PERIPHERAL NERVOUS SYSTEM
The peripheral nervous system (PNS) includes all nervous system structures not located within the brain orspinal cord. In away, the PNSisrelated to the CNS in the same way that the appendicular skeleton is related to the axial skeleton. In both cases, there is a centrally located “main frame” (the CNS and axial skeleton) and a set of more peripherally located structures (the PNS and appendicular skeleton, re- spectively). In general, the major anatomical features of the PNS include nerves, nerve roots, rami, plexuses, ganglia, sensory receptors, and axon terminals.
The PNS contains sensory and motor neurons, and a few interneurons, and it is divided into the somatic and autonomic divisions. In this chapter, we will describe nerves, nerve roots, rami, and plexuses, and their roles in the somatic nervous system. In the next chapter, we will describe details of the autonomic nervous system. Let’s begin with a description of a typical nerve.
OVERVIEW OF NERVES
A nerve is a collection of axons and blood vessels bundled together within connective tissue outside the central nervous system (Figure 18-1).
The outermost connective tissue covering of a nerve is the epineurium (ep-i-NŪ-rē-um) which consists of dense, irregular connective tissue. The epineurium is continuous with the dura mater of the spinal cord and brain. Axons within a nerve exist within bundles called fascicles (FAS-i-klz, “bundles”). An extension of the epineurium, called the perineurium (per-i-NŪ-rē-um), surrounds each fascicle. Blood vessels course between the fascicles and supply nutrients to and remove wastes from the cellular components of the nerve. The perineurium extends within each fascicle as the endoneurium (en-dō-NŪ-rē-um), which surrounds individual axons.
Innervation (e-ner-VĀ-shun) refers to the connection between the CNS and either a sensory structure or an effector in the PNS by way of a nerve. Nerves are classified based on (1) the function of the axons within them, and (2) their origin, the site where the nerve connects to the CNS.
Figure 18-1. Structure of a nerve
Classification based on function
Based on the function of the axons within them, nerves may be sensory, motor, or mixed.
- Sensory nerves contain only axons of sensory neurons; thus, sensory nerves conduct impulses only from the PNS to the CNS.
- Motor nerves contain mostly axons of motor neurons, and therefore conduct impulses mainly to effectors [muscles or glands].
- Mixed nerves contain abundant axons of sensory and motor neurons. These nerves are the most common and function like two-lane highways. Keep in mind that a single axon will conduct impulses in only one direction, so the impulses that are traveling in opposite directions within a mixed nerve are traveling along different axons.
Classification based on origin
Based on their origin along the CNS, nerves are either cranial nerves or spinal nerves. Cranial nerves connect to the brain, while spinal nerves attach to the spinal cord by way of spinal roots (described shortly).
CRANIAL NERVES
Cranial nerves are nerves that arise from the inferior surface of the brain, and include twelve pairs (one nerve on each side of the brain’s midline). All of these nerves, except one pair, the vagus nerves, innervate structures in the head and neck. Most cranial nerves are mixed nerves, and some conduct both somatic and autonomic impulses. Some cranial nerves contain mostly motor axons but also contain a few sensory axons that conduct impulses from proprioceptors (stretch receptors in muscles). Since sensory axons are so few in these particular “mixed” nerves, we will say the nerves are primarily motor nerves.
Neurologists use Roman numerals to number the cranial nerves, with the lower numbers being more anterior and superior to the others. The cranial nerves are as follows:
I: Olfactory (ŌL-fak-to-rē; olfact, smell): the first pair of cranial nerves are actually a number of very short nerves that pass through the olfactory foramina of the ethmoid bone on either side of the nasal septum. These are sensory nerves, which conduct impulses for smell from sensory receptors in the olfactory epithelium to the olfactory bulbs of the brain. The olfactory epithelium is a mucous membrane located in the superior portion of the nasal cavity. The olfactory bulbs are two knob-like structures that rest on the cribriform plate of the ethmoid bone, just on either side of the crista galli.
II: Optic (OP-tik; “eye”): the second pair of cranial nerves arises from the posterior surface of the eyeballs and passes through the optic canals. They are sensory nerves that conduct impulses from the light- sensitive regions of the eyeball to the optic chiasma, which is an X-shaped structure on the inferior surface of the brain. Im- pulses traveling through the optic nerves are necessary for vision.
