A&p Spinal Cord

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Gross anatomical features of the Spine

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occupies the spinal canal of the vertebral column.

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Spinal Cord Origin

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Foramen magnum

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Termination point of the Spinal Cord

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Conus medullaris at the level between L1-L2

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Distribution of spinal nerves

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Pair of spinal nerves delineates each of 31 spinal cord segments. The spinal nerves are connected to the cord via posterior (dorsal) and anterior (ventral roots).

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Sensory (afferent) information enters

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the cord through the posterior roots.

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Motor (efferent) activities leaves the cord

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through the anterior roots.

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Shape of Gray Matter

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H-shaped or butterfly-shaped. central region of the spinal cord contains

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Posterior (dorsal) and anterior (ventral) horns are found

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in all segments

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lateral horns are found in

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segments T1-T12 and L1-L2

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Components of Gray Matter

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nerve cell bodies
axons
synapses

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Types of motor neurons

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sensory relay neurons
anterior motor neurons
interneurons (intermediate neurons)

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Anterior motor neurons located in

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the anterior (ventral) horns of the gray matter

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Axons leave the cord via

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the anterior (ventral) roots and innervate skeletal muscle fibers

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Two types of motor neurons alpha motor neurons

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give rise to large A-alpha nerve axons that innervate extrafusal skeletal muscle fibers and gamma motor neurons

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Alpha innervate

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muscle and innervate the large skeletal fibers.

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Gamma composition

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make up about 1/3 of total motorneurons in the anterior horns. much smaller than alpha motorneurons

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Gamma give rise to

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A-gamma axons that innervate intrafusal fibers in muscle spindles

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Anterior motor neurons located in

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the anterior (ventral) horns of the gray matter

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Axons leave the cord via

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the anterior (ventral) roots and innervate skeletal muscle fibers

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Two types of motor neurons alpha motor neurons

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give rise to large A-alpha nerve axons that innervate extrafusal skeletal muscle fibers and gamma motor neurons

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Alpha innervate

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muscle and innervate the large skeletal fibers.

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Gamma composition

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make up about 1/3 of total motorneurons in the anterior horns.
much smaller than alpha motorneurons

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Gamma give rise to

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A-gamma axons that innervate intrafusal fibers in muscle spindles

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Motor neurons recieve

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direct input from sensory neurons, but input to the motor neurons is derived from interneurons.

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Interneuron main task

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filter information for motor neurons

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Alpha Motor Neurons

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give rise to large A-alpha nerve axons that innervate extrafusal skeletal muscle fibers.

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Alpha Motor Neuron speed

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fast
large diameter and have myelin

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Gamma Motor Neuron Speed and size

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smaller than alpha

fast d/t myelin

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Interneurons found

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in all gray matter

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Interneuron amount

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most numerous cells in gray matter

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Interneuron excitation ability

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Both excitatory and inhibitory

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Interneurons synapse

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with alpha motor neurons and information is delivered via the interneurons

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Interneurons receive information

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from certain sensory nerves

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White matter contains

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numerious tracts of nerve axons

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White matter color comes from

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Most are myelin coated that make up white color.

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White matter Ascending tracts examples

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Lateral spinothalamic tracts and Posterior spinothalamic tracts

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Lateral spinothalamic tracts

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principal pathway for transmission of pain and temperature sensations

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Posterior spinothalamic tracts

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(dorsal columns) principal pathway for transmission of tactile and proprioceptive sensations

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Descending tracts examples

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Corticospinal (pyramidal) tracts and nerves associated with the sympathetic NS.

