Chapt. 13: Spinal Cord and Spinal Nerves

cauda equina

"horse's tail", a fan of nerve fibers below the spinal cord

Motor output to skeletal muscles travels down the spinal cord in two types of descending pathways:

direct and indirect

What initiates a nerve impulse in a sensory neuron? Which branch of the nervous system includes all integrating centers for reflexes?

A sensory receptor produces a generator potential, which triggers a nerve impulse if the generator potential reaches threshold. Reflex integrating centers are in the CNS.

Groups of axons with their endoneurium are held together in bundles called

fascicles, each of which is wrapped in perineurium (peri- = around), the middle layer. The perineurium is a thicker layer of connective tissue. It consists of up to 15 layers of fibroblasts within a network of collagen fibers.

If the effector is skeletal muscle, the reflex is a

somatic reflex

Tracts consisting of axons that carry nerve impulses from the brain are called

motor (descending) tracts

By adjusting how vigorously a muscle spindle responds to stretching, the brain sets an overall level of

muscle tone, which is the small degree of contraction present while the muscle is at rest. Because the stimulus for the stretch reflex is stretching of muscle, this reflex helps avert injury by preventing overstretching of muscles.

The spinal cord has two principal functions in maintaining homeostasis:

nerve impulse propagation and integration of information.

autonomic (visceral) reflexes

which generally are not consciously perceived. They involve responses of smooth muscle, cardiac muscle, and glands

Nerve impulses propagating into, through, and out of the CNS follow specific pathways, depending on the kind of information, its origin, and its destination. The pathway followed by nerve impulses that produce a reflex is a

reflex arc (reflex circuit)

A tendon reflex operates as follows

1. As the tension applied to a tendon increases, the tendon organ (sensory receptor) is stimulated (depolarized to threshold). 2. Nerve impulses arise and propagate into the spinal cord along a sensory neuron. 3. Within the spinal cord (integrating center), the sensory neuron activates an inhibitory interneuron that synapses with a motor neuron. 4. The inhibitory neurotransmitter inhibits (hyperpolarizes) the motor neuron, which then generates fewer nerve impulses. 5. The muscle relaxes and relieves excess tension.

What portion of the spinal cord connects with nerves of the upper limbs?

The cervical enlargement connects with sensory and motor nerves of the upper limbs.

Why is the crossed extensor reflex classified as a contralateral reflex arc?

The crossed extensor reflex is a contralateral reflex arc because the motor impulses leave the spinal cord on the side opposite the entry of sensory impulses.

injuries to the phrenic nerves

breathing stops because the phrenic nerves no longer send nerve impulses to the diaphragm. The phrenic nerves may also be damaged due to pressure from malignant tracheal or esophageal tumors in the mediastinum.

stretch reflex

causes contraction of a skeletal muscle (the effector) in response to stretching of the muscle. This type of reflex occurs via a monosynaptic reflex arc. The reflex can occur by activation of a single sensory neuron that forms one synapse in the CNS with a single motor neuron. Stretch reflexes can be elicited by tapping on tendons attached to muscles at the elbow, wrist, knee, and ankle joints.

Sensory (ascending) tracts

consist of axons that conduct nerve impulses toward the brain.

Unlike the flexor reflex, which is an ipsilateral reflex, the crossed extensor reflex involves a

contralateral reflex arc

Inferior to the lumbar enlargement, the spinal cord terminates as a tapering, conical structure called the

conus medullaris (conus = cone) which ends at the level of the intervertebral disc between the first and second lumbar vertebrae (L1-L2) in adults.

posterior column tracts

convey nerve impulses for touch, pressure, vibration, and conscious proprioception (the awareness of the positions and movements of muscles, tendons, and joints).

