Shared Flashcard Set


Kaplan4a - The Spinal Cord (General organization & First two systems)

Additional Accounting Flashcards




What is the beginning and end of the spinal chord? At what level does it end in adults?
-It goes from the pyramidal decussation to the conus medularis at the second lumbar vertebra
-From there it continues as the cauda equina made up of dorsal and ventral roots
Where are the cervical and lumbar enlargements? How many spinal nerves are there?
-Cervical; C5-T1
-Lumbar; L1-S2
-31 total (and remember, there is one more cervical spinal nerve than there are cervical vertebra)
Describe the general organization of the gray matter in the spinal chord (particularly of ventral horn)?
-The dorsal horn contains sensory neurons (more later)

-The ventral horn contains alpha and gamma motoneurons (alpha go to muscle, gamma to contractile intrafusal spindle fibers)
-The organization is; flexors dorsal to extensors, and neurons innervating distal is lat. to proximal

-The intermediate zone (middle and horn) has all the sympathetics and also Clarke nucleus, which sends unconscious proprioception to the cerebellum
Concerning the dorsal horn, What is in the medial division? Where do they go and what do they do? Which is most heavily myelinated?
-The dorsal root is broken up into a *medial division (with type I & II fibers) and a *lateral division (with type III & IV fibers)

Type I fibers (Ia & Ib)(A-alpha type fibers); Proprioception
-Type Ia fibers innervate spindle fibers
-Type Ib fibers innervate golgi tendon organs
Type II fibers (A-beta fibers); Touch

-Both run medial to the dorsal horn so they can send fibers into the *dorsal column to the CNS
-They also send fibers to the intermediate zone and ventral horn that participate in reflexes
-The proprioceptive fibers (type I) are the fastest and most heavily myelinated fibers (important for reflexes); This is the general trend going from A-alpha to C fibers, with C being unmylinated
-Note that an alphabetic scheme is used more for skin and the roman numeral system more for muscle
What about the lateral division of the dorsal root? Myelination? Synapse?
-These fibers enter the dorsal horn on the upper and lateral aspect and do not send collateral fibers in the dorsal columns
-They synapse in the dorsal horn (mostly 1 and 2)

Type III fibers (A-delta fibers); Sharp pain, cold
-These are very lightly myelinated
Type IV fibers (C fibers); Dull pain, warmth
-These are unmyelinated
What are Rexed laminae, and how are they organized?
-It is just an number system for organizing zones of the gray matter

1-6; Dorsal horn
7; Intermediate zone (middle and horn)
8-9; Ventral horn
What are the neurons of the MOTOR SYSTEM, and where can we find their cell bodies?
-The motor system is one of the three major neural systems in the spinal cord
-It uses a two neuron pathway with the lower motorneuron being the one to synaps directly at a neuromuscular junction

-The lower one has its cell body in the ventral horn OR in a cranial nerve nuclei in the brain stem

-The upper motor neurons are found in various locations;
Cerebral cortex*
Red nucleus
Reticular formation
Lateral vestibular nuclei
(last three all brainstem)
Where do we find axons of the cortical (upper) motoneurons in the spine?
-We find them in the corticospinal tract
-Mostly they are in the lateral corticospinal tract just lateral to the gray matter
-Some run in the anterior corticospinal tract, which only runs until mid-thoracic region

What fibers make up the corticospinal tract?
-About 60% are from the primary motor cortex located in the precentral gyrus of the frontal lobe & the premotor area, just anterior to the primary motor cortex
-The other 40% are from somatosensory cortical areas mostly in the postcentral gyrus
-Note also that "pyramid" is a synonym for corticospinal tract, and they can be used interchangeably

What is the course of from the cerebral cortext to muscle?
-Primary motor cortex (in precentral gyrus)-->internal capsule-->through ventral midbrain in pyramids-->cross at decussation (80-90%)-->corticospinal tract-->ventral horn for synapse-->lower motoneuron then exits in dorsal root-->muscle
What is the positional 'hub' we need to think about when considering upper motoneuron lesions? What about for lower?
-For upper, we need to consider whether it is above or below the decussation of pyramids; below gives ipsilateral, and below gives contralateral paralysis
-Any lower motoneuron lesion produces ipsilateral muscle weakness
What is the pathway for stretch reflex? Define the fibers involved? What is another name for the stretch reflex?
-Ia afferent from spindle is the first in the path; remember, it comes in on the medial aspect of the dorsal horn sending fibers to the dorsal column, but also to the ventral horn and intermediate zone

-Overall; Ia sensory fiber-->alpha motoneuron-->muscle
-The Ia also synapse with interneurons that will go inhibit antagonist alpha motoneurons
-Notice that the CNS is left out completely (except for attenuation of the response)
-Gamma motoneurons also play a role via attenuation of the reflex response by stretching spindle fibers

-Also called the myotatic reflex
What are some important stretch reflexes and chord segments tested (4)?
-Ancle (S1)
-Knee (L2-L4)
-Biceps & Brachioradialis (C5-C6)
-Triceps (C7-C8)

