12-1 structure of nerves (including roots and rami and plexuses) consist of –axon bundles/nerve...
TRANSCRIPT
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Structure of Nerves (including roots and rami and plexuses)
• Consist of– Axon bundles/nerve fibers– Schwann cells– Connective tissue– Blood vessels
• Endoneurium: surrounds individual neurons
• loose CT with capillaries (for neurons)
• Perineurium: Surrounds fascicles
• blood vessels Epineurium: surrounds the entire nerve
• Dense CT
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Spinal Cord and Spinal NervesChapter 12
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Spinal Cord Functions• carry/transmit sensory and
motor impulses between spinal nerves and the brain– in columns/white matter
• integration center for spinal reflexes– in gray matter
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Spinal Cord • Extends from foramen magnum to ~L1-L2
• Vertebral canal continues length of sacrum– a portion of the vertebral canal is not
occupied by the actual spinal cord– creates the opportunity for spinal
tap/lumbar puncture (see next side)
• Gives rise to 31 pairs of spinal nerves– Exit through intervertebral and sacral foramina
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Lumbar puncture
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Spinal Cord • Not uniform in diameter – Cervical enlargement:
• supplies upper limbs
– Lumbar enlargement: • supplies lower limbs
• Conus medullaris: – Tapered/pointed inferior end of
cord.
• Cauda equina: – Roots and nerves extending down
vertebral canal below L2 that exit intervertebral and sacral foramina
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Spinal Nerves• Thirty-one pairs of spinal
nerves• First pair exit vertebral
column between skull and atlas
• Exit vertebral canal through intervertebral and sacral foramina
• 8 pair cervical, 12 pair thoracic, 5 pair lumbar, 5 pair sacral, & coccygeal
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Rami of spinal nerve (thoracic region): rami branch off the spinal nerve
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Cervicalnerves
Thoracicnerves
Lumbarnerves
Sacralnerves
Coccygealnerves
(a) Posterior view
C1
T1
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2345
6
78
9
10
11
12
L1
2
3
45
T2
S1
L5L5
S1
L4 L4
L3L3
L2L2
S3
S3
C8C8
T1T1
C7C7
T3T4T5T6
T7
T8
T9
T10
T11
T12
L1
C6C6
C5
C4
C4C3
C2
S1
C5
T1T1
S4
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Dermatomal Map• Dermatomal map: skin area supplied with somatic
sensory innervation by spinal nerves—general trends
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Plexuses: intermingling nerves arising from multiple anterior rami
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Branches of
Spinal Nerves
Dorsal/posterior Ramus: • innervate deep muscles of the trunk
responsible for movements of the vertebral column and skin near dorsal midline
Ventral/anterior Ramus: • innervates structures anterior and lateral to
spinal cord.– Thoracic region: form intercostal nerves
that innervate the intercostal muscles and the skin over the thorax
– Other regions they form plexuses (intermingling of nerves).
• Ventral rami C1-C4= cervical plexus• Ventral rami C5-T1= brachial plexus• Ventral rami of L1-L4= lumbar plexus• Ventral rami of L4-S4= sacral plexus• Ventral rami S4 & S5= coccygeal
plexus
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Plexus
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Cervical Plexus
• C1-C4 • Innervates superficial
neck structures, skin of neck, posterior portion of head
Selected nerve of cervical plexus--Phrenic nerve
– Innervate diaphragm• Sole motor innervation of
diaphragm
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Brachial Plexus• C5-T1 and some from C4 • Nerves arising from:
– Axillary– Radial– Musculocutaneous– Ulnar– Median
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Lumbar Plexus• Lumbar plexus: ventral rami
of L1-L4• Selected Nerves:
– Femoral– Obturator
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sacral Plexus• Sacral plexus: ventral rami
of L4-S4– sometimes considered
together because of their close relationship
• major nerves exit and enter lower limb
• Selected Nerves:– Sciatic
• Tibial• Common fibular (peroneal)
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Cross Section of Spinal Cord
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Cross Section of Spinal Cord• Gray matter: mostly glial cell, cell bodies, dendrites
Horns• Posterior (dorsal)
– sensory neurons enter the cord
• Anterior (ventral)– cell bodies of somatic motor neurons
• Lateral – associated with ANS—cell bodies of visceral motor neurons– distinct lateral horn not present in all regions of cord
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• Commissures: connections between left and right halves (of CNS)– Gray & White Commisures
• Roots: nerves connecting to the cord– Dorsal (posterior) root
• Sensory Nerve• contains sensory neurons (unipolar)
– Dorsal Root Ganglion: cell bodies of sensory neurons
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• Ventral (anterior) Root– motor nerve
• axons of motor neurons (multipolar)
• Spinal Nerve: Two roots merge to form a spinal nerve– then passes through intervertebral foramen– mixed nerves
• axons of both motor and sensory neurons
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Cross Section of Spinal CordWhite matter: myelinated axons forming nerve tracts
– columns (funiculi): • divided into tracts (fasciculi; pathways)• Carry information:
– to and from the brain (ascending and descending)– to and from other regions of the spinal cord
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Figure 16.4 locations of various neurons within spinal cord (e.g., somatic motor, visceral motor [autonomic], somatic sensory)
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Pathways through roots and cord
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Ascending and Descending Tracts/Pathways
