1. Chapter 14: Peripheral Nervous System

2. SPINAL NERVES Overview
Thirty-one pairs of spinal nerves are connected to the spinal cord (Figure 14-1)
No special names; numbered by level of vertebral column at which they emerge from the spinal cavity
Eight cervical nerve pairs (C1 through C8)
12 thoracic nerve pairs (T1 through T12)
Five lumbar nerve pairs (L1 through L5)
Five sacral nerve pairs (S1 through S5)
One coccygeal nerve pair

3. SPINAL NERVES (cont.)
Lumbar, sacral, and coccygeal nerve roots descend from point of origin to the lower end of the spinal cord (level of first lumbar vertebra) before reaching the intervertebral foramina of the respective vertebrae, through which the nerves emerge
Cauda equina describes the appearance of the lower end of the spinal cord and its spinal nerves as a horse’s tail

5. SPINAL NERVES (cont.) Structure of spinal nerves
Each spinal nerve attaches to spinal cord by a ventral (anterior) root and a dorsal (posterior) root
Dorsal root ganglion: swelling in the dorsal root of each spinal nerve
All spinal nerves are mixed nerves

6. SPINAL NERVES (cont.) Ramus
One of several large branches formed after each spinal nerve emerges from the spinal cavity (Figure 14-2)
Dorsal ramus supplies somatic motor and sensory fibers to smaller nerves that innervate the muscles and skin of the posterior surface of the head, neck, and trunk

7. SPINAL NERVES (cont.) Ventral ramus
Structure is more complex than that of dorsal ramus
Autonomic motor fibers split from the ventral ramus and head toward a ganglion of the sympathetic chain
Some autonomic fibers synapse with neurons that continue on to autonomic effectors through splanchnic nerves; others synapse with neurons whose fibers rejoin the ventral ramus
Sympathetic rami: splitting and rejoining of autonomic fibers
Motor and sensory fibers innervate muscles and glands in the extremities and lateral and ventral portions of neck and trunk

9. SPINAL NERVES: PLEXUSES (cont.)
Nerve plexuses
Plexus: complex network formed by the ventral rami of most spinal nerves (not T2 through T12), subdividing and then joining together to form individual nerves
Each individual nerve that emerges contains all the fibers that innervate a particular region of the body
In plexuses, spinal nerve fibers are rearranged according to their ultimate destination, reducing the number of nerves needed to supply each body part

10. SPINAL NERVES: PLEXUSES (cont.)
Four major pairs of plexuses
Cervical plexus (Figure 14-3)
Located deep within the neck
Composed of ventral rami of C1 through C4 and a branch of the ventral ramus of C5
Individual nerves emerging from cervical plexus innervate the muscles and skin of the neck, upper shoulders, and part of the head
Phrenic nerve exits the cervical plexus and innervates the diaphragm
Brachial plexus (Figure 14-4)
Located deep within the shoulder
Composed of ventral rami of C5 through T1
Individual nerves emerging from brachial plexus innervate the lower part of the shoulder and the entire arm

13. SPINAL NERVES: PLEXUSES (cont.)
Lumbar plexus (Figure 14-5)
Located in the lumbar region of the back in the psoas muscle
Formed by intermingling fibers of L1 through L4
Femoral nerve exits the lumbar plexus, divides into many branches, and supplies the thigh and leg
Sacral plexus and coccygeal plexus (Figure 14-5)
Located in the pelvic cavity in the anterior surface of the piriformis muscle
Formed by intermingling of fibers from L4 through S4
Tibial, common peroneal, and sciatic nerves exit the sacral plexus and supply nearly all the skin of the leg, posterior thigh muscles, and leg and foot muscles

15. SPINAL NERVES (cont.) Dermatomes and myotomes (Figure 14-6)
Dermatome: region of skin surface area supplied by afferent (sensory) fibers of a given spinal nerve (Figure 14-7)
Myotome: skeletal muscle or muscles supplied by efferent (motor) fibers of a given spinal nerve (Figure 14-8)

19. CRANIAL NERVES Overview (Tables 14-2 and 14-3)
12 pairs of cranial nerves connect to the brain, mostly the brainstem (Figure 14-9)
Identified by name (determined by either distribution or function) or number (order in which they emerge, anterior to posterior) or both
Composed of bundles of axons
Mixed cranial nerve: axons of sensory and motor neurons
Sensory cranial nerve: axons of sensory neurons only
Motor cranial nerve: mainly axons of motor neurons and a small number of sensory fibers (proprioceptors)

21. Proprioreception
 the sense of the relative position of one's own parts of the body and strength of effort being employed in movement.

