Ch 14: Nervous System: Spinal Cord and Spinal Nerves

14.4a Overview of Conduction Pathways

14.4a Overview of Conduction Pathways

14.2 Protection and Support of the Spinal Cord

14.2 Protection and Support of the Spinal Cord

14.3 Sectional Anatomy of the Spinal Cord and Spinal Roots

14.3 Sectional Anatomy of the Spinal Cord and Spinal Roots

14.3a Distribution of Gray Matter

14.3a Distribution of Gray Matter

14.3b Distribution of White Matter

14.3b Distribution of White Matter

14.4 Sensory and Motor Pathways

14.4 Sensory and Motor Pathways

four continuous subdivisions (parts) of the spinal cord

(1) the cervical part, which is continuous with the medulla oblongata, (2) the thoracic part, (3) the lumbar part, and (4) the sacral part. (Some references further divide the sacral part into a sacral part and a coccygeal part.)

14.1c spinal nerve idenification and gross anatomy

14.1c spinal nerve identification and gross anatomy

Spinal cord and spinal nerves 2 primary functions

1. provide an essential structural and functional link between the brain and the torso and limbs of the body.(communication between the brain and body) 2. Spinal reflexes; nervous system responses to that do not require the brain the spinal cord is the integration center instead, quickest reactions to stimulus ex pulling hand back from a hot stove

14.1 Overview of the Spinal Cord and Spinal Nerves

14.1 Overview of the Spinal Cord and Spinal Nerves

14.1a General Functions

14.1a General Functions

14.1b Spinal Cord Gross Anatomy

14.1b Spinal Cord Gross Anatomy

14.4b Sensory Pathways

14.4b Sensory Pathways General sense receptors somatic sensory (or somatosensory) receptors tactile receptors or proprioceptors located within joints, muscles, and tendons to detect stretch and pressure relative to position and movement of the skeleton and skeletal muscles and visceral sensory receptors are located in the walls of the viscera (internal organs) and blood vessels. They detect changes to an organ or a blood vessel (e.g., stretch).

14.4c Motor Pathways

14.4c Motor Pathways

14.5 Spinal Nerves

14.5 Spinal Nerves

14.5a General DIstribution of Spinal Nerves

14.5a General Distribution of Spinal Nerves

14.5b Nerve Plexuses

14.5b Nerve Plexuses

14.5c Intercostal Nerves

14.5c Intercostal Nerves

14.5e Brachial Plexuses

14.5e Brachial Plexuses

14.5f Lumbar plexuses

14.5f Lumbar plexuses

14.5g Sacral Plexuses

14.5g Sacral Plexuses

14.6 Reflexes

14.6 Reflexes

14.6a Characteristics of Reflexes

14.6a Characteristics of Reflexes

14..6b Components of a Reflex Arc

14.6b Components of a Reflex Arc

14.6c Classifying Spinal Reflexes

14.6c Classifying Spinal Reflexes

14.6d Spinal Reflexes

14.6d Spinal Reflexes

14.6e Reflex Testing in a Clinical Setting

14.6e Reflex Testing in a Clinical Setting

14.7 Developement of the Spinal Cord

14.7 Development of the Spinal Cord

Spinal Cord

18 in long, about as wide as a piece of rope, 31 associated pairs of spinal nerves

Corticospinal Tracts.

Corticospinal tracts originate within the cerebrum, decussate in the pyramids or the spinal cord, and synapse on lower motor neurons within the anterior horns of the spinal cord. The upper motor neurons are red, and the lower motor neurons are orange.

intervertebral foramen

Each spinal nerve exits the vertebral column through an intervertebral foramen (which is a lateral opening between two adjacent stacked vertebrae; figure 14.3a). Note that each of the more superior spinal nerves (i.e., cervical and thoracic) extends horizontally through its associated intervertebral foramen at the same level (as shown in figure 14.3a). The more inferior spinal nerves (i.e., lumbar, sacral and coccygeal) have roots that extend inferiorly as part of the cauda equina, and each of these spinal nerves then extends through its intervertebral foramen, which is inferior to where the roots are anchored to the spinal cord (see figure 14.1a, c).

phrenic (fren′ik; phren = diaphragm) nerve

One important branch of the cervical plexus is the phrenic (fren′ik; phren = diaphragm) nerve, which is formed primarily from the C4 nerve and some contributing axons from C3 and C5. The phrenic nerve extends through the thoracic cavity to innervate the thoracic diaphragm, which is the primary skeletal muscle of breathing (see section 11.5).

Development of the Spinal Cord

Recall from section 13.1b that the caudal (inferior) part of the neural tube forms the spinal cord. As the caudal part of the neural tube differentiates and specializes, the spinal cord begins to develop (figure 14.24). However, this developmental process is much less complex than that of the brain. A hollow neural canal in the neural tube develops into the central canal of the spinal cord. Note that the neural canal doesn't shrink in size; rather, the neural tube around it grows at a rapid rate. Thus, as the neural tube walls grow and expand, the neural canal in the newborn appears as a tiny channel called the central canal.

classifying a reflex

Spinal reflex or cranial reflex. A reflex may be identified by the specific area of the central nervous system (integration center) that serves as the processing site. Spinal reflexes involve the spinal cord, whereas cranial reflexes involve the brain Somatic reflex or visceral reflex. This classification criterion is determined by the type of effector that is stimulated by the motor neurons involved in the reflex. Somatic reflexes involve skeletal muscle as the effector. Visceral (or autonomic) reflexes involve cardiac muscle, smooth muscle, or a gland as the effector. Monosynaptic reflex or polysynaptic reflex. A reflex may also be classified by the number of neurons participating in the reflex. A monosynaptic (mon′ō-si-nap′tik; monos = single) reflex has only a sensory neuron and a motor neuron (figure 14.20). The axon of the sensory neuron synapses directly on the motor neuron, whose axon projects to the effector. Thus, there is only one synapse between neurons. Monosynaptic reflexes are the simplest, and they are the most rapid. With only one synaptic delay, the response is very prompt. A polysynaptic (pol′ē-si-nap′tik; polys = many) reflex has one or more interneurons positioned between the sensory and the motor neuron. These reflex arcs are more complicated and not as rapid Ipsilateral reflex or contralateral reflex. The reflex may also be classified based upon whether it involves only one side of the body. An ipsilateral reflex is a reflex in which both the receptor and effector organs are on the same side of the spinal cord. A contralateral reflex is a reflex that involves an effector on the opposite side of the body from the receptor that detected the stimulus. Note that this terminology is only applicable to reflexes that involve the limbs. For example, an ipsilateral effect occurs when the muscles in your left arm contract to pull your left hand away from a hot object. In comparison, a contralateral effect occurs when you step on a sharp object with your left foot and then contract the muscles in your right leg to maintain balance as you withdraw your left leg from the damaging object. Innate reflex or acquired reflex. The reflex may be classified based upon whether you are born with it. An innate reflex is a reflex that you are born with, whereas an acquired reflex is one that is developed after birth