If integration occurs in the brainstem rather than the spinal cord, the reflex is called a

cranial reflex

Something else may happen when you step on a tack: You may start to lose your balance as your body weight shifts to the other foot. Besides initiating the flexor reflex that causes you to withdraw the limb, the pain impulses from stepping on the tack also initiate a

crossed extensor reflex to help you maintain your balance; it operates as follows 1. Stepping on a tack stimulates the sensory receptor of a pain-sensitive neuron in the right foot. 2. This sensory neuron then generates nerve impulses, which propagate into the spinal cord. 3. Within the spinal cord (integrating center), the sensory neuron activates several interneurons that synapse with motor neurons on the left side of the spinal cord in several spinal cord segments. Thus, incoming pain signals cross to the opposite side through interneurons at that level, and at several levels above and below the point of entry into the spinal cord. 4. The interneurons excite motor neurons in several spinal cord segments that innervate extensor muscles. The motor neurons in turn generate more nerve impulses, which propagate toward the axon terminals. 5. Acetylcholine released by the motor neurons causes extensor muscles in the thigh (effectors) of the unstimulated left limb to contract, producing extension of the left leg. In this way, weight can be placed on the foot that must now support the entire body. A comparable reflex occurs with painful stimulation of the left lower limb or either upper limb.

ndividual axons within a nerve, whether myelinated or unmyelinated, are wrapped in

endoneurium (endo- = within or inner; -neurium = nerve), the innermost layer. The endoneurium consists of a mesh of collagen fibers, fibroblasts, and macrophages.

The outermost covering over the entire nerve is the

epineurium (epi- = over). It consists of fibroblasts and thick collagen fibers. Extensions of the epineurium also fill the spaces between fascicles. The dura mater of the spinal meninges fuses with the epineurium as the nerve passes through the intervertebral foramen

cervical enlargement

extends from the fourth cervical vertebra (C4) to the first thoracic vertebra (T1). Nerves to and from the upper limbs arise from the cervical enlargement.

lumbar enlargement

extends from the ninth to the twelfth thoracic vertebra. Nerves to and from the lower limbs arise from the lumbar enlargement.

Arising from the conus medullaris is the

filum terminale, an extension of the pia mater that extends inferiorly, fuses with the arachnoid mater, and dura mater, and anchors the spinal cord to the coccyx.

reflex

is a fast, involuntary, unplanned sequence of actions that occurs in response to a particular stimulus. Some reflexes are inborn, such as pulling your hand away from a hot surface before you even feel that it is hot. Other reflexes are learned or acquired.

Because the posterior root contains sensory axons and the anterior root contains motor axons, a spinal nerve is classified as a

mixed nerve. The posterior root contains a posterior root ganglion in which cell bodies of sensory neurons are located.

A reflex pathway having only one synapse in the CNS is termed a

monosynaptic reflex arc

tendon reflex

operates as a feedback mechanism to control muscle tension by causing muscle relaxation before muscle force becomes so great that tendons might be torn.

Injury to the long thoracic nerve results in

paralysis of the serratus anterior muscle. The medial border of the scapula protrudes, giving it the appearance of a wing.

This type of arrangement, in which the components of a neural circuit simultaneously cause contraction of one muscle and relaxation of its antagonists, is termed

reciprocal innervation

The roots (anterior rami) of spinal nerves S4-S5 and the coccygeal nerves form a small

coccygeal plexus

somatic reflexes

which involve contraction of skeletal muscles

tendon (Golgi tendon) organs

which lie within a tendon near its junction with a muscle. In contrast to muscle spindles, which are sensitive to changes in muscle length, tendon organs detect and respond to changes in muscle tension that are caused by passive stretch or muscular contraction.

Thoracic

Small diameter due to relatively small amounts of gray matter; except for first thoracic segment, anterior and posterior gray horns are relatively small; small lateral gray horn is present.

