CH 13 - Spinal Cord, Spinal Nerves, and Somatic Reflexes

(X) are quick, involuntary, stereotyped reactions of glands or muscles to stimulation.

Reflexes

Purely sensory nerves, composed only of afferent fibers, are rare; they include nerves for (X)

smell and vision.

An (X) 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.

intersegmental reflex arc - Note that all of these reflex arcs can function simultaneously to produce a coordinated protective response to pain.

In the medulla, second-order fibers of the gracile and cuneate systems decussate and form the (X), 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.

medial lemniscus

(X) in the spinal cord 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.

White matter

At the level of vertebra S2, the terminal filum exits the lower end of the cistern and fuses with the dura mater, and the two form a (X) that anchors the cord and meninges to vertebra Co1.

coccygeal ligament

The spinal cord serves four principal functions:

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.

Carry sensory signals from receptors to the CNS

afferent fibers

The (X) 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.

cuneate fasciculus

In the spinal cord, these fibers synapse directly with the alpha motor neurons that return to the muscle, thus forming (X)

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.

Nerves usually have a pearly white color and resemble frayed string as they divide into smaller and smaller branches. As we move away from the spinal nerves proper, the smaller branches are called (X)

peripheral nerves - their disorders are collectively called peripheral neuropathy.

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

stretch (myotatic) reflex

4 important properties of reflexes:

1. Reflexes require stimulation—they are not spontaneous actions like muscle tics but responses to sensory input. 2. Reflexes are quick—they generally involve only a 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.

A somatic reflex employs a reflex arc, 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.

There are 31 pairs of spinal nerves: (breakdown)

8 cervical (C1-C8), 12 thoracic (T1-T12), 5 lumbar (L1-L5), 5 sacral (S1-S5), and 1 coccygeal (Co1)

(X) tracts carry sensory information up the cord,

Ascending

The spinal cord serves four principal functions:

Conduction, neural integration, locomotion, and reflexes

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 (X) to compose the nerve as a whole.

Epineurium - The epineurium consists of dense irregular connective tissue and protects the nerve from stretching and injury. Nerves have a high metabolic rate and need a plentiful blood supply, which is furnished by blood vessels that penetrate these connective tissue coverings.

Innervate widespread organs such as muscles, skin, glands, viscera, and blood vessels

General fibers

(X) in the spinal cord 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.

Gray matter

(X) is the nonvisual sense of the position and movements of the body.

Proprioception

Innervate skin, skeletal muscles, bones, and joints

Somatic fibers

For nerves (X) 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 - 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.

Innervate more localized organs in the head, including the eyes, ears, olfactory and taste receptors, and muscles of chewing, swallowing, and facial expression

Special fibers

In two areas, the cord is a little thicker than elsewhere. In the inferior cervical region, a (X) gives rise to nerves of the upper limbs. In the lumbosacral region, there is a similar (Y) that issues nerves to the pelvic region and lower limbs.

X = cervical enlargement Y = lumbar enlargement

Inferior to the lumbar enlargement, the cord tapers to a point called the (X). 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 (Y) for its resemblance to a horse's tail, innervates the pelvic organs and lower limbs.

X = medullary cone Y = cauda equina

The right and left sides of the gray matter are connected by a median bridge called the (X). In the middle of the commissure is the (Y), 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.

X =gray commissure Y = central canal

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.

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 (X)

anterior ramus, 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.

The (X) consists of the arachnoid membrane—five or six layers of squamous to cuboidal cells adhering to the inside of the dura—and a looser array of cells and collagenous and elastic fibers spanning the gap between the arachnoid membrane and the pia mater. This gap, the subarachnoid space, is filled with cerebrospinal fluid

arachnoid mater

The (X) is a vertical stalk that supports the large cerebellum at the rear of the head and, even larger, two cerebral hemispheres that dominate the brain.

brainstem

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 (X)

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.

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 (X)—a posterior (dorsal), lateral, and anterior (ventral) column on each side.

columns or funiculi -Each column consists of subdivisions called tracts or fasciculi

The (X) 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.

corticospinal tracts -

The (X) is the contraction of extensor muscles in the limb opposite from the one that is withdrawn. It extends and stiffens that limb and enables you to keep your balance.

crossed extension reflex

Several of these tracts undergo (X) 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.

decussation

At regular intervals along the spinal cord, extensions of the pia called (X) extend through the arachnoid to the dura, anchoring the cord and limiting side-to-side movements.

denticulate ligaments

Each spinal nerve except C1 receives sensory input from a specific area of skin called a (X)

dermatome.

(X) tracts conduct motor impulses down the cord

descending

The meninges separate the soft tissue of the central nervous system from the bones of the vertebrae and skull. From superficial to deep, they are the (X)

dura mater, arachnoid mater, and pia mater.

The dura mater forms a loose-fitting sleeve called the (X) around the spinal cord.

dural sheath

Motor nerves carry only (X) fibers.

efferent - 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.

