TBL3 - Spinal Nerves

Question 1

What occupy the anterior and posterior horns of the spinal cord?

Answer 1

1) Somatic motor neurons occupy the anterior (ventral) horns of the spinal cord
2) Interneurons occupy the posterior (dorsal) horns of the spinal cord

Question 2

Where are spinal sensory nerves rooted in the spine? Where do spinal nerves emerge from along the spine?

Answer 2

1) A dorsal root ganglion (or spinal ganglion) is a cluster of nerve cell bodies (a ganglion) in a dorsal root (a branch of a nerve carrying mostly sensory signals into the spinal cord). DRGs and roots are protected within the vertebral canal
2) Gray matter has two ventral horns and two dorsal horns connected at the center by an isthmus of gray commissures. Sensory nerve fibers enter the spinal cord via the dorsal horns, and motor nerve fibers exit from the ventral horns in discrete bundles known as spinal nerves
3) Spinal nerves emerge from the intervertebral foramen

Question 3

How can you distinguish white matter from gray matter?

Answer 3

1) The white matter of the spinal cord, unlike that in other CNS areas, is peripherally located; the gray matter occupies an H-shaped central region
2) White matter is so named because of large amounts of myelin, the fatty insulating substance that forms sheaths around individual nerve fibers. White matter consists of ascending and descending tracts of myelinated nerve fibers
3) Gray matter consists chiefly of cell bodies and unmyelinated nerve fibers, so to the naked eye they appear pinkish gray compared with myelinated fibers of the white matter
4) Gray matter has two ventral horns and two dorsal horns connected at the center by an isthmus of gray commissures
5) Sensory nerve fibers enter the spinal cord via the dorsal horns, and motor nerve fibers exit from the ventral horns in discrete bundles known as spinal nerves

Question 4

Summarize how spinal nerves reach their exiting foramina by comparing alphanumeric designations of the cervical spinal nerves to alphanumeric designations of the thoracic, lumbar, and sacral spinal nerves

Answer 4

1) Cervical spinal nerves (except C8) bear the same alphanumeric designation as the vertebrae forming the inferior margin of the IV foramina through which the nerve exits the vertebral canal (foramina exit is superior to vertebrae)
2) The more inferior spinal (T1 through Co1) nerves bear the same alphanumeric designation as the vertebrae forming the superior margin of their exit (foramina exit is inferior to vertebrae)
3) First cervical nerves lack posterior roots in 50% of people, and the coccygeal nerve may be absent

Question 5

What do spinal nerves divide into immediately after emerging from intervertebral foramina?

Answer 5

1) Immediately after emerging from the intervertebral foramina, spinal nerves divide into posterior and anterior rami (branches)
2) Posterior rami supply the posterior body wall
3) Anterior rami supply the anterolateral body wall and extremities

Question 6

Which spinal nerve roots are compressed by disc herniations between vertebrae L4 and L5 and between vertebrae C4 and C5?

Answer 6

1) The general rule is that when an IV disc protrudes, it usually compresses the nerve root numbered one inferior to the herniated disc; for example, the L5 nerve is compressed by an L4–L5 IV disc herniation
2) The cervical IV discs most commonly ruptured are those between C5-C6 & C6-C7, compressing spinal nerve roots C6 & C7. Between vertebrae C4 & C5 would be spinal nerve C5

Question 7

How do neurons communicate with one another?

Answer 7

1) Neurons communicate with each other at synapses, points of contact between neurons
2) The communication occurs by means of neurotransmitters, chemical agents released or secreted by one neuron, which may excite or inhibit another neuron, continuing or terminating the relay of impulses or the response to them

Question 8

What is a characteristic size of multipolar neuron axons?

Answer 8

Multipolar neurons contain long, projecting motor axons (also called fibers)

Question 9

What forms the posterior roots of spinal nerves?

Answer 9

Central fibers from DRG sensory neurons form the posterior roots of the spinal nerves

Question 10

What types of neurons do sensory fibers synapse with?

Answer 10

The sensory fibers synapse with dendrites of the multipolar interneurons in the posterior horns

Question 11

What types of neurons do activated interneurons synapse with?

