Chapter 13 The Spinal Cord, Spinal Nerves, And Somatic Reflexes

Front 1

Spinal Cord serves 4 principal functions:

Back 1

1. Conduction. It contains bundles of nerve fibers that conduct information up and down the cord, connecting different levels of the trunk with each other and with the brain. This enables sensory information to reach the brain, motor commands to reach the effectors, and input received at one level of the cord to affect output from another level.

2. Neural integration. Pools of spinal neurons receive input from multiple sources, integrate the information, and execute an appropriate output. For example, the spinal cord can integrate the stretch sensation from a full bladder with cerebral input concerning the appropriate time and place to urinate and execute control of the bladder accordingly.

3. Locomotion. Walking involves repetitive, coordinated contractions of several muscle groups in the limbs. Motor neurons in the brain initiate walking and determine its speed, distance, and direction, but the simple repetitive muscle contractions that put one foot in front of another, over and over, are coordinated by groups of neurons called central pattern generators in the cord. These neural circuits produce the sequence of outputs to the extensor and flexor muscles that cause alternating movements of the lower limbs.

4. Reflexes. Spinal reflexes play vital roles in posture, motor coordination, and protective responses to pain or injury.

Front 2

spinal cord

Back 2

is a cylinder of nervous tissue that arises from the brainstem at the foramen magnum of the skull. It passes through the vertebral canal as far as the inferior margin of the first lumbar vertebra (L1) or slightly beyond. In adults, it averages about 45 cm long and 1.8 cm thick (about as thick as one’s little finger). Early in fetal development, the cord extends for the full length of the vertebral column. The cord gives rise to 31 pairs of spinal nerves.

Front 3

Spinal Cord regions

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The spinal cord is divided into cervical, thoracic, lumbar, and sacral regions. It may seem odd that it has a sacral region when the cord itself ends well above the sacrum. These regions, however, are named for the level of the vertebral column from which the spinal nerves emerge, not for the vertebrae that contain the cord itself.

Front 4

Sinal Cord Enlargement Areas

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In the inferior cervical region, a cervical enlargement gives rise to nerves of the upper limbs. In the lumbosacral region, there is a similar lumbar enlargement that issues nerves to the pelvic region and lower limbs. Inferior to the lumbar enlargement, the cord tapers to a point called the medullary cone (conus medullaris). Arising from the lumbar enlargement and medullary cone is a bundle of nerve roots that occupy the vertebral canal from L2 to S5. This bundle, named the cauda equina (CAW-duh ee-KWY-nah) for its resemblance to a horse’s tail, innervates the pelvic organs and lower limbs.

Front 5

meninges (meh-NIN-jeez)

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The spinal cord and brain are enclosed in three fibrous membranes called meninges (meh-NIN-jeez)—singular, meninx (MEN-inks). These membranes separate the soft tissue of the central nervous system from the bones of the vertebrae and skull. From superficial to deep, they are the dura mater, arachnoid mater, and pia mater.

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dura mater (DOO-ruh MAH-tur) (superficial layer)

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forms a loose-fitting sleeve called the dural sheath around the spinal cord. It is a tough collagenous membrane about as thick as a rubber kitchen glove. The space between the sheath and vertebral bones, called the epidural space, is occupied by blood vessels, adipose tissue, and loose connective tissue. Anesthetics are sometimes introduced to this space to block pain signals during childbirth or surgery; this procedure is called epidural anesthesia.

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arachnoid (ah-RACK-noyd) mater (middle layer)

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consists of a simple squamous epithelium, the arachnoid membrane, adhering to the inside of the dura, and a loose mesh of collagenous and elastic fibers spanning the gap between the arachnoid membrane and the pia mater. This gap, called the subarachnoid space, is filled with cerebrospinal fluid (CSF). Inferior to the medullary cone, the subarachnoid space is called the lumbar cistern and is occupied by the cauda equina and CSF.

