I. General Anatomy of the Peripheral Nervous System (p. 538)

A. Subdivisions of the Peripheral Nervous System (PNS) (p. 538)

1. The sensory (afferent) division carries sensory signals by way of afferent nerve fibers from receptors to the central nervous system (CNS). It can be further subdivided into somatic and visceral divisions.

a. The somatic sensory division carries signals from receptors in the skin, muscles, bones, and joints.

b. The visceral sensory division carries signals mainly from the viscera of the thoracic and abdominal cavities.

2. The motor (efferent) division carries motor signals by way of efferent nerve fibers from the CNS to effectors (mainly glands and muscles). It can be further subdivided into somatic and visceral divisions.

a. The somatic motor division carries signals to the skeletal muscles.

b. The visceral motor division, also known as the autonomic nervous system, carries signals to glands, cardiac muscle, and smooth muscle. It can be further divided into the sympathetic and parasympathetic divisions.

i. The sympathetic division tends to arouse the body to action.

ii. The parasympathetic division tends to have a calming effect.

3. Nerve fibers of the PNS are classified according to their involvement in motor or sensory, somatic, or visceral pathways. (table 15.1)

a. Mixed nerves contain both motor and sensory fibers.

b. Sensory nerves contain mostly sensory fibers; they are less common and include the optic and olfactory nerves.

c. Motor nerves contain motor fibers.

B. Anatomy of Nerves and Ganglia (p. 539)

1. A nerve is an organ composed of multiple nerve fibers bound together by sheaths of connective tissue. (fig. 15.1; TR 518)

a. The sheath adjacent to the neurilemma is the endoneurium, which houses blood capillaries that feed nutrients and oxygen to the nerve.

b. In large nerves, fibers are bundled into fascicles and wrapped in a fibrous perineurium.

c. The entire nerve is covered with a fibrous epineurium.

2. A ganglion is a cluster of neuron cell bodies enveloped in an epineurium continuous with that of a nerve. (fig. 15.2; TR 519)

a. A ganglion appears as a swelling along the course of a nerve.

b. The dorsal root ganglia associated with the spinal cord contain the unipolar neurons of the sensory nerve fibers that carry signals to the cord. The fiber passes through the ganglion without synapsing.

c. However, in the autonomic nervous system, a preganglionic fiber enters the ganglion and in many cases synapses with another neuron. The axon of the second neuron leaves the ganglion as the postganglionic fiber.

II. The Cranial Nerves (p. 541)

A. The cranial nerves emerge from the base of the brain and lead to muscles and sense organs in the head and neck for the most part. (p. 541; fig. 15.3; TR 520–522)

B. The twelve pairs of cranial nerves are the olfactory, optic, oculomotor, trochlear, trigeminal, abducens, facial, vestibulocochlear, glossopharyngeal, vagus, accessory, and hypoglossal nerves. (pp. 542–49; table 15.2; TR 523–536)

III. The Spinal Nerves (p. 550)

A. There are 31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal. (p. 550; fig. 15.4; TR 537)

B. Proximal Branches (p. 550; figs. 15.5, 15.6; TR 538, 539)

1. Each spinal nerve branches into a dorsal root and a ventral root. The dorsal root ganglion is occupied by cell bodies from afferent neurons. The convergence of dorsal and ventral roots forms the spinal nerve.

2. The cauda equina is formed by the roots arising from segments L2 to Co of the spinal cord.

C. Distal Branches (p. 551)

1. After emerging from the vertebral column, the spinal nerve divides into a dorsal ramus, a ventral ramus, and a small meningeal branch that leads to the meninges and vertebral column. (fig. 15.7; TR 540, 541)

a. The dorsal ramus innervates the muscles and joints of the spine and the skin of the back.

b. The ventral ramus innervates the ventral and lateral skin and muscles of the trunk, plus gives rise to nerves leading to the limbs.

D. Nerve Plexuses (p. 553)

1. The ventral rami merge to form nerve plexuses in all areas except the thoracic region. (fig. 15.8; TR 542–549; tables 15.3, 15.4, 15.5, and 15.6)

E. Cutaneous Innervation and Dermatomes (p. 562)

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

2. A dermatome map is a diagram of the cutaneous regions innervated by each spinal nerve; such a map is an oversimplification, however, (fig. 15.9; TR 550)

IV. Somatic Reflexes (p. 562)

A. The Nature of Reflexes (p. 562)

1. Reflexes are quick, involuntary, stereotyped reactions of peripheral effectors to stimulation.

2. A spinal reflex is made up of a reflex arc, including somatic receptors, afferent nerve fibers, interneurons, efferent nerve fibers, and skeletal muscles.

B. The Muscle Spindle (p. 563; fig. 15.10; TR 551)

1. The muscle spindle is a stretch receptor located in muscle. It is a cigar-shaped organ containing 3–12 modified muscle fibers wrapped in a fibrous capsule.

2. Muscle spindles have three types of nerve fibers: primary afferent, secondary afferent, and gamma motor neurons.

C. The Stretch Reflex (p. 563)

1. When a muscle is stretched, it contracts to maintain tone. This is the stretch (myotatic) reflex.

2. Stretch reflexes involve specific muscles and sometimes feed back to a set of synergists and antagonists. These reflexes are important in coordinating vigorous and precise movements.

