The Spinal Cord, Spinal Nerves, and Spinal Reflexes

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Chapter 13
The Spinal Cord, Spinal Nerves, and Spinal
Reflexes
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Organization
President
VP (dean)
division heads
department chair
faculty
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brain
more processing centers
processing centers
senses
muscles
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fig. 13-1
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Chapter 12 neurons Chapter 13 spinal cord
reflexes Chapter 14 brain and cranial
nerves Chapter 15 brain-spinal cord
interaction Chapter 16 autonomic NS
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Spinal cord anatomy
18 inches (not as long as vertebral
column) ends about L1 or L2 (conus
medullaris) deep groove on anterior (ventral)
side roots attached to it (dorsal and ventral)
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cervical thoracic lumbar
fig. 13-2
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fig. 13-3
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fig. 13-5
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white matter
posterior white columns
lateral white columns
anterior white columns
fig. 13-5
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ascending tracts sensory
descending tracts (motor)
posterior white columns
lateral white columns
anterior white columns
fig. 13-5
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epineurium perineurium endoneurium
fig. 13-6
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Charlie
epaxial
hypaxial
e
axis
h
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fig. 7-22c
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fig. 11-3
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fig 13-7
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distribution of spinal nerves
dorsal and ventral dermatomes
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fig 13-8
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Clinically significant
damage to spinal nerve or DRG may cause sensory
loss to just a restricted part of the skin
by mapping the deficit you may be able to
pinpoint where the nerve has been damaged
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Clinically significant
example shingles
virus attacks DRG painful rash area of
dermatome of that spinal nerve
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Distribution of spinal nerves T1 to T12 is
typical
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fig. 13-9
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cervical plexus
C
3
, 4
and 5
keep the diaphragm alive
fig. 13-10
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brachial plexus
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lumbar plexus
sacral plexus
S
2
, 3
and 4
keep your anus off the floor
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The human body contains
10,000,000 sensory neurons 500,000 motor
neurons 20,000,000,000 interneurons
information to CNS control effectors
(muscles) interpret, plan, coordinate incoming
and outgoing info
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all these neurons are organized into neuronal
pools -
functional groups of interconnected neurons
100s to 1000s ??
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neuronal polls
100s to 1000s ??
each has limited input/output can be excitatory
or inhibitory
contain different circuit patterns
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one neuron (neuronal pool)
Divergence
multiple neurons (neuronal pools)
fig. 13-13a
(vision)
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many neurons
Convergence
single neuron
fig. 13-13b
(muscle control, breathing)
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Serial Processing
stepwise spread of info
fig. 13-13c
(pain)
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Parallel Processing
several neurons (pools) access same info
divergence
serial
fig. 13-13d
(ouch)
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Reverberation
positive feedback loop
fig. 13-13e
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conditions in or around the body can change
suddenly and unexpectedly
reflex
rapid, automatic response to specific stimulus
make adjustments to maintain homeostasis
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remember chapter 1
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Homeostatic regulation
autoregulation extrinsic regulation
adjustment within organ nervous/endocrine
system
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Homeostatic regulation
Three part mechanism receptor
(stimulus) control center effector
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wiring of a reflex is called
reflex arc
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reflex arc
step 1
arrival of stimulus activation of receptor
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reflex arc
step 2
activation of sensory neuron
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reflex arc
step 3
information processing
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reflex arc
step 4
activation of motorneuron
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reflex arc
step 5
response of effector
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fig. 13-14
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Classification of reflexes
developmental origin nature of motor
response complexity site of information processing
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Classification of reflexes
developmental origin
innate acquired
nursing driving
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Classification of reflexes
nature of motor response
somatic visceral
skeletal muscle later (16)
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Classification of reflexes
complexity
monosynaptic polysynaptic
quicker slower
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Classification of reflexes
site of processing site
spinal cord brain
spinal reflexes cranial reflexes (later)
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Monosynaptic reflexes
very little delay rapid responses example stretch
reflex
automatic regulation of length of skeletal muscle
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fig. 13-15
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stretch reflex
stimulus
increases muscle length
activates
sensory neuron receptor
stimulates effector
muscle contracts to counter the stimulus
(within 20-40 msec)
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stretch reflex
receptor is called
muscle spindle
intrafusal fibers
surrounded by
extrafusal fibers
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fig 13-16
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stretch reflex
intrafusal fibers
sensory area in center
sensory branch
has a normal resting length
myofibrils at ends
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stretch reflex
intrafusal fibers
sensory branch
stretching membrane distorts dendrites (AP)
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stretch reflex
sensory axon from intrafusal fiber is always
active (AP)
(membrane in central area of)
if intrafusal fiber is stretched the frequency of
AP is increased

