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Chapter 13 The Spinal Cord & Spinal Nerves Lecture Outline


Chapter 13 The Spinal Cord & Spinal Nerves Lecture Outline INTRODUCTION The spinal cord and spinal nerves mediate reactions to environmental changes. – PowerPoint PPT presentation

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Title: Chapter 13 The Spinal Cord & Spinal Nerves Lecture Outline

Chapter 13
  • The Spinal Cord Spinal Nerves
  • Lecture Outline

  • The spinal cord and spinal nerves mediate
    reactions to environmental changes.
  • The spinal cord has several functions.
  • It processes reflexes.
  • It is the site for integration of EPSPs and IPSPs
    that arise locally or are triggered by nerve
    impulses from the periphery and brain.
  • It is a conduction pathway for sensory and motor
    nerve impulses.
  • The size of the vertebral canal varies in
    different regions of the vertebral column and
    affects spinal cord injuries.

Chapter 13The Spinal Cord Spinal Nerves
  • Together with brain forms the CNS
  • Functions
  • spinal cord reflexes
  • integration (summation of inhibitory and
    excitatory) nerve impulses
  • highway for upward and downward travel of sensory
    and motor information

  • The spinal cord is protected by two connective
    tissue coverings, the meninges and vertebra, and
    a cushion of cerebrospinal fluid.
  • The vertebral column provides a bony covering of
    the spinal cord (Figure 13.1c).

Spinal Cord Protection
By the vertebral column, meninges, cerebrospinal
fluid, and vertebral ligaments.
  • The meninges are three coverings that run
    continuously around the spinal cord and brain
    (Figures 13.1a, 14.4a).
  • The outermost layer is the dura mater.
  • The middle layer is the arachnoid.
  • The innermost meninx is the pia mater, a thin,
    transparent connective tissue layer that adheres
    to the surface of the spinal cord and brain
  • Denticulate ligaments are thickenings of the pia
    mater that suspend the spinal cord in the middle
    of its dural sheath.

Structures Covering the Spinal Cord
  • Vertebrae
  • Epidural space filled with fat
  • Dura mater
  • dense irregular CT tube
  • Subdural space filled with interstitial fluid
  • Arachnoid spider web of collagen fibers
  • Subarachnoid space CSF
  • Pia mater
  • thin layer covers BV
  • denticulate ligs hold in place

Structures Covering the Spinal Cord
  • Vertebrae
  • Epidural space filled with fat
  • Dura mater
  • dense irregular CT tube
  • Subdural space filled with interstitial fluid
  • Arachnoid spider web of collagen fibers
  • Subarachnoid space CSF
  • Pia mater
  • thin layer covers BV
  • denticulate ligs hold in place

  • The subarachnoid space is between the arachnoid
    mater and pia mater and contains cerebrospinal
    fluid (CSF).
  • Inflammation of the meninges is known as
  • Removal of cerebrospinal fluid from the
    subarachnoid space is called a spinal tap (lumbar
    puncture). This procedure is used to diagnose
    pathologies and to introduce antibiotics,
    contrast media, anesthetics, and chemotherapeutic

External Anatomy of the Spinal Cord
  • The spinal cord begins as a continuation of the
    medulla oblongata and terminates at about the
    second lumbar vertebra in an adult (Figure 13.2).
  • It contains cervical and lumbar enlargements that
    serve as points of origin for nerves to the
  • The tapered portion of the spinal cord is the
    conus medullaris, from which arise the filum
    terminale and cauda equina.

