Download CH 13 spinal cord A and P 2017

CH 13
spinal cord and more
the cord
spinal nerves
somatic reflexes
spinal cord
four primary functions
conduction = carries information up and down the cord
integration = spinal neurons receive info from many sources
locomotion = simple repetitive movements are coordinated by
groups of neurons (central pattern generators) in the
spinal cord and pass this information cranially
reflexes = spinal reflexes for posture, muscle tone, protection
31 pairs of spinal nerves
cervical 8, thoracic 12, lumbar 5, sacral 5, coccygeal 1
cervical and lumbar have thicker areas
brain and spinal cord are covered by
three fibrous layers – the meninges
dura mater - lies just under the skull or surrounds cord, thick rubbery
collagenous membrane, 2 layers
arachnoid – simple squamous epithelial membrane lies just under the
dura & has a loosely organized space between the
arachnoid membrane and pia mater filled with CSF & BV
pia mater - delicate transparent membrane which follows closely the
surfaces of the spinal cord and brain, in the lumbar region
becomes terminal filum & at sacral area fuses with dura to
become coccygeal ligament which anchors cord and
meninges to coccyx 1
spinal cord terms
white matter – bundles of axons called tracts, lots of myelin
gray matter – little myelin, lots of somas, dendrites, synapses
tracts – bundles carry specific info to and from specific areas
horns – anterior, posterior, lateral, give gray matter a H shape
central canal – filled with cerebrospinal fluid (CSF)
posterior (dorsal) root – sensory nerves
anterior (ventral) root – motor fibers
lateral horn – sympathetic NS sends axons to anterior root
afferent – towards the CNS, sensory
efferent - towards the periphery, motor
decussation – cross over from one side to opposite body side
contralateral – origin & destination are on opposite body sides
ipsilateral – origin and destination are on same side of body
denticulate ligament,
epidural space - BV, fat, CT
Ganglion – lots of cell bodies
words to know
gracile – long, slender
fasciculus (fascicle) – band, bundle within a funiculus
cuneate – wedge
commissure – bundle that crosses over
tecto – roof, cover
reticulo – brainstem area involved with many involuntary functions
ganglia – groups of nerve cell bodies usually outside of CNS
funiculus – columns of white matter made up of fiber tracts
fissure – anterior groove, not as deep & wider than post. sulcus
sulcus – posterior groove, not as wide & deeper than ant. fissure
spinal tracts
ascending – afferent, sensory, carry info to CNS
descending – efferent, motor, carry info to effectors
specific tracts carry information
from a similar origin
a similar destination
and a similar function
ascending tracts (sensory)
typically 3 neurons
1st order detects stimulus & sends signal to spinal cord or brainstem
2nd order carries information to thalamus
3rd order sends info to cerebral cortex
gracile fasciculus
- carries info from mid thoracic and lower parts of body
- first order neurons travel ipsilateral to gracile nucleus in
medulla oblongata
- 2nd order decussate and form medial lemniscus which carries
information to thalamus
- 3rd order carry info to cerebral cortex
- carries info for visceral pain, vibrations, discriminating touch,
deep touch, proprioception for lower limbs & lower trunk
cuneate fasciculus
- joins gracile fasciculus at T6 level (mid thoracic)
- carries info from T6 and above (upper limbs and chest)
- info to ipsilateral cuneate nucleus in medulla oblongata
- 2nd order fibers decussate, form medial lemniscus and carry
info up the brainstem to the thalamus
- 3rd order carry info to cerebral cortex
- carries same information as gracile fasciculus, visceral pain,
deep and discriminating touch, proprioception from upper
limbs and upper trunk
spinothalamic tract
- form the anterolateral system
- signals for pain, temp., pressure, tickle, itch, light or crude touch
- 1st order end in posterior horn of cord near point of entry
- 2nd order decussate and form contralateral ↑ spinothalamic tract
- 3rd order neurons travel from thalamus to cerebral cortex
spinoreticular tract
- travels up anterolateral system
- pain from injury,
- 1st order enter post. horn and immediately synapse with
- 2nd order neurons which decussate and travel up to reticular
formation in pons and medulla
- 3rd order neurons continue to thalamus and
- 4th order neurons carry info to cerebral cortex
posterior & anterior spinocerebellar tracts
- travel through lateral columns
- carry proprioceptive signals from limbs and trunk to cerebellum
- 1st order from muscles & tendons send info to posterior horn
- 2nd order from post. horn up spinocerebellar tracts to cerebellum
- these are ipsilateral pathways
- anterior tract fibers decussate in cord then cross back over in
brainstem to enter ipsilateral cerebellum
- both tracts send feedback needed for posture and muscle action
descending tracts
- corticospinal
- tectospinal
- lateral & medial reticulospinal
- lateral & medial vestibulospinal
corticospinal
- motor signals from cerebral cortex for finely
controlled motor movements
- form the pyramids of the anterior medulla oblongata
- most fibers decussate in lower medulla
and form lateral corticospinal tract
- ipsilateral fibers form the anterior corticospinal tract which
decussates at the level of the spinal cord, these fibers
disappear at the mid thoracic level
tectospinal tract
- starts in midbrain region the tectum
- crosses to contralateral side of midbrain
- descends through brainstem to upper spinal cord
- involved in reflexes and movements of head
in response to sound and sights
lateral and medial reticulospinal tract
- originates in reticular formation of brainstem
- control muscles of upper and lower limbs
- controls muscles for balance and posture
- contain analgesic signals to reduce pain transmission to brain
lateral and medial vestibulospinal tracts
- inner ear sends balance signals to vestibular nuclei of brainstem