reflex arc includes the following five functional components

1. Sensory receptor 2. Sensory neuron 3. Integrating center 4. Motor Neron 5. effector

A stretch reflex operates as follows

1. Slight stretching of a muscle stimulates sensory receptors in the muscle called muscle spindles. The spindles monitor changes in the length of the muscle. 2. In response to being stretched, a muscle spindle generates one or more nerve impulses that propagate along a somatic sensory neuron through the posterior root of the spinal nerve and into the spinal cord. 3. In the spinal cord (integrating center), the sensory neuron makes an excitatory synapse with, and thereby activates, a motor neuron in the anterior gray horn. 4. If the excitation is strong enough, one or more nerve impulses arises in the motor neuron and propagates, along its axon, which extends from the spinal cord into the anterior root and through peripheral nerves to the stimulated muscle. The axon terminals of the motor neuron form neuromuscular junctions (NMJs) with skeletal muscle fibers of the stretched muscle. 5. Acetylcholine released by nerve impulses at the NMJs triggers one or more muscle action potentials in the stretched muscle (effector), and the muscle contracts. 6. Thus, muscle stretch is followed by muscle contraction, which relieves the stretching.

spinal cord nerves

8 cervical 12 thoracic 5 lumbar 5 sacral 1 coccygeal

What is reciprocal innervation?

Reciprocal innervation is a type of arrangement of a neural circuit involving simultaneous contraction of one muscle and relaxation of its antagonist.

Why is the flexor reflex classified as an intersegmental reflex arc?

The flexor reflex is intersegmental because impulses go out over motor neurons located in several spinal nerves, each arising from a different segment of the spinal cord.

Abdominal reflex

This reflex involves contraction of the muscles that compress the abdominal wall in response to stroking the side of the abdomen.

Babinski sign

This reflex results from gentle stroking of the lateral outer margin of the sole. The great toe extends, with or without a lateral fanning of the other toes.

Achilles reflex

This stretch reflex involves plantar flexion of the foot by contraction of the gastrocnemius and soleus muscles in response to tapping the calcaneal (Achilles) tendon.

plantar flexion reflex, or negative Babinski

a curling under of all the toes

Because the reflex arc includes synapses in lower parts of the brain, the

absence of a normal pupillary light reflex may indicate brain damage or injury.

Indirect motor pathways

also called extrapyramidal pathways, include the rubrospinal, tectospinal, vestibulospinal, lateral reticulospinal, and medial reticulospinal tracts. These tracts convey nerve impulses from the brainstem to cause automatic movements and help coordinate body movements with visual stimuli. Indirect pathways also maintain skeletal muscle tone, sustain contraction of postural muscles, and play a major role in equilibrium by regulating muscle tone in response to movements of the head.

direct motor pathways

also called pyramidal pathways, include the lateral corticospinal, anterior corticospinal, and corticobulbar tracts. They convey nerve impulses that originate in the cerebral cortex and are destined to cause voluntary movements of skeletal muscles

Another reflex involving a polysynaptic reflex arc results when, for instance, you step on a tack. In response to such a painful stimulus, you immediately withdraw your leg. This reflex, called the

flexor reflex or withdrawal reflex, operates as follows 1. Stepping on a tack stimulates the dendrites (sensory receptor) of a pain-sensitive neuron. 2. This sensory neuron then generates nerve impulses, which propagate into the spinal cord. 3. Within the spinal cord (integrating center), the sensory neuron activates interneurons that extend to several spinal cord segments. 4. The interneurons activate motor neurons in several spinal cord segments. As a result, the motor neurons generate nerve impulses, which propagate toward the axon terminals. 5. Acetylcholine released by the motor neurons causes the flexor muscles in the thigh (effectors) to contract, producing withdrawal of the leg. This reflex is protective because contraction of flexor muscles moves a limb away from the source of a possibly damaging stimulus.

Because nerve impulses from one sensory neuron ascend and descend in the spinal cord and activate interneurons in several segments of the spinal cord, this type of reflex is called an

intersegmental reflex arc, Through intersegmental reflex arcs, a single sensory neuron can activate several motor neurons, thereby stimulating more than one effector. The monosynaptic stretch reflex, in contrast, involves muscles receiving nerve impulses from one spinal cord segment only.

polysynaptic reflex arc

involves more than two types of neurons and more than one CNS synapse.

In the reflex arc just described, sensory nerve impulses enter the spinal cord on the same side from which motor nerve impulses leave it. This arrangement is called an

ipsilateral reflex. All monosynaptic reflexes are ipsilateral.

