special senses

Topics: Retina, Sensory system, Cochlea Pages: 17 (1957 words) Published: October 10, 2013
1. Sensation
There are different modalities (forms) of sensation
Sound, pain, pressure, touch, stretch, vibration, heat, cold, vision, taste, smell, proprioreception, hearing, equilibrium, gustation, etc.

Each modality has a specific receptor

Each modality is conducted by sensory (afferent) neurons to the CNS and is the result of different neural pathways and synaptic connections

2. Sensory Pathways

3. Law of Specific Nerve Energy
Each sensory neuron carries information about only one sensation: they usually respond best to one form of stimulus energy (but other stimuli can evoke a response) It ISN’T THE NEURON, it’s WHERE IT GOES IN THE CNS

Normal or Adequate stimulus
Allows receptors to function as filters
Requires the least amount of energy to start an AP
Under normal conditions (no tissue injury) correct CNS interpretation of receptors

4. Nerves
Nerves carry both afferent and efferent axons (mixed nerve)
The axons are all of different sizes
Remember that thicker axons have a faster conduction velocity Decrease in K+ and Cl- leaks through ungated channels
Less resistance to current flow
Myelination allows a decrease in axon diameter with increased conduction velocities Rapid flow through reflex arcs
Increase the number of axons in neural tissue : “miniaturization” of nervous system Fewer APs required so less ion flow: less Na+/K+ ATPase used so the neurons conserve energy

5. Nerve Conduction
Three types of fibers are generally recognized in the sensory subclass: A-beta fibers
▪Largest fibers
▪Mediate touch and mild pressure, as well as vibration and the position of joints ▪Conduction velocity above 30 m/sec.
A-delta fibers
▪Smaller than A-beta fibers
▪Mediate cold and the first components of pain
▪Conduction velocity between 2 and 30 m/sec.
C fibers
▪Slowest and smallest, serve most of autonomic peripheral functions ▪Mediate the sensation of warmth and the main component of pain ▪Conduction velocity less than 2 m/sec.

6. Information Transduction
Transduction: the conversion of stimulus energy into a membrane potential (receptor potential, generator potential); the mechanism by which the body changes energy from one modality to another Often graded potentials

Proportional to strength of stimulus
▪Possible amplification
Reaches threshold to generate AP in afferent neuron 
Receptors, both in organs and in tissues, transduce energies so that the environment can be interpreted by the CNS

7. Sensory Receptors
React to stimuli
Act as “transducers” to change environmental stimulus to action potential in afferent neuron “Generator potential”

Classification by stimulus:
Chemoreceptors (ions or molecules)
Mechanoreceptors (pressure, position, or acceleration)
Photoreceptors (electromagnetic radiation…a.k.a. light) Thermoreceptors (heat or cold)
Nociceptors (pain; severe heat, pressure, or chemicals released by inflammatory response)

8. Sensory Receptors
Alternative Classifications:
General versus Special
▪General sensory receptors
▪Special sensory receptors

Source of stimulus
▪External versus internal receptors

9. Sensory Receptors
Classification: General
Touch, pressure pain, heat, cold stretch, vibration, proprioreception

Classification: Special
Sight, hearing, equilibrium, smell, taste

10. Sensory Receptors: Classification Systems
Sensory Information
Sensory Adaptation

11. Structure of Receptors
Free nerve endings
Specific receptor
Hair cells

12. Sensory Information
Muscle spindles, Golgi tendon organs, joint receptors
Position sense and fine motor control
Cutaneous receptors
Light touch, pressure, thermoreceptors, pain
Sensation of...
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