Transcript Chapter 12 n 13 Spinal Cord and Spinal Nerves

• Spinal cord is enclosed within the
vertebral column from the foramen
magnum to L2
• Provides two-way communication to
and from the brain
• Protected by bone, meninges, and
CSF
• Is composed of
–
–
–
–
Cervical segments
Thoracic segments
Lumbar segments
Sacral segments
• Gives rise to 31 pairs of spinal
nerves
• SC is not uniform in diameter throughout
length
– Cervical enlargement: nerve fibers that
supplies upper limbs enter & leave SC
– Lumbosacral enlargement: nerve fibers
that supplies lower limbs enter & leave
SC
• Conus Medullaris: terminal tapered portion
of the spinal cord below Lumbosacral
enlargement
• Cauda Equina: origins of spinal nerves from
inferior end of lumbosacral enlargement and
Conus Medullaris
• Spinal Cord is surrounded by connective
tissue membranes – Meninges
– Dura mater: Outermost membrane
– Continuous with epineurium of the
spinal nerves
– Arachnoid mater: Middle layer, thin
and wispy
– Pia mater: Deepest layer, bound
tightly to surface of spinal cord
– Filum terminale: fibrous extension of
the pia mater, which anchors the
spinal cord to the coccyx
– Denticulate ligaments: Are sawtoothed shelves of pia mater; attach
the spinal cord to the dura mater
•
Spaces
– Epidural: space between the vertebrae
and dura mater
– anesthesia injected
– Contains blood vessels, areolar
connective tissue and fat
– Subdural: space between dura mater
and arachnoid mater
– Contains small amount of serous fluid
– Subarachnoid: space between
arachnoid and pia mater
– Contains CSF, blood vessels
• Gray Matter
– Consists of neuron cell
bodies, dendrites and axon
– Is present in the interior of
the spinal cord
– Forms an ‘H’ shape
– Ventral (Anterior) Horns:
• Two anterior
projections of gray
matter
• Contain cell bodies of
motor neurons
• Dorsal Horns
– sensory neurons enter
and synapse with
association neurons
• Lateral Horns
– Only visible from T1
to L2
– Contain autonomic
neuron cell bodies
• Gray commissure
– Connects right and left halves
of gray matter
• External fissures
– Anterior median fissure &
– Posterior median sulcus
– Are deep clefts partially
separating left and right halves
• White matter
– Consists of myelinated
axons, forms nerve tracts
– Divided into columns
called columns or funiculi
– Anterior, lateral and
dorsal white columns or
funiculi
• White matter
– Consists of myelinated
axons, forms nerve tracts
in columns
• Ascending tracts: carry
information from spinal cord
to brain
• Descending tracts: carry
information from brain to
spinal cord
• Spinal cord provides means of
communications between brain and
various organs with the help of spinal
nerves
• Conduction of sensory impulses upward
– through ascending tracts to the brain
• Conduction of motor impulses from
brain down
– through descending tracts to the
efferent neurons
– To muscles or glands
• Spinal nerves arises from series
of rootlets from the dorsal and
ventral surface of the spinal cord
• 6-8 rootlets combine to form
ventral root on ventral side and
dorsal root on dorsal side of
spinal cord
• Ventral and dorsal root extend
laterally and join to form Spinal
cord
• Dorsal root ganglion:
• Each dorsal root contains a ganglion called
Dorsal root ganglion
• Dorsal root ganglion are collections of cell
bodies of sensory neurons forming dorsal
roots
• Axons of these neurons extend from various
parts of body to spinal nerve to dorsal root
ganglia to dorsal root to dorsal horn of
spinal cord
• Axons synapse with interneurons in dorsal
horn or pass into white matter
• Ventral root:
– Axons of motor neurons form
ventral roots and pass into spinal
nerves
– Motor neuron cell bodies are in
anterior and lateral horns of spinal
cord gray matter
• Multipolar somatic motor
neurons in anterior (motor)
horn
• Autonomic neurons in lateral
horn
• Dorsal root contains sensory axons
• Ventral root contains motor axons
• Spinal nerves have both sensory
and motor axons
• A reflex is a rapid, predictable motor response to a stimulus
• There are five components of a reflex arc
– Receptor – site of stimulus
– Sensory neuron – transmits the afferent impulse to CNS
– Integration center – either monosynaptic or polysynaptic region within the
CNS
– Motor neuron – conducts efferent impulses from the integration center to an
