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Principles of Sensory Transduction


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Sherrington's Classification of Sensory Systems
1. Proprioceptors provide information about the relative position of the body segments and their position in space.
2. Exteroceptors are sensitive to external stimuli and include taste, touch, pressure, pain and may also include vision, audition and smell.
3. Interoceptors monitor internal bodily events.
4. Teleceptors are concerned with detecting distant external events, and include vision and audition.
Clinical Classification of sensory Systems
1. Special Senses are served by cranial nerves and include smell, taste, rotational & linear acceleration, vision and audition.
2. Superficial or cutaneous senses are those with receptors in the skin.
3. Deep Sensations are position senses in the muscles and joints.
4. Visceral Sensations are those concerned with perception of the internal environment.
Other Classification of sensory systems
functional and descriptive terms such as audition, olfaction, mechanoreceptors, etc.
What is the relationship between the receptor and the adequate stimulus?
A. Secondary and Primary Receptors differ in terms of the embryological origin of the receptor cells.
1. Receptors produce a receptor potential or generator potential.
2. Secondary receptors are of non-nervous origin, e.g., hair cells in taste receptors, and they communicate with the primary afferent neuron.
B. Receptors are specialized for detecting specific kinds of energy and transducing it into electrochemical energy.
C. What Qualities of Stimuli do receptors respond to or what features are extracted?
1. Qualitative
2. Quantitative
3. Temporal
4. Spatial
D. Evolutionary refinement of receptor structure has reached a theoretical limit for some receptor types.
Discuss the receptors, pathways and terminations for crude touch and for fine touch.
Tactile sensations arise from stimulation of receptors in the skin. There are both primary and secondary receptors in the skin which convey information about mechanical distortion of the skin surface. Sensations arising from stimulation of these receptors are frequently divided into discriminative touch, deep touch or pressure, vibration sense and crude touch. Crude touch is phylogenetically older and less topographically organized. Sometimes other cutaneous sensations such as pain and temperature and proprioception are grouped with these senses under the umbrella term, somasthesia.
How are stimuli encoded by sensory systems?
Place Coding - where stimulus is originating from

Frequency Coding - number of action potentials produced
How is the sensory input organized in the thalamus and cortex?
Thalamic and cortical organization is topographical

A. Several thalamic cell groups receive tactile input.
1. Discriminative touch relays are in the ventral posterior nucleus (VPN).
2. Crude touch relays are in the VPN, posterior nuclear group and the intralaminar nucleus.
B. The body surface is represented in at least four separate cortical areas in macaques and presumably also humans.
1. SI contains areas 1,2,3a and 3b which are functionally distinct.
2. SII is located adjacent to SI on the upper bank of the lateral fissure.
How does convergence and divergence of neuronal connections influence information processing?
1. Convergence occurs when many diverse cells send projections to one or a few afferent neurons; this is necessary for feature extraction.

2. Divergence occurs when a single afferent cell projects to multiple cells; also permits feature abstraction.
How does the brain know the spatial position of the body and its various parts?
Proprioceptors provide information about the relative position of the body segments and their position in space.

Dorsal Column (Medial Lemniscal) System carries proprioceptive inputs from the arm

Kinesthetic information from the legs are carried in a separate pathway (Spinocerebellar Tract) which merges with other dorsal column afferents at the level of the brain stem.
7. How are fast adapting and slow adapting fibers important for tactile sensation?
8. What techniques are available for measuring tactile sensation?
9. How does the activity of the muscle spindle allow the organism finer control over muscle contraction?

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