intercostal nerves

The anterior rami of spinal nerves T1-T11 are called intercostal nerves because they are located within the intercostal space sandwiched between two adjacent ribs (figure 14.14). (T12 is Page 558called a subcostal nerve, because it arises inferior to the ribs, not between two ribs.) With the exception of T1, the intercostal nerves do not form plexuses. The intercostal nerves innervate much of the torso wall and portions of the upper limb (see the dermatomal map in figure 14.13). The specific innervation pattern of the T1-T12 nerves is as follows: A portion of the anterior ramus of T1 helps form the brachial plexus, but a branch of it is housed within the first intercostal space. The anterior ramus of nerve T2 emerges from its intervertebral foramen and innervates the intercostal muscles of the second intercostal space. Additionally, a branch of T2 transmits sensory information from the skin covering the axilla and the medial surface of the arm. Anterior rami of nerves T3-T6 follow the costal grooves of the ribs to innervate the intercostal muscles and receive sensations from the anterior and lateral chest wall. Anterior rami of nerves T7-T12 innervate not only the inferior intercostal spaces but also the abdominal muscles and their overlying skin.

cervical plexuses

The left and right cervical plexuses are located deep on each side of the neck, immediately lateral to cervical vertebrae C1-C4 (figure 14.15). They are formed primarily by the anterior rami of spinal nerves C1-C4. The fifth cervical spinal nerve is not considered part of the cervical plexus, although it contributes some axons to one of the plexus branches. Branches of the Page 559cervical plexuses innervate anterior neck muscles (see section 11.3d) as well as the skin of the neck and portions of the head and shoulders. The branches of the cervical plexuses are described in detail in table 14.3. Note that in the tables, Motor branches indicates the portion that relays motor output to skeletal muscles and Cutaneous branches indicates the portion that relays sensory input from the skin.

Cauda Equina

They are so named because they resemble a horse's tail (cauda = tail, equus = horse). Note that the conus medullaris (at the L1 vertebra) represents (1) the inferior end of the spinal cord and (2) the most superior portion of the spinal roots forming the cauda equina.

posterior median sulcus

Two longitudinal depressions extend the full length of the spinal cord a narrow groove on the posterior surface

anterior median fissure

Two longitudinal depressions extend the full length of the spinal cord a slightly wider groove on the anterior surface

reciprocal inhibition.

When the sensory nerve signals reach the spinal cord, some of the sensory axons synapse with interneurons. These interneurons synapse with alpha motor neurons that inhibit antagonistic muscle contraction. In the case of the triceps reflex, the biceps brachii muscle is the inhibited antagonistic muscle. Thus, as the triceps brachii is stimulated, reciprocal inhibition results in the biceps brachii contraction being dampened, so the triceps movement will not be opposed by the biceps brachii. The stretch reflex is a monosynaptic reflex, but the corresponding reciprocal inhibition is polysynaptic, because it uses an interneuron within the circuit.

sciatic (sī-at′ik) nerve

also known as the ischiadic (is-kē-at′ik; hip joint) nerve, is the largest and longest nerve in the body. It is formed from portions of both the anterior and posterior divisions of the sacral plexus. This nerve projects from the pelvis through the greater sciatic notch of the os coxae and extends into the posterior region of the thigh. The sciatic nerve is actually composed of two divisions—the tibial division and the common fibular division—wrapped in a common sheath.

Lateral horns

are both the left and right lateral masses of gray matter. They are located only within the T1-L2 parts of the spinal cord, not the entire length of the spinal cord. The gray matter of the lateral horns is due to the presence of the dendrites and cell bodies of autonomic motor neurons. Collectively, they form the autonomic motor nuclei (orange shaded region), which makes up the entire lateral horn on each side of the spinal cord. The axons of autonomic motor neurons (orange line) extend to and innervate autonomic (or visceral) effectors. Autonomic effectors include those body structures that are not controlled consciously or voluntarily (i.e., cardiac muscle, smooth muscle, and glands). (Examples of autonomic effectors include cardiac muscle of the heart, smooth muscle of the stomach wall, and exocrine glands of the pancreas. Autonomic effectors are components of the autonomic nervous system

Posterior horns

are both the left and right posterior masses of gray matter. The gray matter forming the posterior horns is due to the presence of the dendrites and cell bodies of interneurons (the neurons that are located completely within the CNS; see section 12.2d). Sensory neurons within the spinal nerves extend through the posterior root and synapse with the dendrites and cell bodies of the interneurons within the posterior horn. The posterior horn gray matter on each side of the spinal cord is subdivided regionally into both somatic sensory nuclei and visceral sensory nuclei based upon the specific type of sensory neurons that synapse there

spinal cord meninges

are connective tissue membranes that protect and encapsulate the spinal cord within the vertebral canal. (They are continuous with the cranial meninges, described in section 13.2a.) The spinal cord meninges, layered from innermost to outermost, are: pia mater, arachnoid mater, and dura mater.

Reflexes

are rapid, preprogrammed, involuntary responses of muscles or glands to a stimulus. An example of a reflex occurs when you accidentally touch a hot burner on a stove. Instantly and automatically, you remove your hand from the stimulus (the hot burner), even before you are completely aware that your hand was touching something extremely hot. All reflexes have similar properties: A stimulus is required to initiate a reflex. A rapid response requires that few neurons are involved and synaptic delay (see section 12.3) is minimal. A preprogrammed response occurs the same way every time. An involuntary response requires no conscious intent or preawareness of the reflex activity. Thus, reflexes are usually not suppressed. A reflex is a survival mechanism; it allows us to quickly respond to a stimulus that may be detrimental to our well-being without having to wait for the brain to process the information. Awareness of the stimulus occurs after the reflex action has been completed, in time to correct or avoid a potentially dangerous situation. (This is possible because sensory input has reached the cerebral cortex.

