First Part, Sense and Sensory Coding
Converting a receptor stimulus to a recongnizable sensation is termed sensory coding. All sensory systems code for four elementary attributes of a stimulus: 1.modality, the type of energy transmitted by the stimulus;2.location, the site on the body or space where the stimulus originated;3.intensity, which is signaled by the response amplitude or frequency of action;and 4.duration, that is the time from start to end of a response in the receptor.
There is a very important rule first enunciated by Johannes Muller in 1835, called the law of specific nerve energies. The law is that when the nerve from a particular sensory receptor is stimulated, the sensation evoked is that for which the receptor is specialized no matter how or where along the nerve the activity is initiated. For instance, if the sensory nerve from a Pancinian corpuscle in the hand is stimulated by pressure at the elbow or by irritation from a tumor in the brachial plexus, the sensation evoked is touch.
Table 1 Principle Sensory Modalities
Humans have four basic classes of receptors based on their sensitivity to one predominant form of energy, including mechanoreceptors, thermorecetpros, chemoreceptor, and photoreceptors (mechanical, thermal, electromagnetic, and chemical). The particular form of energy to which a receptor is most sensitive is called its adequate stimulus. For example, the adequate stimulus for rods and cones in the eyes is light (an example of electromagnetic energy).
However, receptors do respond to forms of energy other than their adequate stimuli, but the threshold for these nonspecific responses is much higher. Pressure on the eyeball will stimulate the rods and cones, but the threshold of these receptors to pressure is much higher than the threshold of the pressure receptors in the skin.
The term sensory unit refers to a single sensory axon and all of its peripheral branches. These branches vary in number but may be numerous, especially in the cutaneous senses. The receptive field of a sensory unit is the spatial distribution from which a stimulus produces a response in that unit.
If the skin is carefully mapped, millimeter by millimeter, with a fine hair, a sensation of touch is evoked from spots overlying these touch receptors. None is evoked from the intervening areas. Similarly, temperature sensations and pain are produced by stimulation of the skin only over the spots where the receptors for these modalities are located. Further more, the area supplied by one sensory unit usually overlaps and interdigitates with the areas supplied by others.
One of the most important mechanisms that enable localization of a stimulus site is lateral inhibition. Information from sensory neurons whose receptors are at the peripheral edge of the stimulus is inhibited compared to information form the sensory neurons at the center of the stimulus. Thus, lateral inhibition enhances the contrast between the center and periphery of a stimulated area and increases the ability of the brain to localize a sensory input. Note that two-point discrimination is based on lateral inhibition.
The intensity of sensation is determined by the amplitude of the stimulus applied to the receptor. For example, as a greater pressure is applied to the skin, the receptor potential in the mechanoreceptor increases, and the frequency of the action potentials in a single axon transmitting information to the CNS is also increased. In addition to increasing the firing rate in a single axon, the greater intensity of stimulation also will recruit more receptors into the receptive field.
As the strength of a stimulus is increased, it tends to spread over a large area and generally not only activates the sense organs immediately in contact with it but also "recruits" those in the surrounding area. Furthermore, weak stimuli activate the receptors with the lowest thresholds, and stronger stimuli also activate those with higher thresholds. Some of the receptors activated are part of the same sensory unit, and impulse frequency in the unit therefore increases.
Because of overlap and interdigitation of one unit with another, however, receptors of other units are also stimulated, and consequently more units fire. In this way, more afferent pathways are activated, which is interpreted in the brain as an increase in intensity of the sensation.
If a stimulus of constant strength is maintained on a sensory receptor, the frequency of the action potentials in its sensory never declines over time. This phenomenon is known as receptor adaptation or desensitisation. The degree to which adaptation occurs varies from one sense to another. Receptors can be classified into rapidly adapting (phasic) receptors and slowly adapting (tonic) receptors. Messier and Pacinian corpuscles are examples of rapidly adapting receptors, and Merkel cells and Ruffini endings are examples of slowly adapting receptors. Other examples of slowly adapting receptors are muscle spindles and nociceptors.