Answer 11

Fibers from some of the activated interneurons synapse with dendrites of the multipolar neurons in the adjacent anterior horn

Question 12

What does synaptic activation of the somatic motor neurons cause?

Answer 12

Synaptic activation of the somatic motor neurons causes reflexive movement i.e., reflexive muscle contraction recoils the hand from the hot iron

Question 13

How do interneurons assist the sensory impulse of a reflexive muscle contraction, for example?

Answer 13

Fibers from other activated interneurons help conduct the sensory impulse up the spinal cord to the brain for conscious perception of the painful sensation

Question 14

What are two sympathetic neurons in series designated?

Answer 14

The two sympathetic neurons in series are designated presynaptic and postsynaptic neurons

Question 15

Where are presynaptic neurons located?

Answer 15

1) Presynaptic neurons are located in the intermediolateral (aka lateral) horns of the spinal cord
2) The horns exist only in segments T1-L2

Question 16

Describe the connection between presynaptic fibers, somatic motor fibers, & postsynaptic fibers

Answer 16

1) Presynaptic fibers leave the lateral horns and accompany somatic motor fibers in the anterior roots of spinal nerves T1-L2
2) The visceral motor fibers continue in the anterior rami of spinal nerves T1-L2 and exit the anterior rami to synapse with the postsynaptic neurons in the sympathetic ganglia

Question 17

What are linked sympathetic ganglia?

Answer 17

1) Sympathetic ganglia are linked to form bilateral sympathetic trunks or chains along the vertebral column
2) The linked ganglia are called paravertebral ganglia
3) The 12 thoracic, 5 lumbar, and 5 sacral spinal segments have corresponding paravertebral ganglia but the 8 cervical segments have only 3 bilateral ganglia

Question 18

Describe the communication between white and gray communicating rami

Answer 18

1) White communicating rami, which are formed by presynaptic sympathetic fibers, arise from the anterior rami of spinal nerves T1-L2 and terminate in the corresponding paravertebral ganglia
2) Gray communicating rami, which arise in the paravertebral ganglia, are formed by postsynaptic sympathetic fibers returning to the anterior rami of spinal nerves T1-L2. The postsynaptic fibers also pass onto the nerves’ posterior rami

Question 19

Instead of synapsing in corresponding paravertebral ganglia, what do some presynaptic fibers do?

Answer 19

1) Some presynaptic fibers, rather than synapsing in their corresponding paravertebral ganglia, either ascend the sympathetic trunks to synapse in the cervical ganglia or descend the sympathetic trunks to synapse in the inferior lumbar and sacral paravertebral ganglia
2) Gray communicating rami from these paravertebral ganglia provide postsynaptic sympathetic fibers to anterior and posterior rami of the corresponding spinal nerves

Question 20

What are three places that spinal nerves send visceral motor fibers to? Where do the spinal nerves send somatic sensory fibers to?

Answer 20

1) The spinal nerves deliver visceral motor fibers to the arrector pili muscles, sweat glands, & blood vessels
2) The spinal nerves deliver somatic sensory fibers to the skin of the body wall and extremities

Question 21

How do sympathetic fibers contribute to regulation of blood flow?

Answer 21

The sympathetic fibers stimulate contraction of smooth muscle in the arteries and arterioles that supply the skin, skeletal muscles, and bones of the body wall and extremities; thus, the sympathetic fibers contribute to the regulation of blood flow to the respective tissues

Question 22

Why are white communicating rami only associated with para-vertebral sympathetic ganglia at T1-L2?

Answer 22

There are no lateral horns & anterior sympathetic roots outside of T1-L2 (no pre-synaptic neurons outside of T1-L2)

Question 23

What causes “goose bumps”?

Answer 23

The postsynaptic sympathetic fibers stimulate contraction of the blood vessels (vasomotion) and arrector muscles associated with hairs (pilomotion, resulting in “goose bumps”), and to cause sweating (sudomotion)

Question 24

The anterior and posterior rami of all spinal nerves contain what kind of fibers?

Answer 24

1) The anterior and posterior rami of all spinal nerves contain somatic motor fibers, somatic sensory fibers, and visceral motor fibers
2) The axons are bundled together by sleeves of connective tissue

Question 25

What does a peripheral nerve consist of?