III: Oculomotor (ok-ū-lō-MŌ-tor; oculo, eye; motor, movement): the third pair of cra- nial nerves arises from the midbrain and passes through the superior orbital fissure. They are primarily motor nerves that conduct impulses to most of the extrinsic eye muscles, which attach to the outside of the eyeballs, and all intrinsic eye muscles, which are located inside the eyeball. The extrinsic eye muscles move the eyeball so that a person can look at different objects without moving the head. The intrinsic eye muscles control the diameter of the pupil and change the shape of the eye’s internal lens so that a person can focus clearly on objects at different distances.
IV: Trochlear (TRŌK-lē-ar; trochea, pulley): the fourth pair of cranial nerves arises from the midbrain and passes through the superior orbital fissure. These are primarily motor nerves that stimulate the superior oblique eye muscle, causing the eyeball to point inferiorly and laterally. The nerve gets its name because the muscle it innervates passes through a trochlea (pulley-like structure) on the medial side of the orbit.
V: Trigeminal (trī-JEM-i-nal; “threefold”): the fifth pair of cranial nerves arises from the pons, and it is named for its three ma- jor branches. The ophthalmic branches (of-THAL-mik; ophthal, eye) are sensory nerves that pass through the superior orbital fissure and conduct impulses from the upper eyelids, upper part of the nose, lacrimal (tear) glands in the orbit, and anterior part of the scalp. The maxillary branches (MAKS-i-lar-ē; maxilla, jaw) are sensory nerves that pass through the foramen rotundum and conduct impulses from the lining of the nasal cavity, palate (roof of mouth), upper teeth, and skin of the lower eyelid, cheek, and upper lip. The mandibular branches (man-DIB-ū-lar; mandib, jaw) are mixed nerves that pass through the foramen ovale. They conduct sensory impulses from the tongue (but not its taste buds), the lower teeth, scalp in the temple regions and skin of the chin. They conduct motor impulses to muscles involved in moving the lower jaw for chewing.
VI: Abducens (ab-DŪ-senz; abduct, take away): the sixth pair of cranial nerves arises from the pons and passes through the superior orbital fissure. They are primarily motor nerves that conduct impulses to the lateral rectus eye muscle, which causes the eyeball to look laterally.
VII: Facial: the seventh pair of cranial nerves arises from the pons and passes through the stylomastoid foramen. The facial nerves are mixed. They conduct sensory impulses from taste buds on the anterior two-thirds of the tongue, and conduct motor impulses to muscles of the face, lacrimal glands, most salivary glands (except the largest), and mucous glands in the lining of the nasal cavity.
VIII: Vestibulocochlear (ves-TIB-ū-lō-KOK- lē-ar): the eighth pair of cranial nerves, formerly called the acoustic or auditory nerves, arises from the junction between the pons and medulla oblongata and passes through the internal auditory meatus. These are sensory nerves, which conduct impulses from the inner ear. The inner ear includes two parts: (1) the vestibule, which contains receptors to detecting body position so that the brain can make adjustments in skeletal muscle tension for equilibrium for maintaining balance; and (2) the cochlea, which contains receptors for hearing.
IX: Glossopharyngeal (glos-sō-fa-RIN-jē-al; gloss, tongue; pharynx, throat): the ninth pair of cranial nerves is mixed and arises from the medulla oblongata and passes through the jugular foramen. These nerves conduct sensory impulses from taste buds on the posterior one-third of the tongue, and from chemoreceptors and baroreceptors in the neck’s carotid arteries. Chemo- receptors respond to different chemicals in the blood, while baroreceptors respond to vessel stretch due to blood pressure. These nerves conduct motor impulses to skeletal muscles in the pharynx for swallowing movements and to the largest salivary glands (parotid glands).
X: Vagus (VĀ-gus; “vagabond” or “wander-er”): the tenth pair of cranial nerves arises from the medulla oblongata and passes through the jugular foramen. These are mixed nerves and the only cranial nerves that extend below the neck. They con- duct sensory impulses from external ear, throat, esophagus, heart, lungs, abdominal visceral organs, baroreceptors in the aorta (large artery exiting the heart), and chemoreceptors in the aorta and carotid arteries. They conduct motor impulses to the heart and lungs for regulating breathing and heart rates and to the abdominal visceral organs to regulate digestive and urinary system activity.