effector
– Effector – muscle fiber or gland that responds to the efferent impulse
• Some integrated within spinal cord; some within brain
• Some involve excitatory neurons yielding a response; some
involve inhibitory neurons that prevent an action
•
Major spinal cord reflexes are:
– Stretch Reflex
– Golgi Tendon Reflex
– Withdrawal Reflex
• In stretch reflex, muscles contract
in response to a stretching force
applied to them
• Sensory receptor of stretch reflex
is muscle spindle
• Muscle spindle: Are composed of
3-10 specialized skeletal muscle
cells that lack Myofilaments in
their central regions, are noncontractile, innervated by Sensory
neurons
• Cells are contractile only at the
ends, innervated by Gamma Motor
neurons
• Muscle spindle detect stretch of the
muscle
• Sensory neurons of muscle spindle
conduct AP to the spinal cord
• Sensory neurons of muscle spindle
synapse with motor neurons of the
spinal cord called alpha motor neurons
• Alpha motor neurons transmit AP to
skeletal muscle
• And causes contraction of stretched
muscle, opposes the stretch of muscle
• Eg. A person standing in the upright position begins to lean to one side
– The postural muscles that are closely connected to the vertebral column on
the other side will stretch
– Stretch reflexes are initiated
– Then muscles contract to correct posture
• Eg. The patellar (knee-jerk) reflex
– Tapping the patellar tendon stretches the quadriceps and starts the reflex
action
– Quadriceps tendon stretched  muscle spindles send impulse (muscle
stretching)  spinal cord  multipolar motor neuron  quadriceps muscle
contracts
– Extend the lower leg
• The opposite of the stretch
reflex
• A tendon reflex operates as
follows:
• As the tension applied to a
tendon increases, the Golgi
tendon organ (sensory
receptor) is stimulated
• AP arise and propagate into
the spinal cord via sensory
neuron
• Within the spinal cord (integrating
center), the sensory neuron activates an
inhibitory interneuron that makes a
synapse with a motor neuron
• The inhibitory neurotransmitter
inhibits (hyperpolarizes) the motor
neuron, which then generates fewer
nerve impulses
• The muscle relaxes and relieves excess
tension
• Example: weight lifter suddenly drops
heavy weight
• Function is to remove a body
limb or other part from a painful
stimulus
• Reciprocal innervation:
• Polysynaptic Reflexes
– Require 3 or more sets of
neurons
• Causes relaxation of extensor
muscle when flexor muscle
contracts
– Also involved in stretch reflex,
(eg. Quadriceps contract &
hamstrings relax.)
• Crossed extensor reflex:
• Polysynaptic Reflexes
– Require 3 or more sets of
neurons
– Person steps on a sharp object
– When a withdrawal reflex is
initiated in one lower limb, the
crossed extensor reflex causes
extension of opposite lower
limb for balance
• Reflexes do not operate alone but as a whole
• Divergent and convergent pathways reflex
activities are integrated with Nervous system
• Diverging branches of sensory neuron send
AP along ascending nerve tracts to the brain
• e.g., pain stimulus, Initiates withdrawal
reflex and enable to perceive pain
• Descending tracts from brain carry info to
reflexes
• Neurotransmitters produce either EPSPs or
IPSPs modifying the reflex
• Peripheral nerves consist of:
– Axons
– Schwann cells
– Connective tissue
• Each axon and its Schwann cell
sheath are surrounded by
connective tissue layer:
• Endoneurium: surrounds
individual neurons
• Perineurium: surrounds
axon groups to form
fascicles
• Epineurium: surrounds the
entire nerve
• A dermatome is the area of
skin supplied with sensory
innervation by a pair of
spinal nerves
• All spinal nerves except C1
participate in dermatomes
• Thirty-one pairs of spinal
nerves
• First pair exit vertebral column
between skull and atlas
• Last four pair exit via the sacral
foramina
• Others exit through
intervertebral foramina
• Eight pair cervical, twelve pair
thoracic, five pair lumbar, five
pair sacral, one pair coccygeal
• Spinal nerve is very short (1-2 cm)
and divides into
– Small dorsal ramus
– Larger ventral ramus
– Additional rami, called
Communicating rami is present at
the base of the ventral rami in the
thoracic and upper lumbar spinal
cord regions
– Contain autonomic (visceral)
nerve fibers