When integration takes place in the spinal cord gray matter, the reflex is a

spinal reflex. An example is the familiar patellar reflex (knee jerk)

Five large terminal branches arise from the brachial plexus:

(1) The axillary nerve supplies the deltoid and teres minor muscles. (2) The musculocutaneous nerve supplies the anterior muscles of the arm. (3) The radial nerve supplies the muscles on the posterior aspect of the arm and forearm. (4) The median nerve supplies most of the muscles of the anterior forearm and some of the muscles of the hand. (5) The ulnar nerve supplies the anteromedial muscles of the forearm and most of the muscles of the hand.

The anterior and posterior gray horns divide the white matter on each side into three broad areas called columns:

(1) anterior (ventral) white columns, (2) posterior (dorsal) white columns, and (3) lateral white columns. Each column in turn contains distinct bundles of axons having a common origin or destination and carrying similar information. These bundles, which may extend long distances up or down the spinal cord, are called tracts. Recall that tracts are bundles of axons in the CNS, whereas nerves are bundles of axons in the PNS.

If the effector is smooth muscle, cardiac muscle, or a gland, the reflex is an

autonomic (visceral) reflex

Injuries to the femoral nerve

which can occur in stab or gunshot wounds, are indicated by an inability to extend the leg and by loss of sensation in the skin over the anteromedial aspect of the thigh.

anterior (ventral) root

contain axons of motor neurons, which conduct nerve impulses from the CNS to effectors (muscles and glands).

There are two major reasons for this variation in spinal cord white matter:

(1) As the spinal cord ascends from sacral to cervical segments, more ascending axons are added to spinal cord white matter to form more sensory tracts. (2) As the spinal cord descends from cervical to sacral segments, the motor tracts decrease in thickness as more descending axons leave the motor tracts to synapse with neurons in the gray matter of the spinal cord

The internal organization of the spinal cord allows sensory input and motor output to be processed by the spinal cord in the following way

1. Sensory receptors detect a sensory stimulus. 2. Sensory neurons convey this sensory input in the form of nerve impulses along their axons, which extend from sensory receptors into the spinal nerve and then into the posterior root. 3. Axons of sensory neurons may extend into the white matter of the spinal cord and ascend to the brain as part of a sensory tract. 4. Axons of sensory neurons may enter the posterior gray horn and synapse with interneurons whose axons extend into the white matter of the spinal cord and then ascend to the brain as part of a sensory tract. 5. Axons of sensory neurons may enter the posterior gray horn and synapse with interneurons that in turn synapse with somatic motor neurons that are involved in spinal reflex pathways. Spinal cord reflexes are described in more detail later in this chapter. 6. Motor output from the spinal cord to skeletal muscles involves somatic motor neurons of the anterior gray horn. Many somatic motor neurons are regulated by the brain. Axons from higher brain centers form motor tracts that descend from the brain into the white matter of the spinal cord. There they synapse with somatic motor neurons either directly or indirectly by first synapsing with interneurons that in turn synapse with somatic motor neurons. 7. When activated, somatic motor neurons convey motor output in the form of nerve impulses along their axons, which sequentially pass through the anterior gray horn and anterior root to enter the spinal nerve. From the spinal nerve, axons of somatic motor neurons extend to skeletal muscles of the body. 8. Motor output from the spinal cord to cardiac muscle, smooth muscle, and glands involves autonomic motor neurons of the lateral gray horn. When activated, autonomic motor neurons convey motor output in the form of nerve impulses along their axons, which sequentially pass through the lateral gray horn, anterior gray horn, and anterior root to enter the spinal nerve. 9. From the spinal nerve, axons of autonomic motor neurons from the spinal cord synapse with another group of autonomic motor neurons located in the peripheral nervous system (PNS). The axons of this second group of autonomic motor neurons in turn synapse with cardiac muscle, smooth muscle, and glands.

Functions of the Spinal Cord and Spinal Nerves

1.The white matter of the spinal cord contains sensory and motor tracts, the "highways" for conduction of sensory nerve impulses toward the brain and motor nerve impulses from the brain toward effector tissues. 2.The spinal cord gray matter is a site for integration (summing) of excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs). 3.Spinal nerves and the nerves that branch from them connect the CNS to the sensory receptors, muscles, and glands in all parts of the body.

