1. Chapter 13: The Spinal Cord and Spinal Nerves

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Spinal Cord Anatomy
Protective and stabilizing structures:
Vertebral column
Meninges – continuous around spinal cord and brain
cerebrospinal fluid
denticulate ligament
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Spinal Cord Anatomy
Meninges – 3 layers
Dura mater
most superficial
dense irregular connective tissue
Ends at second sacral vertebra
Cushion of fat and connective tissue in epidural space
Subdural space filled with interstitial fluid
Arachnoid mater
Spider web arrangement of delicate collage and elastic
Subarachnoid space filled with cerebrospinal fluid (CSF)
Pia mater
Thin delicate ct adheres to spinal cord and brain
Extensive blood supply
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6. External Anatomy of the Spinal Cord
Two enlargements: cervical and lumbar
Conus medullaris
Filum terminale
Cauda equina
Posterior (dorsal root) & anterior(ventral) root
Posterior (dorsal root) ganglion
Spinal nerve
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8. Internal Anatomy of the Spinal Cord
Partially divide spinal cord into right and left
Anterior median fissure
Posterior median sulcus
Gray matter
Divided into horns
Anterior, posterior & lateral gray horns
White matter
Columns
Anterior, posterior & lateral white columns
Central canal
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9. Internal Anatomy of the Spinal Cord
Look for (see next slide)
Gray commissure
Central canal
Anterior, posterior and lateral gray horns
Anterior, posterior and lateral white columns
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10. Internal Anatomy of Spinal Cord

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White columns
Ascending tracts – sensory information
Examples: Spinothalamic, spinocortical
Descending tracts – motor impulses
Examples: Corticospinal, vestibulospinal
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13. Sensory and Motor Tracts
The name of the tract often indicates its location in the white matter and where it begins and ends
The white matter contains both sensory and motor tracts
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Spinal Nerves
31 pairs mixed nerves
Cervical (C1-C8)
Thoracic (T1-T12)
Lumbar (L1-L5)
Sacral (S1-S5)
Coccygeal
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Spinal Nerves
Spinal nerves are mixed with motor and sensory axons
Spinal nerves connect to each segment of spinal cord by two roots
posterior or dorsal – sensory
Anterior or ventral – motor
Branches of spinal nerves
Posterior ramus
Anterior ramus
Meningeal branch
Rami communicantes
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17. Spinal Nerves

18. Distribution of Spinal Nerves
Plexus
Anterior rami of spinal nerves, except T2-T12, form networks of nerves
Emerging from a plexus are nerves bearing names that typically describe the general regions they supply or route they follow
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Cervical Plexus
anterior rami of C1 -C5
Muscles and skin of head, neck, upper shoulder
Phrenic nerve formed by cranial nerve XII and cervical plexus innervates diaphragm
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Brachial plexus
anterior rami of C5 – C8 & T1.
Supplies the shoulders and upper limbs.
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Brachial plexus
Major nerves
Axillary
Musculocutaneous
Radial
Median
Ulnar
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Lumbar Plexus
L1 – L4
Supplies the antero-lateral abdominal wall, external genitals, and part of the lower limbs
Femoral & obturator nerves
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Sacral Plexus
L4 – L5 and S1 – S4.
buttocks, perineum, and lower limbs
Sciatic – largest nerve in the body
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Coccygeal Plexus
S4 – S5 and the coccygeal nerves
skin in the coccygeal region
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25. T2 – T12 Anterior rami do not form plexus
Intercostal / thoracic nerves
Distributed directly to structures they supply in intercostal spaces

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Dermatome
Dermatome is the area of the skin that provides sensory input to the CNS via one pair of spinal nerves or the trigeminal nerve
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27. Spinal Cord Functions
White matter tracts represent “highways” for nerve impulse propagation
Sensory input toward brain and motor output to skeletal muscles and effector organs / tissues
Sensory – two main routes
Posterior column
Spinothalamic tract
Motor – two main routes
Direct pathways
Indirect pathways

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29. Spinal Cord Functions Integrating center for spinal reflexes
Integration occurs in gray matter
Reflex
Fast, automatic, and predictable involuntary response to a stimulus
May occur as muscle contractions or glandular secretions
Occurs in response to changes in environment
May be spinal or cranial and somatic or autonomic (visceral)
When the integration takes place in the spinal cord it is a spinal reflex

