CHAPTER 13
The Spinal Cord & Spinal
Nerves
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INTRODUCTION
• Mediate reactions to environmental changes.
• Process reflexes
• Site for integration of EPSPs and IPSPs that arise locally
or are triggered by nerve impulses from periphery and
brain
• Conduction pathway for sensory & motor nerve impulses
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I. SPINAL CORD ANATOMY
• Protective structures
– vertebral column provides a bony covering of spinal cord
• vertebral foramen form vertebral cavity
• added protection from vertebral ligaments & CSF
– meninges
• CT layers surrounding brain & spinal cord
• 3 layers
– dura mater = outermost layer
» epidural space
– arachnoid mater = middle layer
» subdural space contains ISF
– pia mater = innermost layer
» vascularized
» subarachnoid space contains CSF
– denticulate ligaments prevent displacement of cord
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Applications
• Subarachnoid space is between the arachnoid mater &
pia mater and contains cerebrospinal fluid (CSF).
• Inflammation of the meninges is known as meningitis.
• Removal of CSF from the subarachnoid space is called
a spinal tap (lumbar puncture).
– used to diagnose pathologies and to introduce
antibiotics, contrast media, anesthetics, and
chemotherapeutic drugs.
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I-B. Ext. Anat. of Spinal Cord
• Begins as a continuation of medulla oblongata and
terminates at about L2 vertebra in an adult
• Spinal nerves
– 31 pair emerge @ regular intervals from vertebral cav.
– roots = bundles of axons that connect nerves to cord
• posterior (dorsal) root contains only sensory neurons
– sensory info from skin, muscles, organs to CNS
– post. (dorsal) root ganglion = swelling of root that
contains cell bodies of sensory neurons
• anterior (ventral) root contains axons of motor neurons
– CNS to effector organs
– no ventral ganglion
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I-C. Internal Anat. of Spinal Cord
• Gray matter surrounded by white matter
• Gray matter
– cell bodies, unmyelinated axons and dendrites
– central canal = center of gray matter
• filled with CSF
• extends entire length of spinal cord
– nuclei = functional groups of neuronal cell bodies
• sensory
• motor
– horns
• anterior gray horns contain somatic motor neurons
• posterior gray horns contain autonomic/somatic sensory
neurons
• lateral gray horns contain autonomic motor neurons
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I-C. Internal Anat. of Spinal Cord
• White matter
– bundles of myelinated axons of w/ common origin/destination
 form tracts in CNS
• sensory (ascending) tracts carry sensory info to brain
• motor (descending) tracts carry impulses from brain to
effector
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Sensory and Motor Tracts
• Figure 13.4 shows the principal sensory and motor tracts
in the spinal cord. (detailed in Chapter 16)
• Sensory (ascending) tracts conduct nerve impulses
toward the brain.
– the lateral and anterior spinothalamic tracts and the
posterior column tract.
• Motor (descending) tracts conduct impulses down the
cord.
– Direct pathways include lateral and anterior corticospinal
and corticobulbar tracts.
– Indirect pathways include rubrospinal, tectospinal, and
vestibulospinal tracts.
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II. SPINAL NERVES
• Spinal nerves are part of the PNS!!!
• Connective tissue coverings
– endoneurium covers individual axons (superficial to myelin
sheath)
– perineurium surrounds groups of axons (fascicles)
– epineurium is outer covering of entire nerve
– blood vessels associated w/ peri-/epineuria
• Distribution
– branches arise just outside vertebral column
• posterior (dorsal) ramus
• anterior (ventral) ramus
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II. SPINAL NERVES
• Distribution
– plexuses
• network of interlacing, adjacent axons
• do not directly connect to structures they innervate
• 5 plexuses
– cervical: skin/muscles of head/neck/upper chest
– brachial: shoulders/upper limbs
– lumbar: lower limbs/abdominal wall
– sacral: buttocks/perineum/lower limbs
– coccygeal: small area of skin in coccygeal region
– intercostal nerves
• T2-T12  rami do not form plexuses
• directly connected to structures they supply
• Dermatomes provide sensory input from regions of skin
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III. SPINAL CORD PHYSIOLOGY
• Sensory & motor tracts
– naming of tracts indicates direction of information flow
• example = anterior spinothalamic tract
• impulses travel from spinal cord toward brain
– two routes for sensory input from spinal cord to brain
• spinothalamic tracts
• posterior columns
– sensory systems keep CNS apprised of Δ in environ.
