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Chapter 13 – Spinal Cord and Spinal Nerves!
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Chapter 13!The Spinal Cord and Spinal Nerves!
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SECTION 13-1 !The brain and spinal cord make up the central nervous system, and the cranial nerves and spinal nerves constitute the peripheral nervous system!
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Grouping of Neural Tissues (1 of 3)!White matter!• Myelinated (and unmyelinated) neuron processes!• Schwann cells in PNS!• Oligodendrocytes in CNS!
Gray matter!• Nerve cell bodies and dendrites!• Unmyelinated axons!• Neuroglia (e.g. astrocytes)!
Nerve!• Bundle of nerve fibers (axons) outside the CNS!• Has associated CT layers!
Chapter 13 – Spinal Cord and Spinal Nerves!
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Grouping of Neural Tissue (2 of 3)!
Ganglion!• Collection of nerve cell bodies outside of the
CNS!• E.g. Dorsal root ganglion!
Nucleus!• Collection of nerve cell bodies with a
common function found inside the CNS!• In brain or spinal cord!• Unmyelinated!• E.g. Basal nuclei!
5!
Grouping of Neural Tissue (3 of 3)!
Columns are major regions of cord!• E.g., posterior columns!
Tracts are bundles of axons with common:!• Structural characteristics (e.g. axon diameter,
myelinated or not)!• Function/direction!• E.g., spinothalamic tract!
Ascending tracts (in cord) = sensory!Descending tracts (in cord) = motor!Horns!• Major areas of gray matter in spinal cord!
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SECTION 13-2 !The spinal cord is surrounded by three meninges and conveys sensory and motor information!
Chapter 13 – Spinal Cord and Spinal Nerves!
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Gross Anatomy of the Spinal Cord Figure 13-2!
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Spinal Cord Functions/Anatomy!
Spinal cord functions:!• Convey sensory information to brain!• Convey motor information to PNS!• Reflexively integrate sensory and motor info!
Gross anatomical features:!Length!• Extends from medulla to L1−L2 in adults!• 16 to 18 in. long!• About 0.5 in. diameter!
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Spinal Cord Gross Anatomy (1 of 2)!
Enlargements:!Regions where cord is
thicker!• Cervical enlargement!
Fibers to and from arms!• Lumbar enlargement!
Fibers to and from legs!Conus medullaris!• Tapered end of the
cord!• About L1 or L2!
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Chapter 13 – Spinal Cord and Spinal Nerves!
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Spinal Cord Gross Anatomy (2 of 2)!
Filum terminale!• Fibrous extension of pia mater!• Forms part of coccygeal ligament!
Coccygeal ligament!• Dura mater + filum terminale (pia)!• Attaches conus medullaris to coccyx!
Cauda equina (“horse’s tail”)!• Nerves arising from lower portion of cord!• Descend in spinal canal!• Exit spinal canal as spinal nerves!
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Conus Medullaris, Filum Terminale!
Gray’s anatomy!www.bartleby.com/107/illust661.html!
Conus medullaris!
Filum terminale -!Intradural portion!
Filum terminal - !extradural portion!(coccygeal ligament)!1st coccygeal!
vertebra!
Dura mater!
Subarachnoid!space!
2nd sacral vertebra!
1st lumbar!Vertebra!
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Spinal Tap, Spinal Nerves!Spinal tap!• Deliver anesthetics, sample CSF fluid or
pressure, etc.!• Done between L3 and L4 or L4 and L5!
(Cord not present here)!Spinal nerves contain two roots! Spinal nerves are mixed nerves!• Ventral root - motor !• Dorsal root - sensory !• Dorsal root ganglion!
Sensory cell bodies (unipolar neurons)!
Chapter 13 – Spinal Cord and Spinal Nerves!
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Gross Anatomy of the Spinal Cord Figure 13-2b!
motor!
sensory!
mixed!
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The Spinal Meninges Figure 13-3a!
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Spinal Meninges and Associated Structures!
Continuous with cranial meninges at F. magnum!Functions:!• Provide support, stability, shock absorption!
!!Epidural space:!• Between dura mater and walls of vertebral
canal!• Contains areolar CT, blood vessels!• Cushions cord!
Chapter 13 – Spinal Cord and Spinal Nerves!
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Dura Mater!
Dura mater = “tough mother”!• Not firmly attached to bone (as in cranium)!• Dense irregular CT!• Inner lining of simple squamous EPI!• Tubular portion runs from F. Magnum to S2!
Fuses to form coccygeal ligament (with filum terminale) that extends to coccyx!Fuses with CT coverings of spinal nerves!
