Somatosensory System Physiology PDF

Physiology of Somatosensory System Faculty of Dentistry and Faculty of Health Science Prof. Dr. Lamia Pınar Okan University Faculty of Medicine Department of Physiology Somatic nervous system, controls movements of our somatic organs like the musculoskeletal system and provides movements such as ref...

Physiology of Somatosensory System Faculty of Dentistry and Faculty of Health Science Prof. Dr. Lamia Pınar Okan University Faculty of Medicine Department of Physiology Somatic nervous system, controls movements of our somatic organs like the musculoskeletal system and provides movements such as reflexes, walking, running, or carrying out an activity with our arms, legs, or any part of our body. It is activated by the needs of the body. The Somatic nervous system mostly works voluntarily. 9/9/12 Autonomic nervous system, controls the functions of our internal organs like the heart, lungs, stomach, liver, intestines and urogenital organs, etc. It is activated by incoming senses from the internal organs and the needs of the body system. The Autonomic nervous system produces reflex activations and works involuntarily. That means it works automatically. Introduction to the Sensory System Special senses are concerned with the perception of stimuli from the external environment. These are the eye for vision, ear for hearing (audition) and equilibrium, nose for smell (olfaction) and oral mucosa for taste. These are all located in the head. Remaining senses such as touch, pressure, proprioception, and pain are sensed by receptors throughout the skin and underlying tissue (subcutaneous area). These receptors cause the body to be aware of activity in the surrounding area and allow appropriate or protective actions to be taken. Unimportant senses are eliminated before entering the CNS The central nervous system receives millions of bits of information from the different sensory nerves and organs each minute. If the incoming senses are not relevant or not important for the body at a given moment, they are suppressed before reaching the somatic sensory cortex. This means that around 99% of sensory inputs that enter the central nervous system, such as the sensations of clothes touching our body or sounds from the environment, are eliminated at some point in the ascending system. RECEPTORS are Cells or parts of the cells in the surface or interior environment of the body that receive sensory information such as light, sound, touch, pain, heat, cold, etc. They may be proteins or glycoproteins. All of them are connected to a sensory axon When a receptor potential reaches a threshold voltage, an action potential is produced in the sensory axon. Receptors can be classified based on their Stimulus Localization Teleceptors: detect distant senses such as light and sound Exteroceptors: perceive sensations from the outer surface of the body such as touch, pressure, heat, and pain… Interoceptors: perceive stimuli from the body's internal environment, such as blood pressure, blood oxygen level, stretching internal organs *The modality of each receptor is unique. For example; Light cannot activate the auditory system, and sound cannot activate the pressure receptors. This is ‘the law of specific energies.’ Classification of Receptors According to Sensory Types Chemoreceptor: stimulated by taste, smell, and chemicals in the blood. Photoreceptor: stimulated by light. Thermoreceptor: stimulated by cold and heat. Mechanoreceptor: stimulated by mechanical deformation of the cell membrane Nociceptor: stimulated by chemicals that occur in tissue damage (and creates pain) Mechanoreceptors Touch and Pressure Receptors (Tactile receptors) Merkel discs can sense continuous touch (adapt slowly; tonic) They are located in both hairy and glabrous skin. Pacinian corpuscles are found in the skin and subcutaneous areas. They are stimulated by rapid movement on the skin (adapt rapidly; phasic). Detect vibration or rapid changes in the tissues. Meissner’s corpuscles are sensitive to the light touch of glabrous skin. It is found in the most sensitive areas of the body, such as the cornea of the eyes, lips, tongue, tops of fingers, nips of the breasts, face … (adapt rapidly; phasic) Types of mechanoreceptors (continue) Ruffini endings produce signals for continuous deformation, such as heavy touch or pressure (adapt very slowly or not, tonic). They are found in deeper tissues and joint capsules, activated with joint rotation. They form the sense of proprioception. Free Nerve Endings, respond to pain, touch and pressure, and thermal stimuli. The myelin sheath is lost at the endings of the axonal branches. Their adaptation depends on the type and intensity of the stimuli. Hair Follicle Receptors are phasic, free nerve endings that can innervate up to 100 hairs spread over several cm² of skin. Sensory Unit One sensory axon and its peripheral branches constitute a ‘sensory unit’ The area where a stimulus produces a sensory effect in the clusters of neighboring receptors is ‘the receptive field’ The intensity of the stimulus depends on the frequency of the impulse transmission or increasing the number of receptors involved simultaneously But the type of sensation (modality) does not change as intensity increases (Law of Specific Energies) Senses are affected by ‘central excitation’ or ‘central inhibition’ states of the nervous system, due to releasing; of; Opiates, or Calcitonin Gene-Related Peptides (CGRP) secreted from the brain Cutaneous and subcutaneous senses (Tactile senses) Beginning of Action Potential in the Pacinian Corpuscle (Two-point Discrimination) The ability to discriminate clearly between two separate stimuli applied together is highly developed in the skin, and poorly developed in deeper tissues and organs. The two points that can be recognized by the cortex must have a distance of at least 70 mm on the back of the body and 1 to 3 mm on the fingertips or lips. These difference depends on the sensitivity and the amount of the sensory receptors detect the stimuli. Types of somatic receptors (dorsal root ganglion-spinal ganglion) All the somatic afferents have their cell bodies in the spinal ganglion. All the sensory fibers are excitatory fibers and secrete glutamate. All the somatic and part of the autonomic sensory fibers enter the spinal cord via dorsal roots, and all the motor fibers going to the musculature leave the spinal cord from the anterior (ventral) column via ventral roots. This is Bell-Magendie law. Dorsal Column - Medial Lemniscus System (proprioception, fine touch, fine pressure) Receptors: muscle spindles, Golgi tendon organs, Ruffini Endings. Axons: thick, myelinated First neurons: Spinal Ganglion (Dorsal root ganglion) Afferents travel in the dorsal horn of the spinal cord and are called Fasciculus Gracilis and Fasciculus cuneatus Second neurons: Nucleus Gracilis and Nucleus Cuneatus in the Bulbus Fibers cross to the opposite side. Fibers are named Medial Lemniscus and go up. Third neurons are located in the ventral posterolateral nucleus of the Thalamus Fourth neurons are located in the postcentral gyrus (Brodman’s 3,1,2 area) Lesions in the Dorsal Column-Medial Lemniscal System causes; Disorders of Balance (unsteady during standing) Ataxia (drunken-like walking, swinging during eyes closed) Astereognosis: inability to distinguish the shape of the objects handled, when eyes are closed