Front 8

pia (PEE-uh) mater (deep layer)

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is a delicate, transparent membrane that closely follows the contours of the spinal cord. It continues beyond the medullary cone as a fibrous strand, the terminal filum, within the lumbar cistern. At the level of vertebra S2, it exits the lower end of the cistern and fuses with the dura mater, and the two form a coccygeal ligament that anchors the cord and meninges to vertebra Co1. At regular intervals along the cord, extensions of the pia called denticulate ligaments extend through the arachnoid to the dura, anchoring the cord and limiting side-to-side movements.

Front 9

lumbar puncture (or colloquially, spinal tap)

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When a sample of CSF is needed for clinical purposes, it is taken from the lumbar cistern by a procedure called lumbar puncture (or colloquially, spinal tap). A spinal needle is inserted between two vertebrae at level L3/L4 or L4/L5, where there is no risk of accidental injury to the spinal cord (which ends at L1 to L2). CSF drips from the spinal needle into a collection tube; usually 3 to 4 mL of CSF is collected.

Front 10

Gray matter

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has a relatively dull color because it contains little myelin. It contains the somas, dendrites, and proximal parts of the axons of neurons. It is the site of synaptic contact between neurons, and therefore the site of all neural integration in the spinal cord.

Front 11

Gray Matter (continued 1)

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The spinal cord has a central core of gray matter that looks somewhat butterfly- or H-shaped in cross sections. The core consists mainly of two posterior (dorsal) horns, which extend toward the posterolateral surfaces of the cord, and two thicker anterior (ventral) horns, which extend toward the anterolateral surfaces. The right and left sides are connected by a gray commissure. In the middle of the commissure is the central canal, which is collapsed in most areas of the adult spinal cord, but in some places (and in young children) remains open, lined with ependymal cells, and filled with CSF.

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Gray Matter (continued 2)

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Near its attachment to the spinal cord, a spinal nerve branches into a posterior (dorsal) root and anterior (ventral) root. The posterior root carries sensory nerve fibers, which enter the posterior horn of the cord and sometimes synapse with an interneuron there. Such interneurons are especially numerous in the cervical and lumbar enlargements and are quite evident in histological sections at these levels. The anterior horns contain the large somas of the somatic motor neurons. Axons from these neurons exit by way of the anterior root of the spinal nerve and lead to the skeletal muscles.An additional lateral horn is visible on each side of the gray matter from segments T2 through L1 of the cord. It contains neurons of the sympathetic nervous system, which send their axons out of the cord by way of the anterior root along with the somatic efferent fibers.

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White matter

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has a bright, pearly white appearance due to an abundance of myelin. It is composed of bundles of axons, called tracts, that carry signals from one level of the CNS to another. Both gray and white matter also have an abundance of glial cells. The white matter of the spinal cord surrounds the gray matter. It consists of bundles of axons that course up and down the cord and provide avenues of communication between different levels of the CNS. These bundles are arranged in three pairs called columns or funiculi (few-NIC-you-lie)—a posterior (dorsal), lateral, and anterior (ventral) column on each side. Each column consists of subdivisions called tracts or fasciculi (fah-SIC-you-lye).

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Spinal Tracts

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Ascending tracts carry sensory information up the cord, and descending tracts conduct motor impulses down. All nerve fibers in a given tract have a similar origin, destination, and function. Many of these fibers have their origin or destination in a region called the brainstem.

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decussation (DEE-cuh-SAY-shun) (Spinal tracts)

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Several of these tracts undergo decussation (DEE-cuh-SAY-shun) as they pass up or down the brainstem and spinal cord—meaning that they cross over from the left side of the body to the right, or vice versa. As a result, the left side of the brain receives sensory information from the right side of the body and sends motor commands to that side, while the right side of the brain senses and controls the left side of the body. Therefore, a stroke that damages motor centers of the right side of the brain can cause paralysis of the left limbs and vice versa.

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contralateral and ipsilateral

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When the origin and destination of a tract are on opposite sides of the body, we say they are contralateral to each other. When a tract does not decussate, its origin and destination are on the same side of the body and we say they are ipsilateral.

