Chapter 13: The Spinal Cord, Spinal Nerves, and Spinal Reflexes

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Chapter 13 The Spinal Cord, Spinal Nerves,
and Spinal Reflexes
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General Organization of the Nervous System

• Highly organized, very efficient

Figure 131
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Nervous System

• Organization
• CNS brain and spinal cord
• PNS all other neural tissue
• Structures in the PNS
• 1. Ganglia collection of somas together in one
  place
• 2. Nerves bundles of axons
• Structures in the CNS
• 1. Center collection of somas with a common
  function
• 2. Nucleus a center with a visible boundary
• 3. Neural cortex gray matter (somas) covering
  the brains
• Tracts bundles of axons with common origins,
• destinations and function
• 5. Columns/funiculi large tracts in the spinal
  cord
• Pathways centers and tracts that link brain
  with body
• Sensory pathways receptor ? CNS
• Motor pathways CNS ? effector

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Gross Anatomy of the Spinal Cord

• 45cm (18) from brain to L2
• Inside vertebral canal
• Stacked vertebral foramen
• Surrounded by CT
• Spinal Meninges
• Support spinal cord
• Protect spinal cord

Figure 13-2
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Spinal Meninges

• Three Layers
• 1. Dura Mater
• 2. Arachnoid
• 3. Pia Mater
• Functions
• protect spinal cord
• carry blood supply
• All three layers are continuous with cranial
  meninges of the same name and the connective
  tissue around the spinal nerves.

Figure 133
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Spinal Meninges

• Dura Mater Outermost
• Dense collagen fibers
• Attaches to periosteum of occipital bone and
  coccyx by coccygeal ligament
• Surrounded by the epidural space which contains
  blood vessels and adipose
• Arachnoid Middle
• Two layers
• Arachnoid membrane
• - Simple squamous epithelium, contacts dura mater
• Arachnoid tradeculae
• Collagen and elastin fibers that bind to pia
  mater
• Fibers pass through the subarachoid space which
  contains cerebrospinal fluid
• CSF for shock absorption and diffusion medium

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Spinal Meninges

• 3. Pia Mater Innermost
• Fine mesh of collagen and elastin fibers bound to
  neural tissue
• Attached to arachnoid trabeculae
• Has dentriculate ligaments that extend through
  arachnoid to dura mater to prevent lateral
  movement of the cord

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Anatomy of the Spinal Cord
Figure 135a
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Anatomy of the Spinal Cord

• Posterior median sulcus
• Posterior surface bears a shallow longitudinal
  groove
• Anterior median fissure
• Deeper groove along the anterior surface
• Central canal contains CSF for diffusion
• Gray matter somas, neuroglia, unmyelinated axons
• Posterior gray horn
• somatic and visceral sensory nuclei
• Anterior gray horn
• somatic motor nuclei
• Lateral gray horn
• throacic and lumbar only, visceral motor nuclei
  (ANS)
• Gray commissure
• axons for decussation (cross over to other side)

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Anatomy of the Spinal Cord

• White Matter myelinated axons
• Posterior white column/funiculus
• Anterior white column/funiculus
• Lateral white column/funiculus
• All 6 column contains tracts
• Ascending tracts sensory to brain
• Descending tracts motor from brain
• Transverse tracts decussation
• Axons exit as spinal roots (dorsal ventral
  spinal nerve)
• Dorsal root sensory axons from receptor to CNS
• Dorsal root ganglion somas of the sensory
  neurons
• Ventral root motor axons from CNS to effectors

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KEY CONCEPT

• Spinal cord has a narrow central canal
• surrounded by gray matter
• containing sensory and motor nuclei
• Sensory nuclei are dorsal
• Motor nuclei are ventral
• Gray matter
• is covered by a thick layer of white matter
• White matter
• consists of ascending and descending axons
• organized in columns
• containing axon bundles with specific functions

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Anatomy of the Spinal Cord

• Spinal roots exit vertebral canal through
  intervertebral foramen
• Dorsal and ventral roots combine to form spinal
  nerve

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Damage to which root of a spinal nerve would
interfere with motor function?

1. posterior root
2. anterior root
3. dorsal root
4. ventral root

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Where is the cerebrospinal fluid that surrounds
the spinal cord located?

