Chapter 13
Gross Anatomy of the Spinal Cord
▪About 18 inches long, ½ in wide
▪Posterior median sulcus
▪Anterior median fissure
▪Overall Organization
▫Enlargements- expanded amounts of gray matter.
-cervical
-lumbar
▫Conus medullaris- tapered conical region inferior to lumbar enlargement
-Filum terminale
▫Dorsal roots
▫Dorsal root ganglia
▫Ventral roots
▫Spinal nerves
▫Naming- From T1 down, spinal nerves associate with vertebrae above it.
From base of skull to C7, spinal nerves associate with vertebrae below it.
▫Cauda Equina
▪Spinal meninges- series of specialized membranes surrounding the spinal cord
▫Dura mater- tough fibrous layer that forms the outermost covering of the
spinal cord.
-epidural space
-coccygeal ligament
▫Arachnoid mater- middle meningeal layer. Contains a delicate network of
collagen and elastic fibers that extends between the arachnoid membrane
and the surface of the pia mater.
-Subarachnoid space
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▫Pia mater- delicate innermost meningeal layer, bound firmly with the
underlying neural tissue
-Denticulate ligaments
▪Sectional Anatomy of Spinal Cord: Organization of Gray Matter
▫Nuclei- cell bodies of neurons in the gray matter of the spinal cord.
-Sensory Nuclei
-Motor nuclei
▫Posterior gray horns
▫Anterior gray horns
▫Lateral gray horns
▫Gray commissures
▪Sectional Anatomy of Spinal Cord: Organization of White Matter
▫Posterior white column
▫Anterior white column
▫Anterior white commissure
▫Lateral white column
▫Ascending tracts
▫Descending tracts
Spinal Nerves
▪Anatomy
▫Epineurium, fascicles, perineurium, endoneurium
▪Peripheral Distribution of Spinal Nerves
▫Motor Fibers (fig. 13-8)
▫Sensory Fibers (fig. 13-8)
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▫Rami Communicantes-White Ramus-
-Gray Ramus-
▫Dorsal Ramus-
▫Ventral Ramus-
▫Dermatones
▪Nerve Plexuses- complex interwoven network of nerves. 4 major plexuses
▫Cervical plexus- ventral rami of spinal nerves C1-C5 (fig. 13-10)
▫Brachial plexus- ventral rami of spinal nerves C5-T1 (fig. 13-11)
▫Lumbar plexus- ventral rami of spinal nerves T12-L4 (fig. 13-12)
▫Sacral plexus- ventral rami of spinal nerves L4-S4 (fig. 13-12)
Principles of Functional Organization
▪Neuronal pools- functional groups of interconnected neurons. 5 neural circuits
1. Divergence
2. Convergence
3. Serial processing
4. Parallel processing
5. Reverberation
▪Reflexes- rapid, automatic responses to specific stimuli
▫Reflex arc- “wiring” of a single reflex.
-5 Steps in a neural reflex (fig. 13-14)
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▫Classification of reflexes- can be classified by their development, site of information
processing, nature of the resulting motor response, and the complexity of the neural
circuit involved.
-Development of Reflexes
-Innate
-Acquired
-Nature of Response
-Somatic
-Visceral
-Complexity of the Circuit
-Monosynaptic reflex
-Polysynaptic reflex
-Processing Site
-Spinal or Cranial
Spinal Reflexes
▪Monosynaptic Reflexes: The Stretch Reflex- provides automatic regulation of skeletal muscle
length.
▫Muscle spindles- a bundle of small, specialized skeletal muscle fibers called intrafusal
muscle fibers, surrounded by extrafusal muscle fibers.
▫Each intrafusal muscle fiber is innervated by both sensory and motor neurons.
▫Gamma efferent motor neurons- motor neurons innervating intrafusal fibers.
▫Stretching of the sensory region of the intrafusal fibers stimulates the sensory neuron,
which in turn stimulates the motor neuron, causing a contraction.
▫Postural reflexes- stretch reflexes that help to maintain a normal upright posture.
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▫Gamma efferents stimulate the myofibrils at the ends of the intrafusal fibers, causing
them to maintain the normal length of the region, allowing them to remain sensitive to
externally imposed changes to muscle length.
▪Polysynaptic Reflexes
▫Tendon Reflex- monitors the external tension produced during a muscular contraction,
and prevents tearing or breaking of the tendons.
-sensory receptors have not been identified.
-receptors are stimulated when collagen fibers are stretched to a dangerous
degree.
-inhibitory interneurons inhibit muscle contraction, muscles generally cannot
develop enough tension to break their tendons.
▫Withdrawal Reflexes- moves affected parts of the body away from a source of
stimulation.
-Flexor Reflex- affects the muscles of a limb.
-Stimulus is received; sensory neurons activate interneurons in the spinal
cord, which then stimulated motor neurons, resulting in a contraction of
a flexor muscle, flexes the limb.
-Reciprocal inhibition- when one set of motor neurons are stimulated,
those neurons that control antagonistic muscles are inhibited.
-Crossed Extensor Reflexes- motor response occurs on the side opposite the
stimulus, complements the flexor reflex, they occur simultaneously.
-reverberating circuits use positive feedback to ensure that the
movement lasts long enough to be effective.
▫Ipsilateral reflex arcs
▫Contralateral reflex arc
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▫General Characteristics of Polysynaptic reflexes.
1. Involve pools of interneurons
2. Intersegmental in distribution
3. Involve reciprocal inhibition
4. Have reverberating circuits, which prolong the reflexive motor response.
5. Several reflexes may cooperate to produce a coordinated controlled response.
Integration and Control of Spinal Reflexes
▪The brain can facilitate or inhibit reflex motor patterns based in the spinal cord.
▫Descending tracts starting in the brain can produce EPSP’s or IPSP’s at the postsynaptic
membrane, facilitating or inhibiting the reflex.
▪Motor control involves a series of interacting levels.
▫Descending tracts starting in the brain can activate complex patterns of motor activity,
like walking, running, or jumping.
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