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Ascending Tracts

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Ascending tracts carry sensory signals up the spinal cord. Sensory signals typically travel across three neurons from their origin in the receptors to their destination in the brain: a first-order neuron that detects a stimulus and transmits a signal to the spinal cord or brainstem; a second-order neuron that continues as far as a “gateway” called the thalamus at the upper end of the brainstem; and a third-order neuron that carries the signal the rest of the way to the cerebral cortex.

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gracile fasciculus (GRAS-el fah-SIC-you-lus)

(ascending tract)

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carries signals from the midthoracic and lower parts of the body. Below vertebra T6, it composes the entire posterior column. At T6, it is joined by the cuneate fasciculus, discussed next. It consists of first-order nerve fibers that travel up the ipsilateral side of the spinal cord and terminate at the gracile nucleus in the medulla oblongata of the brainstem. These fibers carry signals for vibration, visceral pain, deep and discriminative touch (touch whose location one can precisely identify), and especially proprioception from the lower limbs and lower trunk. Proprioception is the nonvisual sense of the position and movements of the body.

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cuneate (CUE-nee-ate) fasciculus

(Ascending tract)

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joins the gracile fasciculus at the T6 level. It occupies the lateral portion of the posterior column and forces the gracile fasciculus medially. It carries the same type of sensory signals, originating from T6 and up (from the upper limbs and chest). Its fibers end in the cuneate nucleus on the ipsilateral side of the medulla oblongata. In the medulla, second-order fibers of the gracile and cuneate systems decussate and form the medial lemniscus (lem-NIS-cus), a tract of nerve fibers that leads the rest of the way up the brainstem to the thalamus. Third-order fibers go from the thalamus to the cerebral cortex. Because of decussation, the signals carried by the gracile and cuneate fasciculi ultimately go to the contralateral cerebral hemisphere.

Front 20

spinothalamic (SPY-no-tha-LAM-ic) tract

(Ascending Tract)

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some smaller tracts form the anterolateral system, which passes up the anterior and lateral columns of the spinal cord. The spinothalamic tract carries signals for pain, temperature, pressure, tickle, itch, and light or crude touch. Light touch is the sensation produced by stroking hairless skin with a feather or cotton wisp, without indenting the skin; crude touch is touch whose location one can only vaguely identify. In this pathway, first-order neurons end in the posterior horn of the spinal cord near the point of entry. Here they synapse with second-order neurons, which decussate and form the contralateral ascending spinothalamic tract. These fibers lead all the way to the thalamus. Third-order neurons continue from there to the cerebral cortex. Because of decussation, sensory signals in this tract arrive in the cerebral hemisphere contralateral to their point of origin.

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spinoreticular tract

(Ascending Tract)

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also travels up the anterolateral system. It carries pain signals resulting from tissue injury. The first-order sensory neurons enter the posterior horn and immediately synapse with second-order neurons. These decussate to the opposite anterolateral system, ascend the cord, and end in a loosely organized core of gray matter called the reticular formation in the medulla and pons. Third-order neurons continue from the pons to the thalamus, and fourth-order neurons complete the path from there to the cerebral cortex.

Front 22

posterior and anterior spinocerebellar (SPY-no-SERR-eh-BEL-ur) tracts

(Ascending Tract)

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travel through the lateral column and carry proprioceptive signals from the limbs and trunk to the cerebellum at the rear of the brain. Their first-order neurons originate in muscles and tendons and end in the posterior horn of the spinal cord. Second-order neurons send their fibers up the spinocerebellar tracts and end in the cerebellum. Fibers of the posterior tract travel up the ipsilateral side of the spinal cord. Those of the anterior tract cross over and travel up the contralateral side but then cross back in the brainstem to enter the ipsilateral side of the cerebellum. Both tracts provide the cerebellum with feedback needed to coordinate muscle action

Front 23

Descending tracts

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Descending tracts carry motor signals down the brainstem and spinal cord. A descending motor pathway typically involves two neurons called the upper and lower motor neurons. The upper motor neuron begins with a soma in the cerebral cortex or brainstem and has an axon that terminates on a lower motor neuron in the brainstem or spinal cord. The axon of the lower motor neuron then leads the rest of the way to the muscle or other target organ. The names of most descending tracts consist of a word root denoting the point of origin in the brain, followed by the suffix -spinal. The major descending tracts are described here.