External Anatomy of Spinal Cord
  • Flattened cylinder
  • 16-18 Inches long 3/4 inch diameter
  • In adult ends at L2
  • In newborn ends at L4
  • Growth of cord stops at age 5
  • Cervical enlargement
  • upper limbs
  • Lumbar enlargement
  • lower limbs

Inferior End of Spinal Cord
  • Conus medullaris
  • cone-shaped end of spinal cord
  • Filum terminale
  • thread-like extension of pia mater
  • stabilizes spinal cord in canal
  • Caudae equinae (horses tail)
  • dorsal ventral roots of lowest spinal nerves
  • Spinal segment
  • area of cord from which each pair of spinal
    nerves arises

Spinal tap or Lumbar Puncture
  • Technique
  • long needle into subarachnoid space
  • safe from L3 to L5
  • Purpose
  • sampling CSF for diagnosis
  • injection of antibiotics, anesthetics or
  • measurement of CSF pressure

Spinal nerves
  • The 31 pairs of spinal nerves are named and
    numbered according to the region and level of the
    spinal cord from which they emerge (Figure 13.2).
  • 8 pairs of cervical nerves,
  • 12 pairs of thoracic nerves,
  • 5 pairs of lumbar nerves,
  • 5 pairs of sacral nerves, and
  • 1 pair of coccygeal nerves.
  • Spinal nerves are the paths of communication
    between the spinal cord and most of the body.
  • Roots are the two points of attachment that
    connect each spinal nerve to a segment of the
    spinal cord (Figure 13.3a).

Spinal Cord Spinal Nerves
  • Spinal nerves begin as roots
  • Dorsal or posterior root is incoming sensory
  • dorsal root ganglion (swelling) cell bodies of
    sensory nerves
  • Ventral or anterior root is outgoing motor fibers

Internal Anatomy of the Spinal Cord
  • The anterior median fissure and the posterior
    median sulcus penetrate the white matter of the
    spinal cord and divide it into right and left
    sides (Figure 13.3b).
  • The gray matter of the spinal cord is shaped like
    the letter H or a butterfly and is surround by
    white matter.
  • The gray matter consists primarily of cell bodies
    of neurons and neuroglia and unmyelinated axons
    and dendrites of association and motor neurons.
  • The gray commissure forms the cross bar of the
    H-shaped gray matter.
  • The white matter consists of bundles of
    myelinated axons of motor and sensory neurons.

Gray Matter of the Spinal Cord
  • Gray matter is shaped like the letter H or a
  • contains neuron cell bodies, unmyelinated axons
  • paired dorsal and ventral gray horns
  • lateral horns only present in thoracic spinal
  • gray commissure crosses the midline
  • Central canal is continuous with 4th ventricle of

Internal Anatomy of the Spinal Cord
  • Anterior to the gray commissure is the anterior
    white commissure, which connects the white matter
    of the right and left sides of the spinal cord.
  • The gray matter is divided into horns, which
    contain cell bodies of neurons.
  • The white matter is divided into columns.
  • Each column contains distinct bundles of nerve
    axons that have a common origin or destination
    and carry similar information.
  • These bundles are called tracts.

White Matter of the Spinal Cord
  • White matter covers gray matter
  • Anterior median fissure deeper than Posterior
    median sulcus
  • Anterior, Lateral and Posterior White Columns
    contain axons that form ascending descending

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  • The spinal cord has two principal functions.
  • The white matter tracts are highways for nerve
    impulse conduction to and from the brain.
  • The gray matter receives and integrates incoming
    and outgoing information.

Sensory and Motor Tracts
  • Figure 13.12 shows the principal sensory and
    motor tracts in the spinal cord. These tracts
    are discussed in detail in Chapter 16 summarized
    in tables 16.3 and 16.4.
  • Sensory (ascending) tracts conduct nerve impulses
    toward the brain.
  • the lateral and anterior spinothalamic tracts and
    the posterior column tract.
  • Motor (descending) tracts conduct impulses down
    the cord.
  • Direct pathways include lateral and anterior
    corticospinal and corticobulbar tracts.
  • Indirect pathways include rubrospinal,
    tectospinal, and vestibulospinal tracts.

