The nervous system receives millions of bits of information from the sensory organs that are innervated by sensory nerves. The main component of the sensory system is sensory receptors. Although our body contains a very large number of different sensory receptors, the functional features are usually common. Sensory receptors are present as visual receptors in the eyes, tactile receptors on the surface of the body, auditory receptors in the ear and many other kind of sensory receptors. The memory of the sensory receptors is stored in the brain for minutes or even years. Our body needs adequate information of the external environment and the internal environment of the body. This information is collected by our sensory system. Our body gathers the information in the form of different signals and transforms these signals which dictates how you’ll behave in future actions when this event is replicated. The mechanism behind how our sensory system process the sensory information involves the gathering of the sensory information from the environment by sensory receptors that are converted into action potentials in sensory nerves. The information flows through the nerves in the form of action potentials that reach the central nervous system. Each set of specialized sensory receptors are structurally and functionally adopted for its specific role. The sensory information received is then integrated with the previously learned and stored sensory information to influence the judgment about the quality and the relevance of what is being sensed.
Stimulus is defined as something that elicits a physiological response in a cell or an organism. Stimuli can be heat, light, pressure, vibrations, sound waves, molecular shape and size. There are four fundamental attributes of the stimuli such as intensity, modality, time and location. Intensity can be described as the measure of the energy content available to interact with a sensory receptor such as loudness of the sound etc. Modality refers to the kind of sensation such as taste, smell, sight, touch, hearing and even complex sensations. There are specialized receptors for the specific modality such as photoreceptors for visual function and chemoreceptor detects taste and smell. Mechanoreceptors are for touch, hearing and stress associated with tendons and muscles. Heat is detected by thermoreceptors. Receptors are specialized to detect a specific type of stimuli for example the receptors in the eye are specialized to detect light.
Steps in the sensory transduction:
- Detection of stimuli: This step involves the detection of the stimuli at the specific receptors. For example detection of light on the retina.
- Depolarization and hyperpolarization: ions channels are opened to allow the passage of the current. Usually there is inward flow of the current which produces the depolarization of the receptor. Exception is photoreceptors. The change of membrane potential produced due to the stimuli is receptor potential. The depolarizing receptor potential means that the stimuli brings the membrane potential closer to the threshold.
Sensory pathways from the sensory receptor to brain
- Sensory receptors: The environmental stimuli activates the sensory receptors. These receptors can be photoreceptor or taste receptors etc.
- First order neuron: These neurons receives the signals and send them to Central Nervous System. These neurons are also known as primary afferent neurons. Cell bodies of these neurons are located in the dorsal root or spinal cord ganglia.
- Second order neuron: They are located in the spinal cord or brain stem. They receive information from primary afferent neurons and transmit it to the thalamus. The axons of these neurons cross the midline in a relay nucleus which is present in the spinal cord and then moves to the thalamus.
- Third order neuron: They are located in the relay nuclei of thalamus. From here, the information is transmitted to cerebral cortex.
- Fourth order neuron: They are located in the sensory area of the cerebral cortex.
Pain, cold, and heat receptors are naked dendrites
- Cold receptors: located close to the epidermis.
- Warm receptors: located deeper in the dermis.
- Hot receptors: pain experienced by a hot stimulus is sensed by a special nociceptor called a capsaicin.
Touch and pressure receptors have special structures around their dendrites
- Meissner’s corpuscles
- Encapsulated dendrites in connective tissue
- Changes in texture and slow vibrations
- Pacinian corpuscles
- Encapsulated dendrites by concentric lamellae of connective tissue structures
- Deep pressure and fast vibrations
- Ruffini Endings
- Sustained pressure
- Enlarged dendritic endings with open, elongated capsule
- Merkel’s discs
- Expanded dendritic endings
- Sustained touch and pressure
- Slow adapting