-If you can remember the order, it is 1/234/56/78
What is the inverse muscle stretch reflex?
-This is the reflex induced by Ib afferents which innervate the golgi tendon organs (GTOs) at muscle-tendon junctions
-Basically, it monitors force and acts to relax the muscle if the tension becomes too great (protective)
-Same path, but opposite effect of the stretch reflex; so remember, Ia->spindle & stimulatory, Ib->GTO & inhibitory
What mediates muscle tone? How?
-The gamma motoneurons
-When they are stimulated, the spindle in stretched, and the Ia fiber is stimulated to 'shrink' the spindle via contraction of the muscle
-The upper motoneurons, in turn, control the gamma fiber
What is the flexor withdraw syndrome?
-It is a pain response that causes withdrawal of the stimulated limb
What is the difference in symptoms between a lower and upper motoneuron lesion?
-When it is the lower you get **flaccid type paralysis because no contraction is possible
-In flaccid type, you get very pronounced muscle atrophy, *ipsilateral to the lesion and at the *same level

-When it is upper you get **spastic type paralysis, because the reflex's are still intact while voluntary movement is lost
-Here you get less pronounced muscle atrophy from disuse, but *increased muscle tone
-It is below and contralateral or ipsilateral to the lesion
-Reflexes will be **hyperactive because the CNS, overall, is inhibitory to the reflex responses

-Also, spastic tends to be widespread and seen as weakness and decrease in speed (paresis), while flaccid tends to be concentrated and complete (paralysis)
-More or less because it's disease vs. injury
What is the early sign of lower motoneuron damage?
-Fasciculations; twitches that may be visible
-Later you get the faster, and invisible fibrillations
What are the two types of rigidity we see in upper motoneuron damage and when do we see each?
-Decorticate rigidity (postural flexion of arm and extension of leg [mummy position]); from lesion above midbrain

-Decerebrate rigidity (postural extension of both); from lesions below midbrain

-The actual point or change is the red nucleus
What is a sign of upper motor neuron damage, other than the paralysis?
-A reversal of the cutaneous reflexes is classic
-As an example, a **Babinski sign is extending the great toe and fanning of the other toes when the lateral surface of the sole is stroked with a slightly painful stimulus
-Cutaneous reflexes normal yield flexor motor responses
-This is normal in infants (myelination not complete)

-The other sign we see is a "clasp knife reflex" where there is passive resistance to flexing, which then gives way
The corticospinal tracts (lat. and ant.) are the most important bundles for upper motor neurons, but not the only ones. What are some others and locations?
-*Reticulospinal and *vestibulospinal tracts are extensor biased and are mostly ant. (see link)
-More importantly, they innervate **bilaterally so a lesion on one side only will not show symptoms

-The other two are rubrospinal and olivospinal tracts (no info yet) which are lat. and ant. respectively

-Note that motoneurons outside of the corticospinal tract are mostly from nuclei in the brain stem (non-cortical) and are also referred to as *extrapyramidal tracts
What are the other two most important neural systems in the spinal chord (first was motor)? What do they handle? What are their divisions? What kind of system do they use? Where are their tracts?
-Both are ascending (i.e. sensory)

1. Dorsal column medial lemniscal system; discriminative touch, joint position, vibration, and pressure
-Fasciculus gracilis (med.); lower extremities, all levels
-Fasciculus cuneatus (lat.); upper extremities, upper thoracic and higher only

2. Anterolateral (spinothalamic) system; pain and temperature
-Lateral spinothalamic tract
-Anterior spinothalamic tract

-They use a three neuron pathway

-The location of the tracts is exactly where it sounds;
Where are the neuron cell bodies for the ascending pathways?
-1° neuron cell body is in the dorsal root ganglion (DRG)
-2° neuron cell body is ipsilateral, in the spinal cord or medulla, and always crosses the midline near cell body
-3° neuron cell body is contralateral and in the thalamus
What types of fibers does the dorsal column medial lemniscal system have? Where are they coming from?
-It has *medial division fibers that tend to come from specialized, encapsulated receptors such as;
Pacinian corpuscles-pressure & vibration (course touch)
Meissner corpuscles-light touch
Muscle spindles-proprioceptive
Go through the long pathway for the dorsal column medial lemniscal system?
-Encapsulated receptor-->Dorsal root ganglion-->Ipsilateral dorsal column-->*Nucleus cuneatus or gracilis (caudal medulla)-->Cross midline as *internal arcuate fibers-->Ascend as the *medial lemniscus-->Ventral posterolateral (VPL) nucleus (thalamus)-->Continue as *thalamocortical fiber-->*Primary somatosensory cortex (postcentral gyrus; most anterior part of parietal lobe)

-Remember they are also sending some reflex fibers to the lower motor neurons in the ventral horn
What determines whether the nucleus cuneatus or nucleus gracilis will be used?
-Basically, whether the fibers are from upper or lower extremities (and area of trunk)
-Lower extremities primary fibers use the fasciculus gracilis, which is medial in the dorsal column and runs its entire length, eventually meeting the secondary neuron cell body in the nucleus gracilis
-In contrast, the upper extremity fibers use the cuneate fasciculus (lat. in dorsal column and limitied to upper thoracic and higher) & nucleus cuneatus
What is the result of a lesion of the dorsal column pathway-medial lemniscal system? What can we use to test for it, and what do we need to differentiate it from?
-There will be a loss of vibratory sensation, joint position, pressure sensation, and two point discrimination (basically loss of most touch)
-There will be an inability to characterize objects with touch alone; called *astereognosis
-May be contralateral or ipsilateral, depending on location

-We test using the vibratory sense with a 128-Hz fork

-We use the **Romberg sign to distinguish it from a cerebellum lesion (of vermal area)
-The patient puts their feet together and if the posture deteriorates with their eyes closed, this is a positive Romberg sign suggesting dorsal column lesion
-Same with eyes open, it suggests cerebellar damage
-Reason is that proprioceptive input can be compensated for by visual input to the cerebellum
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