3. The tendon reflex (knee jerk) is an example of a monosynaptic reflex arc. (fig. 15.11; TR 552)

4. For reflexes like the knee jerk to work, there must simultaneously be reciprocal inhibition of antagonistic muscles.

D. The Flexor (Withdrawal) Reflex (p.566; fig. 15.12[left]; TR 553)

1. Flexor reflexes are important when a limb must be pulled away from harm.

2. These types of reflexes involve a polysynaptic reflex arc, a pathway in which signals travel over many synapses on their way back to the muscle.

E. The Crossed Extensor Reflex (p. 566; fig. 15.12[right]; TR 553)

1. The crossed extensor reflex involves the cerebellum and the spinal cord. An example is lifting your foot quickly to avoid injury (flexor reflex) while still maintaining your balance.

2. The flexor reflex is ipsilateral; the crossed extensor reflex is contralateral.

3. An intersegmental reflex arc occurs when the nerve signal produces an output from a different segment than the one that received the input.

F. The Golgi Tendon Reflex (p. 567)

1. Golgi tendon organs are proprioceptors located at the junction of a muscle and its tendon. (fig. 15.13; TR 554)

2. Golgi tendon organs produce an inhibitory response called the Golgi tendon reflex when muscle contracts too tightly. This prevents damage to the tendon.

V. The Autonomic Nervous System: Introduction and Anatomy (p. 568)

A. General Properties (p. 568; table 15.7)

1. The visceral reflexes are mediated by the autonomic nervous system (ANS), which has two branches (sympathetic and parasympathetic). (fig. 15.14; TR 555)

2. The target organs of the ANS are glands, cardiac muscle, and smooth muscle; it operates to maintain homeostasis.

3. Control over the ANS is, for the most part, involuntary.

4. The ANS differs structurally from the somatic nervous system in that there are two neurons leading from the ANS to the effector: a preganglionic neuron and a postganglionic neuron. (fig. 15.15; TR 556)

B. Anatomy of the Sympathetic Division (p. 570)

1. The sympathetic division is also called the thoracolumbar division because of the spinal nerves it employs.

2. Paravertebral ganglia occur close to the vertebral column. (figs. 15.16, 15.17; TR 557)

a. Preganglionic neurons are short, while postganglionic neurons, traveling to their effector, are long.

b. When one preganglionic neuron fires, it can excite multiple postganglionic fibers that lead to different target organs (mass activation).

3. In the thoracolumbar region, each paravertebral ganglion is connected to a spinal nerve by two communicating rami: the white communicating ramus and the gray communicating ramus. (fig. 15.18; TR 558)

4. Nerve fibers leave the paravertebral ganglia by spinal, sympathetic, and splanchnic nerves. (fig. 15.19; TR 559; table 15.8)

C. The Adrenal Glands (p. 574)

1. The pyramid-shaped adrenal glands lie atop each kidney and consist of a glandular adrenal cortex surrounding an adrenal medulla made of modified sympathetic neurons.

2. When stimulated, the adrenal medulla produces catecholamines (as hormones) that complement the action of sympathetic postganglionic neurotransmitters.

D. Anatomy of the Parasympathetic Division (p. 574; fig. 15.20; TR 560)

1. The parasympathetic division is also referred to as the craniosacral division because its fibers travel in some cranial and sacral nerves.

2. The parasympathetic ganglion (terminal ganglion) lies in or near the target organs.

3. The parasympathetic fibers leave the brainstem by way of the oculomotor, facial, glossopharyngeal, and vagus nerves.

4. The parasympathetic system uses long preganglionic and short postganglionic fibers.

E. Comparison of the Sympathetic and Parasympathetic Divisions (p. 577; table 15.9)

VI. The Autonomic Nervous System: Physiology (p. 577)

A. Neurotransmitters and Receptors (p. 57; fig. 15.21; TR 561)

1. The autonomic nervous system has cholinergic fibers that secrete ACh, and adrenergic fibers that secrete norepinephrine (NE). (table 15.10)

a. Preganglionic fibers of both divisions are cholinergic, as are the postganglionic fibers of the parasympathetic branch.

b. Postganglionic fibers of the sympathetic branch are usually adrenergic.

c. ACh binds to muscarinic and nicotinic receptors.

d. Nicotinic receptors occur on all postganglionic somas of the ANS, in the adrenal medulla, and on skeletal muscular fibers.

e. Muscarinic receptors occur on all cholinergic receptors of the ANS.

2. Different classes of adrenergic receptors account for the different effects of norepinephrine on its target cells.

a. Binding to alpha-adrenergic receptors is usually excitatory. b. Binding to beta-adrenergic receptors is usually inhibitory.

B. Dual Innervation (p. 578; fig. 15.22; TR 562)

1. Both divisions have nerves leading to most of the visceral organs (dual innervation).

2. The sympathetic and parasympathetic divisions may have antagonistic effects or cooperative effects.

C. Control Without Dual Innervation (p. 580)

1. Control of organ function can be achieved without dual innervation.

2. Sympathetic control of vasomotor tone can shift blood flow from one organ to another according to the body’s changing needs.

D. Central Control of Autonomic Function (p. 580)

1. Control of the ANS is accomplished by several levels of the CNS.

a. Conscious processes in the cerebral cortex can produce autonomic effects.

b. The hypothalamus is the most important area for integrating autonomic function because it has centers (nuclei) for numerous functions, such as sweating, vasodilation, and cardiac and pulmonary function.

c. The reticular formation contains centers for cardiac, vasomotor, respiratory, and gastrointestinal function.

d. The spinal cord controls the defecation and urination reflexes without the involvement of the brain.

E. Effects of the Sympathetic and Parasympathetic Nervous Systems (pp. 578–79; table 15.11; TR 564)