In: Biology
3. The human body responds to both external and internal signals of all types including light, pressure, sound, temperature, and chemical signals. The human nervous system depends on a network of neurons to transmit information from one area of the body to another.
a. Describe how the body detects such a wide variety of external and internal signals.
b. Describe how information moves through a single neuron starting with the dendrite and ending at the synapse.
A) All impulses from the receptors transmit as nerve signals and what ultimately determines how we perceive the stimulus is where the nerve fiber terminates in the central nervous system. It is important to realize that what one senses is dependent on the receptor and any damage occurring from the beginning of the path to its end.
The following is a detailed discussion of major sensory receptor types.
Receptors of vision
Retinal is the principal molecule of vision in the retina. It can absorb different frequencies of light. Its isomer (Cis-retinal) is present in rhodopsin, which is a photosensitive transmembrane G-protein that exists in rods and cones; it contains both cis-retinal and opsin.
Receptors of hearing
To discuss how sound receptors work, first, we must mention the order of events. Sound waves travel to the ear creating a vibration in the tympanic membrane. This energy transforms into mechanical energy to the malleus, incus, and stapes. The stapes is in close proximity to the oval window, and it amplifies the mechanical energy to the cochlea, a fluid-filled structure with a fluid called perilymph, by directly pushing on it. The cochlea has three layers called scala vestibuli (the ascending portion), scala media, and scala tympani (the descending portion). The organ of Corti is on the basilar membrane surface, and it contains hair cells which are the primary receptors in sound signal creation. There are two varieties of hair cells: inner and outer. Inner cells transmit information to the auditory nerve, and outer cells mechanically amplify low-level sound entering the cochlea.
Receptors of balance
The inner ear senses balance. With head motion or pressure impulses of sound, the endolymph vibrates and creates a stimulus for the receptors of the vestibular system - the utricle and saccule. Inside the utricle and saccule are maculae containing hair cells with a membranous covering of microscopic otoconia that detect motion of the endolymph. Those in the saccule help sense vertical accelerations whereas those in the utricle sense horizontal accelerations. With changes in position, and thus changes in fluid motion, the shifting of these hair cells causes opening of receptor channels leading to action potentials propagating from the hair cells to the auditory nerve. The rate of fluid motion, plus the quality of the fluid, gives us more information about the motion. While the utricle and saccule detect linear motion, the semicircular ducts detect rotations in a similar fashion.
Receptors of taste
Taste buds on the tongue and oropharynx help us enjoy and discriminate what we ingest.[3] The different tastes include sweet, salty, bitter, umami, and sour. A taste bud is a collection of taste cells that elongate at a tip to create a pore where stimuli may enter. Along these elongations are microvilli that protrude into the lumen of the mouth. On the other side of taste cells, there are nerve fibers that will eventually transmit the chemical gustatory message to the brain.
Receptors of smell
Smell occurs by binding of odorant molecules to receptors on the membrane of the cilia, causing an action potential that sends this information to the brain. These systems utilize G-protein receptors along with adenylate cyclase. Initially, scientists believed that molecules bound directly to receptors and that each receptor potentially identified a specific type of smell. However, Yoshioka et al. proposed a more plausible theory, because hydrogen and its isotope are sensed as entirely different smells. The authors relate this to a postulate called the "molecule vibration model." When a substance is bound to its receptor, the substrate allows electrons to go down their gradient, and through their specific vibrational energies, it causes a flow of chemical changes and subsequent signaling to the brain.
Receptors on the skin
What follows is a discussion of the various receptors in the skin. Signals from the skin may be conveyed by physical change (mechanoreceptors), temperature (thermoreceptors), or pain (nociceptors). Sensory receptors exist in all layers of the skin.
Mechanoreceptors
There are six different types of mechanoreceptors detecting innocuous stimuli in the skin: those around hair follicles, Pacinian corpuscles, Meissner corpuscles, Merkel complexes, Ruffini corpuscles, and C-fiber LTM (low threshold mechanoreceptors).[5] Mechanoreceptors respond to physical changes including touch, pressure, vibration, and stretch. Hair follicles can detect light touch; Meissner corpuscles in the dermal papillae detect indentation and slipping of objects; Pacinian corpuscles in the deeper dermis detect vibration; Merkel complexes in the basal epidermis create an understanding of structure and texture; Ruffini corpuscles detect stretch; C-fiber LTMs detect pleasant, light tactile sensations.Encapsulated receptors include Meissner corpuscle and the Pacinian corpuscle. In receptors that respond to stretch, there is a presence of “stretch-activated channels” that leads to depolarization via sodium influx. With smaller receptive fields, there is more precision in the detection of shape, form, and texture of stimuli.
Proprioceptors are also mechanoreceptors. Examples include muscle spindles and the Golgi tendon organ which respond to muscle contraction/relaxation and muscle strain respectively.
