Sensation
Key Definitions:
Sensation : is defined as the conscious or subconscious awareness of changes in the external or internal environment. Perception : is defined as the conscious awareness and interpretation of sensations. This function is primarily the responsibility of the cerebral cortex.
Stimulus : a change in the environment that can activate certain sensory receptors.
Sensory Receptor : either a specialized cell or dendrites of a sensory neuron.
Selectivity : a sensory receptor responds only weakly or not at all to other stimuli.
Sensory Modality : each unique type of sensation such as touch, pain, vision, or hearing. A given sensory neuron carries information for only one sensory modality.
Sensory modalities are grouped into 2 classes: General and Special senses.
General Senses : refer to both somatic and visceral senses.
Somatic Senses : tactile sensations (touch, pressure, vibration, itch, and tickle), thermal sensations (warm & cold), pain sensations, and proprioceptive sensations (joint and muscle position sense)
Visceral Senses : sensations about conditions within internal organs.
Special Senses : sensory modalities of smell, taste, vision, hearing, equilibrium or balance.
For a sensation to arise, the following 4 events generally must occur:
1. Stimulation of a sensory receptor 2. Transduction of the stimulus: the conversion (transduction) of stimulus into a graded potential 3. Generation of a nerve impulse: graded potential reaches threshold 4. Integration of sensory input, occurs in the CNS
Sensory receptors produce 2 kinds of graded potentials :
Generator Potential : free nerve endings, encapsulated nerve endings, and receptive part of the olfactory receptors. If a generator potential is large enough to reach threshold, it triggers one or more nerve impulses in neuron. This impulse propagates into the CNS. Generator potentials generate action potentials.
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