Are second-order neurons also internuncial neurons

Human sensory reception with which people respond to changes in external and internal environments.

Ancient philosophers called the human senses "the windows of the soul," and Aristotle described at least five senses - sight, hearing, smell, taste, and touch. Aristotle's influence was so lasting that many people still speak of the five senses as if there were no others. The modern catalog of senses now includes receptors in the muscles, tendons, and joints that evoke the kinesthetic sense (i.e., the sense of movement), as well as receptors in the vestibular organs in the inner ear that evoke the sense of balance. Sensory receptors are found within the circulatory system that are sensitive to carbon dioxide in the blood or to changes in blood pressure or heart rate, and there are receptors in the digestive tract that seem to convey such experiences as hunger and thirst. Some brain cells can also be involved as hunger receptors. This is especially true of cells in the lower part of the brain (like the hypothalamus), where some cells have been found to be sensitive to changes in blood chemistry (water and other digestive products) and even to changes in temperature in the brain itself.

General considerations on sensation

Basic characteristics of sensory structures

One way to classify sensory structures is by the stimuli to which they normally respond; so there are photoreceptors (for light), mechanoreceptors (for warping or bending), thermoreceptors (for heat), chemoreceptors (e.g. for chemical smells) and nociceptors (for painful stimuli). This classification is useful because it makes it clear that different sensory organs may share characteristics in the way in which they convert stimulus energy (converting) nerve impulses. So auditory cells and vestibular (balance) receptors are in the ear and some receptors in the skin all react similarly to mechanical displacement (distortion). Since many of the same principles apply to other animals, their receptors can be studied as models of the human senses. In addition, many animals are equipped with special receptors that enable them to recognize stimuli that humans cannot perceive. The pit viper, for example, has a receptor that is extremely sensitive to "invisible" infrared light. Some insects have receptors for ultraviolet light and for pheromones (chemical sex attractants and aphrodisiacs unique to their own species), which also exceed human sensory abilities.

Regardless of their specific anatomical shape, all sense organs have basic features in common:

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(1) All sensory organs contain receptor cells that are specifically sensitive to a class of stimulus energies, usually within a limited range of intensity. Such selectivity means that each receptor has its own "appropriate" or correct or normal stimulus since, for example, light is the appropriate stimulus for vision. However, other energies (“insufficient” stimuli) can also activate the receptor if they are sufficiently intense. So you can "see" pressure, for example when the thumb is placed on a closed eye and you see a bright spot (phosphene) in the field of vision at a position opposite the touched area.

(2) The sensitive mechanism for each modality is often in the body on a receptive membrane or surface (such as the retina of the eye) where transducer neurons (sensory cells) are located. The sense organ often contains accessory structures to direct the stimulating energy to the receptor cells. Thus, the normally transparent cornea and lens in the eye focus the light on the sensory neurons of the retina. Retinal nerve cells themselves are more or less protected from non-visual energy sources by the structure surrounding the eye.

(3) The primary transducers or sensory cells in each receptor structure normally connect (synapse) with secondary, incoming (afferent) nerve cells that carry the nerve impulse. In some receptors, such as the skin, the individual primary cells have thread-like structures (axons) that can be meters long and wind through subcutaneous tissue from below the surface of the skin until they reach the spinal cord. Each axon from the skin ends here and synchronizes with the next neuron (second order) in the chain. In contrast, each primary receptor cell in the eye has a very short axon that is entirely contained in the retina and synchronized with a network of different types of second-order neurons - internal uncial cells, which in turn synchronize with third-order neurons - all of which are bipolar cells - all still in the retina . The axons of the bipolar cells afferently extend beyond the retina, allowing the eyeball to form the optic nerve, which enters the brain to make further synaptic connections. When this visual system is viewed as a whole, the retina can be described as an expanded part of the brain that light can fall directly on.

From such afferent nerves, higher-order neurons make increasingly complex connections with anatomically separate pathways in the brain stem and deeper parts of the brain (e.g., the thalamus), which eventually end in certain receiving areas in the cerebral cortex (the convoluted outer covering of the brain). Different sensory reception areas are located in certain regions of the cortex, e.g. B. Occipital lobes at the back of the brain for vision, temporal lobes at the sides for hearing, and parietal lobes at the top of the brain for tactile function.