if intrafusal fiber is compressed the frequency
of APs is decreased
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stretch reflex
sensory axon from intrafusal fiber is always
active (AP)
sensory axon synapses in spinal cord
motorneurons that innervate the extrafusal fibers
of that muscle
collaterals that send info to brain
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stretch reflex
stretch muscle
increase spindle sensory neuron AP
increase activity of motorneuon to extrafusal
fiber
increase muscle tone (contraction)
increase resistance to being stretched
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stretch reflex
Many stretch reflexes are postural
postural muscles have firm muscle tone very
sensitive stretch receptors
fine adjustments are always being made
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stretch reflex
What is the role of the gamma efferents?
myofibrils at ends
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stretch reflex
What is the role of the gamma efferents?
adjust the tension on the sensory membrane area
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stretch reflex
What is the role of the gamma efferents?
if muscle is contracted
gamma efferents stimulate contraction of
myofibrils of spindleshorten spindle
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stretch reflex
What is the role of the gamma efferents?
if muscle is lengthened
gamma efferents stop stimulation of myofibrils
of spindlerelax spindle
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stretch
relax
start
muscle
sr
spindle
contract
compress
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stretch reflex
regulates the length of muscle monosynaptic
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polysynaptic reflexes
more complicated responses can involve multiple
muscle groups can be inhibitory or
excitatory (ipsps) (epsps)
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tendon reflex
?? receptors (different than spindles,
etc) sense if collagen (tendon) is being
stretched too much if so, will stimulate
inhibitory interneurons in spinal cord that will
inhibit motorneuron activity
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tendon reflex
sensory neuron

too much stretch
inhibitory interneuron
-
-
motorneuron
reduce tension
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withdrawl reflex
move affected parts of the body away from a
stimulus
pain, touch, pressure
eg., flexor reflex
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sensory neuron AP
activate interneuron
stimulate flexors
fig. 13-17
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sensory neuron
stimulates
interneuron
stimulates
motorneuron
contraction of flexor
stretch of extensor ?
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sensory neuron AP
activate interneuron
stimulate flexors
inhibit extensors
reciprocal inhibition
fig. 13-17
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What kind of processing did you see in the
withdrawl reflex ?
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divergence
parallel
fig. 13-17
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stretch tendon withdrawl
reflexes
all ipsilateral (same side)
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crossed extensor reflex
contralateral reflex arc (opposite side)
this reflex compliments the flexor reflex
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(crossed) contralateral extensor reflex
ipsilateral flexor reflex
reverberation (positive feedback)
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characteristics of polysynaptic reflexes

• use pools of interneurons
• are intersegmental
• involve reciprocal inhibition
• have reverberating circuits
• several reflexes cooperate

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Integration of reflexes
all these reflexes occur without input from the
brain (higher centers)
but the higher centers do have an influence
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Integration of reflexes
facilitation
epsp ipsp
move a cell closer to threshold move cell
further from threshold
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Integration of reflexes
higher centers
stimulate excitatory or inhibitory interneurons
adjust sensitivity of reflexes
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Integration of reflexes
reinforcement
when excitatory synapses are chronically active
inhibition
when reflexes are inhibited
example
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Integration of reflexes
plantar reflex
stroke sole of foot
toes curl down (normal for adults)
(negative Babinski)
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Integration of reflexes
in infants
stroke sole of foot
fanning of toes
Babinski
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Integration of reflexes
Babinski disappears normally (is inhibited) with
development of descending motor pathways
If Babinski shows up in adult, it means that
descending pathways are damaged