BSCI353 Midterm Study Guide Keep in mind exam also includes content from Chapters 5 6 and 7 Chapter 9 The Somatic Sensory System Touch Proprioception Lecture 7 on ELMS February 27th and March 4th lectures Khan academy video for overview http www khanacademy org science health and medicine nervous system and sensory infor somatosensation 2014 03 27T18 45 57 777Z v somatosensation 1 Overview of the somatosensory system o Responsible for touch mechanoception pressure vibration limb position proprioception temperature thermoception and pain nociception o Dr Kanold focuses on the three receptors mechanoreceptors nociceptors and thermoceptors Mechanoreceptors are in chapter 9 Nociceptors are in chapter 10 Thermoceptors are not really touched on in the book but can be found on pages 210 211 Sensory transduction converting energy of a stimulus into an electrical signal o Afferent fibers in the somatic sensory system Convey information from the skin surface to central circuits Cell bodies of the fibers are located along the spinal cord o Sensory receptors for the face are found in the trigeminal ganglia near brainstem Figure 9 1 Neurons are pseudounipolar Afferent fibers are continuous attached to the cell body in the ganglia by a single process Afferent fibers are encapsulated by mechanoreceptors Result in lower thresholds for action potential generation more sensitive to stimulation Fibers without these receptor cells are free nerve endings carrying pain and temperature Dermatome is the area of the skin that is supplied by a single spinal nerve o Dermatomes overlap so that injury to an individual dorsal root does not lead to complete loss of sensation in the relevant skin region o Most overlap in sensations for touch pressure and vibration than for pain and temperature Transduction in a mechanosensory afferent o Once a capsule is deformed the membrane of the fiber stretches opening cation channels leading to depolarization Figure 9 2 Properties that differentiate somatic sensory afferents o Axon diameter table 9 1 Largest to smallest muscle touch pain temperature itch Larger diameter means greater conduction speed o Size of receptive field The area of the skin surface over which stimulation results in a significant change in the rate of action potentials Receptive fields with dense innervation finger toes are relatively small Limits the spatial accuracy with which tactile stimuli can be sensed which is why two point discrimination varies across the skin Two point discrimination is the minimum interstimulus distance required to perceive two simultaneously applied stimuli as distinct Mechanosensory afferent fibers can have overlapping receptive fields If two point discrimination stimuli are closely spaced there is a single focus of neural activity with one afferent firing most actively If the stimuli are further apart then the activity in the outer afferents fire more figure 9 3 Smaller two point discrimination threshold in fingers than wrist because smaller receptive field TAKE AWAY since fingers have small threshold receptor field it takes less of a distance to recognize two different foci o Temporal dynamics figure 9 4 Rapidly adapting afferents Become inactive in the face of continued stimulation Effective in conveying information about changes in ongoing stimulation Ex receptors have adapted to you sitting down in a chair or wearing clothes Slowly adapting afferents Provide information about the spatial attributes of the stimulus such as size and shape o 4 classes of mechanoreceptors table 9 2 Merkel cell afferents Slowly adapting Account for 25 of afferents in hand Enriched in fingertips only afferent located in epidermis lying in the tips of epidermal ridges fingerprints Excitable cells required for synaptic vesicle release Process information about form and texture Meissner afferents Pacinian afferents Rapidly adapting fibers Account for 40 of innervation in hand Lie in the tops of the dermal papillae adjacent to the epidermal ridges Comprised of Schwann cells Four times more sensitive to skin deformation that Merkel Larger receptor fields thus they transmit signals with reduced spatial resolution Responsible for detecting slippage between the skin and an object held in the hand control of grip Rapidly adapting fibers 10 15 innervation of hand Located deep in the dermis subcutaneous tissue Acts as a filter allowing only disturbances at high frequencies Lower response threshold Detect vibrations transmitted through objects being held ex cutting bread with a knife using a wrench Slowly adapting fibers Located deep in skin also in ligaments and tendons Located parallel to stretch lines in skin making them sensitive to the skin stretching produced by digit limb movement Ruffini afferents 20 of hand innervation Associated with finger position conformation of the o Joint Receptors hand Mechanoreceptors specialized for proprioception Give information about the position of the limbs and other body parts in space Two types muscle spindles golgi tendons Muscle spindles signals stretch of the muscle inside the muscle Golgi is on the outside of the muscle You would lose all sensation without these two systems o Ian Waterman had a virus that killed the Muscle spindles necessary nerves Found in skeletal muscles consisting of 4 8 specialized intrafusal muscle fibers Intrafusal muscle fibers are surrounded by a capsule of connective tissue and are in a parallel arrangement with the extrafusal fibers When the muscle is stretched the tension on the intrafusal fibers activates mechanically gated ion channels in the nerve endings generating action potentials Innervation of the muscle spindles arrive from two classes of fibers o Primary endings Group Ia afferents Largest myelinated sensory axons Rapidly adapting responses to changes in muscle length Transmit info about limb dynamics the velocity and direction of movement o Secondary endings Group II afferents Produce sustained responses to constant muscle lengths Provide info about static position of limbs Muscle fibers are controlled by motor neurons o motor neurons lie in the ventral horn of the spinal cord and are connected to the intrafusal fibers Level of gamma activity allows to determine limb position and movement o Alpha motor neurons are attached to the outside Golgi tendon organs connective tissue Mechanoreceptors inform the CNS about changes in muscle tension as opposed to muscle length like muscle spindles do Low threshold receptors