Answer 25

1) A peripheral nerve consists of one or more bundles of nerve fibers 2) Each bundle, or fascicle, contains a mixture of fibers, either efferent (motor) or afferent (sensory) 3) In peripheral nerves consisting of more than one fascicle, an outer layer of dense irregular connective tissue, the epineurium, binds the fascicles together and forms a strong cylindrical sheath around the whole nerve 4) Surrounding each fascicle is a very condensed layer of specialized connective tissue called the perineurium, which is made of multiple concentric layers of flattened perineurial cells with intervening, longitudinal collagen fibrils 5) Individual nerve fibers and their support cells within each fascicle are firmly embedded in a delicate packing of loose connective tissue called endoneurium

Question 26

How does the perineurium contribute to blood brain barrier?

Answer 26

1) The perineurium acts as a selective, metabolically active diffusion barrier. It restricts passage of many macromolecular substances, thereby regulating the internal microenvironment of the nerve 2) Perineurial cells are modified fibroblasts, most likely of mesenchymal origin, which are linked together by tight junctions and help contribute to a blood-nerve barrier between highly permeable blood vessels in the exterior of each fascicle and the interior tight capillaries

Question 27

Are fibers within fascicles myelinated or unmyelinated?

Answer 27

1) Mixed fibers within the fascicles are either myelinated or unmyelinated 2) Most of the axons are myelinated 3) Loose connective tissue between the fibers constitutes the endoneurium

Question 28

How is myelin in white matter formed?

Answer 28

1) Lipoprotein-rich myelin sheaths are formed by a continuous series of Schwann cells and gaps between the cells create nodes (of Ranvier) 2) In the PNS, Schwann cells form myelin sheaths by wrapping around a single axon in a “jelly-roll-like” manner

Question 29

What are examples of myelinated (white) fibers?

Answer 29

1) Somatic motor axons 2) Somatic sensory axons conducting sensations of touch 3) Presynaptic axons of white communicating rami are myelinated fibers

Question 30

What part of a Schwann cell contributes to a node of Ranvier?

Answer 30

The basal lamina that surrounds the Schwann cells is continuous across each node of Ranvier

Question 31

Explain how Schwann cells assist in the regeneration of damaged nerve fibers

Answer 31

1) Schwann cells also aid debris removal (phagocytosis of dysfunctional axons) and serve as guides for sprouts of regenerating axons after injury 2) Damage to myelin is common in neurologic diseases and leads to blocked axonal conduction, secondary damage to axons, and possibly permanent neurologic deficits

Question 32

What are examples of unmyelinated (gray) fibers?

Answer 32

1) When Schwann cells engulf multiple axons without wrapping them with myelin sheaths, the fibers are unmyelinated 2) The postsynaptic axons of gray communicating rami, axons of interneurons, and somatic sensory axons conducting pain and temperature sensations are unmyelinated fibers

Question 33

Compare the impulse conduction speeds of myelinated vs. unmyelinated fibers

Answer 33

1) Myelin acts as an electrical insulator and permits nerve impulses to be transmitted rapidly by saltatory conductance along nodes of Ranvier 2) A direct relationship exists among speed of nerve conduction, axon size, and thickness of the myelin sheath (number of concentric myelin lamellae) 3) In myelinated fibers, the speed of conduction may vary from 5 to 100 m/s, which is much higher than that in smaller unmyelinated fibers, with conduction speeds of 0.2-2 m/s

Question 34

What is saltatory conductance?

Answer 34

1) Saltatory conduction is the propagation of action potentials along myelinated axons from one node of Ranvier to the next node, increasing the conduction velocity of action potentials 2) The uninsulated nodes of Ranvier are the only places along the axon where ions are exchanged across the axon membrane, regenerating the action potential between regions of the axon that are insulated by myelin

Question 35

What are differences between Schwann cells and Oligodendrocytes?

Answer 35

1) Oligodendrocytes and Schwann cells are responsible for synthesis and maintenance of myelin in the CNS and PNS, respectively 2) Nonmyelinating Schwann cells collectively ensheath groups of several small axons, while myelinating Schwann cells are most often associated with one large axon 3) Oligodendrocytes produce a myelin sheath by wrapping around axons. Unlike oligodendrocytes that wrap around numerous axons, one Schwann cell myelinates one segment of an axon 4) Oligodendrocytes are not enveloped by a basal lamina, which may contribute to relatively poor regeneration after CNS injury

Question 36

Why can MS affect both afferent and efferent axons?

