An axon or nerve fiber is a long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action potentials away from the nerve cell body. The function of the axon is to transmit information to different neurons, muscles, and glands. In certain sensory neurons, such as those for touch and warmth, the axons are called afferent nerve fibers and the electrical impulse travels along these from the periphery to the cell body and from the cell body to the spinal cord along another branch of the same axon. Axon dysfunction can be the cause of many inherited and acquired neurological disorders that affect both the peripheral and central neurons. Nerve fibers are classed into three types – group A nerve fibers, group B nerve fibers, and group C nerve fibers. Groups A and B are myelinated, and group C are unmyelinated. These groups include both sensory fibers and motor fibers. Another classification groups only the sensory fibers as Type I, Type II, Type III, and Type IV.
The sensory nervous system is a part of the nervous system responsible for processing sensory information. A sensory system consists of sensory neurons, neural pathways, and parts of the brain involved in sensory perception and interoception. Commonly recognized sensory systems are those for vision, hearing, touch, taste, smell, balance and visceral sensation. Sense organs are transducers that convert data from the outer physical world to the realm of the mind where people interpret the information, creating their perception of the world around them.
Stimulus modality, also called sensory modality, is one aspect of a stimulus or what is perceived after a stimulus. For example, the temperature modality is registered after heat or cold stimulate a receptor. Some sensory modalities include: light, sound, temperature, taste, pressure, and smell. The type and location of the sensory receptor activated by the stimulus plays the primary role in coding the sensation. All sensory modalities work together to heighten stimuli sensation when necessary.
A cutaneous receptor is the type of sensory receptor found in the skin. They are a part of the somatosensory system. Cutaneous receptors include mechanoreceptors, nociceptors (pain), and thermoreceptors (temperature).
A mechanoreceptor, also called mechanoceptor, is a sensory receptor that responds to mechanical pressure or distortion. Mechanoreceptors are innervated by sensory neurons that convert mechanical pressure into electrical signals that, in animals, are sent to the central nervous system.
The dorsal column–medial lemniscus pathway (DCML) is a sensory pathway of the central nervous system that conveys sensations of fine touch, vibration, two-point discrimination, and proprioception from the skin and joints. It transmits information from the body to the primary somatosensory cortex in the postcentral gyrus of the parietal lobe of the brain. The pathway receives information from sensory receptors throughout the body, and carries this in nerve tracts in the white matter of the dorsal column of the spinal cord to the medulla, where it is continued in the medial lemniscus, on to the thalamus and relayed from there through the internal capsule and transmitted to the somatosensory cortex. The name dorsal-column medial lemniscus comes from the two structures that carry the sensory information: the dorsal columns of the spinal cord, and the medial lemniscus in the brainstem.
Tactile corpuscles or Meissner's corpuscles are a type of mechanoreceptor discovered by anatomist Georg Meissner (1829–1905) and Rudolf Wagner. This corpuscle is a type of nerve ending in the skin that is responsible for sensitivity to pressure. In particular, they have their highest sensitivity when sensing vibrations between 10 and 50 hertz. They are rapidly adaptive receptors. They are most concentrated in thick hairless skin, especially at the finger pads.
Sensory neurons, also known as afferent neurons, are neurons in the nervous system, that convert a specific type of stimulus, via their receptors, into action potentials or graded receptor potentials. This process is called sensory transduction. The cell bodies of the sensory neurons are located in the dorsal ganglia of the spinal cord.
In neuroscience and anatomy, nodes of Ranvier, also known as myelin-sheath gaps, occur along a myelinated axon where the axolemma is exposed to the extracellular space. Nodes of Ranvier are uninsulated and highly enriched in ion channels, allowing them to participate in the exchange of ions required to regenerate the action potential. Nerve conduction in myelinated axons is referred to as saltatory conduction due to the manner in which the action potential seems to "jump" from one node to the next along the axon. This results in faster conduction of the action potential.
A dorsal root ganglion is a cluster of neurons in a dorsal root of a spinal nerve. The cell bodies of sensory neurons known as first-order neurons are located in the dorsal root ganglia.