What is the difference between a horn and a column in the spinal cord?

A horn is an area of gray matter, and a column is a region of white matter in the spinal cord.

Why are all spinal nerves classified as mixed nerves?

All spinal nerves are classified as mixed because their posterior roots contain sensory axons and their anterior roots contain motor axons.

DEEP (LARGELY MOTOR) BRANCHES

Ansa cervicalis Superior root Inferior root Phrenic Segmental branches

Lateral gray horns are found in which segments of the spinal cord?

Lateral gray horns are found in the thoracic and upper lumbar segments of the spinal cord.

Lumbar

Nearly circular; very large anterior and posterior gray horns; small lateral gray horn is present in upper segments; relatively less white matter than cervical segments.

Cervical

Relatively large diameter, relatively large amounts of white matter, oval; in upper cervical segments (C1-C4), posterior gray horn is large but anterior gray horn is relatively small; in lower cervical segments (C5 and below), posterior gray horns are enlarged and anterior gray horns are well developed.

Sacral

Relatively small, but relatively large amounts of gray matter; relatively small amounts of white matter; anterior and posterior gray horns are large and thick.

Coccygeal

Resembles lower sacral spinal segments, but much smaller.

Why does complete severing of the spinal cord at level C2 cause respiratory arrest?

Severing the spinal cord at level C2 causes respiratory arrest because it prevents descending nerve impulses from reaching the phrenic nerve, which stimulates contraction of the diaphragm, the main muscle needed for breathing.

What are the signs of femoral nerve injury?

Signs of femoral nerve injury include inability to extend the leg and loss of sensation in the skin over the anterolateral aspect of the thigh.

spinal meninges

Surround the spinal cord and are continuous with the cranial meninges.

Which spinal nerve branches serve the upper and lower limbs?

The anterior rami serve the upper and lower limbs.

What five important nerves arise from the brachial plexus?

The axillary, musculocutaneous, radial, median, and ulnar nerves are five important nerves that arise from the brachial plexus.

Dura mater (= tough mother)

The most superficial of the three spinal meninges is a thick strong layer composed of dense irregular connective tissue. The dura mater forms a sac from the level of the foramen magnum in the occipital bone, where it is continuous with the meningeal dura mater of the brain, to the second sacral vertebra. The dura mater is also continuous with the epineurium, the outer covering of spinal and cranial nerves.

Which is the only spinal nerve that does not have a corresponding dermatome?

The only spinal nerve without a corresponding dermatome is C1.

What is the origin of the sacral plexus?

The origin of the sacral plexus is the anterior rami of spinal nerves L4-L5 and S1-S4.

Based on its name, list the origin and destination of the spinothalamic tract. Is this a sensory or a motor tract?

The spinothalamic tract originates in the spinal cord and ends in the thalamus (a region of the brain). Because "spinal" comes first in the name, you know it contains ascending axons and thus is a sensory tract.

What are the superior and inferior boundaries of the spinal dura mater?

The superior boundary of the spinal dura mater is the foramen magnum of the occipital bone. The inferior boundary is the second sacral vertebra.

A short distance after passing through its intervertebral foramen, a spinal nerve divides into several branches

These branches are known as rami

The presentation of this injury is characterized by an upper limb in which the shoulder is adducted, the arm is medially rotated, the elbow is extended, the forearm is pronated, and the wrist is flexed

This condition is called Erb-Duchenne palsy or waiter's tip position. There is loss of sensation along the lateral side of the arm.

Pia mater (pia = delicate).

This innermost meninx is a thin transparent connective tissue layer that adheres to the surface of the spinal cord and brain. It consists of thin squamous to cuboidal cells within interlacing bundles of collagen fibers and some fine elastic fibers.