Motor pathways 14.4c

are the descending pathways that originate within the brain and act to control effectors. Here we discuss the motor pathways that specifically control skeletal muscle of the torso and limbs. These motor pathways originate from the cerebral cortex, the cerebral nuclei, or the brainstem At least two motor neurons are present within the motor pathway to transmit signals from the brain to the body: an upper motor neuron and a lower motor neuron. An upper motor neuron is the first neuron in a chain of neurons. The cell body of the upper motor neuron is housed within the cerebral cortex, cerebral nuclei, or a specific nucleus within the brainstem. Axons of the upper motor neuron synapse either directly upon lower motor neurons (in direct pathways) or upon interneurons that ultimately synapse upon lower motor neurons (in indirect pathways). The upper motor neurons either excite or inhibit the activity of lower motor neurons. The lower motor neuron is the last neuron in the chain of neurons. The cell body of a lower motor neuron is housed within the anterior horn of the spinal cord (as described in section 14.3a). Axons of the lower motor neurons exit the spinal cord through the anterior root and project to and innervate a specific skeletal muscle. The lower motor neuron always excites the skeletal muscle fibers to contract. two types of motor pathways: the direct pathway and indirect pathway. The direct pathway is responsible for conscious control of skeletal muscle activity; the indirect pathway is responsible for subconscious (or reflexive) control of skeletal muscle

Denticulate (den-tik′ū-lāt; dentatus = toothed) ligaments

are the numerous paired, triangular extensions present along the spinal cord. These pia mater extensions suspend and anchor the spinal cord laterally to the arachnoid and dura mater

dura mater

composed of dense irregular connective tissue. The dura mater associated with the spinal cord has only one layer (unlike the dura mater covering the brain, which is composed of both a periosteal layer and meningeal layer; see section 13.2a). Extensions of the dura mater ensheathe the spinal nerve roots and merge with the connective tissue layer that surrounds the spinal nerves (i.e., the epineurium; see section 12.1c). Two spaces are associated with the dura mater: the subdural space and the epidural space. The subdural space is a potential space internal to the dura mater (between the arachnoid mater and dura mater). The epidural space is a space external to the dura mater. The epidural space is a clinically significant area that houses adipose and areolar connective tissue, as well as blood vessels. Epidural anesthetics, such as may be used to lessen pain during childbirth, are introduced into this space.

funiculus

axons within each funiculus are organized into smaller structural units (or bundles of myelinated axons) called fasciculi (fă-sik′ū-lī; fascis = bundle) (figure 14.5). White matter on each side of the cord can also be referred to as tracts, which have common functions. Individual tracts are either (1) sensory (or ascending) tracts, which conduct nerve signals from the spinal cord to the brain, or (2) motor (or descending) tracts, which conduct nerve signals from the brain to the spinal cord sensory input is relayed to the brain within each funiculus, whereas motor output from the brain is relayed only within the lateral and anterior funiculi Tracts are myelinated axons that have a common origin, a common destination, and a similar function. The name of each tract reflects its origin and destination. For example, sensory tracts usually begin with the prefix spino-, indicating that they originate in the spinal cord. The second part of the name provides its destination. An example is the sensory spinothalamic tract, which extends from the spinal cord to the thalamus. Another example is the spinocerebellar tract, which extends from the spinal cord to the cerebellum. (The fasciculus gracilis and fasciculus cuneatus are exceptions to how sensory tracts are named.) Motor pathways begin either with cortico-, indicating an origin in the cerebral cortex, or with the name of a brainstem nucleus (such as rubro-, indicating an origin within the red nucleus of the midbrain; see section 13.5a). Thus, the corticospinal tract extends from the cerebral cortex to the spinal cord. How tracts within the spinal cord function in sensory and motor pathways is discussed in section 14.4.

Anterior horns

both the left and right anterior masses of gray matter. The gray matter of the anterior horns is due to the presence of the dendrites and cell bodies of somatic motor neurons. Collectively, they form the somatic motor nuclei (red shaded area), which forms the entire anterior horn on each side of the spinal cord. The axons of somatic motor neurons (red line) extend to and innervate a somatic effector. The somatic effector includes only the muscle that can be controlled consciously or voluntarily (i.e., skeletal muscle). Note: A type of virus that specifically targets somatic motor neurons within the spinal cord and potentially result in muscle paralysis is the poliovirus

posterior funiculus

white matter that lies between the posterior gray horns on the posterior side of the cord and the posterior median sulcus

anterior root

contains motor neurons that extend to effectors (muscle or glands; figure 14.2). These motor neurons relay nerve signals from the spinal cord and control muscles and glands. Observe that the motor neurons that compose the spinal nerves are multipolar neurons (see section 12.2d). Both the dendrites and the cell bodies of motor neurons, unlike those of sensory neurons, are housed within the spinal cord; thus, the anterior root does not contain a ganglion along its length.

ulnar nerve

descends along the medial side of the arm. It extends posterior to the medial epicondyle of the humerus and then extends along the ulnar side of the forearm. It innervates some of the anterior forearm muscles (the medial region of the flexor digitorum profundus and all of the flexor carpi ulnaris). It also innervates most of the intrinsic hand muscles, including the hypothenar muscles, the palmar and dorsal interossei, and the medial two lumbricals (see section 11.8e). It receives sensory input from the skin of the dorsal and palmar aspects of the medial 1½ fingers (the pinky finger and the medial half of the ring finger).

pia mater

directly adheres to the external surface of the spinal cord. It is the delicate, innermost meningeal layer, which is a meshlike membrane composed of both elastic and collagen fibers. Pia mater extensions form two structures: denticulate ligaments and the filum terminale.

terminal branches

emerge from the three cords: the axillary nerve (from the posterior cord), median nerve (from the medial and lateral cords), musculocutaneous nerve (from the lateral cord), radial nerve (from the posterior cord), and ulnar nerve (from the medial cord). The nerves are compared in table 14.4.

median nerve

extends along the midline of the arm and forearm, and deep to the carpal tunnel in the wrist. It innervates most of the anterior forearm muscles, the thenar muscles, and the lateral two lumbricals (see section 11.8e). It receives sensory nerve signals from the palmar side of the lateral 3½ fingers (thumb, index finger, middle finger, and the lateral half of the ring finger) and from the dorsal tips of these fingers.

