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The Spinal Cord and Spinal Nerves

General Organization of the Nervous System

Divisions of the Nervous System


    Brain and spinal cord

    In the white matter, axons arranged in tracts and columns


    Remainder of nervous tissue

Gross Anatomy of the Spinal Cord

Adult spinal cord

      Localized enlargements provide innervation to limbs

      31 segments

    each segment has a pair of dorsal roots and a pair of ventral roots

      Filum terminale

      Conus medularis

      Spinal nerves extend off cord

    Mixed nerves


Spinal meninges

      Provide physical stability and shock absorption

      Three layers

    Dura mater


    Pia mater


Dura mater

      Covers spinal cord

      Tapers to coccygeal ligament

      Epidural space separates dura mater from walls of vertebral canal



      Interior to dura mater are the subdural space, the arachnoid and the subarachnoid space

    Subarachnoid space contains CSF


Pia mater

      Meshwork of elastin and collagen fibers

      Innermost meningeal layer

      Denticulate ligaments extend from pia mater to dura mater


Sectional anatomy of the spinal cord

      White matter is myelinated and unmyelinated axons

      Gray matter is cell bodies, unmyelinated axons and neuroglia

    Projections of gray matter toward outer surface of cord are horns


Horns of spinal cord

      Posterior gray horn contains somatic and visceral sensory nuclei

      Anterior gray horns deal with somatic motor control

      Lateral gray horns contain visceral motor neurons

      Gray commissures contain axons that cross from one side to the other


White matter

      Divided into six columns (funiculi) containing tracts

      Ascending tracts relay information from the spinal cord to the brain

      Descending tracts carry information from the brain to the spinal cord

Spinal Nerves

31 pairs of spinal nerves

      Nerves consist of:





Spinal nerves

      White ramus (myelinated axons)

      Gray ramus (unmyelinated axons that innervate glands and smooth muscle)

      Dorsal ramus (sensory and motor innervation to the skin and muscles of the back)

      Ventral ramus (supplying ventrolateral body surface, body wall and limbs)

      Each pair of nerves monitors one dermatome


Nerve plexus

      Complex interwoven network of nerves

      Four large plexuses

    Cervical plexus

    Brachial plexus

    Lumbar plexus

    Sacral plexus



Principles of Functional Organization

General organization

      Sensory neurons

    Deliver information to CNS

      Motor neurons

    Distribute commands to peripheral effectors


    Interpret information and coordinate responses


Neuronal pools

      Functional group of interconnected neurons

      Neural circuit patterns



    Serial processing

    Parallel processing



An introduction to reflexes

      Reflexes are rapid automatic responses to stimuli

      Neural reflex involves sensory fibers to CNS and motor fibers to effectors

Reflex arc

      Wiring of a neural reflex

      Five steps

    Arrival of stimulus and activation of receptor

    Activation of sensory neuron

    Information processing

    Activation of motor neuron

    Response by effector


Reflex classification

      According to


   Site of information processing

   Nature of resulting motor response

   Complexity of neural circuit

      Innate reflexes

    Result from connections that form between neurons during development

      Acquired reflexes

    Learned, and typically more complex


More reflex classifications

      Cranial reflexes

    Reflexes processed in the brain

      Spinal reflexes

    Interconnections and processing events occur in the spinal cord

      Somatic reflexes

    Control skeletal muscle

      Visceral reflexes (autonomic reflexes)

    Control activities of other systems

      Monosynaptic reflex

    Sensory neuron synapses directly on a motor neuron

      Polysynaptic reflex

    At least one interneuron between sensory afferent and motor efferent

    Longer delay between stimulus and response

Spinal Reflexes

Spinal Reflexes

      Range from simple monosynaptic to complex polysynaptic and intersegmental

    Many segments interact to form complex response


Monosynaptic Reflexes

      Stretch reflex automatically monitors skeletal muscle length and tone

    Patellar (knee jerk) reflex

      Sensory receptors are muscle spindles

      Postural reflex maintains upright position


Polysynaptic reflexes

      Produce more complicated responses

    Tendon reflex

    Withdrawal reflexes

    Flexor reflex

    Crossed extensor reflex


Polysynaptic reflexes

      Involve pools of interneurons

      Are intersegmental in distribution

      Involve reciprocal inhibition

      Have reverberating circuits to prolong the motor response

      Several reflexes may cooperate to produce a coordinated response

Integration and Control of Spinal Reflexes

Control of spinal reflexes

      Brain can facilitate or inhibit motor patterns based in spinal cord

      Motor control involves a series of interacting levels

    Monosynaptic reflexes are the lowest level

    Brain centers that modulate or build on motor patterns are the highest


Reinforcement and inhibition

      Reinforcement = facilitation that enhances spinal reflexes

      Spinal reflexes can also be inhibited

    Babinski reflex replaced by planter reflex