Subdural “space”!• Probably not present in life!
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Arachnoid Membrane!
Arachnoid = “spider-like” = refers to trabeculae!• Avascular!• Simple squamous EPI + CT trabeculae!
Collagen + elastin fibers form trabeculae!• EPI adheres to dura across “subdural space”!• Extends with dura to end of intradural portion
of filum terminale, so roots of nerves making up the cauda equina are in subarachnoid space!
Subarachnoid space!• Contains cerebrospinal fluid (CSF)!
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Pia Mater!
Pia mater = “soft” or “delicate” mother!• Adheres to surface of brain and cord!• Delicate collagen and elastin fibers!• Highly vascular!• Runs to coccyx as filum terminale (with dura,
forms coccygeal ligament)!Denticulate (dentate) ligaments (Fig. 13-5)!• Formed from pia mater!• Extend through arachnoid to attach to dura!• Suspend cord within subarachnoid space!
Chapter 13 – Spinal Cord and Spinal Nerves!
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The Spinal Cord and Spinal Meninges!
Figure 13-3b!
Note orientation compared to that in Figure 13-5!
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SECTION 13-3 !Gray matter is the region of integration and command, and white matter carries information from place to place!
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Sectional Organization of Cord Figure 13-5b!
Note orientation compared to that in Figure 13-3!
Chapter 13 – Spinal Cord and Spinal Nerves!
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Internal Anatomy of the Cord!
• Ventral (anterior) median fissure !(A fissure is larger than a sulcus.)!
• Dorsal (posterior) median sulcus !(smaller than fissure)!
• Gray commissures!Commissure = crossing from one side of the
body to the other!Gray = gray matter!
• Central canal!Contains CSF!
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Gray Matter in the Cord see Figure 13-5a!
Nuclei - collections of cell bodies with a similar function!
Ventral (anterior) gray horns!• Somatic motor nuclei!• Efferent information to skeletal muscles!
Lateral gray horns!• Visceral (autonomic) motor nuclei!• Only in thoracic and lumbar segments!
Dorsal (posterior) gray horns (see slide #63)!• Sensory area!• Somatic and autonomic (visceral) nuclei!
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Sectional Organization of Cord Figure 13-5a!
Note interneurons in dorsal horns!
Chapter 13 – Spinal Cord and Spinal Nerves!
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White Matter – Columns and Tracts!
Columns:!• Large collections of fibers conveying
information to or from the brain!• Have similar origin and similar destinations!
Tracts:!• Collections of fibers within a column!• Carry same type of information in the same
direction to the same destination!
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Examples of Columns and Tracts!
Posterior columns!• Sensory (ascending)!
Spinothalamic tracts!• Sensory (ascending)!
Corticospinal tracts!• Motor (descending)!• Voluntary movements!
Vestibulospinal tracts!• Motor (descending)!• Automatic (involuntary) movements!
Note that the name usually tells you the direction of information
flow.!
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Sensory Columns and Tracts Figure 15-4!
Note organization and direction. Do not try to memorize locations.!
Posterior column pathway
Spinothalamic pathway
Chapter 13 – Spinal Cord and Spinal Nerves!
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Motor Columns and Tracts Figure 15-8!
Lateral corticospinal tract
Vestobulospinal tract
Anterior corticospinal tract
Note organization and direction. Do not try to memorize locations.!
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SECTION 13-4 !Spinal nerves form plexuses that are named according to their level of emergence from the vertebral canal!
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Spinal Nerves!
Spinal nerves are mixed nerves!• Contain both sensory and motor fibers!
Exist in 31 pairs!Exit vertebral canal via intervertebral foramina!• Named for where they exit vertebral column!
Cord stops at level of L2!• Nerves exiting below this level form the cauda
equina!
Chapter 13 – Spinal Cord and Spinal Nerves!
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Spinal Nerve Categories Figure 13-9!Pairs of spinal nerves:!! 8 cervical !
• (C1 exits between atlas and occipital bone)!
12 thoracic! 5 lumbar! 5 sacral! 1 coccygeal!31 pairs!
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Peripheral Nerve Anatomy Figure 13-6!
Connective tissue coverings!1. Epineurium!• Surrounds entire nerve!
2. Perineurium!• Delineates fascicles
(bundles)!3. Endoneurium!• Surrounds individual
nerve fibers!
Does this terminology seem familiar?!
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Rami (Branches) of Spinal Nerves!
Ramus = “branch”; plural = rami!1. Dorsal ramus!• Information to and from skin, skeletal muscles
of back!• Somatic and visceral, both motor and sensory!
