Sensory Receptors PDF - Neurophysiology

Sensory Receptors PHYL 206-Neurophysiology Salim MOUSSA, PhD 1  Sensory system: Ability to be in interaction with the environment (internal and external) 2  Stimulus & Mod...

Sensory Receptors PHYL 206-Neurophysiology Salim MOUSSA, PhD 1  Sensory system: Ability to be in interaction with the environment (internal and external) 2  Stimulus & Modalities A stimulus is a change detectable by the body. Stimuli exist in a variety of energy forms, or modalities, such as heat, light, sound, pressure, and chemical changes. Sometimes we perceive sensory signals when they reach a level of consciousness, but other times they are processed completely at the subconscious level. All the information regarding all these senses is sent to the CNS via AFFERENT NEURONS. 3  Because the only way afferent neurons can transmit information to the CNS about stimuli is via action potential propagation, these forms of energy must be converted into electrical signals. THE CONVERSION OF STIMULUS ENERGY INTO A GRADED POTENTIAL IS CALLED SENSORY TRANSDUCTION AND IS DONE BY SENSORY RECEPTORS. 4  Sensory Receptors Specialized structures which respond to changes in their environment (stimuli) Some receptors are simply ends of sensory nerve fibers. Other receptors are cells adjacent to sensory nerve fibers. Other receptors are sensory nerve fiber endings plus specialized supporting cells and/or extracellular material Receptor activation results in graded potentials that may trigger action potentials Sensation (awareness of stimulus) and perception (interpretation of the meaning of the stimulus) occur in the brain Transduction: the process by which a stimulus of any form is transformed into electrical responses 5  Classifying receptors Receptors have been classified according to: – The type of stimulus they detect – Receptor location in the body – Structural complexity of the receptor 6  Classification by Stimulus Type Mechanoreceptors—respond to touch, pressure, vibration, stretch, and itch Thermoreceptors—sensitive to changes in temperature Photoreceptors—respond to light energy (e.g., retina) Chemoreceptors—respond to chemicals (e.g., smell, taste, changes in blood chemistry) Nociceptors—sensitive to pain-causing stimuli (e.g. extreme heat or cold, excessive pressure, inflammatory chemicals) 7  Classification by Location Exteroceptors Respond to stimuli arising outside the body: receptors in the skin for touch, pressure, pain, and temperature; also most special sense organs (eyes, ears, etc) Interoceptors (visceroceptors) Respond to stimuli arising in internal viscera and blood vessels: chemical environment, tissue stretch, temperature Proprioceptors Respond to stretch in skeletal muscles, tendons, joints, ligaments, and connective tissue coverings of bones and muscles; inform the brain of one’s movements 8 9 Unencapsulated (free) dendritic endings 10 Encapsulated dendritic endings 11 12  Receptor potential A receptor potential, also known as a generator potential, a type of graded potential, is the transmembrane potential difference produced by activation of a sensory receptor. 13 14 15 16 17 18 19 20 Researchers have enough data now to indicate that the origin of phantom limb sensations must be the brain itself. 21  Special Proprioceptors in skeletal muscles Proprioceptors: – monitor the positions of joints and muscles – the most structurally and functionally complex of general sensory receptors 22 Proprioceptors (not shown – Joint kinesthetic receptors.) 23  Muscle Spindles Interspersed among most skeletal muscle fibers and aligned parallel to them. Measure muscle stretching. Consists of intrafusal muscle fibers- specialized muscle fibers with sensory nerve endings and motor neurons called gamma motor neurons. Extrafusal muscle fibers- surrounding muscle fibers supplied by alpha motor neurons. 24  Tendon Organs Located at the junction of a tendon and a muscle. Protect tendons and their associated muscles from damage due to excessive tension. Consists of a thin capsule of connective tissue that encloses a few tendon fascicles. 25  THE MUSCLE SPINDLE AS LENGTH DETECTOR(SENSORY FIBERS) TYPE Ia NERVE FIBERS: TRANSMIT INFORMATION ABOUT LENGTH AND VELOCITY TO THE CNS TYPE II NERVE FIBERS:TRANSMIT INFORMATION ABOUT MUSCLE LENGTH TO CNS 26 TWO TYPES OF INTRAFUSAL FIBERS TYPE IA SENSORY FIBER TYPE II SENSORY FIBER NUCLEAR NUCLEAR CHAIN BAG FIBER FIBER 27 STRETCHING AN INTRAFUSAL FIBER SENDS SIGNALS TO CNS 28  GOGLI TENDON ORGANS: TENSION RECEPTORS IN SERIES WITH EXTRAFUSAL FIBERS TRANSMITS INFORMATION ABOUT FORCE OR TENSION TO CNS FREQUENCCY CODING Each tendon organ is innervated by a single afferent type Ib sensory nerve fiber (Aɑ fiber) that branches and terminates as spiral endings around the collagen strands. The Ib afferent axon is a large diameter, myelinated axon. 29 30  Joint Kinesthetic Receptors Found within or around the articular capsules of synovial joints. Free nerve endings and Ruffini corpuscles in the capsules of joints respond to pressure. Pacinian corpuscles respond to acceleration and deceleration of joints during movement. 31 32 33  Somatic Sensory Pathways First-order neuron(somatic receptor to the brain stem/spinal cord) → second order neuron(brain stem/spinal cord to the thalamus; decussate) → third-order neuron(thalamus to the primary somatosensory area of the cortex). 34 35  Major Somatic Sensory Pathways The posterior column-medial lemniscus pathway. The anterolateral (spinothalamic) pathway. The trigeminothalamic pathway. The anterior and posterior spinocerebellar pathway. 36 The somatosensory pathways 37 Trigeminothalamic Pathway Conveys nerve impulses for most somatic sensations from the face, nasal cavity, oral cavity and teeth to the cerebral cortex. 38 Mapping of the Primary Somatosensory Area The Homunculus On the surface of the brain is an area known as the somato-sensory cortex. It is the brain’s map of the body, known familiarly as the “homunculus.” As might be expected, touch increases activity here (Young et al 2004). When someone touches you, receptors in the skin and or in the muscles transmit a signal via the spinal cord and medulla to this area of your brain; this corresponds to an increase in the activity of the neurons in this area. Touch receptors in skin are distinct from those in muscle. There may be measurable differences in brain response to different depths and duration of touch. This is an as yet unexplored area. 39 40