The star-nosed mole is a small semiaquatic mole found in moist, low elevation areas in the northern parts of North America. It is the only extant member of the tribe Condylurini and genus Condylura, and it has more than 25,000 minute sensory receptors in touch organs, known as Eimer's organs, with which this hamster-sized mole feels its way around. With the help of its Eimer's organs, it may be perfectly poised to detect seismic wave vibrations.
Merkel cells, also known as Merkel-Ranvier cells or tactile epithelial cells, are oval-shaped mechanoreceptors essential for light touch sensation and found in the skin of vertebrates. They are abundant in highly sensitive skin like that of the fingertips in humans, and make synaptic contacts with somatosensory afferent nerve fibers. It has been reported that Merkel cells are derived from neural crest cells, though more recent experiments in mammals have indicated that they are epithelial in origin.
Merkel nerve endings are mechanoreceptors, a type of sensory receptor, that are found in the basal epidermis and hair follicles. They are nerve endings and provide information on mechanical pressure, position, and deep static touch features, such as shapes and edges.
The tactile corpuscles of Grandry or Grandry corpuscles are mechanoreceptors found in the beak skin and oral mucosa of aquatic birds. They were first described by Grandry in 1869 in the bill skin of ducks and geese. Their general structure includes the flattened endings of an afferent nerve fiber sandwiched between two or more somewhat flattened sensory cells called Grandry cells, all surrounded by a layer of satellite cells and a partial capsule of collagen protein. Electrophysiological studies have shown that Grandry corpuscles function as rapidly adapting velocity detectors. In birds, Grandry and Merkel corpuscles share many morphological similarities, which has led to some confusion in the literature over their classification.
Microneurography is a neurophysiological method employed to visualize and record the traffic of nerve impulses that are conducted in peripheral nerves of waking human subjects. It can also be used in animal recordings. The method has been successfully employed to reveal functional properties of a number of neural systems, e.g. sensory systems related to touch, pain, and muscle sense as well as sympathetic activity controlling the constriction state of blood vessels. To study nerve impulses of an identified nerve, a fine tungsten needle microelectrode is inserted into the nerve and connected to a high input impedance differential amplifier. The exact position of the electrode tip within the nerve is then adjusted in minute steps until the electrode discriminates nerve impulses of interest. A unique feature and a significant strength of the microneurography method is that subjects are fully awake and able to cooperate in tests requiring mental attention, while impulses in a representative nerve fibre or set of nerve fibres are recorded, e.g. when cutaneous sense organs are stimulated or subjects perform voluntary precision movements.
Abraham Vater was a German anatomist from Wittenberg.
Eimer's organs are sensory organs in which the epidermis is modified to form bulbous papillae. First isolated by Theodor Eimer from the European mole in 1871, these organs are present in many moles, and are particularly common in the star-nosed mole, which bears 25,000 of them on its unique tentacled snout. The organs are formed from a stack of epidermal cells, which is innervated by nerve processes from myelinated fibers in the dermis, which form terminal swellings just below the outer keratinized layer of epidermis. They contain a Merkel cell-neurite complex in the epidermis and a lamellated corpuscle in the dermal connective tissue.
Pallesthesia, or vibratory sensation, is the ability to perceive vibration. This sensation, often conducted through skin and bone, is usually generated by mechanoreceptors such as Pacinian corpuscles, Merkel disk receptors, and tactile corpuscles. All of these receptors stimulate an action potential in afferent nerves found in various layers of the skin and body. The afferent neuron travels to the spinal column and then to the brain where the information is processed. Damage to the peripheral nervous system or central nervous system can result in a decline or loss of pallesthesia.
Touch is perceiving the environment using skin. Specialized receptors in the skin send signals to the brain indicating light and soft pressure, hot and cold, body position and pain. It is a subset of the sensory nervous system, which also includes the visual, auditory, olfactory, gustatory and vestibular senses.
A sense is a biological system used by an organism for sensation, the process of gathering information about the surroundings through the detection of stimuli. Although, in some cultures, five human senses were traditionally identified as such, many more are now recognized. Senses used by non-human organisms are even greater in variety and number. During sensation, sense organs collect various stimuli for transduction, meaning transformation into a form that can be understood by the brain. Sensation and perception are fundamental to nearly every aspect of an organism's cognition, behavior and thought.