Front 24

corticospinal (COR-tih-co-SPY-nul) tracts

(Descending Tracts)

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carry motor signals from the cerebral cortex for precise, finely coordinated limb movements. The fibers of this system form ridges called pyramids on the anterior surface of the medulla oblongata, so these tracts were once called pyramidal tracts. Most corticospinal fibers decussate in the lower medulla and form the lateral corticospinal tract on the contralateral side of the spinal cord. A few fibers remain uncrossed and form the anterior corticospinal tract on the ipsilateral side. Fibers of the anterior tract decussate lower in the cord, however, so even they control contralateral muscles. This tract gets smaller as it descends and usually disappears by the mid-thoracic level.

Front 25

tectospinal (TEC-toe-SPY-nul) tract

(Descending Tracts)

Back 25

begins in a midbrain region called the tectum and crosses to the contralateral side of the midbrain. It descends through the brainstem to the upper spinal cord on that side, going only as far as the neck. It is involved in reflex turning of the head, especially in response to sights and sounds.

Front 26

lateral and medial reticulospinal (reh-TIC-you-lo-SPY-nul) tracts

(Descending Tracts)

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originate in the reticular formation of the brainstem. They control muscles of the upper and lower limbs, especially to maintain posture and balance. They also contain descending analgesic pathways that reduce the transmission of pain signals to the brain

Front 27

lateral and medial vestibulospinal (vess-TIB-you-lo-SPY-nul) tracts

Back 27

begin in the brainstem vestibular nuclei, which receive signals for balance from the inner ear. The lateral vestibulospinal tract passes down the anterior column of the spinal cord and facilitates neurons that control extensor muscles of the limbs, thus inducing the limbs to stiffen and straighten. This is an important reflex in responding to body tilt and keeping one’s balance. The medial vestibulospinal tract splits into ipsilateral and contralateral fibers that descend through the anterior column on both sides of the cord and terminate in the neck. It plays a role in the control of head position.

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nerve

Back 28

is a cordlike organ composed of numerous nerve fibers (axons) bound together by connective tissue. If we compare a nerve fiber to a wire carrying an electrical current in one direction, a nerve would be comparable to an electrical cable composed of thousands of wires carrying currents in opposite directions. A nerve contains anywhere from a few nerve fibers to (in the optic nerve) a million.

Front 29

Nerve (continued)

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Nerve fibers of the peripheral nervous system are ensheathed in Schwann cells, which form a neurilemma and often a myelin sheath around the axon. External to the neurilemma, each fiber is surrounded by a basal lamina and then a thin sleeve of loose connective tissue called the endoneurium. In most nerves, the fibers are gathered in bundles called fascicles, each wrapped in a sheath called the perineurium. The perineurium is composed of up to 20 layers of overlapping, squamous, epithelium-like cells. Several fascicles are then bundled together and wrapped in an outer epineurium to compose the nerve as a whole. The epineurium consists of dense irregular connective tissue and protects the nerve from stretching and injury.

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Types of Nerves

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sensory nerves, composed only of afferent fibers, are rare; they include nerves for smell and vision. Motor nerves carry only efferent fibers. Most nerves, however, are mixed nerves, which consist of both afferent and efferent fibers and therefore conduct signals in two directions. However, any one fiber in the nerve conducts signals in one direction only. Many nerves commonly described as motor are actually mixed because they carry sensory signals of proprioception from the muscle back to the CNS.

Front 31

ganglion

Back 31

If a nerve resembles a thread, a ganglion resembles a knot in the thread. A ganglion is a cluster of neurosomas outside the CNS. It is enveloped in an epineurium continuous with that of the nerve. Among the neurosomas are bundles of nerve fibers leading into and out of the ganglion.