Answer 36

1) Multiple sclerosis (MS) is a chronic inflammatory disease of the CNS characterized by a loss of myelin; damaged patches called plaques appear in seemingly random areas of the white matter 2) During periods of MS activity, leukocytes (T cells) are drawn to regions of the white matter, which initiates an inflammatory response accompanied by loss of oligodendrocytes and axon demyelination

Question 37

What is the clinical significance of the histological observation that Schwann cells but not oligodendrocytes are invested by basement membranes?

Answer 37

Oligodendrocytes are not enveloped by a basal lamina, which may contribute to relatively poor regeneration after CNS injury

Question 38

Why does a cutting nerve injury but not a crushing nerve injury require surgery?

Answer 38

1) A crushing nerve injury damages or kills the axons distal to the injury site; however, the nerve cell bodies usually survive, and the nerve’s connective tissue coverings remain intact. No surgical repair is needed for this type of nerve injury because the intact connective tissue coverings guide the growing axons to their destinations 2) Regeneration is less likely to occur in a severed nerve. Sprouting occurs at the proximal ends of the axons, but the growing axons may not reach their distal targets. A cutting nerve injury requires surgical intervention because regeneration of the axon requires apposition of the cut ends by sutures through the epineurium. The individual nerve bundles are realigned as accurately as possible

Question 39

Where do synapses occur between in the CNS?

Answer 39

Most synapses in the CNS are between axons and dendrites (e.g., axons from interneurons in posterior horns of the spinal cord and dendrites of motor neurons in the adjacent anterior horns)

Question 40

What exactly occurs within a synapse?

Answer 40

1) Typical synapse in the CNS consists of three major components: presynaptic terminal, synaptic cleft, and postsynaptic membrane 2) The presynaptic terminal aligns closely with the postsynaptic membrane of the target cell. In the area of membrane apposition, presynaptic and postsynaptic membranes are separated by a narrow synaptic cleft 12-30 nm wide 3) Clusters of large numbers of synaptic vesicles in the presynaptic terminal contain neurotransmitter that is released by exocytosis to mediate synaptic transmission 4) An action potential causes presynaptic vesicles to fuse with the presynaptic membrane and discharge neurotransmitter into the synaptic cleft. Neurotransmitter then diffuses across the cleft to interact with receptor molecules on the postsynaptic membrane, which changes postsynaptic membrane conductance

Question 41

What are ganglia?

Answer 41

1) Ganglia are discrete aggregations of neuron bodies located outside the CNS. All derived from neural crest, they include sensory ganglia of cranial nerves, dorsal root ganglia of spinal nerves, and autonomic ganglia at various peripheral sites 2) Regardless of ganglion site and size, an outer, dense connective tissue capsule continuous with epineurium and perineurium associated with entering or emerging nerve fibers invests all ganglia 3) Neuron bodies within cranial or spinal sensory ganglia are usually pseudounipolar, whereas those in autonomic ganglia are multipolar

Question 42

What important cells surround dorsal root ganglia (DRG)?

Answer 42

1) Within the ganglion, a single layer of neural crest–derived satellite cells usually surrounds, to form a continuous investment around, each neuron body— arranged like satellites around a central planet 2) Satellite cells are flattened, modified Schwann cells with heterochromatic nuclei that are small compared with those of neurons 3) A basement membrane encloses the outer aspect of the satellite cells, which are linked by gap junctions

Question 43

What types of neurons occupy the DRG?

Answer 43

Central and peripheral axons of the sensory neurons occupy the DRG

Question 44

Why do postsynaptic neurons of sympathetic ganglia lack a complete investment of satellite cells?

Answer 44

1) Postsynaptic neurons of the sympathetic ganglia lack a complete investment of satellite cells to allow preganglionic fibers to form synapses with postsynaptic neurons that occupy the ganglia 2) The investment is usually less complete in autonomic ganglia than in dorsal root ganglia, which allows passage of terminal parts of preganglionic axons that form synapses on ganglion cells