- lateral tract goes down anterior column of spinal cord and helps
extensor muscles of limbs to straighten and stiffen for balance
- medial tract splits into ipsi and contra tracts which descent the
anterior column on both sides of cord and end in neck to
control head position
spinal nerves
- this is how the brain via the spinal cord communicates
with the body
- a nerve can have a few to thousands of individual nerve fibers
- most nerves are a mix of afferent (sensory) and efferent
(motor) fibers
- ganglions contain many neurosomas (cell bodies) outside of CNS
- peripheral nervous system (PNS) have myelinated fibers
spinal nerves
- there are 31 pairs of spinal nerves
- cervical = 8, thoracic = 12, lumbar = 5, sacral = 5, coccyx = 1
- C1 originates between skull and atlas
- C2 to C8 originate above the numbered vertebrate (eg C5
nerve originates below C4 and above C5)
- BUT thoracic, lumbar, sacral, and coccygeal originate below
the numbered vertebrate (eg T6 originates below T6
and above T7)
- spinal nerves leave through intervertebral foramen
- L2 to Cox1 become the cauda equine (horses tail)
classification of nerves
afferent = carry sensory info from receptors to CNS
efferent = carry motor info from CNS to effectors
somatic = innervate skin, bones, joints, skeletalmuscles
visceral = innervates blood vessels, glands, and viscera
general = innervates blood vessels, glands, viscera, skin, & muscles
special = innervate more localized organs in the head, eyes, ears,
smell, taste, and muscles for chewing, smiling, expression,
anatomy of a nerve
nerve – bunch of individual nerve fibers held together by CT
endoneurium – CT covering of an individual nerve fiber
unmyelinated nerve – one coat of myelin (neurilemma)
myelinated - many wrappings of myelin, last one = neurilemma
fascicle – a bundle of nerve fibers surrounded by CT
perineurium - CT wrapping about a bundle of nerve fibers
spinal nerve - 31 pairs leaving the spinal cord at different levels
anterior root – carry motor fibers to effectors
posterior root – carry sensory fibers to cord and CNS
ganglion – in posterior root filled with somas of sensory nerves
rootlets – 6 to 8 nerves which together form the ant & post roots
epineurium – outer covering of a neuron
branches of spinal nerves
spinal nerve – 31 afferent and efferent fibers of the spinal cord
meningeal nerve – reenters vertebral canal and innervates the
meninges, vertebrae, and spinal ligaments with
sensory and motor fibers
ramus (rami) – branch, spinal nerves split into ant & post rami
communicating rami – fibers leave ant ramus to join sympathetic
chain of ganglia
sympathetic ganglia – thoracolumbar division from T1 to L2 but
communicating fibers travel up and down so that all
levels of the cord have sympathetic fibers
nerve plexus
– multiple anterior rami spinal nerves anastomosing,
communicating with each other, and generating multiple
peripheral functions
- they provide somatosensory function from bones, joints,
muscles, skin, for touch, heat, cold, stretch, pressure, pain,
proprioception
- their motor functions is primarily for skeletal muscles, bones,
and ANS fibers to the viscera and for control of BV
dermatomes
- each spinal nerve except C1 receives sensory input from a
specific area of the skin
- dermatome maps show which area of the skin a spinal nerve
receives sensory information
- the maps are not perfect because there is always some
overlap sometimes as much as 50% with the
dermatome above and below
reflexes
- reflexes are quick, involuntarily, stereotypical reactions of
glands and muscles to a stimulation
- reflexes need stimulation therefore not spontaneous
- are quick, there are few or no interneurons
- involuntary so difficult to suppress
- stereotypical so the same every time unless pathological
somatic reflex - reflexes of skeletal muscles
visceral reflexes – reflexes of heart and intestines
the simplest reflex involves a receptor, a sensory
neuron, and an effector (monosynaptic reflex arc)
usually there are one or more interneurons and
information is used for the reflex as well as
integration with the spinal cord and brain
muscle spindle reflex (myotatic reflex)
- nerve ending or muscle spindle is excited
- primary afferent nerve sends info to posterior root & cord
- efferent fiber in cord activated
- which stimulates muscle to act
- branch of afferent fiber stimulates inhibitory motor neuron
- which inhibits muscle antagonistic to initial reflex,
known as reciprocal inhibition
muscle spindle (somatic reflex)
- stretch receptors embedded in normal (extrafusal) muscles
- spindle info used to monitor body part position & movement
- a bundle of 7 or 8 modified muscle fibers (intrafusal fibers)
- spindles have a few sarcomeres and have a fibrous capsule
- are concentrated at ends of muscles near tendons
- gamma motor neuron innervates each end of the spindle
- 30% of all spinal motor neurons are gamma motor neurons
- primary afferents monitor muscle length & speed of change
- secondary afferents monitor length of spindle only
- through spindle fibers the brain is consciously and
unconsciously made aware of the length and
tension on skeletal muscles as well as the speed
with which these changes take place
- this information is used to control posture, motor
control, and corrective reflexes
flexor & crossed extension reflex
- when a reflex causes you to pull away from a source of injury
your body suddenly becomes unstable
- so as one flexor reflex is used to pull a body part away from
injury a second extension reflex acts on the opposite
side of the body to stabilize and support the body
tendon reflex
- proprioceptors at junction of tendons and muscles
- when muscle contracts tendon organs let CNS know
- if too strong/rapid CNS inhibits alpha motor neurons
to muscle does not contract as strongly
- if one part of a muscle contracts much more than another
part it inhibits those fibers so it is in line with the
contractions of the rest of the muscle