Front 32

Spinal Nerves (31 in total)

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There are 31 pairs of spinal nerves: 8 cervical (C1–C8), 12 thoracic (T1–T12), 5 lumbar (L1–L5), 5 sacral (S1–S5), and 1 coccygeal (Co1) (fig. 13.10). The first cervical nerve emerges between the skull and atlas, and the others emerge through intervertebral foramina, including the anterior and posterior foramina of the sacrum and the sacral hiatus. Spinal nerves C1 through C7 emerge superior to the correspondingly numbered vertebrae (nerve C5 above vertebra C5, for example); nerve C8 emerges inferior to vertebra C7; and below this, all the remaining nerves emerge inferior to the correspondingly numbered vertebrae (nerve L3 inferior to vertebra L3, for example).

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Proximal Branches

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Each spinal nerve arises from two points of attachment to the spinal cord. In each segment of the cord, six to eight nerve rootlets emerge from the anterior surface and converge to form the anterior (ventral) root of the spinal nerve. Another six to eight rootlets emerge from the posterior surface and converge to form the posterior (dorsal) root. A short distance away from the spinal cord, the posterior root swells into a posterior (dorsal) root ganglion, which contains the somas of sensory neurons. There is no corresponding ganglion on the anterior root.

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Spinal Nerve (signal breakdown)

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The spinal nerve is a mixed nerve, carrying sensory signals to the spinal cord by way of the posterior root and ganglion, and motor signals out to more distant parts of the body by way of the anterior root. The anterior and posterior roots are shortest in the cervical region and become longer inferiorly. The roots that arise from segments L2 to Co1 of the cord form the cauda equina. Some viruses invade the CNS by way of the spinal nerve roots

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Distal Branches

Back 35

Distal to the vertebrae, the branches of a spinal nerve are more complex. Immediately after emerging from the intervertebral foramen, the nerve divides into an anterior ramus, a posterior ramus, and a small meningeal branch. Thus, each spinal nerve branches on both ends—into anterior and posterior roots approaching the spinal cord, and anterior and posterior rami leading away from the vertebral column.

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Distal Branches (Continued)

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The meningeal branch reenters the vertebral canal and innervates the meninges, vertebrae, and spinal ligaments with sensory and motor fibers. The posterior ramus innervates the muscles and joints in that region of the spine and the skin of the back. The larger anterior ramus innervates the anterior and lateral skin and muscles of the trunk, and gives rise to nerves of the limbs.

The anterior ramus differs from one region of the trunk to another. In the thoracic region, it forms an intercostal nerve, which travels along the inferior margin of a rib and innervates the skin and intercostal muscles (thus contributing to breathing). It also innervates the internal oblique, external oblique, and transverse abdominal muscles. All other anterior rami form the nerve plexuses described next.

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Nerve Plexuses

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Except in the thoracic region, the anterior rami branch and anastomose (merge) repeatedly to form five webs called nerve plexuses: the small cervical plexus in the neck, the brachial plexus near the shoulder, the lumbar plexus of the lower back, the sacral plexus immediately inferior to this, and finally, the tiny coccygeal plexus adjacent to the lower sacrum and coccyx. The spinal nerve roots that give rise to each plexus are indicated in violet in each table. Some of these roots give rise to smaller branches called trunks, anterior divisions, posterior divisions, and cords, which are color-coded and explained in the individual tables.

Front 38

Nerve Plexuses (continued)

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The nerves tabulated here have somatosensory and motor functions. Somatosensory means that they carry sensory signals from bones, joints, muscles, and the skin, in contrast to sensory input from the viscera or from special sense organs such as the eyes and ears. Somatosensory signals are for touch, heat, cold, stretch, pressure, pain, and other sensations. One of the most important sensory roles of these nerves is proprioception, in which the brain receives information about body position and movements from nerve endings in the muscles, tendons, and joints. The brain uses this information to adjust muscle actions and thereby maintain equilibrium (balance) and coordination.The motor function of these nerves is primarily to stimulate the contraction of skeletal muscles. They also innervate the bones of the corresponding regions, and carry autonomic fibers to some viscera and blood vessels, thus adjusting blood flow to local needs.