Tracts of the Spinal Cord
  • Function of tracts - highways for sensory motor
  • sensory tracts ascend
  • motor tracts descend
  • Naming of tracts
  • indicates position direction of signal
  • example anterior spinothalamic tract
  • impulses travel from spinal cord towards brain
  • found in anterior part of spinal cord

Location of Tracts inside Cord
  • Motor tracts Sensory tracts
  • pyramidal tract (corticospinal) ---spinothalamic
  • extrapyramidal tract ---posterior column

  • ---spinocerebellar

Function of Spinal Tracts
  • Spinothalamic tract
  • pain, temperature, deep pressure crude touch
  • Posterior columns
  • proprioception, discriminative touch, two-point
    discrimination, pressure and vibration
  • Direct pathways (corticospinal corticobulbar)
  • precise, voluntary movements
  • Indirect pathways (rubrospinal, vestibulospinal)
  • programming automatic movements, posture muscle
    tone, equilibrium coordination of visual

Reflexes and Reflex Arcs
  • The spinal cord serves as an integrating center
    for spinal reflexes. This occurs in the gray
  • A reflex is a fast, predictable, automatic
    response to changes in the environment that helps
    to maintain homeostasis.
  • Reflexes may be spinal, cranial, somatic, or

Reflex Arc
  • Specific nerve impulse pathway
  • 5 components of reflex arc
  • receptor
  • sensory neuron
  • integrating center
  • motor neuron
  • effector

Reflex Arc
  • A reflex arc is the simplest type of pathway
    pathways are specific neuronal circuits and thus
    include at least one synapse.
  • Reflexes help to maintain homeostasis by
    permitting the body to make exceedingly rapid
    adjustments to homeostatic imbalances.
  • Somatic spinal reflexes include the stretch
    reflex, tendon reflex, flexor (withdrawal)
    reflex, and crossed extensor reflex all exhibit
    reciprocal innervation.

Stretch Reflex (patellar reflex)
  • It operates as a feedback mechanism to control
    muscle length by causing muscle contraction.
  • Prevents injury from over stretching because
    muscle contracts when it is stretched
  • Monosynaptic,ipsilateral reflex arc
  • Events of stretch reflex
  • muscle spindle signals stretch of muscle
  • motor neuron activated muscle contracts
  • Brain sets muscle spindle sensitivity as it sets
    muscle tone (degree of muscle contraction at
  • Reciprocal innervation (polysynaptic-
  • antagonistic muscles relax as part of reflex

Illustration of the Stretch Reflex
Tendon Reflex
  • It operates as a feedback mechanism to control
    muscle tension by causing muscle relaxation when
    muscle force becomes too extreme.
  • ipsilateral polysynaptic reflex
  • Golgi tendon organs are in tendon
  • activated by stretching of tendon
  • inhibitory neuron is stimulated (polysynaptic)
  • motor neuron is hyperpolarized and muscle relaxes
  • Both tendon muscle are protected
  • Reciprocal innervation (polysynaptic)
  • causes contraction of ipsilateral muscle group

Illustration of Tendon Reflex
Flexor and Crossed Extensor Reflexes
  • The flexor (withdrawal) reflex is ipsilateral and
    is a protective withdrawal reflex that moves a
    limb to avoid pain (Figure 13.16).
  • This reflex results in contraction of flexor
    muscles to move a limb to avoid injury or pain.
  • The crossed extensor reflex, which is
    contralateral, helps to maintain balance during
    the flexor reflex.
  • This is a balance-maintaining reflex that causes
    a synchronized extension of the joints of one
    limb and flexion of the joints in the opposite
    limb (Figure 13.17).

