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Neural Integration : Sensory Pathways and the Somatic Nervous System

An Overview of Sensory Pathways and the Somatic Nervous System

Neural pathways

      Afferent pathways

    Sensory information coming from the sensory receptors through peripheral nerves to the spinal cord and on to the brain

      Efferent pathways

    Motor commands coming from the brain and spinal cord, through peripheral nerves to effecter organs

Sensory Receptors and their Classification

Sensory receptor

      Specialized cell or cell process that monitors specific conditions

      Arriving information is a sensation

      Awareness of a sensation is a perception



      General senses



    Physical distortion

    Chemical detection

   Receptors for general senses scattered throughout the body

      Special senses

    Located in specific sense organs

    Structurally complex


Sensory receptors

      Each receptor cell monitors a specific receptive field


    A large enough stimulus changes the receptor potential, reaching generator potential



      Tonic receptors

    Always active

    Slow acting receptors

      Phasic receptors

    Provide information about the intensity and rate of change of a stimulus

    Fast acting receptors


    Reduction in sensitivity in the presence of a constant stimulus


The general senses

      Three types of nociceptor

    Provide information on pain as related to extremes of temperature

    Provide information on pain as related to extremes of mechanical damage

    Provide information on pain as related to extremes of dissolved chemicals

    Myelinated type A fibers carry fast pain

    Slower type C fibers carry slow pain


Thermoceptors and mechaniceptors

      Found in the dermis


    Sensitive to distortion of their membrane

   Tactile receptors (six types)


   Proprioceptors (three groups)




    Carotid bodies

    Aortic bodies




Neural Integration I: Sensory Pathways and the Somatic Nervous System

The Organization of Sensory Pathways

First, second, and third order neurons

      First order neurons

    Sensory neurons that deliver sensory information to the CNS

      Second order neurons

    First order neurons synapse on these in the brain or spinal cord

      Third order neurons

    Found in the thalamus

    Second order neurons synapse on these


Somatic sensory pathways

      Three major pathways carry sensory information

    Posterior column pathway

    Anterolateral pathway

    Spinocerebellar pathway


Posterior column pathway

      Carries fine touch, pressure and proprioceptive sensations

      Axons ascend within the fasciculus gracilis and fasciculus cuneatus

      Relay information to the thalamus via the medial lemniscus



Anterolateral pathway

      Carries poorly localized sensations of touch, pressure, pain, and temperature

      Axons decussate in the spinal cord and ascend within the anterior and lateral spinothalamic tracts

      Headed toward the ventral nuclei of the thalamus


Spinocerebellar pathway

      Includes the posterior and anterior spinocerebellar tracts

      Carries sensation to the cerebellum concerning position of muscles, tendons and joints


Visceral sensory pathways

      Carry information collected by interoceptors

      Information from cranial nerves V, VII, IX and X delivered to solitary nucleus in medulla oblongata

      Dorsal roots of spinal nerves T1 L2 carry visceral sensory information from organs between the diaphragm and pelvis

      Dorsal roots of spinal nerves S2 S4 carry sensory information below this area

The Somatic Nervous System

Somatic motor pathways

      Upper motor neuron

    Cell body lies in a CNS processing center

      Lower motor neuron

    Cell body located in a motor nucleus of the brain or spinal cord


The corticospinal pathway

      Provides voluntary skeletal muscle control

      Corticobulbar tracts terminate at cranial nerve nuclei

      Corticospinal tracts synapse on motor neurons in the anterior gray horns of the spinal cord

    Visible along medulla as pyramids



      Most of the axons decussate to enter the descending lateral corticospinal tracts

      Those that do not cross over enter the anterior corticospinal tracts

      Provide rapid direct method for controlling skeletal muscle


medial and lateral pathways

      The medial and lateral pathways

    Issue motor commands as a result of subconscious processing

      Medial pathway

    Primarily controls gross movements of the trunk and proximal limbs

    Includes the vestibulospinal tracts, tectospinal tracts and reticulospinal tracts


lateral pathways

      Lateral pathway

    Controls muscle tone and movements of the distal muscles of the upper limbs

    Rubrospinal tracts


The basal nuclei and cerebellum

      Basal nuclei adjust motor commands issued in other processing centers

      Provide background patterns of movement involved in voluntary motor movements

      Cerebellum monitors proprioceptive information, visual information and vestibular sensations


control and responses

      Levels of processing and motor control

    Spinal and cranial reflexes provide rapid, involuntary, preprogrammed responses

      Voluntary responses

    More complex

    Require more time to prepare and execute


During development

      Spinal and cranial reflexes are first to appear

      Complex reflexes develop as CNS matures and brain grows