physiology of sensory system

Receptor Physiology
Sensory receptors are specialized structures that detect a specific form of energy in the external environment. Each of the principal types of sensation that we can experience like pain, touch, sight, sound, and so forth-is called a modality of sensation. Each receptor is sensitive and respond to one modality ex. nociceptors respond only to painful stimuli and will not be stimulated by pressure, but if pressure become so intense and causes damage to the tissue, it will activate the pain receptors and perceived as painful stimulus.
The particular form of energy to which a receptor is most sensitive is called its adequate stimulus. The adequate stimulus for the rods and cones in the eye, for example, is light (an example of electromagnetic energy).
Receptor potentials
When a small amount of pressure is applied to a sensory receptor like Pacinian corpuscle, a non-propagated depolarizing potential resembling an excitatory postsynaptic potential (EPSP) is recorded in the receptor. This is called the receptor potential. This potential results from converting some form of energy like mechanical or thermal energy into an electrical response (change in membrane potential), the magnitude of which is proportional to the intensity of the stimulus. As the pressure is increased, the magnitude of the receptor potential is increased. Thus, the responses are described as graded potentials rather than all-or-none as is the case for an action potential.
The intensity of sensation is determined by the amplitude of the stimulus applied to the receptor. 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 is also increased, activation of receptors with higher threshold, because of overlap and interdigitation of one receptive unit with another, receptors of other units are also stimulated, and consequently more units fire. Somatosensory pathways
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.
Dorsal Column-Medial Lemniscal System
1. Touch sensations requiring a high degree of localization of the stimulus
2. Touch sensations requiring transmission of fine gradations of intensity
3. Phasic sensations, such as vibratory sensations
4. Sensations that signal movement against the skin
5. Position sensations from the joints
6. Pressure sensations related to fine degrees of judgment of pressure intensity.
Anterolateral System
1. Pain
2. Thermal sensations, including both warmth and cold sensations
3. Crude touch and pressure sensations capable only of crude localizing ability on the surface of the body
4. Tickle and itch sensations
5. Sexual sensations