Flexor (withdrawal) Reflex
  • Step on tack (pain fibers send signal to spinal
  • Interneurons branch to different spinal cord
  • Motor fibers in several segments are activated
  • More than one muscle group activated to lift foot
    off of tack

Crossed Extensor Reflex
  • Lifting left foot requires extension of right leg
    to maintain ones balance
  • Pain signals cross to opposite spinal cord
  • Contralateral extensor muscles are stimulated by
    interneurons to hold up the body weight
  • Reciprocal innervation - when extensors contract
    flexors relax, etc

Clinical Considerations
  • Checking a patients reflexes may help to detect
  • Plantar flexion reflex -- stroke the lateral
    margin of the sole
  • normal response is curling under the toes
  • abnormal response or response of children under
    18 months is called Babinski sign (upward fanning
    of toes due to incomplete myelination in child)

  • Spinal nerves connect the CNS to sensory
    receptors, muscles, and glands and are part of
    the peripheral nervous system.
  • The 31 pairs of spinal nerves are named and
    numbered according to the region and level of the
    spinal cord from which they emerge (Figure 13.2).
  • Roots of the lower lumbar, sacral, and coccygeal
    nerves are not in line with their corresponding
    vertebrae and thus form the cauda equina (Figure
  • Spinal nerves connect to the cord via an anterior
    and a posterior root (Figure 13.3a). Since the
    posterior root contains sensory axons and the
    anterior root contains motor axons, a spinal
    nerve is a mixed nerve.

Spinal Nerves
  • 31 Pairs of spinal nerves
  • Named numbered by the cord level of their
  • 8 pairs of cervical nerves (C1 to C8)
  • 12 pairs of thoracic nerves (T1 to T12)
  • 5 pairs of lumbar nerves (L1 to L5)
  • 5 pairs of sacral nerves (S1 to S5)
  • 1 pair of coccygeal nerves
  • Mixed sensory motor nerves

Connective Tissue Covering of Spinal Nerves
  • Spinal nerve axons are grouped within connective
    tissue sheathes (Figure 13.4a).
  • A fiber is a single axon within an endoneurium.
  • A fascicle is a bundle of fibers within a
  • A nerve is a bundle of fascicles within an
  • Numerous blood vessels are within the coverings.

Connective Tissue Coverings
  • Endoneurium wrapping of each nerve fibers
  • Perineurium surrounds group of nerve fibers
    forming a fascicle
  • Epineurium covering of entire nerve
  • dura mater blends into it at intervertebral

Endoneurium Perineurium -- Epineurium
Branching of Spinal Nerve
  • Spinal nerves formed from dorsal ventral roots
  • Spinal nerves branch into dorsal ventral rami
  • dorsal rami supply skin muscles of back
  • ventral rami form plexus supply anterior trunk
  • meningeal branches supply meninges, vertebrae

A Nerve Plexus
  • Joining of ventral rami of spinal nerves to form
    nerve networks or plexuses
  • Found in neck, arm, low back sacral regions
  • No plexus in thoracic region
  • intercostal nn. innervate intercostal spaces
  • T7 to T12 supply abdominal wall as well

  • The cervical plexus supplies the skin and muscles
    of the head, neck, and upper part of the
    shoulders connects with some cranial nerves and
    supplies the diaphragm (Figure 13.6, Exhibit
  • Damage to the spinal cord above the origin of the
    phrenic nerves (C3-C5) causes respiratory arrest.
  • Breathing stops because the phrenic nerves no
    longer send impulses to the diaphragm.

Cervical Plexus
  • Ventral rami of spinal nerves (C1 to C5)
  • Supplies parts of head, neck shoulders
  • Phrenic nerve (C3-C5) keeps diaphragm alive
  • Damage to cord above C3 causes respiratory arrest

Phrenic Nerve
  • The brachial plexus constitutes the nerve supply
    for the upper extremities and a number of neck
    and shoulder muscles (Figures 13.7a and b,
    Exhibit 13.2).
  • A number of nerve disorders may result from
    injury to the brachial plexus.
  • Erb-Duchene palsy or waiters tip palsy
  • Klumphes palsy
  • wrist drop
  • carpal tunnel syndrome
  • claw hand
  • winged scapula

Brachial Plexus
  • Ventral rami from C5 to T1
  • Supplies shoulder upper limb
  • Passes superior to 1st rib under clavicle
  • Axillary n. deltoid teres m.
  • Musculocutaneous n. elbow flexors
  • Radial n. shoulder elbow extensors
  • Median ulnar nn. flexors of wrist hand