Different types of sensory adaptation likely have some value to the individual. Light touch would be distracting if it were persistent; and, conversely, slow adaptation of spindle input is needed to maintain posture. Similarly, inputs from nociceptor provides a warning that it would lose its value if it is adapted and disappeared.
Second Part, the Ascending Tracts
The sensation evoked by impulses generated in a sensory receptor depends in part on the specific part of the brain they ultimately activate. The ascending pathways from sensory receptors to the cortex are different for the various sensations. All somatosensory (touch, proprioception, temperature, pain, and itch) are mediated by two ascending sensory pathways.
Touch, vibratory sense, and proprioception are mediated by dorsal column medial lemniscal pathway. Pain and temperature are mediated by ventrolateral spinothalamic pathway.
Dorsal Column Pathway
The principal pathway to the cerebral cortex for touch, vibratory sense, and proprioception is the dorsal column pathway. Fibres mediating these sensations ascend ipsilaterally in the dorsal columns of the spinal cord to the medulla, where they synapse in the gracious nuclei (lower body, medial) and cuneate nuclei (upper body, lateral). Also, from peripheral to cerebral cortex, there are total three orders of neurons, like shown in the right figure.
Therefore, the second-order neurons from gracious and cuneate nuclei cross the midline and ascend in the medial lemniscus to the end in the contralateral ventral posterior lateral (VPL) nucleus and related specific sensory relay nuclei of the thalamus. Finally, the third-order neurons from VPL ascend to the cerebral cortex.
One interesting thing of dorsal column pathway is that there is a obvious characteristic, that is, the somatotopic organization. That is, within the dorsal columns, fibres arising from different levels of the cord are somatotopically organised. For instance, fibers from the sacral cord are positioned most medially (gracious nuclei) and those from the cervical cord are positioned most laterally (cuneate nuclei).
Somatotopic organization continues through the thalamus and cerebral cortex. VPL thalamic neurons (third-order neutrons) carrying sensory information project in a highly specific way to the primary somatosensory cortex in the post central gyrus of the parietal lobe. The arrangement of projections to this region is such that the parts of the body are represented in order along the post central gyrus, with the legs on top and the head at the foot of the gyrus. Also, the size of the cortical receiving area for impulses from a particular part of the body is proportional to the use of the part.
Finally, in addition to the primary somatosensory cortex, there are two other cortical regions that contribute to the integration of sensory information, including sensory association area (located in the parietal cortex) and the secondary somatosensory cortex (located in the wall of the lateral fissure/sylvian fissure), which receive input from the primary somatosensory cortex.
Ventrolateral Spinothalamic Tract
Fibers from nociceptors and thermoreceptors synapse on neurons in the dorsal horn of the spinal cord (shown int the figure at right side). The axons from these dorsal horn neurons (second-order neurons) cross the middling and ascend in the ventrolateral quadrant of the spin cord, where they form the ventrolateral spinothalamic pathway. Then, fibers of second-order neurons synapse in the VPL and the third-order neurons from VPL project to the cortex. Whereas, some dorsal horn neutrons (second-order) that receive nociceptive input synapse in the reticular formation of the brain stem (spinoreticular pathway) and then project to the ventrolateral nucleus of the thalamus.
Studies show that noxious stimuli that did not induce a change in affect caused an increased metabolism in the primary somatosensory cortex, whereas stimuli that elicited motivational-affective responses activated a larger portion of the cortex. This showed that the pathway to the primary somatosensory cortex is responsible for the discriminative aspect of pain. In contrast, the pathway that includes synapses in the brain stem reticular formation and ventrolateral thalamic nucleus projects to the frontal lobe, limbic system, and insular cortex. This pathway mediates the motivational-affective component of pain.
Visceral sensation travels along the same central pathways as somatic sensation in the spinothalamic tracts and thalamic radiations, and the cortical receiving areas for visceral sensation are intermixed with the somatic receiving areas.