XI: Accessory: the eleventh pair of cranial nerves, also called the spinal accessory nerves, are the only cranial nerve pair that arises partly from the spinal cord; it also connects to the medulla oblongata and passes through the jugular foramen. These nerves are primarily motor nerves that conduct impulses to muscles of the throat, neck, and upper back.
XII: Hypoglossal (hī-pō-GLOS-al): the twelfth pair of cranial nerves arises from the medulla oblongata and passes through the hypoglossal canal. They are motor nerves that conduct impulses to the tongue to change its shape and position during chewing and talking.
Figure 18-2. The cranial nerves
SPINAL ROOTS
The spinal roots connect the spinal cord to a spinal nerve and include dorsal and ventral roots (Figure 18-3).
Note: Use the following to remember the order of the cranial nerves: “On Old Olympus Towering Tops, A Finn Very Gladly Viewed Some Hops.” Use the following to remember the primary function of each pair of cranial nerves in their proper order; S=sensory; M=motor; B=both (mixed): Sister Says Marry Money But My Brother Says Bad Boys Marry Money.
The dorsal (posterior) roots connect to the posterior-lateral sides of the spinal cord and contain only axons of sensory neurons. The cell bodies of sensory neurons exist within a swollen region called the dorsal root ganglion, located on the dorsal root. The ventral (anterior) roots connect to the anterior-lateral sides of the cord and contain only axons of motor neurons. There are no neuron cell bodies in the ventral root. Most spinal roots are very short, extending no farther out than the lateral end of a vertebra’s transverse process. However, roots that attach to the spinal cord in the lumbar region are extremely long, some of which extend inferiorly through the vertebral foramina all the way to the coccyx. This mass of long spinal roots in the section of the vertebral column below the conus medullaris (inferior end of the spinal cord) is called the cauda equina (KAW-da, “tail”; ē-KWĪ-na; “horse”), so named for its horse tail appearance. Nerve roots give rise to spinal nerves at different levels of the vertebral column and exit the column through intervertebral foramina.
SPINAL NERVES
The dorsal and ventral roots unite laterally to form a very short spinal nerve, which is only about 2 cm long. The spinal nerve then extends peripherally from the spinal cord and divides into a dorsal and ventral ramus (described below). There are 31 pairs of spinal nerves, and neurologists group them based on the region of the spine to which they attach. All spinal nerves are mixed nerves and include eight cervical pairs (C1–C8 ), twelve thoracic pairs (T1–T12 ), five lumbar pairs (L1–L5 ), five sacral pairs (S1–S5 ), and one coccygeal pair (C0 ).
RAMI, PLEXUSES, and PERIPHERAL NERVES
A few centimeters lateral to the vertebral column, a spinal nerve branches into a dorsal ramus (RĀ mus; “branch”) and ventral ramus. Along most of the vertebral column (but not in the thoracic region), ventral rami (RĀ-mī) arising from several levels along the spinal cord come together to form braid-like networks called plexuses (PLEK-sus-ez; “braids”). In turn, plexuses give rise to peripheral nerves, which innervate muscles, glands, skin, and other structures. There are four pairs of plexuses (one plexus of each pair is on opposite sides of the body) (Figure 18-4).
Figure 18-3. Spinal roots, spinal nerves, and rami
Table 18-1. Summary of the cranial nerves
Cervical plexuses
The cervical plexuses arise from spinal segments C1–C5 , and give rise to five major pairs of nerves:
- Auricular nerves (aw-RIK-ū-lar; auricle, ear) innervate the external ear.
- Occipital nerves (ok-SIP-i-tul; occip, back of head) innervate the scalp on back of head.
- Phrenic nerves (FRE-nik; phren, diaphragm) innervate the diaphragm, the skeletal muscle separating the thoracic and abdominopelvic cavities.
- Cervical nerves innervate muscles of neck and upper back.
- Suprascapular nerves innervate muscles of upper back, shoulder, and chest.
Figure 18-4. Spinal plexuses and selected nerves
Brachial plexuses
The brachial plexuses arise from spinal segments C4-T1 , and give rise to five major pairs of nerves (Figure 18-5):
- Axillary nerves (AKS-i-lar-ē; “armpit”) innervate the deltoid and teres minor muscles.
- Median nerves innervate the flexor muscles in the forearm and skin of the palm; inflammation of this nerve is responsible for carpal tunnel syndrome.