1. epidural space
2. subarachnoid space
3. above the dura mater
4. between the pia mater and the nervous tissue of
   the brain

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Spinal Nerves

• 31 pairs
• Exit via intervertebral or sacral foramen
• Name for location of exit on spin beginning
  between skull and C1
• Nerves
• C1-C8
• T1-T12
• L1-L5
• S1-S5
• Co1

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Spinal Nerves

• Cord and column grow together until age 4
• Column continues to grow but cord does not
• Roots stretch to reach foramen
• Adult cord ends at L1-L2
• stretched spinal roots after L2
• cauda equina
• Lumbar puncture
• spinal tap at L3-L4 draw CSF from subarachnoid
  space

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Intervertebral foramen Maintained by
intervertebral Discs between vertebrae
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• Herniated disc
• Nucleus pulposus ruptures through anulus fibrosis
• Compresses nerves in intervertebral foramen
  and/or spinal cord in vertebral canal
• Slipped disc
• Intervertebral disc distorted or displaced
• Causes pressure

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Nerve Structure

• Dorsal root ventral root blood vessels
  connective tissue
• Epineurium
• Outermost layer
• Dense collagen fibers
• Perineurium
• - Partitions that extend inward from epineurium
  and divide axons into bundles called fascicles
• Endoneurium
• Innermost layer
• Thin collagen fibers that surround each axon

Figure 136
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Nerve Structure

• Axons repair if cut , it will follow original
  path
• Severed nerves do not usually repair
• Axons do not line up correctly
• Spinal nerves branch off cord near to what they
  innervate
• Cervical and lumbar enlargements of cord house
  cell bodies of motor neurons for muscles of
  appendages
• Dermatome Region of skin surface
• Innervate by one pair spinal nerves

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Dermatomes
Figure 138
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Nerve Plexus

• Most spinal nerves do not go directly to target
• Axons from multiple nerves intermingle in a nerve
  plexus

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Spinal Nerve Plexus

• Nerve plexus
• Interwoven network of nerves
• 1. Cervical plexus
• Nerves C1-C5
• Innervate muscles of neck and diaphragm
• 2. Brachial plexus
• Nerve C5-T1
• Innervate pectoral girdle and upper limbs
• 3. Lumbar plexus
• Nerves T12-L4
• Innervate pelvic girdle and lower limbs
• 4. Sacral plexus
• Nerves L4-S4
• Innervate lower limbs

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Trauma and Disorders

• Often result form damage or pressure
• 1. Paralysis
• Loss of motor function
• Disorder of ventral root or anterior gray horn
• 2. Paresthesias
• Sensory loss
• Disorder of dorsal root or posterior gray horn
• Complete transection results in loss of both
  motor and sensory below injury
• 3. Paraplegia
• Sever between T1 and L4
• Loss of lower limb function
• 4. Quadriplegia
• Sever in cervical
• Loss of all limb function (above C5 can kill)

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Organization of Neural Pathways

• 10 million sensory neurons
• receptor to CNS
• 500 thousand motor neurons
• CNS to effector
• 20 billion interneurons
• coordinate sensory and motor, they are between
  sensory and motor neurons located in the CNS
• Interneurons organized into neuronal pools
• functional groups with limited input sources
  (sensory) and output locations (motor)
• Spread of info organized into neural circuits

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5 Neural Circuits
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Reflexes

• Rapid, automatic nerve responses triggered by
  specific stimuli
• Used to maintain homeostasis
• Simple reflex
• Sensory perception in, motor response out
• Simple reflexes can be grouped together for
  complex actions
• Reflex arc single reflex

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5 Steps in a Neural Reflex
Figure 1314
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5 Steps in a Neural Reflex

• Step 1 Stimulus Activates Receptor
• Receptor specialized cell or dendrite of
  sensory neuron
• Receptor responds to a particular type of stimuli
• physical or chemical changes
• Step 2 Sensory Neuron is Activated
• Stimulation causes action potential on axon of
  sensory neuron
• Nerve impulse travels into spinal cord via dorsal
  root

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5 Steps in a Neural Reflex

• Step 3 Information Processing
• Simple case sensory neuron synapses on motor
  neuron
• More complex sensory neuron synapses on an
  interneuron
• Sensory neuron causes EPSP which is integrated
  with other stimuli
• Step 4 Motor Neuron is Activated
• Motor neuron is stimulated to threshold
• Action potential travels down motor neuron axon
  to effector
• Step 5 Peripheral effector responds
• Neurotransmitters released by motor neuron
  trigger events in effector
• E.g. muscle contraction