22. CRANIAL NERVES (cont.) Olfactory nerve (I) Optic nerve (II)
Composed of axons of neurons whose dendrites and cell bodies lie in nasal mucosa and terminate in olfactory bulbs
Carries information about sense of smell
Optic nerve (II)
Composed of axons from the innermost layer of sensory neurons of the retina
Carries visual information from the eyes to the brain

23. CRANIAL NERVES (cont.) Oculomotor nerve (III)
Fibers originate from cells in the oculomotor nucleus and extend to some of the external eye muscles
Efferent autonomic fibers are also present, extending to the intrinsic muscles of the eye to regulate amount of light entering eye and help focus on near objects
Sensory fibers from proprioceptors in the eye muscles are also present

24. CRANIAL NERVES (cont.) Trochlear nerve (IV)
Motor fibers originate in cells of the midbrain and extend to the superior oblique muscles of the eye
Also contains afferent fibers from proprioceptors in the superior oblique muscles of the eye

25. CRANIAL NERVES (cont.) Trigeminal nerve (V) (Figure 14-10)
Has three branches: ophthalmic nerve, maxillary nerve, and mandibular nerve
Sensory neurons carry afferent impulses from skin and mucosa of head and teeth to cell bodies in the trigeminal ganglion
Motor fibers originate in trifacial motor nucleus and extend to the muscles of mastication through the mandibular nerve

27. CRANIAL NERVES (cont.) Abducens nerve (VI)
Motor nerve with fibers originating from a nucleus in the pons on the floor of the fourth ventricle and extending to the lateral rectus muscles of the eye
Contains afferent fibers from proprioceptors in the lateral rectus muscles

28. CRANIAL NERVES (cont.) Facial nerve (VII)
Motor fibers originate from a nucleus in lower part of pons and extend to superficial muscles of the face and scalp (Figure 14-11)
Autonomic fibers extend to submaxillary and sublingual salivary glands
Also contains sensory fibers from taste buds of anterior two thirds of the tongue

30. CRANIAL NERVES (cont.) Vestibulocochlear nerve (VIII)
Two distinct divisions that are both sensory: vestibular nerve and cochlear nerve
Vestibular nerve fibers originate in the semicircular canals in inner ear and transmit impulses that result in sensations of equilibrium
Cochlear nerve fibers originate in organ of Corti in the cochlea of the inner ear and transmit impulses resulting in sensations of hearing

31. CRANIAL NERVES (cont.) Glossopharyngeal nerve (IX) Vagus nerve (X)
Composed of sensory, motor, and autonomic nerve fibers
Supplies fibers to tongue, pharynx, and carotid sinus (Figure 14-12)
Vagus nerve (X)
Composed of sensory and motor fibers with many widely distributed branches
Sensory fibers supply pharynx, larynx, trachea, heart, carotid body, lungs, bronchi, esophagus, stomach, small intestine, and gallbladder (Figure 14-13)
Somatic motor fibers innervate the pharynx and larynx and are mostly autonomic fibers

34. CRANIAL NERVES (cont.) Accessory nerve (XI) Hypoglossal nerve (XII)
Motor nerve that is an “accessory” to the vagus nerve
Innervates thoracic and abdominal viscera, pharynx, larynx, trapezius, and sternocleidomastoid (Figure 14-14)
Hypoglossal nerve (XII)
Composed of motor and sensory fibers
Motor fibers innervate the muscles of the tongue
Contains sensory fibers from proprioceptors in muscles of the tongue

36. DIVISIONS OF THE PERIPHERAL NERVOUS SYSTEM
Two functional divisions of the peripheral nervous system
Afferent (sensory) division
Efferent (motor) division
Efferent division is divided further into the somatic motor nervous system and the efferent portions of the autonomic nervous system

37. DIVISIONS OF THE PERIPHERAL NERVOUS SYSTEM (cont.)
Basic principles of somatic motor pathways
Somatic nervous system includes all voluntary motor pathways outside the central nervous system
Somatic effectors: skeletal muscles