radial nerve

extends along the posterior side of the arm and then along the radial side of the forearm. The radial nerve innervates the posterior arm muscles (forearm extensors) and the posterior forearm muscles (extensors of the wrist and digits, and the supinator of the forearm, see sections 11.8c and d). It receives sensory nerve signals from the posterior arm and forearm surface and the dorsolateral side of the hand

Each spinal nerve (except C1 and Co1)

extends through an intervertebral foramen to exit the vertebral column (as described in section 14.2). Each spinal nerve splits almost immediately into two primary branches, termed rami (figure 14.12). The posterior (dorsal) ramus (rā′mŭs; pl., rami, rā′mī; branch) is the smaller of the two main branches. It innervates the deep muscles of the back (e.g., erector spinae and transversospinalis; see section 11.4) and the skin of the back. The anterior (ventral) ramus is the larger of the two main branches. The anterior ramus splits into multiple other branches, which innervate skin and skeletal muscles of the anterior and lateral portions of the trunk, the upper limbs, and the lower limbs. Many of the anterior rami go on to form nerve plexuses, which are described in the following sections. Additional rami, called the rami communicantes, are also associated with spinal nerves. These rami contain axons associated with the autonomic nervous system (ANS). Each set of rami communicantes extends between the spinal nerve and a spherical structure called the sympathetic trunk ganglion. These ganglia are interconnected and form a beaded necklace-like structure called the sympathetic trunk that extends parallel and lateral to the vertebral column

deep fibular (deep peroneal) nerve

extends through the anterior compartment of the leg and terminates between the first and second toes. It innervates the anterior leg muscles (which dorsiflex the foot and extend the toes) and the muscles on the dorsum of the foot (which extend the toes, see sections 11.9c and d). In addition, this nerve receives sensory input from the skin between the first and second toes on the dorsum of the foot.

superficial fibular (superficial peroneal) nerve

extends through the lateral compartment of the leg. Just proximal to the ankle, this nerve becomes superficial along the anterior part of the ankle and dorsum of the foot. The superficial fibular nerve innervates the lateral compartment muscles of the leg (foot evertors and weak plantar flexors, see sections 11.9c and d). It also receives sensory input from most of the dorsal surface of the foot and the anteroinferior part of the leg.

spinal cord

extends through the vertebral column to the inferior border of the L1 vertebra (figure 14.1). The superior end of the spinal cord is continuous with the medulla oblongata of the brain, and its inferior end tapers (narrows) to form the conus medullaris (kō′nŭs med′ū-lār′is; kōnos = cone, medulla = middle). does not extend the entire length of the vertebral column, but ends at approximately the superior border of the small of your back. This is because growth of the individual vertebrae continues longer than the growth of the spinal cord; thus, an adult spinal cord is shorter than the vertebral column.

Gray matter

first discussed in section 13.1c, where it was described as (1) being primarily composed of the dendrites and cell bodies of neurons and (2) functioning as a processing center. The gray matter within the spinal cord is centrally located, and its shape resembles a letter H or a butterfly. The gray matter is subdivided into the following components on each side of the spinal cord: a posterior horn, a lateral horn, an anterior horn, and a bar of gray matter that connects the left and right sides called the gray commissure. The gray matter of each horn is discussed first.

vertebral column

formed by 26 stacked vertebrae and the intervertebral discs between vertebrae. The vertebral column physically protects the spinal cord and is flexible enough to allow for movement of the torso. All of the stacked vertebral foramina collectively form the vertebral canal, which houses both the spinal cord and the cauda equina Note that the different parts of the spinal cord described in section 14.1b do not match up exactly with the vertebrae of the same name (figure 14.1a). For example, the lumbar part of the spinal cord is actually closer to the inferior thoracic vertebrae than to the lumbar vertebrae. This apparent discrepancy is due to the continued growth of individual vertebrae after spinal cord growth is complete.

spinal nerve forms

forms where the posterior root (containing sensory neurons) and the anterior root (containing motor neurons) join. Thus, both sensory and motor neurons compose each spinal nerve, and it is classified as a mixed nerve (see section 12.1c). If you compare a spinal nerve to a cable composed of multiple wires, the "wires" within a spinal nerve are the sensory and motor axons, and each "wire" transmits signals in one direction only.

posterior root

houses sensory neurons (see section 12.2d) that extend from sensory receptors. These sensory neurons relay nerve signals from the sensory receptors to the spinal cord. Observe that the sensory neurons that compose the spinal nerves are unipolar neurons (see section 12.2d). The dendrites of Page 541these sensory neurons form the sensory receptors (see section 16.1b), and their axons extend from the dendrites to the spinal cord. It is critical to note that the cell bodies of these sensory neurons (which are positioned along the length of the axon) are located external to the spinal cord and form the posterior root ganglion

Sensory pathways

include the sensory neurons that relay sensory input to the brain. Sensory pathways are also called ascending pathways because the nerve signals are relayed from the sensory receptors superiorly to the brain

Motor pathways

include the series of motor neurons that relay motor output from the brain. Motor pathways are also called descending pathways because the nerve signals are relayed from the brain inferiorly to the body's muscles and glands. Most conduction pathways—whether sensory or motor—share several general characteristics: Paired tracts. All pathways are composed of paired tracts. Thus, a pathway on one side of the CNS has a matching tract on the other side of the CNS. Composed of two or more neurons. Most pathways are composed of a series of two or three neurons that form the pathway. Common location of neuron cell bodies. Neuron cell bodies are located in one of three general places: the posterior root ganglion, the gray horns within the spinal cord, or nuclei within the brain along the pathway. Common location of axons. The axons of the different neurons extend through spinal nerves, the spinal cord (as named tracts or fasciculi), and the brain. Decussation. Most pathways include neurons that cross over, or decussate (dē-kŭ-sāt′; decusseo = to make in the form of an X), from one side of the body to the other side at some point along the pathway—within either the spinal cord or the brain. This means that the left side of the brain receives sensory input from or initiates motor output to the right side of the body, whereas the right side of the brain receives sensory input from or initiates motor output to the left side of the body. The term contralateral (kon-tră-lat′er-ăl; contra = opposite, latus = side) is used to indicate the relationship to the opposite side. Over 90% of all neurons within pathways decussate. Limited ipsilateral pathway. Pathways may have some neurons (about 10%) that remain on the same side of the body. The term ipsilateral (ip-si-lat′er-ăl; ipse = same) is used to indicate the relationship to the same side.

