Neuroscience/Cellular Neurobiology/Somatic Sensory System
There are many different sensory systems. The somatic sensory system mediates the sense of touch, and the proprioceptive division relays information about internal parts of the body. The auditory system provides hearing, and located in the same area is the vestibular system, which provides a sense a balance. The olfactory system and the gustatory system mediate the sense of smell and taste. The conversion of outside signals to chemical signals in the brain is referred to as transduction, and sensory transduction in the various systems shares a few characteristics. Sensory receptors (neurons, in the somatic sensory and olfactory systems, and epithelial cells in the visual, auditory, gustatory systems) synapse on an interneuron that relays signals to the brain. Perceptions can vary in four ways: modality (auditory, visual, etc.), intensity, duration and location. Variations in sensation are encoded in variations in action potentials.
There are five major modalities: vision, touch, smell, taste and hearing. In each modality, there may be submodalities (touch, pressure and temperature, in touch.) Each modality has a specific receptor. Vision transduction requires photoreceptors; audition, mechanoreceptors; taste and smell, chemoreceptors, and the touch system uses mechanoreceptors, thermoreceptors and nociceptors (pain receptors.) Each one of these modalities has its own pathway, and a relay through the subnuclei of the thalamus, and eventually terminate in a specific area of the cortex.
The intensity of a stimulus has a threshold; that is, there is a minimum stimulus required for its perception. The intensity is encoded in two ways: frequency of action potential and population. A stronger stimulus will cause a higher frequency of action potentials. In addition a greater stimulus will affect a larger area, causing a larger number of cells to respond to the stimulus, and this is perceived as a more intense stimulus.
Skin receptors include four types of mechanical receptors that have different field sizes and speeds of adaptation. There are two mechanoreceptors with large field sizes: pacinian corpuscles and ruffini's endings. While they both respond to stimuli from a large area of the skin, and therefore are not useful for locating stimuli accurately, pacinian corpuscles' speed of adaption is fast, and ruffini's endings adaptation is slow. The term speed of adaptation refers to how quickly the mechanoreceptor respond to stimuli. Slow adaptation means that the receptor is slow to give a response to a stimuli, and when a stimulus is removed, the response falls slowly. Fast adaptation means that the receptor will respond quickly, but will not give a sustained response. Two mechanoreceptors have a small receptive field: merkel's discs and meissner's corpuscles. Merkel's discs respond slowly, and meissner's corpuscles respond quickly. In addition to these four receptors, the skin contains nociceptors (pain receptors, usually free nerve endings) and thermoreceptors (sensitive to temperature.)
Dermatomes are the receptive field of a particular section of the spinal cord. Specifically, the area of the skin that makes up the receptive field of a spinal nerve, dorsal root and spinal segment.
Speed of Adaption
All receptors will adapt to stimulus, i.e. the receptor's potential amplitude decreases with continued stimulation. The slowly adapting receptors experience tonic (continued) activation while the rapidly adapting receptors respond only with the onset and offset of the stimulus, but not in between. This difference discriminates between transient and persistent sensory signals.
Types of Nerve Projections
There are four types of primary afferents (nerves fibers): the largest is the A alpha, whose diameter is usually between 13 and 20 micrometers. Signals travel between 80 and 120 m/s in A alpha afferents and carry information from proprioceptors in the skeletal muscles. A beta afferents are 6 - 12 micrometers in diameter and carry information from the skin's mechanoreceptors at 35 - 75 m/s. A gamma afferents are 1 - 5 micrometers in diameter and carry information about pain and temperature at 5 - 30 m/s. All three A type afferents are myelinated, which increases the speed of transmission. The fourth type of afferent is C, which is unmyelinated, .2 - 1.5 micrometers in diameter, and carries information about temperature, pain, and itch at .2 - 5 m/s.
Somatic Sensory System Paradigm
There are a few elements shared by every modality of the somatic sensory system. Starting with a receptor, the sensory system transfers information via a primary sensory neuron to a sensory pathway which then carries the information to the brain. Receptors that relay information on touch and pressure are mechanical; for temperature, there are thermoreceptors, and pain receptor are called nociceptors. The primary sensory neurons are the first neurons in the sensory system to respond and are called the first order neurons. These project through the dorsal root ganglia to synapse inside the spinal cord, and on the other (peripheral end) is a receptor. At this point, each modality follows its own pathway. Mechanoreceptors follow the dorsal column-medial lemniscus pathway and the second neuron in the pathway is found in the medulla. Thermoreceptors and nociceptors follow the spinothalamic pathway and their second order neurons are found in the dorsal horn of the spinal segment corresponding to their dermatome.
Dorsal Column-Medial Lemniscal Pathway
The dorsal column-medial leminiscal pathway, used to transmit touch and proprioceptive information, first goes up the ipsilateral (same side as input) dorsal column until it reaches the medulla. The projection synapses in the dorsal column nuclei (includes the Gracile nucleus and the cuneate nucleus.) Next, the pathway crosses to the contralateral side (opposite side of the input) and continues to the ventral posterior nucleus of the thalamus. After synapsing in the thalamus, still on the contralateral side, the pathway continues to the primary somatic sensory cortex.
The spinothalamic pathway, relaying information about temperature and pain, enters the spinal cord through the ipsilateral dorsal horn and immediately crosses, and the projection synapses on the secondary neuron in the contralateral side. The pathway proceeds all the way up the spine to the ventral posterior nuclei of the thalamus. After the thalamus, the pathway continues to the primary somatic sensory cortex (on the contralateral side.) In the somatic sensory cortex, every part of the body is represented by a particular area, i.e. each part of the body constitutes the receptive field of some group of neurons in the somatic sensory cortex.