Branches off Brachial Plexus
  • The lumbar plexus supplies the anterolateral
    abdominal wall, external genitals, and part of
    the lower extremities (Figure 13.9a and b,
    Exhibit 13.3).
  • The largest nerve arising from the lumbar plexus
    is the femoral nerve.
  • Injury to the femoral nerve is indicated by an
    inability to extend the leg and by loss of
    sensation in the skin over the anteromedial
    aspect of the thigh.
  • Obturator nerve injury is a common complication
    of childbirth and results in paralysis of the
    adductor muscles of the leg and loss of sensation
    over the medial aspect of the thigh.

Lumbar Plexus
  • Ventral rami of L1 to L4
  • Supplies abdominal wall, external genitals
    anterior/medial thigh
  • Injury to femoral nerve causes inability to
    extend leg loss of sensation in thigh
  • Injury to obturator nerve causes paralysis of
    thigh adductors

Branches of Lumbar Plexus
  • Notice Femoral and Obturator nerves
  • Found anterior and medial to hip joint

  • The sacral plexus supplies the buttocks,
    perineum, and part of the lower extremities
    (Figure 13.10, Exhibit 13.4).
  • The largest nerve arising from the sacral plexus
    (and the largest nerve in the body) is the
    sciatic nerve.
  • Injury to the sciatic nerve (common peroneal
    portion) and its branches results in sciatica,
    pain that extends from the buttock down the back
    of the leg.
  • Sciatic nerve injury can occur due to a herniated
    (slipped) disc, dislocated hip, osteoarthritis of
    the lumbosacral spine, pressure from the uterus
    during pregnancy, or an improperly administered
    gluteal injection.

Sacral Plexus
  • Ventral rami of L4-L5 S1-S4
  • Anterior to the sacrum
  • Supplies buttocks, perineum part of lower limb
  • Sciatic nerve L4 to S3 supplies post thigh
    all below knee
  • Peroneal nerve injury produces foot drop or
  • Tibial nerve injury produces calcaneovalgus (loss
    of function on anterior leg dorsum of foot)

Branches of Sacral Plexus
  • Notice Sciatic nerve origins

Sciatic Nerve Branches
  • Notice Common Peroneal nerve and Tibial nerve
    behind the knee
  • Notice Sciatica pain extends from the buttock
    down the leg to the foot

  • The skin over the entire body is supplied by
    spinal nerves that carry somatic sensory nerves
    impulses into the spinal cord.
  • All spinal nerves except C1 innervate specific,
    constant segments of the skin the skin segments
    are called dermatomes (Figure 13.11).
  • Knowledge of dermatomes helps a physician to
    determine which segment of the spinal cord or
    which spinal nerve is malfunctioning.
  • Skin on face supplied by Cranial Nerve V

  • Damaged regions of the spinal cord can be
    distinguished by patterns of numbness over a
    dermatome region
  • Infusing local anesthetics or cutting roots must
    be done over 3 adjacent spinal nerves.
  • Spinal cord transection
  • injury that severs the cord loss of sensation
    motor control below the injury

  • Neuritis
  • inflammation of nerves
  • caused by injury, vitamin deficiency or poison
  • Shingles
  • infection of peripheral nerve by chicken pox
  • causes pain, skin discoloration, line of skin
  • Poliomyelitis
  • viral infection causing motor neuron death and
    possible death from cardiac failure or
    respiratory arrest

Clinical Correlations
  • Erb-Duchene palsy
  • waiters tip position
  • fall on shoulder
  • Radial nerve injury
  • improper deltoid
  • injectionor tight cast
  • wrist drop
  • Median nerve injury
  • numb palm fingers inability to pronate flex
  • Ulnar nerve injury (clawhand)
  • inability to adduct/abduct fingers, atrophy of
  • Long thoracic nerve injury (winged scapula)
  • paralysis of serratus anterior, cant abduct
    above horizontal

  • end