|Anatomical terms of neuroanatomy|
A sensory nerve is an enclosed, cable-like bundle of sensory nerve fibers in the peripheral nervous system (PNS) that link sensory receptors on the body surface or deeper within it with relevant processing circuits in the central nervous system (CNS). Sensory nerves are composed of afferent nerve fibers that travel from the sensory receptors to the CNS. These are often paired with motor nerves, which are efferent nerve fibers that travel from the CNS to the PNS. Sensory nerves receive signals from the sensory receptors that stimuli impinge on the receptors and alter the potentials, which is known as sensory transduction.
Input into the CNS
Types of Sensory Receptors
For a signal to be sent down the sensory nerve, it must first be transduced from an external stimulus into action potential. This occurs at the site of the sensory receptors. There are different kinds of sensory receptors that respond to different stimuli. These sensory receptors include chemorecptors, photoreceptors, mechanoreceptors, thermoreceptors, and nociceptors. The different receptors respond to the different stimuli exist and transduce the energies into action potentials that are generated at the sensory neuron.
Chemoreceptors, or chemosensors, detect certain chemical stimuli and transduce that signal into an electrical action potential. There are two primary types of chemoreceptors:
- Distance chemoreceptors are integral to receiving stimuli in the olfactory system through both olfactory receptor neurons and neurons in the vomeronasal organ.
- Direct chemoreceptors include the taste buds in the gustatory system as well as receptors in the aortic bodies which detect changes in oxygen concentration.
Photoreceptors are capable of phototransduction, a process which converts light (electromagnetic radiation) into, among other types of energy, a membrane potential. There are three primary types of photoreceptors: Cones are photoreceptors that respond significantly to color. In humans the three different types of cones correspond with a primary response to short wavelength (blue), medium wavelength (green), and long wavelength (yellow/red). Rods are photoreceptors that are very sensitive to the intensity of light, allowing for vision in dim lighting. The concentrations and ratio of rods to cones is strongly correlated with whether an animal is diurnal or nocturnal. In humans, rods outnumber cones by approximately 20:1, while in nocturnal animals, such as the tawny owl, the ratio is closer to 1000:1. Ganglion Cells reside in the adrenal medulla and retina where they are involved in the sympathetic response. Of the ~1.3 million ganglion cells present in the retina, 1-2% are believed to be photosensitive.
Mechanoreceptors are sensory receptors which, respond to mechanical forces, such as pressure or distortion. While mechanoreceptors are present in hair cells and play an integral role in the vestibular and auditory system, the majority of mechanoreceptors are cutaneous and are grouped into four categories:
- Slowly Adapting type 1 Receptors have small receptive fields and respond to static stimulation. These receptors are primarily used in the sensations of form and roughness.
- Slowly Adapting type 2 Receptors have large receptive fields and respond to stretch. Similarly to type 1, they produce sustained responses to a continued stimuli.
- Rapidly Adapting Receptors have small receptive fields and underlie the perception of slip.
- Pacinian Receptors have large receptive fields and are the predominant receptors for high frequency vibration.
Thermoreceptors are sensory receptors, which respond to varying temperatures. While the mechanisms through which these receptors operate is unclear, recent discoveries have shown that mammals have at least two distinct types of thermoreceptors:
- The End-Bulb of Krause, or bulboid corpuscle, detects temperatures above body temperature
- Ruffini’s end organ detects temperatures below body temperature
Nociceptors respond to potentially damaging stimuli by sending signals to the spinal cord and brain. This process, called nociception, usually causes the perception of pain. They are found in internal organs as well as on the surface of the body. Nociceptors detect different kinds of damaging stimuli or actual damage. Those that only respond when tissues are damaged are known as "sleeping" or "silent" nociceptors.
- Thermal nociceptors are activated by noxious heat or cold at various temperatures.
- Mechanical nociceptors respond to excess pressure or mechanical deformation.
- Chemical nociceptors respond to a wide variety of chemicals, some of which are signs of tissue damage. They are involved in the detection of some spices in food.
Spinal Cord Entry
Sensory information carried by the afferent axons of the spinal nerves enters the spinal cord via the dorsal roots, and motor commands carried by the efferent axons leave the cord via the ventral roots. Once the dorsal and ventral roots join, sensory and motor axons (with some exceptions) travel together in the segmental spinal nerves).
Input into the CNS
Information from the sensory receptors in the head enters CNS through cranial nerves. Information from receptors below the head enters the spinal cord and passes towards the brain through the 31 spinal cord nerves. The sensory information traveling through the spinal cord follows well-defined pathways. The nervous system codes the differences among the sensations in terms of which cells are active.
Damage to the sensory nerve causes a wide range of symptoms because of the amount of functions performed by the nerve. Traumatic injuries and other damages to the sensory nerves can lead to peripheral neuropathy, which in turn can lead to things such as chronic liver disease, kidney disease, cancer, vitamin B deficiency, etc.
The ability to feel pain or changes in temperature can be affected by damage to the fibers in the sensory nerve. This can cause a failure to notice injuries such as a cut or that a wound is becoming infected. There may also be a lack of detection of heart attacks or other serious conditions. The lack of detection of pain and other sensations is a particularly large problem for those with diabetes, which contributes to the rate of lower limb amputations among this population. Overall, the poor sensation and detection may lead to changes in skin, hair, joint, and bone damage over the years for many people.
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