Arachnoid mater (arachn- = spider; -oid = similar to)

This layer, the middle of the meningeal membranes, is a thin, avascular covering comprised of cells and thin, loosely arranged collagen and elastic fibers. It is called the arachnoid mater because of its spider's web arrangement of delicate collagen fibers and some elastic fibers. It is deep to the dura mater and is continuous through the foramen magnum with the arachnoid mater of the brain. Between the dura mater and the arachnoid mater is a thin subdural space, which contains interstitial fluid.

spinal tap (lumbar puncture)

a local anesthetic is given, and a long hollow needle is inserted into the subarachnoid space to withdraw cerebrospinal fluid (CSF) for diagnostic purposes; to introduce antibiotics, contrast media for myelography, or anesthetics; to administer chemotherapy; to measure CSF pressure; and/or to evaluate the effects of treatment for diseases such as meningitis

From this plexus arises the

anococcygeal nerves

The white matter tracts in the spinal cord

are highways for nerve impulse propagation. Sensory input travels along these tracts toward the brain, and motor output travels from the brain along these tracts toward skeletal muscles and other effector tissues.

Spinal nerves

are the paths of communication between the spinal cord and specific regions of the body. The spinal cord appears to be segmented because the 31 pairs of spinal nerves emerge at regular intervals from intervertebral foramina. Indeed, each pair of spinal nerves is said to arise from a spinal segment

denticulate ligament (= small tooth)

are thickenings of the pia mater. They project laterally and fuse with the arachnoid mater and inner surface of the dura mater between the anterior and posterior nerve roots of spinal nerves on either side

meninges

are three protective, connective tissue coverings that encircle the spinal cord and brain. From superficial to deep they are the (1) dura mater, (2) arachnoid mater, and (3) pia mater.

The roots (anterior rami) of spinal nerves C5-C8 and T1 form the

brachial plexus, which extends inferiorly and laterally on either side of the last four cervical and first thoracic vertebrae

Injury to the tibial portion of the sciatic nerve results in dorsiflexion of the foot plus eversion, a condition called

calcaneovalgus

Injury to the radial (and axillary) nerve

can be caused by improperly administered intramuscular injections into the deltoid muscle. The radial nerve may also be injured when a cast is applied too tightly around the mid-humerus. Radial nerve injury is indicated by wrist drop, the inability to extend the wrist and fingers (Figure 13.9c). Sensory loss is minimal due to the overlap of sensory innervation by adjacent nerves.

The principal plexuses are the

cervical plexus, brachial plexus, lumbar plexus, and sacral plexus. A smaller coccygeal plexus is also present.

posterior (dorsal) gray horns

contain axons of incoming sensory neurons as well as cell bodies and axons of interneurons. Recall that cell bodies of sensory neurons are located in the posterior (dorsal) root ganglion of a spinal nerve.

posterior (dorsal) root

contain only sensory axons, which conduct nerve impulses from sensory receptors in the skin, muscles, and internal organs into the central nervous system.

anterior (ventral) gray horns

contain somatic motor nuclei, which are clusters of cell bodies of somatic motor neurons that provide nerve impulses for contraction of skeletal muscles.

spinothalamic tract

conveys nerve impulses for sensing pain, temperature, itch, and tickle

In the axillae, the divisions unite to form

cords called the lateral, medial, and posterior cords

The gray matter of the spinal cord is shaped like the letter H or a butterfly; it consists of

dendrites and cell bodies of neurons, unmyelinated axons, and neuroglia.

The area of the skin that provides sensory input to the CNS via one pair of spinal nerves or the trigeminal (V) nerve is called a

dermatome (derma- = skin; -tome = thin segment)

Posterior to the clavicles, the trunks diverge into

divisions, called the anterior and posterior divisions.

Damage to the common fibular nerve causes the foot to be plantar flexed, a condition called

foot drop, and inverted, a condition called equinovarus

gray commissure

forms the crossbar of the H. In the center of the gray commissure is a small space called the central canal; it extends the entire length of the spinal cord and is filled with cerebrospinal fluid.

he gray matter on each side of the spinal cord is subdivided into regions called

horns

The anterior rami of spinal nerves T2-T12 do not enter into the formation of plexuses and are known as

intercostal nerves or thoracic nerves.

anterior median fissure of the spinal cord

is a wide groove on the anterior (ventral) side

cervical plexus

is formed by the roots (anterior rami) of the first four cervical nerves (C1-C4), with contributions from C5

spinal cord

is roughly oval in shape, being flattened slightly anteriorly and posteriorly. In adults, it extends from the medulla oblongata, the inferior part of the brain, to the superior border of the second lumbar vertebra

The first layer of protection for the central nervous system

is the hard bony skull and vertebral column. The second protective layer is the meninges, three membranes that lie between the bony encasement and the nervous tissue in both the brain and spinal cord.