gray commissure

within the spinal cord forms a bar of gray matter connecting the left and right sides of the posterior, lateral, and anterior horns. The gray commissure is an unusual gray matter region because it primarily houses unmyelinated axons (which lack the whitish-colored myelin and, thus, appear gray in color). This bar of gray matter serves as a communication route between the right and left sides of the spinal cord.

reflex arc

includes a sensory receptor, an effector, and the neural wiring between the two. It always begins at a receptor in the PNS, communicates with the CNS, and ends at a peripheral effector, either a muscle or a gland. The number of intermediate steps varies, depending upon the complexity of the reflex. Generally, five steps are involved in a reflex, as illustrated in figure 14.19 and described here: A stimulus activates a sensory receptor. A sensory receptor (dendritic endings of a sensory neuron or specialized receptor cells) responds to external and internal stimuli, such as temperature, pressure, or tactile changes. Proprioceptors are sensory receptors found in muscles and tendons, and a stimulus to a proprioceptor (such as the tapping of tendon) may initiate a reflex as well. The sensory neuron transmits a nerve signal to the CNS. A sensory neuron transmits a nerve signal from the receptor to the spinal cord (or brain). Information from the nerve signal is processed in the integration center by interneurons. More complex reflexes may use a number of interneurons within the CNS to integrate and process incoming sensory nerve signals and transmit information to the motor neuron. The simplest reflexes do not involve interneurons; rather, the sensory neuron synapses directly on a motor neuron in the CNS. The motor neuron transmits a nerve signal from the CNS to an effector. A motor neuron transmits a nerve signal from the CNS to a peripheral effector organ—a gland or a muscle. The effector responds to the nerve signal from the motor neuron. An effector is a muscle or a gland that responds to the nerve signal from the motor neuron. This response is intended to counteract or remove the original stimulus.

musculocutaneous nerve

innervates the anterior arm muscles (coracobrachialis, biceps brachii, and brachialis), which flex the humerus, flex the forearm, or both (see sections 11.8b and c). It also receives sensory information from the lateral surface of the forearm.

withdrawal (flexor) reflex

involves muscles contracting to withdraw the body part away from a painful stimulus. This reflex involves pain receptors, termed nociceptors (see section 16.1d). It is initiated by a painful stimulus, such as touching something very hot or painful (figure 14.23). This stimulation initiates a nerve signal that is transmitted by a sensory neuron to the spinal cord. Interneurons receive the sensory nerve signal and stimulate motor neurons to the flexor muscles. These flexor muscles contract in response.

nerve plexus

is a network of interweaving anterior rami of spinal nerves. The anterior rami of most spinal nerves form nerve plexuses on both the right and left sides of the body. These nerve plexuses then split into multiple "named" nerves that innervate various body structures. The main plexuses are the cervical plexuses, brachial plexuses, lumbar plexuses, and sacral plexuses Nerve plexuses are organized such that axons from each anterior ramus extend to body structures through several different branches. In addition, each terminal branch of the plexus houses axons from several different spinal nerves. Thus, damage to a single segment of the spinal cord or damage to a single spinal nerve generally does not result in complete loss of innervation to a particular muscle or region of skin. Most of the thoracic spinal nerves, as well as nerves S5-Co1, do not form plexuses. We discuss the anterior rami of thoracic spinal nerves (called intercostal nerves) first, followed by the individual nerve plexuses.

muscle spindle

is a proprioceptor that detects changes in stretch within a muscle; for this reason, a muscle spindle is also known as a stretch receptor (figure 14.21). A muscle spindle is composed of intrafusal muscle fibers surrounded by a connective tissue capsule. These intrafusal muscle fibers lack myofilaments in their central regions and are contractile only at their distal regions. (Actin and myosin are found only at the ends of these fibers.) These muscle fibers are innervated by both sensory neurons (which relay nerve signals to the spinal cord) and gamma (γ) motor neurons, so named because gamma refers to motor neurons with small-diameter axons. Gamma motor neurons stimulate the contractile fibers at the distal ends of the intrafusal muscle fibers to contract, which elongates the inner portion of the muscle spindle fiber, causing the muscle spindle to be more sensitive to any additional stretch. (Thus, the gamma motor neurons function prior to the reflex to increase the sensitivity of a muscle to stretch.) Around the muscle spindle are extrafusal muscle fibers, which are innervated by alpha (α) motor neurons, so named because these motor neurons have the largest diameter axons. (One study tip to remember the difference between the two neuron types is that gamma goes within the muscle, and alpha wraps around the muscle.) A muscle spindle is associated with a type of reflex called a stretch reflex. Alpha motor neurons stimulate the extrafusal muscle fiber of a skeletal muscle to contract.

Golgi tendon reflex

is a reflex that is initiated by a Golgi tendon organ proprioceptor. A Golgi tendon organ is composed of sensory nerve endings within a tendon or near a muscle-tendon junction and detects change in tension (stretch) in a muscle tendon when a muscle contracts. Whereas the stretch reflex prevents muscles from stretching excessively, the Golgi tendon reflex prevents muscles from doing the opposite: tensing or contracting excessively. The Golgi tendon reflex is a polysynaptic reflex that results in muscle relaxation in response to increased tension at a Golgi tendon organ As a muscle contracts, its associated tendon stretches, resulting in increased tension in the tendon and activation of the Golgi tendon organ. Sensory neurons in the Golgi tendon organ transmit nerve signals to interneurons in the spinal cord, which in turn inhibit the alpha motor neurons in the same muscle. When the motor neurons are inhibited, the associated muscle is allowed to relax, thus protecting the muscle and tendon from excessive tension damage. Note that the sensory neurons also communicate with other interneurons in the spinal cord that stimulate alpha motor neurons for the antagonistic muscles. This process is called reciprocal activation. Page 575So, for example, if a Golgi tendon organ in the quadriceps femoris muscle detects excessive tension, then the Golgi tendon reflex ultimately relaxes the quadriceps femoris muscle, and reciprocal activation results in the hamstrings being stimulated to contract.