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Corticospinal (pyramidal) tracts

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motor nerve tracts that originate in the motor cortex and terminate primarily on interneurons in the spinal cord gray matter

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Nerve tracts associated with the sympathetic nervous system

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originate in the brainstem and terminate in spinal segments T1-T12 and L1-L2

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Three layers of connective tissue surround the cord

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pia mater
arachnoid mater
dura mater (also surround the brain)

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the dura and arachnoid extend to level

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S2

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Subarachnoid space contains

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CSF

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Epidural space contains

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fat connective tissue and venous plexuses

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Epidural space extends from

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the foramen magnum to the sacral hiatus

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Ligaments

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 Ligamentum flavum – lies just posterior to the epidural space
 Interspinous ligament
 Supraspinous ligament

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The cord receives its blood supply from

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the vertebral arteries and from the thoracic and abdominal aorta

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Anterior spinal artery

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single midline vessel that supplies the anterior two-thirds of the cord. originates from the vertebral arteries at the base of the skull

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Posterior spinal arteries

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paired vessels that supply the posterior one-third of the cord. arise from the posterior cerebellar arteries

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Radicular arteries

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supply blood to the anterior and posterior spinal arteries

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Artery of Adamkiewicz (arteria radicularis magna)

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Single branch of the aorta -- almost always arises on the left side

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Supplies nearly all of the blood flow to the lower thoracic and lumbar cord

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Artery of Adamkiewicz

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Injury to Artery of Adamkiewicz

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Spinal cord ischemia may result from injury to this artery surgical dissection or aortic cross-clamping) paraplegia

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Spinal Nerves are apart of what Nervous System?

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Peripheral

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Spinal Nerves are Formed by the joining of

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Anterior and posterior roots at each of the spinal cord segments

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The spinal nerves exit

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the vertebral column via openings.
the intervertebral foramina between the vertebrae.

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How many spinal nerves?

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31

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Breakdown of Spinal Nerves

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 cervical, 8
 thoracic, 12
 lumbar, 5
 sacral, 5
 coccygeal, 1

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the lower lumbar and sacral nerves descend as

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the cauda equina before exiting the vertebral column

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The spinal nerves are mixed nerves they contain

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somatic (sensory and motor). autonomic nerve fibers

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Each spinal segment supplies a specific

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region of skin (dermatome) and skeletal muscle

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Two major branches (rami)

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Anterior primary division (anterior ramus): major portion of the spinal nerve.

Posterior primary division (posterior ramus): supplies long muscles of the back and tissue overlying them

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Nerve Plexuses General anatomy

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A plexus is an intermingling collection of nerves that emanate from different cord segments

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A plexus typically contains

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sensory and motor nerve fibers that innervate a particular region of the body

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Cervical plexus

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Anterior rami of spinal nerves C1-C4 join to form the cervical plexus in the neck

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Peripheral nerves emerging from the cervical plexus and supply

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 supply skin and muscles of the neck
 posterior scalp

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Innervates the diaphragm

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phrenic nerve

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Brachial Plexus

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oAnterior rami of spinal nerves C5-C8 and T1 join to form the brachial plexus
o Peripheral nerves (eg, musculocutaneous n., median n., ulnar n.) emerge from the plexus innervate the upper limbs, shoulders, and pectoral muscles

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Lumbosacral Plexus

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oAnterior rami of spinal nerves L1-L5 and S1-S4 join to form the lumbosacral plexus
oPeripheral nerves (eg, obtrurator n., femoral n., sciatic n.) emerge from this plexus to innervate the lower limbs and pelvis

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Muscle receptors provide

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•Muscle receptors (proprioceptors)
oProvide continuous feedback information to the CNS

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Muscle Spindles are found in the

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skeletal msucle

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Spindles function as

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stretch receptors and detect muscle length

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Spindle is made up of

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each spindle is made up of several intrafusal muscle fibers

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Spindles are attacched to

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glycocalyx of surrounding extrafusal fibers

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Central region of the spindle is innervated by

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types Ia and II; this central region is extremely sensitive to stretch of the muscle

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The end portion of spindles contain

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actin and myosin
innervated by A-gamma motor fibers
regulate tension

