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Receptors

As the name implies, receptors are structures that receive information.  More specifically, receptors receive information about stimuli or physical changes in the internal or external environment.  With receptors, our nervous system monitors various environmental conditions.  When changes occur, these can warrant a response(motor output to effectors) to maintain homeostasis.

Receptors are classified broadly as interoreceptors(visceroreceptors) or exteroreceptors.  Interoreceptors monitor internal body conditions whereas exteroreceptors monitor changes in the external environment.

More specific classification involves reference to the type of stimulus monitored by a receptor.  For example, your body has chemoreceptors, baroreceptors, photoreceptors, and tactile receptors.  These monitor shifts in chemistry, blood pressure, light, and touch respectively.

All receptors are linked to sensory neurons.  When a receptor responds to a stimulus, a signal is sent along the sensory neuron to the CNS(brain or spinal cord).  Within the CNS, the stimulus is identified and if a response is required to maintain homeostasis, signals are sent to effectors along motor neurons.  

Meissner's and Pacinian Corpuscles

As an example of receptors we can look at the functions of two types of encapsulated tactile receptors, the Meissner's and Pacinian corpuscles in skin.

Meissner's corpuscles are fine-touch receptors.  If you close your eyes and have a friend place an object in the open palm of your hand, chances are good you will be able to detect the object but you will not be able to identify it.  By moving the object to your finger tips where Meissner's corpuscles are abundant, you gather information about its shape, texture, and density, information your brain uses to identify the object.

In the palm of your hand you possess fewer fine-touch receptors. Its the Pacinian Corpuscles or pressure receptors located deeper in the skin that enable you to detect the object due to its weight. These are less abundant and without sufficient receptor density, you cannot gather more specific information about the object.

Meissner's and Pacinian corpuscles are both encapsulated receptor types, surrounded by concentric layers of a specialized connective tissue.  Looking like a minute onion cut in half, the concentric layers help in the identification of the Pacinian corpuscle.

Here is an example of a Pacinian corpuscle in the dermis of skin.  See if you can locate the small nerve cut in cross-section here.  For a labelled view, click here!

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