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The Spinal Cord and Spinal Nerves

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Caudae equinae (horse's tail) dorsal and ventral roots of lowest spinal nerves. Spinal segment ... Step on tack (pain fibers send signal to spinal cord) ... – PowerPoint PPT presentation

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Title: The Spinal Cord and Spinal Nerves


1
Chapter 13
The Spinal Cord and Spinal Nerves
2
INTRODUCTION
  • 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
    (excitatory) and IPSPs (inhibitory) 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.

3
External (Gross) Anatomy of Spinal Cord
  • Flattened cylinder
  • Continuation of the medulla oblongata
  • 42 to 45 cm (16-18 inches) long and 2cm (.75
    inch) in diameter
  • Cervical enlargement
  • upper limbs
  • Lumbar enlargement
  • lower limbs
  • 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 and ventral roots of lowest spinal nerves
  • Spinal segment
  • area of cord from which each pair of spinal
    nerves arises

4
Structures Covering and Protecting the Spinal Cord
  • Vertebrae
  • The vertebral column provides a bony covering of
    the spinal cord.
  • Epidural space filled with fat
  • The meninges are three coverings that run
    continuously around the spinal cord and brain
  • Dura mater (outer layer)
  • dense irregular connective tissue tube
  • Subdural space filled with interstitial fluid
  • Arachnoid (middle layer)
  • - spider web of collagen fibers
  • Subarachnoid space filled with cerebrospinal
    fluid
  • Pia mater (inner layer)
  • thin layer covers blood vessels
  • denticulate ligiments hold in place
  • Spinal cord-fixed in place
  • Anchored -superiorly to the brain
  • -laterally to the denticulate ligaments
  • inferiorly to the coccyx

5
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.
  • 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.

6
Spinal Cord and Spinal Nerves
  • Spinal nerves begin as roots
  • Dorsal or posterior root entry for sensory fibers
  • dorsal root ganglion (swelling) cell bodies of
    sensory nerves
  • Ventral or anterior root site of outgoing motor
    fibers

7
Internal Anatomy Gray Matter 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.
  • 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.
  • paired dorsal and ventral gray horns
  • lateral horns only present in thoracic spinal
    cord
  • gray commissure crosses the midline
  • Central canal is continuous with fourth ventricle
    of brain

8
Internal Anatomy White Matter of the Spinal Cord
  • White matter covers gray matter
  • The white matter consists of bundles of
    myelinated axons of motor and sensory 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.
  • Anterior median fissure deeper than Posterior
    median sulcus
  • Anterior, Lateral and Posterior White Columns
    contain axons that form ascending and descending
    tracts

9
SPINAL CORD PHYSIOLOGY
  • The spinal cord has two principal functions.
  • The white matter tracts are pathways for nerve
    impulse conduction to and from the brain.
  • The gray matter receives and integrates incoming
    and outgoing information.

10
Sensory and Motor Tracts
  • 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.
  • Naming of tracts
  • indicates position and direction of signal
  • example anterior spinothalamic tract
  • impulses travel from spinal cord towards brain
    (thalamus)
  • found in anterior part of spinal cord

11
Functions of Spinal Tracts
  • SENSORY
  • Spinothalamic tract
  • pain, temperature, deep pressure and crude touch
  • Posterior columns
  • proprioception, discriminative touch, two-point
    discrimination, pressure and vibration
  • MOTOR
  • Direct pathways (corticospinal and corticobulbar)
  • precise, voluntary movements
  • Indirect pathways (rubrospinal, vestibulospinal)
  • programs automatic movements, posture and muscle
    tone, equilibrium and coordination of visual
    reflexes

12
Reflexes and the Reflex Arc
  • The spinal cord serves as an integrating center
    for spinal reflexes. This occurs in the gray
    matter.
  • 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
    autonomic
  • Specific nerve impulse pathway
  • 5 components of reflex arc
  • receptor
  • sensory neuron
  • integrating center
  • motor neuron
  • effector
  • Somatic spinal reflexes include the stretch
    reflex, tendon reflex, flexor (withdrawal)
    reflex, and crossed extensor reflex all exhibit
    reciprocal innervation.

13
Stretch (Patellar Reflex) 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 and muscle contracts
  • Brain sets muscle spindle sensitivity as it sets
    muscle tone (degree of muscle contraction at
    rest)
  • Reciprocal innervation (polysynaptic-
    interneuron)
  • antagonistic muscles relax as part of reflex

14
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 and muscle are protected
  • Reciprocal innervation (polysynaptic)
  • causes contraction of ipsilateral muscle group

15
Flexor and Crossed Extensor Reflexes
  • The flexor (withdrawal) reflex is ipsilateral and
    is a protective withdrawal reflex that moves a
    limb to avoid pain.
  • 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.

16
Flexor (withdrawal) Reflex
  • Step on tack (pain fibers send signal to spinal
    cord)
  • Interneurons branch to different spinal cord
    segments
  • Motor fibers in several segments are activated
  • More than one muscle group activated to lift foot
    off of tack

17
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.

18
Clinical Considerations
  • Reflexes-
  • diagnosis of nervous system disorders or damaged
    tissue
  • Superfiscial reflexes
  • withdrawal reflexes-tactile or noxious stimuli
  • corneal reflex abdominal reflex gag reflex
    pharyngeal reflex.
  • Deep tendon reflexes (stretch stimulus to a
    tendon)
  • integrity or function of reflex arc and spinal
    cord
  • patellar reflex(knee jerk) Achilles reflex
    (ankle jerk) abdominal reflex.

19
Dermatomes
  • 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
  • Knowledge of dermatomes helps determine which
    segment of the spinal cord or which spinal nerve
    is malfunctioning.
  • Skin on face supplied by Cranial Nerve V

20
Connective Tissue Coverings of Spinal Nerves
  • 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
    foramen
  • Numerous blood vessels are within the coverings

21
Spinal Nerves
  • Spinal nerves connect the CNS to sensory
    receptors, muscles, and glands and are part of
    the peripheral nervous system.
  • 31 Pairs of spinal nerves
  • Named and numbered by the cord level of their
    origin
  • 8 pairs of cervical nerves (C1 to C8)
  • 12 pairs of thoracic nerves (T1 to T12) (T2-12
    do not enter into plexuses)
  • 5 pairs of lumbar nerves (L1 to L5)
  • 5 pairs of sacral nerves (S1 to S5)
  • 1 pair of coccygeal nerves
  • Mixed sensory and motor nerves

22
Branching of Spinal Nerves
  • Spinal nerves formed from dorsal and ventral
    roots
  • Spinal nerves branch into dorsal and ventral rami
  • dorsal rami supply skin and muscles of back
  • ventral rami form plexus supply anterior trunk
    and limbs
  • meningeal branches supply meninges, vertebrae and
    blood vessels
  • rami-communicantes branches associated with the
    autonomic nervous system

23
A Nerve Plexus
  • Joining of ventral rami of spinal nerves to form
    nerve networks or plexuses
  • Found in neck, arm, low back and sacral regions
  • No plexus (T2-T12) in thoracic region
  • intercostal nerves
  • innervate intercostal spaces
  • T7 to T12 supply abdominal wall as well

24
Disorders
DISORDERS HOMEOSTATIC IMBALANCES
  • Neuritis
  • inflammation of nerves
  • caused by injury, vitamin deficiency or poison
  • sciatica injury to the sciatic nerve and its
    branches results in sciatica, pain that extends
    from the buttock down the back of the leg
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