot, and toes I and II; knee joint

Motor innervation: Short head of biceps femoris, fibularis tertius, fibularis brevis, fibularis longus, tibialis anterior, extensor hallucis longus, extensor digitorum longus, and extensor digitorum brevis

Pudendal Nerve

Composition: Motor and somatosensory

Sensory innervation: Skin of penis and scrotum of male; clitoris, labia majora and minora, and lower vagina of female Motor innervation: Muscles of perineum

Coccygeal Nerve

Composition: Motor and somatosensory Sensory innervation: Skin over coccyx Motor innervation: Muscles of pelvic floor

Posterior divisions fiif

Inferior gluteal nerve

S4

v

Ï1

S5

A

Co1

Common fibular nerve

-Tibial nerve

Common fibular nerve

-Tibial nerve

Sciatic nerve

  • ij__—i--Posterior cutaneous femoral nerve
  • Internal pudendal nerve

Figure 13.18 The Sacral and Coccygeal Plexuses.

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Cutaneous Innervation and Dermatomes

Each spinal nerve except C1 receives sensory input from a specific area of skin called a dermatome.22 A dermatome map (fig. 13.19) is a diagram of the cutaneous regions innervated by each spinal nerve. Such a map is oversimplified, however, because the dermatomes overlap at their edges by as much as 50%. Therefore, severance of one sensory nerve root does not entirely deaden sensation from a dermatome. It is necessary to sever or anesthetize three successive spinal nerves to produce a total loss of sensation from one dermatome. Spinal nerve damage is assessed by testing the dermatomes with pinpricks and noting areas in which the patient has no sensation.

22derma = skin + tome = segment, part

Before You Go On

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

  1. What is meant by the dorsal and ventral roots of a spinal nerve? Which of these is sensory and which is motor?
  2. Where are the somas of the dorsal root located? Where are the somas of the ventral root?
  3. List the five plexuses of spinal nerves and state where each one is located.
  4. State which plexus gives rise to each of the following nerves: axillary, ilioinguinal, obturator, phrenic, pudendal, radial, and sciatic.

Somatic Reflexes

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.

-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 CI 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 CI 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

Spindle Nerve Structure Mcgraw

Figure !3.20 A Muscle Spindle and Its Innervation.

Figure !3.20 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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@ 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

Image Delayed Tendon Reflex

(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
Gamma Motor Neurons Muscle Contraction
  • 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
Ipsilateral Labia

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)

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