Thermoreceptors
The body has both warm and cold thermoreceptors. These receptors display a constant discharge to their specific temperatures, and when an experience of the opposite temperature occurs, there is a sudden ceasing of receptor discharge.
Nociceptors
Nociceptors help signal pain that is related to temperature, pressure, and chemicals. As Dubin et al. discusses, most sensory receptors have low sensitivity to dictate all sensations to the brain. However, when it comes to pain, nociceptors only signal when the body has reached a point of tissue damage. Inflammatory markers increase during tissue damage, bind to receptors, and initiate pain signaling either externally or in the viscera. One of the ion channels families that are present on nociceptive neurons is called TRP (transient receptor potential) ion channels. Those signals that activate nociceptive receptors include extremes of temperatures, high pressures, and chemicals causing tissue damage . Different fibers relay pain information; these are A-delta and C fibers. These fibers differ in their myelination and nerve diameter and thus speed of transmission. Painful temperatures, uncomfortable pressures, and chemicals mostly use C-fibers. C-fibers vary to be able to sense all three types of stimuli. A-delta fibers are small and unmyelinated and are primarily involved in thermal and mechanosensitive pain. Nociceptors utilize mostly glutamate but also substance P, calcitonin gene-related peptide, and somatostatin to signal pain.
Additionally, the gate theory of pain proposes that innocuous stimuli may trump painful stimuli if both are present simultaneously.
All sensory signals begin as receptor potentials. These potentials lead to a release of a neurotransmitter that excites its corresponding nerve to send information to the brain. Just as with regular nerve signal transduction, creating a receptor potential requires surpassing a threshold level in the membrane potential. Interestingly, with sensory receptors, the more the threshold is exceeded, the higher the frequency of action potentials. All receptors share the property that they can detect signals that are weak and intense. However, there is a drop-off, or plateau when the stimulus has reached a level of maximum stimulation. At that point, the receptor is unable to increase its firing potential.
Mechanism:
Receptive field
The site of a sensory neuron within its surrounding neuronal population is vital to determine the location of its neural message, whether tactile, visual, auditory, or others. The bodily area where a stimulus can affect a sensory receptor is called receptive field. This attribute in form of a physical dimension is vital to encode an accurate location of a stimulus. Areas that contain a higher number of small receptor fields can achieve better spatial resolution, evident in the fovea of the retina and portions of the skin such as fingertips and lips.
Labelled line principle
Sensory systems function by responding only to stimuli they are specific for and subsequently transducing it into a neural message which follows a discrete path to the brain. This constitutes the labelled line principle, which reserves the specificity of a receptor class in encoding a sensory modality to the designated brain area. This applies to somatosensory systems, as well as other specialized systems such as visual and auditory.
Adaptation
Adaptation is a common property of all sensory receptors. As a stimulus constantly excites the receptor, there will be a decrease in the rate of action potentials. Although receptors can adapt to a constant, unchanging stimulus, if there is a change, whether loss of the stimulus or change in intensity, the receptor is able to respond.
Topographical representation
Primary sensory cortical areas contain neurons that construct a location-specific or a quality-specific organization. Somatotopic representation displays in the primary sensory cortex by representing a distorted anatomical version of the body called sensory homunculus. Another example is the auditory system, where it displays a tonotopic map in the primary auditory cortex pertaining to sound frequencies.
B) Dendrites bring information to the cell body and axons take information away from the cell body. Information from one neuron flows to another neuron across a synapse. The synapse contains a small gap separating neurons. ... a presynaptic ending that contains neurotransmitters, mitochondria and other cell organelles.
Neurons talk to each other across synapses. When an action potential reaches the presynaptic terminal, it causes neurotransmitter to be released from the neuron into the synaptic cleft, a 20–40nm gap between the presynaptic axon terminal and the postsynaptic dendrite (often a spine).
Synapses can be thought of as converting an electrical signal (the action potential) into a chemical signal in the form of neurotransmitter release, and then, upon binding of the transmitter to the postsynaptic receptor, switching the signal back again into an electrical form, as charged ions flow into or out of the postsynaptic neuron.
These are respectively termed excitatory and inhibitory inputs, as they promote or inhibit the generation of action potentials (the reason some inputs are excitatory and others inhibitory is that different types of neuron release different neurotransmitters; the neurotransmitter used by a neuron determines its effect).
Neuroscientists often refer to action potentials as ‘spikes’, or say a neuron has ‘fired a spike’ or ‘spiked’. The term is a reference to the shape of an action potential as recorded using sensitive electrical equipment.
(NOTE: Synapse can be defined as functional junction between parts of two different neurons. Presynaptic region is mostly contributed by axon and postsynaptic region may be contributed by dendrite or soma (cell body) or axon of another neuron.)