stretch reflex

is a reflex that is initiated by a muscle spindle proprioceptor and involves a muscle reflexively contracting in response to stretching of a muscle (figure 14.21). When the muscle spindle is stretched, the sensation is detected by sensory neurons that are wrapped around the intrafusal muscle fibers of the muscle spindle. The sensory neurons transmit nerve signals to the spinal cord (CNS), where they synapse with the alpha motor neurons associated with that muscle. The alpha motor neurons then transmit nerve signals to the extrafusal muscle fibers, which causes the muscle to contract and thus resist the stretch. ex triceps reflex

central canal

is a small, internal channel that extends through the center of the gray commissure along the entire length of the spinal cord. As with the ventricles of the brain, the central canal is formed during embryonic development from the neural canal within the neural tube (see section 14.7). The central canal contains cerebrospinal fluid (CSF), which enters this space from the fourth ventricle of the brain

dermatome

is a specific segment of skin innervated by a single spinal nerve. All spinal nerves except for C1 innervate a segment of skin, and each area of the skin that is innervated by a specific spinal nerve has been mapped. Collectively, this map is called a dermatome map (figure 14.13). The dermatome map follows a segmental pattern along the body (although there is slight overlap between adjacent spinal nerves). For example, the horizontal segment of skin around the umbilicus (navel) region is supplied by the anterior ramus of the T10 spinal nerve.

filum terminale

is a thin strand of pia mater that anchors the conus medullaris to the coccyx bone. It extends within the cauda equina and can be viewed in figure 14.1a, c. Both types of pia mater extensions help stabilize the spinal cord within the vertebral canal.

Conduction pathway

is an inclusive term that refers to all of the series of neurons (and their associated structures) that relay signals between the brain and the body. Pathways that extend between the brain and the torso and limbs include components that are (1) within the brain (e.g., cerebrum), (2) within the spinal cord (either a fasciculus or tract that extends through the spinal cord), and (3) the individual neurons within spinal nerves. Pathways also include integration and processing centers (gray matter) at different locations along each type of pathway where the neurons synapse. identified as either sensory or motor pathways, depending upon the direction nerve signals are relayed relative to the brain

anterior funiculus

is composed of white matter that occupies the space on each anterior side of the cord between anterior gray horns and the anterior median fissure; the anterior funiculi are interconnected by the white commissure.

white matter of the spinal cord

is external to the gray matter and on each side of the cord is partitioned into three distinct anatomic structural regions based upon their location within the spinal cord. Each of these regions is called a funiculus

common fibular (common peroneal) nerve

is formed from the posterior division of the sciatic nerve. As the common fibular division of the sciatic nerve, it innervates the short head of the biceps femoris muscle (see section 11.9b). Along the lateral knee, as it wraps around the neck of the fibula, this nerve splits into two main branches: the deep fibular nerve and the superficial fibular nerve

indirect pathway

is so named because upper motor neurons originate within brainstem nuclei and take a complex, circuitous route through the Page 553brain to the spinal cord that involves more than one upper motor neuron. The indirect pathway modifies or helps control the pattern of somatic motor activity by exciting or inhibiting the lower motor neurons that innervate the muscles. The different tracts of the indirect pathway are grouped according to their primary functions as either a lateral pathway or a medial pathway. The lateral pathway regulates and controls precise, discrete movements and tone in flexor muscles of the limbs—for example, the type of movement required to gently lay a baby in a crib. This pathway consists of the rubrospinal (rū′brō-spī′năl; rubro = red) tracts that originate in the red nucleus of the midbrain (see section 13.5a). The medial pathway regulates reflexive muscle tone and gross movements of the muscles of the head, neck, proximal parts of the limbs, and trunk. Within the medial pathway, three groups of tracts originate in the midbrain, pons, or medulla oblongata. The reticulospinal (re-tik-ū-lō-spī′năl) tracts originate from the reticular formation in the midbrain (see section 13.5a). They help control reflexive movements related to posture and maintaining balance. The tectospinal (tek-tō-spī′năl) tracts extend from the superior and inferior colliculi in the tectum of the midbrain to help regulate reflexive positional changes of the upper limbs, eyes, head, and neck as a consequence of visual and auditory stimuli. The vestibulospinal (ves-tib′ū-lō-spī′năl) tracts originate within vestibular nuclei of the brainstem. Nerve signals conducted within these tracts regulate reflexive muscular activity that helps maintain balance during sitting, standing, and walking.

visceral sensory nuclei

is the location for synapses between visceral sensory neurons (dark blue line) that extend from visceral sensory receptors (e.g., baroreceptors Page 546of the urinary wall) to the interneurons within the posterior horns of the spinal cord.

somatic sensory nuclei

is the site for synapses between somatic sensory neurons (light blue line) that extend from somatic sensory receptors (e.g., tactile receptors within the skin; see section 16.2a) and the interneurons within the posterior horns of the spinal cord.

lateral funiculus

is the white matter on each lateral side of the spinal cord

cervical enlargement

is the wider area in the cervical part

lumbosacral enlargement

is the wider area in the lumbar and sacral parts. These regions of the spinal cord are enlarged due to the presence of a greater number of neurons within the spinal nerves extending from these spinal cord parts to innervate the upper and lower limbs, respectively.

brachial plexuses

left and right brachial plexuses are networks of nerves that supply the upper limb. Each brachial plexus is formed by the anterior rami of spinal nerves C5-T1 (figure 14.16). The components of the brachial plexus extend laterally from the neck, pass superior to the first rib, and then continue into the axilla. Each brachial plexus innervates the pectoral girdle and the entire upper limb of one side. brachial plexus is more complex than a cervical plexus and is composed of anterior rami, trunks, divisions, and cords when examined from a medial to lateral perspective. The anterior rami (sometimes called roots) of the brachial plexus are simply the continuations of the anterior rami of spinal nerves C5-T1. These rami emerge through the intervertebral foramina and extend to the neck. The five rami unite in the posterior triangle of the neck to form the superior, middle, and inferior trunks. Nerves C5 and C6 unite to form the superior trunk; nerve C7 remains as the middle trunk; and nerves C8 and T1 unite to form the inferior trunk. Portions of each trunk divide deep to the clavicle into an anterior division and a posterior division (shown in green and purple, respectively, in figure 14.16). These contain axons that primarily innervate the anterior and posterior parts of the upper limb, respectively. At the axilla, these anterior and posterior divisions converge to form three cords. They are named with respect to their position near the axillary artery: The posterior cord is posterior to the axillary artery and is formed by the posterior divisions of the superior, middle, and inferior trunks; therefore, it contains portions of C5-T1 nerves. The medial cord is medial to the axillary artery and is formed by the anterior division of the inferior trunk; it contains portions of nerves C8-T1. Page 561The lateral cord is lateral to the axillary artery and is formed from the anterior divisions of the superior and middle trunks; thus, it contains portions of nerves C5-C7. In general, nerves from the anterior division of the brachial plexus tend to innervate muscles that flex the parts of the upper limb. Nerves from the posterior division of the brachial plexus tend to innervate muscles that extend the parts of the upper limb.