38. DIVISIONS OF THE PERIPHERAL NERVOUS SYSTEM (cont.)Q?
Somatic reflexes
Nature of a reflex
Reflex: action that results from a nerve impulse passing over a reflex arc; predictable response to a stimulus
Cranial reflex: center of reflex arc is in the brain
Spinal reflex: center of reflex arc is in the spinal cord
Reflex consists of either muscle contraction or glandular secretion
Somatic reflex: contraction of skeletal muscles
Autonomic (visceral) reflex: either contraction of smooth or cardiac muscle or secretion by glands

39. DIVISIONS OF THE PERIPHERAL NERVOUS SYSTEM (cont.)
Somatic reflexes of clinical importance: reflexes deviate from normal in certain diseases, and reflex testing is a valuable diagnostic aid (Figure 14-16)
Abdominal reflex: drawing in of the abdominal wall in response to stroking the side of the abdomen; superficial reflex; mediated by arcs with sensory and motor fibers in T9 through T12 and centers in these segments of the cord; decreased or absent reflex may involve lesions of pyramidal tract upper motor neurons

40. Other examples of reflexes
Other reflexes that provide us with biofeedback (information about what’s wrong with the nerves based off of their reflex action)
Knee jerk
Plantar reflex
Corneal reflex
Spinal cord
Many others

43. Ganglia
In vertebrates the ganglion is a cluster of neural bodies outside the central nervous system.
A spinal ganglion, for instance, is a cluster of nerve bodies positioned along the spinal cord at the dorsal and ventral roots of a spinal nerve.

44. Ganglia

45. AUTONOMIC NERVOUS SYSTEM
Overview
Contains afferent (sensory) and efferent (motor) components (the efferent components are emphasized here)
Carries fibers to and from the autonomic effectors
Major function: to regulate heartbeat, smooth muscle contraction, and glandular secretions to maintain homeostasis
Two efferent divisions: sympathetic division and parasympathetic division
Sympathetic division consists of neural pathways separate from parasympathetic pathways
Many autonomic effectors are dually innervated, which allows remarkably precise control of effector

46. AUTONOMIC NERVOUS SYSTEM (cont.)
Structure of the autonomic nervous system
Basic plan of efferent autonomic pathways (Figure 14-17)
Each pathway is composed of autonomic nerves, ganglia, and plexuses, which are made of efferent autonomic neurons
All autonomic neurons function in reflex arcs
Efferent autonomic regulation ultimately depends on feedback from sensory receptors
Relay of two efferent autonomic neurons conducts information from central nervous system to autonomic effectors
Preganglionic neuron: conducts impulses from the central nervous system to an autonomic ganglion
Postganglionic neuron: efferent neuron with which a preganglionic neuron synapses within autonomic ganglion

48. AUTONOMIC NERVOUS SYSTEM: STRUCTURE (cont.)
Structure of the sympathetic pathways
Sympathetic chain ganglia
Most ganglia of the sympathetic division lie along either side of the anterior surface of the vertebral column and are joined with the other ganglia located on the same side
Each chain runs from the second cervical vertebra to the level of the coccyx
22 sympathetic chain ganglia are usually on each side of vertebral column: three cervical, 11 thoracic, four lumbar, and four sacral
Thoracolumbar division
Sympathetic preganglionic neurons with dendrite and cell bodies in lateral gray horns of the thoracic and lumbar segments of the spinal cord
Axons leave the cord by way of the ventral roots of the thoracic and first four lumbar spinal nerves and split away from other spinal nerve fibers by the white ramus to a sympathetic chain ganglion

49. AUTONOMIC NERVOUS SYSTEM: STRUCTURE (cont.)
Preganglionic fiber may take one of three paths once inside the sympathetic chain ganglion
Synapse with sympathetic postganglionic neuron
Send ascending or descending branches through the sympathetic trunk to synapse with postganglionic neurons in other chain ganglia
Pass through one or more chain ganglia without synapsing
Sympathetic postganglionic neurons
Dendrites and cell bodies are mostly in sympathetic chain ganglia or collateral ganglia
Gray ramus: short branch by which some postganglionic axons return to a spinal nerve
In the sympathetic division, preganglionic neurons are relatively short, and postganglionic neurons are relatively long
Axon of one sympathetic preganglionic neuron synapses with many postganglionic neurons, terminating in widely spread organs (Figure 14-18)

