Somatosensory System PDF 2020

Somatosensory System MDSC2002: Neuroscience & Behaviour Farid F. Youssef Somatosensory System Somatic senses refer to the perception of: i. touch ii. temperature iii. proprioception (position sense) iv. pain This information is received from the skin, joints, muscl...

Somatosensory System MDSC2002: Neuroscience & Behaviour Farid F. Youssef Somatosensory System Somatic senses refer to the perception of: i. touch ii. temperature iii. proprioception (position sense) iv. pain This information is received from the skin, joints, muscles and viscera. What is it? Sensory The sensory receptors for Receptors touch are specialized epithelial cells or endings of neurons that innervate the skin. There are two major submodalities for touch: i. fine or discriminatory touch, including two- point discrimination ii. crude touch - describes sensations such as itch and tickle; these are poorly localized. CUTANEOUS Sensory receptors There are four distinct types of receptors that transduce touch within the skin. These are: i. Pacinian Corpuslce ii. Meissner’s Corpuslce iii. Ruffini’s Corpuslce iv. Merkel’s Disc  The Pacinian corpuscle is Pacinian Corpuscle an encapsulated receptor found deep within the subcutaneous layers of both hairy and non-hairy skin. It consists of a group II myelinated fibre enclosed in a lamellar structure approximately 1mm in diameter  When this capsule is Pacinian Corpuscle deformed by mechanical stimulation it causes cell depolarization and generates a receptor potential However these receptors are extremely rapidly adapting, a feature associated with the physical properties of the lamellae. Pacinian Corpuscle Each stimulus only generates a few actions potentials therefore it is ideally suited to detecting vibrations. Adaptation is a result of the fact that the lamellae adjusts its shape as the deformation is continued and so does not generate the necessary depolarization for signaling. Pacinian Corpuscle Frequency of the vibratory signal equal the frequency of firing and intensity is a reflection of the number of action potentials generated. In addition to being rapidly adapting these receptors have large receptive fields and therefore are poorly localizing.  Meissner's corpuscles is consist of group II myelinated fibres inserted Meissner’s into a small capsule. They have extremely small Corpuscle receptive fields and therefore are utilized during two-point discrimination. They are found in high concentrations in the fingertips and the lips (non- hairy skin). Meissner's corpuscles are also rapidly adapting receptors that can also encode tapping, and flutter.  These are unencapsulated mechanoreceptors located in the epidermis of non-hairy skin. The nerves end in disc enclosed by a specialised epithelial cell – Merkel’s cell They have small receptive fields and are slowly adapting, accounting for 25 % of mechanoreceptors in hand and particularly dense in finger tips and lips. Receptor potentials are only generated if pressure is applied to the disc itself and so it is ideal for detecting the location of a stimulus. Merkel’s discs Ruffini’s corpuscles Nerve terminal ends in a liquid filled collagen capsule. Any deformation of the skin causes the nerve cells to depolarize. Slowly adapting receptor Large receptive field Encodes information about the magnitude of the stimulus  Rapidly adapting Slowly adapting Meissner's Merkel Pacinian Ruffini Thermoreceptors These are found on free nerve endings of myelinated (Ad) and unmyelinated (C) fibres. There are separate receptors with unique receptive fields for cold and warm sensations. Cold receptors – optimum sensitivity at 25oC Hot receptors - optimum sensitivity at 45 oC At 34 oC both receptor types are activated Very cold (45 oC) activate pain fibres (i.e. thermal nociceptors) Thermoreceptors These receptors transiently change their rate of firing as the temperature changes in a particular direction and so the initial skin temperature is important.  One of the primary functions of the somatosensory system is to provide the brain with information about what part of the body is being stimulated. Try to imagine losing your sense of touch. What might some of the effects be? Where is it? Labelled line code Remember the answer to the question where is it is provided by the labelled line code. Dermatome: Refers to the body area innervated by the neurons in a single spinal nerve Where is it?  