lumbar plexuses

left and right lumbar plexuses are formed from the anterior rami of spinal nerves L1-L4 located lateral to the L1-L4 vertebrae and along the psoas major muscle in the posterior abdominal wall (figure 14.17). This plexus innervates the inferior abdominal wall, anterior thigh, medial thigh, and skin of the medial leg. The lumbar plexus is structurally less complex than the brachial plexus. However, like the brachial plexus, the lumbar plexus is subdivided into an anterior division and a posterior division. The primary nerves of the lumbar plexus are listed in table 14.5. main nerve of the posterior division of the lumbar plexus is the femoral nerve. This nerve innervates the anterior thigh muscles, such as the quadriceps femoris (knee extensor) and the sartorius, psoas, and iliacus (hip flexors, see sections 11.9a and b). It also receives sensory input from the skin of the anterior and inferomedial thigh as well as the medial aspect of the leg. The main nerve of the anterior division is the obturator nerve, which extends through the obturator foramen of the os coxae to the medial thigh. There, the nerve innervates the medial thigh muscles (which adduct the thigh, see section 11.9a) and conducts sensory input from the superomedial skin of the thigh. Smaller branches of each lumbar plexus innervate the abdominal wall, portions of the external genitalia, and the inferior portions of the abdominal muscles (table 14.5; see also section 11.6

sacral plexuses

left and right sacral plexuses are formed from the anterior rami of spinal nerves L4-S4 and are located immediately inferior to the lumbar plexuses (figure 14.18). The lumbar and sacral plexuses are sometimes considered together as the lumbosacral plexus. The nerves emerging from a sacral plexus innervate the gluteal region, pelvis, perineum, posterior thigh, and almost all of the leg and foot. The anterior rami of the sacral plexus are organized into an anterior division and a posterior division. The nerves that are formed from the anterior division tend to innervate muscles that flex (or plantar flex) parts of the lower limb, whereas the posterior division nerves tend to innervate muscles that extend (or dorsiflex) part of the lower limb. Table 14.6 lists the main and smaller nerves of the sacral plexus

arachnoid mater

lies external to the pia mater. It is partially composed of a delicate web of both collagen and elastic fibers termed the arachnoid trabeculae. Immediately deep to the arachnoid mater is the subarachnoid space. Cerebrospinal fluid (CSF) circulates within this space (both around the spinal cord and around the brain). Cerebrospinal fluid can be analyzed (e.g., for infectious agents) following its removal from the subarachnoid space by the clinical procedure called a lumbar puncture

crossed-extensor reflex

often occurs in conjunction with the withdrawal reflex, usually in the lower (weight-bearing) limbs (figure 14.23). In essence, when the withdrawal reflex is occurring in Page 576 one limb, the crossed-extensor reflex occurs in the other limb. So, when the sensory neurons transmit nerve signals to the spinal cord, some sensory branches synapse with interneurons involved in the stretch reflex, whereas other sensory branches synapse with interneurons involved in the crossed-extensor reflex. These latter interneurons cross to the other side of the spinal cord through the gray commissure and synapse with motor neurons that control antagonistic muscles in the opposite limb. These motor neurons are stimulated and cause these antagonistic muscles to contract. Figure 14.23 illustrates how the withdrawal reflex causes the right lower limb to flex at the knee, due to contraction of the right hamstring muscles. In contrast, the crossed-extensor reflex stimulates the left quadriceps femoris muscle to contract, so the left limb remains extended and supports the body weight. Thus, the crossed-extensor reflex helps us maintain balance and shift body weight accordingly in these situations.

anterolateral pathway

or spinothalamic pathway) uses a chain of three neurons to communicate with the brain about a specific stimulus (figure 14.8). This pathway originates at tactile somatosensory receptors within both the skin and mucous membranes. This sensory input is providing information to the brain (specifically, the cerebral cortex) about crude touch and pressure as well as pain and temperature. Typically, sensations that require us to act in response to the stimulus (such as either an itch that makes us want to scratch or tickling that makes us jerk away) are relayed through the anterolateral pathway. Three sensory neurons compose the chain of neurons within this pathway: The axon of the primary neuron (purple line) extends from the somatosensory receptor into the spinal cord (via the posterior root). The primary neuron synapses with the secondary neuron within the posterior horn of the spinal cord, as described in section 14.3a. (Note: The synapsing of the primary neuron at the level of the spinal cord is the most significant structural difference of the anterolateral pathway when compared to the posterior funiculus-medial lemniscal pathway.) The axon of the secondary neuron (blue line) extends from the spinal cord to the thalamus. The axons project within the spinothalamic tract—either within the anterior funiculus (via the anterior spinothalamic tract) or the lateral funiculus (via the lateral spinothalamic tract) to the thalamus. The thalamus "filters" the incoming sensory input as described in section 13.4b. (Decussation occurs to the opposite side within the spinal cord as the axons extend into the spinothalamic tract.) The axon of the tertiary neuron (green line) extends from the thalamus to the cerebrum (specifically, to a location within the primary somatosensory cortex housed within the postcentral gyrus of the parietal lobe (see figure 13.13 in section 13.3c). Conscious perception of the tactile or proprioceptor sensory input occurs within the parietal lobe. principal name of this pathway (anterolateral pathway) is derived from the location of the two funiculi through which it ascends (anterior funiculus and lateral funiculus). Its secondary name (spinothalamic pathway) is derived from the tracts that relay the nerve signals within the spinal cord to the thalamus.

Somatosensory pathways

process stimuli received from somatosensory receptors (e.g., tactile receptors within the skin, proprioceptors)

viscerosensory pathways

process stimuli received from visceral sensory receptors (i.e., from the receptors of internal organs). We limit our discussion to somatosensory pathways.