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Reflex Arcs

• Negative Feedback
• Action opposes stimulus as form of defense
• Fast response, but not always coordinated

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Reflex Classification 4 Ways to Classify
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4 Classifications of Reflexes

1. By early development
2. By type of motor response
3. By complexity of neural circuit
4. By site of information processing

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Response

• Nature of resulting motor response
• somatic reflexes
• involuntary control of nervous system
• superficial somatic reflexes
• Stimuli originate at skin or mucous membranes
• stretch reflexes (deep tendon reflexes)
• Stimuli from overstretched tendon
• e.g., patellar reflex
• visceral reflexes (autonomic reflexes)
• control systems other than muscular system

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Complexity

• Complexity of neural circuit
• monosynaptic reflex
• sensory neuron synapses directly onto motor
  neuron
• polysynaptic reflex
• at least 1 interneuron between sensory neuron and
  motor neuron
• Response delayed by each synapse but capable of
  more complex output

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Examples of Common Spinal Reflexes

• Patellar Reflex
• Withdrawal Reflex

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1. Patellar Reflex

• Monosynaptic stretch reflex
• Carried on type A fibers largest myelinated
  axons
• Carries action potential at the fastest rate
• Sudden stretch of patellar ligament activates
  muscle spindle ? signal quadriceps group to
  contract

Figure 1315
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Stretch Reflex

• Muscle spindles sensory receptors involved in
  the stretch reflex
• Consist of a bundle of small, specialized
  skeletal muscle fibers called intrafusal muscle
  fibers
• Innervated by sensory and motor neurons
• Surrounded by a larger extrafusal muscle fiber
  responsible for
• The resting muscle tone
• The contraction of the entire muscle, when
  greater levels of stimulation are present

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A Muscle Spindle
Axons of motor neurons innervating intrafusal
fibers
Figure 1316
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Muscle SpindleSpecialized Muscle Fiber

• Constantly signal CNS
• Relaxed signal less
• Stretched signal more ?
• threshold, trigger reflex arc
• Prevent overstretching of muscle and tendons
• Aid in maintaining upright position

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2. Withdrawal Reflexes

• Complex polysynaptic spinal reflex
• Consists of three parts
• Flexor reflex flex to withdraw
• Reciprocal Inhibition inhibit extensors
• Crossed extensor reflex maintain balance

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Withdrawal Reflexes

• Pain ? flexor muscles pull limb away
• ? extensors same limb inhibited
• to prevent opposition to flexion
• ? limbs on opposite side extend
• to provide balance for sudden
• flexion

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A Flexor Reflex
Figure 1317
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Reflexes

• Reflexes automatic but can be impacted by higher
  brain centers
• Fine tune or combine reflexes
• Take cues from reflex for coordinated voluntary
  movements
• Facilitate or inhibit reflexes
• Reflexes serve as diagnostic tool to assess
  health and function of spinal cord and brain

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Integration and Control of Spinal Reflexes

• Though reflex behaviors are automatic
• processing centers in brain can facilitate or
  inhibit reflex motor patterns based in spinal cord

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Voluntary Movements and Reflex Motor Patterns

• Higher centers of brain incorporate lower,
  reflexive motor patterns
• Automatic reflexes
• can be activated by brain as needed
• use few nerve impulses to control complex motor
  functions
• walking, running, jumping

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KEY CONCEPT

• Reflexes
• rapid, automatic responses to stimuli
• buy time to plan and execute complex, conscious
  responses
• Somatic motor reflexes
• fastest reflexes
• involve myelinated axons
• involve only 1 spinal cord segment or brain
  nucleus
• are monosynaptic

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SUMMARY

• General organization of nervous system
• CNS, PNS
• Afferent (sensory) and efferent (motor) fibers
• Structures and functions of spinal meninges
• Gray matter and horns of spinal cord
• 4 major nerve plexuses
• cervical, brachial, lumbar, sacral
• Neuronal pools and neural circuit patterns
• divergence, convergence, serial, parallel,
  reverberation

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SUMMARY

• Reflexes and reflex arcs
• Classifications of reflexes
• innate vs. acquired
• somatic vs. visceral
• cranial vs. spinal
• monosynaptic, polysynaptic, or intersegmental
• Characteristics of monosynaptic reflexes
• stretch reflex, postural reflex, muscle spindles
• Characteristics of polysynaptic reflexes
• tendon, withdrawal, flexor, and crossed extensor
  reflexes