- Musculocutaneous nerves (MUS-kū-lō kū-TĀN-ē-us; musculo, muscle; cutan, skin) innervate the flexor muscles in arm and the skin of the lateral forearm.
- Radial nerves innervate the extensor muscles and skin of the arm and forearm.
- Ulnar nerves innervate the skin of the medial forearm; called the “funny bone” because of the tingling arm sensation one gets after hitting it.
Lumbar plexuses
The lumbar plexuses arise from spinal segments T12-L4 , and gives rise to two major pairs of nerves (Figure 18-6):
- Femoral nerves innervate the quadriceps muscles of the thigh and the skin on medial aspect of the leg.
- Obturator nerves innervate the adductors in the thigh and the skin on medial aspect of the thigh.
Sacral plexuses
The sacral plexuses arise from spinal segments L4-S4, and gives rise to several nerves:
- Sciatic nerves (sī-AT-ik; “hip”) are the largest nerves in the body. Each sciatic nerve is actually two nerves: the tibial nerve innervates muscles and skin along the medial portion of the leg, and the fibular nerve innervates muscles and skin along the lateral portion of the leg.
- Gluteal nerves (GLŪ-tē-al; “buttock”) innervate the buttocks muscles.
- Pudendal nerves (pū-DEN-dal; “feel ashamed”) innervate the external genitalia (sex organs) and the muscles and skin of the lower pelvis.
Figure 18-5. Nerves from the brachial plexus
Figure 18-6. Nerves from the lumbar and sacral plexuses
Figure 18-7. Parts of a reflex arc
There are no plexuses in the thoracic region. Instead, ventral rami in this region become intercostal nerves that innervate skin of the thorax and intercostal muscles between the ribs.
REFLEXES
A reflex is a rapid, involuntary response to a stimulus. Some reflexes are somatic reflexes that involve “voluntary” skeletal muscles. Reflexes involving cardiac and smooth muscle are visceral (autonomic) reflexes. You will learn about somatic reflexes in this chapter and visceral reflexes in the next chapter. An example of a somatic reflex is the knee-jerk reflex that occurs when a physician taps your knee with a percussion hammer. Jerking your hand away from a hot stove is another example. The pathway of impulses in a basic reflex occurs in a reflex arc, and it includes a receptor, sensory (afferent) neuron, integration center (brain or spinal cord), motor (efferent) neuron, and effector (muscle) (Figure 18 7).
Reflexes that use the spinal cord as the integration center are called spinal reflexes. A spinal reflex that does not utilize an association neuron within the spinal cord is a monosynaptic reflex arc; that is, the sensory neuron synapses directly with the motor neuron in the spinal cord. In a polysynaptic reflex arc, the impulses pass through at least one association neuron in the spinal cord.
Figure 18-8. Parts of a stretch reflex
SOMATIC REFLEXES
There are three types of somatic reflexes: stretch, tendon, and flexor (withdrawal) reflexes.
Figure 18-9. Parts of a tendon reflex
Stretch Reflex
A stretch reflex is a monosynaptic reflex in which a stretched muscle contracts to help prevent overstretching of the muscle (Figure 18-8). This often works to help a person maintain balance. Modified skeletal myofibers called muscle spindles act as the receptors for a stretch reflex. When a muscle stretches, its muscle spindles depolarize and initiate action potentials on sensory neurons wrapped around the spindle. The sensory neuron, in turn, conducts the impulse into the spinal cord and transmits it to motor neurons that innervate the stretched muscle. In response, the motor neurons send efferent impulses to the stretched muscle, causing it to contract.
At the same time the stretched muscle contracts as part of the stretch reflex, its antagonistic muscle relaxes. While this action is not necessary for the stretch reflex, it ensures that the stretched muscle will be able to contract and not have to work against a contraction from the antagonistic muscle. During the stretch reflex, the sensory neuron transmitting the impulse from the muscle spindles to the spinal cord stimulates association neurons that inhibit motor neurons of the antagonistic muscles; consequently, the antagonistic muscle relaxes. The phenomenon is reciprocal inhibition of the antagonistic muscle and increases the efficiency of the stretch reflex.