!2. Ventral ramus!• Information to and from ventrolateral body
surface, body wall and limbs!• Somatic and visceral, both motor and sensory!• Form plexuses or thoracic (intercostal) nerves!
Chapter 13 – Spinal Cord and Spinal Nerves!
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Nerve Roots and Rami modified from Fig. 13-3b!
Dorsal!
Ventral!
Dorsal root!Dorsal root
ganglion!
Ventral root! Denticulate ligament!
Sympathetic chain ganglion!
Ventral ramus!
Dorsal ramus!
Rami communicantes!
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Rami Communicantes (much more in Ch. 16)!
3. Rami communicantes!• Communicate between ventral rami and
sympathetic ganglia!• Visceral sensory and visceral motor!• Information to and from interoceptors, smooth
muscle, visceral organs, glands!3a. White ramus!
• Myelinated preganglionic fibers!3b. Gray ramus!
• Unmyelinated postganglionic fibers!Gray and white rami unite at sympathetic ganglia!
• Form sympathetic nerves!
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Thoracic nerves
Thoracic (Intercostal) Nerves!
• Formed by ventral rami of T2 - T12 spinal nerves!
• Go directly to innervated structures.!
• Most axons from these segments do NOT join plexuses!
Chapter 13 – Spinal Cord and Spinal Nerves!
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Distribution of Sensory Nerve Fibers Fig. 13-8a!
Exteroceptors in skin are:!• Ambient
temperature!• Pain!• Light touch,
pressure!
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Distribution of Motor Nerve Fibers Fig. 13-8b!
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Dermatomes Figure 13-7!
Dermatome = specific region of skin innervated by a single pair of spinal nerves.!!!Clinically important for diagnoses of specific neuropathies.!
Chapter 13 – Spinal Cord and Spinal Nerves!
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Nerve Plexuses!
Plexus = “network”!Formed from ventral rami !• T2 - T12 do not participate in plexuses!
Plexuses are formed by nerve fibers from different spinal nerves.“During development, small skeletal muscles
innervated by different ventral rami fuse to form larger muscles with compound origins. The anatomical distinctions between the component muscles may disappear, but separate ventral rami continue to control each part of the compound muscle.” p. 439!
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Peripheral Nerves and Nerve Plexuses!
Figure 13-9!
Cervical plexus!Brachial plexus!
Lumbar plexus!
Sacral plexus!
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Brachial Plexus – 1 Figure 13-11a!
Chapter 13 – Spinal Cord and Spinal Nerves!
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The Brachial Plexus – 2 See your copy of Netter!
Netter Plate 416
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Brachial Plexus – 3 Figure 13-11b!
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SECTION 13-5!Interneurons are organized into functional groups called neuronal pools!
Chapter 13 – Spinal Cord and Spinal Nerves!
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The Organization of Neuronal Pools!
Figure 13-13!
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SECTION 13-6 !Reflexes are rapid, automatic responses to stimuli!
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Reflexes!
Definition:!• Unconscious, rapid, stereotyped responses
to a stimulus. !• Involve a reflex arc!
!Advantages of reflexes. Why important?!
1. Fast response!• Don’t have to think about it!
2. Predictable!• Absence indicates damage to N.S.!
Chapter 13 – Spinal Cord and Spinal Nerves!
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Reflex Arc – 1!
Spinal cord is the integrating center.!Components of a Reflex Arc:!1. Sensory receptor!• Responds to stimulus!• Generates signal to be sent to integrator!
2. Sensory neuron!• Cell body in dorsal root ganglion!• Carries info to integrating center!• Info enters via dorsal root!
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Reflex Arc – 2!
3. Integrating center = spinal cord!• Usually involves interneurons!
Polysynaptic reflex (most reflexes)!• May not involve interneurons!
Monosynaptic reflex!4. Motor neuron!
• Transmits impulses to muscle or gland!5. Effector!
• Muscle or gland that responds to motor neuron!
• Effects a change in controlled variable!
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Events of a Reflex Arc Figure 13-14a!
Chapter 13 – Spinal Cord and Spinal Nerves!
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Classification of Reflexes Figure 13-15!
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Types of Spinal Reflexes!
“Wiring diagrams”!• Ipsilateral!• Contralateral!• Intersegmental!
Reflex responses initiated by:!1. Changes in muscle length!
• Stretch reflex - e.g. patellar reflex!
2. Changes in tendon tension!• Golgi tendon reflex!
3. Pain!• Flexor reflex, crossed extensor reflex!
54!
