Sensory Pathways Overview: Neural Integration
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Presentation Transcript
Chapter 15, part 1 Neural Integration I: Sensory Pathways and the Somatic Nervous System
Learning Objectives • Specify the components of the afferent and efferent divisions of the nervous system, and explain what is meant by the somatic nervous system. • Explain why receptors respond to specific stimuli and how the organization of a receptor affects its sensitivity. • Identify the major sensory pathways.
Learning Objectives • Explain how we can distinguish among sensations that originate in different areas of the body. • Describe the components, processes and functions of the somatic motor pathways. • Describe the levels of information processing involved in motor control.
SECTION 15-1An 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
Figure 15.1 An Overview of Neural Integration Figure 15.1
Sensory receptor • Specialized cell or cell process that monitors specific conditions • Arriving information is a sensation • Awareness of a sensation is a perception
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
Sensory receptors • Each receptor cell monitors a specific receptive field—the larger the field, the poorer the spatial resolution
Receptors transduce stimulus signals to neural signals • Transduction • A large enough stimulus changes the receptor potential, reaching generator potential
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
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
Figure 15.2 Receptors and Receptive Fields Figure 15.2
Thermoceptors and mechaniceptors • Found in the dermis • Mechaniceptors • Sensitive to distortion of their membrane • Tactile receptors (six types) • Baroreceptors • Proprioceptors (three groups)
Figure 15.3 Tactile Receptors in the Skin Figure 15.3a-f
Touch • Types of classification: • Capsulated/non-capsulated • Fast-adapting/slow-adapting; amount of myelin • Superficial/Deep layer
Touch Receptor Cell Types • Merkel cell: Slow adapting, superficial, non-capsulated • Meissner cell: Fast adapting, superficial, capsulated • Pacinian: Fast adapting, deep, capsulated • Ruffini: Slow adapting, deep, non-capsulated
Chemoreceptors • Chemoreceptors—function is to allow for regulation of pH, PCO2, and PO2 • Carotid bodies • Aortic bodies
Figure 15.5 Chemoreceptors Figure 15.5
Figure 15.4 Baroreceptors and the Regulation of Visceral Function Figure 15.4
Nociceptors (Pain Receptors) • Useful to indicate something is amiss (adaptive advantage to know when something is hurting you) • Normally produced by intense stimuli that can damage tissue • BUT, hypersensitivity to pain can occur
Hypersensitivity to Pain • Hyperalgesia: Sensitization such that normal pain becomes excruciating pain • Allodynia: Normal, non-pain (e.g. light touch) becomes painful • How and why would hypersensitivity occur?
Hypersensitivity to Pain • Can occur due to nociceptor sensitization—may result from changes in membrane permeability, membrane receptors, post- and pre-synaptic modulation, etc. • Peripheral sensitization: occurs in PNS; increased responsiveness/lower threshold to stimuli • E.g. sunburn • Central sensitization: occurs in CNS; increased excitability of neurons—often triggered by increase in nociceptor activity • E.g. Following an injury • Actually a manifestation of abnormal sensory processing within the CNS (not on the periphery) • E.g. migraines, post-surgical hypersensitization, “tender” areas surrounding injury • And MAYBE fibromyalgia, irritable bowel syndrome
Silent Nociceptors • Often can involve so-called “silent nociceptors”—nociceptors that normally don’t respond except under conditions of sensitization • Up to 30% of nociceptors are silent • May add to pain further, as the functioning density of nociceptors has essentially increased
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
Figure 15.6 Sensory Pathways and Ascending Tracts in the Spinal Cord Figure 15.6
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—”crossing over”
Figure 15.8 The Posterior Column Pathway and the Spinothalamic Tracts Figure 15.8a, b
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
Figure 15.8 The Posterior Column Pathway and the Spinothalamic Tracts Figure 15.8c
Spinocerebellar pathway • Includes the posterior and anterior spinocerebellar tracts • Carries sensation to the cerebellum concerning position of muscles, tendons and joints
Figure 15.9 The Spinocerebellar Pathway Figure 15.9
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
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
Figure 15.10 Descending (Motor) Tracts in the Spinal Cord Figure 15.10
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
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
Figure 15.11 The Corticospinal Pathway Figure 15.11
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