50. AUTONOMIC NERVOUS SYSTEM: STRUCTURE (cont.)
Structure of the parasympathetic pathways
Parasympathetic preganglionic neurons: cell bodies are located in nuclei in the brainstem or lateral gray columns of the sacral cord; extend a considerable distance before synapsing with postganglionic neurons
Parasympathetic postganglionic neurons: dendrites and cell bodies are located in parasympathetic ganglia, which are embedded in or near autonomic effectors
Parasympathetic postganglionic neurons synapse with postganglionic neurons that each lead to a single effector (Figure 14-18)

51. Q?

52. Para v sympathetic
Sympathetic is responsible for the response commonly referred to as "fight or flight," while parasympathetic is referred to as "rest and digest.
BOTH parts of autonomic nervous system

54. THE BIG PICTURE: THE PERIPHERAL NERVOUS SYSTEM AND THE WHOLE BODY
The peripheral nervous system is composed of all the afferent nervous pathways coming into the central nervous system and all the efferent pathways going out of the central nervous system
Peripheral pathways lead from the integrator central nervous system to the effectors
Peripheral motor pathways serve as an information-carrying network that allows the central nervous system to communicate regulatory information to all the nervous effectors in the body
Every major organ is influenced, directly or indirectly, by peripheral nervous system output

55. AUTONOMIC NERVOUS SYSTEM: STRUCTURE (cont.)
Autonomic neurotransmitters (Figures and 14-20)
Axon terminal of autonomic neurons releases either of two neurotransmitters: norepinephrine or acetylcholine
Adrenergic fibers: release norepinephrine; axons of postganglionic sympathetic neurons
Cholinergic fibers: release acetylcholine; axons of preganglionic sympathetic neurons and of preganglionic and postganglionic parasympathetic neurons
Norepinephrine affects visceral effectors by first binding to one of two types of adrenergic receptors in plasma membranes: alpha receptors or beta receptors
Binding of norepinephrine to alpha receptors in smooth muscle of blood vessels is stimulating, causing the vessels to constrict
Binding of norepinephrine to beta receptors in smooth muscle of blood vessels is inhibitory, causing blood vessels to dilate; in cardiac muscle, has stimulating effect

56. AUTONOMIC NERVOUS SYSTEM: STRUCTURE (cont.)
Autonomic neurotransmitters (cont.)
Epinephrine also stimulates adrenergic receptors, enhancing and prolonging effects of sympathetic stimulation
Effect of a neurotransmitter on any postsynaptic cell is determined by characteristics of the receptors, not by the neurotransmitter
Termination of actions of norepinephrine and epinephrine
Monoamine oxidase (MAO): enzyme that breaks up neurotransmitter molecules taken back up by the synaptic knobs
Catechol-O-methyl transferase (COMT): enzyme that breaks down the remaining neurotransmitter
Acetylcholine binds to two types of cholinergic receptors: nicotinic receptors and muscarinic receptors
Termination of action of acetylcholine is by the enzyme acetylcholinesterase
Autonomic neurotransmitters and receptors may influence different types of presynaptic and postsynaptic receptors at synapses with dually innervated effectors; this summation of effects increases precision of control

58. AUTONOMIC NERVOUS SYSTEM (cont.)
Functions of the autonomic nervous system
Overview of autonomic function
The autonomic nervous system functions to regulate effectors in ways that tend to maintain or quickly restore homeostasis
Sympathetic and parasympathetic divisions are tonically active, often exerting antagonistic influences on visceral effectors
Doubly innervated effectors continually receive both sympathetic and parasympathetic impulses; summation of the two determines the controlling effect

59. AUTONOMIC NERVOUS SYSTEM: FUNCTIONS (cont.)
Functions of the sympathetic division
Under resting conditions, the sympathetic division can act to maintain the normal functioning of doubly innervated autonomic effectors
Sympathetic impulses function to maintain normal tone of the smooth muscle in blood vessel walls
Major function of sympathetic division is as an “emergency” system—the “fight or flight” reaction (Table 14-7)
Functions of the parasympathetic division (Table 14-6)
Dominant controller of most autonomic effectors most of the time
Acetylcholine: slows heartbeat and promotes digestion and elimination