Direct Pathway

pyramidal) pathway uses a chain of only two motor neurons to communicate between the brain and the skeletal muscles (figure 14.10). This pathway originates in the primary motor cortex of the cerebral frontal lobe. The direct pathway name is derived from the presence of only one upper motor neuron and one lower motor neuron. The name pyramidal is derived from the pyramid-like shape of the cell bodies of the upper motor neurons within gray matter of the cerebral cortex. The axon of the upper motor neuron extends from the frontal lobe of the cerebral cortex through the internal capsule and cerebral peduncles of the brain (see section 13.3d) and through a corticospinal tract within the spinal cord. This axon synapses on the lower motor neuron within the anterior horn of the spinal cord. The dendrites and cell bodies of the lower motor neurons form the gray matter of the anterior horn. The axon of the lower motor neuron extends from the spinal cord through the anterior root into the spinal nerve to innervate the target skeletal muscle. The direct pathways are housed within one of two pathways within the spinal cord: the lateral corticospinal tract and anterior corticospinal tract. These two pathways differ in several significant ways, including the specific muscles they innervate and control: The lateral corticospinal tracts (which composes 85% of the direct pathway) innervates skeletal muscles that control skilled movements in the limbs, such as playing a guitar, dribbling a soccer ball, or typing on your computer keyboard. The anterior corticospinal tracts (which composes the other 15% of the direct pathway) innervate axial skeletal muscle. Decussation of the lateral corticospinal tracts occurs to the opposite side within the brain at the medulla oblongata (specifically, at pyramids of the medulla oblongata; see section 13.5c), whereas the anterior corticospinal tracts decussate through the anterior gray commissure at the level of a spinal cord segment.

hypoactive reflex

refers to a reflex response that is diminished or absent. A hypoactive spinal reflex may indicate damage to a segment of the spinal cord, or it may suggest muscle disease or damage to the neuromuscular junction.

hyperactive reflex

refers to an abnormally strong response. A hyperactive spinal reflex may indicate damage somewhere in either the brain or spinal cord, especially if it is accompanied by clonus (klō′nŭs; klonus = tumult), rhythmic oscillations between flexion and extension, when the muscle reflex is tested

spinal nerves

spinal cord is associated with 31 pairs of spinal nerves (figure 14.1a). Each spinal nerve is typically identified by the first letter of the spinal cord part to which it attaches, followed by a number. Thus, each side of the spinal cord contains 8 cervical nerves (called C1-C8), 12 thoracic nerves (T1-T12), 5 lumbar nerves (L1-L5), 5 sacral nerves (S1-S5), and 1 coccygeal nerve (Co1). Spinal nerve names are readily distinguished from cranial nerve names (discussed in section 13.9) because cranial nerves are designated by either CN followed by a roman numeral (e.g., CN I, CN II) or a specific name (e.g., olfactory nerve, optic nerve). The nerve plexuses (which are labeled in figure 14.1a, such as the brachial plexus) are extensions of spinal nerves and are discussed in section 14.5. Each spinal nerve anchors to the spinal cord by two roots, a posterior root and an anterior root, and each of these roots is composed of multiple rootlets

axillary nerve

traverses through the axilla and posterior to the surgical neck of the humerus. The axillary nerve innervates both the deltoid and teres minor muscles (see section 11.8b). It receives sensory nerve signals from the superolateral part of the arm

Sensory pathways 14.4b

use a series of two or three neurons to transmit nerve signals from the sensory receptors to the brain, which are the primary neuron, secondary neuron, and tertiary neuron. The primary neuron (or first-order neuron) is the first neuron in the chain of neurons. The primary neuron extends from the sensory receptor to the CNS (brain or spinal cord), where it synapses with a secondary neuron. The secondary neuron (or second-order neuron) is an interneuron that extends from the primary neuron to either the tertiary neuron or the cerebellum. The tertiary neuron (or third-order neuron) is also an interneuron. It extends from the secondary neuron to the cerebrum (specifically, the primary somatosensory cortex of the parietal lobe; see section 13.3c). Pathways that lead to the cerebellum do not have a tertiary neuron.

spinocerebellar pathway

uses a chain of only two neurons to communicate with the brain about a specific stimulus (figure 14.9). This pathway originates at proprioceptors within joints, muscles, and tendons at different locations in the body. This sensory input is providing information to the brain (specifically, the cerebellum) related to subconscious postural input, which helps in maintaining balance and posture Two sensory neurons compose the chain of neurons within this pathway. (There is no tertiary neuron.) The axon of the primary neuron (purple line) extends from a proprioceptor into the spinal cord (via the posterior root). The primary neuron synapses with the secondary neuron within the posterior horn of the spinal cord. (This is similar to the synapse between primary and secondary neurons within the anterolateral pathway.) The axon of the secondary neuron (blue line) extends from the spinal cord within the spinocerebellar tract—either within the anterior or within the posterior portion of the lateral funiculus to the cerebellum. The name of this pathway (spinocerebellar pathway) is derived from the origin of its tracts that ascend from the spinal cord to the cerebellum.

posterior funiculus-medial lemniscal pathway

uses a chain of three sensory neurons to communicate with the brain about a specific stimulus (figure 14.7). This pathway originates at either of the two types of somatosensory receptors: (1) tactile receptors housed within both the skin and mucous membranes or (2) proprioceptors within joints, muscles, and tendons. This sensory input is providing information to the brain (specifically, the cerebral cortex) about discriminative touch, precise pressure, and vibration sensations from the tactile receptors of the skin and with conscious perception of the skeleton and skeletal muscles from proprioceptors. For example, this pathway provides the information to your brain to identify an object in your hand (even if your eyes are closed) and where your arms are positioned (even if your eyes are closed). Three sensory neurons compose the chain of neurons within this pathway: The axon of the primary neuron (purple line) extends from the somatosensory receptor into the spinal cord (via the posterior root) and ascends within the posterior funiculus within the spinal cord. (The specific fasciculus is either the fasciculus cuneatus or the fasciculus gracilis.) The primary neuron synapses with the secondary neuron within the gray matter of the medulla oblongata of the brain (specifically, the nucleus cuneatus and nucleus gracilis, respectively). Page 550The axon of the secondary neuron (blue line) extends from the medulla oblongata and projects within the medial lemniscus (see section 13.5a) to the thalamus. The thalamus "filters" this incoming sensory input as described in section 13.4b. (Decussation occurs to the opposite side within the brain just prior to the medial lemniscus.) The axon of the tertiary neuron (green line) extends from the thalamus to the cerebrum (specifically to a location within the primary somatosensory cortex housed within the postcentral gyrus of the parietal lobe; see figure 13.13 in section 13.3c). Conscious perception of the tactile or proprioceptor sensory input occurs within the parietal lobe. The name of this pathway (posterior funiculus-medial lemniscal pathway) is derived from the two components of white matter that it extends through: the posterior funiculus within the spinal cord and medial lemniscus within the brain.