– motor pathways
• direct: info from cortex; results in controlled voluntary
movements (corticospinal/corticobulbar)
• indirect: info from brain for autonomic movements
– Fig 13.12 summarizes functions of cord & nerves
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III-B. Spinal Reflexes & Reflex Arcs
• Spinal cord = integrating center for spinal reflexes
• Terms
– reflex =fast, automatic response occurring in response to
particular stimulus
– Reflexes may be spinal, cranial, somatic, or autonomic.
– reflex arc = pathway followed by nerve impulses in
generation of reflex
• sensory receptor produces graded potential in response to
stimulus
• sensory neuron sends info to gray matter of cord
• integrating ctr = synapse btwn sensory/motor neurons
• motor neuron carries impulses from integrating ctr to
effector organ
• effector = body part that responds to motor impulse
– somatic effectors = skeletal muscles
– autonomic effectors = visceral organs
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III-B. Spinal Reflexes & Reflex Arcs
Somatic spinal reflexes (4)
1) Stretch reflex
–
–
–
–
causes contraction of skeletal muscle in response to stretching
prevents overstretch of muscle (feedback mechanism)
sensory receptors = muscle spindles
spindles detect Δ in length  sends info to CNS  if strong
enough, motor neuron generates a.p.  results in ACh
release  muscle contracts to prevent overstretch
– antagonistic muscles relax (via inhibitory potentials) as part
of reflex (reciprocal innervation)
– brain sets overall level of muscle tone by adjusting sensitivity
of spindles
• tone = degree of contraction present in resting muscle
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III-B. Spinal Reflexes & Reflex Arcs
2) Tendon reflex
– controls muscle tension by causing muscle relaxation when
muscle force becomes too extreme
– sensory receptors = Golgi tendon organs
• detect change in tension caused by stretch/contract.
– activation of inhibitory interneuron results in release of
inhibitory NT @ NMJ (hyperpolarization of postsynaptic neuron)
 muscle relaxation results
– in this case reciprocal innervation causes contraction of
antagonist muscle
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III-B. Spinal Reflexes & Reflex Arcs
3) Withdrawal (flexor) reflex
– protective reflex that moves a limb to avoid pain
– results in contraction of flexor muscles to move a limb to
avoid injury or pain
• EX: stepping on tack, touching hot burner
• ACh release causes contraction of muscle to w/draw
from stimulus
4) Crossed extensor reflex helps maintain balance during
withdrawal reflex
– causes synchronized extension of the joints of one limb and
flexion of the joints in the opposite limb (Figure 13.9)
• EX: step on tack, one leg flexes, other leg extends to
maintain balance
• ACh release @ NMJ results in contraction of extensors
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Clinically Useful Reflexes
Presence or absence of certain reflexes is useful in diagnosing
disorders/injuries in nervous tissue
• Patellar reflex: extension of knee results from tapping
patellar ligament
– blocked by damage to L2-L4 segment of spinal cord
– absent in some cases of diabetes mellitus
– exaggerated in certain brain injuries
• Achilles reflex: plantar flexion of foot in response to
tapping of Achilles tendon
– absence indicates damage to nerves supplying leg muscles
– may be absent in diabetics, alcoholics & as result of subarachnoid
hemorrhage
– exaggerated reflex indicates spinal cord compression in cervical
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Clinically Useful Reflexes
• Babinski sign: pressure on sole of foot results in
dorsiflexion of big toe and fanning of others
– normal under age 1½  incomplete myelination
– if present after age 1½  damage to corticospinal tract
– (--) Babinski results in curling under of all toes
• Abdominal reflex: contraction of muscles in abdominal
wall causes umbilicus to move in direction of stimulus
– absence indicates lesions in corticospinal tract
• Pupillary light reflex (autonomic reflex)
– pupils of both eyes ↓ in diameter when either eye is exposed
to light
– absence indicates brain damage or injury
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