The afferent neuron that transmits information from the periphery to the CNS is called the first-order neuron. A single first-order neuron may diverge within the CNS and communicate with several interneurons. In addition, interneurons may receive converging input from several first-order neurons. Some of these interneurons transmit the information to the thalamus, the major relay nucleus for sensory input; such interneurons are examples of second-order neurons. In the thalamus, these second-order neurons form synapses with third-order neurons that transmit information to the cerebral cortex, where sensory perception occurs. Different sensory pathways travel through different areas of the thalamus and cortex.
Dorsal column medial leminscal pathway
Fibers ascend ipsilaterally in the dorsal columns of the spinal cord to the medulla after sending collateral fibers to the dorsal horn cells, where they synapse in the Gracilus and Cuneate nuclei. The second-order neurons from these nuclei cross the midline and ascend in the medial lemniscus to end in the specific sensory relay nuclei of the thalamus. This ascending system is called the dorsal column or medial lemniscal system . The fibers within the dorsal column pathway are joined in the brain stem by fibers mediating sensation from the head via the main sensory and mesencephalic nuclei of the trigeminal nerve.
Somatotopic organization
Within the dorsal columns, fibers arising from different levels of the cord are somatotopically organized. Specifically, fibers from the sacral cord are positioned most medially and those from the cervical cord are positioned most laterally. This arrangement continues in the medulla.
Somatotopic organization continues through the thalamus and cortex. Thalamic neurons 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. The size of the cortical receiving area for impulses from a particular part of the body is proportional to the use of the part. The cortical areas for sensation from the trunk and back are small, whereas very large areas are concerned with impulses from the hand and the parts of the mouth concerned with speech.
In addition to the primary somatosensory cortex, there are two other cortical regions that contribute to the integration of sensory information. The sensory association area is located in the parietal cortex and the secondary somatosensory cortex is located in the wall of the sylvian fissure that separates the temporal from the frontal and parietal lobes. These regions 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. The axons from these dorsal horn neurons cross the midline and ascend in the ventrolateral quadrant of the spinal cord, where they form the ventrolateral spinothalamic pathway. Fibers within this tract synapse in the thalamic nuclei where third order neuron transmit information to the somatosensory cortex.
PAIN
is an unpleasant sensory and emotional experience associated with actual or potential tissue damage. This is to be distinguished from the term nociception which is defined as the unconscious activity induced by a harmful stimulus applied to sense receptors.
Pain is frequently classified as physiologic or acute pain and pathologic or chronic pain, which includes inflammatory pain and neuropathic pain. Acute pain typically has a sudden onset and recedes during the healing process; it can be regarded as “good pain” as it serves an important protective mechanism. The withdrawal reflex is an example of the expression of this protective role of pain.
Chronic pain can be considered “bad pain” because it persists long after recovery from an injury and is often refractory to common analgesic agents, including non-steroidal anti-inflammatory drugs (NSAIDs) and opioids. Chronic pain can result from nerve injury (neuropathic pain) including diabetic neuropathy, toxin-induced nerve damage, and ischemia.
Hyperalgesia and allodynia
Pain is often accompanied by increased sensitivity of nociceptors to painful stimuli (hyperalgesia and allodynia). Hyperalgesia is an exaggerated response to a noxious stimulus, and allodynia is a sensation of pain in response to a normally innocuous stimulus. An example of the latter is the painful sensation from a warm shower when the skin is damaged by sunburn.
hyperalgesia and allodynia might be caused by 1- release chemical mediators like K+, bradykinin and substance p from injured cells leading to sensitization of the pain receptors.
2- In addition to sensitization of nerve endings by chemical mediators. The nerve growth factor NGF released by tissue damage is picked up by nerve terminals and transported retrogradely to cell bodies in dorsal root ganglia where it can alter gene expression and increases production of substance P and converts non-nociceptive neurons to nociceptive neurons.
3- Another change in the spinal cord is due to the activation of microglia near afferent nerve terminals in the spinal cord. This, in turn, leads to the release of pro-inflammatory cytokines and chemokines that modulate pain processing.
Deep or visceral pain
Afferent fibers from visceral structures reach the CNS via sympathetic and parasympathetic nerves. Their cell bodies are located in the cranial nerve ganglia (facial, glossopharyngeal, and vagus nerves); in the thoracic, lumbar and sacral dorsal roots.
The receptors in the walls of the hollow viscera are specially sensitive
Duration and adaptation
If a stimulus of constant strength is maintained on a sensory receptor, some receptor types continue to respond to the stimulus as long as its applied while others adapt, that is mean the frequency of the action potentials in their sensory nerve declines over time. This phenomenon is known as receptor adaptation or desensitization. The degree to which adaptation occurs varies from one sense to another. Receptors can be classified into rapidly adapting receptors like olfactory receptors and Pacinian corpuscles and slowly adapting receptors like muscle spindles and nociceptors.
Transmission of sensory information to the spinal cord
When a specific stimuli activate its own receptor, receptor potential will be generated, this receptor potential have to be transmitted through peripheral sensory nerve to the spinal cord where it will relay it to the specific areas of the cerebral cortex. According to the speed of conduction, various types of nerve fibers exist (table 2).

Some information need to be transmitted to or from the central nervous system extremely rapidly; otherwise, the information would be useless. An example of this is the sensory signals that apprise the brain of the momentary positions of the legs at each fraction of a second during running. At the other extreme, some types of sensory information, such as that depicting prolonged, aching pain, do not need to be transmitted rapidly, so slowly conducting fibers is sufficient.
Somatosensory pathways
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.
Dorsal Column-Medial Lemniscal System
1. Touch sensations requiring a high degree of localization of the stimulus
2. Touch sensations requiring transmission of fine gradations of intensity
3. Phasic sensations, such as vibratory sensations
4. Sensations that signal movement against the skin
5. Position sensations from the joints
6. Pressure sensations related to fine degrees of judgment of pressure intensity.