SUPERFICIAL (SENSORY) BRANCHES

lesser occipital, great auricular, transverse cervical, supraclavicular

The roots (anterior rami) of spinal nerves L1-L4 form the

lumbar plexus

Injury to the median nerve may result in

median nerve palsy, which is indicated by numbness, tingling, and pain in the palm and fingers.

In the gray matter of the spinal cord and brain, clusters of neuronal cell bodies form functional groups called

nuclei

branches

of the brachial plexus form the principal nerves of the brachial plexus.

Axons from the anterior rami of spinal nerves, except for thoracic nerves T2-T12, do not go directly to the body structures they supply. Instead, they form networks on both the left and right sides of the body by joining with various numbers of axons from anterior rami of adjacent nerves. Such a network of axons is called a

plexus

Each posterior root has a swelling, the

posterior (dorsal) root ganglion, which contains the cell bodies of sensory neurons.

Other branches of a spinal nerve are the

rami communicantes, components of the autonomic nervous system

Sensory nuclei

receive input from receptors via sensory neurons, and motor nuclei provide output to effector tissues via motor neurons.

The gray matter of the spinal cord

receives and integrates incoming and outgoing information.

Injuries to the obturator nerve

result in paralysis of the adductor muscles of the thigh and loss of sensation over the medial aspect of the thigh. It may result from pressure on the nerve by the fetal head during pregnancy.

Two bundles of axons, called

roots, connect each spinal nerve to a segment of the cord by even smaller bundles of axons called rootlets

The roots (anterior rami) of spinal nerves L4-L5 and S1-S4 form the

sacral plexus

Injury to the sciatic nerve results in

sciatica, pain that may extend from the buttock down the posterior and lateral aspect of the leg and the lateral aspect of the foot.

posterior (dorsal) ramus

serves the deep muscles and skin of the posterior surface of the trunk

anterior (ventral) ramus

serves the muscles and structures of the upper and lower limbs and the skin of the lateral and anterior surfaces of the trunk.

. Between the arachnoid mater and pia mater is a space, the

subarachnoid space, which contains shock-absorbing cerebrospinal fluid.

anterior corticospinal tract is located in

the anterior white column; it begins in the cerebral cortex (superficial gray matter of the cerebrum of the brain) and ends in the spinal cord.

posterior column consists of two tracts:

the gracile fasciculus and the cuneate fasciculus

Nerve impulses from sensory receptors propagate up the spinal cord to the brain along two main routes on each side:

the spinothalamic tract and the posterior column.

The spinal cord is located within

the vertebral canal of the vertebral column.

Compression of the brachial plexus on one or more of its nerves is sometimes known as

thoracic outlet syndrome

The roots of several spinal nerves unite to form

trunks in the inferior part of the neck. These are the superior, middle, and inferior trunks.

Injury to the ulnar nerve may result in

ulnar nerve palsy, which is indicated by an inability to abduct or adduct the fingers, atrophy of the interosseous muscles of the hand, hyperextension of the metacarpophalangeal joints, and flexion of the interphalangeal joints, a condition called clawhand

lateral gray horns

which are present only in thoracic and upper lumbar segments of the spinal cord. The lateral gray horns contain autonomic motor nuclei, which are clusters of cell bodies of autonomic motor neurons that regulate the activity of cardiac muscle, smooth muscle, and glands.

When the arm is raised, the vertebral border and inferior angle of the scapula pull away from the thoracic wall and protrude outward, causing the medial border of the scapula to protrude; because the scapula looks like a wing, this condition is called

winged scapula