Figure 18-10. Parts of a withdrawal reflex
Tendon Reflex
The tendon reflex is a polysynaptic reflex in which a contracted muscle relaxes in response to excessive stretch in its tendon (Figure 18-9). The excessive stretch of the tendon could be the result of muscle contraction. The tendon reflex is important in preventing damage to a muscle or its tendons. The receptor for this reflex is a Golgi tendon organ, which is a group of specialized neuron endings located within the tendon.
When excessive stretch of the tendon occurs, the tendon organs initiate sensory neuron impulses that travel to the spinal cord. In the cord, the sensory neuron stimulates an association neuron that hyperpolarizes (inhibits) motor neurons of the contracted muscle. As a result, impulses to the contracted muscle cease and the muscle relaxes. To ensure the efficiency of the tendon reflex, impulses from the sensory neurons stimulate association neurons that, in turn, stimulate motor neurons of the antagonistic muscle, causing this muscle to contract. This is reciprocal activation of the antagonist muscle.
Figure 18-11. Parts of a crossed-extensor reflex
Flexor (Withdrawal) Reflex
The flexor (withdrawal) reflex is a polysynaptic reflex in which a flexor typically contracts in response to a pain stimulus. An example is the reflex you might expect if you step on a tack. Suppose you step on a tack with the left foot. A pain receptor in the left foot depolarizes and initiates impulses that move along sensory axons into the spinal cord. In the cord, the sensory neuron stimulates association neurons that, in turn, stimulate motor neurons for flexor muscles of the left thigh. As a result, the left leg flexes, which pulls the left foot away from the pain stimulus. During this time, reciprocal inhibition occurs in the extensor muscles of the left thigh.
During a flexor reflex, impulses can pass across the spinal cord and stimulate extensor muscles on the opposite side of the body; this is a crossed-extensor reflex. In our example, sensory impulses entering the spinal cord on the left side will stimulate motor neurons for extensor muscles in the right thigh, causing them to contract. Therefore, when the person lifts the left foot off the tack, the right leg extends to prevent the person from falling. At the same time, the sensory neuron stimulates an association neuron that inhibits the motor neurons leading to the flexor muscles on the right side. In this way, reciprocal inhibition of flexor muscles in the right thigh prevents the right leg from flexing.
TOPICS TO KNOW FOR CHAPTER 18
(Peripheral Nervous System (Part 1): Nerves and Somatic Reflexes)
auricular nerves
axillary nerves
baroreceptors
brachial plexuses
carpal tunnel syndrome
cauda equina
cervical nerves (C1 C8)
cervical plexuses
chemoreceptors
classification of nerves
CN I: olfactory
CN II: optic
CN III: oculomotor
CN IV: trochlear
CN V: trigeminal
CN VI: abducens
CN VII: facial
CN VIII: vestibulocochlear
CN IX: glossopharyngeal
CN X: vagus
CN XI: accessory
CN XII: hypoglossal
coccygeal nerve (C0)
cochlea
cranial nerves
crossed-extensor reflex
dorsal (posterior) roots
dorsal ramus
dorsal root ganglion
endoneurium
epineurium
extrinsic eye muscles
fascicles
femoral nerves
fibular nerve
flexor (withdrawal) reflex
foramen ovale
foramen rotundum
gluteal nerves
Golgi tendon organ
hypoglossal canal
innervation
intercostals nerves
internal auditory meatus
intervertebral foramina
intrinsic eye muscles
IV: trochlear
IX: glossopharyngeal
jugular foramen
knee-jerk reflex
lumbar nerves (L1–L5)
lumbar plexuses
mandibular branches
maxillary branches
median nerves
mixed nerves
motor nerves
muscle spindles
musculocutaneous nerves
nerve
obturator nerves
occipital nerves
olfactory foramina
ophthalmic branches
optic canals
perineurium
peripheral nerves
peripheral nervous system
phrenic nerves
plexuses
polysynaptic reflex arc
pudendal nerves
radial nerves
reciprocal activation
reciprocal inhibition
reflex
reflex arc
sacral nerves (S1–S5)
sacral plexuses
tendon reflex
sciatic nerves
sensory nerves
somatic reflexes
spinal nerve
spinal reflexes
spinal roots
stretch reflex
stylomastoid foramen
superior orbital fissure
suprascapular nerves
EOC Questions
thoracic nerves (T1–T12)
tibial nerves
ulnar nerves
ventral (anterior) roots
ventral ramus
vestibule
visceral (autonomic) reflex
EOC Questions