Remember the receptive field is the area of the body which when stimulated activates the sensory receptor. Two - point discrimination = Minimal distance to perceive two simultaneous stimuli as two separate events Two point discrimination Two point discrimination Receptive fields differ in size on different parts of the body Small and numerous on hands and face Large and few on back, leg, arm Spatial discrimination ability - highest where receptive fields are small and numerous, e.g., hands. Lateral inhibition A stimulus activates some neurones whilst inhibiting adjacent neurones (inhibitory and excitatory receptive fields overlap) in order to sharpen the edges of the perceived sensation. This process improves feature extraction of the stimulus. Lateral inhibition occurs at each relay point in the sensory pathway i.e. dorsal horn, thalamus & cortex. Mach Bands Mach Bands  The are two primary Sensory pathways pathways that convey sensory information from the periphery through the spinal cord to the brain 1. The dorsal column pathway transmits information pertaining to fine touch, two-point discrimination, pressure and proprioception. 2. The anterior lateral system conveys information pertaining to pain, temperature and crude touch. Dorsal Columns Anterolateral System Touch sensations requiring a high Pain degree of localization of the stimulus Thermal sensations, including both Touch sensations requiring transmission of fine gradations of warmth and cold sensations intensity Crude touch and pressure sensations Phasic sensations, such as vibratory capable only of crude localizing ability sensations on the surface of the body Sensations that signal movement against the skin Tickle and itch sensations Position sensations from the joints Sexual sensations Pressure sensations having to do with fine degrees of judgment of pressure intensity DORSAL COLUMNS Receptor → Spinal nerve, enter via dorsal root → ascend ipsilateral to gracilis or cuneatus nucleus in the medulla → cross and ascend through medial lemniscus to Thalamus (VPL nucleus) → Somatosensory Cortex ANTEROLATERAL SYSTEM Receptor → Spinal nerve, enter via dorsal root → Synapses and cross in spinal tract to ascend contralateral → Ventral posterolateral n. of thalamus → Somatosensory Cortex Clinical Application A young man in his early twenties got into a fight at a bar and received a stab wound on the right side of his lower back that appears to involve his spinal cord. Clinical examination demonstrated hemiparesis of his right leg, loss of discriminatory touch on his right side and loss of pain and temperature on his left side below the site of injury. Thalamus Thalamus – is a relay station to the cerebral cortex for all sensory input, except olfaction.  There are different cortical areas involved in the processing of somatosensory information. 1. SI – Primary somatosensory cortex post-central gyrus Brodmann’s areas 1, 2, 3 with a topographic map or homunculus. 2. SII – secondary somatosensory cortex superior bank of the lateral fissure has another topographic map but much more diffuse 3. Somatosensory association Cortical Processing cortex Brodmann’s area 5 and 7 Primary Somatosensory Cortex Somatic senses are represented contra- laterally, except for few fibers from face. Larger areas for lips, thumb, & face, smaller areas for other body parts like trunk & lower parts Primary Somatosensory Cortex  Functionally the somatosensory cortex arranged in vertical columns extending all the way through Primary the six layers of the cortex, Somatosensory containing perhaps 10,000 neuronal cell Cortex bodies. Each of these columns serves a single specific sensory modality i.e some respond to stretch around joints, some to stimulation of tactile hairs, others to discrete localized pressure points on the skin Some neurons in the cortex have orientation- sensitive and direction-sensitive properties. Somaotsensory association cortex Receives synthesized connections from the primary and secondary sensory cortices. Involved in complex associations. Electrical stimulation in a somatosensory association area can occasionally cause an awake person to experience a complex body sensation, sometimes even the "feeling" of an object such as a knife or a ball. Also involved in integrating information from different sensory sytems. Damage to Somatosensory Cortex Somatosensory System MDSC2002: Neuroscience & Behaviour Farid F. Youssef