Carry motor signals from the CNS to effectors

efferent fibers

External to the neurilemma, each fiber is surrounded by a basal lamina and then a thin sleeve of loose connective tissue called the (X)

endoneurium. In most nerves, the fibers are gathered in bundles called fascicles, each wrapped in a sheath called the perineurium.

The space between the sheath and vertebral bones, called the (X), is occupied by blood vessels, adipose tissue, and loose connective tissue.

epidural space

A (X) is the quick contraction of flexor muscles resulting in the withdrawal of a limb from an injurious stimulus.

flexor reflex

Each end of an intrafusal fiber has a few sarcomeres. A (X) of the spinal cord innervates each end and stimulates its contraction.

gamma motor neuron - This maintains tension and sensitivity of the intrafusal fiber, preventing it from going slack like an unstretched rubber band when a muscle shortens. Spinal motor neurons that supply the extrafusal muscle fibers are called alpha motor neurons.

A (X) is a cluster of neurosomas outside the CNS.

ganglion - 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.

The (X) 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,

gracile fasciculus

The anterior ramus differs from one region of the trunk to another. In the thoracic region, it forms an (X), which travels along the inferior margin of a rib and innervates the skin and intercostal muscles (thus contributing to breathing).

intercostal nerve - Sensory fibers of the intercostal nerve branches to the skin are the most common routes of viral migration in the painful disease known as shingles. Motor fibers of the intercostal nerves innervate the internal oblique, external oblique, and transverse abdominal muscles. All other anterior rami form the nerve plexuses,

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 (X),

intrafusal fibers - whereas those that make up the rest of the muscle and do its work are called extrafusal fibers.

The (X) 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

lateral and medial reticulospinal tracts

The (X) begin in the brainstem vestibular nuclei, which receive signals for balance from the inner ear.

lateral and medial vestibulospinal tracts - 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.

Most corticospinal fibers decussate in the lower medulla and form the (X) on the contralateral side of the spinal cord.

lateral corticospinal tract - 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 gives off nerve fibers, and usually disappears by the midthoracic level.

An additional (X) 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.

lateral horn

The upper motor neuron begins with a soma in the cerebral cortex or brainstem and has an axon that terminates on a (X) 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.

lower motor neuron

Inferior to the medullary cone, the subarachnoid space is called the (X) and is occupied by the cauda equina and CSF.

lumbar cistern

The spinal cord and brain are enclosed in three fibrous membranes called (X)

meninges

Many somatic reflexes involve stretch receptors called (X) embedded in the muscles.

muscle spindles - 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. Hand and foot muscles 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.

A (X) is a cordlike organ composed of numerous nerve fibers (axons) bound together by connective tissue

nerve

The (X) is a delicate, transparent membrane composed of one or two layers of squamous to cuboidal cells and delicate collagenous and elastic fibers. It closely follows the contours of the spinal cord.

pia mater - It continues beyond the medullary cone as a fibrous strand, the terminal filum, within the lumbar cistern.

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. This circuit is part of a (X)—a pathway in which signals travel over many synapses on their way back to the muscle.

polysynaptic reflex arc

The (X) carry proprioceptive signals from the limbs and trunk to the cerebellum at the rear of the brain. .

posterior and anterior spinocerebellar tracts - 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

Stretch reflexes and other muscle contractions often depend on (X), a reflex that prevents muscles from working against each other by inhibiting antagonists.

reciprocal inhibition

Each spinal nerve arises from two points of attachment to the spinal cord.

rootlets - In each segment of the cord, six to eight nerve (X) 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.

The cord gives rise to 31 pairs of spinal nerves. Although the spinal cord is not visibly segmented, the part supplied by each pair of nerves is called a

segment.

Reflexes include glandular secretion and contractions of all three types of muscle. The reflexes of skeletal muscle are called (X)

somatic reflexes - since they involve the somatic nervous system.

The (X) 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.

spinal cord

The (X) carries pain signals resulting from tissue injury.

spinoreticular tract - 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.

The (X) 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.

spinothalamic tract - 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.

The (X) begins in a midbrain region called the tectum and crosses to the contralateral side of the midbrain.

tectospinal tract - 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.

The (X) 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.

tendon reflex - This serves to moderate muscle contraction before it tears a tendon or pulls it loose from the muscle or bone.

The cord gives rise to 31 pairs of spinal nerves. Although the spinal cord is not visibly segmented, the part supplied by each pair of nerves is called a segment. The cord exhibits longitudinal grooves on its anterior and posterior sides—

the anterior median fissure and posterior median sulcus, respectively

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.

White matter is composed of bundles of axons, called (X), that carry signals from one level of the CNS to another. Both gray and white matter also have an abundance of glial cells.

tracts

Descending tracts carry motor signals down the brainstem and spinal cord. A descending motor pathway typically involves two neurons called the (X)

upper and lower motor neurons.

Innervate blood vessels, glands, and viscera

visceral fibers