Classfication and Properties of Receptors Notes

Receptors Definition

• Receptors are the sensory (afferent) nerve endings that terminate in the periphery as bare unmyelinated endings or in the form of specialized capsulated structures.
• The receptors give a response to the stimulus. When stimulated, receptors produce a series of impulses which are transmitted through the afferent nerves.
• Actually, receptors function like a transducer. Transducer is a device, which converts one form of energy into another.
• So, the receptors are often defined as the biological transducers which convert (transduce) various forms of energy (stimuli) in the environment into action potentials in nerve fiber.

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Classification Of Receptors

Generally, the receptors are classified into two types.

1. Exteroceptors
2. Interoceptors.

 

1. Exteroceptors: Exteroceptors are the receptors that give response to stimuli arising from outside the body. The exteroceptors are divided into three groups.
   • Cutaneous Receptors: The receptors situated in the skin are called the CMtaneoua 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 a response to vibration also. The different types of cutaneous receptors are given.
   • Chemoreceptors: The receptors, which give response to chemical stimuli, are called the chemoreceptors.
   • Telereceptors: Telereceptors are the receptors that give a response to stimuli arising away from the body. These receptors are also called the distance receptors.
2. Interoceptors: Interoceptors are the receptors that give response to stimuli arising from within the body. Interoceptors are of two types:
   • Visceroceptors: The receptors situated in the viscera are called visceroceptors. Different interoceptors are listed in.
   • 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 Muller’S Law
   • Specificity of response or Muller’s 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 a 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 the doctrine of specific nerve energies.
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, the receptors are divided into two types.
   1. Phasic receptors, which get adapted rapidly. Touch and pressure receptors are the phasic receptors
   2. Tonic receptors, which adapt slowly. Muscle spindle, pain receptors and cold receptors are the tonic receptors.
3. Response To Increase In The Strength Of Stimulus
   • During the stimulation of a receptor, if the response given by the receptor is to be doubled, the strength of the stimulus must be increased 100 times. This phenomenon is called Weber-Fechner law, which states that the change in response of a receptor is directly proportional to the logarithmic increase in the 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 a response by sending information about the stimulus to CNS. A series of events occur to carry out this function such as the development of receptor potential in the receptor cell and the development of action potential in the sensory nerve.
   • The sensory transduction varies depending upon the type of receptor. For example, the chemoreceptor converts chemical energy into action potential in the sensory nerve fiber. The touch receptor converts mechanical energy into action potential in the sensory nerve fiber.
5. Receptor Potential
   • Receptor potential is a nonpropagated transmembrane potential difference that develops when a receptor is stimulated, it is also called generator potential. The receptor potential is short-lived and hence, it is called transient receptor potential.
   • The receptor potential is not an action potential. It is a graded potential. It is similar to excitatory postsynaptic potential (EPSP) in the synapse, endplate potential in the neuromuscular junction, and electrotonic potential in the nerve fiber.
   • Properties of Receptor Potential: Receptor potential has two important properties.
     1. Receptor potential is nonpropagated. It is confined within the receptor itself
     2. It does not obey all or none law.
   • Significance of Receptor Potential: When the receptor potential is sufficiently strong (when the magnitude is about 10 mV), it causes the development of action potential in the sensory nerve.
   • Mechanism of Development of Receptor Potential
     • The pacinian corpuscles are generally used to study the receptor potential because of their large size and anatomical configuration. These corpuscles can be easily dissected from the mesentery of experimental animals.
     • In the Pacinian corpuscle, the tip of the nerve fiber is unmyelinated. This unmyelinated nerve tip extends through the corpuscle as center core fiber. The concentric layers of the corpuscle surround the core fiber.
     • Pacinian corpuscles give a response to pressure stimulus. When pressure stimulus is applied, the Pacinian corpuscle is compressed. This compression causes elongation or change in shape of the corpuscle.
     • The change in shape of the corpuscle leads to the deformation of center core fiber of the corpuscle. This results in the opening of mechanically gated sodium channels. So, the positively charged sodium ions enter the interior of the core fiber. This produces a mild depolarization, i.e. receptor potential.
   • Generation of action potential in the nerve fiber
     • The receptor potential causes development of a local circuit of current flow which spreads along the unmyelinated part of the nerve fiber within the corpuscle.
     • When this local circuit of current reaches the first node of Ranvier within the corpuscle, it causes opening of voltage-gated sodium channels and the entrance of sodium ions into the nerve fiber. This leads to the development of action potential in the nerve fiber.