Process of Sensation

As discussed in the last post, sensations begin at the sensory receptors in our bodies.  These receptors can either be a specialized cell or the dendrites of a sensory neuron.  An example of a specialized cell would be one of the special senses.  The dendrites associated with a sensory neuron would be more common with the general senses.  There are four events that typically occur when a sensation arises, and these begin with the stimulation of the sensory receptor.  Then the transduction of the stimulus occurs, there is a generation of impulses, and then there is the integration of sensory input. 

In order for the stimulation of a sensory receptor to occur, it has to happen within the sensory receptor's receptive field.  The body region where the stimulation activates a receptor and can elicit a response.  So this would include something like a pin touching the tip of your finger.  This is in an area of the body that can perceive the pin touching the finger, and thus the sensory receptors in that area can react to that stimulus.  

The transduction of the stimulus happens when a sensory receptor transduces energy from the stimulus into a graded potential.  Graded potentials vary in amplitude depending on the strength of of the stimulus that evokes them, and they are not propagated.  An example of this would be odorant molecules in the air stimulate olfactory receptors in the nose.  These then transduce the molecule's chemical energy into electrical energy in the form of a graded potential.

After transduction happens, there is a generation of impulses.  When a graded potential in a sensory neuron reaches its threshold, it then triggers one or more action potentials.  Impulses are then propagated toward the central nervous system. The sensory neurons that conduct impulses from the central nervous system to the peripheral nervous system are called first order neurons.

There is a particular region of the central nervous system that receives and integrates the sensory nerve impulses.   This is where the conscious sensations or perceptions are integrated in the cerebral cortex .  How this works is that you see with your eyes, you hear with your ears, and feel pain in the injured part of the body.  This happens because sensory impulses from each part of the body arrive in a specific region of the cerebral cortex, which then interprets the sensation as coming from the stimulated sensory receptors.