Receptors are sensory (afferent) nerve endings that terminate in periphery as bare unmyelinated endings or in the form of specialized capsulated structures. Receptors give response to the stimulus. When stimulated, receptors produce a series of impulses, which are transmitted through the afferent nerves.
Biological
Transducers
Actually receptors function like a transducer. Transducer is a device, which converts one form of energy into another. So, receptors ae often defined as the biological transducers, which convert (transducer) various forms of energy (stimuli) in the environment into action potentials in nerve fiber.
CLASSIFICATION OF RECEPTORS
Generally,
receptors are classified into two types:
A. Exteroceptors
B. Interoceptors
EXTEROCEPTORS
Exteroceptors are the
receptors, which give response to stimuli arising from outside
the body.
Exteroceptors are divided
into three groups:
1. Cutaneous
Receptors or Mechanoreceptors
Receptors situated in the
skin are called the cutaneous receptors. Cutaneous receptors are also called
mechanoreceptors because of their response to mechanical stimuli such as
touch, pressure and pain. Touch and pressure receptors give response to vibration also.
2. Chemoreceptors
Receptors, which give
response to chemical stimuli, are called the chemoreceptors.
3. Telereceptors
Telereceptors are the receptors
that give response to stimuli arising away from the body. These
receptors are also called the distance receptors.
INTEROCEPTORS
Interoceptors are the
receptors, which give response to stimuli arising from within the
body.
Interoceptors are of two types
which are as follows:
1. Visceroceptors
Receptors situated in the
viscera are called visceroceptors.
2. Proprioceptors
Proprioceptors are the
receptors, which give response to change in the position of
different parts of the body.
PROPERTIES OF RECEPTORS
1. SPECIFICITY OF
RESPONSE – MÜLLER LAW
Specificity
of response or Müller law refers to the response given by a particular
type of receptor to a
specific
sensation. For example, pain receptors give response only to pain sensation.
Similarly, temperature receptors give response only to
temperature sensation. In addition, each type of
sensation depends upon the part of the brain in which
its fibers terminate. Specificity of response is also called Müller’s
doc
2. ADAPTATION –
SENSORY ADAPTATION
Adaptation is the decline
in discharge of sensory impulses when a receptor is stimulated continuously
with constant strength. It
is also called sensory adaptation or desensitization.
Depending upon adaptation
time, receptors are divided into two types:
i. Phasic receptors,
which get adapted rapidly. Touch and pressure receptors are the
phasic
receptors
ii. Tonic
receptors, which adapt slowly. Muscle spindle,
pain receptors and cold receptors are
the tonic receptors.
3. RESPONSE TO
INCREASE IN STRENGTH OF STIMULUS – WEBERFECHNER
LAW
During the stimulation of
a receptor, if the response given by the receptor is to be doubled, the
strength of stimulus must be increased 100 times. This phenomenon is called
WeberFechner law, which states that intensity of response (sensation) of a receptor
is directly proportional to logarithmic increase in the intensity of stimulus.
Derivation of
Weber-Fechner Law
WeberFechner
law is derived as follows:
R = k log S
Where,
R = Intensity of response (sensation)
k = Constant
S = Intensity of stimulus
4. SENSORY
TRANSDUCTION
Sensory transduction in a receptor is a process by which the energy (stimulus) in the environment is converted into electrical impulses (action potentials) in nerve fiber (transduction = conversion of one form of energy into another). When a receptor is stimulated, it gives response by sending information about the stimulus to CNS. Series of events occur to carry out this function such as the development of receptor potential in the receptor cell and development of action potential in the sensory nerve. Sensory transduction varies depending upon the type of receptor. For example, the chemoreceptor converts chemical energy into action potential in the sensory nerve fiber. Touch receptor converts mechanical energy into action potential in the sensory nerve fiber.
5. RECEPTOR POTENTIAL
Definition
Receptor potential is a nonpropagated transmembrane potential difference that develops when a receptor is stimulated. It is also called generator potential. Receptor potential is short lived and hence, it is called transient receptor potential. Receptor potential is not action potential. It is a graded potential. It is similar to excitatory postsynaptic potential (EPSP) in synapse, endplate potential in neuromuscular junction and electrotonic potential in the nerve fiber.
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