= sensory
= motor
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Specific Ascending Pathways within spinal cord white matter
• Anteriolateral System — within anterior and lateral columns– Spinothalamic Tracts: somatic sensory information
from cutanous receptors to Thalamus• pain, crude touch, temperature
• Dorsal Column System — in dorsal columns– proprioception, fine touch, two point discrimination
• Spinocerebellar--periphery of lateral and dorsal columns– proprioception to cerebellum
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• Anteriolateral System —Spinothalamic Tracts: somatic sensory information from cutanous receptors to Thalamus
• pain, pressure, crude touch, temperature
• Dorsal Column System – proprioception, fine-touch,
two point discrimination, pressure
• Spinocerebellar--periphery of lateral and dorsal columns– proprioception to cerebellum
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Descending Pathways through spinal cord white matter
• Corticospinal (pyramidal)—within dorsal and anterior columns– voluntary movements
• Indirect Pathways– involuntary movement, upright posture,
balance, walking, reflexive movements of head and neck in response to visual and auditory stimuli
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roots
Spinal nerve
ramusplexus“nerves”
e.g., femoral, median, phrenic, sciatic
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Spinal Meninges and Protection of cord
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Cross Section of Spinal Cord
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Figure 16.2
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Meninges & Associated Spaces • Connective tissue membranes
surrounding spinal cord and brainEpidural Space: – Contains blood vessels, areolar CT
and fat.Dura mater: – continuous with epineurium of the
spinal nervesArachnoid mater: thin and wispySubarachnoid space: – Contains CSF and blood vessels
within web-like strands of arachnoid tissue
Pia mater: – bound tightly to surface of brain
and spinal cord. • filum terminale: anchors spinal
cord to coccyx—longitudinal support
• denticulate ligaments: attach the spinal cord to the dura mater laterally—lateral support
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Protection of the Spinal Cord
Physical Protection• Vertebrae
– rigid protection• Epidural Space with adipose
– padding/cushioning• Meninges• CSF
– cushioning• Filum Terminale
– longitudinal support• Denticulate Ligaments
– lateral support
Chemical Protection• Blood Brain Barrier—blood CNS barrier
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Somatic Motor and Sensory:--Single sensory neurons carry sensory impulses from the receptor/site of stimulus all the way into the spinal cord.
--Single motor neurons carry motor impulses from the spinal cord all the way to the effector muscle
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Pathways through roots and cord
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ReflexesAutomatic responses to specific stimuli (do
not require conscious thought/processing)
– Higher brain centers can influence, suppress, or exaggerate reflex responses
Types:• Learned• Innate (typically homeostatic)
– Spinal, integrated in spinal cord– Cranial, integrated in brain
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Spinal Reflexes
• represent some of the most basic nerve pathways and CNS integration
• Brain not necessary for spinal reflexes to occur– Although brain can modify (suppress or enhance )
spinal reflexes.
• The nerve pathway is called a reflex arc
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Reflex Arc Components1. Electrical Impulse (Action potentials) produced in sensory receptors
transmitted to2. Sensory neuron. To-3. Interneurons—in most cases. To-4. Motor neuron. To-5. Effector organ which responds with a reflex
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Stretch/Extensor Reflexes• Monosynaptic:
– Two neurons– One synapse– Sensory neuron synapses directly with motor
neuron
1. Stretch Receptor (Muscle spindle):2. Sensory neuron3. synapse with motor neurons of the spinal cord4. Motor neuron innervates muscle that was
stretch causing contraction
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Stretch/Extensor Reflex
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Withdrawal/Flexor Reflex:Function is to remove a body limb or other part from a painful stimulus.
• Polysynaptic:– 2+ synapses– 3+ neurons– Interneuron(s) between sensory and motor neuron
Variations:• Reciprocal inhibition
– causes relaxation of antagonistic extensor muscle when flexor muscle contracts.
• Crossed extensor reflex: – when a withdrawal reflex is initiated in one lower limb, the crossed extensor reflex causes extension of
opposite lower limb.
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Withdrawal Reflex
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Withdrawal Reflex with Reciprocal Inhibition
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Relationship of Brain and Spinal Cord Reflexes
• Sensory information goes to brain; e.g., pain.
• Descending tracts from brain carry info to reflexes.
• Neurotransmitters produce inhibitory or excitatory effects modifying the reflex.
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PNS Disorders• General disorders
– Anesthesia: loss of sensation• Hyperesthesia: increased sensitivity to pain, pressure, light• Paresthesia: tingling, prickling, burning• Neuralgia: nerve inflammation causing stabbing pain• Sciatica: pain radiating down back of thigh and leg
• Infections– Herpes: skin lesions– Shingles or herpes zoster: adult disease of chickenpox, virus latent in
peripheral ganglia– Poliomyelitis: infantile paralysis– Anesthetic leprosy: bacterial infection of peripheral nerves– Diptheria: demylenation, motor/sensory decline, resp and heart failure
• Genetic and autoimmune disorders– Myasthenia gravis: results in fatigue and muscular weakness due to
inadequate ACh receptors