Structure of Nerves (including roots and rami)

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Structure of Nerves (including roots and rami)

• 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 Nerves

• Chapter 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 L1-L2
• Vertebral canal continues length of sacrum
• there is a portion of the vertebral canal 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

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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
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Dermatomal Map

• Dermatomal map skin area supplied with somatic
  sensory innervation by spinal nervesgeneral
  trends

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Plexuses intermingling nerves arising from
multiple anterior rami
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Branches of Spinal Nerves

• Dorsal Ramus
• innervate deep muscles of the trunk responsible
  for movements of the vertebral column and skin
  near dorsal midline
• Ventral 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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Cervical Plexus

• C1-C4
• Innervates superficial neck structures, skin of
  neck, posterior portion of head
• Phrenic nerve
• Innervate diaphragm
• Sole motor innervation of diaphragm

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Brachial Plexus

• C5-T1 and some from C4
• Branches/nerves
• Axillary
• Radial
• Musculocutaneous
• Ulnar
• Median

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Lumbar Plexus

• Lumbar plexus ventral rami of L1-L4
• Major Nerve
• 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
• Sciatic
• Tibial
• Common fibular (peroneal)

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Cross Section of Spinal Cord
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Cross Section of Spinal Cord

• White 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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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 ANScell bodies of visceral motor
  neurons
• distinct lateral horn not present in all regions
  of cord

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Cross section of Spinal Cord, cont.

• 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)
• ganglion cell bodies of sensory neurons

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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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Cross section of Spinal Cord, cont.

• 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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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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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 brain
• Epidural Space
• Contains blood vessels, areolar CT and fat.
• Dura mater
• continuous with epineurium of the spinal nerves
• Arachnoid mater thin and wispy
• Subarachnoid 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
  coccyxlongitudinal support
• denticulate ligaments attach the spinal cord to
  the dura mater laterallylateral 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 Barrierblood 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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Reflexes

• Automatic 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 Components

1. Electrical Impulse (Action potentials) produced
   in sensory receptors transmitted to
2. Sensory neuron. To-
3. Interneuronsin 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
• Stretch Receptor (Muscle spindle)
• Sensory neuron
• synapse with motor neurons of the spinal cord
• Motor neuron innervates muscle that was stretch
  causing contraction

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Stretch/Extensor Reflex
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Withdrawal/Flexor ReflexFunction 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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Withdrawal Reflex with Crossed Extensor Reflex
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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