Neural Integration
Chapter 15

Introduction
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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Specific ascending pathways (lemniscal) are concerned with precise, straightthrough 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
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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
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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
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Sensory input evokes a behavioral responses
Human decide to act or not to act on the basis of the information
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The choice we make depends, in part, on our past experience with similar sensory inputs

Aspects of Sensory Perception
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The main aspects of sensory perception are
– Perceptual detection
– Magnitude estimation
– Spatial discrimination
– Feature abstraction
– Quality discrimination
– Pattern recognition

Perceptual Detection
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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
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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
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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
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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
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Quality discrimination is the ability to differentiate the submodalities of a particular sensation
Each sensory modality has several submodalities
– Taste (sweet, salt, bitter, sour)
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The ability to discriminate is one of the major achievements of our sensory system

Quality Discrimination
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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
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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

Motor Integration From
Intention to Effect