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Reflex Arc
The pathway followed by nerve impulses that produce a reflex is a reflex arc
A reflex arc components:
sensory receptor
sensory neuron
integrating center
motor neuron
effector
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31. Reflex Arc 1 2 3 4 5 1 2 3 4 1 2 3 1 2 1 SENSORY NEURON
(axon conducts impulses from
receptor to integrating center)
SENSORY RECEPTOR
(responds to a stimulus
by producing a generator
or receptor potential)
INTEGRATING CENTER
(one or more regions within the CNS
that relay impulses from sensory to
motor neurons)
MOTOR NEURON
integrating center to effector)
EFFECTOR
(muscle or gland that
responds to motor
nerve impulses)
Interneuron 2 3 4 5 1
SENSORY NEURON
(axon conducts impulses from
receptor to integrating center)
SENSORY RECEPTOR
(responds to a stimulus
by producing a generator
or receptor potential)
INTEGRATING CENTER
(one or more regions within the CNS
that relay impulses from sensory to
motor neurons)
MOTOR NEURON
integrating center to effector)
Interneuron 2 3 4 1
SENSORY NEURON
(axon conducts impulses from
receptor to integrating center)
SENSORY RECEPTOR
(responds to a stimulus
by producing a generator
or receptor potential)
INTEGRATING CENTER
(one or more regions within the CNS
that relay impulses from sensory to
motor neurons)
Interneuron 2 3 1
SENSORY NEURON
(axon conducts impulses from
receptor to integrating center)
SENSORY RECEPTOR
(responds to a stimulus
by producing a generator
or receptor potential) 2 1
SENSORY RECEPTOR
(responds to a stimulus
by producing a generator
or receptor potential)

32. Somatic Spinal Reflexes
Stretch (monosynaptic)
Tendon (polysynaptic)
Flexor / withdrawal (polysynaptic)
Crossed extensor (polysynaptic)

33. Stretch Reflex Stretching stimulates muscle spindles
Impulse along somatic sensory neuron through posterior root into spinal cord
Activation of motor neuron in anterior gray horn through integrating center
Strong enough stimulus activates impulses out motor neuron through anterior root stimulating muscle
Monosynaptic (2 neuron)
Reciprocal innervation – components of neural circuit simultaneously cause contraction of on muscle and relaxation of antagonists
Ipsilateral maintaining muscle tone
Patellar reflex

34. Stretch Reflex

35. Tendon Reflex
Polysynaptic – more than one synapse and more than two different neurons involved
Golgi tendon organ is sensory organ
Sensory neuron synapses with two interneurons
Inhibitory interneuron relaxes effector
Stimulatory neuron contracts antagonist
Ipsilateral preventing damage to muscles and tendons when muscle force become too extreme

36. Tendon Reflex

37. Flexor (withdrawal) Reflex
Ipsilateral and polysynaptic
Inter-segmental reflex arc
Reciprocal innervation
Single sensory neuron activates several motor neurons
Stimulates more than one effector

38. Flexor (withdrawal) Reflex

39. Crossed Extensor Reflex
Contralateral reflex arc
Polysynaptic
Synchronizes the extension of contralateral limb with withdrawal (flexion) of stimulated limb
Reciprocal innervation

40. Crossed Extensor Reflex

41. Reflexes and Diagnosis
Patellar reflex
Absent in damage to sensory or motor nerves or integrating center in 2nd, 3rd, 4th lumbar
Absent in chronic DM and neurosyphilis
Exaggerated in disease or injury involving certain descending motor tracts from brain

42. Reflexes and Diagnosis
Achilles reflex – plantar flexion through contraction of gastrocnemius and soleus
Absence indicates damage nerves of posterior leg or neurons of lumbo-sacral region
DM, neurosyphilis, alcoholism, subarachnoid hemorrhage
Exaggerated
Cervical cord compression or lesion of motor tracts of 1st and 2nd sacral segments

43. Reflexes and Diagnosis
Babinski sign – gentle stroking of the lateral outer margin of the sole
Great toe dorsiflexes / extends with fanning of other toes
Normal in children under 1 ½ due to incomplete myelination corticospinal tract
Abnormal after 1 ½ indicating interruption of corticospinal tract
Negative Babinski – normal response is plantar felxion – curling under of toes

44. Reflexes and Diagnosis
Abdominal reflex – contraction of muscles that compress abdominal wall in response to stroking side of abdomen
Causes contraction moving umbilicus toward stimulus
Absence indicates lesions of corticospinal tract
Multiple sclerosis
Absence of normal pupillary light reflex – associated with brain injury or damage

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End of Chapter 13
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