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dermatome

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Each spinal nerve except C1 receives sensory input from a specific area of skin called a dermatome. A dermatome map 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.

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Reflexes

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

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

2. Reflexes are quick—they generally involve few interneurons, or none, 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.

4. Reflexes are stereotyped—they occur in essentially the same way every time; the response is very predictable, unlike the variability of voluntary movement.

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somatic reflexes

Back 41

The reflexes of skeletal muscle are called somatic reflexes, since they involve the somatic nervous system. Somatic reflexes have traditionally been called spinal reflexes, but this is a misleading expression for two reasons: (1) Spinal reflexes are not exclusively somatic; visceral reflexes also involve the spinal cord. (2) Some somatic reflexes are mediated more by the brain than by the spinal cord.

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reflex arc

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in which signals travel along the following pathway:

1. somatic receptors in the skin, muscles, and tendons;

2. afferent nerve fibers, which carry information from these receptors to the posterior horn of the spinal cord or to the brainstem;

3. an integrating center, a point of synaptic contact between neurons in the gray matter of the cord or brainstem;

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

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

Front 43

Muscle Spindles

Back 43

Many somatic reflexes involve stretch receptors called muscle spindles embedded in the muscles. These are among the body’s proprioceptors, sense organs specialized to monitor the position and movement of body parts. The function of muscle spindles is to inform the brain of muscle length and body movements. This enables the brain to send motor commands back to the muscles that control muscle tone, posture, coordinated movement, and corrective reflexes (for example, to keep one’s balance). Spindles are especially abundant in muscles that require fine control. wand none at all in the middle-ear muscles.

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Muscle Spindles (continued)

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A muscle spindle is a bundle of usually seven or eight small, modified muscle fibers enclosed in an elongated fibrous capsule about 5 to 10 mm long. Spindles are especially concentrated at the ends of a muscle, near its tendons. The modified muscle fibers within the spindle are called intrafusal fibers, whereas those that make up the rest of the muscle and do its work are called extrafusal fibers.

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gamma motor neuron

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of the spinal cord innervates each end and stimulates its contraction. This maintains tension and sensitivity of the intrafusal fiber, preventing it from going slack like an unstretched rubber band when a muscle shortens.

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alpha motor neurons

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Spinal motor neurons that supply the extrafusal muscle fibers are called alpha motor neurons.

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stretch (myotatic) reflex

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When a muscle is suddenly stretched, it “fights back”—it contracts, increases tone, and feels stiffer than an unstretched muscle. This response, called the stretch (myotatic) reflex, helps to maintain equilibrium and posture

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monosynaptic reflex arcs

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In the spinal cord, these fibers synapse directly with the alpha 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, so there is little synaptic delay and a very prompt response. The alpha motor neurons excite the quadriceps, making it contract and creating the knee jerk.

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reciprocal inhibition

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a reflex that prevents muscles from working against each other by inhibiting antagonists. In the knee jerk, for example, the quadriceps 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 quadriceps muscle spindles stimulate spinal interneurons that, in turn, inhibit the alpha motor neurons of the hamstrings. The hamstring remain relaxed and allow the quadriceps to extend the knee.

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flexor reflex

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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. 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.

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polysynaptic reflex arc

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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.

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crossed extension reflex

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is the contraction of extensor muscles in the limb opposite from the one that is withdrawn. 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 alpha motor neurons to the muscles of the contralateral limb.

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ipsilateral reflex arc

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one in which the sensory input and motor output are on the same side of the spinal cord

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ontralateral reflex arc

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in which the input and output are on opposite sides

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intersegmental reflex arc

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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.

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Tendon organs

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are proprioceptors located in a tendon near its junction with a muscle. A tendon organ is about 0.5 mm long. It consists of an encapsulated bundle of small, loose collagen fibers and one or more nerve fibers that penetrate the capsule and end in flattened leaflike processes between the collagen fibers. As long as the tendon is slack, its collagen fibers are slightly spread and put little pressure on the nerve endings. 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.

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tendon reflex

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is a response to excessive tension on the tendon. It inhibits alpha 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.