All sensory signals are subject to extensive control at the various synapses along the ascending pathways before they reach higher levels of the central nervous system. Much of the incoming information is reduced or even abolished by inhibition from collaterals from other neurons in ascending pathways (lateral inhibition, discussed earlier) or by pathways descending
Descending pathways may control sensory information by directly inhibiting the central terminals of the afferent neuron (an example of presynaptic inhibition) or via an interneuron that affects the ascending pathway by inhibitory synapses. Arrows indicate the direction of action-potential transmission.
PART TWO Biological Control Systems
PART TWO Biological Control Systems
TABLE 9-1 Principles of Sensory System Organization
1. Specific sensory receptor types are sensitive to certain modalities and submodalities.
2. A specific sensory pathway codes for a particular modality or submodality.
3. The ascending pathways are crossed so that sensory information is generally processed by the side of the brain opposite the side of the body that was stimulated.
5. Information is organized such that initial cortical processing of the various modalities occurs in different parts of the brain.
6. Ascending pathways are subject to descending controls.
In some cases (for example, in the pain pathways), the afferent input is continuously inhibited to some degree. This provides the flexibility of either removing the inhibition (disinhibition) so as to allow a greater degree of signal transmission or of increasing the inhibition so as to block the signal more completely.
We conclude our general introduction to sensory system pathways and coding with a summary of the general principles of the organization of the sensory systems (Table 9-1). We now present the individual systems.
SECTION A SUMMARY
Sensory processing begins with the transformation of stimulus energy into graded potentials and then into action potentials in nerve fibers. Information carried in a sensory system may or may not lead to a conscious awareness of the stimulus.
I. Receptors translate information from the external world and internal environment into graded potentials, which then generate action potentials.
a. Receptors may be either specialized endings of afferent neurons or separate cells at the end of the neurons.
b. Receptors respond best to one form of stimulus energy, but they may respond to other energy forms if the stimulus intensity is abnormally high.
c. Regardless of how a specific receptor is stimulated, activation of that receptor always leads to perception of one sensation. Not all receptor activations lead, however, to conscious sensations.
II. The transduction process in all sensory receptors involves—either directly or indirectly—the opening or closing of ion channels in the receptor. Ions then flow across the membrane, causing a receptor potential.
a. Receptor-potential magnitude and action-potential frequency increase as stimulus strength increases.
b. Receptor-potential magnitude varies with stimulus strength, rate of change of stimulus application, temporal summation of successive receptor potentials, and adaptation.
Neural Pathways in Sensory Systems
I. A single afferent neuron with all its receptor endings is a sensory unit.
a. Afferent neurons, which usually have more than one receptor of the same type, are the first neurons in sensory pathways.
b. The area of the body that, when stimulated, causes activity in a sensory unit or other neuron in the ascending pathway of that unit is called the receptive field for that neuron.
II. Neurons in the specific ascending pathways convey information to specific primary receiving areas of the cerebral cortex about only a single type of stimulus.
III. Nonspecific ascending pathways convey information from more than one type of sensory unit to the brainstem reticular formation and regions of the thalamus that are not part of the specific ascending pathways.
Association Cortex and Perceptual Processing
I. Information from the primary sensory cortical areas is elaborated after it is relayed to a cortical association area.
a. The primary sensory cortical area and the region of association cortex closest to it process the information in fairly simple ways and serve basic sensory-related functions.
b. Regions of association cortex farther from the primary sensory areas process the sensory information in more complicated ways.
c. Processing in the association cortex includes input from areas of the brain serving other sensory modalities, arousal, attention, memory, language, and emotions.
I. The type of stimulus perceived is determined in part by the type of receptor activated. All receptors of a given sensory unit respond to the same stimulus modality.
II. Stimulus intensity is coded by the rate of firing of individual sensory units and by the number of sensory units activated.
III. Perception of the stimulus location depends on the size of the receptive field covered by a single sensory unit and on the overlap of nearby receptive fields. Lateral inhibition is a means by which ascending pathways emphasize wanted information and increase sensory acuity.
IV. Stimulus duration is coded by slowly adapting receptors.
Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition
The Sensory Systems CHAPTER NINE
The Sensory Systems CHAPTER NINE
V. Information coming into the nervous system is subject to control by both ascending and descending pathways.
SECTION A REVIEW QUESTIONS
sensory system sensory information sensation perception sensory receptor stimulus stimulus transduction adequate stimulus receptor potential adaptation sensory pathway ascending pathway sensory unit receptive field specific ascending pathway somatic receptor somatosensory cortex visual cortex auditory cortex nonspecific ascending pathway polymodal neuron cortical association area modality recruitment acuity lateral inhibition rapidly adapting receptor slowly adapting receptor
Distinguish between a sensation and a perception. Describe the general process of transduction in a receptor that is a cell separate from the afferent neuron. Include in your description the following terms: specificity, stimulus, receptor potential, neurotransmitter, graded potential, and action potential.
List several ways in which the magnitude of a receptor potential can be varied. Describe the relationship between sensory information processing in the primary cortical sensory areas and in the cortical association areas. List several ways in which sensory information can be distorted.
How does the nervous system distinguish between stimuli of different types?
How is information about stimulus intensity coded by the nervous system?
Make a diagram showing how a specific ascending pathway relays information from peripheral receptors to the cerebral cortex.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.