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Golgi tendon organs

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sensory receptors (nerve endings) located in the muscle tendons near the point of attachment of the tendons to the skeletal muscle fiber

the receptors are sensitive to tension developed within the muscle

Type Ib sensory nerves transmit information from the Golgi tendon organs to the CNS; signals are directed to the spinal cord, the cerebellum, and the cerebral cortex

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Stretch Reflex

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Stretch activates muscle spindles
-sensory information delivered via type Ia sensory fibers to dorsal root of spinal cord segment
-from there to alpha motor neurons
-activation of alpha motor neurons that innervate the same muscle that was stretched
-muscle contraction to oppose stretch

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Flexor

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 Elicited most often by a painful stimulus
 polysynaptic reflex
 Activation of sensory (eg, pain) receptors 
o sensory information directed to interneurons in spinal cord  excitation of alpha motorneurons that innervate
o appropriate flexor muscles 
o contraction of flexor muscles to remove affected body area from the painful stimulus
Crossed extension

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Crossed Extension

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 Flexion of one limb is accompanied by extension of the opposite limb; delayed reflex (200-500 msec)
 Occurs during flexor response to a painful stimulus eg, step on tack with right foot 
o flexor muscles in right leg contract and extensor muscles in left leg contract
o crossover of sensory input in the spinal cord
 Crossed extension also acts over long distances of the spinal cord to control reciprocal limb movements during locomotion

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Muscle Spasms

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 May represent a reflex response to irritation of muscle
 Pain stimulus  spinal cord  muscle contraction
o Fractures  local spasm
o Peritonitis, abdominal surgery  contraction of abdominal muscles
o Ischemia, excess exercise  local spasm; cramp

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Cord Injury

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 Lesions can result from a variety of insults
o Trauma
o Degenerative and demyelinating disorders, eg, multiple sclerosis, ALS
o Tumors
o Infections
o Ischemia
o Part or all of the cord may be affected at the site of injury

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Spinal Cord Transection

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 Initial period of spinal shock
o both somatic and autonomic reflexes below the lesion are temporarily interrupted
o blood pressure can fall precipitously, but usually recovers within a few days
 Gradually, over a period of several weeks or more
o spinal neurons below the lesion regain excitability and at least some reflex activity returns
o there is loss of sensation and voluntary movement below the transection

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Automonic Hyperreflexia

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o Overactivity of the sympathetic nervous system -- commonly seen in patients with cord transection at T5 or above; unusual with lesions below T10
o Cutaneous (eg, incision) or visceral (eg, distended bladder) stimulation below the level of the transection can trigger strong autonomic responses:
 sympathetic discharge causes intense vasoconstriction below the lesion
 reflex bradycardia and vasodilatation occur above the lesion
 hypertension results
 cardiac dysrhythmias may occur

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Spinal Block

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o Local anesthetics are injected into the subarachnoid space (CSF) of the spinal cord; injection is usually performed below the end of the spinal cord, eg, at L3-L4 or L4-L5
o Uptake of local anesthetics into
 Spinal nerve roots – main site of action
 Dorsal root ganglia
 Spinal cord itself
o CSF circulation does not affect distribution of the drug
o Complications
 Hypotension: sympathetic nervous system blockade can cause venous pooling of blood and decreased venous return and cardiac output
 Postspinal headache may occur as a result of CSF leakage from puncture site; reduction of CSF pressure in the subarachnoid space likely causes tension in the meninges

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Epidural Block

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o Placement of local anesthetics into the epidural space of the spinal cord
 typically at the lumbar level; the epidural space of the spinal cord contains fat, connective tissue, and venous plexuses, but not free-moving fluid as in the subarachnoid space
o Major site of action is the spinal nerve roots; the dura covering the roots is relatively thin
o Sympathetic, sensory, and motor nerve blockade occur;
 onset of sympathetic blockade is often slower than occurs with spinal block, thus an abrupt fall in blood pressure usually does not occur (but hypotension is still a potential complication)