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Neural Integration. Chapter 15. Introduction. Through the chapters covered to date we have looked at the nervous system from its component pieces However, our nervous system typically acts as an integrated whole, gathering sensory inputs, processing information and effecting a motor response
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Neural Integration Chapter 15
Introduction • Through the chapters covered to date we have looked at the nervous system from its component pieces • However, our nervous system typically acts as an integrated whole, gathering sensory inputs, processing information and effecting a motor response • Integration is the process by which the nervous system processes and interprets sensory input and makes decisions about what should be done at each moment
Sensory Integration • Our survival needs depend upon • Sensation • Awareness of changes in our internal and external environment • Perception • Conscious interpretation of stimuli • Perception determines how we will respond to the stimuli
Organization of Somatosensory System • The somatosensory system receives inputs from exteroreceptors, proprioceptors, and interoreceptors • It transmits information about several different sensory modalities via a system of ascending sensory pathways between receptors and the cerebral cortex • The pathways between receptor and cerebral cortex follow a chain of three neurons
Organization of Somatosensory System • The typical chain of neurons involves • The cell body of the afferent (1st order) neuron in the dorsal root ganglion • The second order neuron in the dorsal horn of the spinal cord or medulla oblongata • The third order neuron in the thalamus • Although this is the basic pattern for the upward flow of information, collateral synapses occur along the pathway
Organization of Somatosensory System • First order • Cell body of the afferent neuron in the dorsal root ganglion • Second order • Neuron in the dorsal horn of spinal cord or medulla oblongata • Third order • Neuron in the thalamus
Organization of Somatosensory System • There are three main levels of neural integration in the somatosensory system • Level I: The receptor level corresponding to the sensory receptors • Level II: The circuit level corresponding to the ascending pathways • Level III: The perceptual level corresponding to the awareness of the incoming stimuli
Organization of Somatosensory System • Receptor level • Information about internal and external environment • Circuit level • Ascending pathways to the brain • Perceptual level • Allows us to be aware of stimuli and discriminate about their characteristics
Processing at the Receptor Level • Information about the external and internal environments presents itself as different forms of energy (sound, light) • The sensory receptors respond to these stimuli; the rest of the nervous system responds chiefly to neurotransmitters • The process by which a stimulus is transformed into an electrical response at a receptor is called transduction
Processing at the Receptor Level • If the receptor potential is of threshold strength, it triggers an action potential along the axon of the afferent neuron • Nerve impulse conduction continues as long as a threshold stimulus is applied • The strength of the stimulus is encoded in the frequency of the impulses sent, stronger stimuli cause more impulses per second to reach the CNS
Processing at the Circuit Level • The central processes of sensory neurons, carrying impulses from cutaneous receptors of the skin and proprioceptors branch diffusely as they enter the spinal cord • Some branches take part in local spinal cord reflexes and directly initiate motor activities • The remaining sensory afferents synapse with second order neurons in the dorsal horn or continue upward in the dorsal column to synapse in medullary nuclei
Processing at the Circuit Level • Small-diameter pain fibers synapse with superficially located substantia gelatinosa neurons in the dorsal horn • The large myelinated fibers from pressure and touch receptors make collateral synapses with interneurons in the dorsal horns
Processing at the Circuit Level • The somatosensory cortex receives sensory information via two major ascending routes; the nonspecific and specific pathways • These pathways relay information to the brain for three purposes: perception, arousal and motor control
Nonspecific Ascending Pathways • The nonspecific ascending pathways receive inputs many different types of sensory receptors and make multiple synapses in the brain stem • These anteriolateral pathways are formed by the lateral and anterior spinothalamic tracts Anterior Spinothalmic
Specific Ascending Pathways • Specific ascending pathways (lemniscal) are concerned with precise, straight- through transmission of inputs from a single (or related) sensory receptor • These pathways are formed by the dorsal column of the spinal cord and the medial lemniscal tracts Lemniscal
Processing at the Perceptual Level • Perception is the final stage of sensory processing which allows us to be aware of stimuli and discriminate about their characteristics • As sensory information reaches the thalamus, they are perceived in a crude sense • Full appreciation of their qualities and sharp localization awaits the arrival of the input at the somatosensory cortex
Processing at the Perceptual Level • The entire sensory cortex is arranged in columns of cortical neurons with each column representing a building block of sensory perception • The thalamus projects fibers (sorted out by sensation type) to the primary somatosensory cortex and to sensory association areas • First to those restricted to the same modality and then to those considering more than one • This allows for parallel processing of the various inputs, and results in an internal, conscious image of the stimulus
Processing at the Perceptual Level • Sensory input evokes a behavioral responses • Human decide to act or not to act on the basis of the information • The choice we make depends, in part, on our past experience with similar sensory inputs
Aspects of Sensory Perception • The main aspects of sensory perception are • Perceptual detection • Magnitude estimation • Spatial discrimination • Feature abstraction • Quality discrimination • Pattern recognition
Perceptual Detection • Detecting that a stimulus has occurred is the simplest level of perception • As a general rule, several receptor impulses must be summated (accumulated) for perceptual detection to occur
Magnitude Estimation • Magnitude estimation is the ability to detect how much of the stimulus is acting on the body • Because of frequency coding, perception increases as stimulus intensity increases
Spatial Discrimination • Spatial discrimination allows us to identify the site or pattern of stimulation • Two point discrimination test • The test is a crude map of the density of tactile receptors in the various regions of the skin
Feature Abstraction • The mechanism by which a neuron or circuit is tuned to one feature in preference to others is called feature abstraction • This implies that a unit of perception is tuned to a coordinated set of several stimulus properties called a feature • The skin integrates receptor inputs in parallel fashion to appreciate the “texture” of an object
Quality Discrimination • Quality discrimination is the ability to differentiate the submodalities of a particular sensation • Each sensory modality has several submodalities • Taste (sweet, salt, bitter, sour) • The ability to discriminate is one of the major achievements of our sensory system
Quality Discrimination • Quality discrimination may be analytic or synthetic • Analytic / each quality retains its individual nature (sugar and salt) • Synthetic / our perception of the qualities merge into a new sensation (chocolate) • Synthetic discrimination is important in sight as we merge red, green and blue into the myriad of colors we see • Vision and olfaction use only synthetic
Pattern Recognition • Pattern recognition refers to our ability to take in the scene around us and recognize • A familiar pattern • An unfamiliar pattern • A pattern that has special significance to us
End of Material for Test 3 Chapter 15
