Transcript Document

Chapter 13
The Spinal Cord, Spinal
Nerves, and Spinal Reflexes
Organization:
President
VP (dean)
division heads
department chair
faculty
brain
more processing
centers
processing
centers
senses
muscles
fig. 13-1
Chapter 12
Chapter 13
Chapter 14
Chapter 15
Chapter 16
neurons
spinal cord reflexes
brain and cranial nerves
brain-spinal cord interaction
autonomic NS
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)
cervical
thoracic
lumbar
fig. 13-2
fig. 13-3
fig. 13-5
white matter
fig. 13-5
ascending
tracts
sensory
fig. 13-5
descending
tracts
(motor)
epineurium
perineurium
endoneurium
fig. 13-6
Charlie
e
h
axis
fig. 7-22c
fig. 11-3
fig 13-7
distribution of spinal nerves
dorsal and ventral
dermatomes
fig 13-8
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
Clinically significant:
example:
shingles
virus attacks DRG
painful rash area of dermatome
of that spinal nerve
spinal cord
Distribution of spinal nerves
T1 to T12 is “typical”
fig. 13-9
cervical plexus
C
3, 4 and 5 keep the diaphragm alive
fig. 13-10
brachial plexus
lumbar plexus
S
sacral plexus
2, 3 and 4 keep your anus off the floor
The human body contains:
10,000,000 sensory neurons
information to CNS
500,000 motor neurons
control effectors (muscles)
20,000,000,000 interneurons
interpret, plan, coordinate
incoming and outgoing info
all these neurons are organized into
neuronal pools -
functional groups of
interconnected neurons
100’s to 1000’s ??
neuronal polls
100’s to 1000’s ??
each has limited input/output
can be excitatory or inhibitory
contain different circuit patterns
Divergence
one
neuron
(neuronal pool)
multiple
neurons
(neuronal pools)
fig. 13-13a
(vision)
Convergence
many
neurons
single
neuron
fig. 13-13b
(muscle control, breathing)
Serial Processing
stepwise
spread
of
info
fig. 13-13c
(pain)
Parallel Processing
divergence
several
neurons
(pools)
access
same
info
serial
fig. 13-13d
(ouch)
Reverberation
positive
feedback
loop
fig. 13-13e
conditions in or around the body can
change suddenly and unexpectedly…
reflex
rapid, automatic response to
specific stimulus
make adjustments to
maintain homeostasis
remember chapter 1…
Homeostatic regulation:
autoregulation
adjustment within organ…
extrinsic regulation
nervous/endocrine system
Homeostatic regulation:
Three part mechanism:
receptor (stimulus)
control center
effector
wiring of a reflex is called…
reflex arc
reflex arc
step 1
arrival of stimulus
activation of receptor
reflex arc
step 2
activation of sensory neuron
reflex arc
step 3
information processing
reflex arc
step 4
activation of motorneuron
reflex arc
step 5
response of effector
fig. 13-14
Classification of reflexes
developmental origin
nature of motor response
complexity
site of information processing
Classification of reflexes
developmental origin
innate
nursing
acquired
driving
Classification of reflexes
nature of motor response
somatic
skeletal muscle
visceral
later (#16)
Classification of reflexes
complexity
monosynaptic
quicker
polysynaptic
slower
Classification of reflexes
site of processing site
spinal cord
spinal reflexes
brain
cranial reflexes (later)
Monosynaptic reflexes
very little delay
rapid responses
example: stretch reflex
automatic regulation of
length of skeletal muscle
fig. 13-15
stretch reflex
stimulus
increases muscle length
activates
sensory neuron receptor
stimulates effector
muscle contracts to
counter the stimulus
(within 20-40 msec)
stretch reflex
receptor is called
muscle spindle
intrafusal fibers
surrounded by
extrafusal fibers
fig 13-16
stretch reflex
gamma
efferent
intrafusal
fibers
sensory area
in center
has a
“normal”
myofibrils
resting
at ends
length
sensory
branch
gamma
efferent
stretch reflex
intrafusal
fibers
sensory
branch
stretching
membrane
distorts
dendrites
^(AP)
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 AP’s is decreased
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
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
stretch reflex
Many stretch reflexes are postural
postural muscles have firm muscle tone
very sensitive stretch receptors
fine adjustments are always being made
stretch reflex
gamma
efferent
What is the role of the
gamma efferents?
myofibrils
at ends
gamma
efferent
stretch reflex
What is the role of the
gamma efferents?
adjust the tension on the
sensory membrane area
stretch reflex
What is the role of the gamma
efferents?
if muscle is contracted…
…gamma efferents stimulate
contraction of myofibrils of
spindle…shorten spindle
stretch reflex
What is the role of the gamma
efferents?
if muscle is lengthened…
…gamma efferents stop
stimulation of myofibrils of
spindle…relax spindle
relax
start
sr
spindle
contract
stretch reflex
regulates the length of muscle
monosynaptic
polysynaptic reflexes
more complicated responses
can involve multiple muscle groups
can be inhibitory or excitatory
(ipsp’s)
(epsp’s)
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
tendon reflex
too
much
stretch
reduce
tension
sensory
neuron
+
inhibitory
interneuron
motorneuron
withdrawl reflex
move affected parts of the body
away from a stimulus
pain, touch, pressure
eg., flexor reflex
sensory
neuron AP
stimulate
flexors
fig. 13-17
activate
interneuron
sensory neuron
stimulates
interneuron
stimulates
motorneuron
contraction of flexor
stretch of extensor ?
sensory
neuron AP
stimulate
flexors
activate
interneuron
inhibit
extensors
“reciprocal
inhibition”
fig. 13-17
What kind of processing did you
see in the withdrawl reflex ?
divergence
fig. 13-17
stretch
tendon
withdrawl
reflexes
all ipsilateral
(same side)
crossed extensor reflex
contralateral reflex arc
(opposite side)
this reflex compliments the
flexor reflex
ipsilateral
flexor
reflex
(crossed)
contralateral
extensor
reflex
reverberation
(positive
feedback)
characteristics of
polysynaptic reflexes
•use pools of interneurons
•are intersegmental
•involve reciprocal inhibition
•have reverberating circuits
•several reflexes cooperate
Integration of reflexes
all these reflexes occur
without input from the brain
(higher centers)…
…but the higher centers
do have an influence
Integration of reflexes
facilitation
epsp
move a cell closer to threshold
ipsp
move cell further from threshold
Integration of reflexes
higher centers
stimulate excitatory or
inhibitory interneurons
adjust sensitivity of reflexes
Integration of reflexes
reinforcement
when excitatory synapses are
chronically active
inhibition
when reflexes are inhibited
example:
Integration of reflexes
plantar reflex
stroke sole of foot
toes curl down
(normal for adults)
(negative Babinski)
Integration of reflexes
in infants
stroke sole of foot
fanning of toes
Babinski
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