Flashcards in TBL3 - Spinal Nerves Deck (44):
What occupy the anterior and posterior horns of the spinal cord?
1) Somatic motor neurons occupy the anterior (ventral) horns of the spinal cord
2) Interneurons occupy the posterior (dorsal) horns of the spinal cord
Where are spinal sensory nerves rooted in the spine? Where do spinal nerves emerge from along the spine?
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
How can you distinguish white matter from gray matter?
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
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
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
What do spinal nerves divide into immediately after emerging from intervertebral foramina?
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
Which spinal nerve roots are compressed by disc herniations between vertebrae L4 and L5 and between vertebrae C4 and C5?
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
How do neurons communicate with one another?
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
What is a characteristic size of multipolar neuron axons?
Multipolar neurons contain long, projecting motor axons (also called fibers)
What forms the posterior roots of spinal nerves?
Central fibers from DRG sensory neurons form the posterior roots of the spinal nerves
What types of neurons do sensory fibers synapse with?
The sensory fibers synapse with dendrites of the multipolar interneurons in the posterior horns
What types of neurons do activated interneurons synapse with?
Fibers from some of the activated interneurons synapse with dendrites of the multipolar neurons in the adjacent anterior horn
What does synaptic activation of the somatic motor neurons cause?
Synaptic activation of the somatic motor neurons causes reflexive movement i.e., reflexive muscle contraction recoils the hand from the hot iron
How do interneurons assist the sensory impulse of a reflexive muscle contraction, for example?
Fibers from other activated interneurons help conduct the sensory impulse up the spinal cord to the brain for conscious perception of the painful sensation
What are two sympathetic neurons in series designated?
The two sympathetic neurons in series are designated presynaptic and postsynaptic neurons
Where are presynaptic neurons located?
1) Presynaptic neurons are located in the intermediolateral (aka lateral) horns of the spinal cord
2) The horns exist only in segments T1-L2
Describe the connection between presynaptic fibers, somatic motor fibers, & postsynaptic fibers
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
What are linked sympathetic ganglia?
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
Describe the communication between white and gray communicating rami
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
Instead of synapsing in corresponding paravertebral ganglia, what do some presynaptic fibers do?
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
What are three places that spinal nerves send visceral motor fibers to? Where do the spinal nerves send somatic sensory fibers to?
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
How do sympathetic fibers contribute to regulation of blood flow?
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
Why are white communicating rami only associated with para-vertebral sympathetic ganglia at T1-L2?
There are no lateral horns & anterior sympathetic roots outside of T1-L2 (no pre-synaptic neurons outside of T1-L2)
What causes “goose bumps”?
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)
The anterior and posterior rami of all spinal nerves contain what kind of fibers?
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
What does a peripheral nerve consist of?
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
How does the perineurium contribute to blood brain barrier?
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
Are fibers within fascicles myelinated or unmyelinated?
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
How is myelin in white matter formed?
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
What are examples of myelinated (white) fibers?
1) Somatic motor axons
2) Somatic sensory axons conducting sensations of touch
3) Presynaptic axons of white communicating rami are myelinated fibers
What part of a Schwann cell contributes to a node of Ranvier?
The basal lamina that surrounds the Schwann cells is continuous across each node of Ranvier
Explain how Schwann cells assist in the regeneration of damaged nerve fibers
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
What are examples of unmyelinated (gray) fibers?
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
Compare the impulse conduction speeds of myelinated vs. unmyelinated fibers
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
What is saltatory conductance?
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
What are differences between Schwann cells and Oligodendrocytes?
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
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
Why can MS affect both afferent and efferent axons?
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
What is the clinical significance of the histological observation that Schwann cells but not oligodendrocytes are invested by basement membranes?
Oligodendrocytes are not enveloped by a basal lamina, which may contribute to relatively poor regeneration after CNS injury
Why does a cutting nerve injury but not a crushing nerve injury require surgery?
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
Where do synapses occur between in the CNS?
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)
What exactly occurs within a synapse?
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
What are ganglia?
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
What important cells surround dorsal root ganglia (DRG)?
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
What types of neurons occupy the DRG?
Central and peripheral axons of the sensory neurons occupy the DRG