Rapidly-adapting nerve receptors deliver information on changes that occur at the skin, such as occurs as you move your finger over a gritty surface. Slowly-adapting receptors deliver information regarding prolonged touch, such as occurs when you are grasping a tool for an extended period of time. The receptors also differ in the size of their receptive fields. Smaller receptive fields contribute to acuity. For example, the receptive field size of sensory nerves on the human back is much larger than on the fingertip.
If two needles spaced a millimeter apart are touched to a large receptive field, both points will lie within the receptive field and will be encoded as a single point. If the needles are touched to two smaller receptive fields, such that each needle point contacts a separate receptive field, the nervous system can distinguish these as two separate points. Combined, these receptors allow for the sense of touch and discrimination of shapes, textures, and objects, are resonsible for the sensations of heat and pain, and give information on where the hand and fingers are in relation to the body i.
Am I holding a bat or a ball? There are also sensory nerves encircling hair follicles that give information on when the hair is displaced. However, it is the sensory nerve endings on glabrous non-hairy skin that likely contribute to the proprioceptive and tactile control necessary for fine touch discrimination and tool use.
Image Nerve endings associated with hair follicles in the skin of a mouse. Sensory neurons are the brain's portal to the external world. However the sense of touch, which is conveyed by general somatic sensory neurons, is much less well defined.
These neurons reside in discrete ganglia that lie peripheral to the brainstem and spinal cord, including the trigeminal ganglia that receive signals from the face and head, and the dorsal root ganglia that serve the trunk and limbs. Traditionally, somatic sensory neurons have been divided into three broad subtypes: nociceptors for sensing pain , mechanoreceptors for sensing touch , and proprioceptors which sense body position.
Nociceptors and mechanoreceptors terminate in the skin, whereas proprioceptors terminate in muscles and tendons. It has long been recognized that nerve endings in the skin display a diverse range of forms, but prior studies have generally used histological methods in tissue sections that do not reveal the complete morphology of each neuronal axon.
The results of the Johns Hopkins experiment are largely descriptive in nature, and we might assume that the results reported are already buried somewhere in the literature, but they are not. Thus we are reminded that our knowledge of even well-studied experimental systems is still very fragmentary.
Drawings of neurons and nerve endings made more than a century apart. The cells were stained with potassium dichromate and silver nitrate.
The trace on the right shows nerve endings in the skin of a mouse. A combination of genetic and histochemical techniques were used to record the image from which the trace is taken Wu et al. This latest work was made possible by the Cre-Lox system—a widely-used approach in which a Cre recombinase enzyme is used to remove chromosomal DNA flanked by two genetically engineered loxP recognition sequences.
Wu, Williams and Nathans used this method to excise a signal sequence blocking the expression of a histochemical marker gene which had been previously engineered into the chromosomal location of a transcription factor Brn3a that is important in the development of the sensory nervous system. That was a stunning example. The images produced by the team at EMBL are notable because although skin samples are easy to collect and maintain in a lab environment, they're notoriously resistant to analysis by microscopy.
It also has a background fluorescence. SNAP-tagging overcomes this issue by using a special protein that can bind itself to artificial dies and is produced in the skin of genetically-engineered mice. The next step is to image whole circuits in action in the brain or spinal cord. Heppenstall says his ultimate aim is to not only identify individual neurons, but record them in action. There are a few types of hair receptors that detect slow and rapid hair movement, and they differ in their sensitivity to movement.
Some hair receptors also detect skin deflection, and certain rapidly adapting hair receptors allow detection of stimuli that have not yet touched the skin. In addition to Krause end bulbs that detect cold and Ruffini endings that detect warmth, there are different types of cold receptors on some free nerve endings: thermoreceptors, located in the dermis, skeletal muscles, liver, and hypothalamus, that are activated by different temperatures.
Their pathways into the brain run from the spinal cord through the thalamus to the primary somatosensory cortex. Warmth and cold information from the face travels through one of the cranial nerves to the brain. You know from experience that a tolerably cold or hot stimulus can quickly progress to a much more intense stimulus that is no longer tolerable. Any stimulus that is too intense can be perceived as pain because temperature sensations are conducted along the same pathways that carry pain sensations.
Pain is the name given to nociception , which is the neural processing of injurious stimuli in response to tissue damage. Pain is caused by true sources of injury, such as contact with a heat source that causes a thermal burn or contact with a corrosive chemical. But pain also can be caused by harmless stimuli that mimic the action of damaging stimuli, such as contact with capsaicins, the compounds that cause peppers to taste hot and which are used in self-defense pepper sprays and certain topical medications.
Nociception starts at the sensory receptors, but pain, inasmuch as it is the perception of nociception, does not start until it is communicated to the brain. There are several nociceptive pathways to and through the brain.
Most axons carrying nociceptive information into the brain from the spinal cord project to the thalamus as do other sensory neurons and the neural signal undergoes final processing in the primary somatosensory cortex. Interestingly, one nociceptive pathway projects not to the thalamus but directly to the hypothalamus in the forebrain, which modulates the cardiovascular and neuroendocrine functions of the autonomic nervous system.
Recall that threatening—or painful—stimuli stimulate the sympathetic branch of the visceral sensory system, readying a fight-or-flight response. Somatosensation includes all sensation received from the skin and mucous membranes, as well as from the limbs and joints. Somatosensation occurs all over the exterior of the body and at some interior locations as well, and a variety of receptor types, embedded in the skin and mucous membranes, play a role.
There are several types of specialized sensory receptors.
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