• Dorsal rami : innervate deep
muscles of the dorsal trunk
responsible for movements of the
vertebral column
•
Ventral rami : Distributed in 2
ways:
– Thoracic region: Ventral rami
(T1-T12) form intercostal
nerves that innervate the
intercostal muscles and the skin
over the thorax
– Remaining Spinal nerve Ventral
rami : form five plexuses
(interlacing nerve networks )
• Nerve Plexus:
– Ventral rami of spinal
nerves C1-C4= cervical
plexus
– Ventral rami of C5-T1=
brachial plexus
– Ventral rami of L1-L4=
lumbar plexus
– Ventral rami of L4-S4=
sacral plexus
– Ventral rami of S4 and S5=
coccygeal plexus
• The cervical plexus is formed
by ventral rami of C1-C4
• Most branches are cutaneous
nerves of the neck, ear, back of
head, and shoulders
• The most important nerve of
this plexus is the phrenic nerve,
from C3-C5 (cervical and
brachial plexuses)
• The phrenic nerve is the major
motor and sensory nerve of the
diaphragm
• Formed by C5-C8 and T1
(C4 and T2 may also
contribute to this
plexus)
• It gives rise to the
nerves that innervate
the upper limb
• C4 from cervical plus C5-T1
• “Robert Taylor Drinks Cold Beer”
• “Roots Trunks Divisions Cords
Branches”
• Five ventral rami (roots C5-T1) form three
trunks (upper/ middle/ lower) that separate
into six divisions (ant./post.) then form 3
cords (lat./ med./ post.)from which five
branches or nerves of upper limbs emerge
• Terminal branches/ nerves
– Axillary innervates part of shoulder
– Radial innervates post. arm
– Musculocutaneous innervates ant. arm
– Ulnar & Median innervates ant.
Forearm and hand
– Smaller nerves such as pectoral, long
thoracic, thoracodorsal, subscapular,
suprascapular innervates shoulder and
pectoral muscles.
• Axillary nerve innervates
the deltoid and teres minor
• Laterally rotate arm - teres
minor
• Abducts arm – deltoid
• Cutaneous (Sensory)
Innervation: inferior lateral
shoulder
• Innervates essentially all
extensor muscles
• Movements at elbow and
wrist, thumb movements
• Cutaneous (Sensory)
Innervation - posterior
surface of arm and forearm,
lateral 2/3 of dorsum of hand
• Movements at shoulder,
elbow and wrist
• Sends fibers to the biceps
brachii and brachialis
• Cutaneous (Sensory)
Innervation - lateral surface
of forearm
• Movements at wrist,
fingers, hand
• Supplies the flexors (flexor
carpi ulnaris and part of the
flexor digitorum profundus)
• Cutaneous (Sensory)
Innervation - medial 1/3 of
hand, little finger, and
medial 1/2 of ring finger
• Movement of hand, wrist,
fingers, thumb
• Branches to most of the
flexor muscles of arm.
• Cutaneous (Sensory)
Innervation - lateral 2/3
palm, thumb, index and
middle fingers; lateral 1/2
of ring finger and dorsal
tips of same fingers
• Small nerves that innervate muscles acting on scapula
and arm
– Pectoral
– Long thoracic
– Thoracodorsal
– Subscapular
– Suprascapular
• Supply cutaneous innervation of medial arm and forearm
• Lumbar plexus: ventral rami of L1L4
• Sacral plexus: ventral rami of L4-S4
• Usually considered together because
of their close relationship
• Four major nerves exit and enter
lower limb
–
–
–
–
Obturator innervates medial thigh
Femoral innervates ant. thigh
Tibial innervates post. thigh
Common fibular (peroneal) innervates
post. Thigh, ant & lateral leg and foot
• Obturator nerve
supplies the muscle that
adduct the thigh and
knee
• Cutaneous (Sensory)
Innervation - superior
middle side of thigh
• Femoral nerve innervates
iliopsoas, Sartorius,
quadriceps femoris group
muscles
•
Movements of hip and
knee
• Cutaneous (Sensory)
Innervation: anterior and
lateral thigh; medial leg and
foot
• The Tibial and Fibular nerves
together referred to as the
sciatic (ischiadic) nerve
• Tibial
– Movement of hip, knee, foot,
toes
– Cutaneous (Sensory)
Innervation: none
• Common fibular
– Anterior and lateral
muscles of the leg and foot
– Cutaneous (Sensory)
Innervation: lateral and
anterior leg and dorsum of
the foot
• Nerves that innervate the skin of the suprapubic area, external
genitalia, superior medial thigh, posterior thigh are:
– Gluteal nerves
– Pudendal nerve
– Iliohypogastric nerve
– Ilioingual nerve
– Genitofemoral nerve
– Cutaneous femoral
• Coccygeal plexus formed by S5 and coccygeal
nerve (Co)
• Innervates muscles of pelvic floor
• Sensory Cutaneous Innervation : over coccyx