Neural pathways

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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

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Figure 15.1 An Overview of Neural Integration
Figure 15.1
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Sensory receptor

• Specialized cell or cell process that monitors
  specific conditions
• Arriving information is a sensation
• Awareness of a sensation is a perception

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Senses

• General senses
• Pain
• Temperature
• Physical distortion
• Chemical detection
• Receptors for general senses scattered throughout
  the body
• Special senses
• Located in specific sense organs
• Structurally complex

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Sensory receptors

• Each receptor cell monitors a specific receptive
  field
• Transduction
• A large enough stimulus changes the receptor
  potential, reaching generator potential

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Receptors

• Tonic receptors
• Always active
• Slow acting receptors
• Phasic receptors
• Provide information about the intensity and rate
  of change of a stimulus
• Fast acting receptors
• Adaptation
• Reduction in sensitivity in the presence of a
  constant stimulus

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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

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Figure 15.2 Receptors and Receptive Fields
Figure 15.2
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Thermoceptors and mechaniceptors

• Found in the dermis
• Mechaniceptors
• Sensitive to distortion of their membrane
• Tactile receptors (six types)
• Baroreceptors
• Proprioceptors (three groups)

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Figure 15.3 Tactile Receptors in the Skin
Figure 15.3a-f
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Chemoreceptors

• Chemoreceptors
• Carotid bodies
• Aortic bodies

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Figure 15.4 Baroreceptors and the Regulation of
Visceral Function
Figure 15.4
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Figure 15.5 Chemoreceptors
Figure 15.5
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SECTION 15-3 The Organization of Sensory
Pathways
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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

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Somatic sensory pathways

• Three major pathways carry sensory information
• Posterior column pathway
• Anterolateral pathway
• Spinocerebellar pathway

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Figure 15.6 Sensory Pathways and Ascending
Tracts in the Spinal Cord
Figure 15.6
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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
• Decussation

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Figure 15.8 The Posterior Column Pathway and the
Spinothalamic Tracts
Figure 15.8a, b
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Figure 15.8 The Posterior Column Pathway and the
Spinothalamic Tracts
Figure 15.8c
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SECTION 15-4 The Somatic Nervous System
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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

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Figure 15.10 Descending (Motor) Tracts in the
Spinal Cord
Figure 15.10
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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

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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

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Figure 15.11 The Corticospinal Pathway
Figure 15.11
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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

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Figure 15.12 Centers of Somatic Motor Control
Figure 15.12