1. Muscle Spindle (Stretch) Reflex!
Responds to changes in muscle length• A.K.A. knee-jerk or patellar reflex!• A monosynaptic reflex!
Receptor (muscle spindle) located in muscle!Cord sends excitatory message to the muscle
being stretched!• Adjusts stretch of muscle!• Adjust muscle tone at rest!• Prevents overstretch!
Chapter 13 – Spinal Cord and Spinal Nerves!
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The Patellar (Knee-jerk) Reflex Figure 13-17!
= excitatory synapse!+
+
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Muscle Spindles – 1!Intrafusal fibers!• Specialized muscle cells!• Surrounded by extrafusal (“normal”)
muscle fibers !• (Extrafusal fibers innervated by alpha
motor neurons)!• Innervated by both motor and sensory
neurons!A. Gamma motor neurons (γ motor
neurons)!• Innervate ends of intrafusal fibers!• Contraction adjusts sensitivity of
spindle
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Muscle Spindle – Intrafusal Fibers – 2!B. Sensory innervation of intrafusal fibers!• Innervate center of intrafusal fibers!
(no actin or myosin here)!• Synapse in cord with motor neurons that
innervate extrafusal fibers in the same muscle!• Collaterals inform brain of degree of spindle
stretch!Function to adjust muscle tone!
1. Stretch → ↑ AP frequency → stimulate extrafusal fibers → ↑ muscle tone!
2. Compress → ↓ AP frequency → ↓ stimulation of extrafusal fibers → ↓ muscle tone!
Chapter 13 – Spinal Cord and Spinal Nerves!
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Intrafusal Fibers From 7th edition!
Note that gamma afferent fires!to some degree even at normal length.!
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2. (Golgi) Tendon Reflex!
Function:!• Senses changes in muscle tension (via tendon
tension)!• Receptor located in tendon (Golgi tendon
organ)!• Prevents injury to tendons!• Ipsilateral reflex!• Sends inhibitory message to muscle under
tension!
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Golgi Tendon Organ!
https://humanphysiology2011.wikispaces.com/11.+Muscule+Physiology!
Chapter 13 – Spinal Cord and Spinal Nerves!
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3. Flexor Reflex – a.k.a. Withdrawal Reflex!
Characteristics – responds to PAIN1. Polysynaptic!2. Ipsilateral!3. Intersegmental!
• Synapses occur within multiple cord segments!
• Activates several muscle groups!(i.e. stimulates more than one effector)!
4. Reciprocal innervation!(i.e. flexors and extensors)!
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Flexor Reflex – a.k.a. Withdrawal Reflex – 2!
Mechanism:!1. Stimulate pain-sensitive sensory neuron!2. Message sent to cord!3. Activate interneurons!4. Relay to several segments!5. Several flexors activated, extensors inhibited!
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Flexor Reflex – 3 Figure 13-14!
Be able to draw this! !See study guide.!
++
+
_
indicates excitatory synapse indicates inhibitory synapse+_
Back to #23!
Chapter 13 – Spinal Cord and Spinal Nerves!
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Crossed Extensor Reflex!
Characteristics:!1. Polysynaptic!2. Contralateral!3. Reciprocal innervation!
• Activates flexors on one side! Inhibits extensors on this side!• Activates extensors on the other side! Inhibits flexors on this side!
4. Intersegmental!
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Crossed Extensor Reflex – 2 Figure 13-18!
Mechanism:!1. Pain!2. Sensory info to cord!3. Contract flexors on
one side, extensors on other side!!
+ - -+
+ +
66!
Flexor and Crossed Extensor Reflexes!
Comparison Figures 13-19, -20 from 9th edition!
Chapter 13 – Spinal Cord and Spinal Nerves!
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SECTION 13-8 !The brain can affect spinal cord-based reflexes!
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Integration and Control of Spinal Reflexes!
Higher centers can modify spinal reflexes!• Inhibit or facilitate reflex patterns!
Continuous inputs (IPSP, EPSP) to neurons !involved in reflexes!
• Allows a few neurons from brain to control complex motor functions!
e.g. Walking, running based on spinal reflexes! Modified by higher centers!
Example: decerebrate cat!
![Yoga Therapy and Integrative Medicine - LotusEd...and consciousness” (Yogananda, 1995) or “[nerve] plexuses that are the storage places of pranic energy” (Vishnudevananda, 1959)](https://img.pdfslide.net/doc/110x75/5ea9f2f9ae7ce426c21c0a1a/yoga-therapy-and-integrative-medicine-and-consciousnessa-yogananda-1995.jpg)
